Finer points of tyre tread measurement

[jimzzr]

Hi,
I know that the law say that there must be 1.6mm of tread over the central 75% and around the entire circumference.

As I understand it the central 75% actually refers to normal contact patch rather than tyre width, so how is this actually defined/measured.

Also presumably it is only the full depth grooves that are measured?

Looking at one of my tyres it appears that the full width grooves only extend to around the central 2/3 of the tyre. When these grooves are down to the wear bars the thread on the outer edges of the tyre would be gone. The outer grooves extend to within ~5mm of the full depth ones so it's pretty clear they are cut to different depths. If I was to run these tyres down to the wear bars would they be illegal as only the central 2/3 would have tread or legal as the only grooves that count within the 75% have tread. Or perhaps I'm over estimating the contact patch?

Hope that makes sense to someone.

Also what is the case with uneven wear on motorcycle tyres where one side of the groove is at the wear bar and the other 2mm above it - lowest or take an average?

Thanks

Jim

[Arryace]

it's 75% of the treaded area that must have 1mm or more the remaining 25% need only have a visible tread pattern (for motorcycles the minimum tread depth is only 1mm) so if you liked to ride round and round your local roundabout trying to get yer knee down and wore down the right hand 25% of the treaded area to below 1mm the tyre would still be lagal.
contact patch is irrelevant

[fatboyjim154]

Buy a moped.

Only needs visable tread.

[jimzzr]

it's 75% of the treaded area that must have 1mm or more the remaining 25% need only have a visible tread pattern (for motorcycles the minimum tread depth is only 1mm) so if you liked to ride round and round your local roundabout trying to get yer knee down and wore down the right hand 25% of the treaded area to below 1mm the tyre would still be lagal.
contact patch is irrelevant

Sorry didn't make that very clear I was mainly refering to car tyres.

The bit about motorcycle tyres was an aside (due to irregular wear patterns that you don't seem to get with car tyres) - the assumtion in the above diagram is that the depth measured to one side of the groove equals the depth on the other side i.e. 2mm|_|2mm whereas you sometimes get 1mm¬_|2mm. Probably quite hard to visualise without a picture, basically one side of a tread bock gets worn down more than the other.

Thanks

Jim

[peodude]

Some more finer points, what happens if the wire is showing on the very inner edge, and the rest of the tyre still has lots of tread? I assume any wires showing make the tyre illegal, although the 75% rule says not.

[nilrem]

From memory the tyre should not show any of the inner structure or cord (wire), and is illegal/dangerous if it does, much the same as it shouldn't have any lumps/bumps, regardless of if the tyre still has oodles of tread left.

Basically damage over rides the normal wear and tear rules from what I understand, which is sensible, as damage can occur to a tyre at any time regardless of if it's age/wear.

[edit]
I can't find the specific rule in the highway code online, except that the tyre should be free of "defects".

[oldcodger]

From memory the tyre should not show any of the inner structure or cord (wire), and is illegal/dangerous if it does, much the same as it shouldn't have any lumps/bumps, regardless of if the tyre still has oodles of tread left.

Basically damage over rides the normal wear and tear rules from what I understand, which is sensible, as damage can occur to a tyre at any time regardless of if it's age/wear.

I am sure that is all correct.

I think you will find that the rules with regard to tyres are contained in the Construction and Use Regulations. From memory I think its regulation 27. And dozens of other later amendments to reg 27.

[spinlondon]

Hi,
Also what is the case with uneven wear on motorcycle tyres where one side of the groove is at the wear bar and the other 2mm above it - lowest or take an average?

This generally indicates something up with the suspension. Could be the fork oil needs changing, or a heavier grade is needed.

[jimzzr]

I am sure that is all correct.

I think you will find that the rules with regard to tyres are contained in the Construction and Use Regulations. From memory I think its regulation 27. And dozens of other later amendments to reg 27.

Bad form answering ones own post, but the MOT testers manual states:

The tread pattern excludes any tie- bars, tread wear indicators, or features designed to wear out substantially before the remainder of the pattern, and other minor features. Grooves that had not been cut as deep as those containing the wear indicators when new, are not to be considered as part of the tread pattern.

This would seem to suggest that with the particular tyres I had - as there were no full depth grooves beyond the central two thirds and thus for legal purposes no grooves (and no wear indicators) to begin with even when new would still be legal provided the outermost full depth grooves were above the 1.6m even if the outer third was bald.

Can anyone confirm that the CU regs will mirror the MOT and visa versa?

Thanks

Jim

[fatboyjim154]

The tread depth for m/cycles is only 1mm and not 1.6mm

I sure this is from the wording of Reg 27, covering details condition and maintenance part 2 cover bike tyres. I could find out tomorrow for sure.

Any vehicle with a gross vehicle weight (GVW) or gross train weight (GTW) over 3500kgs or a motorcycle above 50cc must, either:

1. The grooves of the tread pattern must have a depth of a least 1 mm throughout a continuous band measuring at least three-quarters of the breadth of the tread and round the entire outer circumference of the tyre, OR
2. if the grooves of the original tread pattern of the tyre do not extend beyond three-quarters of the breadth of the tread (i.e. common with motorcycle tyres) any groove of the original pattern must have a minimum depth of at least 1 mm.

[jimzzr]

The tread depth for m/cycles is only 1mm and not 1.6mm

I sure this is from the wording of Reg 27, covering details condition and maintenance part 2 cover bike tyres. I could find out tomorrow for sure.

Any vehicle with a gross vehicle weight (GVW) or gross train weight (GTW) over 3500kgs or a motorcycle above 50cc must, either:

1. The grooves of the tread pattern must have a depth of a least 1 mm throughout a continuous band measuring at least three-quarters of the breadth of the tread and round the entire outer circumference of the tyre, OR
2. if the grooves of the original tread pattern of the tyre do not extend beyond three-quarters of the breadth of the tread (i.e. common with motorcycle tyres) any groove of the original pattern must have a minimum depth of at least 1 mm.

I was refering to car tyres. The separate question I raised about M/C tyres related to how the deth is defined if a tread block wears unevenly so that one side of a groove is say 1 mm and the other side 3mm. Hard to explain without a picture. This never seems to be an issue with car tyres. In any case I guess the wording would be the same for bike tyres i.e. grooves that are not full depth are not grooves for the purposes of the MOT (and presumably CU regulations) - lots of motorcycle tyres have grooves that are clearly not as deep (when new) as the main ones with the TWIs in.

The question is is a groove defined in the CU regs the same as it is for the MOT?

Presumably normal police practice would be to only measure depth in grooves with TWI in, otherwise how could you know that the groove was originally full depth or not (without taking a lot more measurements to establish a suitable datum or a new tyre to compare with)