tyre tread depth | The Back Room Archive | Back Room Forum

Mon 7 Jan 2002 22:21

tyre tread depth - fred bear

I'd like to add a new angle to the tyre debates which feature on this site. We all know the legal tyre requirement and the supposed use of the tread wear indicator (TWI). However most modern tyres have at least two depths of tread when new. The shallow. fine channels wear away long before the deeper grooves with the result that the tyre becomes a "slick" with only the three or so main grooves remaining - those remaining grooves are still well within the TWI but the tyre is now very different in overall pattern from its new state. When this happened to me three separate tyre fit companies assure me the tyres are legal but admit the fine tread is functional, not decorative!
I believe the law doesnt refer to the TWI but to the depth of tread across the tyre so I assume, TWI or not, that tyres worn in this way are illegal - meaning the effective life is much reduced. I wonder if anyone has been pulled by plod for having tyres in this state and whether they are as dangerous as they look?

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Tue 8 Jan 2002 13:26

Re: tyre tread depth - Derek

Good question. I have to admit that I hadn't seen it that way. On the other hand, I believe that the law refers only to tread depth, not tread design. Presumably, that would mean that one of the old 'square block' treads would be legal on a Porsche, so long as they had enough depth AND the necessary speed rating. Or maybe there are other laws about tyre suitability that I don't know about?

BTW, m/c tyres can get away with a lower thickness, which perhaps is just as well because they do s**-all mileage as it is!

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Tue 8 Jan 2002 13:27

Re: tyre tread depth - Derek

Good question. I have to admit that I hadn't seen it that way. On the other hand, I believe that the law refers only to tread depth, not tread design. Presumably, that would mean that one of the old 'square block' treads would be legal on a Porsche, so long as they had enough depth AND the necessary speed rating. Or maybe there are other laws about tyre suitability that I don't know about?

BTW, m/c tyres can get away with a lower tread depth, which perhaps is just as well because they do s**-all mileage as it is! Anyway, they start off with less when new.

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Tue 8 Jan 2002 14:26

Re: tyre tread depth - mike harvey

The tread depth indicators on car tyres are at 2.0mm, I think, but the law says a minimum of 1.6 mm round the whole circumference, and across the central 3/4. Down to the TWI is not therefore illegal. It is also legal to have the area outside the central 3/4 totally devoid of tread.
Mike

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Tue 8 Jan 2002 21:36

Re: tyre tread depth - Brian

I agree with your point about motorcycle tyre mileage, Derek. However, what surprises me is the discrepancy between motorcycle front and rear tyre wear.
I have just changed the front tyre on my bike for the first time at 40,000 miles.
I have lost count of the back tyres it has gone through in the same time but it must be at least four. And the back tyre always gets the punctures, I haven,t had a front tyre puncture for thirty years.
It seems that the design spec. for rear tyres leaves a lot to be desired, or the basic bike design needs to be changed to even out the stresses.

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