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[LarThaL]

As the title says:

We obscess a lot over the effects of unsprung weight on handling, but how much of a difference does shedding 6-8 lbs from a wheel/tire combo really make?

Let me also add that this is all theory based on what I have read and some basic physics, but I'm hoping we can have a lively discussion to get the facts on the table once and for all.

A 3600lb M3 with a 51/49 weight distribution has, then, a 918 lb load on each front corner. The front brake assembly is 37lbs and the stock front wheel-tire is approximately 48lbs (depending of course on the specific wheel and size).

You therefore have approximate 85lbs unsprung weight. Now, the shock/spring assembly is neither sprung, nor unsprung so lets take it out of the equation. I don't know for sure, but I will estimate the weight at 25lbs.

Net result is 85 lbs unsprung weight vs 808 lbs sprung weight. The ratio of unsprung to sprung weight is 0.105. Lets say you are able to take 7lbs off per corner by going to a lighter wheel and tire combo. You would now have 78 lbs unsprung weight and ratio would be 0.096. A difference of 0.009 which is basically nothing.

Since the implication with regards to handling has to do with the unsprung/sprung weight ratio, it doesn't seem that a lighter wheel/tire combo really makes much of a difference. It will make more of a difference on a lighter car, or maybe on a rear engine car that doesn't have as much mass over the front wheels. Basically, the heavier the car, the less the wheel/tire weight will affect overall handling, for a given suspension stiffness.

This can really also be applied to ride. Going to a lighter wheel/tire combo, all other things being equal, the impact on ride will be felt less on a heavier car due to the fact that the unsprung weight is a smaller fraction of the total weight at that corner.

I think, in the end, it is the suspension and tire rubber composition that will have a far greater impact on handling, not the unsprung weight on a car in the M3's (or heavier) weight range. Ride will be affected by the suspension as well as the tire sidewall height and stiffness more than a decrease in unspung weight.

Oh....and this would apply to the issue of lighter brakes too, although if you were to go all out with like ceramics and BBS F1's, for example, that might be enough weight savings to actually start to see a difference.

Discuss......

[IMG]

On a street driven car you will not noticed much of anything if at all. You will notice tire sizes more than anything in terms of grip and turn in.

On the track, unless youre a pro, you will likely not notice much either except for tire sizes as above.

[Z K]

It definitely makes a difference, you can feel the difference in wheel weight when it is decreased. A lighter wheel makes it easier for the car to rotate. The result is better throttle response, faster reaction of suspension and better gas mileage. You don't gain horsepower but you make better use of it as it's being put to the ground better than being used up to turn the wheel.

I've ran lighter and heavier wheels on my previous cars, sometimes back to back and the lighter wheel is always preferrable. The heavier wheels are just less responsive and it feels slow. I've gone back to stock wheels before because the performance was so much better over larger heavy wheels. Even in normal street driving, you'll notice less throttle response and the car reaction is just slower to driver inputs.


Check out this article.
http://www.tirerack.com/wheels/tech/...jsp?techid=108

A vehicle's total weight is the sum of all of its parts and affects its ability to accelerate, brake and corner. Reducing the total weight will enhance the vehicle's performance because less weight needs to be controlled and therefore, less energy is required. Unsprung weight is the weight under the springs which moves up and down as the vehicle rides over uneven roads and leans in the corners. Reducing unsprung weight allows the springs and shock absorbers to be more effective in controlling the suspension's movement. Additionally, a vehicle's rotational weight includes all parts that spin including everything in the vehicle's driveline from the engine's crankshaft to its wheels and tires. This affects the energy required to change speed as the vehicle accelerates and brakes. As you would guess, reducing the weight of any of these rotating components will enhance the vehicle's performance because less energy will be required to increase or decrease their speed.

[wooderson]

I think you will notice more of a difference when you start changing the diameters of the wheel and even rotors. 8lbs per wheel is pretty hefty. I have to think that the average driver will notice a big difference adding or subracting that much weight at each corner.

[Dave07997S]

I think an 8lb difference per wheel will be felt. If it was in the 2-4lb range per wheel than obviously the effects will be quite a bit less. On a street car I don't think it would matter that much. I would try to keep the weight change to be as minimal as possible. Also look at the tradeoffs, yes you are a few pounds heavier but now you have a wider wheel that can support a wider tire and such.

Dave

[IND-Distribution]

I believe that weight is not the only factor in a wheel's performance, so it is very very difficult to perform a truly accurate test.

A lighter wheel may be structurally weaker than a heavier counterpart, and may therefore lead to a decrease in performance, or perhaps the heavier wheel may reach a point of diminishing returns where adding weight does not increase rigidity by a useful amount. Wheel width and offset can also have an effect on turn in and overall mechanical grip.

I know of only one well-known test performed by a popular Japanese publication where a given wheel was manufactured with the exact same overall design but with some material removed for weight savings specifically for this magazine test. The car in question did produce an overall slower lap time with the same driver, same tires, on the same day with the lighter weight wheels due to their overall weaker structure.

The article didn't intend to illustrate that lighter wheels are never better, but simply that low weight is not the only factor in a wheel's performance. Therefore it's difficult to make a statement about a wheel's exact effect on performance without considering many factors - weight included.


With that being said, having tried some wheel and tire setups that vary by at least 10 lbs per corner back to back when switching between spares at the track, I have noticed that lighter wheels make a dramatic difference in how nimble and tossable the car feels. This is of course just anecdotal, but I am a firm believer that a well engineered, strong, and light weight wheel in the appropriate size will dramatically improve a car's performance and feel.

[Nautik]

It can make a big difference and it is more complex than you might realize, especially when it comes to rotating mass vs just unsprung weight like the brake assembly. Like the previous poster mentions, there are many factors which can include things such as:

• Weight of the wheel
• Where the weight is in the wheel relative to the axis (more weight further from the axis is less desirable in terms of being able to accelerate quicker)
• The rotating speed of the wheel (1 lb of weight reduction will have different effects at different speeds of rotation)
• Strength of the wheel

Pilfered this off a Porsche forum:

Olaf Manthey explains in an interview : “Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a 1:6 factor has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with the 1:6 ratio, as Porsche considers it to be 1:7.

So you can assume 1 lb of weight out of your wheel is worth 6-7 lbs out of your car. But again, where this weight is in the wheel and the speed of the wheel will make a difference as well.


From personal experience I can tell you that I had a civic si years ago with heavy wheels on it. The wheels were stolen and I put some lighter weight wheels with a smaller diameter on it (smaller diameter rim). The difference was IMMEDIATELY noticeable to me in terms of acceleration.

So according to the above, you might be able to simulate the difference in terms of acceleration of braking by putting your 200 lb buddy in your passenger seat and seeing if you notice a difference... I do.

[disapr]

Based on experience.

I put a set of aluminum Marchesini wheels on my CBR600RR and the bike felt ‘punchier’ than before and seemed to accelerate and brake a lot better than the stock wheels with the same tires. Before I sold the bike I put the stock wheels back on with the same tires and the bike felt like a boat, super dull and even acceleration and the braking took more input for the same reaction.

Obviously that’s a much smaller scale but still an example.

Now with my M3 – I don’t have any hard vBox data to back this up but going from the stock 220M wheels with OEM sized tires to 18” HRE R40’s with much wider tires the acceleration ‘feels’ noticeably more responsive and ‘quick’. This is all per the but-o-meter but if feels quicker and next summer I plan to do some vBox runs with the different wheels on the car so that I have some data to back up the experience.