How does ET affect a car?

[Captain Muppet]

Originally Posted by Vova

Courtesey to Car Bibles : The Wheel and Tyre Bible Page 2 of 2

If we look at his maths for a "standard car and tyre"

Standard car weight = 1500kg
Standard tyre width = 18.5cm
Standard tyre pressure = 30psi
Standard tyre pressure in Kg/cm^2 = 2.11Kg/cm^2
Weight on one tyre = 375kg
Contact patch = 177.7cm^2
length of flat at the bottom of the tyre = 9.6cm

Fair enough.

Now we let the air out...

Standard car weight = 1500kg
Standard tyre width = 18.5cm
Standard tyre pressure = 1psi
Standard tyre pressure in Kg/cm^2 = 0.07Kg/cm^2
Weight on one tyre = 375kg
Contact patch = 5331.7cm^2
length of flat at the bottom of the tyre = 288.2cm

Oh, that's a bit frigging huge isn't it? Tends towards infinity at zero pressure, if you're wondering why I used 1 psi instead of 0psi.

Now we fit the same flat tyre to, oh, I don't know, maybe an S1 Elise

Standard car weight = 750kg
Standard tyre width = 18.5cm
Standard tyre pressure = 1psi
Standard tyre pressure in Kg/cm^2 = 0.070Kg/cm^2
Weight on one tyre = 187.5kg
Contact patch = 2665.8cm^2
length of flat at the bottom of the tyre = 144.1cm

Now, I happen to know that with old Pirelli P Zeros and no pressure in the 185 front tyres an S1 Elise front tyre still doesn't go flat.

What does this tell us? His equation has no expression for the structure of the tyre, no sidewalls, no steel banding, no inch thick rubber with deformable blocks. All the bits which are carefully engineered in to a modern tyre. Great for getting his point across, but not the whole truth. An equation that asymptotes for flat tyres is one that doesn't reflect the real world.

You may think I being pedantic, but really I'm just a little bored and playing cut and paste in Excel while having a coffee I haven't a clue why fitting wider tyres doesn't increase grip, but at the same time seems to increase grip. It's all magic to me

Good excuse to wheel out the word "asymptote" though, which is one of my favourites.

[Keefe]

WOAH, whole heap of shit going on here.

Contact patch WILL change when using different size tyres.

From what i've heard, i'd listen to Captain Muppet on this one

And about 20%. But it's slip. Not grip.

[Captain Muppet]

Originally Posted by Keefe

From what i've heard, i'd listen to Captain Muppet on this one

Don't drag me in as some kind of authority...

Originally Posted by Captain Muppet

It's all magic to me

[Keefe]

HAHA i'm not. But when people question what you've said which is perfectly right, and then dispute it using information from TeH All KnOWin INderneTz then i'm likely to side with you.

[Mattyboi]

This thread is funny! And why engineers get paid alot of money and normal home mechanics build shit cars when they mod using stuff they read on the internet.

[steve.82]

Originally Posted by Bladerider

Touring cars are much better pointers to real world as they have not had the same restrictions.

I would go with hillclimb cars to give an example of what happens with less regs and without fashion dictated tyre sizes...


These are hillclimb specific tyres too, so aren't just hand me downs from highly regulated circuit formulae, so high sidewalls *can* be good.

Also they are huge, and barely have time to get warm. Skinny tyres would heat up much faster which would be good in a hillclimb car (standing start no tyre warmers allowed), but clearly wider tyres are grippier.

Originally Posted by royal

Even with high pressures the wide tyres make the car much more snatchy and its easier to just use narrower ones

Narrow tyres with higher sidewalls can generally support a higher slip angle before break away.

Weight (well pressure) also has an effect on slip angle possible. The more weight on a tyre the sharper it can turn without breakaway, which is why race cars often have reverse ackerman. Which brings me on to...

Originally Posted by royal

'once the car lets go then you have no control whatsoever'

Most norms don't have much control, me included
But looking through some of the pictures here most of the time the front wheels are pointing the same way they would be during normal cornering. Does the Ackerman still work out ok on opposite lock? I guess it would work out to be effectively reverse Ackerman on a normal road car based upright?

Originally Posted by Vova

Lambo and McLaren have 300 wide rear tires however only 225 front's, because the rear wheels heat up way more than the rolling, cornering and braking fronts. Than there are also the issues of resistance, scrub and many others but leaving those aside for a bit.

I disagree, I think it's largely because they're midengined and a bit tail heavy and powerful so need big tyres to get the power down (hot or cold!)

If you look at front engined RWD GT cars, front and rear wheels are relatively similar sized compared to their mid-engined equivalents.

Note: most of these comments are based on vague memories of lectures since when I've consumed too much beer and lost most of these memories... Liking this thread though

[royal]

Originally Posted by steve.82

Narrow tyres with higher sidewalls can generally support a higher slip angle before break away. Weight (well pressure) also has an effect on slip angle possible. The more weight on a tyre the sharper it can turn without breakaway,

So with narrow tyres you have more weight on a specific area as per vova point? that make sense to me.

Originally Posted by steve.82

which is why race cars often have reverse ackerman.

Which brings me on to...

Most norms don't have much control, me included
But looking through some of the pictures here most of the time the front wheels are pointing the same way they would be during normal cornering. Does the Ackerman still work out ok on opposite lock? I guess it would work out to be effectively reverse Ackerman on a normal road car based upright?

Its a long time point of conjecture on here. Some people reckon less ackerman like on autograss cars is good (Declan runs zero) whereas others such as Phil think its better to have it because the trailing wheel drags and helps to control the car in a slide. I suppose its down to driver preference and style? I'd like to try less ackerman just to see.
I didn't know people do reverse ackerman. I wonder how that would drift with the trailing wheel pointing in even more when on full opposite lock? I'd imagine it would make transitioning strange as the weight shifts from one wheel to the other and you have to change the wheel to the other lock?

[steve.82]

I don't think reverse ackerman would be much use on a drifter really, as running normal ackerman geometry is effectively reversed when drifting, i.e. normally the inside wheel is actually outside on the corner, getting all the weight shift?

[steve.82]

But then I guess it's not necessarily all about ultimate grip, more feel, in which case I have not a clue! haha

[Vova]

Originally Posted by steve.82

I disagree, I think it's largely because they're midengined and a bit tail heavy and powerful so need big tyres to get the power down (hot or cold!)

If you look at front engined RWD GT cars, front and rear wheels are relatively similar sized compared to their mid-engined equivalents.

Note: most of these comments are based on vague memories of lectures since when I've consumed too much beer and lost most of these memories... Liking this thread though

Well doesn't more weight, contribute, to more heating of the tire...? Thus, what it all comes down to and is kinda my point?

[steve.82]

Originally Posted by Vova

Well doesn't more weight, contribute, to more heating of the tire...? Thus, what it all comes down to and is kinda my point?

Not when the tyres are cold no

[Vova]

How do you mean 'when cold' ? Just rolling resistance of a heavier rear contributes to more heat, just as power transfer.

[mitto]

ET made my dick get bigger...

think about it

[Vova]

Good article about Wider = more grip?

Browser Warning

[steve.82]

I love mathematical models as much as the next chap, but I struggle to believe it. All this assumes mu is a constant, which I highly doubt that something as deformable as a tyre it is.

[Keefe]

Nothing to do with the deformability of the tyre. It's the surface it's rolling on. Models will normally quote an agreed figure for "decent paving" i.e. good tarmac.

[steve.82]

Ok then, well as deformable as tyre rubber, I was a bit misleading with that.

Quick search on google...

The value of u is dimensionless when applied to sliding friction of hard surfaces. In situations where the surfaces deform or there is molecular adhesion, the friction is not independent of the areas in contact. In these cases surface area usually comes into play. This is also true for rolling and fluid friction. Although the standard friction equation still holds, the coefficient of friction may have area, shape and other factors included in it.

from forum chat I know, but clarifies what I was trying to say

edit: this was mentioned earlier by someone else too. So, ET's?

[steve.82]

Another point... camber wouldn't matter (on cold tyres) if that theory was correct

[shaiz]

Just had a quick skim through this. Someone mentioned that width doesn't affect the area of the contact patch, just the shape of it. If that was true (which I'm skeptical about but would need to lok into further) surely the analogy of rally cars using narrow tyres in snow to increase the force per unit area of the contact patch would go out the window. I'd get stuck into this further but I'm going to be late for work.

[Keefe]

Originally Posted by shaiz

Just had a quick skim through this. Someone mentioned that width doesn't affect the area of the contact patch, just the shape of it. If that was true (which I'm skeptical about but would need to lok into further) surely the analogy of rally cars using narrow tyres in snow to increase the force per unit area of the contact patch would go out the window. I'd get stuck into this further but I'm going to be late for work.

Yeah it's bullshit. Of course it changes. That was just some internet fodder someone took as gospel as per usual. Which was wrong. As per usual.