Why do tyres have less grip as they wear in dry.

Not really sure what the answer to all that is Chaz, but i think i'm right in saying that different compounds react in different ways as they wear.
I always use to use Falken ZR rated tyres, but they were quite a hard compound and really needed some heat in them to work well. As they wore, they contained the heat less and used to slide very easily. Quite scary with 300HP and RWD as you come on boost.
I am surprised you use Kuhmo's though. I always thought they were quite a middle range rated tyre.
I currently use Uniroyal RainSports 245/45/17's. They are very very grippy in both the wet and dry, but i do tend to go through a set in about 10 months.

 

I would say it's either

a)Because the tread depth is reduced there's less flexibility in the tread, reducing it's ability to mould to the road

or

b)Because there's less rubber in the tyre it's ability to absorb the heat created during normal driving is reduced, which cooks the rubber more easily

Or a combination of both?

Or neither?

 

Never heard of Kumhos.

Here's a tyre survey that I saw elsewhere on fchat.

http://www.tirerack.com/tires/surveyresults/surveydisplay.jsp?type=MP

 

I hope you're right cos I'm doing the Brands track day on a set!

 

I've got Kumho's on my Ruf 911, great tyres at a reasonable price, I found P Zero's offered little in the way of extra grip on the road but cost almost double! and havent looked back since switching (on set number 4!) they also do the awkard sizes for Testarossa's for a very very reasonable price (under £450 a set the last time I ordered some)

 

don't know if you can get them for all F-cars, but Toyo T1-R's or for the more 'enthusiastic' user the Toyo R888

Honestly these tyres are top notch and are being used for all sorts of sprints etc.

check out www.mytyres.co.uk they are very good value also.

 

These guys are much cheaper - www.johnsonstyres.com
Call them up as they don't have everything on their website.

 

Chaa is right on this one Khumo's are good. I race an E30 bmw in the Khumo / Miltek championship and all the cars run on Khumo's as they are a very good tyre at a reasonable price. Never thought of putting them on my F car though, but anything has got to be better than the Bridgestone SO1's I've got on it at the moment.

 

The Toyo R888's are supposed to be very good but you dont want to run them in the winter as they are semi slick trackday tyres with a soft compound they can be quite scary if there is any standing water.

My Exige runs bespoke Yokohama AO48's which are also semi slicks. The grip on them is unbelievable but my rears only lasted 4000 miles.

 

I know what you mean, I have done a set of AO46's which are unavailable over here, in 3500 miles!

 

Chaa, as you know Im equally intrigued by this effect. Shall we try posting this in the tech-heads section?

Graham's theory does sound very plausable. As I suppose 'slicks' effective can't flex and mould to the road in the same way a tread does, so they perhaps rely more on heat in the tyre to melt the compound to the road.

This fits in with my observation that I have very low grip on worn tyres for the first 10 minutes of driving especially.

 

Only 4000??

My Lotus Sunbeam trashed a set of Yokohama AO32R's in three trackdays. Gonna go slicks for the dry, Goodyear Eagle F1's in the wet.

Chaa, I have found that harder compound tyres work less efficiently when worn down and softer ones work even better (up to the point of "going off") when worn down. I can only put this down to the "blocks" rubbing against each other when new on the harder compound, creating heat and consequently grip, and when they are not able to later in their life, grip suffers. I have used Bridgestones on both my Porker and 355 and found that they maintain grip right up to the point of being bald...the pic is after a trackday at Brunters. And I drove the car home blissfully unaware of the cords showing!
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have your tyre filled with nitrogen rather than air and the tyres will last upto 25% longer...

 

I tried to fill my Ferrari tyres with water once. Pulled the wrong hose out of the combined air/water machine at the garage.

I just ended up covered in water as it all splashed back off the valve

Well they both looked the same to me

 

was it proceeded by one of those comedy moments where you looked down the 'barrel of the gun' to check its ok, and subsequently power washed you face in public with a rather large jet stream of water.

I try to look natural in these kind of senarios, and swiftly return naked with a bar of soap and towel to finish the job off.

 

I think the 2 points made earlier are probably pretty close to the mark.

Also age may have something to do with it. Tyres go off, as people who garage their cars for years discover. I would guess the heat and pressure they are exposed to in the process of wearing the tread may well accelerate this process. Something which is undoubtedly acceptable as they would be illegal to use on the road at this point anyway.

I believe there is an inverse relationship between the stability of the tyre compound and its effective lifetime.

 

Heat (or lack of it).

If you cut a tread into a slick then it becomes an intermediate. ie good for the damp, but if you use it in the dry then - in general - it will fall apart. This is because the slick is made of very soft rubber and the tread you cut into it enables friction to generate heat which over heats the tyre with no water to cool it. A slick needs to be soft so that the small movement generated in the carcass generates a small(er) amount of heat to get the compound upto temperature.

A treaded tyre that is designed to be used in the dry and have a good life will be made from hard(ish) rubber. The movement of the tread blokes generates friction which heats the compound to its operating temperature which gives you grip (makes the rubber sticky).

As the tyre wears the tread blokes become less flexible and generate less heat (less movement=less friction) so you end up with less grip.

There are road tyres used for racing/rallying where reducing the tread depth gives better characteristics (not neccesarily grip) but these tyres are usually soft in compound and have a short life even with full tread (they are "mock" road tyres - designed for production racing but badged as "road" so production series can get away with running them). People tend to buff these tyres down to create a semi-slick. Doing this can also help with "feel" from the tyre and help its life during a race as there is less movement from the tyres contact patch with the ground.

Manufacturers also use differing compunds in different layers of the tyre as mentioned above (some have also started putting softer compounds on the outsides - harder inner which helps with the grip/wear ratio as long as you run the pressures a little high).

Didn't know Kumho did the right sizes, I can get them free - for personal use - cool! were you at SeeRed angelis? (see Donington Park pic below)
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I know this is an old thread but my view is this:

The available grip from a given tyre varies massively according to tyre temperature. i.e cold tyres = low grip

GrahamS touched on the subject (point b) but the wrong way around (IMHO). The fully treaded tyre heats up quickly due to the movement of the tread blocks under cornering etc. This enable the tyre to get closer to its optimum working temperatutre on the road where it is obviously difficult to generate any heat. The 'bald' tyres take longer to heat up and more to the point are very unlikely to heat up to any great effect as there is less movement to cause this heat build up. With the roads being how they are these days most of us are driving around on stone cold tyres.

Bear in mind the above applies to road driving. Hard acceleration and braking doesn't heat tyres as quickly as hard on the limit cornering and how many places can you do that on the road now.

On the race track it becomes a different issue. Fully treaded tyres will often overheat after 10 laps or so and drift off to the tune of tenths of a second to more than a second a lap (will obviously vary depending on car, tyre etc). The slicker tyre will take a little longer to heat up by a lap or so but will take much longer to overheat.

This is why on a wet/damp track with no standing water most race cars will still run full tread tyres as opposed to low tread. In these conditions getting heat into tyres is very difficult and you need all the help you can get.

Edited: Only just saw page 2 of the thread and Ricards post. I agree!