Grip and Unsprung Weight

[Mycroft]

Well you DID ask!

We first look at what causes tyres to get traction and what makes them to 'break away' and slide. This will make you realise that your inputs with the accelerator, brake pedal, and steering wheel are all limited by the Nature of the Physics involved. This bit of background knowledge will also allow you to slide the tyre slightly and by transferring the weight of the car to it bring the car under control. Driving is not just pressing hard on the pedals, knowledge of the physical limits combined with a good feel for the weight of each corner of the car will help you predict the consequences of your actions, developing a good feel for these limits will keep you out of trouble. Tazio Nuvolari, probably the greatest racing driver of all time, said he knew the weight on each corner of his car to within a kilo. If he had not this 'ability' then driving that damned Auto Union would have killed him.

Lets look at real time static friction, (you will like this) take a wheel off your car, go and weigh it and take 5lbs of air out of it, my rear wheel (265/40×17 on a 9½"wide rim) weighs 19.9kgs.
Now make a 'cradle' for it, I made my cradle out of a couple of coathangers and 4 pieces of wood, I know this is a little 'Blue Peter' but bear with me, it'll be worth it, honestly, it will. The cradle allows the tyre to stand upright but unable to roll, onto this assembly you put a strain gauge marked in Kilos, now with the help of a friend (who is by this time convinced you are well on the way to losing your mind and is only helping so as to ensure when you do finally lose the plot and take hostages he has a bargaining chip) you attempt drag this along the ground at right angles to the natural 'roll' you pull from fairly low down and you pull smoothly, your neighbours contribution is to ensure the tyre stays very upright and doesn't 'skip', you then read the gauge, on my car my reading with F1 GSD2 the reading was 36.4kgs, this was on dry tarmac, on concrete it was 35.7kgs on some high friction anti skid surface it 43.4kgs

These figures give the following potential g-force under braking and acceleration of 1.82, 1.79 and 2.18 G and that is the limit of your adhesion.

"HANG ON, HANG ON it can't be as simple as that, what about heat and weight and all the other 'things' " I hear you say well you are right there are lots of other 'things' and weight and heat but the strangest thing is that they all cancel each other out, I have a truly wonderfully complex Computer program for this and the results mirror the results of our little driveway trick almost exactly. you can even do it with tyre and no wheel, but remember that, the less the weight the greater the margin of percentage error.

You can pull from the front and there is sometimes a difference my tyre numbers were 37.1, 36.0 and 48.4kgs (you work out the Gs')

So this little experiment has given you the limit of you adhesion, this is all you have, you will never have more and unless you can make the suspension perfect you will always have less.

Do this and you will know your limits, that tyre on tarmac if I had enough power and perfect suspension could give me a take off the square root of that 1.87G, about 1.37G I have done a 1.0G start, but i believe that a 1.2G start is possible, in reality this is about as good as I'm ever gonna get.

Whilst we are on the subject of starts, read this and ponder... a good start at 0.5G will get you about 8 feet in the first second (2.45metres) a 0.7g start will take you 16 feet (4.9metres) down the line, a 1G start takes you 32feet (9.8metres) and as I said all in that first second.

This is the first part of many, now if you want ask some questions we can expand into grip specifics then this time next week we move on to the next part.

Bet you really can't wait eh!

[Ahmed]

very interesting and informative thank you mycroft,

as you mentioned

but remember that, the less the weight the greater the margin of percentage error.

forgive me for being dumb, but arent we missing a very big weight, that being the weight of the car itself ?
Surely with the increased weight factored into this experiment, the the adhesion of the tyres would increase greatly?

Cheers,
AK
The Orginial Scouser

[Peter]

Nice one Iain.

I guess weight is tied up with contact area of the tyre so as the weight increases, the contact area of the tyre increases but overall there is the same weight distributed per mm2...?

Tyre pressure is critical to grip and handling. Too high and the contact area decreases leading to more kg/mm2, too low would lead to less kg/mm2 but sidewalls would not be stiff enough...?

Can't get my head around the heat issue though, more heat should make the tyre stickier initially before they start 'going off'...?

[Mycroft]

I know this is hard to get your head around but this test developed by Dunlop in the 1950s' is still far more accurate than we like to admit.

The weight of the car is immaterial as is the number of tyres! You get the same result just slightly more accurate.

I have seen footage of guys doing this at Indianapolis so as to get readings for different parts of the curcuit.

Now to heat, this is gonna be difficult to swallow but heat does not improve the maximum traction available. It does allow you to use what you have as the speed increases and the tyre has to gain and then release its grip.

Think, with cold hands, can you grab and release anything as quickly as you can on a warm day, is your apparent strength and grip lessened with the cold, are you actually weaker?, No, your ability to use the strength you have is impaired by the cold, now take the corollary on and with the tyre we are trying pull the tyre across the surface, the molecules have time to search out the imperfections of the tarmac and interact with the surface molecules it finds there, heat only means this searching happens at a faster rate, in this test we don't need heat because we have extended the time frame allowing the tyre time to do what we ask to happen normally in a split second.

[Mycroft]

Originally posted by PeterE

I guess weight is tied up with contact area of the tyre so as the weight increases, the contact area of the tyre increases but overall there is the same weight distributed per mm2...?

No.

Originally posted by PeterE

Tyre pressure is critical to grip and handling. Too high and the contact area decreases leading to more kg/mm2, too low would lead to less kg/mm2 but sidewalls would not be stiff enough...?

See post above.

Originally posted by PeterE

Can't get my head around the heat issue though, more heat should make the tyre stickier initially before they start 'going off'...?

Irrelevent, don't confuse dynamic usage with static parameters

[Peter]

Cool, 0/3.

[RoboPope]

Peter,
You werent the only one batting those ideas around in your mind... looks like we failed as a class

Paul

[Mycroft]

Don't "beat yourself up" too badly lads, I catch even the most experienced engineers with these things.

[RoboPope]

Mycroft,
Im not disapointed, i have a mind like a sponge and i am on a quest voraciously absorbing information,

Paul

[dan0h]

Mycroft,

Fantasticly interesting and informative, and with a practical exercise too! (Did it in the workshop at tea break).

Good shout mentioning Tazio Nuvolari too, hardly anyone has even heard of him, but for sure his driving ability and car control was bordering on super-human. Hopefully people might do a bit of research into him... Makes for good reading.

Looking forward to the next part - its great having all this stuff to make you think while driving...

Cheers,

Dan

[Harry]

think about it...I'm still trying to picture the cradle

[Mycroft]

Rent some Blue Peter' videos, you'll get there.

[lightspeed]

Haven't heard of Nuvolari!!! The legendary flying Mantuan.

I wonder if people will say the same of Schumacher in 60 years?

[Mycroft]

Are we ready to move on?

[Peter]

Sitting comfortably....