Help settle an argument please

[Entai]

Hi all, here is a good one for you, a guy at work, wants to get wider wheels and tyres so that at traffic light races he will get better traction, and a better launch, with less wheel spin.

I said, most likely that will not happen, if anything you will be worse off I said. As the friction is still the same between the tyres and the road surface, no matter how big the tyres.

My reasoning is, that surely the size of the surface area of a contact patch between tyre and road, has no bearing on the friction coefficient of that contact.
The friction coefficient is only determined by the types of material that the friction is occuring between, (tyre rubber and road tarmac) and to some extent the load applied to those materials.

So in this example, the contact patch is still between the same two materials, and if anything, the load on the contact patch will be less than he already has (he is changing from steelies to alloys as well, so lighter wheels), and getting wider tyres so the contact patch is larger spreading the load more. Therefore by my reckoning in a straight line he will possibly have less traction than before, but in cornering he will be much better off.

He still insits that bigger tyres equals more rubber on the road so it has to be more grip.

So who is right??

 

[Entai]

Surely bigger surface area = more grip ?

But the friction coefficient is constant between the two materials so surface area does not come into it.

The friction coefficient between metal and glass is the same wether the glass is 2 cm squared or 200cm squared.

 

[Entai]

If the rolling radius is the same the grip will be worse from a standing start. A thinner tyre will have approx the same contact patch to a wider tyre with the same rolling radius. The thinner tyre will have a higher aspect ratio so when the car squats the tyre will squish enlarging the contact patch (just realising Im talking about RWD here and dont know what type of drive his car has). A wider tyre with a smaller aspect ratio will resist that squishing.

Exactly what I was thinking.

He has an Astra GSi Turbo, but don't let that put you off him, he is a decent enough bloke really.

Again he is FWD so the weight transfer will also diminish the squish of front tyres.

 

[Entai]

If you are correct, why do dragsters have such large tyres?

Dragsters have wide tall soft tyres so that on launch they rise and form a very long thin contact patch to launch the car off the start line.

 

[Entai]

Fox was talking about width not height.

Anyway, at actual launch point the tyres dont stretch tall, that happens after launch. At launch the tyre squidge down and spread over the road surface.

The initial width helps create the height once power is applied, because the sidewalls are so soft, that is why one rotation of a tyre gives a distance travelled of over 12 feet.

 

[Entai]

It will deffinatley give more grip as more surface area means more grip, thus why f1 use as wider wheels as possible and ice skates are a sharp as possible. Also going on more surface area = no better grip then surley a slick provides no advantage over a treaded tyre? If his car is standard then he is a **** but if he has modified it to go faster (not with stipes) then fair enough.

F1 have big wide tyres to help them go round corners fast, wide tyres do not help in a straight line at all.
Look at Thrust 2 the wheels were only 6 inches wide on that and it did mach 1, but it would never go round a corner at any speed.

Again in a straight line you are perfectly correct a slick tyre does not offer any advantage at all over a treaded one, but in a corner it helps tremendously, that is why F1 now use treaded (read grooved) tyres so that the corner speeds are vastly reduced compared to what they would be if they were running true slick tyres.

 

[Entai]

But that has nothing to do with the original question though does it. Thrust 2 was designed for maximum top speed, not maximum acceleration from stand-still. Don't forget that Thrust 2 was powered by jet engines that were not driving through the wheels (such as in a car).

It's obviously an interesting question, but surely if wide tyres help a car with cornering speeds, then surely it's the same thing as pulling away from a stand-still i.e grip.

But those wheels are being "pushed" not driven. When you pull off quickly and break traction you don't skid the rears do you? Nope they just roll along I'd have thought that would make a difference?

Ok, Ok, maybe Thrust was the wrong example.

What I was trying to say is that wide tyres and more importantly slick tyres are on racing cars to help them go round corners fast, they do not help with straight line speed. The tyres are wide and slick to generate lateral grip not longitudinal. The benefits of grip (if there are any) are far outweighed by the negatives in regard to drag and rolling resistance.

In fact and I am sure NicktheNorse will help out here the huge wide rear tyres on an F1 are a hinderance to high speed because of the drag they impart to the vehicle. I would be fairly sure that if an F1 car was designed just to go in a straight line, then the tyres would not be as wide as they are now.

 

[Entai]

Slicks do have an advantage in a straight line. Firstly more rubber on the road for the same width tyre and second no tread blocks to roll over so a slick will grip better in a straight line and in bends.

As NicktheNorse said F=mu*N, the area of rubber in contact with the road plays no part in that equation.
So extra wide slick tyres makes no difference to the amount of friction(grip) compared to thin treaded tyres, when both are made from the same compound of rubber, and have the same constuction techniques employed, because the size of contact patch has no relation to the coefficient of friction between the two materials.

I think really there are way to many other variables involved here to get a straight yes or no answer, it would be great to have the resources to test it out properly, because I think many people woould be very surprised at the results.