Advantages to taller tires?

So, in looking at the setup on the challenge car, I need to determine Tire size.

Right now, it has 26 inch tall tires on it. It was initially going to stay 26 inch tall tires.

But then we found the world of Circle track. The size of their tires comes in at 27.5.

Special miata comes in at 23.

We want to stay with a 15x8 wheel. We also want cheap and readily available tires.

So really, the 205/50/15 or 27x8x15 are our best options for future proof, and price.

I know a shorter Tire will help with gearing on the autocross and turn in, etc, but what are the benefits of a taller tire?

What is power band, peak torque, 2nd gear and final drive ratios etc. before anyone can answer that?

Harder to overheat is about the only advantage I can think of for a challenge car.

I know nothing about your challenge build, but why would a 25" tire not also be an option? TONS of tire sizes in a 25" diameter. 26" is pretty common also.

Ford aod transmission First: 2.400:1 Second: 1.467:1 Third: 1.00:1 Overdrive: 0.667:1 Reverse: 2.00:1

Rear gears will be 3.73-4.11 depending upon what we can score.

PowerBar should be 1500-5000 based upon cam and heads.

In reply to ProDarwin:

What 15 inch inexpensive tires do you know of in 25 and 26? That are narrow enough for a 15x8.

SM7 take offs are very cheap. The shorter tire will give more advantageous gearing for acceleration. The only advantage I see to a tall tire is if you are running out of rpm at the top of 2nd during autocross.

KyAllroad wrote: SM7 take offs are very cheap. The shorter tire will give more advantageous gearing for acceleration. The only advantage I see to a tall tire is if you are running out of rpm at the top of 2nd during autocross.

Hmmm, this actually gets me thinking. My MR2 usually floats up toward the top of the RPM band in 1st for quite a lot of a run, would be nice to drop that down a bit.

OK, so 2nd gear tends to be the most important for autocross. Let’s calculate based on 5000 rpm.

With the 27.5” tire and the 3.73 you’ll be doing approx.. 75mph at the top of 2nd. That seems high.
With the 27.5” tire and the 4.11 you’ll be doing approx.. 68mph at the top of 2nd. Again, that seems a bit high, but I don’t know what kind of beast this is. If it’s some stonking great big V8 and an open course it could be good.
With the 25” tire and the 3.73 you’ll be doing approx.. 68mph at the top of 2nd. That seems a nice sweet spot.
With the 25” tire and the 4.11 you’ll be doing approx.. 62mph at the top of 2nd. Seems good for a smaller car.

All opinion here is worth what you’ve paid for it, but if it’s small car I’d be looking at the 25” tire, if it’s got some serious grunt the 27.5” tire and the 4.11 (or is it 4:10?) seems better. Serioulsy though, get other opinions and I know nothing about drag racing other than it’s around a ¼ mile with no left or right stuff. Calculating trap speeds is an art I know nothing of.

Dusterbd13 wrote: In reply to ProDarwin: What 15 inch inexpensive tires do you know of in 25 and 26? That are narrow enough for a 15x8.

Doh, I missed the 15x8 wheel part.

It's a built ford 302 v8 in a cP classed amc spirit. With an automatic. The rear will be an explorer rear. Most common are 3.73 and 4.11 with limited slip.

Seems like, based upon your post, that the shorter Tire and reviews limiter may be our best bet. Engine should run to 6k, but probably stop making power at 5.

Adrian_Thompson wrote: Serioulsy though, get other opinions and I know nothing about drag racing other than it’s around a ¼ mile with no left or right stuff. Calculating trap speeds is an art I know nothing of.

I spent too much time and too much math building a calculator to do such a thing.

Post about it

Calculator

Taller tires also equal larger contact patch.

However, you've got to have more power to turn them.

I don't think it offsets the gearing advantage for the average Challenge car.

What about contact patch vs gearing for its post challenge life in c-prepared?

SVreX wrote: Taller tires also equal larger contact patch. However, you've got to have more power to turn them. I don't think it offsets the gearing advantage for the average Challenge car.

Nope. The only thing that changes the size of the contact patch is tire pressure and vehicle weight.

Tire diameter and width change the shape, but never the area.

In reply to Adrian_Thompson:

So a circle 10' in diameter has the exact same amount on the ground as a circle 2 inches in diameter? You sure about that??

A bigger tire at the same pressure will often deform less due to having a higher weight carrying capacity, so the contact patch would be about the same size. But the bigger, higher weight rating tire will typically need less pressure on a given car, so you'd get a bigger contact patch once the pressure was optimized for each tire.

SVreX wrote: In reply to Adrian_Thompson: So a circle 10' in diameter has the exact same amount on the ground as a circle 2 inches in diameter? You sure about that??

yes, at the same pressure.

It is very simple PSI = pounds per square inch (I know you know this - haha). if PSI is the same in both tires, and the weight on the tire is the same, then the square inches of contact patch has to be the same.

Now, it has already been mentioned about the ability to change pressure, and then change contact patch. You can also vary the 'shape' of the contact patch.

Also, I'm sure there are some other effects from the fact that a bigger tire is heavier, a stiffer tire might concentrate force better, etc. but assuming the two tires are the same except for size...

I have a couple "computers" that I purchased from Ed. Iskenderian Racing Cams years ago.

One is for gear ratio, tire diameter and rpm.

The other computes average speed. This is mostly for track use.

Both will fit in your back pocket.

They may not be as accurate as the newer electronic versions but they are close enough.

SVreX wrote: In reply to Adrian_Thompson: So a circle 10' in diameter has the exact same amount on the ground as a circle 2 inches in diameter? You sure about that??

Yes, 0

(If its truly a circle its tangent :) )

In reply to ProDarwin:

Right. Except it's not. There's a flat spot on the bottom. ;-)

Advantages of taller tires:

Smoother ride

Better obstacle clearing capability

Lower bearing speeds

Robbie wrote:

SVreX wrote: In reply to Adrian_Thompson: So a circle 10' in diameter has the exact same amount on the ground as a circle 2 inches in diameter? You sure about that??

yes, at the same pressure. It is very simple PSI = pounds per square inch (I know you know this - haha). if PSI is the same in both tires, and the weight on the tire is the same, then the square inches of contact patch has to be the same. Now, it has already been mentioned about the ability to change pressure, and then change contact patch. You can also vary the 'shape' of the contact patch. Also, I'm sure there are some other effects from the fact that a bigger tire is heavier, a stiffer tire might concentrate force better, etc. but assuming the two tires are the same except for size...

I disagree with your math.

Contact patch is NOT determined by PSI. It is determined by deflection of the rubber. The specific characteristics of the compound determine the contact patch, and are adjusted by the pressure.

The contact patch is the flat segment created by a specific weight on a specific compound at a specific pressure. So, if 30 psi creates a deflection in the rubber of 1/2" off the diameter creating a 6" contact patch on a 13" wheel, the same 1/2" deflection on a 15" diameter would be a longer contact patch.

UNLESS the construction of the different diameter tires counteract the effects. If the weight ratings are different, they will act different.

increased ride height for superior rallycross ground clearance.

Can we agree that contact patch is largely determined by PSI and weight, with an additional smaller influencing factor of tire casing construction?

It can't be solely up to the casing of the tire, because the contact patch of a flat tire is very different from the contact patch of an inflated tire, and they have the same casing. I think it is also a big assumption to say that two tires are going to have an identical carcass, with the same stiffness and same internal friction when deforming under load.

I use smaller tires because I like the gearing (usually).

I'd never considered the contact patch (as it relates to diameter) until "Fast Phil" Currin spent some time with us setting up the car last year. He liked the larger dia. tires because of the contact patch.

Now, I'll be the first to admit Phil is not an engineer, but there are very few racers that have as many wins under their belt as he does, or as much time setting up cars. He has some solid input.

Since we don't run tires autocross tires half flat, the point is out there. A fully inflated 15" dia tire generally has more footprint than an equal 13" with equal weight on them.

Agree or disagree is fine by me. I spent a lot of time arguing with Phil too.

The question at hand is what are the advantages to taller tires. I am just answering the question. To some very experienced racers, contact patch is an advantage. Even if it is perhaps only perceived.