Hello,
I am wondering with the large 18" and therefore heavier wheels/tires combo (loads of unsprung weight), isn't it detrimental to the RX8's low torque performance? I like to autoX with this car (should I buy it...still mulling over the choices). Can a 17" work? Or even a 16"...for winter rubbers?
thanks.

 

The more mass and the farther away it is from the center of rotation the more torque it will take to spin the wheel. That is simple physics. Won't bore you with the math but for instance a tire's interia is estimated by

Inertia=mass of tire * the radius squared. Interia is related to kinetic energy and acceleration, therefore torque(force).

The wheel is far more difficult because it's a hoop on the outside (like a tire) but the spokes make it difficult to estimate without actual measurements.

So, yes a lighter and smaller wheel/tire combo will improve performance. The question becomes by how much?!?

Just going to a 17" or even 16" wheel will not mean better performance if you keep the same overall tire diameter. The raduis will stay the same and since tires are usually heavier or as heavy as the wheel you maybe making things worse by pushing the mass farther out from the center.

You can go to a smaller overall diameter, like a 225/45ZR17, but this will change the overall gearing, effect the DSC and cause the speedo to be inaccurate.

For Auto-X this may not be a problem, but on the street it will be a problem.

There have been posts on what size wheel will fit, do a search. But 16" is unlikely with a 12.7" rotor.

 

Here's a nice site where you can see how changing the wheel/tire effects gearing and speedo.

http://gs.tolan-hoechst.com/tirecalc.htm


As far as unsprung weight, the stock wheel weighs 20lbs. The tire 26lbs.

Remember, reducing the unsprung weight of the wheel only helps when the wheel loads the spring. In other words, the tire has to be moving up towards the body in reference to the road.

So, if the tire stays in the same position in regards to the road there will be no benefit. Hitting a bump for instance causes the wheel to accelerate up away from the road.

 

keep in mind that if you have the big rotors, 16" rims will not fit.

 

also keep in mind that the gears in the transmission multiply torque, which actually puts it's at the wheel toque on par with most of its competitors (and those which have more wheel torque are heavier).

 

According to the dyno chart on this forum torque at the wheels is 130 ft-lb. It is FAR below that of the competitors. The 350Z which mazda views as a direct competitor put out twice as much. Sorry to tell you but no matter what you do to the 13B n/a, you won't get low end performance unless you go for forced induction. The lighter rotors of the renesis aren't helping the cause either. I test drove one last week and it was exactly what I expected.

 

You miss the point. The 130 ft-lb was in 3rd or 4th gear. There is much more torque in 1st gear at the wheels!

 

no, no I get the point, really. I deal with power transmission products every day. You are correct that the lower gears produce more torque at the wheels but the competitors also have the same set-up except that their input torque from the motor is a lot higher. I love rotaries, that's all I drive but let's face it, a little more torque would be fun. If you want a car with decent performance looks and practicality then the 8 is your car. Just don't expect it to win many street/drag races.

 

I agree ith TurboSE. TQ is TQ. If the engine doesn't spin as much twitsting force, the problem will compound itself with heavier unsprung weight. Now anyone notice how it is with the car loaded up with passengers?

 

Hey guys,

So, there may be some misinformation floating around here that I hope I can clear up for you. The first thing pertains to some chassis dyno knowledge that a lot of folks don't know...maybe you guys all know this, but from some of the posts I wasn't 100% sure:

The RX-8 may measure 130 lb-ft of torque on the chassis dyno, but the reason has nothing to do with what gear the car was in. The actual reason is that chassis dynos don't report the torque at the wheels. I know, you're thinking "so why does everyone call it torque at the wheels then?" Well, it's because somebody called it by the wrong name, and the wrong name stuck. What chassis dynos are really reporting is torque at the engine, corrected for drivetrain losses. So when you see that one engine makes 1.85x as much torque as another engine on the chassis dyno (e.g. RX-8 vs. 350Z), the information is useless if you don't also know the gear ratios of the two cars you are comparing. And I'm talking about ALL the gear ratios...not just the particular gear that the dyno run was made in. This is the point that Wakeech was making.

Ok, so the 350Z doesn't make 1.85x the torque at the wheels as the RX-8, but it still makes 1.85x torque at the engine after drivetrain losses (240 vs. 130)...doesn't that mean something? Well, it does mean something, but it is far from the whole story. Another thing that a lot of people seem to assume is that gearing from one car to another is the same. In the case of the RX-8 vs. the 350Z, the RX-8 has a sizeable gearing advantage. I'm sure you've seen these numbers in countless threads before, but for convenience, here are the ratios (transmission and differential combined) for comparison:

Gear 350Z RX-8 RX-8 advantage
1st 13.42 16.71 1.25x
2nd 8.21 10.08 1.23x
3rd 5.73 7.31 1.28x
4th 4.50 5.28 1.17x
5th 3.54 4.44 1.25x
6th 2.80 3.75 1.34x

Average RX-8 gearing advantage: 1.25x

Looking at the ratios, you can see that the drivetrain of the RX-8 multiplies torque 1.25 times more than the drivetrain of the 350Z. So instead of the torque at the wheels being 1.85x different between the two cars, the actual difference is closer to 1.48x.

But wait, there's even more to the picture! And this one I know you guys already know....Torque at the wheels isn't the only thing that effects acceleration, and acceleration is ultimately what is important, right? Weight is just as important, and the RX-8 has an advantage there too. If you factor in the RX-8's weight advantage, you'll find that the advantage the 350Z has over the RX-8 is reduced to about 38%.

The point I'm trying to make is that there is a whole lot more to performance than the torque number you get from the dyno. It seems to me like a lot of people ignore other factors that are just as important. In the end, the 350Z still has an advantage, but it's no where near as much as people would think by just comparing the torque between the two engines.

Finally, lets not forget that ultimately the difference may be even smaller than the 38% I've stated above: We should stick by the 130 lb-ft dyno number for now since it's the only dyno result we have, but I can't be the only one here that smells something fishy when I calculate the drivetrain losses for that run (above 20%). Either mazda has mis-rated the power of the engine, or there was something wrong with that car/dyno. If we see higher numbers from RX-8s in the future then the actual difference between the RX-8 and the 350Z would be even smaller...I'd venture to guess around 30% advantage to the Z based on the 15% drivetrain losses we'd expect from the 8...Only time will tell....

 

Even by your own numbers a performance advantage of 30% is significant since they are in the same price range and people inevitably want to compare the 2 cars. In the largest auto market, where more/bigger is always better mentality rules, mazda is at a great disadvantage. I know this car handles better than anything out there and probably is a better value for money. I just want a rotary to be a commercial success and not get a cult status so that they can bring back the rx7 :D
GO RX-8!!!!

 

BACK TO THE SUBJECT, yes ligher wheel's tires will help... but that is going to cost you some bucks... some rays engineering should help you out
they have a new line, Grams Lite, which are good wheels ... not as light as volk's but a lot cheaper i believe

 

The other benefit of getting a smaller wheel and tire combo, aside from the weight savings, is that the smaller combo gets you closer to the ground. Moving the car down a even just half of an inch can make a difference in Auto-X ing.

 

Huh? Compared to which cars?

 

Compared to even a 3rd gen rx-7 which is one of the best handling cars out there.