How does ET affect a car?

[royal]

What he said ^. Changing the camber doesn't change the scrub radius.

I find there's a very noticeable point where there are too much spacers and the cars handling gets ropey. Toe'ing the front in a bit helps a little but I dont reckon its good to over spacer as it still feels sh*t.

[Vova]

Originally Posted by howler

Vova, surely though more surface area generates more friction as there is more tyre in contact with the road?

I get what you are saying about pressure, but that doesn't explain why if you drift on very skinny tyres you get less grip

I guess there is a width vs weight balance to account for?

To reply to you and royal;

Grip is generated in three ways; - Adhesion, Deformation and to a small extent shearing. Deformation is 85% of the contribution, adhesion 10 and 5 for shearing.

Deformation.
Force pressing the tire into road generates grip because rubber, seeks the imperfections in the pavement, in which it flows into and pushes itself from, to create resistance in the opposite direction of turning.

Imagine this is the road: /\/\/\/\/\/\/\/\/\ ... like a hacksaw blade. The tire, fills the valleys in between and pushs itself off, against the sides of the mountains.

Like this, taken for the Carbibles page.



The more FORCE is pushing the rubber into the valley, the more resistance the tire can excert onto the sides..the more grip. A softer tire, can conform (get pushed into) much easier than a hard one, which is why compound is the second key factor of grip. Now if you widen the tire, the force per "mountain" becomes smaller, but there are more mountains used in total. So the grip remains equal, providing the tire operates in the optimum temperature zone in both situations. Which is another key... Too wide tires, get too much cooling, don't operate in the designed temperature range, and have reduced grip.

To anwser the drift-tire-width-question

To be honest, I can't explain that to the full extent. In theory, like above, it shouldn't make a difference, however by changing width, you change multiple factors. Like sidewall stiffness, maybe compound, cooling capacity etc which could provide more grip on theselves, contributing to a total greater grip package. But the width itself, or the presumed "more rubber on the road equals more grip" is not the cause of the grip increase. Maybe placebo also plays a part but let's not get carried away.

p.s. , a good example, look at snow-rally's, they use skinny tires as the REALLY need to dig, thus have maximum pressure per square mm.

[Kit]

I know wiki sucks:

Scrub radius - Wikipedia, the free encyclopedia

Scrub_radius

"The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel. This means that if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the the scrub radius, this alters the width and offset of tires you can safely run on your car."

But apparently it was quoted from: Reimpell, Jornsen; Helmut Stoll, Jurgen W. Betzler. The Automotive Chassis Engineering Principles

*shrug*

this is why I wanted ot start this thread, there are so many differing opinions!

[royal]

I'm sure there's something more fundamental going on there Vova. Try doing a burnout on skinny tyre's then on wide ones and its very obvious you can'd do it as easy on the wide ones.
Its the same at high speeds drifting, wide tyres will grip too much and bog you down, whereas skinnys will keep spinning. We put more pressure in very wide tyres to help keep them spinning through the bog down areas but there's only so far you can go. Even with high pressures the wide tyres make the car much more snatchy and its easier to just use narrower ones.

[howler]

That makes a bit more sense now, thanks Vova

You mention snow rallies where you need to 'dig' presumably to get to the packed down surface underneath, but on dry tarmac cars tend to use wide tyres, touring cars for example?

Though the force argument makes sense

[steve.82]

Struts with camber adjusted most likely by drilling out the holes on the upright In which case yes that will change the scrub radius

[Vova]

Originally Posted by howler

That makes a bit more sense now, thanks Vova

You mention snow rallies where you need to 'dig' presumably to get to the packed down surface underneath, but on dry tarmac cars tend to use wide tyres, touring cars for example?

Though the force argument makes sense

Hit the nail on the head with the snow thing. Just like rain, wider tires have less grip as there is less force pushing the water out of the rubbers way.

On (dry) tarmac, heat issues come into play. A car that tries to transfer horsepower, braking and cornering into friction, will heat up the tire in the same proportion as the applied input. Wider tires with more surface area in contact and more frequently with the wind, get cooled more than skinnies.

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.

Touring car don't use overly wide tires as there is no need for it. They find the balance between cooling capacity, sidewall stiffness, weight and some others to determine the optimum width and stick with it (litteraly!).

[Vova]

Originally Posted by royal

skinnys will keep spinning

The only explanation I could offer is skinnies overheating to such an extent they lose all possible grip as whereas wider tires stay cooler and retain some grip.

However sidewall stiffness is not to be underestimated, as skinnies tend to have high sidewalls vs low wide tires that provide better grip characteristics.

[royal]

Nope, there's something else. They do it hot or cold

[Captain Muppet]

If tyre grip was generated just by friction (so total grip = vertical load * friction coefficient) then wider tyres wouldn't grip more than narrow tyres.

But...

If tyre grip is also generated by covalent bonding between the rubber and the tarmac then not only would wider tyres grip more than narrow ones but you would get some other weird effects like small stones being picked up by sticky tyres and rubber bonding to the tarmac during burnouts (neither of which can be explained by friction and deformation).

[howler]

Interesting reading

Royal, black magic is the something else you are looking for I reckon

[Bladerider]

Sorry me ET (elapsed time for drag racing) humour got lost,

But here are some other key things to think about.

What Vova is saying is generally regarded as the "contact patch". If you consider two tyres of extremes for a moment and only consider the surface area of the tyre touching the tarmac, you could have a vry slim tyre, say a 165 for example, which is being pushed into the tarmac by the 1000kg car on top meaning you have a long slim patch deforming into the roadsurface. The 1000kg car next to it has some hoofing great 315 tyres, but its still the same weight car and the psi's in the tyres is all the same so it has a short and very wide contact patch, but the key is that the surface area of tyre is actually the same as the skinny one thats touching the ground.

Trouble is that as with alot of things, this is all well and good in atext book, but in real life the 165 tyre will have 40psi in it and a 70+ % sidewall ratio, and the 315 will have 25psi in it and a 30% or less sidewall ratio. In general lower psi = more grip for the same size tyre, but it also leads to more deformation, more heat and less sidewall resistance and easier rollover of the treadblocks.

This then leads us to slip and the other strange effects we see in road tyres such as tread block shuffle. If you have more width going against the pull of the car, ie the cornering force generated when you are being a hero around clearways for example, then it is beneficial to be running a tyre that has a wider pattern so the when the outemost tread blocks start to give up there are more blocks on the inside to help maintain friction, this is not available on a slim tyre with perhaps only a couple of rows of slimmer treadblocks. There is also the sidewall ratio that was mentioned earlier, lower stiffer sidewalls force the tyre to remain in contact with the tarmac whereas higher sidewall tyres allow the treads to grip enough to make the edges of the tyre fall over and roll under the rim giving you sidewall scrub, on a piece of tyre not designed to generate grip !!! And before anyone mentions F1 its only because lower sidewalls were banned that the tyres look the way they do !! Touring cars are much better pointers to real world as they have not had the same restrictions.

Theres lots more to say on this subject but I will be back later to help some more.

J.

[royal]

Good info. Amusing quote in a book I have at home about suspension and shizzle:

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

I guess their idea of 'letting go' is different to ours?!

[Vova]

Captain Muppet:

Friction is the sum of adhesion (the covalent bonding you mention), deformation and shearing of the tire. Now I see your point, wider tire should mean more contact area and more adhesion? Sound fair. However, think of the contact patch shape. This is a bit more tricky to gasp when not illustrated so I'll refer you to:

/Edit: Damn it, he doesn't allow direct irl's from his site.

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

A wider tire has a shorter but wider contact patch. A skinny has a long and skinnier CP. However the total sufrace area stays the same, so there is not "more" road/rubber-contact.

/Edit:

@ Bladerider

[Bladerider]

I just said that !!

[Vova]

Yup, just saw it, and said it better than I could have

[royal]

Originally Posted by Bladerider

Sorry me ET (elapsed time for drag racing) humour got lost,

But here are some other key things to think about.

What Vova is saying is generally regarded as the "contact patch". If you consider two tyres of extremes for a moment and only consider the surface area of the tyre touching the tarmac, you could have a vry slim tyre, say a 165 for example, which is being pushed into the tarmac by the 1000kg car on top meaning you have a long slim patch deforming into the roadsurface. The 1000kg car next to it has some hoofing great 315 tyres, but its still the same weight car and the psi's in the tyres is all the same so it has a short and very wide contact patch, but the key is that the surface area of tyre is actually the same as the skinny one thats touching the ground.

Trouble is that as with alot of things, this is all well and good in atext book, but in real life the 165 tyre will have 40psi in it and a 70+ % sidewall ratio, and the 315 will have 25psi in it and a 30% or less sidewall ratio. In general lower psi = more grip for the same size tyre, but it also leads to more deformation, more heat and less sidewall resistance and easier rollover of the treadblocks.

This then leads us to slip and the other strange effects we see in road tyres such as tread block shuffle. If you have more width going against the pull of the car, ie the cornering force generated when you are being a hero around clearways for example, then it is beneficial to be running a tyre that has a wider pattern so the when the outemost tread blocks start to give up there are more blocks on the inside to help maintain friction, this is not available on a slim tyre with perhaps only a couple of rows of slimmer treadblocks. There is also the sidewall ratio that was mentioned earlier, lower stiffer sidewalls force the tyre to remain in contact with the tarmac whereas higher sidewall tyres allow the treads to grip enough to make the edges of the tyre fall over and roll under the rim giving you sidewall scrub, on a piece of tyre not designed to generate grip !!! And before anyone mentions F1 its only because lower sidewalls were banned that the tyres look the way they do !! Touring cars are much better pointers to real world as they have not had the same restrictions.

Theres lots more to say on this subject but I will be back later to help some more.

J.

Just read that for the fifth time and think its sinking in now great post, I like being schooled.

[Bladerider]

Right Im off, but here's a question for you, and no googling the answer you dirty cheating mofo's..........

At what percentage of total grip does a tyre actually generate the most cornering forces ???

J.

[Vova]

Does the anwser involve 'slip angle' ?

[sideways.taff]

Now I remember why I gave up wanting to be an engineer after graduating and went all namby pamby and started out as an artist instead lol.. Engineering takes way too much thought.. can you imagine doing that sort of shit day in day out?! haha fuck that. I rather make pretty cars in pretty racing games!

Saying that learning the principles and theories for things such as tyre characteristics and force analysis during my years at University was interesting. Very much pointless though because in reality you just bosh some numbers into a computer after doing a few tests and it works it all out for you instead!