Bald tyres

They grip better on dry roads. Mike.

All tyres work in exactly the same way regardless of compound hardness. Grip is maximised when the amount of rubber in contact with the tarmac is maximised. On dry roads that means no tread pattern at all is the optimum tread pattern. A completely bald road tyre will have more grip than a new or part worn one. In race series where treaded road tyres are mandatory the competitors buff the tyres down to remove most of the tread depth which reduces the amount of flex and squirm in the tread blocks.

In wet conditions the grip is optimised when the tread pattern best matches the amount of standing water. The more water the more grooves you need in the tread pattern. That can range from a few small sipes cut into the tyre when it's merely damp to many large channels when it's flooded. Every tyre manufacturer chooses a different tread pattern based on their own perception of these compromises between optimal wet and dry grip.

That's a fairly concise answer Dave, but, a bald normal road tyre must be pretty close to the canvas so to speak. Does this mean that for a while because it is completely smooth, it will grip better than a new road tyre in the dry? I would have thought that because the bald tyre has no rubber left under the tiny bit left it would "patter" more under normal dry road conditions, also would the rubber left be less grippy than the new tyre? I'm not trying to contradict what you said, I'm interested in the physics of it. I know that more rubber on the road is better in the dry, but it's the quality of the rubber left on a bald tyre that interests me. For instance, on my bike, new tyres with minimal tread pattern are very grippy in the dry, but use them for 8000 miles and they go "off". It's like the sticky rubber tread has been worn away leaving a harder compound that doesn't stick as well. Are normal car tyres prone to this when the legal tread has been used up? ie. does a normal car tyre have better rubber on the outside, and once you have worn through it do you get the dodgy hard compound undeneath? I am talking about normal tyres under normal road use that don't get a lot of heat to make them sticky.

Stuart

Also sprach snipped-for-privacy@aol.comNoEmails (Dave Baker):-

And, of course, vitally important, to look pretty so the marketing department are happy.

You certainly do get tyres with different rubber compounds at the carcass level and at the outer diameter of the tread so that would affect grip as the tread wears away. It isn't a factor you can account for in a generalised argument though. It may also be necessary to alter the tyre pressure as the tread wears away to add a bit of carcass flexibility which the tread blocks would provide on a new tyre.

The worst tyre I have come across for grip altering as the tread wore was Pirelli P6000s (and also very marked with P6's). When new the wet grip was very good but by half worn it was almost non existent and to dangerous levels. The closest analogy I can think of is what bikes felt like in the wet in the 70s with the OE Japanese nylon tyres which were stone killers and have no doubt accounted for many fatalities. I can only assume the Pirelli compound changed radically with tread depth and I won't be troubling them for tyres again. I've covered it on here in detail before. Assuming compound stays the same and carcass flexibility is not an issue then dry grip will be best with no tread pattern though.

One thing I've noticed on my current car and tyres (Focus 2.0 ESP, Michelin Pilot Primacy front and Firestone rears) is the change in wet grip with tyre pressure. I usually overinflate, 40 psi, to get best fuel consumption and this doesn't affect dry grip as measured by ultimate cornering speed on my test corners. Wet grip gets measurably worse with excessive pressure though, mainly at the back end, and in winter I put the pressures back to book specs, 32/34 psi, to increase the contact patch area. It's a gilding the lily kind of thing though because wet or dry the Focus can run rings round most other cars in the corners whatever the tyre pressures are set to.

I agree with you also Stuart as I went to a conference once at which a tyre forensic tyre expert gave a lecture. He was basically saying on this point that road tyres are designed to have tread/rubber on them. This helps to dissipate the heat and maintain the correct tyre pressure and handling. Whereas competition slicks are designed to run bald.

I've seen some ordinary tyres advertised as having a softer compound exposed when the tyre is half worn to maintain wet grip through its life. Bridgestone I think.

Which reminds me. Did you ever get a report back from Ferrodo on those dodgy brake pads? Or have I just missed a posting of yours?

In the end, as is often the way of the world, I got swamped with work and other matters and couldn't be arsed to go through all the hassle of sending them back and getting into correspondence for the sake of the £12 or so they cost me. However, the Motorcraft ones I replaced them with have been a huge improvement although still not quite as good as the OE factory fitted ones. I could have had another set of those in a Ford box rather a Motorcraft one but at about twice the price. The Motorcraft ones will fade under hard use but they are fine in all normal situations, i.e. just one high speed panic stop which is all I was really concerned about. If I was doing track days or any other really hard driving I'd suggest paying the extra for Ford ones or more race oriented aftermarket pads. For my normal pottering about the Motorcraft ones are an acceptable compromise between cost and performance.

Bridgestone S02s (now S03s but not as good.) The S02s were the best tyre I ever had and were grippier in the wet at the legal limit of tread than new budget tyres. The S03s aren't quite so grippy but last longer.