Tyres

Crasher

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There is an interesting aside to the quality of the tyre not related to braking but rolling resistance. Some cars come with very expensive low drag tyres that have a significant affect on CO2 output and possibly incur a lower tax bracket but if the expensive low drag covers are replaced by some Yingtong Ditch Finders that CO2 figure is irrelevant and could be well into the next bracket but unfairly (and unknowingly) the car still incurs the lower tax. A number of times I have fitted Uniroyal Rainsport XL rated tyres to a car which had some nasty Chinky rubbish on it and you can actually feel the significant difference when coasting. Another one that surprised me was I had the original Rain Sports on my Octavia and after a while the front sidewalls looked very compressed and they were noisy. The next tyres were the RS2 but I fitted the XL versions and I could not believe the difference in rolling resistance and the sidewalls stay pert longer although the noise was terrible when they were not even half worn.
 

SadClouds

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There is an interesting aside to the quality of the tyre not related to braking but rolling resistance
Exactly, there is whole spectrum of tyre performance characteristics that get overlooked. I personally find the EU kindergarten labelling system with its letters of alphabet, not very useful. What exactly does dry/wet braking performance A vs B actually mean? They also need to publish the actual test results - car weight and speed, surface type and temperature, 0%, 50% and 100% tread wear braking distances, abrasion resistance, etc.
 

SadClouds

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Results were:-
Brand new = 25.8 metres / 3mm = 31.0 metres / 1.6mm = 36.2 metres.
So what is the significance of these numbers? At tread depth of around 8mm braking distance was 26m, but at tread depth of 3mm braking distance increased by 5m. Is this safe enough? Who says so? Well how about we now start changing tyres at 5mm tread depth, surely that is even safer, right?

There is no such thing as "no change in characteristics", as the stopping distance will be hugely affected by many difference factors - weight, speed, temperature, road surface conditions, etc. The point is there is no safe stopping distance - you can kill a pedestrian at 5m and 25m and any other distance. You can safely drive with tyres of 0.5mm tread depth if you watch your speed and slow down when road conditions demand it. That is the most important factor, not some magic number that gives you a false sense of security.
 

Le Touriste

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“You can safely drive with tyres of 0.5mm tread depth if you watch your speed and slow down when road conditions demand it”????
This would certainly be untrue in heavy rain, particularly on winding roads and sharp bends. I think it has already been proven that steering and braking efficiency reduces as tyres wear down. Most drivers are incapable of judgement in sensing the situation just prior to their vehicle breaking away into a skid, and that would be even more critical at 0.5mm tread depth. As for braking distance, how many times have their been accidents because drivers do not look far enough ahead, and drive too close to stop if a vehicle in front has to brake suddenly.
If every driver was fully capable of maintaining control under any situation and was strictly self-disciplined to control his/her speed/steering to suit any road conditions, it might be feesible to say that tyres need have no tread at all. But we all are well aware that this is an impossible scenario, so it is vital that tyres are designed to give maximum reliability and that must also include specifying the limits at which the tyres fall below acceptable standards.
We drivers all have variable degrees of ability, and the suitability of tyres cannot be based upon how the most capable of drivers perform - some average level of ability has to be applied. And this is where tyre manufacturers and safety organisations need, through their combined research and development teams, produce advisory information for the government to pass into law.
Although a 1.6mm tread depth is our current legal minimum, reading the data provided by tyre manufacturers and safety organisations such as ROSPA and BRAKE, it seems that 3mm is generally agreed by these bodies as being the most reliable minimum tread depth for road safety.

For myself, I look at it this way. If my vehicle had tyres with 1.6mm tread depth and was involved in an accident and someone was fatally injured, and an investigation produced an opinion that, had my tyres been at 3mm tread depth, there was a very good chance that I would have stopped in time and the fatally been avoided, I would be devastated by such findings.
Also, for peace of mind, were I to be stopped by the police for any reason, their reaching for their tyre depth gauge would not bother me. In fact, one look at my tyres would show measuring the depth to be superfluous.
 
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SadClouds

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Although a 1.6mm tread depth is our current legal minimum, reading the data provided by tyre manufacturers and safety organisations such as ROSPA and BRAKE, it seems that 3mm is generally agreed by these bodies as being the most reliable minimum tread depth for road safety.
Is that a fact? Can you please answer one simple question - why do you think that a brand new budget tyre is considered safer and more reliable than a premium tyre worn down to 1.6mm which also has been proven in tests to have a shorter stopping distance due to better design and construction?

I understand and agree with the premise of "more tread depth can give slightly shorter stopping distance, which is good". What I don't agree with is the misguided assertion that 3mm tread depth is the minimum safe, and completely disregarding all other factors. Before people embark on safety crusades, they really need to understand the problem and do their own research, instead of copy and paste other peoples' findings. Just because somebody calls themselves "The Royal Society for the Prevention of Accidents" does not mean they are engineers and experts in various safety fields. Here is the exact quote from some of their tyre safety literature:

In 2003, the British Rubber Manufacturers Association (BRMA) commissioned MIRA to study the effects of tread depth on stopping distances...

So the study was done almost 19 years ago and ROSPA got the figures from BRMA, who in turn got the figure from MIRA. We are almost in the year 2022 and how relevant is this study? Why not do a study on the tyres manufactured back in the 1950s then we can really scare people and drum up even more business for rubber manufacturers.

So I am not saying that replacing tyres when they are down to 3mm tread depth is a really, really bad advice. If it makes you feel safe, then feel free to do it, nobody is going to judge you. But trying to coerce people to do this under some dubious safety campaign can get a bit irritating.
 

Crasher

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I do agree that a brand new Ying Tong ditch finder will have better wet grip than an on the limit premium but that’s about it. It also reminds me of that other question I ask people who praise four wheel drive, “would you rather make an emergency stop in a front wheel drive car on winters or a four wheel drive on summers?
 

Le Touriste

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I did not even rate budget tyres because I simply would not buy them, so that is a misquote. The stopping distance between a 3mm tyre and a 1.6mm tyre is not slight.
What puzzles me is that, whenever research and safety bodies provide advice from a position of knowledge, there are always that section of society that will challenge the findings of the professionals.
I spent all my life in engineering, and most of that in engineering development. I do not claim to be an expert in tyre design or serviceability, but I can see the logic of the argument which recommends a minimum tread depth of 3mm.
One cannot change the law of physics - in heavy rain a bow wave will develop in front of the front wheels (not so much at the rear wheels because the front wheels will have displaced much of the water in the tracks that the rear wheels will follow), but the front wheels will ride on the bow wave if the treads are insufficient to displace the water via their grooves.

But, at the end of the day, we all have a free choice - 1.6mm is legal for those who wish to go down that route, and they are free do do so without adverse criticism. Likewise, those such as myself are free to take the more cautious option and go for a 3mm minimum tread.
 

Le Touriste

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I have found an article on line by Road Safety GB in association with THINK!
It is headed “Drivers advised to follow blue light example.”

A FOI request was made by tyre retailer Kwik Fit to every police force, fire and rescue services and ambulance services in the UK - with 95% responding.
Findings:-
16% allow their tyre tread depths to go below 2.5mm
73% change their tyres at between 2.6mm and 3mm
10% go even further and change their tyres at between 3.1mm and 3.5mm(6%), and between 3.6mm and 4mm(4%)

There was one comment, the poster saying, “I’d put more credence on the report if it wasn’t commissioned by one of the biggest tyre suppliers in the UK.”
Maybe the public would have preferred an independent commissioning source but, even so, the facts of those findings still remain.
I would fully agree with anyone pointing out that the emergency services use speed as a necessity on a regular basis.
But many drivers drive at the highest speed possible for road conditions as they see them. Also, whilst emergency service drivers may travel at still higher speeds, their special training outclasses the average driver. We “ordinary” drivers may not face the regular need for swift reaction BUT, in the odd occasion when we might just meet that requirement, the performing ability of our tyres could be the one factor which decides success or failure in our attempt to avoid involvement in an accident.
Police investigations into road accidents, particularly where serious injuries or fatalities are concerned are very thorough, so in the hypothetical event of a fatality I’ll just ask this, “How would you feel if your tyres were just inside the legal limit, but the investigating officers said that things would very possibly have been much less serious had your tyres treads been much less worn?” I know how I would feel.
 

SadClouds

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Why would four wheel drive make any difference to the stopping distance? As far as I know, it is down to friction and how well the rubber compound can grip the surface. The technology is always evolving, so whatever opinions people had 10 years ago, may not be true today. There are all season tyres that work very well in summer and winter and come pretty close to the performance of dedicated summer or winter tyres. There is so much science that goes into tyre design that you can't just look at one factor in isolation. So one set of tyres can have a different rubber compound that doesn't go stiff during cold temperatures and different groove design that displaces water much better. Two different tyres from the same manufacturer could have very different performance characteristics. The problem is, much of the test data is not readily available, so it is difficult to tell exactly how tyres will behave under different conditions, especially when they are worn. See these slides from Michelin that describe some of the factors affecting the grip - wet grip of worn tyre
 

Crasher

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That’s what I mean, a lot people think a four wheel drive stops faster when in fact two identical cars, say an A3 on the same tyres one fwd and the other not a quattro “quattro” the latter may actually have a slightly longer stopping distance due to the extra weight.
 

Le Touriste

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That’s interesting, Crasher - the vehicle weight being a factor in stopping distance. Being a caravanner, I have always had fairly heavy vehicles, and I do prefer a weighty vehicle around me. Conversely, our grandchildren, through financial considerations, have small, much lighter cars.
I have often wondered, would a lighter vehicle take a longer stopping distance because it’s lighter weight results in less downward force and reduce grip between tyre and road? This is opposite to your observations, though.
Perhaps not as simple as that, though. I imagine that it must include a calculation based on tyre size which determines the area that is actually in contact with the road.
 

SadClouds

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I think it depends on the situation. Heavier vehicle can potentially have more kinetic energy (given the same speed) and needs more friction to stop, so on a slippery road surface it can travel a longer distances. However, more weight on the tyres can result in more friction against the road surface. I guess it depends on road conditions - for ice/gravel you want less weight, for mud/water you want more weight. Tyre rubber compound may also play a big factor - too soft or too hard and tyres will glide.
 

Le Touriste

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Even whackier than Top Gear - think I’ll play safe and stick with tyre makes that I already use, that have proven trustworthy for years, and my usual “when-to-change” approach.
 

Crasher

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And the heavier vehicle will probaly have wider tyres and bigger brakes, as Professor Jordan Petersen says "it's complicated man"
 
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