Unsprung weight...lighter rims; how does it help?
 

Hello..
just how much can a lighter rim 18/19lbs vs. 25/26 lbs. aid in off the line acceleration? 1/4 mile? handling? is it significant? is it worth paying A LOT of extra $$ for fikse or HREs?

Unsprung weight is always good, but just through lighter wheels wont help. The lighter wheels, used in conjunction w/ thousands of other lighter pieces like carbon fiber body panels and only the essentials inside the cabin while everything else is stripped out would help, but buying mega bucks HREs by themselves wont help you out too much.

I always had the impression that lighter wheels (thus lighter unsprung weight) had to do mostly with keeping the wheels on the road (or off-road) surface. Heavy wheels have more momentum and when bounced up take longer for the suspension springs to bring them back down into contact with the road. Lighter wheels get back down on the road again quicker so as to propel you or steer you.

It also has an impact on the ride quality because when a heavy wheel is bounced up, it takes more force from the car's body to stop it and throw it back down again. Force that you can feel because it throws up the whole car.

Off-the-line acceleration improvement due to lighter wheels is mostly due only to the overall vehicle weight savings - not that significant.

Motorcycle design is a lot more involved with trying to minimize unsprung weight because it is a bigger proportion of the vehicle.

I really like the alloy wheels on the old 300D, but if it came with steel wheels I'd never replace them because they'd cost more than the car did. :)

Ken

to add to Ken300D, lighter wheels also improve ride quality, since less momentum is transferred to the chassis on the wheel's upward movement.

lighter wheels also have less rotational mass, so they are easier to spin, but this only becomes a factor in ultra-high-performance cars. However, a larger wheel will have a higher rotational moment of inertia than a smaller wheel.

I have installed Forgeline wheels on my car and I noticed an improved ride, and quicker acceleration at speed but not from dead stop.

Are they worth the money? It all depend on how sensitive you are to your car and how much money are you willing to spend. Will I do it again? You bet.

I read about this test SCC did. They dyno'ed a honda civic hatchback with the stock steel wheels and marked the HP. Then they put on a set of 17 inch rims and marked the HP. Seems like there was a 20hp difference between the steel wheels and the 17 inch rims.

This is quite in interesting discussion, because I know I read in article involving an E55 or CLK55 that had major fenders flared. They used light 3-piece 17" rims from BBS or HRE and had 315/35/17 in the rear to handle the power band by adding more rubber to get the power down. The fronts used 235/45/17 in the front. The weight savings was probably there. AMG monoblocks are light, but some of the multipiece wheels offered by others are increbily light.

The issue with the civics using 17" rims and so forth, also a factor is putting power on to the surface. As te CLK and E AMG modles produce much power, this prodces quicker times without actually making any other modifications.

I read in a recent article that a 1 lb. reduction in unsprung weight had the same effect as 3lb. reduction in vehicle weight on acceleration. I recall that it was in European Car Magazine. Some authority on the whole topic was consulted before assembling the lightweight package.

That is true, and for every 11lbs saved, its like gaining 1 horsepower is also another one I heard. However, the benefits in handling are far more greater. Lets say a 5lbs savings on a wheel/tire combo at each corner could make the difference of taking a turn at 40 to 55.