contact patch shape

[barryogen]

Hi there guys,
From what I have read, a stock tyre normaly has a rectangular contact patch, longer from the front to back(of the tyre).

If you put a wider tire on than stock, the size of the contact patch remains practically the same, but you widen and shorten the contact patch making it more square, or to the extreme, a wide flat rectangle.

Firstly, I realise that there are more to tyres than just the contact patches, but I'm trying to learn one thing at a time... So, can anyone explain what the charactoristics of a stock tyre contact patch vs the other two examples I have given above?

From my own thinking(which may well be wrong), the stock should give a more predictable and forgiving when moving into a brake enforced skid(meaning to slam on the brakes), where as the other two would be more forgiving on lateral movements.

although, from all accounts in pretty much all motorsport they put big fat tyres on everything and it appears to "make it better".

I'd rather not see guesses in this thread, so ideally only facts, but hey, once the thread is out there, it's not like I can stop anyone.

[Rick]

I think this should help:

http://forums.toyspeed.org.nz/viewtopic.php?t=39835

[barryogen]

thanks, have read it in the past, but re-reading it now.

[barryogen]

Firstly, I realise that there are more to tyres than just the contact patches, but I'm trying to learn one thing at a time... So, can anyone explain what the characteristics of a stock tyre contact patch vs the other two examples I have given above?

after re-reading the link provided, it pretty much all makes sense.
wider tyres have smaller slip zones, and basically just have more grip available.. and shopping trolleys have "normal sized" tyres because of cost, expected usage, etc.

I guess the answer to my quoted question above is there is no characteristic that can be assigned to skinny or fat tyres, as each tyre is different compound/sidewall flex/heat range.

either way, once again toyspeed taught me something

[Stealer Of Souls]

Hmmm....

There are a few characteristics that could be assigned to skinny Vs fat tyres, IF you assume that the variables associated with the rubber and tread design are constant.

Skinny "long" tyres should ride over bumps better, and should provide better braking and accelerating performance.

Fat "short" tyres are the opposite.

Basically the article talked primarily about cornering forces. Of course the opposite applies when talking longitudinal forces.

[fivebob]

Hmmm....

There are a few characteristics that could be assigned to skinny Vs fat tyres, IF you assume that the variables associated with the rubber and tread design are constant.

You're forgetting one of the most important points in that article, inflation pressure

Skinny "long" tyres should ride over bumps better,

Only at the same inflation pressure, and I'm not sure that "better" is an apt description. Sure they will have more give in the sidewall so provide more springing in the suspension, but matched to the correct shocks wide low prfile tyres would provide the same reaction, except that most people would consider the suspension too soft then

and should provide better braking and accelerating performance.

Fat "short" tyres are the opposite.

And just how do you figure this to be the case???

Given that the correct tyre pressure for a low profile wide tyre should be lower, thereby providing a bigger contact patch, the wider tyre will have better braking and acceleration characteristics.

Basically the article talked primarily about cornering forces. Of course the opposite applies when talking longitudinal forces.

I suggest you read the article again and try to undersand what it is talking about. To save you wading through it here's the most appropriate bit to counter your argument.

Anyway, if it is approximately true that A = Fn/P , it follows that a wide tyre will have greater vertical stiffness, or tyre spring rate, than a narrow one, at any given inflation pressure. It will also have a smaller static deflection at a given load, which is why the contact patch is shorter. The flip side of this is that for a given static deflection, or tyre spring rate, a wide tyre needs a lower inflation pressure. Consequently, if we compare wide and narrow tyres at similar static deflection, or tyre spring rate, rather than similar pressure, they will have similar length contact patches and the wider one really will have more rubber on the road

And if you think about it the article is mainly talking about circuit racing, a larger percentage of which involves braking and acceleration in a straight line. If a wider tyre was worse in these situations, then all the increased cornering performance would go to waste

[touge rolla]

Skinny "long" tyres should ride over bumps better, and should provide better braking and accelerating performance.

Fat "short" tyres are the opposite.

not true, My ideal launch rpm increased by almost 1000 when i went from 195/60R14 (32psi) to 245/40R17 (36psi) therefore proving forward traction improved by wider low profile tyres.

[Stealer Of Souls]

Skinny "long" tyres should ride over bumps better, and should provide better braking and accelerating performance.
Fat "short" tyres are the opposite.

not true, My ideal launch rpm increased by almost 1000 when i went from 195/60R14 (32psi) to 245/40R17 (36psi) therefore proving forward traction improved by wider low profile tyres.

Or was it that your 14's would bulge out into an unusal shape under launch therefore altering the patch shape/adhesion.
And again. That generalisation is based up leaving ALL other variables constant. You also changed pressure, rim size and no doubt compound, and tread pattern.


Fivebob: Fair enough. But pressure IS a variable. And if kept constant (to be fair on the fact that the real question is whether a wider tyre, with everything else constant, does provide more grip), that changes the story again. Also for general argument sake, we've gotta keep the suspension the same too. Can't go changing too many variables.
I don't have any personal experience with changing just rim sizes. But my friend once commented that when he went from 13s up to 14s on his old sentra, the ride quality was remarkably different.
Same comments from a friend with a commodore going from 14s to 15s.
Now in both cases the tyres changed too, but pressures used were the same. BUT we've also go the problem that the rim size changed, therefore sidewalls changed. To be fair, we need to compare a 175/70-13, compound A, 30psi, tread pattern A to a 245/50-13, compount A, 30psi, tread pattern A.

This is a pretty hairy subject, since there are so many factors involved. It's not like comparing hokey pokey with mint choc chip ice cream. In that situation there really is normally only one variable that changes. So it's down to subjective preferences. With tyre/suspension, it's hard to change just one variable.

My rule of thumb has always been to pick the narrowest tread that fits suitable on the rim. E.G. I've gone with a 195/50-15 on my rims rather than a 215/45-15 (15x6.5in rims).

[blackie]

...... wide tyres ftw !

[barryogen]

My rule of thumb has always been to pick the narrowest tread that fits suitable on the rim. E.G. I've gone with a 195/50-15 on my rims rather than a 215/45-15 (15x6.5in rims).

Interesting in that I've always gone for pretty much the opposite.
Currently on 205/50-16(16x7{I think})

I must say that a while back I tried 18s on it and the ride quality was awful.
Made it feel like it had concrete tyres, and made the steering shudder for the short drive I had them on it. Same tyres, same pressure, same width, different sidewall/sizes obviously.

[fivebob]

Fivebob: Fair enough. But pressure IS a variable. And if kept constant (to be fair on the fact that the real question is whether a wider tyre, with everything else constant, does provide more grip), that changes the story again. Also for general argument sake, we've gotta keep the suspension the same too. Can't go changing too many variables.

You can't do both, tyres are part of the suspension. By keeping the pressures constant you're actually changing something else, the spring rate of the tyre. To be honest I don't think anyone who understands how to set up a performance car would keep the same tyre pressures when going to a wider tyre, however they might keep the same spring rate.

To be fair, we need to compare a 175/70-13, compound A, 30psi, tread pattern A to a 245/50-13, compount A, 30psi, tread pattern A.

No, that's not being fair at all. What you need to do is optimise each tyre to see which one gives the best performance. After all that's what you should be doing with any item that affect the performance of your vehicle.

Trying to rig a test to justify your opinion is not going to tell you which tyre is better performing. e.g. Say 25psi was the optimum pressure for the wider tyre and 30 psi was optimum for the skinny tyre. If you did the test at 25psi (according to you that's keeping all variables constant) then the wider tyre would be better.

This is a pretty hairy subject, since there are so many factors involved. It's not like comparing hokey pokey with mint choc chip ice cream. In that situation there really is normally only one variable that changes. So it's down to subjective preferences. With tyre/suspension, it's hard to change just one variable.

Not really a hairy subject, just one filled with hearsay and opinions

Like I said before, you have to optimise each tyre then compare the results. In which case I think you will find the wider tyre will be better, at least in terms of traction and handling.

My rule of thumb has always been to pick the narrowest tread that fits suitable on the rim. E.G. I've gone with a 195/50-15 on my rims rather than a 215/45-15 (15x6.5in rims).

Then IMHO you are probably giving away performance potential by sticking to the dogma that narrow is best

[Stealer Of Souls]

I'm going to lose since I obviously don't know as much about the subject as you.

Any true scientific experiment changes one variable at a time.

To me, the fundamental thing here is whether we're asking "is Package A (with narrow tyre) better than Package B (with wide tyre)" (If that is the question, then there is little doubt that package B optomised is going to be better than package A) or "is a narrow tyre better than wider tyre".
In some respects it's a bit like my choice with video cards on my home computer. Dual AMD MP2000, 266FSB, AGP 4x slot, 2gb 266 ECC Registered Ram. I used to have a geforce 4 ti4200 but wanted a bit more grunt. I decided that I didn't feel like buying a whole new machine (the smartest and best performance option, but more than I wanted to spend) so I decided to buy a geforce 6600GT second hand. This gave me plenty of graphics processing power, but left me with the AGP 4x limitation. So yeah.. it was better, but if I'd bought a newer machine I could have gotten a GF 7600GT PCI-e which would've completely thrashed the living whatsits out of the 6600gt in my existing machine. Or just gotten a computer with an AGP 8x slot.
I only changed "one" variable and didn't optomise the solution. To test which video card was actually better (did I just waste $100 buying a 2nd hand video card) I could've run benchmarks. But from usage alone it was obvious that the new card provided better performance.
Obviously this is not exactly like tyres. You can and would adjust tyre pressures and WHY to match. But was that the question? By virtue of testing alone one can realise that a wider tyre pattern gives better performance. Just like one can determine that a more engine displacement makes a car go faster.
So, why not just slap on some race rubber and be done with it. Superior sidewall construction, superior grip compound, road legal. Even cold many of them perform adequately for "street" use, and the hotter they get the more they grip.

[barryogen]

So, why not just slap on some race rubber and be done with it. Superior sidewall construction, superior grip compound, road legal. Even cold many of them perform adequately for "street" use, and the hotter they get the more they grip.

for the same reason you went for the second hand video card... cost.

I'm all for safety and more grip in cars, but any more than about $200 a tyre and I start to wonder why.

[Stealer Of Souls]

Ha ha ha....
Yeah. Race tyres get a bit pricey.
I had some priced up once, can't remember if it was yokos or dunlop, and they were around about $250 a corner. Which wasn't much more than the tyres I ended up with at the time. But the life expectancy is half, and they told me the rubber is only rated for so many "heat cycles". So basically every time you actually drove the car, and then stopped so the tyres cooled off, that was one cycle. The tyre guy said it wasn't recommended to exceed about 500 cycles as the rubber started to "go off".
Anyways, doing that on a street driven car means that realistically, you've probably got only two years use in them. And considering how little distance I actually drive, it would be some VERY pricey tyres.