As we all know, the legal limit on truck tyres is 1mm, and on cars it’s 1.6mm’s. But my brain really doesn’t like remembering it that way around, and naturally wants to switch them, as logically when I scale up, it seems more reasonable for the truck tyres to be 1.6mm minimum.
So can someone explain to me why the limit is lower for trucks and buses?
The only logical explanation I can come up with is that due to the extra weight of a truck, it’s more capable of dispersing water in the wet, and therefore minimum tread depth is less relevant/critical…
On a side issue, recut tyres - From what I know, truck tyres can be recut once, and this is done by hand. Now I find this a little scary, as how (as a driver) am I meant to know that a set of tyres has only been recut once, and I’m not driving a truck with dangerous tyres?
I’m as green as grass on this stuff, and it’s not like it’s keeping me awake at night, I’m just an inquisitive sod and would like to know ! 
Evil8Beezle:
On a side issue, recut tyres - From what I know, truck tyres can be recut once, and this is done by hand. Now I find this a little scary, as how (as a driver) am I meant to know that a set of tyres has only been recut once, and I’m not driving a truck with dangerous tyres?
Because a tyre fitter wouldn’t cut a tyre that had been cut already.
Evil8Beezle:
As we all know, the legal limit on truck tyres is 1mm, and on cars it’s 1.6mm’s. But my brain really doesn’t like remembering it that way around, and naturally wants to switch them, as logically when I scale up, it seems more reasonable for the truck tyres to be 1.6mm minimum.
So can someone explain to me why the limit is lower for trucks and buses?
The only logical explanation I can come up with is that due to the extra weight of a truck, it’s more capable of dispersing water in the wet, and therefore minimum tread depth is less relevant/critical…
On a side issue, recut tyres - From what I know, truck tyres can be recut once, and this is done by hand. Now I find this a little scary, as how (as a driver) am I meant to know that a set of tyres has only been recut once, and I’m not driving a truck with dangerous tyres?
I’m as green as grass on this stuff, and it’s not like it’s keeping me awake at night, I’m just an inquisitive sod and would like to know !
IF a tyre has been recut more than once or recut too deep if you look in bottom of tread pattern you will see the pattern of the tyre cords under the rubber,this obviously would need changing, sorry if this does not make sense Evil but its been a long crap week! Cheer’s Pete
Tread used to be much more , think it was 3 mm .
But those swines the europeans were upset at getting fined for bald tyres here which were legal in their own countries .
If a tyre has been recut it should have a mark/notch in panel on sidewall .
Used to have saturday morning work recutting tyres but not heard of drivers doing it in last 10 years .
Reason for recutting is if tread was down to maximum level the blocks would be so tall they would break off .
Not something i worry my pretty little head about tho .
If its hard , got tight nuts and a little scratch showing i just drive em .
Cars have 1.6 mm due to Danger of Aquaplaning as they lighter and First,higher Speed.
Dont worry. I had aquaplaning with full Load too.
You can not re-cut Tyres twice. Its technically un-possible.
edit
deleted on the grounds it was nonsense.
My understanding of the legal requirement is the truck is 1 as opposed to 1.6 because a truck has more wheels than a car therefore there is a lot more tyre actually in contact with the road.
The difference is fairly minimal and as we all know any decent haulage company would change a tyre when it reached 3 and any car driver when it got to 2.
Not sure of the logic on the different tread depths, it’s only 1mm for bikes too, and mopeds are legal with visible tread only.
IIRC the minimum depth was set at 1mm for all vehicles, it then increased to 1.6mm for vehicles under 3.5T in the 1990’s. It is rare that one of my truck tyres gets lower than 3mm before it is removed, as there is a greater risk of cuts to the cords with low tread depth.
I saw fitter re cutting a tyre yesterday, he pointed out these tiny holes in the bottom of the treads, if you can still see the holes then it’s ok to cut. Obviously don’t cut deeper than the holes though!
Thanks for the answers people!
I kind of knew that if you could see the cord, the tyre had been overcut. But I’ve no idea how dangerous this could be, or if some more unscrupulous people would cut it down to this, to try and make the tyres last a bit longer.
Cheers to GORDON 50, I didn’t know about the little holes!
As for the main question, it appears to me that vehicle weight, and possibly a lower vehicle speed are the only reasons.
I can’t get my head around the spinning speed having any effect on clearing water, and the whole reason for the tread.
Evil8Beezle:
Thanks for the answers people!
I kind of knew that if you could see the cord, the tyre had been overcut. But I’ve no idea how dangerous this could be, or if some more unscrupulous people would cut it down to this, to try and make the tyres last a bit longer.
Cheers to GORDON 50, I didn’t know about the little holes!
As for the main question, it appears to me that vehicle weight, and possibly a lower vehicle speed are the only reasons.
I can’t get my head around the spinning speed having any effect on clearing water, and the whole reason for the tread.
I’m pretty sure that it’s down to the weight of the vehicle. It may also have been due to lobbying from the transport industry at the time of the change (i.e. operators telling MPs to exempt commercial vehicles from the change (from 1 mm to 1.6 mm) to keep their costs down.
I don’t think that the “spinning speed” makes much difference (in fact, the higher rotational speed would mean that the water would be “flung” off the tyre more readily). However, I can think of a possible justification for why a larger wheel might be less likely to aquaplane.
If you think of how the contact patch changes as the wheel rotates, then you have a leading edge at the front where water is being sandwiched between the tyre and the road. The weight of the vehicle pushing down then squeezes this water out into either the grooves in the tyre or away from the contact patch entirely. As the tyre rolls, the front part of the contact patch will have a layer of water between the tyre and road, but at some point between the leading and trailing edge, the water will be squeezed out to the point where the tyre is in direct contact with the road. The faster you go, the further back this point becomes. You start to aquaplane when the combination of the depth of the water arriving at the leading edge and the speed at which it is arriving means that it doesn’t get squeezed out before it reaches the trailing edge of the contact patch, and therefore the whole of the contact patch has a layer of water separating the tyre and road.
A larger diameter tyre will have a larger contact patch (front to back), and therefore there is more time for the water to be squeezed out.
That’s my guess, anyway.
I can sort of understand about the 1mm tread with wider tyres and more of them etc, but why the hell are motorbikes only 1mm as well? A much smaller contact patch, plus only 2 wheels! Just doesn’t make sense to me.
GORDON 50:
I can sort of understand about the 1mm tread with wider tyres and more of them etc, but why the hell are motorbikes only 1mm as well? A much smaller contact patch, plus only 2 wheels! Just doesn’t make sense to me.
Obviously there are extreme examples but in general they are narrower and have a rounded profile rather than the flat profile of motor vehicles.
With trucks my understanding was always the weight makes it harder for the water to lift the tyre out of contact with the tarmac. Also they effectively have a permanent tread groove cut into the middle of their drive axles by having twin wheels.
GORDON 50:
I can sort of understand about the 1mm tread with wider tyres and more of them etc, but why the hell are motorbikes only 1mm as well? A much smaller contact patch, plus only 2 wheels! Just doesn’t make sense to me.
The reason may be that motorcycle tyres are made of a far softer compound than car or truck tyres and therefore give a lot more grip which is needed when you are getting your knee down round a bend. 
when I have given it some proper thought, I think my earlier post about tyre diameter was nonsense, so deleted it. 
don’t think my brain was firing on all cylinders this morning …
albion1971:
GORDON 50:
I can sort of understand about the 1mm tread with wider tyres and more of them etc, but why the hell are motorbikes only 1mm as well? A much smaller contact patch, plus only 2 wheels! Just doesn’t make sense to me.
The reason may be that motorcycle tyres are made of a far softer compound than car or truck tyres and therefore give a lot more grip which is needed when you are getting your knee down round a bend.
This; actually on a dry surface a bald tyre can actually stick better than a treaded one. That’s why racers use them. Wouildn’t recommend it on the road though.
In old Times Car were slow,Heavy and had small Tyres.
Then Tyres went wider,Cars faster and lighter,which increased the Danger of aquaplaning.
Lorries are Heavy and slow,why the Risk of Aquaplaning is much lower.
Motorbikes have small Tyres,and as far as you dont speed you shall be safe too.
The Gov does Tests,and theres Decision is on an Average Circumstances. But,on Places were that were not enough are Speed Limits put in Place to keep you safe.
