Why have those BIG BIG wheels??????

[normski]

You can\'t go down that road regards brakes! you bring into play things like maerials tech ie carbo fiber heat Dispersion and alike. we are on about road rubber here. Fact, same car, same contions, same road, same compound, but 1 has wider wheels and rubber than the other, stopping distance ie friction brake point will be the same, It\'s the Law!! can\'t get around physics man! :burnout

[MikeLMR]

you can\'t simplify physics to the point you have either ... we will never fully understand the "laws" and there will always be exceptions.

[T11MNM]

Decent pad material and a good set of calipers is all that really matters imho. The last time i chcked RX7\'s come with 4 pots as standard which is more than enough to stop the car.

If you scrimp on pads then there is no wonder you\'re getting brake fade. Either that or your on the brakes alot longer than you should be. Short/Hard braking for track will pro-long brake fade. I only attend open pit events so you can go out for 10 minutes and then come back in to let things cool down before going out again.

[marco]

a quick mental experiment. you have a length of 2x4 wood flying on the floor and you have to push it along the ground. in order to account for the way in which the rubber lightly beds in to the tarmac (rubber is squishy, and tarmac has a number of small holes in it, so its going to happen) by your account then pushing the wood sideways will required the same force as if you were pushing it  length ways. I am sure you will agree that from experience from shifting heavy stuff you move it in the direction of the smallest side. so the value of mu has changed even though NOTHING has changed in the two bodies, just the direction of movement. now consider you get a piece of plywood that is the same weight as the 2x4. and try pushing that flat on the carpet. again by your statement then because the weight is the same the force required will be the same. again i hope you can see that this is intuitively wrong.
from your first post, your original equation is wrong, f does not = r, it is in fact f=mu*r if f=r then it would mean that all materials have the same frictional coefficient- all materials would require the same force to move them. it is \'mu\' that is the key to this, and as i have said before they calculate this empirically for a system.
\'ok so this thing weighs 10kg, so thats 100newtons. on this surface, at this temperature, etc, etc, etc, it takes 50 newtons to move it, therefor \'mu\' is 2\' for an example. in A level problems they give you mu to allow you do do a nice sum that can be marked. but by changing the size of the tyres, of the pressure in them, or the temperature of the day, or the surface they are on, or the make up of the tyre, or any other of hundreds of variable, you change \'mu\' therefore you cannot compare a tyre of 3inch width with one of 10inch, as there aren\'t like for like. all \'mu\' does is give a conversion factor between the weight (or normal force) to the maximal frictional force. and even then it is a rough estimate as i doubt the weight/frictional relationship is linear in real systems.

marco

(again the usual, \'if you know better, shoot me down\' applies:o

[normski]

Your bits of wood, of course there will be a Difference, as it will take more energy to move the whole weight of it than pushing it sideways, as you will not have the weight of length to overcome. So that doesn’t work! Weight is different from friction which is what you would feel when pushing length ways opposed to sideways.

Anyway, it depends how pedantic you wish to be regards width of rubber, of course you may find very, very slight differences if measured precisely, but the point still remains, if one takes two things of exact equals ie car weight, conditions, everything except width of rubber you will find almost no difference in the point of friction loose, that is a fact! Put it to the test if you don’t believe it, but things have to be as near as possible to each other except width of rubber, and don’t forget the compound, which also has to be the same.

[marco]

i don\'t think you have understood me. there is a plank of wood on the carpet. you push it one way (so it travels with its longest edge leading, lets call the sideways) and 90 degrees to that (so its shortest edge leads, lets call that forwards) going forward will take less effort than going sideways, even though the weight remains constant (same wood) and contact area remains same (same wood). try the same with a piece of plywood of the same weight (constant weight, but different contact area) again would expect the effort required to vary from intuition. do you agree with this?
another way of thinking, it is well known that over inflating tyres leads to a reduction in grip available, no? and why do you think this is the case? it is because the increasing the pressure tends the tyre to become more bulbous, and as such has less contact with the ground. a small change in pressure can give a noticeable change in grip, not measurable, i mean noticeable as in you can tell by driving. if your statement is correct, then how come such a small change in pressure in the tyres (which will produce a change in the contact area of only a square cm at best) can affect grip, whilst increasing the width of the tyre by inches cannot? or are you implying that changing tyre pressure affects grip on another way? if so can you please tell me how it does so?

[Grizzly]

The way i see it, on a road car there is an Optimum contact area to weight, if you exceed this you will start to suffer from Aquaplaning.

It works the same in the dry but obviously the rubber tyres will heat up and give a better Grip (another reason you shouldn\'t go to wide, as the tyres won\'t warm up properly unless your Flat out i.e. racing)

Don\'t get Racing on a track and Road use mixed up, you don\'t see F1 cars driving down the road.

An example http://www.youtube.com/watch?v=I0O7gFycPho