RonCT

I responded to another thread after doing some quick research (a poster asked if there was any documentation on the effects of unsprung weight)... I had read untold articles on it, so when I did a google to find something quick, I came up with the following...

Lower weight = higher performance. At what point does weight become important? The relationship is linear, meaning 1 lb is certainly not helping performance, 5 is 5x worse, and 10 is 10x...

Here's the simple wiki on it:
http://en.wikipedia.org/wiki/Unsprung_weight

http://www.bluegooseriders.org/riders/roadracing.html
This is a Biker's site, but the theories are sound.

Weight is directly related to horsepower. It is estimated that every seven pounds you eliminate, gains one horsepower.
There are many benefits to reducing unsprung weight. One of the most noted is that, 1 kg of reduction in unsprung weight is approximately equal to 6kg reduction in sprung weight.

One easy way to reduce unsprung weight is the use of lighter weight wheels. By reducing the weight of the wheels, you also reduce the gyroscopic effect. The gyroscopic effect is the force that is generated from a spinning object. To get an idea of what this is, hold a bicycle tire in your hands and spin it. Now try and change the direction of the wheel. You will notice it is hard to do especially if the wheel is heavy. By reducing weight in the wheels you will reduce this effect resulting in better handling during cornering and improved acceleration. Magnesium wheels are the choice for most Superbike racers, and some are even going to carbon fiber but these are very expensive and typically not as durable as cast aluminum.
+ + +

So, if you can remove 70 lbs from your car, it's like adding 10 hp. Better yet, if you remove 20 lbs of unsprung weight (say 5 per wheel), that's like removing 120 lbs of weight, which is like adding 17 hp.

This is why performance minded racers and drivers are constantly seeking ways to reduce not only weight but more importantly unsprung weight. I'm chasing 2-piece rotors like the PFC ones currently made for the base 997. I'm on a notification list from PFC for when they start making rotors for the 997S because they should be about 4 lbs lighter per wheel. One of the reasons I stuck with Pilot Sport Cups is they are the lightest R-comp out there at 25 lbs each (vs. upwards of 30 for something from Toyo / Nitto). Same with wheels - light weight 18s at say 20 vs. others at 25 or 30 lbs.

I'm just saying - don't underestimate the impact of weight all around. Tires, wheels, rotors, stuff in the trunk (some guys snicker when they see me taking everything out of my car at the track - the tool kit, compressor, maps, etc.). It all goes in my track box and probably weighs 50 lbs, equivalent to 7 more hp.

Land Jet

Wow, I need lighter weight wheels. Who knows wheel weights and lets hear them. I now have 18 " technology wheels that I think are aftermarket. Wonder what they weigh?
Don't mean to hijack your thread, but the light bulb went on when I read that Wikipedia piece. Wheel mass plays a huge role in good handling, and that's not something I understood until I read that article. Thanks Ron!

RonCT

Cool - a reply! I think my Fikse Profil 13s in 18x8.5 and 18x11 are in the 18 lb front and 20 lb rear range. That's pretty typical of forged after market wheels... There are plenty of threads listing wheel weights up in the Racing / DE forum.

malammik

RonCT, I got this very nice book from amazon called "How to make your car handle" It's a suspension theory, tuning book. It talks quite a but about unsprug weight and I think you would enjoy reading it. It has all kinds of cool formulas that will help you calculate exactly what results you can expect from x changes in unsprung weights.

JayP

I bought my 944S with 16x8s all around. They're 20# each conservatively. Now I have 15s, 7" Cookie Cutters and 8" Fuchs. 12# and 15#, saving me 26# total. Pretty significant and should make a difference.

jkb

good write up, thanks!!

ibkevin

Land, your aftermarkets increase your fronts ~4lb's and rears ~5lb's.

Got to Tire Rack and check your tire weights also.

moorepower

My 924S had some Mille Miglia 17X7.5's on it when I bought it (46lbs. each wheel and tire on a shipping scale). When I sold them and put the stock 15X6 phones on it (35lbs. per tire and wheel on same scale), it was like a different car. The suspension worked like it was supposed to, and it accelerated noticeably quicker.

Unsprung mass mainly affects handling, as previously described, but rotating mass (wheels, tires, hubs, rotors, spacers, lugs, etc.) mainly affects acceleration. Any way you look at it, lighter is MUCH better.

911pcars

Unsprung weight not only includes the wheels but the rotors, calipers and a portion of the shock and suspension bits.

Here's a chart I created on wheel weights. Let me know if you have add'l data points, add'l wheel info, etc.

http://members.rennlist.org/911pcars/WheelWts.html

Thanks,
Sherwood

auto-xr

Very good info!

96C4S

would unsprung weight also make horse power with the lighter flywheel.

911pcars

If the weight is supported by the vehicle springs, then it's sprung weight, flywheel too, even a lighter one.

With less mass, the engine will rev. faster (not higher) due to less inertia. Power remains the same although throttle response will be quicker. However, on a street-based vehicle, the reduced mass will result in more sensitive clutch engagement. Starting from a standstill might require slipping the clutch to create a smooth start. It depends on the amount of weight reduction.

The late 911s with dual mass flywheels are very heavy. Replacing it with a regular heavy flywheel is an improvement, but it's still relatively heavy. That's the purpose of a flywheel.

Sherwood

bruinbro

Don't forget the tires. There is a large difference in weight between different tire brands and models for equivalent sizes. For example, IIRC my Dunlop SSR's were about 4lb heavier than my MPSCs.

Bro

Chris M.

Check my math here but by that logic if I go from 15" Fuchs 7s and 8s that weigh 12 and 14lbs to aftermarket 17s which will weigh at least 20 pounds each (not including the extra tire weight) I'll effectively add 280 pounds (28 unsprung) to the car and lose 40hp? If so why does basically everyone racing an SC or Carrera run at least 17s and some even run 18s?

RonCT

It's not just the wheels, but the wheels and tires in an apples to apples comparison. When I talk about wheels I'm comparing a before / after package where the tires pretty much remain constant. So if with the same basic tires I can save 70 lbs on wheels, it's a good thing performance wise. Everyone I talk with seeks the best balance between wheel and tire size for weight and brake clearance. My 997S requires at least an 18" wheel to clear the big brakes. So when I look at my options, it's pretty much oem 19" size from the S or 18" oem size from the base 997. If I can keep the same 19" tire on the car but drop 70 lbs of weight by swapping the oem wheel for a forged one, then I'm ahead of the game. Probably the best performing package for my car is an 18" forged wheel (I'm running Fikse now at about 18 lbs front and 20 lbs rear) and Michelin PSCup (trying RA1s now, which are much heavier than PSCs). I really can't speak to your 15" vs. 17" example -- no experience there -- but the science of unsprung weight should remain constant. It's just a matter of finding a combination that saves weight.