Learn Me: Wider tires = more Grip? = Faster cornering?

First off, let me state that I am an Engineer.

What that means is: I am very smart with some things. However, I am TOTALLY ignorant to some other things.

I hear people say that wider tires give you more grip. I am not sure about that. The frictional forces involved only take into account the weight on the contact patch AND the coefficient of Friction. Nowhere in the equation does surface area come into play.

My question is: Do wider tires actually produce more grip? OR, Do wider tires allow higher cornering speeds?

My understanding:

The tires sidewall is going to hold up a certain amount of weight without ANY air in the tire. Say it is 10% of the weight of the car, or 2.5% the weight of the car per corner. With a 2000# car, that is 50 pounds (this car is corner balanced to have equal weight on every wheel).

The other 450 #s is "held up" by the air in the tire. Assuming we have 30# of air in the tire, or 30 psi, the contact patch would be 450# / 30lbs/sq in = 15 sq in of contact patch.

Again, nowhere in there does the size of the tire come into play. It seems to me that the contact patch is going to stay 15sq in until either the load changes or the air pressure changes. (Or to a much lesser degree, the tire sidewall stiffness changes).

I am left to conclude that the only difference in running wider OR narrower tires is the shape of the contact patch.

How does shape of the contact patch change the level of grip? How would change the "allowable" cornering speeds?

Thanks for sticking with me on this. I have no idea if this line of thinking is correct.

Thanks,

Rob R.

Hard physics ahead!

This thread has a good discussion on the topic:

http://www.physicsforums.com/showthread.php?t=330790

GameboyRMH wrote: http://www.physicsforums.com/showthread.php?t=330790

Translation of above linkie..... yes

I would have said that the translation would be more like:

If road racing, temperature and pressures will determine your correct tire width.

It seems some people in that discussion are saying that wider -smaller sidewall tires give better heat dissipation.

I did not see where the article specifically detailed how a wider tire helps cornering grip.

Rob R.

You run lower pressures with wider tires.

It's about lateral force vs. slip angle, check out the post by GreenV8S in this thread:

http://www.pistonheads.com/gassing/topic.asp?t=78848

So, I run lower pressures. My contact patch expands.

Why can't I just run lower pressures on narrower tires and get a larger contact patch?

Because you start running on the sidewall rather than the tread

^And your tire will start to fold inwards in the center, costing you contact patch area.

I am not sure I understand. Can't I just add more camber if that is the case?

I think I get it now. The lateral grip vs. slip angle kinda "does it" for me.

It would be nice if there was a chart with tire width, diameter, pressures, and vehicle weight that helped me pick out the right size tire for my car. I don't have enough money for all of that testing!

Rob R.

wvumtnbkr wrote: I am not sure I understand. Can't I just add more camber if that is the case?

Nope, then you'd just be driving more on one sidewall than the other. You'd have the same problem even if you had a car with a perfect camber curve where the wheel stood at a right angle to the ground at all times. Try to reduce pressure to gain traction, and as soon as you go below the optimal pressure you start putting more pressure on the sidewalls and less on the center of the tread.

(BTW: This does work somewhat for mud and rock crawling since allowing the tire to deform doesn't really cost grip. Thus the use of near-zero pressures and beadlockers.)

In reply to wvumtnbkr:
The bit you are missing with your standard engineering approach is that the surface of the tire does physically deform and 'grab' the lumps and bumps of the surface. This is affected by temperature and tread compound. If the temperature and tread compound are held constant the contact path area increase with wider tires presents more surface to grab all those little bits of track.
The bottom line is the contact patch of the tire reacts on a macroscopic level as you described AND at a much smaller microscopic level.

Here's my understanding: compare a narrow tire and a wide tire of the same diameter, both use the same tread compound. WARNING WARNING The next crap is grossly oversimplified END WARNING END WARNING

The wider tire will have a longer sideways 'footprint' than the narrower tire. So you have a wider contact patch. That equates to more square inches on the surface to be gripped.

Here's where this becomes important: just picking figures, say the narrow tire has 6 sq in of contact patch and the wide tire has 10 sq in. Since each tire is on the same car, we'll use 1000 lbs load. The tire with 6 sq in is aking each inch to handle a load of 166.66 pounds. The tire with 10 sq in is putting 100 pounds on each inch.

Again just picking a figure, let's say each tire is capable of handling 175 pounds per square inch, the 6 in tire can handle 1050 pounds total and the 10 in can handle 1750. That means the wider tire has a much larger margin of error.

That's important because unlike the conventional notion of friction tire traction is NOT static, it changes. Remember the circle of friction: the tire can develop grip in more than one direction at the same time, say turning and braking, but once the total grip of that tire is exceeded it's skidsville. Also, the contact patch changes as the suspension rises and falls or when going over a bump or through a turn. This of course changes the total amount of grip the tire can develop, remember that number of square inches thing.

Remember that infrared onboard of the F1 tire test? It shows just how much tire contact patch changes in turns, etc.

http://www.youtube.com/watch?v=24iacFcrVOg

Okay, everybody pile on and tell me how dumb I am.

How many threads can end with "it depends"?

Along with what Stafford1500 said, the end effect of the deformation and interlocking on static friction is that the friction coefficient does not remain constant as the normal force applied is varied. This causes a non-linear relationship between the two, which occurs in favor of a higher friction coefficient at lower unit loading on the tire. Neglecting the effect on getting heat into the tires to increase adhesion, you can increase relative grip by either decreasing the vertical load on the contact patch, or increasing the area of the contact patch.

In reply to N Sperlo:

All of them.

Generally, the answer is yes but depending on the car, you can go too wide at which point the car will handle like a pig. Alignment matters too.

N Sperlo wrote: How many threads can end with "it depends"?

It depends.

Driven5's response is straight to the point.

The ultimate grip of a tire (maximum lateral force) is not generated solely by simple friction. The tire material interacts with the surface microscopically and macroscopically, physically interlocking with features of the track/road surface.

This interaction produces a non-linear relationship between vertical/normal force and ultimate grip, whereby the ultimate grip increases less and less for a given increase in normal force.

This means that a tire of a given contact patch size with 1000N of normal force might produce 1000N of ultimate lateral force, but the same tire with 1200N of normal force might only produce 1100N of ultimate lateral force. Increasing the size of the contact patch reduces unit loading and brings the force back to a more favorable range in the normal-to-lateral-force relationship.

If you don't understand how/why tires interact with the surface this way, imagine the ultimate extreme: an off-road tire driving on a lumpy solid surface. The tire's tread blocks could interlock with the lumps in the surface such that the tread blocks would have to be sheared off or completely deflected in order for the tire to slip sideways. The same thing happens on tarmac; the features of the tire and road surface are just smaller. Throw your basic friction out the window. This is a much more complex interaction and hard to model.

Now you also know why cars with a large rear weight bias need such massive tires in the rear compared to the front to be controllable at the limit. Since there is a lot of weight over the rear, the rear tires need to have proportionally larger contact patches so they behave the same as the fronts.

I think of it in terms of a couple examples. Think of why a car can go past 1g on the skidpad, with simple friction it can't, you can do this because the tire is effectively adhering to the track.

That tearing sound a drag car on slicks makes is bits of the tire stuck to the track ripping off the tire.

The problem with the "constant pressure and constant weight means constant area" approach is that like many physics models it assumes perfect, frictionless, massless objects, and tires are lot further from that ideal than many other things that are successfully analyzed in this fashion. This is going to manifest in many ways, but the most obvious one is that the sidewall rigidity will tend to resist the contact patch being long and narrow, but will not resist it being short and wide, so short and wide is going to give a larger contact patch.

To the first part of your question:

My question is: Do wider tires actually produce more grip? OR, Do wider tires allow higher cornering speeds?

The two are the same in this context. Higher cornering speeds requires a larger lateral acceleration force, and assuming you have a non-downforce car that can only come from more mechanical grip from the tires.

I have this discussion 4x a week. Sigh...

It's all about matching temperature and pressure. After that it's slip angle / driving style. Lower pressures are faster, but less fun, less responsive, and won't teach you good habits like a higher pressure. Everyone should start out with a Miata on 165s.

Warren v wrote: I have this discussion 4x a week. Sigh... It's all about matching temperature and pressure. After that it's slip angle / driving style. Lower pressures are faster, but less fun, less responsive, and won't teach you good habits like a higher pressure. Everyone should start out with a Miata on 165s.

Agreed. I started out in an MGA with, what, 4" treads? Not to mention the rubber composition!

I sort of get why the wider tires help in cornering, but why do the drag racers use them? That's something I still don't understand.

In reply to blueafro:

See my example above, the tires are sticky, like tape, more area=more adhesion. If you listen closely to a high power car you can actually hear the tearing noise the tires make when the car makes a pass once it gets a few hundred feet away and the exhaust isn't as loud.

Knobby tires on dirt bikes rely on a macro model of an asphalt tire to create grip. The knobs act like spikes, they sink into the soft loose surface and that's pretty much all they can do. The shape and deformation of the tire carcass (like asphalt tires it's tuned by varying tire pressure, I have run as low as 5 psi with rim locks) will force more knobs into the surface but that only goes so far.

You wanna go nuts trying to figure out tire selection? Ride an enduro where you start out on, say, soft loam which needs damn near a paddle tire then suddenly it changes to hard packed clay which needs a rigid carcass with closely spaced knobs then a gravel road that requires stiff knobs with open spacing then back to loam then back to hardpack then to loose sand etc so forth and so on. AAAAAUUUUGGH.