Friction Force

OMG... something has taken posession of Del the cat...

Sounds like he's been into the fermented catnip again.

You know when a cat starts talking 'theory', that something is up!

because that isn't covered under the usual boilerplate "the Squirrel made me do it!"

Chris

GA

I am simply amazed to be the first to give you a GA......did nobody else here do physics in high school or later?

If the rear tyres was same as the front tyres, would that mean to you that "friction is independent of the contact area"?

What about if the front tyres were wider than the rear ones?

C'mon you are just being argumentative for the hell of it.
You know what he means... sports cars have wide tyres 'to give them more gip' discuss.
Del

I must say I was a little argumentative :-)

But I was thinking about braking too. And I was thinking that in most cars the size of the front is the same with the rear tyre.

So, to rephrase the original question, why in some kind of vehicles, like F1 for example, the rear tyres are so much wider than the front ones. Isn't acceleration and deceleration of the same importance or effect to the car?

Because the rear tyres are the ones that have to transfer the 1,000hp produced by the engine to the ground.

If the vehicle were 4 wheel drive, the tires would be the same size all around.

In a F1 car, the engine power is only applied to the rear tyres. They accelerate the car.

But, a large amount of the braking effort is always (on almost all cars around) done by the front tyres.

The rear tyres are also braked and in a perfect world (car balance under braking), the rear tyres do 50% of the braking....having the engine weight at the rear assists in the good balance.

But just "how perfect" the brake balance REALLY is, is only known by McClaren, Ferrari and the rest!!!

Suffice to say that due to the needs of the rear tyres to do both acceleration AND braking, they need a larger area of rubber to "balance" the wear as far as possible between the front and rear tyres.....

Guessing only, I would imagine that the area of the surface of a rear tyre would be almost double that of a front tyre......but due to the way race circuits are round (Suzuki is different as it crosses "figure of eight" over and evens the wear out more I would imagine), there is almost always more wear on one side, or the front or back than on the other....its not a perfect world. Its the job of the designers to try and get that as even as possible....

The TOTAL force is independent of the area. If an object of a given weight has 4 contact points, or 3 contact points, or two -- the total frictional force will be the same.

For automobile stability you want to reduce the chance that the rear tires will slip. In that case, the larger the contact area, the lower the likelihood of slippage.

For some cars where the engine is rear-mounted, the rear tires are bearing a greater load, in which cases larger tires are needed to support that load.

And...aside from weight transfer during acceleration, it looks cool, ask Ed Roth.

unredundant's reply seems more logical to me. The goal in F1 is to achieve the best accelaration (using mainly the rear tires), or the best decelaration (using mainly the front tires). As they say, the highest acc. is around 1.5 g, while the dec. reaches 4-5 g.

How can this happen when the front tires are smaller than the rear?

I did not include the "pucker factor" for which no formula exists, but does have a psychological effect on deceleration.

If the goal is solely "best acceleration", then a drag chute (on the car, not the body) is effective.

While the friction force may be independent of contact area, the stress on the material (tire and road) is not. A narrower tire will have a smaller contact area (An)than a wider tire (Aw) both having the same friction force (Ff). Force Ff is being applied over area An will result in a much higher stress to the tire material causing it to fail (resulting in slipping) sooner than Ff being applied over area Aw.

It's a simplification put in Freshman books. The friction force is, in fact, directly proportional to the real area, which generally has little to do with the apparent area.

In the case of tyres on many surfaces (say sand) the real area and apparent area become approximately proportional because the sand is free to flow; then bigger helps.

There's a couple of things going on here. First the idea of Ff being independent of area applies to normal solids, not sticky solids, like adhesive tape, or tires composed of soft rubbers that are formulated to become very sticky when hot. The trade off for this stickiness is rapid wear, so you want as much surface as possible to give allow for large amounts of material to wear. You would be pretty honked if you paid 6 grand for a set of tires for your car and they were shot after 300Km, which is what happens in an F1 race.

Martin

So, the phrase "friction force is independant of the contact area" is not valid for the tires and especially for the F1 tires.

What about thesecond half of the origianl question? Why the rear tires of some kind of cars, like F1 cars, is wider than the front ones?

Transfer of the engine power could not be an answer to the above question, because during deceleration the forces are much bigger when compared with accelaration.

I think this stems from early days of design,when turning the vehicle the track width was altered & to keep it within certain boundary's the front track was slightly reduced, also the wider the track the more stable the vehicle.

I am not sure that many of these posts are on topic. I understand that generally the friction is independent of area, but the temperature of the tyre is important - look at how much time is spent getting the tyre pressures right on a race car. The tyres heat as they work and if the area is too small they cannot dissipate the heat and they start to melt.

Wide tyres are a disadvantage if your suspension is not set up correctly as the camber changes can cause big changes in area and poor handling as acceleration out of a corner turns into a spin, for example.

Wide tyres also equals more unsprung weight, so poorer surface following. That is why rear tyres are wider on the back for a rear wheel drive car.

Acceleration and deceleration aside... I think the fact that cars pivot around the front wheels might have something to do with it. Wider tires on the rear might help preventing the car from slipping sideways / fishtailing. I know, I know, this doens't answer the question "is ff independent of area?", but i thought i'd throw this out there.

Because there are tensions involved. The forces must be distributed on surfaces and this will influence in the generation of heat, strength of materials and system balance.