Okay. I'm new here, but have been searching all over this site and haven't been able to find it.

Does, anybody have the specs on what lighter wheels, performance wise do for your car?

It would be cool if there was a calculation or something to help you estimate what it would mean. For instance I have the 17"x7" S-Lite wheels. They roughly 25lbs. What would it mean performance wise is I went to a 17"x7" wheel that weighed 19lbs.

I would assume lighter wheels mean. Higher Speeds, Faster Acceleration, and better gas mileage. But I haven't been able to find the cold hard facts and being an engineer, I like the cold hard facts.

Thanks. Sorry if this is a dupe post. If it is can you just point me the right direction?

 

I haven't seen that either. There ought to be some good approaches to it- total weight, revolving weight, horse power and/or torque. What a fun toy that would be. I bet the software is out there. Maybe not "MINI" associated but racers would want to know.

 

I heard, roughly, that every pound off of the unsprung rotating mass is equal to about 6 pounds off of the sprung mass of the car.

But also, loss of weight off of the unsprung mass (weight below the the suspension) will first be felt in braking, then in acceleration, then in handling. At least thats what I've gathered researching wheels. . .

The car must loose some surefooted-ness, but gain some quickness.

-[[[S]]]

 

Hey sandtoast500,
Thanks for the comment.
Your use of the term unsprung rotating mass was exactly what I was looking for. I searched that term on google and found a flood of information. I am going to spend some time, sift through the junk, and then report back what I have found.

But, here's what I found so far. Apparently surefooted-ness, is improved. Because there is less weight to move up and down and the suspension can react more quickly. Faster reacting suspensions results in the tire getting back on the surface quicker, resulting in more tread on the ground and increased "surefooted-ness". This is best for applications like autocross, and other road course type stuff. For things like ovals and drag racing not so much because the suspension doesn't move much in these applications.

This all makes sense to me, but stay tuned. I want to get down to the cold hard facts. Once I figure it all out I'll be back. Hopefully, with a nice little calculation/calculator for us to play with.

 

Cold hard facts are in short supply here at NAM

 

There is no calculator for what you want because it is very very complicated to take into consideration all of the potential variables that come into play from the tire and it's design and the wheel and it's design.

Lighter wheels if strong do a lot of good but will cost more. As far as wheels and tires go, lighter is a good direction but it's not that simple all the time.
They will provide for faster acceleration, shorter braking distances, better suspension dynamics (handling) since less energy or effort is needed to change speeds or to respond to throttle changes.

Heavy wheels feel sluggish by contrast but don't hurt if you are going downhill or constant speed. I don't think higher speeds are affected by wheel weight itself.

Changing rims from 25 to 19 pounds drops weight 6 pounds per corner for a total of 24 pounds and is similar to loosing about 48 pounds from the car itself. This improves power to weight ratio. Try running in heavy hiking boots then run in light track/sprinting shoes. It's a big improvement.

The more power you have the less effect slightly heavier wheels will have. For the base Cooper it's really important to keep wheel and tire weight to a minimum if possible. You might have a bit better mpg with lighter wheel/tire combo as long as rolling resistance is kept the same or improved.(see the tirerack link).

Wheel design is important. Forged wheels tend to be lighter and stronger than similarly sized gravity cast alloy wheels. Stronger is good for high performance driving and for durability but there is a limit as to how much abuse wheels can take. Some track only wheels are not designed to take the punishment from general street use. Larger rims also require lower sidewalls and a stiff sidewalled tire will transmit road irregularities to the rim. potholes and 35 or 40 series tires are not friendly to lightweight rims. For the wheel design, more weight at the outer circumference is less good- a very wide rim that adds weight isn't as good unless you gain from having sticky wide tires that provide a larger contact tire patch to the road. (for example, two rims each 17x8 weighing the same have different designs, one with minimal spokes with beefy hub and the other with a thick deep dish design and minimal hub.

So those that have light rims still have to be careful about road hazzards, especially so with performance oriented stiff low profile tires. Weight on the tire is at the outer most edge of the wheel tire combo and that makes a difference but that is countered by the tire dimensions/size, tread design, rubber compound, etc. Just because a tire can weigh 2 pounds more in total weight doesn't mean it cannot outperform a lighter tire with less sticky rubber. A good example of an excellent but a little heavy tire is the Goodyear Eagle F1 GS-D3.

Some threads or links found:
http://sccaforums.com/forums/thread/240975.aspx
http://corner-carvers.com/forums/sho...ung+and+weight
http://www.tirerack.com/wheels/tech/...jsp?techid=108


Some discussions on lighter wheels-
https://www.northamericanmotoring.co...ad.php?t=90561
https://www.northamericanmotoring.co...ad.php?t=40723
https://www.northamericanmotoring.co...ad.php?t=35659
https://www.northamericanmotoring.co...ad.php?t=95865
https://www.northamericanmotoring.co...ad.php?t=69678

Threads on wheels/weights
https://www.northamericanmotoring.co...ad.php?t=34730
https://www.northamericanmotoring.co...ad.php?t=21312

and if that is not enough then try these-
http://www.rx8club.com/showthread.php?t=51865

have fun.

 

Yep, rotating weight will obviously have greater inertia based on the distance away from the axis. But the generalizations for acceleration/braking performance are:

1lb of tire weight hurts as much as ~2lbs on the car
1lb of wheel weight is equivalent to ~1.5lbs on the car

so 1lb more per tire x4 tires is the same as adding 8lbs to the car, and adding 6lbs of wheel x4 is like adding 36lbs to the car. Since it is generally accepted that 100lbs of car weight costs approximately one tenth second to the quarter mile time of a 3,000lb car, going from S-lites to 19lb wheels can theoretically gain you a 0.036 second improvement--not much

But the main gains are probably the "surefootedness"--less sprung mass means the existing shocks can control motion better, so lighter wheels essentially make your shocks act stiffer without the corresponding ride penalty. Plus it may be easier to modulate wheelspin because the wheel speed reacts more quickly to changes in throttle, and ABS may be less likely to upset the chassis since it can work faster.

 

--Some people mentioned on this and other MINI forums that the heavy weight of the S-lite wheels along with the heavy weight of the Run Flat tires contributes to the mushrooming (damaging) of the shock towers in our MINIs.

Could this be true?

They cite that originally the MINI was designed for lighter smaller wheels with non-run flat tires. At the last minute BMW added in the heavier and bigger wheel/tire combos.

I'm rather fond of my S-lite wheels, but I know that if I track my MINI on the mainland I'll have to pick up an extra racing set of wheels and tires to change out for racing just like Minihune does. Regardless though I'm going to replace my Run Flat tires with non-run flats to reduce the weight and to reduce spinal trauma on bumpy roads.

You might want to check out SSR wheels and Motegi Track Lite wheels. I think they claim between 11 and 14 pounds for a 17 inch wheel! Yowza!

 

Mushrooming is basically too much force is transmitted to the top of the strut towers causing damage.

Runflats are stiff and have low sidewalls- 45 series, S-lytes are heavy and somewhat strong. Most times you aren't going that fast and hit a pothole, it will sound awful but not big damage is done. But sometimes you don't see the pothole and you hit it hard and will damage not only your tire, your rim, your suspension and your strut tower.

Using any large rim that requires use of lower profile 35 to 45 series sidewalls is going to be hard on your suspension and chassis. Just be careful when driving in unfamiliar places or during bad weather conditions. Look before you drive.

SSR wheels and Motegi Track Lite wheels are both good wheels to consider.

Ways to minimize risk for mushrooming damage to the strut towers:
Use tires with greater sidewall height like 55 or 60 series tires (fits 15" or 16" rims)
Use the correct tire pressure at all times (not too low)
Watch where you are going and don't hit bumps or potholes except at very slow speeds
M7 tower plates +/- strut bar
Shocks tuned for some softer dampening/rebound or stock shocks or Koni FSD
Softer springs (not too firm or low)
Keep your passenger or cargo load light
Use non runflat tires

 

Thanks guys this is all really good information. I believe these generalizations for the most part. I mean hey they had to come from "somewhere." It's just that this "somewhere", is what I'm looking for.

This is a great crash course for me in understanding this element of a vehicles performance, which will ultimately help me not crash on the course.

My search countinues.

 

The closest I've been able to find is:
There have been attempts to create spreadsheets with the calculations embedded, but at best they can only be approximations considering the wide variety of available wheel shapes (and thus moments of inertia at any r).

Since you are an engineer, you really should be posting this question on the engineers' forum.

 

Wow, great. Now we're getting somewhere. Thanks BFG9000.

The quote, the excel, and the link to the engineering forum are all great pieces of information.