6x4 or 6x2

[kyrbo]

wire:
When i first arrived here i was on flat deck work and i used to wonder why i was struggling with tarps when a tilt or Euroliner would have been better but then i realized that most trailers don’t have chassis. I pull a reefer now and if i had a picture to hand i could show you that the body of the trailer actually is the only thing to it.

That’s quite surprising. How rigid that kind of structure is? What I mean with this is that how easily they twist when laden and driving over uneven surfaces and does that structure maintain it’s rigidity as trailer ages?

newmercman:
Think about this, Scandinavia has some pretty extreme weather, the roads are not exactly flat either & they run at 62.5ton with a 6x2 configuration, why then do we need double drive at almost half the weight? The answer is, we don’t, but that’s the way it’s always been done so that’s the way it always will be.

Well, actually I think it’s legally 60 tonnes both in Finland and Sweden, but at least here (Finland) you can be somewhat reliable that you wont get done for weighing 62.5 tonnes. But for the 6x2 vs. 6x4 debate that is irrelevant and even Finland isn’t very flat when that weight is pulled by 420 hp to 500 hp lorry which often is the case. There are more hills on northern parts of Finland and Sweden and despite that 6x4 is very rare on a freight hauling lorry as, like you mentioned, 6x4 isn’t needed when driving mostly on firm surfaces.

Lets take three lorries into consideration (let that be artics, wagon and drags or rigids as it doesn’t really matter). One of them is 4x2, one is 6x2 (tag axle, not midlift) and one 6x4 and they all are loaded so that weight on rear axle(s) is maximum allowed weight and, for the sake of the argument, let that be 9 ton on a drive axle and 18 ton on a rear axles. Next lets assume firm ground, like ice or packed snow on asphalt, which is so slippery that 4x2 can barely get moving and 6x2 can’t as one extra axle gives enough drag to get drive axle spinning despite both of them having same weight on a single drive axle. Like you probably know, 6x2 can get moving by dumping air or lifting tag axle up. 6x4 probably can get moving, as it has same weight on both drive axles as 4x2 lorry.

Now lets make conditions bit worse so that 4x2 can’t get moving anymore. As 4x2 has has same weight on drive axle than 6x4 has on both drive axles, it means that also 6x4 can’t get moving any more, but 6x2 can still get moving by dumping air or lifting tag up. Now why is that so? It’s because even lack of driver acceptance can’t outweight physics

In a short, a force which drive axle can transfer into the ground is directly proportional to the weight on the drive axle, meaning more weight you have on a single drive axle more grip you have (if friction coefficient between tyre and ground stays same). This means that when you lift tag axle up with 6x2 you transfer more weight on a single drive axle and this allows you to transfer more force to the ground opposed to 6x4 or 4x2 lorries. Actually I think that at least in Europe most 4x2 can transfer more force to the ground per single drive axle than 6x4’s can. If I recall correctly in Finland maximum weight on a single drive axle is 11.5 tonnes and 18 tonnes for two rear axles (19 t with road friendly suspension) and that equation gives you only 9 tonnes per drive axle on 6x4!

Now keep in mind this is just a coarse approximation, but as long as ground is firm, tyres are good AND you can lift free rolling axle up you’ll get more grip with 6x2 at slow speeds than you’ll get with 6x4. Grip is also increased with 6x2 midlift but with tag axle increase is much more dramatic. When ground is soft 6x4 is often superior as there are also other contributing factors than just the ability to transfer more force into the ground.

As a comment for those who are still thinking that 6x4 is superior to 6x2 on snowy hills: think about Norway. There definitely are small and winding roads climbing up the mountains and they have fair share of snow too and most of the Norwegian lorries are 6x2’s with tag axles which will raise from the ground almost despite the load. I know that at least Volvo has special Norway-suspension available which is rated for something like 19 tonnes on a single drive axle. I’m not sure about the exact amount, but I was like when a fitter talked me about it at Volvo carage some years ago.

[carryfast]

Kyrbo:

wire:
When i first arrived here i was on flat deck work and i used to wonder why i was struggling with tarps when a tilt or Euroliner would have been better but then i realized that most trailers don’t have chassis. I pull a reefer now and if i had a picture to hand i could show you that the body of the trailer actually is the only thing to it.

That’s quite surprising. How rigid that kind of structure is? What I mean with this is that how easily they twist when laden and driving over uneven surfaces and does that structure maintain it’s rigidity as trailer ages?

newmercman:
Think about this, Scandinavia has some pretty extreme weather, the roads are not exactly flat either & they run at 62.5ton with a 6x2 configuration, why then do we need double drive at almost half the weight? The answer is, we don’t, but that’s the way it’s always been done so that’s the way it always will be.

Well, actually I think it’s legally 60 tonnes both in Finland and Sweden, but at least here (Finland) you can be somewhat reliable that you wont get done for weighing 62.5 tonnes. But for the 6x2 vs. 6x4 debate that is irrelevant and even Finland isn’t very flat when that weight is pulled by 420 hp to 500 hp lorry which often is the case. There are more hills on northern parts of Finland and Sweden and despite that 6x4 is very rare on a freight hauling lorry as, like you mentioned, 6x4 isn’t needed when driving mostly on firm surfaces.

Lets take three lorries into consideration (let that be artics, wagon and drags or rigids as it doesn’t really matter). One of them is 4x2, one is 6x2 (tag axle, not midlift) and one 6x4 and they all are loaded so that weight on rear axle(s) is maximum allowed weight and, for the sake of the argument, let that be 9 ton on a drive axle and 18 ton on a rear axles. Next lets assume firm ground, like ice or packed snow on asphalt, which is so slippery that 4x2 can barely get moving and 6x2 can’t as one extra axle gives enough drag to get drive axle spinning despite both of them having same weight on a single drive axle. Like you probably know, 6x2 can get moving by dumping air or lifting tag axle up. 6x4 probably can get moving, as it has same weight on both drive axles as 4x2 lorry.

Now lets make conditions bit worse so that 4x2 can’t get moving anymore. As 4x2 has has same weight on drive axle than 6x4 has on both drive axles, it means that also 6x4 can’t get moving any more, but 6x2 can still get moving by dumping air or lifting tag up. Now why is that so? It’s because even lack of driver acceptance can’t outweight physics

In a short, a force which drive axle can transfer into the ground is directly proportional to the weight on the drive axle, meaning more weight you have on a single drive axle more grip you have (if friction coefficient between tyre and ground stays same). This means that when you lift tag axle up with 6x2 you transfer more weight on a single drive axle and this allows you to transfer more force to the ground opposed to 6x4 or 4x2 lorries. Actually I think that at least in Europe most 4x2 can transfer more force to the ground per single drive axle than 6x4’s can. If I recall correctly in Finland maximum weight on a single drive axle is 11.5 tonnes and 18 tonnes for two rear axles (19 t with road friendly suspension) and that equation gives you only 9 tonnes per drive axle on 6x4!

Now keep in mind this is just a coarse approximation, but as long as ground is firm, tyres are good AND you can lift free rolling axle up you’ll get more grip with 6x2 at slow speeds than you’ll get with 6x4. Grip is also increased with 6x2 midlift but with tag axle increase is much more dramatic. When ground is soft 6x4 is often superior as there are also other contributing factors than just the ability to transfer more force into the ground.

As a comment for those who are still thinking that 6x4 is superior to 6x2 on snowy hills: think about Norway. There definitely are small and winding roads climbing up the mountains and they have fair share of snow too and most of the Norwegian lorries are 6x2’s with tag axles which will raise from the ground almost despite the load. I know that at least Volvo has special Norway-suspension available which is rated for something like 19 tonnes on a single drive axle. I’m not sure about the exact amount, but I was like when a fitter talked me about it at Volvo carage some years ago.

But krybo in practice it all is’nt quite as simple as the physics suggest.Those conditions which you get regularly over there do sometimes happen here too sometimes more.Using extra weight on less drive axles versus the same weight but more driven wheels usually,if not always,comes out in favour of more driven wheels wins.You could get snow even deep enough to almost reach the tops of the wheels and there is a point where that extra weight on a single drive axle just makes no difference any more whereas having ENOUGH weight on double drive with cross locks and diff locks will get you through.But in general soft ground is no different to driving on deep snow and it’s that scenario which those yanks are probably rightly thinking of and if you don’t know how much deep snow North America can get in the winter then you probably won’t understand their way of thinking.But as for me I’ve seen what a 6x4 Foden snow plough can do in snow up to the doors over here in southern england not even Scotland where 4x2 and 6x2 artics were stuck sometimes for days because their drivers had given up trying to move them before it had even reached their wheelnuts.There’s also a lot of other advantages to double drive in addition to that as well but you’d need to have grown up with the American way to understand them all as I’ve said I was just unlucky to have been a Brit truck driver when I’d have been happier as an American one.And by the way Rikki double drive can often get through most stuff without even bothering with the crosss and diff locks but it will usually turn from a straight line given some time and coaxing or you’d find all those expensive Highways Agency Gritter/Ploughs abandoned at the first motorway roundabout junction that they come to when we get the next snow event.

[remy]

Rikki-UK:
OK you may be technically overweight on both axles, but unless you are really dumb enough to do it while your on a weighbridge your never going to get done for it

I hate to say this about my fellow yank truckers, but some would be dumb enough to do it,
One of the troubles with driving for a large fleet like i do, is they buy trucks that the dumbest driver can drive, that he has the least chance of screwing up in. My 6x4 has a power divider but that only helps some of the time, to lock both diffs up and have all 4 wheels turning would be great but then how long would they last when the dumb driver hammers down the hiway like that
Talking of lift axles, there were several debates at toll booths when drivers would lift the axle up and ask for a lower class (cheaper) ticket.

[carryfast]

remy:

Rikki-UK:
OK you may be technically overweight on both axles, but unless you are really dumb enough to do it while your on a weighbridge your never going to get done for it

I hate to say this about my fellow yank truckers, but some would be dumb enough to do it,
One of the troubles with driving for a large fleet like i do, is they buy trucks that the dumbest driver can drive, that he has the least chance of screwing up in. My 6x4 has a power divider but that only helps some of the time, to lock both diffs up and have all 4 wheels turning would be great but then how long would they last when the dumb driver hammers down the hiway like that
Talking of lift axles, there were several debates at toll booths when drivers would lift the axle up and ask for a lower class (cheaper) ticket.

Maybe but they probably are clever enough to understand that to have the same co efficient of friction provided by 8 wheels in contact with the road you’ll need to load four up to double the weight to even match it.So your firm would be happy with buying 4x2 or 6x2 units and then letting those drivers just double the weight on the tractor drive axle every time it snows and then go through the scales like that and tell the DOT that a Brit told them that’s how it should be done.

[remy]

Carryfast:

remy:

Rikki-UK:
OK you may be technically overweight on both axles, but unless you are really dumb enough to do it while your on a weighbridge your never going to get done for it

I hate to say this about my fellow yank truckers, but some would be dumb enough to do it,
One of the troubles with driving for a large fleet like i do, is they buy trucks that the dumbest driver can drive, that he has the least chance of screwing up in. My 6x4 has a power divider but that only helps some of the time, to lock both diffs up and have all 4 wheels turning would be great but then how long would they last when the dumb driver hammers down the hiway like that
Talking of lift axles, there were several debates at toll booths when drivers would lift the axle up and ask for a lower class (cheaper) ticket.

Maybe but they probably are clever enough to understand that to have the same co efficient of friction provided by 8 wheels in contact with the road you’ll need to load four up to double the weight to even match it.So your firm would be happy with buying 4x2 or 6x2 units and then letting those drivers just double the weight on the tractor drive axle every time it snows and then go through the scales like that and tell the DOT that a Brit told them that’s how it should be done.

If it’s me that gets nabbed at the scale then i’ll show them this post.

[newmercman]

Anyway,

A 6x4 will use more diesel than a 6x2

[carryfast]

newmercman:
Anyway,

A 6x4 will use more diesel than a 6x2

But the word is that they are going to bring UK diesel prices into line with US ones after the election so then we’ll be able to afford to run with 3 drive axles on our artics instead of 1 and if Rikky thinks that it’s difficult to make a 6x4 go round corners with all it’s locks in just wait til he has to drive one of those but there’ll be no more convoys of stuck artics on our motorways the next time we get a few inches of snow.

[newmercman]

Carryfast:

newmercman:
Anyway,

A 6x4 will use more diesel than a 6x2

But the word is that they are going to bring UK diesel prices into line with US ones after the election so then we’ll be able to afford to run with 3 drive axles on our artics instead of 1 and if Rikky thinks that it’s difficult to make a 6x4 go round corners with all it’s locks in just wait til he has to drive one of those but there’ll be no more convoys of stuck artics on our motorways the next time we get a few inches of snow.

What colour is the sky in your world?

UK fuel prices coming down, yeah right…

Everybody running heavier, more expensive to buy, run & maintain 6x4s, I don’t think so…

Nobody getting stuck in the snow, oh come, please, how is an extra drive axle going to give somebody the ability to drive in conditions that are, for the most part, very different to what they are used to?

Even if you wanted to be Supertrucker & get there no matter what, a truck with caterpillar tracks would be about as much use as mudflaps on a tortoise if the road in front was blocked by other vehicles, which it would be face facts, England & snow do not mix very well, it happens for a week or two at most every year, just accept that it’s going to be a nightmare, oh & don’t eat yellow snow

[carryfast]

newmercman:

Carryfast:

newmercman:
Anyway,

A 6x4 will use more diesel than a 6x2

But the word is that they are going to bring UK diesel prices into line with US ones after the election so then we’ll be able to afford to run with 3 drive axles on our artics instead of 1 and if Rikky thinks that it’s difficult to make a 6x4 go round corners with all it’s locks in just wait til he has to drive one of those but there’ll be no more convoys of stuck artics on our motorways the next time we get a few inches of snow.

What colour is the sky in your world?

UK fuel prices coming down, yeah right…

Everybody running heavier, more expensive to buy, run & maintain 6x4s, I don’t think so…

Nobody getting stuck in the snow, oh come, please, how is an extra drive axle going to give somebody the ability to drive in conditions that are, for the most part, very different to what they are used to?

Even if you wanted to be Supertrucker & get there no matter what, a truck with caterpillar tracks would be about as much use as mudflaps on a tortoise if the road in front was blocked by other vehicles, which it would be face facts, England & snow do not mix very well, it happens for a week or two at most every year, just accept that it’s going to be a nightmare, oh & don’t eat yellow snow

I was only young at the time but it snowed like it does around the Great Lakes States over here in the winter of 1963 and it lasted a lot longer than a week or two and my old mum says that we all probably would have starved to death if it had’nt have been for drivers then like me now driving good old fashioned 8x4 rigids getting the loads through.But then the roads were’nt blocked by stuck 6x2 and 4x2 artics like they would be today.But the bit about fuel prices coming down was just a joke or was it a dream.

[newmercman]

Carryfast,

Back in 63 men were men, you got a bag of nuts & a thick ear for xmas, happy days

[carryfast]

newmercman:
Carryfast,

Back in 63 men were men, you got a bag of nuts & a thick ear for xmas, happy days

But we did also get our christmas dinner because of wagons like this one built in the days when double drive was respected as much over here as it still is in the states.

www.truck-photos.net/picture/number1059.asp

[kyrbo]

Carryfast:

Kyrbo:
Now keep in mind this is just a coarse approximation, but as long as ground is firm, tyres are good AND you can lift free rolling axle up you’ll get more grip with 6x2 at slow speeds than you’ll get with 6x4. Grip is also increased with 6x2 midlift but with tag axle increase is much more dramatic. When ground is soft 6x4 is often superior as there are also other contributing factors than just the ability to transfer more force into the ground.

But krybo in practice it all is’nt quite as simple as the physics suggest.Those conditions which you get regularly over there do sometimes happen here too sometimes more.Using extra weight on less drive axles versus the same weight but more driven wheels usually,if not always,comes out in favour of more driven wheels wins.You could get snow even deep enough to almost reach the tops of the wheels and there is a point where that extra weight on a single drive axle just makes no difference any more whereas having ENOUGH weight on double drive with cross locks and diff locks will get you through.But in general soft ground is no different to driving on deep snow…

Well, I’d say it depends. About the deep enough snow being practically same as soft ground and that in soft ground (i.e. deep snow) double drive with enough weight on it is superior to single drive axle I totally agree with you (unless, of course, snow isn’t deep enough and ground below snow is firm and you can lift tag axle up and… ), but that wasn’t my point. I also gladly plead guilty on making conditions described in my post such that you rarely encounter such.

Despite the above, I still stand my ground on that you get more grip on a firm ground even with some or moderate amount of snow on it, with 6x2 with tag axle lifted up compared to 6x4 having similar weight on it. When I was just rereading my post I realized that I had left out the specific reason why this happens (and now I realized that I don’t know the English word for it and Finnish-English dictionaries in web didn’t know it). Nevertheless, when you take weight on a tyre and divide it by the area you get a variable which practically is behind this (direct translation from Finnish would be something like “pressure on a surface”). That variable affects the friction coefficient between tyre and the ground and increasing it increases friction coefficient which in turn increases the amount of force tyre is able to transfer to the ground. So lifting tag axle up increases weight on a drive axle which increases pressure between tyre and ground which then increases friction coefficient between them which in turn allows tyre to transfer more force. That’s what allows 6x2 to have more grip than 6x4 on firm but slippery ground as long as you can lift tag axle up (i.e. empty or light load) and speeds are relatively slow.

When speeds raise (like above 30 km/h) you obviously can’t drive with heavy load and tag axle up because of stability problems. At high speeds 6x4 also offers more grip but at those speeds you usually doesn’t have enough power under your hood to outrun the available traction even with 6x2 while having tag axle on ground. You can also dump air from tag to get some grip improvement when speed is too high or load too heavy to lift the tag.

Please keep in your mind that I’m not going to prove 6x2 being superior choice over 6x4 but just trying to point out that you can get considerable amount of grip even with 6x2. Also note that I’m speaking from Finnish perspective and here you rarely need anything else than that for 60 tonne on a Finnish main road network (which mostly look quite like Canadian roads in this diary). Also Swedes and Norwegians use lot of 6x2 lorries. I guess Norway is closest equivalent to Rocky Mountains in Scandinavia and Norway definitely has some heavy snowfalls and steep, small and winging roads and still they mostly use 6x2 with tag axle, lots of horses and chains for 50 tonnes so 6x2 likely offers enough traction for them (here’s one picture of Norwegian road and I’m sure you find plenty more from Marting Benjamins thread in photos-forum).

Then I can’t help mentioning that many 6x4 loggers (with iron springs) in Finland have system allowing them to transfer weight from front and rearmost axles to first drive axle. Some of those are factory made and some are retrofitted, but common thing for them is that they cause same effect as dumping air from tag would on 6x2 with air suspension. What I’m not sure is if those are mainly to improve grip or to improve manoeuvrability in forest (I’d guess the latter is more important).

Carryfast:
… if you don’t know how much deep snow North America can get in the winter then you probably won’t understand their way of thinking. … There’s also a lot of other advantages to double drive in addition to that as well but you’d need to have grown up with the American way to understand them all …

I don’t know how much and how often heavy snowfalls occur there, but I’d guess they are worse than in Finland because of differences in local climate. I also guess that road administrations response to snowfalls is faster in Finland than in US as I think US has a) more roads and b) distances between cities are longer so you are more likely to encounter unploughed snow on the roads so 6x4 might be handy when crossing such sections. Also US is quite huge country meaning lorries drive can drive through vast amount of different weather conditions so 6x4 gives some versatility 6x2 wouldn’t as with 6x4 company doesn’t have to invest that much into winter tyres which wear out fast in warm climate. I’m not that familiar with US states, but I’m quite sure that even in winter weather difference between Great Lakes region and California is quite big and that kind of routes probably occur. Another situation where 6x4 probably has an advantage over 6x2 is when running light as 6x4 is likely more stable at speed they are using in the US than 6x2 with tag up.

There are also cons I could mention, like are increased fuel consumptions and tyre costs worth of these advantages in all lorries everywhere in US, how negligible the difference in stability is and how much worth improved turning circle of 6x2 would be, but I won’t touch these as after all it’s local conditions, laws and user preferences which mostly dictate what type of vehicles are used.

Btw. one could use Robson drive on 6x2 to make poor mans 6x4. At time that was very popular in Finnish loggers and tippers but currently double drive has eaten much form Robson drives market share. I don’t know how often conditions requiring 6x4 are met in US but if they are not that often that kind of device might be handy.

remy:

Rikki-UK:
OK you may be technically overweight on both axles, but unless you are really dumb enough to do it while your on a weighbridge your never going to get done for it

I hate to say this about my fellow yank truckers, but some would be dumb enough to do it,
One of the troubles with driving for a large fleet like i do, is they buy trucks that the dumbest driver can drive, that he has the least chance of screwing up in. My 6x4 has a power divider but that only helps some of the time, to lock both diffs up and have all 4 wheels turning would be great but then how long would they last when the dumb driver hammers down the hiway like that
Talking of lift axles, there were several debates at toll booths when drivers would lift the axle up and ask for a lower class (cheaper) ticket.

I guess that is also one pros for 6x4 which I wont understand as I haven’t grown up with the American way of thinking…

[driveroneuk]

Robson drive, translation:

http://tinyurl.com/y9ty43l

[carryfast]

Kyrbo:

Carryfast:

Kyrbo:
Now keep in mind this is just a coarse approximation, but as long as ground is firm, tyres are good AND you can lift free rolling axle up you’ll get more grip with 6x2 at slow speeds than you’ll get with 6x4. Grip is also increased with 6x2 midlift but with tag axle increase is much more dramatic. When ground is soft 6x4 is often superior as there are also other contributing factors than just the ability to transfer more force into the ground.

But krybo in practice it all is’nt quite as simple as the physics suggest.Those conditions which you get regularly over there do sometimes happen here too sometimes more.Using extra weight on less drive axles versus the same weight but more driven wheels usually,if not always,comes out in favour of more driven wheels wins.You could get snow even deep enough to almost reach the tops of the wheels and there is a point where that extra weight on a single drive axle just makes no difference any more whereas having ENOUGH weight on double drive with cross locks and diff locks will get you through.But in general soft ground is no different to driving on deep snow…

Well, I’d say it depends. About the deep enough snow being practically same as soft ground and that in soft ground (i.e. deep snow) double drive with enough weight on it is superior to single drive axle I totally agree with you (unless, of course, snow isn’t deep enough and ground below snow is firm and you can lift tag axle up and… ), but that wasn’t my point. I also gladly plead guilty on making conditions described in my post such that you rarely encounter such.

Despite the above, I still stand my ground on that you get more grip on a firm ground even with some or moderate amount of snow on it, with 6x2 with tag axle lifted up compared to 6x4 having similar weight on it. When I was just rereading my post I realized that I had left out the specific reason why this happens (and now I realized that I don’t know the English word for it and Finnish-English dictionaries in web didn’t know it). Nevertheless, when you take weight on a tyre and divide it by the area you get a variable which practically is behind this (direct translation from Finnish would be something like “pressure on a surface”). That variable affects the friction coefficient between tyre and the ground and increasing it increases friction coefficient which in turn increases the amount of force tyre is able to transfer to the ground. So lifting tag axle up increases weight on a drive axle which increases pressure between tyre and ground which then increases friction coefficient between them which in turn allows tyre to transfer more force. That’s what allows 6x2 to have more grip than 6x4 on firm but slippery ground as long as you can lift tag axle up (i.e. empty or light load) and speeds are relatively slow.

When speeds raise (like above 30 km/h) you obviously can’t drive with heavy load and tag axle up because of stability problems. At high speeds 6x4 also offers more grip but at those speeds you usually doesn’t have enough power under your hood to outrun the available traction even with 6x2 while having tag axle on ground. You can also dump air from tag to get some grip improvement when speed is too high or load too heavy to lift the tag.

Please keep in your mind that I’m not going to prove 6x2 being superior choice over 6x4 but just trying to point out that you can get considerable amount of grip even with 6x2. Also note that I’m speaking from Finnish perspective and here you rarely need anything else than that for 60 tonne on a Finnish main road network (which mostly look quite like Canadian roads in this diary). Also Swedes and Norwegians use lot of 6x2 lorries. I guess Norway is closest equivalent to Rocky Mountains in Scandinavia and Norway definitely has some heavy snowfalls and steep, small and winging roads and still they mostly use 6x2 with tag axle, lots of horses and chains for 50 tonnes so 6x2 likely offers enough traction for them (here’s one picture of Norwegian road and I’m sure you find plenty more from Marting Benjamins thread in photos-forum).

Then I can’t help mentioning that many 6x4 loggers (with iron springs) in Finland have system allowing them to transfer weight from front and rearmost axles to first drive axle. Some of those are factory made and some are retrofitted, but common thing for them is that they cause same effect as dumping air from tag would on 6x2 with air suspension. What I’m not sure is if those are mainly to improve grip or to improve manoeuvrability in forest (I’d guess the latter is more important).

Carryfast:
… if you don’t know how much deep snow North America can get in the winter then you probably won’t understand their way of thinking. … There’s also a lot of other advantages to double drive in addition to that as well but you’d need to have grown up with the American way to understand them all …

I don’t know how much and how often heavy snowfalls occur there, but I’d guess they are worse than in Finland because of differences in local climate. I also guess that road administrations response to snowfalls is faster in Finland than in US as I think US has a) more roads and b) distances between cities are longer so you are more likely to encounter unploughed snow on the roads so 6x4 might be handy when crossing such sections. Also US is quite huge country meaning lorries drive can drive through vast amount of different weather conditions so 6x4 gives some versatility 6x2 wouldn’t as with 6x4 company doesn’t have to invest that much into winter tyres which wear out fast in warm climate. I’m not that familiar with US states, but I’m quite sure that even in winter weather difference between Great Lakes region and California is quite big and that kind of routes probably occur. Another situation where 6x4 probably has an advantage over 6x2 is when running light as 6x4 is likely more stable at speed they are using in the US than 6x2 with tag up.

There are also cons I could mention, like are increased fuel consumptions and tyre costs worth of these advantages in all lorries everywhere in US, how negligible the difference in stability is and how much worth improved turning circle of 6x2 would be, but I won’t touch these as after all it’s local conditions, laws and user preferences which mostly dictate what type of vehicles are used.

Btw. one could use Robson drive on 6x2 to make poor mans 6x4. At time that was very popular in Finnish loggers and tippers but currently double drive has eaten much form Robson drives market share. I don’t know how often conditions requiring 6x4 are met in US but if they are not that often that kind of device might be handy.

remy:

Rikki-UK:
OK you may be technically overweight on both axles, but unless you are really dumb enough to do it while your on a weighbridge your never going to get done for it

I hate to say this about my fellow yank truckers, but some would be dumb enough to do it,
One of the troubles with driving for a large fleet like i do, is they buy trucks that the dumbest driver can drive, that he has the least chance of screwing up in. My 6x4 has a power divider but that only helps some of the time, to lock both diffs up and have all 4 wheels turning would be great but then how long would they last when the dumb driver hammers down the hiway like that
Talking of lift axles, there were several debates at toll booths when drivers would lift the axle up and ask for a lower class (cheaper) ticket.

I guess that is also one pros for 6x4 which I wont understand as I haven’t grown up with the American way of thinking…

That’s all interesting stuff krybo but at the end of the day there’s only 1 way that your case can go if it’s taken to it’s logical conclusion.To get a similar type of traction as a typical yank 6x4 has to run with at all times in rain,snow or dry running you’d need to run along an interstate highway at the regular speeds which they use out there in those conditions (you would’nt believe how fast they drive in conditions where we would be stopped or running much slower than they do) or climb and descend their mountains fully freighted at their weight limits but in the typical British 4x2 configuration with your tag axle or mid lift raised.But the highway patrol,state trooper,and the DOT are going to take a very dim view of it all if/when they see a tractor unit running in that configuration when it should have 3 axles in contact with the road and under load not 2 just as they would here if you were trying to drive a fully freighted 44 tonner along a motorway or any other road with only 2 axles on the unit in operation not 3.The only question to still answer is would it be better to be running with 2 driven axles or 1.The issue of trying to get round the 6x2’s obvious weakness when it comes to traction etc by saying just run with the third axle raised is nothing but a red herring in the real world because you can’t do it ‘unless’ you want to run at 44 tonnes here and at nearly 37 tonnes in the states with effectively what would be a 4x2 unit but even then the questions of stability under braking and resistance to jacknifing would remain and as any tipper driver knows if that snow is deep enough that heavy single drive axle and it’s four wheels will just dig you into a deeper hole not get you out of it.

[wheel_nut]

Driveroneuk:
Robson drive, translation:

http://tinyurl.com/y9ty43l

I am not certain that the Robson Drive is the answer, look what my dynamo did to my bike tyre.

But I can see where they got the idea from

[carryfast]

Wheel Nut:

Driveroneuk:
Robson drive, translation:

http://tinyurl.com/y9ty43l

I am not certain that the Robson Drive is the answer, look what my dynamo did to my bike tyre.

But I can see where they got the idea from

Wheelnut that looks like the answer to all of the weight and cost issues.Why go to all the trouble of gearboxes,propshafts,and diffs and locks when we could just use one of those on each driven wheel?.The savings would surely pay for the wrecked tyres.

[driveroneuk]

That’s an old fashioned bit of kit isnt it? Didn’t they move them into the hub later? Then weren’t there legality issues because the lights went out when you stopped?

Good to see you keep 'er nicely polished up Malc.

[wheel_nut]

Carryfast:

Wheel Nut:

Driveroneuk:
Robson drive, translation:

http://tinyurl.com/y9ty43l

I am not certain that the Robson Drive is the answer, look what my dynamo did to my bike tyre.

But I can see where they got the idea from

Wheelnut that looks like the answer to all of the weight and cost issues.Why go to all the trouble of gearboxes,propshafts,and diffs and locks when we could just use one of those on each driven wheel?.The savings would surely pay for the wrecked tyres.

As you will know if you energise a dynamo it becomes an electric motor, my plan here was to connect my dynamo to me mums torch and use the dynamo to power me up the hills, unfortunately our neighbour Mr Robson saw the bike in our coal shed and copied it. It didn’t work very well cos it kept shredding tyres

[kyrbo]

Carryfast:
That’s all interesting stuff krybo but at the end of the day there’s only 1 way that your case can go if it’s taken to it’s logical conclusion.To get a similar type of traction/stability as a typical yank 6x4 has to run with at all times in rain,snow or dry running you’d need to run along an interstate highway at the regular speeds which they use out there in those conditions (you would’nt believe how fast they drive in conditions where we would be stopped or running much slower than they do) or climb and descend their mountains fully freighted at their weight limits but in the typical British 4x2 configuration with your tag axle or mid lift raised.But the highway patrol,state trooper,and the DOT are going to take a very dim view of it all if/when they see a tractor unit running in that configuration when it should have 3 axles in contact with the road and under load not 2 just as they would here if you were trying to drive a fully freighted 44 tonner along a motorway with only 2 axles on the unit in operation not 3.The only question to still answer is would it be better to be running with 2 driven axles or 1.The issue of trying to get round the 6x2’s obvious weakness when it comes to traction etc by saying just run with the third axle raised is nothing but a red herring in the real world because you can’t do it ‘unless’ you want to run at 44 tonnes here and at nearly 37 tonnes in the states with effectively what would be a 4x2 unit.

I’m not sure if you got my post so that I suggest running with tag axle up with 6x2 all the time when traction is reduced, but that’s not what I meant.

Situations I’m talking about where tag is “meant” (at least from drivers point of view ) to be lifted, or air dumped from, are basically when taking off on a slippery surface and on uphills when your speed drops much and road surface is (or you suppose it be) slippery. When driving at normal highway speeds axles stay down unless it’s safe (stability) and legal (axle weights) to lift them up. At those speeds you usually have more traction available than is the power you can get out of the engine so there’s no wheel spin even at full throttle and if there is then you’re obviously driving way too fast for weather conditions.

Still all the above applies only to Finland (and I’d believe that mostly in Sweden also) and in Finnish conditions 6x2 offers enough traction for normal winter weather while speeds remains between 80 and 90 km/h. With normal weather I mean anything between dry asphalt and ice and loose or packed snow and snowfall. I also haven’t never heard of plods stopping lorries in Finland due to weather conditions and when speed drops it’s usually because road is slippery (black ice, water on ice, etc) or snowfall is really heavy, not because of a ordinary snowfall or unploughed road.

Out of interest, what are normal speeds for lorries in US highways? I’ve understood that they are allowed to drive same speed as cars at least in majority of states, but how fast it is I don’t know. Something like 110 or 120 km/h?

Wheel Nut:

Driveroneuk:
Robson drive, translation:

http://tinyurl.com/y9ty43l

I am not certain that the Robson Drive is the answer, look what my dynamo did to my bike tyre.

But I can see where they got the idea from

I guess your dynamo would work much better if there were powerful HYDRAULICS pushing it towards the tyre I’m also sure you understood from translation that there isn’t any motors in Robson drive, it’s just piece of metal rolling freely and in theory you can turn it by hand, just like your dynamo should in theory be able to produce some electricity

Seriously speaking, I’d think Robson drive works pretty well or else all tippers and loggers here would have double drive but it’s still common thing to see even on new vehicles. Loggers often have double drive here nowadays, but you still see Robson drive on tippers quite often. With tippers I don’t mean just road going tippers, like ones hauling stuff from farms, but also “proper” tippers tipping regularly on building, etc. sites with soft ground. Sure, you get some tyre wear when pushing rough, round looking thing by force between axles, but then how often such conditions occur that double drive is needed depends quite much about the continent you live in, I guess

[carryfast]

Kyrbo:

Carryfast:
That’s all interesting stuff krybo but at the end of the day there’s only 1 way that your case can go if it’s taken to it’s logical conclusion.To get a similar type of traction/stability as a typical yank 6x4 has to run with at all times in rain,snow or dry running you’d need to run along an interstate highway at the regular speeds which they use out there in those conditions (you would’nt believe how fast they drive in conditions where we would be stopped or running much slower than they do) or climb and descend their mountains fully freighted at their weight limits but in the typical British 4x2 configuration with your tag axle or mid lift raised.But the highway patrol,state trooper,and the DOT are going to take a very dim view of it all if/when they see a tractor unit running in that configuration when it should have 3 axles in contact with the road and under load not 2 just as they would here if you were trying to drive a fully freighted 44 tonner along a motorway with only 2 axles on the unit in operation not 3.The only question to still answer is would it be better to be running with 2 driven axles or 1.The issue of trying to get round the 6x2’s obvious weakness when it comes to traction etc by saying just run with the third axle raised is nothing but a red herring in the real world because you can’t do it ‘unless’ you want to run at 44 tonnes here and at nearly 37 tonnes in the states with effectively what would be a 4x2 unit.

I’m not sure if you got my post so that I suggest running tag axle up with 6x2 all the time when traction is reduced, but that’s not what I meant.

Situations I’m talking about where tag is “meant” (at least from drivers point of view ) to be lifted or air dumped from are basically when taking off on a slippery surface and on uphills when your speed drops much and road surface is (or you suppose it be slippery). When driving at normal highway speeds axles stay down unless it’s safe (stability) and legal (axle weights). At those speeds you usually have more traction available than is the power you can get out of the engine so there’s no wheel spin even at full throttle and if there is then you’re obviously driving way too fast for weather conditions.

Still all the above applies only to Finland (and I’d believe that mostly in Sweden also) and in Finnish conditions 6x2 offers enough traction for normal winter weather while speeds remains between 80 and 90 km/h. With normal weather I mean anything between dry asphalt and ice and loose or packed snow and snowfall. I also haven’t never heard of plods stopping lorries in Finland due to weather conditions and when speed drops it’s usually because road is slippery (black ice, water on ice, etc) or snowfall is really heavy, not because of a ordinary snowfall or unploughed road.

Out of interest, what are normal speeds for lorries in US highways? I’ve understood that they are allowed to drive same speed as cars at least in majority of states, but how fast it is I don’t know. Something like 110 or 120 km/h?

:

krybo
But that’s not how I’m looking at it in practice and I think that most yanks would agree with me but I’m only guessing from the typical winter conditions here which can cause total gridlock and a stoppage on our roads and motorways most years in one part of England or another.Traction is something which remains an issue at most speeds certainly all the way from standstill up to 80kph.The average British artic has usually given up trying to keep moving in what I’d guess that you’d call loose snow although those depths in those Norwegian photos are’nt as deep as some which we’ve had over here some years.But it’s common to see trucks loaded at all types of weights lose traction while running at normal speed for the conditions (40 ish mph) even on a motorway and then lose speed until it’s momentum has gone and then it’s just a case of spinning drive wheels digging themselves deeper into the snow and sometimes the unit will swing sideways if it’s a 4x2 trying to jacknife itself when the drive axle loses grip.However maybe you’re relating more to a drawbar prime mover/rigid and trailer outfit which in my experience always behaves better in bad conditions than a tractor and semi does?.Which is why I always reckon that a 6x4 prime mover and drag is the best combination if it’s well loaded but an empty truck in bad weather is always going to find things even more difficult but those 6x4 motorway gritters of ours seem to manage ok even empty and as I said if 6x2 was ok then that’s how they’d spec them because the government does’nt like spending money on anything but offices.By the way the speed that they drive trucks at in the States depends on how fast the driver feels like he wants to run at but 110 kmh would be about right if he’s not in a hurry to get home or to the cafe but they often don’t slow down as much as we do in most bad weather conditions.