Pittdawg

It's my understanding that for every pound of unsprung weight you remove, for example utilizing lighter wheels, it is equivalent to removing approx. 5lbs of sprung weight (i.e. everything above the axle and hub assembly).

Additionally, I believe the common figure is for every 10lbs of weight removed off a vehicle it is equivalent to a 1 horsepower increase.

So, my question is, does this all translate directly, for instance were I to purchase wheels that each weigh 5lbs less than each stock wheel, then this would be equivalent to removing a 100lbs [5 unsprung = 25 sprung X 4 = 100]from my vehicle's overall weight, thus yielding an effective 10 horsepower increase (I know in terms of accelaration not dyno gains)?

PharmISee

thats very interesting. Is this because the main energy is transferred first to the wheels? Therefore much less energy is need to spin the rear wheels?

vraa

iTrader: (2)

Something along those lines.
The biggest gain from losing unsprung weight isn't 'horsepower' though, it's much better handling.

Koz

As Ricky stated unspung weight has a greater effect on handling. When looking at wheels it is also important to look at rotating mass and its location. If you could move 5 lbs more towards the center of the wheel, it might be better then reducing 5 lbs all together, on a stock vehicle. The suspension on a stock vehicle is designed and tuned for specific unsprung weight and can not be adjusted. If you change it, one way or the other and can not adjust the compression, rebound and dampening, you MAY affect the handling in a negative way. It's sort of like adding an intake and exhaust and not re-mapping the fuel delivery.

If you don't plan on coilovers (adding adjustability) I would look at wheels that are approx. the same weight and have moved the rotating mass more towards the center of the wheel. The only exception would be if someone else had reduced the weight and had positive results, other wise you may have to add coilovers.

Koz

al503

First of all, there are two distinct types of weight in discussion here:
1. Unsprung weight is basically what is says it is. It's any weight that is not supported by the cars springs. Brake calipers would be an example.
2. Rotational weight refers to those things that spin/rotate on the car such as the pulleys, flywheel, driveshaft etc.

Finally, the above are not always mutually exclusive. Weight can be unsprung and rotational such as the wheels/tires, brake rotors, etc.

There really is no general guideline in regards to what 1 lb of unsprung weight is equivalent to x lbs of dead weight. As Koz mentioned above, where the mass is reduced in relation to the axis will play a significant role in the overall effect.

You also have to consider that the diameter of the wheels play an important part. A 20 lb 19" wheel may not have any advantage over 24 lb 18" wheel as more of the mass in the former is further away from the axis of the wheel which increases it's moment of inertia.

With all that said, I like to think that on a wheel of the same diameter, 1 lb of unsprung rotational weight is equivalent ~4 or 5 lbs of dead weight. Going up in size to a 19 or a 20 will diminish the multiplier greatly or even hurt performance even though its lighter.

So, to address the original question, removing a total of 20 lbs of unsprung weight may shave off a tenth in the 1/4 mile. (Good rule of thumb is 100 lbs. of dead weight = .1)

Doesn't seem like much but reducing that weight will help pretty much every other aspect of the cars performance (braking, handling, etc.) Conversely, going to larger wheels (unless they weight significantly less) will hurt pretty much all aspects of the cars performance; the tradeoff for bling.

vraa

iTrader: (2)

To an extent correct?
A set of 19" HRE C21's would result in better handling than say your stock 17" rims.

But other then that I think I agree with everything you posted.

To the original poster, if you want to reduce unsprung weight brake rotors and rims are the easiest things to replace. For the E55 for example, I'd pick the evosport multi-piece rotors (I believe they reduce 34lb total) and some lightweight rims such as the HRE C-series, CCW's, or anything along those lines.

I don't know what the policy is on groupbuys, but judging from jbrady's attempt on getting custom stuff done, I don't think CL would mind. If the 2IS or 3GS crew was really adament on getting some lightweight parts, I could try to talk to evosport in getting us some parts for a few folks (or I'll just do it on my own for you guys).

al503

Depends. If you went with 19"s with the correct overall diameter, you'd be running 225/35/19" and 255/35/19"s in the rear. With 17"s and if you were staggered, you'd be running the same widths on 45 series tires. You'd have stiffer sidewalls on the 19"s but tires in 45 series today are strong enough for the track.

With the heavier wheels and tires at the extreme corners of the car, you're also going to increase your MOI.

BTW, another good way to decrease rotational weight is to choose lighter tires. IE: the PS2's were about 3-4 lbs lighter than the GS-D3's in the sizes I was looking at.

AudiMan

Please correct me if i'm wrong but it makes more intutive sense (to me anyways) that you would want more weight at the center rather than the outter of the rim. Its easier to spin a wheel if it has less weight at the rim and more in the center. For this reason is why when you can buy a lighter wheel that is larger you effectively negate the power to the ground implications. And most people that buy larger wheels don't spend the 3-4k on forged wheels that are light, they buy big heavy rims that rob power and make the car less nimble.

uschardcor

you're wasting your time trying to figure this out.

AudiMan

no i have figured it out, and so have other people....i have a degree in aerospace engineering dammit! :-)

vraa

iTrader: (2)

PS2's I've heard have horrible balancing issues (wheelexperts).
Although, I still prefer them over anything else (but if you're at the track use the PS Cups)...

I'm going to see what it takes to get a composite rotor made for the 2IS which is in stock size so it's a clean drop in.

lobuxracer

iTrader: (2)

What's really interesting about this is an inertial dyno like a Dynojet will register the weight change as a gain or loss in hp. So under certain conditions, yes, you will see more horsepower. Personally I think it's crap because top speed won't change one bit - and that's the most pure test of power there is - but the time it takes to get to top speed is shorter with lighter gear.

Unfortunately, Toyota builds heavy cars. They're very often heaviest in class vehicles. There is a lot you can do to make them lighter.

AFA the wheels - There is a LOT of really great tire technology from 15" to 17". The choices narrow dramatically in larger diameters.