No real application in mind, I just noticed that the tire size the Datsun was running in this month's magazine seemed uber wide.
Since then I've been wondering about wider tires. If the rim diameter, sidewall height, vehicle alignment, etc all remain the same (I guess for the sake of removing variables) do wider tires automatically equal more grip?
why or why not?
(I'm sure this has been asked before, but I didn't see what I was looking for in my quick search)
If you get them on an appropriate width wheel, then the simple answer is yes. Put more rubber of the same compound down and you get more grip. The problem comes when you're limited with wheel width or some other variable that changes the tire's contact patch shape. If you can use all of an 8 inch wide tire but only 80% of a 9 inch wide tire because it gets distorted, then you're better off with 8.
That's what I figured, but for some reason I remember the "answer" being more difficult than that...
I stumbled across a physics forum where a good debate was going on this subject, unfortunately I didn't understand much of it.
Another situation where it doesn't equal more grip is if you have a tire that needs heat and you can't get them hot enough. Wider tires don't heat up so easily.
(Usually only a problem with really wide tires on really light cars)
The physics discussion would have been theoretical, and the argument would have gone something like this: A tire with a smaller contact patch has higher pounds per square inch than a tire with a larger patch. Therefore, grip will be equal.
The theory falls down when you don't include the tearing of the rubber off the tire. That's where the larger tire gives you more grip, since less psi lets the tire stay intact.
Well, in technical terms it IS more complicated than that, but in my opinion it's complicated enough that the vast majority of enthusiasts (myself included) are better off getting decent results with rules of thumb than putting time and energy into learning abstract concepts so you can get 2% better results from your mundane automobile when there are eight million gillion idiots getting better results without knowing a damn thing.
Some people get off on that kind of learning for little tangible reward. I would rather have fun using what modicum of understanding i already have. At some point that way of thinking becomes a barrier to learning, but i dont think im there just because i dont want to learn more about how your forceXarea total doesnt change when you put wider tires on and there are other more complicated reasons for why you get more lateral traction (or dont).
Gotta be careful with the physicists. I had one argue that tire size makes no difference because friction yada yada. Unfortunately for that argument, tires work off of traction, not friction.
Knurled wrote: Gotta be careful with the physicists. I had one argue that tire size makes no difference because friction yada yada.
Yeah, I had a similar situation when one told me that rewinding an electric motor would not be capable giving it more power. The discussion was WRT EVs, but I knew he was wrong because my dad used to rewind the motors in his slot cars. He used thinner wire with more windings.
When I described that technique, the physicist told me that this would not increase power because it would increase resistance. Naturally, he was right about the increased resistance in the wire. (That's why NEDRA guys use the shortest, fattest wires they can to reduce resistance.)
Perhaps there was a mis-communication due to terminology, but I have no doubt that it makes for a faster slot car....or a faster EV. Again, some the NEDRA guys use custom windings. Physics is not my strong point, but I think the extra windings increase the field effect, letting the motor make more power, but do it less efficiently. (I'm sure someone here will chime in with the correct details.)
it clearly becomes apparent when some physicists are are talking about things that they have no empirical knowledge of. It is dangerous to try to apply book learning to situations where you have no experience......you frequently misunderstand the nature of the situation, or over estimate the importance of something that has a minor effect.
(Kind of like my high school classmate who swore that a supercharger sticking up out of the top of a big block engine made the drag boat slower because of drag. Yes, it makes drag, but that's irrelevant when compared to what it does.)
Knurled wrote: Unfortunately for that argument, tires work off of traction, not friction.
I'm not sure about that particular argument. I would think that "friction" is too muddy of a term. I'd say "traction" is a form of friction that we want, and that he was worried about rolling resistance (which we deal with by modifying the car to deliver more power to the ground.)
It's my understanding that tires create grip by interlocking with the road surface. Thus why concrete has more grip than asphalt, soft tires have more grip than hard ones, and maximum grip occurs at a certain percentage of slippage.
Tires are neat to think about.
From a practical and real-world standpoint, I think there are a few variables missing from this discussion.
1.) Available tread width in the "ideal" circumference vs. the tradeoff of going larger/smaller in circumference to gain width.
2.) The increase in sprung-mass, rolling-resistance and inertia from going to wider(and/or) larger tires vs. the increased grip.
3.) The effect on steering effort/feedback, scrub-radius, and bump-steer vs. the increased grip.
Other than classing limits, those factors seem to be the most likely considerations in real life.
Traction is due to friction, the two can't be separated.
Knurled is right, tires work on the following principle: the rubber deforms and 'wraps around' the tiny imperfections in pavement, the friction of the rubber against those imperfections is what gives a tire its grip. That's also why a rubber tire of a given width will grip but a hard plastic one of the same dimensions would just slide on the same surface.
As far as the size of a tire, the larger the 'contact patch' the better it will grip, BUT: there are other forces at work too. Since the tire has to deform, there's two ways to accomplish that: add weight or make the rubber softer. That's why light cars generally run much softer compounds than if the same tire were on a heavier car. Or the tire can be heated to make it softer.
wider tires decrease mpg.
Unfortunately for that argument, tires work off of traction, not friction.
Vigo wrote:Unfortunately for that argument, tires work off of traction, not friction.
I second that image. Tire theory is completely nonlinear and hard, guys. One of my friends was a tire wizard, took multiple week-long seminars on the topic, designed a few very successful FSAE suspension systems, and worked with the best software in the industry. He was hired by Michelin, and they wouldn't let him touch anything until he graduated from their own internal tire school which took many months. He is adamant that he didn't know jack before that specialized training, and his best answer to "is wider better?" is "depends on what the laptimes say on that day". He is one of the lead guys behind the Rival and the BFG Sport Comp II, and he works many ALMS teams.
If you haven't at least read a few books on the topic, please, please don't contribute to the misunderstanding of what's going on. The term " coefficient of friction" is misleading in a world of non-linearity. You're going to have different values of the coefficient depending on the actual load, longitudinal torque, later grip, camber, kingpin torque, pressure, temperature, wear, road surface. The terms that should tip you off on an intelligent discussion on tires includes the terms:
Van der Waals Force, Keying, Membrane Theory, Bias Angles, Slip, Hysteresis
Here's the best explanation I've been able to find accessible to a layman:
http://www.carbibles.com/tyre_bible_pg3.html
Rules of thumb:
The physics of tires are complicated.
The static area of the tire patch is overwhelmingly determined simply by the air pressure inside, it's just how pneumatic tires work.
At pressures of interest for most applications, carcass stiffness is not significant.
In a perfect world the shape of the contact patch would be a rounded rectangle.
But the world is not perfect.
The static shape of the tire patch is largely determined by the carcass design, the treadwidth, and the camber.
So the tire design becomes very important to deliver the desired contact patch shape.
Examine the carcass of a deflated racing tire (especially a bias ply, if you can still find one) to appreciate the things that they do to achieve that goal.
The shoulders of the tread do not deform the same way as the center of the tread.
Tires are not static when used in ways we are interested in, ie rolling around on cars.
When we accelerate, or brake, or turn, the tire is transmitting forces between the car and the
road surface, and there is always some slippage going on at the contact patch (it is not uniform).
As said above, at a micro level, the tire surface and the road surface are meshing, and grabbing.
It's why a tire's coefficient of friction can be greater than 1 (impossible in classical physics).
Traction is a measure of how much force can be transmitted before the surfaces begin to substantially slide (the definition of 'substantially is squishy here).
Turns out that maximum force is transmitted when the surfaces are sliding 'some'.
That sliding creates friction and heat, it is parasitic energy loss, ie work.
The amount of work that a tire and wheel combination can perform with 'some' slippage becomes the determining factor in how much traction it will generate.
For most tire/wheel setups on most surfaces, a contact patch that is a wider rectangle can perform more work,
and therefore deliver more traction than a narrower rectangle.
It's ability to 'work' under those conditions is more effective.
I don't know if the technobabble above even came close to answering your question.
The fact is, 'it depends'.
Camber control becomes more important with wider tires, some cars do a better job than others.
For a given car, and a given track, you use the wheel/tire combination that's fastest.
It might not be the widest, but often it is.
For street cars, it's more complicated, because street cars have to perform well under a far wider range of conditions,
and the measure of 'merit' usually involves more than lap times.
My criteria for tire/wheel selection is heavily weighted towards:
1) Wet traction and stability.
2) Price.
3) Dry traction and stability
4) Treadwear
HTH
Carter
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