Could someone place clarify the rules surrounding the minimum ground clearance? For larger axle interspaces I believe it is 190mm, but is that specific to the trailer? So things such as side skirts can be lower than that? If so would that end up being problematic if going over speed bumps?
Thanks!
Didn’t realise there were rules?
I’d think you can go as low as want but risk grounding. Going low from weight, nows that’s an issue
There must be a minimum ground clearance of 160 millimetres if the trailer has an axle interspace of more than 6 metres but less than 11.5 metres, and a minimum clearance of 190 millimetres if the trailer has an axle interspace of 11.5 metres or more.
Rear bumpers - ground to lower part of bumper ground clearance = minimum 500mm ,
and or any ancillary equipment fitted inclusive such as taillifts - moffats - the rear bumper clearance as above =500mm.
Car/van/4x4 /light commercial/etc , Rear fitted tow balls/hitches will normally equate to 500mm ground clearance to the ball also.- hence swan-neck hitches.
Coaches in some areas is a minimum of 100mm,
Rules Regs - Comes under construction and use regs iirc. 
Minimum Ground Clearance Risk of Grounding
Trailers subject to the minimum ground clearance rules must keep to the minimum at all times when in use on a road. Exceptions are provided:
for a trailer fitted with a suspension system with which it may be lowered or raised:
while that system is being operated to enable the trailer to pass under a bridge or other obstruction; and
as long as no part of the trailer (excluding wheels) touches the ground or is likely to do so; or
while loading or unloading.
To minimise the risks of grounding on railway level crossings etc,
every trailer manufactured on or after 1 April 1984 must meet minimum ground clearance rules.
That’s perfect, thanks!
How about the car transporters then Trux? Some of them seem to have less than a handspan of clearance.
Does that mean all these lowered cars I see are illegal? Seems to be a new pastime in my area to get them as low as possible.
dcgpx:
Does that mean all these lowered cars I see are illegal? Seems to be a new pastime in my area to get them as low as possible.
Nope ,
provided that the work done to modify/customise the vehicle has been done /completed by a competent engineer and all is safe - crack on.
How low can you go 
The 1st traffic calmer they come across will determine that it will either see-saw or rip the exhaust off 
.
Some insurance companies may ask the Customer' for an engineers inspection report from a bonafide workshop for the file before insuring , all depends who is stated as having completed the alterations and what the mods are ,
and what the insurer/broker requires.
: