Unsprung Weight: Will five lbs / wheel make a diff?
 

I'm thinking of some lighter wheels for my daily driver BMW 325i which I occasionally track. Will losing five lbs per wheel/tire on each corner be felt in terms of handling, acceleration, braking? Is it even worth it? The wheel size and tire size would stay the same.

Thanks...

I don't know the weight diff. between 50 & 55 series tires, but when I went from 50s to 55s on my Datsun Z years ago the diff was VERY pronounced - and not good!

Post in the Tech section, sit back with some popcorn and enjoy.

My personal opinion is for a street/daily driver no biggie. Also depends on the general condition of your local roads. Lots of potholes or good condition.

Buy the wheels and toss em on for track days.

Absolutely worth it. Reducing unsprung weight is one of the single most effective ways to better performance in all areas. The only thing better is reducing rotating unsprung weight, which is what you would be doing. Five pounds per wheel, when stated as a percentage of wheel weight, is significant. You are actually getting a "two for one" deal when lightening wheels, because you are also reducing rotating weight. That has as much of an affect as reducing unsprung weight. In other words, losing five pounds per wheel will be an even more dramatic improvement than five pounds off of, say, a lower control arm. You are geeting the maximum benefit possible by taking the weight out of the wheels.

Absolutely!!
I noticed a huge difference in the feel of my SC just by eliminating the spacers required to run factory Cup wheels. My replica Fuchs weighed about the same as the Cups. No change in tire size at 205/55/16 and 245/45/16.

Yup, lighter gyroscopes vs heavier gyroscopes.

I tried to get that point across numerous times on the various "ROTA FOX" wheel threads with no response whatsoever - couldn't even learn what the "ROTA FOX" wheels weigh (I'm sure significantly more than Fuchs).

I got to be well disliked by the fanatically enthusiastic true believers on those threads by posting factual info and asking a lot of questions :confused::(.

Somewhere around 19 and 21 lbs, about the same for the same size BBS wheels. It was hidden deep inside one of the Rota threads, I'll probably never find it again.

Being an outsider, I'm picking up on some snobbery in those threads. People ask for test data on the new Rotas, where's the test data on the 20+ year old Fuchs to compare it to...... And the PCA's rule of only allowing T(umulat)V approved wheels....well, that's idiotic. It's like the German Porsche Club only allowing DOT approved wheels. Do people think that you can't buy cheap wheels in Germany and that all the after market items available are somehow up to FIA specs?

They're inexpensive, and they look good. Occasional track days, sure....but let's not get crazy on either side of the scale.

Thanks guys. I realize it's a good thing as far as overall weight is concerned, I just wanted to know if I would be able to feel it in the "seat of the pants". Anybody else who would be able to say first hand, having experienced a similar weight reduction, please chime in.

..

Well, we all have our opinions.

Proper tool for the job, I say. I honestly believe they're not junk, but I wouldn't trust them like some other brands. Plus if you were really that sort of user, you should be x-raying your wheels anyways.

You can't ignore the fact that they're just over 1/3 of the price of a set of BBS in the same sizes. This is both a pro and a con. You'd be silly to expect the same sort of over-the-top quality, but subydude DOES stand behind his products and will provide replacements. He and Rota have been in business together for a while and their wheels haven't put him out of business yet.

I don't know if you'll notice 5 pounds, but I put some 40 lb wheel & tires on our 328i in place of the 56 and 60 lb wheel & tires and it felt like another 20 hp especially at lower speeds.

Wheels are not just unsprung weight, they are mass that needs to be rotated as well. The driven wheels feel the effect of the mass.

I remember some time back reading a tuning magazine where they had some modded honda on the dyno. Car put down about 150whp. Then they put on bigger heavier wheels/tires (same rolling circumference) and the car lost 8-10 whp.......

Remember, bigger brakes also need to be accelerated, it is not just wheels...

Cheers

125shifter,

I think I may have seen your thread on one of the Bimmer boards and was shocked. I can't believe the weights of the wheel/tire combos that BMW is putting on the new cars.

I don't know if you will be able to tell the difference on the street, but if you track your car you will definitely notice 5 #/wheel. I went from a 17# wheel to a 14# on my 240Z, and there was a noticable, seat of the pants difference, not a minor difference on a datalogger. It was most noticable under braking.

John

The Miata guys have a table of wheel weights and tire weights as a sticky on their website. They are more fanatical about weight than us Pelicanheads. A couple of pounds makes a big difference when you've got 115 hp from a rip-snorting 1.6 litre engine.

dropping 17lbs of rotational mass is like dropping 100 lbs of unsprung weight

What's that 17:1 based on?

"I got to be well disliked by the fanatically enthusiastic true believers on those threads by posting factual info and asking a lot of questions"

- You'e a Bling Killer!

Ooooh, woe is me, Iyam,Iyam! - forged by slavery to my spartan ways, a mere lightweight with relentless pressure put upon me by the shining and glittering hordes who have cast me out with great gravity! :( :rolleyes: