Wide vs Narrow Tires Options

[sjfehr]

http://www.physlink.com/Education/askexperts/ae200.cfm
Mark took the words right out of my mouth, so I'll shamelessly plagiarize:

Q. As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction?

A. This is a good question and one which is commonly asked by students when friction is discussed. It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance.

A. Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap.

I know the first response is the textbook answer and makes perfect sense, but I'm not completely satisfied with it. Nor does the second really sit right either. I'm comfortable with the statics of it (that tire size should largely be irrelevant to performance), but there seems to be much more to it in the dynamics of the tire as it deforms under stress, otherwise we'd all be using much smaller wheels than we are now. I know there are other considerations involved, too; hydroplane resistance, for instance. But for strict performance, what's going on differently in a wide tire than a thin tire that warrants the use of wide tires on higher performance cars?

[Orkboyz]

Duh! Wider tires mean your car is soo much more cool! Which, I suppose is why my car is not cool since one of the comments I always get is something along the lines of "You are only using 245 tires instead of 275s?"

All things being equal I think a wider tire offers more of a chance of initial bite when putting high hp/tq to the ground. And maybe a slight bit more stability.

Then there is possibly the cornering benefit. Maybe having a wider contact patch on an unloaded corner of the car in a severe turn helps. Unless of course you, uh, only corner on three wheels.

As for me, I prefer sticky to wider. But wider and stickier is always good.

[sjfehr]

I imagine it has something to do with the shape of the patch, too. Given the same psi, the contact patch is going to be the exact same # of square inches regardless of the dimension of the tire, but the shape will be different for wider tires.

30 psi supporting 1/4 of a 2900lb car is 24 square inches per tire, regardless of what wheel/tire combo you're using. It's very simple physics. Which could be a 6x4" patch on a 6" wide tire, or a 12x2" patch on a 12" performance tire. My physics textbook would say both of these give the exact same friction, but there's obviously something fancier involved, as the 12" tires are well known to give much more responsive handling and stability in the corner than 6" tires. Is it that the wider tires are closer to cylindrical, and don't have to deform as much, allowing them to be stiffer with less flex, and better able to transfer those forces to the ground?

[dunk]

I imagine it has something to do with the shape of the patch, too. Given the same psi, the contact patch is going to be the exact same # of square inches regardless of the dimension of the tire, but the shape will be different for wider tires.

30 psi supporting 1/4 of a 2900lb car is 24 square inches per tire, regardless of what wheel/tire combo you're using. It's very simple physics. Which could be a 6x4" patch on a 6" wide tire, or a 12x2" patch on a 12" performance tire. My physics textbook would say both of these give the exact same friction, but there's obviously something fancier involved, as the 12" tires are well known to give much more responsive handling and stability in the corner than 6" tires. Is it that the wider tires are closer to cylindrical, and don't have to deform as much, allowing them to be stiffer with less flex, and better able to transfer those forces to the ground?

A few notes, sorry for them being disorganized. You need to be very careful when talking about tire grip not to box yourself in to the "Contact Area Box." Remember that two other main factors play into grip; material deformation at the road surface (tire compound determined) and hysteresis of the compound as it slides across the road surface.

Wide flat tires have higher lateral stiffness at the contact patch and thus operate at less of a slip. This can make the car easier to control at the edge of traction. Think about it this way. Tires develop their side force because of slip angle (You can think of this as the tread being pulled sideways by the track surface) The contact patch shape will change under cornering, small at the front increasing in size towards the rear. As the force on the tire sideways starts to increase it reaches a point where it exceeds the friction between the tire surface and the road, and the tire beings to slip. As this slip increases, the sideways deflection builds up working the front of the contact patch increasingly hard, while the back starts to slide and lose grip. At some point during this increase you reach the level where more slip angle on the tire means less side force because you are losing more grip at the back than you gain at the front. You hear racers refer to different tires' slip angles all the time. This is what they're talking about. So how does this tie into the static shape of the contact patch between two different size tires?

The longer it is (narrow tire) the more gradually this process will occur. If you shorten it the ramp up to max slip angle will occur more abrubtly but you have more grip. Why? Because less variation in distortion between the front and back contact patches means that more of the contact patch reaches max grip and starts to slide at the same point.

At this point, you may be thinking that tire pressure changes the shape of the contact patch, and it does. So on a theorectical basis, increasing the tire pressure should allow for more grip, because we've shortened and widened the shape of the patch, right? Well, to a point, but we have to balance this with the fact that tire pressure is better for grip from deformation and hysteresis at lower pressures. So we have contact patch length (pressure up) with contact patch pressure or tire deflection (pressure down).

So, to tie it back together, the best way to increase grip at the limit is a wide tire (shorter contact patch) for the same inflation pressure.

Now, if you want, we can start to add the effect from different corner weights, alignments, spring and damper rates, track temp, etc........(IMG:style_emoticons/default/smile.gif)

-Duncan

Also, shorter wider contact patch's deform less lowering rolling resistance.

[prsrcokr]

I'll agree with Duncan here and you can feel the difference if you've driven on varying setups. Keep in mind, a lot of people run wider tires in auto-x because they're stuck with a specific wheel size based on classing. I've seen people running 275's on a 7.5" wheel. Like Duncan said, the speed at which slip angle increases is greater than would be with a wider wheel.

You'll see arguments for both sides on this, in my opinion running the widest wheel possible for the given tire is the best option as you'll have more support behind the given tread width (less deformation)
I'd imagine this is the idea behind drift setups, as they need to keep maximum control of the tire but also need the tire to be able to break loose at will so maximum traction is not the goal.
Keeping the tire and wheel as 'square' as possible will get the most control and traction out of the tire, while reducing that ramp up effect compared to a wide tire on a narrow wheel.

Brian

[mr.beachcomber]

Probably the best, short explanation is from Paul Haney's book "Racing & High Performance Tire" (available on-line at his website for $55 shipped.

"For the same vertical load and internal pressure, a tire with a wider tread has a shorter, wider contact patch than a narrower tire. The area of both contact patches is the same if the internal pressure and the load are the same. . . A shorter contact patch at the same slip angle begins to slip at roughly the same distance from the leading edge as with a long contact patch. But the shorter contact patch has more of its length stuck to the road than the longer, narrower contact patch; and therefore a larger portion of its overall area is gripping." - Why Wider Tires Are Better, p. 101

Paul's book is excellent, although it requires more serious study than most would like to give. I highly recommend it! (IMG:style_emoticons/default/checkeredflag.gif)

[fitz1tj]

How much of the load is supported by the tire carcass and not the air inside the tire? It seems to me with low profile performance tires with stiff sidewalls it would be incorrect to say that the air pressure is the only factor in determining the contact patch area...take a 35 series tire and let all the air our of it and the tire still keeps the rim off the road.

[dunk]

How much of the load is supported by the tire carcass and not the air inside the tire? It seems to me with low profile performance tires with stiff sidewalls it would be incorrect to say that the air pressure is the only factor in determining the contact patch area...take a 35 series tire and let all the air our of it and the tire still keeps the rim off the road.

For all intents and purposes, the tire mainly serves as a vessel to hold pressurized air. The air supports the weight of the vehicle. The carcass design and sidewall stiffness determines how it deforms with vehicle weight, acceleration, braking, and cornerning forces. This deformation changes the shape of the contact patch.

-Duncan

[sjfehr]

The longer it is (narrow tire) the more gradually this process will occur. If you shorten it the ramp up to max slip angle will occur more abrubtly but you have more grip. Why? Because less variation in distortion between the front and back contact patches means that more of the contact patch reaches max grip and starts to slide at the same point.

I has this exact epiphany this afternoon, and was all excited to come in here and solve the mystery that the internet, in all my google searches, seemed incapable of answering (never giving the same explanation twice)... only to discover it's apparently common knowledge on autocross.us, lol! (IMG:style_emoticons/default/drinks.gif) Great information guys, this is really helping me understand what's going on where the tire hits the concrete, thanks!

I'll agree with Duncan here and you can feel the difference if you've driven on varying setups. Keep in mind, a lot of people run wider tires in auto-x because they're stuck with a specific wheel size based on classing. I've seen people running 275's on a 7.5" wheel. Like Duncan said, the speed at which slip angle increases is greater than would be with a wider wheel.

You'll see arguments for both sides on this, in my opinion running the widest wheel possible for the given tire is the best option as you'll have more support behind the given tread width (less deformation)
I'd imagine this is the idea behind drift setups, as they need to keep maximum control of the tire but also need the tire to be able to break loose at will so maximum traction is not the goal.
Keeping the tire and wheel as 'square' as possible will get the most control and traction out of the tire, while reducing that ramp up effect compared to a wide tire on a narrow wheel.

How is this quantified with respect to tire width? Is it a linear relationship where a 265 tire will corner 29% better than a 205 of the same tire? Given the option of 205/50-17 or 225/45-17, both of which will fit on my wheel, is the wider tire better?

This post has been edited by sjfehr: Jan 13 2009, 09:31 PM

[dunk]

Brian
How is this quantified with respect to tire width? Is it a linear relationship where a 265 tire will corner 29% better than a 205 of the same tire? Given the option of 205/50-17 or 225/45-17, both of which will fit on my wheel, is the wider tire better?

How wide is the wheel? If it's a 7.5 or 8 inch wheel I would consider 235/40/17 as well.

[sjfehr]

How wide is the wheel? If it's a 7.5 or 8 inch wheel I would consider 235/40/17 as well.

It's a 7" wheel up front; 225/45-17 is as big as will fit for the tires I've looked at. Not that I'm not tempted to buy a set of 18" wheels and go bigger, though! In fact, that's what originally drove my curiosity in this subject (IMG:style_emoticons/default/smile.gif)

This post has been edited by sjfehr: Jan 13 2009, 07:36 PM

[lightningd]

i wish i was as smart as duncan.

[Stealth TDI]

Hello,

I went through this decision when I moved to R compounds for my 14x6" wheels. My choice was between 205/55R14s and 225/50R14s. The concensus seemed to be that I should "overtire" the wheels and run with the 225s... even with them being cantilevered on the relatively narrow wheel. The tires are great; but I've never been able to compare them to 205/55R14. My only gripe about the tires is that I can feel the delayed reaction during transitions as the wheel works from one end of the cantilever to the other (my guess of what's going on). 205/55R14s MIGHT transition faster since they're a better fit on 6" wheels. But I don't know if I'd lose overall grip. DECISIONS! (IMG:style_emoticons/default/wink.gif)

Thoughts?

Scott

[Driverdog]

i wish i was as smart as duncan.

Don't worry....be (IMG:style_emoticons/default/biggrin.gif) happy!

[sjfehr]

Thoughts?

That's an awfully wide tire for a 6" rim! How well does it resist rollover when you're cornering?

This post has been edited by sjfehr: Jan 13 2009, 09:53 PM

[mpowers]

One other thought to consider, if you can't get the wider tire up to temp where the narrower tire would be working at the ideal temp-then the narrower tire has more grip (for as long as you talking small margins of width changes). Vice versa you could have a too narrow of a tire over heating as well...

[Peter Florance]

i wish i was as smart as duncan.

Me too, although I don't mind correcting his grammer.

Should be: "all intents and purposes"


(IMG:style_emoticons/default/biggrin.gif)

[OneBakerEleven]

Me too, although I don't mind correcting his grammer.

Should be: "all intents and purposes"
(IMG:style_emoticons/default/biggrin.gif)

At least you weren't correcting his spelling. Grammer = grammar (IMG:style_emoticons/default/tongue.gif)

[Slantnose]

It's a 7" wheel up front; 225/45-17 is as big as will fit for the tires I've looked at. Not that I'm not tempted to buy a set of 18" wheels and go bigger, though! In fact, that's what originally drove my curiosity in this subject (IMG:style_emoticons/default/smile.gif)

...FWIW, most of the quick Boxsters are running 245x45's & 275x40's (Hoosiers) with your factory rims.
Check out Rennlist sometime...

[dunk]

Me too, although I don't mind correcting his grammer.

Should be: "all intents and purposes"
(IMG:style_emoticons/default/biggrin.gif)

Good catch. That will teach me not to proof read things I type in a hurry.

-Duncan