Wheel Weight
 

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...
http://www.bcautodesign.com/racing%2...arcoviperR.jpg

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.

Re: Wheel Weight
 

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

Re: Re: Wheel Weight
 

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.

Re: Re: Re: Wheel Weight
 

...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...

Re: Wheel Weight
 

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.

Re: Wheel Weight
 

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

Re: Re: Wheel Weight
 

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

No, but reducing rotational weight will have a greater increase on acceleration/deceleration rates.
Sorry, I guess I missed it.

---jps

LOL :D I actually heard a story about this in the Audi Forums. There's this one guy who's trying to break into the 12 seconds, and he pretty much has pushed his car to the limits with what he has and was trying to think of ways to save weight, so while they we're in this process one of his friends mentioned maybe you should loose some weight and you'll bust into the 12's, since he was pretty close. LOL he mentioned what an excellent idea this was and he is now working to loose weight =D.

While we are on the subject of rotational weight, we also have to consider tires. It's sometimes funny how people worry so much about rim weight but they also forget the factor of tire weight. I've seen some people spend tons of money on light weight rims but then slab on a fat tire. I think the lightest tire I've seen so far with excellent performance are the Toyo Proxes T1-S.

Right on! This is exactly the point i was trying to get at! But was jus kidding, it just felt funny to me bcos besides the wheel weight issue, i believe it is much more fruitful reducing our own weight (if we are on the heavy side) rather than the measley few pounds we can shave off our wheels.

Besides, judging from the type of wheels/tyres u fellas love and are likely to buy for ur brand new RX-8 (OZ, VOLK, PIRELLI, FALKEN etc), i believe such performance wheels/tyres dun come any lighter than they already are, so in terms of wheel/tyre weights, how much weight savings can u have to actually make a difference in car performance? Unless of course you are talking abt comparing GENERIC brand rims to the more upmarket brands as i have stated above... then , its a whole different story.

As to my weight, i am abt 147lbs cant really go any lighter for my body frame. ;)

Does anyone know the stock tire weight on the rx-8. The Toyo Proxes T1-S are aprox 21 lbs for 225 40 18's.

There are so many design factors affecting a tires performance that far outweigh their mass. For instance the low lateral compliance of a lightweight sidewall will make steering very imprecise and lead to delayed turn in. When one experiences delayed turn in the feeling is very similar to understeer and the driver compensates by applying more steering angle. After the extra steering input the tires grab and then you have an oversteer situation.

The Toyo T1S is about the lightest and gripiest tires around. They also have probably the most laterally compliant sidewalls I have ever experienced. I hate these tires with a passion. They make a sports car totally unenjoyable to drive. Who cares if a car can get that extra 0.02 G's on a skidpad if the car has piss poor transient response? I don't. Excellent transient response is what a makes a car fun to drive.

I much prefer Bridgestone S-03, Michelin Pilot Sport and especially Goodyear GS tires. I have heard good things about BF Goodrich but have not experienced them myself. I am not happy with Toyo or Dunlop.

Sorry but saving 3.7lbs per wheel doesn't equate to a 24 Horsepower increase. 3.7lbs pounds per wheel would equate to no more than 2 horsepower increase. But it is a complicated equation because it depends on knowing the "moment" of the wheel or where that extra 3.7lbs is. If it's at the hub than there is nothing different about it than general weight savings. But if the 3.7lbs is in the outer rim of wheel, then it takes more energy to get that mass rotating (and to bring it to a stop as well).

24HP would be almost 10% more horespower and would drop about a half second off the 0-60 time. I would expect it to be more of a 2HP gain and a .05 second better 0-60 and better overall handling and feel.

Also as Sputnik points out, it doesn't matter whether it is all wheel, front wheel or rear wheel. It takes the same amount of energy to get a wheel rotating whether it is a driving wheel or a dead rotating one.

The factory 20lb 18inch wheels is pretty darn good. And for those who think they might get a little extra performance by going with a 17lb 17inch wheel, don't forget that the higher profile tire is going to be heavier by about 3 lbs more than likely.

My friend has the centerline RPM's which are 15 lbs. He sware by them and he hit a nasty pothole once that he thought would have bent them but didn't. You might get 3HP by using a set of those but they are only 7 inches so a 225 tire just barely fits.

I could work the actual math for those interested, but I'm afraid the actual horsepower increases would be worse than what I have stated here. So I think in this case ignorance is bliss :)

-Mr. Wigggles

Alright I did the math anyway and it is about what mentioned earlier.

Assuming that all weight reduced from a using a lighter wheel is coming from the outer rim of the wheel (i.e. reduces moment of inertia the most); for an 18inch wheel with 26 inch tires it works out that each pound you lose from the wheel is equivalent to 1.5 pounds of car weight.

So for a 3000 pound car, 4 wheels X 1.5 lbs eq./wheel = 6 lbs equivalent, and with HP/lb being the key to performance.

3000lb / 2994lb eq = X HP eq / 250HP

Solving for X gives 250.5HP equivalent or 0.5HP increase equivalent by using a 1lb lighter wheel, best-case.

The reason I say "best-case" is my analysis assumes that the weight is removed from the outer rim of the wheel and that the wheels are the only items in the car storing rotational energy. I neglected the flywheel, the crank shaft, etc, and even the weight of the original wheels.

But any estimate over 0.5HP per 1lb is not accurate, 0.5HP per lb reduced is best-case. (at least for 18 inch wheels and a 3000lb car :))

-Mr. Wigggles

Gents,

This is a very interesting discussion. One thing I thought I would add is that the factory rims are far stronger than the light weight items you are looking at. In fact, I just broke one and both the tow truck driver and dealer stated that had I been running lightweight aftermarket rims... it would have been ugly.

Even though there was one hell of an impact, the factory cracked but did not completely break. I was able to maintain control of the car (do to the excellent suspension) and steer the car off the interstate and to a stop. The rim had a semi-circular section missing that went from one spoke to the next.

I, like many of you, have been looking at ways to improve my car. Rims were an option but now, I do not know if I want to give up the safety.

To each his own.

Not sure where the 20 lbs for the stock wheel came from, by most people who have done the weighing most came up with 22-23 lbs. Also the stock tire (Bridgestone Potenza RE040 225/45/18) weigh 27 lbs. The Toyo T1S weigh 24 lbs.

The SSR Comps supposedly weigh 16.3 lbs for the 18" x 8", or if you have more money than sense Autovelocity makes a carbon fiber/magnesium wheel that weighs about 7 lbs. ! At about $1500/wheel. Read where an NSX driver dropped .5 sec. off his 1/4 mile time from switching to these rims, but not sure how much NSX rims weigh.

I hear the upsrung weight vs. sprung weight argument all the time. I really think 10X is very optimistic. I heard 4X, hell I have heard 1.5X.