If I swap my stock rims (21lbs), for Gram Lights (14lbs), saving approximately 7lbs per wheel, 28lbs total - This equals (multiply by 7) 196lbs of unsprung weight savings?

How does this effect performance? My newbie mind can only see 28lbs weight savings - however, I've been told that I can expect "increased performance"....I would like to know if there are more exact ways to describe what is happening performance wise

 

Let me help a little. Unsprung weight is only wieght not held up by the spring, hence "unsprung." So, when you save 7lbs per wheel the unsprung weight is only 3.5 lbs. You total is only 14lbs. (only half the wheel is sprung at any one time)

 

Think of it like this... if you had to push a wheel, lets say 10 miles, but you had to stop it at every mile mark, would you want to push a wheel that weighed 1,000,000lbs, or one that weighed 1lb? I know the numbers are extreme, but I just want you to see the concept.

Your engine has to turn those wheels and your brakes have to stop them, it is much easier to do both if they weigh less.

 

So, I think I'm getting this (I went to public schools ) I'm having to move substantially less weight over time....So, can I expect to feel any appreciable difference in the cars accelration or braking ability?

 

No part of the wheel is sprung by the springs, so the entire wheel is "unsprung"

Half of the lower suspension components is only unsprung however.

Saving of few pounds per corner is a significant advantage in terms of vehicle dynamics and agility. 7 lbs per corner is litterally very significant. However, the down side of lower weight wheels at 17"+ sizes is that you can bend them much more easily going over pot holes; whether they are forged or not. Every gain brings a compromise. Such low weight wheels are great for track however, but for street... think twice

 

Theoretically, yes. You should feel quicker acceleration and quicker (shorter distance to stopping) braking.

The amount of weight you saved may not make a significant difference, but all will be better.

 

why isn't anyone talking about rotational inertia and the fact that each pound of rotational mass saved translates into approx 7 or 8 pounds in actuality?

 

what he said.

 

Good reminder... I wasn't thinking too hard

 

The problem here is that the Gram Lights you want are nowhere near 14lbs (I know, I just bought a set and they weigh 19.6 lbs apiece), so the weight savings over stock is negligible.

 

EvoNick,

Which model and size of Gram Light wheel did you buy?

 

57C, 17x8.5, 30mm offset, gunmetal.

 

Unsprung weight is important with wheels. Rotational mass is even more important. It has to do with Newton’s 2nd law and moment of inertia. I don’t want to bore anyone with physics that I don’t remember any more, so I won’t. I am sure you guys can look it up if you are really interested.

In simple terms, it takes more power to spin a heavier wheel. It also takes more braking to stop.

Example 1: Light weight fly wheel and clutch. If you have ever driven a car with one installed you will notice that the car revs really quickly when gas is applied and the revs drop again really quickly when gas is removed. Good example of reduced rotational mass. This also means that you get more harshness such as engine vibration. You will notice this with light wheels as someone pointed out earlier.

Example 2: Bike racers are **** about wheel weight. The heavier their wheels, the more power it takes them to accelerate. An extreme example would be a bike with car wheels and tires on it. Just think how hard that would be to peddle even on a flat surface.

Example 3: An Escalade with 26” wheels. And estimate from my local tire store for the weight of a wheel/tire combination is about 120 lbs per corner. These wheels have an appalling effect on performance and can almost double stopping distances.

Boring, I know. But light wheels and tires can make a big difference in performance.

BTW, the weight of the rear wheels on my Caterham is 6.8 lbs. They are magnesium with a spun alloy outside -- 13 x 8.

 

oh, and Flames - 666 , the Gram Lights are more like 17-18lbs, unless you get the forged ones.

 

many times when you get a new wheel it's wider... which puts more rotational mass at the rim which kinda negates the rotational mass savings of it being lighter. anything to the far edge of a rotating object will always create more inertia even if it is lighter.