Unsprung Wheel Mass

[A_Whelan]

Winter is upon us, which for some means parking the e30 and driving something else. I'm the parking category because 1. My swap still isn't finished and 2. I want to keep my car as healthy as possible, fuggin road salt

In preparation for the spring I am trying to figure out my budget and thinking ahead to what I need to do and want to do to the car. Which brings me to the topic, unsprung wheel mass.

I can't help but love the look of Diamond Racing Wheels. There are, however, three problems that I have. One is they are considered a "soft" wheel and easy to bend, the second is the lugcentric balancing and the third is the weight.

A lot of people say it's night and day between DRW's and a lighter wheel. This made me think about the extra weight and at most it's going to add 10lbs per wheel for a total of 40lbs. And maybe it's the location of this extra weight but I can't see 40lbs making a huge different for a DD car that I will never ever track. I don't really notice when I have a person in my car so I can see 40lbs in wheels doing that much. Am I wrong on this or is it a personal preference thing?

Cheers,
Aaron

[Sagaris]

I was reading a chassis engineering book which stated that the effect of rotating mass on performance is 3x what it is compared to non-rotating mass.

So the performance decrease due to 40lbs in extra wheel weight would be like having a small adult in your car with you.

[jlevie]

The ratios quoted for the affect of unsprung mass range from 1:2 to 1:4. My experience favors the 1:4 end. Even on a daily driver adding 40lb of unsprung weight would be equivalent to carrying a 160lb passenger, which is highly noticeable.

But there's a bigger problem with the DRW wheels. The odds are that off the car balancing will never be successful. Meaning that the tires will have to be trued and the wheels balanced on the car and that will have to be repeated each time the wheels are dismounted. Been there, done that, and it is no fun and not cheap.

Go with the lightest hub-centric wheel you can find. 15x7 wheels can be had that weigh 13lb. And keep in mind that you can't see the wheels from the driver's seat.

[A_Whelan]

Yeah I wasn't sure if the rotation of the mass would have made it heavier or not. But that's good to know, thanks for the posts. I guess I'll be choosing my backup choice!

[AE86]

Bumping this thread for 318 Drivers, I'm hoping to solicit peoples experience with lower HP cars and lighter wheels.

I know Miata guys attest to the value of lighter wheels, but I'm just not convinced yet. I saw this article on Car and Driver which makes sense and has a lot of good tech info, but the car in the article has more TQ & HP over 318's, so I dont know how applicable it can be?

I'm thinking of going 14x7 Enkei RPF1 8.9lbs vs OEM 14" Basketweave 14lbs = about 20lbs of weight savings for all four wheels. What do you guys think?

[jeffnhiscars]

Bumping this thread for 318 Drivers, I'm hoping to solicit peoples experience with lower HP cars and lighter wheels.

I know Miata guys attest to the value of lighter wheels, but I'm just not convinced yet. I saw this article on Car and Driver which makes sense and has a lot of good tech info, but the car in the article has more TQ & HP over 318's, so I dont know how applicable it can be?

I'm thinking of going 14x7 Enkei RPF1 8.9lbs vs OEM 14" Basketweave 14lbs = about 20lbs of weight savings for all four wheels. What do you guys think?

I've had 13 lb BBS RK 16's on my 318i w Bilstein Sports & Eibachs (among some other tweaks). I had to take one in for repair so put the bottle caps off my Cabrio on it and it was like the difference between doing the 100 yd dash in running shoes or work boots.

Note..those are the wheels on the Cabrio in the pic.

As far as rolling mass, correct me if Im wrong, but I would think the larger the wheel diameter the greater the impact due to higher centrifugal force (more wheel mass further from the spindle). There may be a sweet spot beyond which you no longer gain.

[bastianshaw]

another think to consider here is that these heavier wheels are heavier because the overall diameter is usually bigger making a higher final drive ratio.

[jeffnhiscars]

another think to consider here is that these heavier wheels are heavier because the overall diameter is usually bigger making a higher final drive ratio.

You should be reducing tire weight when you switch to larger wheels because you are suppose to use a lower profile (lighter weight) tire to maintain the same rolling diameter. ie 195/65/14 or 205/50/16.

[nrubenstein]

another think to consider here is that these heavier wheels are heavier because the overall diameter is usually bigger making a higher final drive ratio.

Larger wheels do move more mass to the outside - mass on the rim is much more important than mass at the center. Larger wheels do not inherently impact the overall diameter of the wheel/tire assembly.

Less weight in smaller wheels is a matter of diminishing returns. It's good, but I wouldn't run 14" wheels for the weight savings. The tire selection is shit.

[Mr. Tasty]

You should be reducing tire weight when you switch to larger wheels because you are suppose to use a lower profile (lighter weight) tire to maintain the same rolling diameter. ie 195/65/14 or 205/50/16.

reducing tire weight, generally increasing over all weight of the tire/wheel combination

[jeffnhiscars]

reducing tire weight, generally increasing over all weight of the tire/wheel combination

That REALLY depends. My 16" BBS RK's weigh 13 lbs which I'm sure is less than bottle caps and surely the 205\50/16s weigh less than stock tires.

[nando]

Bumping this thread for 318 Drivers, I'm hoping to solicit peoples experience with lower HP cars and lighter wheels.

I know Miata guys attest to the value of lighter wheels, but I'm just not convinced yet. I saw this article on Car and Driver which makes sense and has a lot of good tech info, but the car in the article has more TQ & HP over 318's, so I dont know how applicable it can be?

I'm thinking of going 14x7 Enkei RPF1 8.9lbs vs OEM 14" Basketweave 14lbs = about 20lbs of weight savings for all four wheels. What do you guys think?

I'm not sure why you aren't convinced. a mere 4lbs in weight per wheel costs 2/tenths in the 1/4 mile, 3/tenths 0-60 and 2.2mpg. also, if your car has less power then it will have an even larger effect. seems pretty obvious to me.

also, haven driven with many sets of wheels/tires of different weight, you can feel it, even just driving around on the street.

Larger wheels do move more mass to the outside - mass on the rim is much more important than mass at the center. Larger wheels do not inherently impact the overall diameter of the wheel/tire assembly.

Less weight in smaller wheels is a matter of diminishing returns. It's good, but I wouldn't run 14" wheels for the weight savings. The tire selection is shit.

this.

the largest tires/wheels in that chart are bigger in diameter, but you would expect cruising MPG to go UP with a taller effective gearing, but it still goes down because the wheels are heavier.

[visionaut]

FWIW - I just recently dropped a little over 6 lbs/wheel, and it's definitely something you can feel...