tyre->road coefficient of friction

anyone got a ballpark figure? dry road. Force acting at 90 degrees to my direction of travel. My little Corsa has been written off by some guy crashing into the side. He managed to spin it through 360 degrees. Im trying to have a guesstimate of the impact speed .
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If you hit a FWD towards the back they'll spin quite easily. You'd also need to know the weight of the other vehicle. Can't the police help? - sorry, silly question. ;-(
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Ouch. Get one of those "heat up in a microwave" sausage shaped things, as I suspect you`re going to be sore as hell within 2 days.
I was hit by a HGV in 2001 and still suffer now - leaning my head back to relax is one thing I can`t do any more because the pain kicks in within 5 seconds...
I was off for 6 or 7 weeks IIRC, and seriously couldn`t do a thing - i.e. get dressed for the first 3 weeks or so. If it hadn`t been for internet shopping we would have starved to death.
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He
Key thing is what was the road condition. Downforce on back of a Corsa or other small FWD hatchback can be as little as 0.1g (which is why the inside wheel lifts in fast cornering) so it probably didn't take as much as you might think.
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Mike wrote:

Downforce? 0.1g?? And in normal physics this translates to?
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Grunff

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Utter gibberish.
Firstly, downforce is an aerodynamic term and very few road cars have any of that. Most generate lift at speed.
If you meant weight distribution then still gibberish. The average FWD car has 55% to 65% weight on the front and 35% to 45% on the rear.
Finally, neither weight distribution nor downforce have anything to do with why a FWD car lifts its inside rear wheel during fast cornering. That's to do with the relative roll stiffness front to rear. FWD cars have high rear roll stiffness to keep the front (driving wheels) on the ground in corners. RWD cars have high front roll stiffness to keep the rear (driving wheels) on the ground in corners.
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Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

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or
inside
you
of
has
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 ?

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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.
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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.
--
Roger

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contains these words:

could be

to
when
?
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 ?
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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.
--
Roger

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contains these words:

due
back
such as

now
acceleration
force
mass
by
0.6
spin.
Then don't call other's people opinions gibberish.

I made a simplification often made in automotive circles. I agree it wouldn't be obvious to all but should have been to most.

The two first mentions of aerodynamic forces I can find are in the 60s by Dan Gurney referring to "an increase in downforce" and Colin Chapman referring to an "aerodynamic gain", both referring to the wings added to the rears of their cars.
To my mind, this implies the term downforce already applied to those cars, i.e. the weight of the car. Also most school textbooks refer to the "force down" through a wheel which isn't too far away.
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Gibberish it was and gibberish it remains.

Wierder and weirder. Recipe for complete confusion between vertical and horizontal forces.
--
Roger

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contains these words:

And a prat you obviiusly choose to remain. The End.
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Whatever your opinion of me the fact remains that what you posted was not correct. Your explanation of why you used such wording was unconvincing and I don't for a moment believe that your usage is common usage in the automotive world except perhaps in the realms of car salesmanship. I am surprised that you should consider this thread at an end when your other fantasies are taking such a pounding but that is your choice. I too have a choice - welcome to my killfile.
plonk
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Except that my killfile refused to work on such a bland address.
Oh well it did at least give me the chance to see that Mike had started a new thread to grovel to Dave, seemingly oblivious to the fact that it was Dave who originally called his random noise gibberish.
--
Roger

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That's the average IQ of those reading this lowered back down again then.
If you won't accept references to Dan Gurney or Colin Chapman on what downforce is then there is little point.
Wonder if my killfile will accept you ?

Hardly. I checked his facts, compared them with mine and accepted that modern road tyres do have a far sharp fall-off in performance on lockup than I expected. If I could find some modern data on racing tyres I expect the data would be different but that is rather beside the point.
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Arrogant little arse aren't you.

You haven't made any as yet but if you post an easily accessed reliable source I will look at it. But accept hearsay from you of all people, no chance.

Why not try it. On the other hand we could call it a draw at this stage and give both of us the chance to get on with what is left of our lives.

Seems like a grovel to me. Particularly that bit - "What I tried to say, obviously not at all well..." but elementary tactics I suppose not wanting to fight on 2 fronts but letting Dave get away with calling your bit of nonsense gibberish smacks of lack of gumption.
--
Roger

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roger wrote:

Thats is untrue. acceleration can be expressed in the force per unit mass needed to achieve it.
Which is what the OP was doing.
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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

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Mike wrote:

wheels on

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