Unsprung weight and rotational inertia, as we know, is more important than sprung weight. The general rule of thumb says that 1 pound rotational or unsprung weight is approx equal to 10 pounds static weight.

With this in mind, I asked mazda for the wieghts of the stock wheels. Their response:

For the 16" wheels, the weight is 16.3 lbs.
For the 18" wheels, the weight is 20.0 lbs.

The 16 incher isn't too bad, it is around the same weight as the FC and FD stock rims.

The 18 incher is a bit weighty. The lightest 18 incher I've found out are the Racing Sparco Viper. At 18x8.5 they weight in at 16.3. That's a 3.7 lbs savings... translated into roughly a 37 lbs static weight savings.
Another rule of thumb is that every six
pounds of weight reduction is like adding an additional horsepower.

So it's time for some crazy shade-tree math...

3.7 lbs savings
x 10 static rule of thumb
x 4 wheels
x 6 horsepower rule of thumb
--------
about 24 horspower gain

Now, lets bring in the cost of these bad boys...


According to their website, they sell for 74,000Yen -- a little over $620 each.

So $620 x 4 = $2480 for a set
and $2480 / 24 HP = $103 dollars per horsepower gain.

 

:p... it's not a 4WD/AWD car Dan. ahahaha...

 

Ok..

3.7 lbs savings
x 10 static rule of thumb
x (2 wheels rotoational + 2 wheels unsprung)
x 6 horsepower rule of thumb
--------
about 24 horspower gain

Lookie! Still the same.

 

...what i meant was that on a dyno, you're only going to see the benefits of the lighter rear wheels (as far as "HP gained" goes)...

but you're right, lighter is better... sheesh, i was just kidding :p ahahaha...

 

Your math and assumptions are in the very far end of the optimistic spectrum. You generalized a couple of things, like the relativity and comparison between unsprung weight and dead weight. 1lb weight saving on the center of the rim is not the same as 1lb saving near the lip or on the tire. 1lb of unsprung weight is = 6lbs of dead weigth at the most.

So lets do the math again,

Assuming the RX-8 weights 2900lbs and have 250hp, the power to weight ratio is 2900/250= 11.6 to 1, meaning that if you lose 11.6lbs, its same as gaining 1hp.

3.7 lbs unsprung weight per wheel
x 6 estimate for dead weight
x4 wheels
/ 11.6lb per hp.
---------------------------
7.7 hp

Even this is a little optimistic.

I went from 19lbs 16" factory wheels to 13.7lb SSR Comp 17", and most of the difference felt is the handling and steering.

 

RX-8 rim width? offset?

 

20lbs is not bad for stock alloy 18" wheels, I was actually expecting them to be heavier.

Btw, anyonw know how much an 18" Volk TE-37 weighs?

 

16x7 on the auto, 18x8 on the 6-speed. No mention of offset, yet. I've emailed rx8orders for more information.

 

I've never heard of a ratio THAT high. The highest that I've ever heard is 4:1.

And you're calculations don't take tire weight into consideration (whether higher or lower).
That's irrelevant.

---jps

 

According to a previous thread Mazda has said the stock offset is 50mm with 5X114.3mm bolt pattern.

I am the most ardent "add lightness" guy one will find. Especially reducing unsprung and rotational weight. However, these "rules of thumb" are worthless. In fact the original math posted in this thread seems to follow the pattern of if I add a header + intake + sticker + wing they seem to be power multipliers and now we have a 60 hp gain. In fact, reducing unsprung weight has no greater increase of acceleration potential than reducing the weight of the engine. Reducing unsprung weight will increase the ability of the suspension to accurately respond to road surface changes.

There are several wheels lighter than Racing Sparco. Check out www.wheelweights.net Some wheels are:

18'X8" RS Watanabe Cyclone = 14.6 lbs
18"X8" SSR Competition = 15.0 lbs
18"X8" BBS RG-R = 17.2 lbs
18'X8" Team Dynamics Visage = 17.4 lbs
18'X8.5" Volk TE37 Magnesium = 12.9 lbs
18"X8.5" Centerline RPM = 15.0 lbs

 

A 20lb OEM 18" wheel is actually pretty good.. My OEM Honda 16 inchers are about 19lbs.. I'm happy with a 20lb weight..

 

er... besides all these wheels weight thingies, in your pursuit of optimum performance for ur RX-8s, i believe u fellas (who are the drivers) might be better off shedding some pounds?? :D

A diet programme should be implemented for all of us ardent RX-8 fans before receipt of our cars... with support groups to encourage the weak minded and demonstrate our resolve by feasting only on apples and water..

just imagine the total weight savings which will translate into less fuel consumption, releasing of more horsepower, money savings from using less fuel and burgers/fries & beer. Not forgetting the benefit to the fragile environnent with less CO2 emmissions... :loco: b

Wouldnt the above be a more worthy cause? :p

 

since i weigh barely 140lbs now. i dont think i will be shedding any pounds any time soon. if anything i'll try to work my way up to 150lbs.

the environment isn't fragile. it will take care of itself! the problem is, will we still be able to breathe?


i guess that depends on how much weight someone actually has to lose. what do you weigh?

 

But how much do YOU weight? I'm hoping your not one of those guys as stated on car and driver's website that "has a 40lb bear gut and boasts about his 5 lb savings by changing out his exhaust to an aluminium exhaust"!

So since I lost 25 pounds since January and so has my girlfriend, I have now 50lbs less dead weight in my car, so I gained 8.3 horsepower? Nah.... :D

 

i was joking man!! sheesh... ahahahaa...