</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (Sp!ke @ Oct. 22 2002,00:12)</td></tr><tr><td id="QUOTE">Now, can you explain the laws of coefficients of friction in the same laymans terms .....especially the bit regarding tyre widths ignoring the rules and providing more grip with a larger contact patch that one really fazes me a little.</td></tr></table><span id='postcolor'> In answer to Spikes recent request I sent him a mail covering it in the best detail I could (tyres are not my speciality) - and he said that I should stick it here - so here it is ! Tyres & friction etc Hi Sp!ke This is a combination of my own work and a piece nicked off the web, all set in laymans terms the best I can ! The modern motoring world has two fallacies that are believed as gospel 1. Wider tyres have a larger contact patch than narrow tyres What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to: 1. the weight on the wheel 2. the tyre pressure. For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres 2. A larger contact patch = more grip Most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but this is very close to 100% accurate, - the size of the contact patch is irrelevant. The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated Think of it in this way - two tyres fitted to a wheel - one tyre is made of rubber and is 3 inches wide the other is made of oiled glass - but is 40" wide - which one has most grip? It doesnt matter how wide the tyre is - its the co-efficient of friction of the rubber compunds of the tyre that counts - and this factor is the same across the contact patch - ie whenever the tyre lets go - the whole tyre lets go ! therefore in this case the 3" tyres has more grip than the 40" tyre! An important factor - The coefficient of friction of rubber tyres is temperature dependant! Now there has to be a minimum contact patch size - or the wear rate would be enormous, - but once you reach the minimum size for a particular weight/condition thats it! - a wider contact patch on its own wont help - but a wider tyre will have a lower sidewall to retain the overall diameter (rolling radius) - and its the reduced sidewall height that helps here, the main benefit is one of handling - the lower profile sidewalls give reduced sidewall deformation under lateral loading, which results in improved steering response and a more stable contact patch, whilst keeping the temperature down - therefore not upsetting the friction coeffciency. Taking my wifes complaints of the "harsh" 17's on our SL into account - the tyres have a lower sidewall and therefore flex less, the original tyres had a higher sidewall which absorbed more of the road shock - turning it into heat in the tyre - exactly the same way as a shock absorber works! - but I digress! Taking the contact patch to be basically rectangular (though it is actually partially oval in shape), then the area of that patch will be its length times its width. Now, for a narrow tyre, the contact patch will be quite long compared with a wide tyre. To get that long flat section to give the required contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as it flattens and then the tread curves again. This bending and unbending process results in a lot of heat being generated. (Think about bending and unbending a piece of metal rapidly, and how hot it gets as you do so. If you bend it less, but at the same frequency, less heat will be generated). Obviously, the more it needs to bend, the greater the amount of heat generated. the next problem relates to the length itself. There will be a greater percentage of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater percentage of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool. - Heating a tyre above its optimum temperature will reduce its coefficient of friction - reducing its grip <img src="http://www.mbclub.co.uk/iB_html/non-cgi/emoticons/sad.gif" border="0" valign="absmiddle" alt=''> Therefore in summmary - a wide tyre has lower sidewalls and therefore less deflection - this generates less heat therefore the tyres can be made of a "stickier" compound - because it wont be overheated. Because the wheels are bigger - they look better, the wider tyres look better and the overall aesthetics of the vehicle are improved! BTW A simple way to find out approximately what pressure is optimal for your combination is to draw a chalkline across the width of the tyre, go for a quick drive , and look at the wear pattern of the chalkmark. Wearing more quickly in the centre indicates pressure that is too high, and wear on the edges indicates too low a pressure. Wear on just the inside or outside edges means you're geometry is out and its time to get it checked! Cheers Mark There are 10 types of people who understand Binary - Those who do - and those who dont!