The Sad, Sad Truth...
 

Having just gotten my '83 RX-7 up and running, I had a unique opportunity to do some back to back comparisons regarding lost HP due to wheel choice. Sorry, but this is all butt-dyno stats this time...:p:

The red '84 RX-7 had a low mile rebuild with a freshly rebuilt carb (This was done before Sterling and Carl started their service...no hatin' please!!! ;)) and all cats removed and replaced with one big one for a 400 cid V8...

The silver '83 RX-7, now named Furious Rex, is a completely stock and original 147,000 mile survivor with the infamous dowel pin leak. It has all the exhaust stuff still on it and the ACV works perfectly.

The red car never seemed as quick as I thought it should be, but I just reasoned to myself that I was spoiled by the now-deceased '80 RX-7 that had the RP big streetport. Furious Rex, the silver '83, was much, much quicker right off the bat once I got it running and stuck an Optima in there. I reasoned that it was because the engine tolerances were a bit looser (It doesn't smoke at all, however, not even on start-up..) and the smaller diameter wheel package. Now, the sad part...

I've been hauling around some 15" X 7" Weds Autobahn rims since the '79 RX-7 (also deceased...) and they were on the red car...I switched them to Furious Rex and oh, no....

When you're only dealing with a 100 HP or so, they are absolute HP killers...:(...I mean BAD....Furious Rex is still fast, but now the red car could beat it which was not the case before...

Just another thing to think about when it comes to bigger rims...do you want performance? How much $$$ have ya got? Or do you just want bling-bling? The Weds were not all that expensive when I got them all those years ago, but now I gotta wonder about them. It's gonna take a 100 shot just to be happy again with them...

Still, they're just too pretty to chunk in the name of ultimate performance...

Furious Rex...the beginning...

https://www.rx7club.com/forum/attach...postid=2211162

Clean 7 mario.

...So you need a SterlingCarb just to turn them?

Wheel weight is a killer. In general larger diameter wheels are heavier, requiring more torque to get them rolling, but not all of them are heavier.

Does anyone know an equasion or chart that demonstrates the centripital force as pertains to mass, RPMs and diameter? (Like the same shit I was asking before I bought my aluminum flywheel...)

I need to be cleared up on something....

Okay... lugnuts... they arent the only force holding the wheel... but.... wouldnt like then farther apart give more tq to the wheels.... you get what im saying... like more leverage... like 4X100 compared to like 5x4.5 or what ever...


i dunno... lol

You tend to stop slower with bigger wheels, too.
It has to do with more rotational inertia to overcome. Same reason that you can accelerate quicker with a lighter flywheel.

Since the wheel isn't moving relative to the hub its like a solid assembly, and the torque the wheels see would be the same. However the larger the bolt pattern the less sheering force the lugs see. There will be some limit to power where you could potentially sheer the lugs, but I'm sure it would be in the thousands of horsepower range. The hub is what really supports the weight on our cars, the lugs just hold the wheel on.

Don't forget sheer force also will spin the wheel inside the tire. Thats why drag racing slicks (14x32's) are screwed to the wheels.

A solid disc of mass 'M' has rotational inertia 'I' according to the following equation:

I = 1/2*M*R^2, where R is the radius of the disc.

You really have to model the entire tire/wheel combo though, as a combination of discs. I don't have a good model for the a tire's rotational inertia, but for every inch of wheel the rotational inertia of the wheel alone increases around 25%. If the overal height of the tire/wheel combo stays the same, the inertia won't change that much... I'm guessing around 5-10% per inch due to the metal haveing higher density than rubber.

Also assuming the spinning mass is balanced right.
Here's an example, i'll use a flywheel. One gram 'out'
at 6000 rpm's equel 60 lbs trying to fly 'out'. out of balance.

I'm not absolutely sure about the math here, but I think it holds up...

From what I can figure, there are a bunch of factors here...

1) as noted in an earlier post, it is the mass at the distance from the centre of the axle that kills for rotational inertia.

2) Bigger outside diameter, all other things being equal, will decrease performance

3) Standard rims are much lighter per unit of "radius" than are tires themselves, so despite their higher density, you should still come out ahead by going to low-profile tires.

4) Point 3) is balanced by the fact that all tires are different, so a fat-ass heavy rim or tire can overwhelm all other factors.

5) Bigger Diameter tires are still OK if you have lots of low-end torque, as torque (not HP) overcomes rotational inertia

6) The rotary engine has low low-end torque, and performs better at higher RPMs - this means any rim/ tire combo with high rotational inertia WILL kill your performance.

This means....

7) Best performance with a stock engine will likely come with rim + tire radius equal to stock, with tires and rims that have the mass distributed as close as possible to the axle, and

8) as TORQUE, not HP increases, you can afford to go to a slightly larger radius for tire & rim (still keeping the overall mass low, and as close as possible to the axle)

Point 7) is an assertion based upon the fact that the dudes at the factory know this shit inside and out; if they made the size they did, they did it for the best balance between power and price. (This point can typically be used with all discussions around a stock engine, barring improvements in technology).

Well, it's alos a lot more fun to catch 'em at the top end, anyhoos, so here comes the juice...:D

You can't save them all Hasselhoff. Either ditch the wheels or go turbo. Or just be happy that you have 2 nice 7's.

why not just install a 4.444 ring and pinion? gearing now much better. engine spins up faster. cha-ching!

still have heavy tires and wheels but the gears will really help. safer than NOS too.

A good point but i want this car to hit 130+ mph like the '80 used to when it had the streetport kickin' it out...besides, I finally have all the nitrous components assembled now. It's just a matter of deciding which car will get it first. I'm leaning towards the red car becuase the clutch is pretty worn out on Furious Rex and due for a change in the immediate future. The nitrous install and day at the track will all be posted here in this Forum...:cool: