[sparoz]

Originally Posted by heli_ben

Who here is a good enough driver to feel 5lbs a corner on unsprung mass? (on a car with street / road focused suspension)

Anyone who pays attention to turn-in and how the suspension behave on road.

Originally Posted by neech

Well the common thought is that (at least for handling) every pound of unsprung rotational mass you shave off is equivalent to 10x that in sprung weight. so 5lbsx4 cornersx10 = 200lbs in total chassis weight. Can you feel the difference around the road course between having a passenger or not?

Now imagine shaving off 10 lbs off each corner. Yes, wheel weight is a big deal.

Unsprung weight by itself (shocks, suspension components, brake calipers) is a 4x ratio IIRC.

Originally Posted by blueshark

Interesting.. How does the math work?

Originally Posted by neech

Well i'm not sure where the 10x comes from but the general principal has to do with the moment of inertia (MOI) / angular momentum. MOI is the amount of force required to change the angular momentum of an object about a fixed axis, or change the axis of rotation. These are what "gyroscopic" forces are.
When you accelerate/brake/turn, not only are you changing the velocity of the wheel (as if it were in your trunk) but you must change the angular velocity AND also the axis of that velocity, which both require significant energy. Think about when you're holding a spinning bike wheel by the hub, how hard it is to change the orientation... And that's 3-4 pounds at a relatively slow angular velocity.. Now take 60 lb car wheel/tires, multiply that by 4 corners, at 20-100mph... That is a significant moment of inertia.

I think the 10x is a pretty rough guesstimate though, not sure where that came from.

After doing a whole lot of research before I purchase my wheels, I think it is almost like Chinese whispers and ended up with something some dramatic like 10x the weight. I have heard people talked about 5x 8x and 10x. The math on angular velocity once you do the sum it range from 1 at the centre of the wheel to about 2.5 times (IIRC) at the edge of the wheel at an average around 2x. So, the more accurate saying should be, "if you save each kg in each wheel, that's equivalent to saving 8x that weight". That is calculating by 2kg in each wheel with a total of 8kg, and not 8kg of unsprung mass saved and equivalent to 32kg.

However, the other thought is that it is about the suspension behaviour when reducing unsprung that people are talking about the x5 x10 improvements.

Originally Posted by heli_ben

I can certainly feel 200lbs. However we've changed rims on my race car with a similar %'age mass reduction and we couldn't really see it in laptimes, and I don't think I can feel it.

I don't think it's that simple.

For sure it's always good to cut mass wherever you can, especially in rotational and non-rotation un-sprung mass... but I think on a road car, it's primary use is in the bar, not on the track! :-)

The advantage of reducing unsprung is not weight reduction from the total mass but suspension behaviour. Now, theres where probably the x5 or x10 possibly comes in. The suspension controls the sprung and the unsprung mass - if there is a proper suspension tuned to the reduction in weight, there is where the advantage comes in. My car has 40kg of unsprung reduced from brakes and wheels, and you can certainly feel the difference in ride and grip. Your suspension basically works better as there is less momentum to deal with. However, to truely extract the performance on track, you will need to tune the suspension accordingly. You will probably see more improvement on the road than track when there are more bumps.

Originally Posted by gatorfast

People put way too much emphasis on wheel weights. Ive switched from "heavy" factory wheels to lighter after market wheels and noticed no difference.

How much lighter?

Originally Posted by Dackelone

I met a guy in Switzerland who went with 18" wheels - he had a P3 gauge vent and he claimed his 0-100 kph (62 mph) was under 4 secs with the 18's. So he runs them in the summer months. He said his car accelerates faster with the lighter 18" wheels. He also said the OE wheels were quite heavy.

+1.