Rolling resistance and tire width

[kachok25]

Assuming equal ridigity in the sidewalls and pressure how much more rolling resistance would a 12 inch wide tire have than a 6 inch wide? I don't think this is a liner formula since the weight on each tire would be equal.

[MishaA]

Not sure how weight is related to rolling resistance. And the difference in your example will be exactly two times.

[kachok25]

weight has alot to do with rolling resistance, the more weight you have on a tire the more deformation you will have (loss of it's perfectly round shape which is what causes rolling resistence) but a tire twice the width would in theory have less deformation with the same pressure since it has twice the area to distribute the force. For example try putting a tire from a toyota carolla on the axle of a 3500 HD chevy, with the same pressure as the other tires you would notice a much greater change of shape in the wimpy 155 series tire than the 235s at the other three corners. No doubt that the wider tire would have a higher rolling resistence but I do not think it would be anywhere near twice as much, does anyone have any scientific feedback?

[MishaA]

Oops, I gues I misread your post. For some reason I was sure you were talking about weight OF the tire. No, I don't have a formula handy, sorry.

[GreyGoose006]

heres an interesting question.
when you say that thepressure is equal, i'm sure you mean that they were inflated to the same pressure while mounted on the car with the full weight of the car on them.

but...

how much would the pressure go up in a 6 inch wide tire vs a 12 inch tire if they were inflated to 25 psi off the car, while not bearing any weight...

get the question?

i've always wondered about that.

[Ian Szgatti]

Originally Posted by GreyGoose006

heres an interesting question.
when you say that thepressure is equal, i'm sure you mean that they were inflated to the same pressure while mounted on the car with the full weight of the car on them.

but...

how much would the pressure go up in a 6 inch wide tire vs a 12 inch tire if they were inflated to 25 psi off the car, while not bearing any weight...

get the question?

i've always wondered about that.

The pressure would be the same... no matter the tire size or load. Case in point... I have a car on the hoist, check all tire pressures, lower car to shop floor, check pressures, still the same.

[GreyGoose006]

but it shouldnt be.
a gasses pressure will increase if the volume of its container is decreased while the volume of the gas and its temperature remains constant.
same with temperature. as a gasses temperature increases, the pressure will increase if the volume and ammount of gas remains constant.

[RobertM87]

The volume of both the container and the gas remain the same. The shape of the tire does not affect the volume, as it is still contained within the inflatable tube.

[Ian Szgatti]

Originally Posted by RobertM87

The volume of both the container and the gas remain the same. The shape of the tire does not affect the volume, as it is still contained within the inflatable tube.

correct...hence the tire buldge when the car is lowered

[grndslm]

Sorry for resurrecting an old thread, but I'm just checking to make sure I've got this 100% correct.

From what I remember of the physics class I took a few years back:

Asking about the width of the tire is nonsense because surface area doesn't affect friction or "rolling resistance".

What you *should* be asking is how much torque does this wheel require? Find out how much the wheel weighs, and how far away from the axle is the majority of this weighs. Then you'll have the answer to rolling resistance.

For example, the less torque the wheel requires to roll... the less rolling resistance you have.

14" 185s < 15" 185s < 15" 205s < 17" 205s

I dunno how much of a difference there is in rolling resistance between these 4 tires, but I can answer that a jump from the lower end of the spectrum to the higher end gave me a ~10mpg decrease in fuel efficiency (29% drop from 35 to 25mpg in a torque-less Corolla). If your vehicle has plenty of torque available, you prolly won't notice as drastic of a change in fuel consumption. I'd still imagine that even torquey vehicles would notice a 10-25% decrease in consumption, tho.

EDIT: I'm curious about momentum, specifically in terms of safety and braking. I'm assuming going right on the spectrum is always safer, but do larger rims mean easier or harder braking??

[J-Ri]

Originally Posted by grndslm

Sorry for resurrecting an old thread, but I'm just checking to make sure I've got this 100% correct.

From what I remember of the physics class I took a few years back:

Asking about the width of the tire is nonsense because surface area doesn't affect friction or "rolling resistance".

What you *should* be asking is how much torque does this wheel require? Find out how much the wheel weighs, and how far away from the axle is the majority of this weighs. Then you'll have the answer to rolling resistance.

For example, the less torque the wheel requires to roll... the less rolling resistance you have.

14" 185s < 15" 185s < 15" 205s < 17" 205s

I dunno how much of a difference there is in rolling resistance between these 4 tires, but I can answer that a jump from the lower end of the spectrum to the higher end gave me a ~10mpg decrease in fuel efficiency (29% drop from 35 to 25mpg in a torque-less Corolla). If your vehicle has plenty of torque available, you prolly won't notice as drastic of a change in fuel consumption. I'd still imagine that even torquey vehicles would notice a 10-25% decrease in consumption, tho.

EDIT: I'm curious about momentum, specifically in terms of safety and braking. I'm assuming going right on the spectrum is always safer, but do larger rims mean easier or harder braking??

I'm not going to agrue with what you said about physics. You seem wrong, but I'm ceartainly no physicist myself. What I am qualified to comment on, however, is the mechanical aspect of what you said. Why would you put 17" wheels on a COROLLA? They don't weigh enough to flex the sidewalls on the stock 185/65/14 tires even on the sharpest turns. I'm also not going to argue that you got 10MPG less, if you say that, I'm not going to call you a liar without proof. However, I will say that on my '86 Silverado, going from 235/85/16 to 35x12.5x15 makes absolutely no difference, given equal inflation pressure. Although, that would be one of the more "torquey" vehicles.

And about the braking, the less rotating mass there is, the easier it is to stop. Especially on a car with brake rotors the size of yours. In addition, I imagaine the diameter of your tires is greater than stock, which is also detrimental to braking. The wheel is basically a big lever trying to rotate the rotor. The longer the lever, the more force put on the rotor. IMO, that's why Toyota is putting bigger brake rotors on their newer cars. People are installing the absolute largest wheel on that will fit, making the older factory brakes extremely undersized.

[grndslm]

Originally Posted by J-Ri

Why would you put 17" wheels on a COROLLA? They don't weigh enough to flex the sidewalls on the stock 185/65/14 tires even on the sharpest turns.

I actually had a mixed set of tires some with 165 & some with 185 width (think I got a rim or two from the junk yard and didn't pay attention to the tires and drove with them like that for a year or so), and i knew I wanted at least 205 width because I've experienced the safety benefits thru handling. I purposefully try to hydroplane and jerk the wheel going over 90mph when it's raining. It's too hard to lose control with 205s, however they still don't help much against loose gravel.

Anyway, I don't regret getting the 205s, but again... 17s are just too much for my car. They look great and handle great, but if I had bought the 15s, I'd prolly save at least $500 every year I keep driving this car and gain a good bit faster jumps off the start.

Originally Posted by J-Ri

I'm also not going to argue that you got 10MPG less, if you say that, I'm not going to call you a liar without proof. However, I will say that on my '86 Silverado, going from 235/85/16 to 35x12.5x15 makes absolutely no difference, given equal inflation pressure. Although, that would be one of the more "torquey" vehicles.

Yup... Corollas aren't torquey. I guess if you've got a really torquey vehicle, tire size prolly doesn't matter too much at all then. It's alllll about torque!

Originally Posted by J-Ri

And about the braking, the less rotating mass there is, the easier it is to stop.

Yea, I always assumed that. Makes total sense, but a friend of mine who is more into mechanicical stuff told me that larger rims help braking. It does seem like it brakes slightly faster to me for some reason, tho... but it doesn't logically make sense because of the momentum.