tyre->road coefficient of friction

[The Natural Philosopher]

Cos an unlocked wheel:-

- provides more than a locked one

- gives you directional control.

The physics of rubber tyres is very complex.

[The Natural Philosopher]

Well, you sort of can, but its no longer a simple relationship :-)

[The Natural Philosopher]

You can do a 360 at 0 mph with a rear wheel drive :-)

I've seen a car fully overturned from a sub 30mph impact.

Frankly you can spin a car from a 15mph impact if its all 'about right'.

>

[Mike]

Downforce is the force acting downwards on the mass of the car. It could be aerodynamic for a racing or performance car but in this case is just due to gravity. Although there is 35% of the weight of a FWD though the back when stationary, this drops to as low as 10% under dynamic conditions such as heavy braking which is where ny 0.1g comes from. Do you understand now ?

I presume your pumas don't race too well then. A FWD lifts it's rear inside wheel because the front outside is heavily loaded by cornering forces and you get suspension compression. On a road car driven fast this is far in excess of how fast the rear suspension can compensate as this is often constrained by either a rear anti-roll system or some form of dead axle connection and the wheel simply lifts. But if you can reduce forward weight transfer under heavy cornering then the effect is reduced considerably - the Focus RS is a prime example of how to do this correctly.

As for RWDs the high front roll stiffness is to keep both front wheels on the ground during braking. Adjusted properly, squat is eliminated and braking is optimal. It has no effect on the driven rear wheels in corners - even on an old Mk2 Escort ! For a fully independent rear suspension the position of the front wheels is nowadays almost irrelevant until you reach the front suspension travel end point - which for a rally car is enormous but for an F1 a few cm.

[Mike]

That depends on the other surface. If wet then a turning treaded wheel will clear water and be more effective. But on dry course abrasive tarmac the locked tyre will give more braking grip until the rubber overheats and goes off. Of course with some competition compounds this can be less than the time of the skid but for road cars it won't be.

Always, no matter what the conditions.

Sure are. Used to play with all sorts of compounds in the 80s and I expect things are far more complex now.

[Rob Morley]

If road tyres don't melt when they skid why do they leave a trail of rubber stuck to the road?

[roger]

The message from "Mike" contains these words:

Still gibberish though. Last time I looked g was still the acceleration due to gravity and very definitely not measured in units of either force or mass.

[Mike]

Oh stop being such a prat. You know very well what I mean - take the mass of the car and multiply it by 0.1*9.81 to get the active downforce, then by the co-efficient of friction of the tyre to the road (typically between 0.6 and 0.95) to get the level of grip needed to be overcome to start the spin. Happy now ?

[Mike]

Didn't say they didn't melt. But that's not the same as overheating. When a section of the tyre overheats it 'cures' and from then on will have little grip on this section until it wears away. That is rarely the case with a road car as a) you aren't usually going fast enough and b) most brakes simply cannot maintain the skid long enough before they overheat and release the wheel a bit. Most road skids do not keep the tyre in a constant position if you watch side angle footage carefully.

[The Natural Philosopher]

Not sure that is totally true. I thought the sliding friction of a dry slick was about 85% of peak grip. Whih occurs not at the slide, but at some rather smaller 'creep' relative velocity.

I.e. maximum traction is achieved with a very limited and little wheelspin on most cars. Too much and you go nowhere.

Indeed.

[The Natural Philosopher]

Because it gets scrubbed off.

Pencils don't melt when they leave a mark on a piece of paper either.

[The Natural Philosopher]

Thats is untrue. acceleration can be expressed in the force per unit mass needed to achieve it.

Which is what the OP was doing.

[Dave Plowman (News)]

Err, once the wheel has locked and pressure continues from the pedal, the brake does no more work. You've obviously never driven an early Jag - this was the only way to stop them in an emergency from speed. Try to avoid locking the wheels and the brakes would fade to nothing long before you came to rest.

[Rob Morley]

If it's just abraded why does it stick to the road surface? Road tyre compound isn't exactly sticky is it?

[roger]

The message from The Natural Philosopher contains these words:

Is it? F=MA so A=F/M but you are stretching the English language into the absurd by claiming it is a way of expressing acceleration. It is a way of calculating it when acceleration happens to be the single unknown in the equation.

Anyway that is besides the point. The units of acceleration are length per time^2. Not a lbf or a slug in sight. Ergo my statement above is true.

I haven't a clue what the op was doing when he came up with 0.1g. My kindest interpretation is that he was just trying to be clever and had a senior moment. (Don't we all from time to time?) 0.1g is 3.22 ft/sec^2. What he meant was 0.1*(mass of vehicle)*g and omitting the mass is just bizarre when calculating force.

[roger]

The message from "Mike" contains these words:

No. I don't like being called a prat by an arrogant arse who can't admit to a simple mistake.

On the subject of 'downforce' I have always understood it to relate only to aerodynamic forces but its usage is so new that none of the dictionaries I can consult here (I am away from home atm) do so much as mention it. I dare say that such usage would be useful to the pedants who argue that 'weight transfer' is incorrect terminology on the dubious grounds that the transfer involved is not actually weight but 'downforce transfer' doesn't trip off the tongue quite so easily and, as you have so amply proved, somewhat harder to calculate.

[Dave Baker]

A locked tyre won't give more grip on tarmac in the wet 'or' the dry. The coefficient of sliding friction is always less than the coefficient of static friction. The only surfaces where locked wheels are better than unlocked ones are where there is loose material which builds up into a wedge between the tyre and the road and absorbs some of the kinetic energy. Gravel and snow are examples of this.

[Dave Baker]

[e-mail]

overheats and

brakes

Now what are you babbling on about. Once a wheel has locked the brake on that wheel is doing no more work and can maintain the situation ad infinitum. The kinetic energy that the brake was absorbing by turning it into heat is now being absorbed by the tyre as that abrades and/or melts.

[Dave Baker]

wheels on

What on earth does roll stiffness have to do with braking? There is no roll under braking.

[Dave Baker]

Weight transfer can't overshoot anything. It transfers from one wheel to other wheels but it doesn't disappear anywhere. The car still weighs the same in total, it's just that total weight is not acting on the same wheels as when the car is stationary. It makes no difference if a car is FWD or RWD. The weight transfer is purely a function of the forces acting on the centre of mass, the height of that centre of mass above the ground and the distance between the wheels.