Page **1** of **1**

### Tall vs. Shorter Tire theory and data to back it up?

Posted: **Sat Oct 01, 2016 6:45 pm**

by **FastBuick**

If you have say a 31"x14" tire coupled with a 5.14 gear vs. a 33"x 14" tire with a 5.43 gear and both combinations yield the same shift and trap rpm, which combination is better to have and why?

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Mon Oct 03, 2016 7:09 pm**

by **F-BIRD'88**

It depends on a lot of things.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Mon Oct 03, 2016 7:54 pm**

by **FastBuick**

F-BIRD'88 wrote:It depends on a lot of things.

Ok like what? If suspension is set up for both to have adequate traction and the only differences being the height of the tire and the rear gear, will one be quicker than the other? Will one react better? Will one be more consistent? Etc. Etc.

I was wondering also if the starting line ratio (2.10x5.14=10.79) vs. (2.10x5.43=11.40) makes a difference in reaction and ET by giving a mechanical advantage to move the heavy car or if the radius of the taller tire lessens that and makes it even with the smaller tire and less gear?

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Mon Oct 03, 2016 11:54 pm**

by **ijames**

FastBuick wrote:F-BIRD'88 wrote:It depends on a lot of things.

Ok like what? If suspension is set up for both to have adequate traction and the only differences being the height of the tire and the rear gear, will one be quicker than the other? Will one react better? Will one be more consistent? Etc. Etc.

I was wondering also if the starting line ratio (2.10x5.14=10.79) vs. (2.10x5.43=11.40) makes a difference in reaction and ET by giving a mechanical advantage to move the heavy car or if the radius of the taller tire lessens that and makes it even with the smaller tire and less gear?

First, the easy physics. Your two formulas give the torque multiplication between engine and axle for each combo. Torque is equal to force tangent to the circle times the radius of the circle, so the force is torque/radius. If we take your torque multiplication expressions and divide by the radius, 10.79/15.5=.696 and 11.40/16.5=.691, that says that the force pushing the car forward is basically the same. If you want a complete answer, say the engine is making 500 ft lbs of torque, and 500 * .696 = 348 lbs of force pushing the car forward at launch (I think that's per tire, but the important thing is that it's the same for both combos). So the 5.43 gear applies more torque to the axle but then the taller tire results in the same net push to the car. Since the formulas for speed vs. tire height vs. gear ratio and torque multiplication vs. gear ratio are all linear in the gear ratio and tire height, any combination of tire height and gear that give the same speed vs. rpm as your two combos will launch the car with the same force.

Second, your tires are the same width but different heights. That means the contact patch between tire and track will the same width for both but longer for the taller tire, which should give a traction advantage to the taller tire. From talking with friends who have used 26" tires with 3.42 gears and 28" tires with 3.73 gears, that is what they saw. More consistent and slightly better 60's. Can't say about reaction time.

Third, more physics. The torque required to changer the rotational speed of an object is proportional to the moment of inertia of that object, and for a wheel and tire the moment of inertia is roughly proportional to the square of the radius, not to the radius as before. That means that there is slightly more parasitic loss in spinning up the larger diameter tire, which would show up as a slightly slower et and mph. How much I don't know, someone who has done the experiment themselves will have to say.

Hope that's a start for you.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Thu Oct 06, 2016 4:32 am**

by **Geoff2**

David Vizard tested different weight wheels on a late model Mustang & the heavier wheels lost et & mph over the quarter mile. Not much, but measurable. Not sure if the wheels were the same diameter.

Wheels should be viewed as a flywheel at each corner of the car. If you make them heavier, or increase the diameter, the acceleration is reduced.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Fri Oct 07, 2016 2:17 pm**

by **j-c-c**

"That means that there is slightly more parasitic loss in spinning up the larger diameter tire, which would show up as a slightly slower et and mph."

But, that would only be true when the available power was not overcoming the available traction, correct? We are also assuming here the larger diameter tire has a larger footprint, but no one has mentioned TP changes, or tire growth, which would alter the footprint and radius, I suppose.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Fri Oct 07, 2016 6:10 pm**

by **MadBill**

FastBuick wrote:If you have say a 31"x14" tire coupled with a 5.14 gear vs. a 33"x 14" tire with a 5.43 gear and both combinations yield the same shift and trap rpm, which combination is better to have and why?

The overall 'gearing' to the road is the same for both cases in your scenario, so if the trap RPM is the same it would appear that the combinations are equal. (Unless subtle unstated differences affect time-to-distance without affecting trap speed.)

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Fri Oct 07, 2016 7:20 pm**

by **ijames**

j-c-c wrote:"That means that there is slightly more parasitic loss in spinning up the larger diameter tire, which would show up as a slightly slower et and mph."

But, that would only be true when the available power was not overcoming the available traction, correct?

The difference in moment of inertia shows up any time the rotational speed of the tire changes, whether it is rolling or spinning, but I'm not sure what you are thinking here?

j-c-c wrote: We are also assuming here the larger diameter tire has a larger footprint, but no one has mentioned TP changes, or tire growth, which would alter the footprint and radius, I suppose.

Right, there are lots of smaller effects, I just started with the big ones.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Fri Oct 07, 2016 9:25 pm**

by **j-c-c**

ijames wrote:j-c-c wrote:"That means that there is slightly more parasitic loss in spinning up the larger diameter tire, which would show up as a slightly slower et and mph."

But, that would only be true when the available power was not overcoming the available traction, correct?

The difference in moment of inertia shows up any time the rotational speed of the tire changes, whether it is rolling or spinning, but I'm not sure what you are thinking here?

My thinking, since you mentioned et/mph, when one has more power then traction, and the tire is spinning, any extra power to accelerate the greater rotational mass, does not detract from accelerating the car, its only when 100% traction comes into play that the rotational mass consumes/stores additional power that ideally would accelerate the car. This condition would only apply normally? in the first 60'? Make sense?

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Sat Oct 08, 2016 3:48 pm**

by **FastBuick**

ijames, that is the same formula I came up with for figuring out the torque available at the tires. It was still hard to put my mind around the fact that the lower ratio in the rear would not help the engine by giving it a mechanical advantage, however I guess it is not enough to overcome the weight of the taller tires. Which in this case is about 4 lbs per tire. Also with the contact patch being longer and not needed would increase friction going down the track. So I understand what you are talking about. Thank you for the info.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Sat Oct 08, 2016 3:56 pm**

by **FastBuick**

j-c-c wrote:ijames wrote:j-c-c wrote:"That means that there is slightly more parasitic loss in spinning up the larger diameter tire, which would show up as a slightly slower et and mph."

But, that would only be true when the available power was not overcoming the available traction, correct?

The difference in moment of inertia shows up any time the rotational speed of the tire changes, whether it is rolling or spinning, but I'm not sure what you are thinking here?

My thinking, since you mentioned et/mph, when one has more power then traction, and the tire is spinning, any extra power to accelerate the greater rotational mass, does not detract from accelerating the car, its only when 100% traction comes into play that the rotational mass consumes/stores additional power that ideally would accelerate the car. This condition would only apply normally? in the first 60'? Make sense?

If both tires have 100% traction, the smaller lighter tire with the equivalent gear ratio will be faster and quicker all the way down the track just from less parasitic losses everywhere. Doesn't just affect 60 ft. I was more concerned with that scenario. Not having trouble hooking the smaller tire at all, was just looking for areas to make gains but doesn't seem like that will be possible in this case. This is what I have gathered from the responses so far.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Sat Oct 08, 2016 11:27 pm**

by **ijames**

FastBuick wrote:ijames, that is the same formula I came up with for figuring out the torque available at the tires. It was still hard to put my mind around the fact that the lower ratio in the rear would not help the engine by giving it a mechanical advantage, however I guess it is not enough to overcome the weight of the taller tires. Which in this case is about 4 lbs per tire. Also with the contact patch being longer and not needed would increase friction going down the track. So I understand what you are talking about. Thank you for the info.

Actually, I didn't mention tire weight at all. The lower rear ratio gives more torque at the axle, but then the TALLER (not heavier) tire exactly cancels that advantage. Normally we think about torque in the "forward" direction, like using a torque wrench that is 2 feet long and applying 20 lbs of force to the handle to get 40 ft-lbs of torque. Here, with the axle, we know the torque and the length of the wrench (radius of the tire) so we have to go "backwards" to calculate the force which will push on the car.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Thu Nov 03, 2016 12:30 pm**

by **AMXstocker1**

IF you don't have trouble hooking on a smaller tire it will be faster, wheel speed also plays a role in why the smaller tire will be quicker. to be honest i can't quantify in terms of physics why that is its just what i've heard from guys with class cars. if your bracket racing you might give up some et for the extra contact patch and assurance that the car isn't going to spin witch is death in a bracket car(why you see a bunch of 10.90 and 9.90 cars with tires way bigger than it would take to hook a car at that power level.)

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Wed Nov 23, 2016 8:15 am**

by **steve316**

the lighter comb. will be faster if traction is the same.

### Re: Tall vs. Shorter Tire theory and data to back it up?

Posted: **Mon Nov 28, 2016 10:50 am**

by **pdq67**

Maybe this will help?

I know, probably already known, but ----

http://www.offroaders.com/info/tech-cor ... ph-rpm.htm
As always, just trying to help.

pdq67