Safe Speed Forums

hi guys
just slightly off topic a bit..i work on a road safety publication for fleets called roadsaafe.....www.roadsafe.com..they are advocating that fleets should change tyres at 3mm tread depth instead of 1.6mm
this has the support and backing of the tyre industry council,and associated
tyre bodies.... in the trade press there has been some comments any thoughts? the idea is that stopping on 3mm in the wet is safer than at 1.6
goldy_n17

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Audere Est Facere.....To Dare Is To Do

I split this from a Brainstorming topic... This is a better place for it.

It's a worry, isn't it, that a group who stand to benefit financially are advocating much earlier tyre changes.

I remember a chart of stopping distances in the wet -v- tread depth (from the 'AA Book of the Car' circa 1970) which purported to show a fairly linear relationship between stopping distance on a soaked road and tread depth. At first I was going to reply, yes, there might well be a significant safety advantage, but on reflection I'm not so sure.

It seems to me that either the read depth is adequate to clear water, or the vehicle aquaplanes with not a great deal in between. We need proper data to verifiy the point. I reserve judgement.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I could be wrong, but from my watching of motorsport, it's my understanding that the tread depth isn't such a huge feature in how grippy it is in the wet. It's more the nature of the tread that's important - the wetter it is, the more grooves you need, and having the existing ones deeper isn't going to significantly improve your traction.

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-mike[F]
Caught in the rush of the crowd, lost in a wall of sound..

What's more important for a tyre to disperse water, tread depth or tread pattern? Or is one more important up to a point after which the other one takes over? The reason I ask is that I wonder if a really expensive up to date tyre with a tread pattern carefully designed on a computer might disperse water more efficiently with 1.6mm of tread than a cheap tyre with 3mm, but a tread pattern designed on the back of a fag packet in 1980 (okay, I exaggerate, but you get my drift ).

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Make everything as simple as possible, but not simpler - Einstein

Our job cars are changed at 3mm primarily for that reason. If we had to drive at 100mph+ in rain, I'd rather have at least 3mm to channel the water through, if I need to stop or change direction.

This was originally posted on the brainstorming thread after the original post

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Fixed ideas are like cramp, for instance in the foot, yet the best remedy is to step on them.

Ian

I can only laugh at this. What did you expect?

It reminds me of the colgate ads showing massive dollops of toothpaste on toothbrushes when they are criticised by the dental associations for doing so. (Too much of the abrasive stuff in the paste can damage your teeth apparently).

I've no technical expertise on the subject, but I'd have thought that the faster you travel, the more water builds up in front of the tyre to be displaced. If there is inadequate opportunity for that water to flow through the grooves in the tread then the tyre lifts. I would have thought that cross sectional area of groove and pumping ability of the tyre tread design would be the two relevant factors.

As always I'd be happy to be corrected by the engineers

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Fixed ideas are like cramp, for instance in the foot, yet the best remedy is to step on them.

Ian

Makes sense. Why is the limit 1.6mm for every tyre then?

Yes, the most significant issue here is high speed aquaplane risk. Clearly the faster you go the greater the risk and the shallower the tread depth the greater the risk.

But for non-emergency services vehicles the law says 1.6mm based on (one hopes) a reasonable analysis of the risks. So to have tyre companies saying 1.6mm is 'too late' is a real worry about vested interest. Let's face it - either the law is wrong or the tyre companies are greedy. I don't think there's much in between.

If the original tread depth is 9mm for law we're allowed 7.4mm of wear. If the tyre companies say 3mm we're only allowed 6mm of wear. Sounds like they stand to sell 23% more tyres where this recommendation is followed.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I think the problem here is what constitutes a reasonable compromise.

When a tyre is in contact with the road the degree of grip depends on the area of the contact patch and the compound of the rubber. Tread is there for one reason alone, and that is to prevent water from moving the tyre off the road. Clearly the point at which the tyre loses contact and aquaplanes depends on several factors, including...

1. The mass pressing down on the tyre,
2. The size of the contact area
3. The tread depth
4. The tread pattern
5. The depth of the water
6. The speed of the vehicle
7. Whether the wheel is turning
8. The slip angle (ie how much steering effect is being demanded)

Clearly all these factors interact, and my feeling is that most are a bit like Hookes Law and "elastic limits", in that they are fairly linear effect up to a certain critical point. On a wet road reducing tread depth will gradually decrease the speed at which aquaplaning starts down to a certain point, after which performance degrades drastically. My belief was that this was generally around the 1 to 1.5mm mark, which is why the previous change in the law yielded a real improvement in wet weather performance. I don't honestly believe that a further increase would yield a similar benefit.

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

Used to be 1mm, (still is for bikes and HGVs).

I'd imagine the answer has to be a compromise between safety and motoring costs.

The difference between 1.6mm and 3mm will probably not offer much of a safety dividend, but may add 20% to your tyre costs.

I change my tyres at 2mm but I am aware of their potential limited adhesion in the wet, and obviously drive accordingly. I am also a tight Scot, so I don't want to pay more for my decent tyres than I have to.

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Fixed ideas are like cramp, for instance in the foot, yet the best remedy is to step on them.

Ian

I'm sure you're about spot-on here Ian. Tyre design is quite a fine art as these often overlooked component parts of our cars literally 'pump' water from between themselvs and the road surface. We must remember that the tyre must have sufficient surface area in contact with the road to offer adequate mechanical grip and yet provide a tread pattern and depth which best disperses the water beneath the wheel.



KISS - Keep It Simple Stupid? Providing the general public with a variety of legal tread depths would confuse huge numbers of people whose technical acumen extends to knowing where to put the petrol in!

And some even stuggle with that, judging by the amount of people who mis-fuel and then try and blame the petrol stations and car manufacturers for not having different shaped holes and filler nozzles. Is it just me or does it remind you of toddlers fitting the shapes through the right holes?

Back on topic and I've had aquaplaning in my car at about 50mph with about 6-7mm of tread all round and loaded up, so personally don't see it making that much difference. Admittedly it was monsoon like weather but that just goes to show that even tyres with lots of meat on them can suffer. Actually I think whenever you get that much standing water all tyres will suffer, despite having more or less depth to them.

Oh yes Makes you wonder how they've got the brains to drive the things doesn't it?



Yep, whenever the amount of water the tyre is having to cope with exceeeds its ability to disperse it, you'll get aquaplaning.

From IanH - "I change my tyres at 2mm but I am aware of their potential limited adhesion in the wet, and obviously drive accordingly. I am also a tight Scot, so I don't want to pay more for my decent tyres than I have to."

No we're not Ian, we're generous Yorkies

Read years ago that at 60mph in certain depth of water front end on rear drive lifts off road and aquaplanes, irrespective of depth of tread.
Certainly know that with 1mm on mini ( ye olde badly made car version) on wet roads you could get wheelspin due to aquaplaning on front end

Sorry chaps,I'm not with you on this one. 1.6 mm may be the legal limit, but it MAY not be the absolutely safe limit. I normally change when the tyres are getting to around 2-2.5, or on the recommendation of my local tyre and service man, who I trust. Incidentally, my last front set lasted so long they became slightly oval which caused a wobble through the steering wheel at low speeds, (OK over 40 mph). Apparently this is common; I changed them !! around 40k from a tyre is not bad in my opinion, they were Continentals by the way.

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Good manners maketh a good motorist

Well of course the tyre industry would want you to change tyres at 3 bloody mm, what self interest group wouldn't. That's like a mobile phone company conducting a survey on the safety of phone masts, they're not going to say they cause health issues are they?

Thagt said, I can't understand why they don't just use an even number to the 'safe' limit. 1.6mm is stupid. 2mm would be more realistic.

Anyone know for sure what tyre 'tread wear indicators' are set at for UK market tyres? Is it still 2mm?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

The idea of changing at 3mm is nothing new.

I have a copy of the "Porsche Driving Guide" ( can't remember exact title at moment ) by John Lyon of HPC. This was written around 20 years ago, when minimum tread depth on cars was 1mm ( i think ), and he recommends changing at 3mm.

It may be the case that modern tyres are more consistent in their performance when wearing, but i change them around 2.5-3mm - my last set did over 30k, tyres do a very important job so would rather err on the side of caution and change bit too early.

I'm pretty sure that's a legal requirement for American tyres and they must be 2mm for that market.

Don't think the UK has a legislative ruling on the depth of TWIs but I'd hazard a guess that they'll be at the same level.

Not much use, but I can't find anything online to confirm.

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Fixed ideas are like cramp, for instance in the foot, yet the best remedy is to step on them.

Ian