I just changed my 19" to 17" wheels, the car feels to accelerate better now.

Has anyone got fact information on the effect of increased rotating and unsuspensed mass?

Ive heard that increasing 1 rotating kg is virtually the same as putting 3,5 to 5 kg on car. So using for example super leggeras instead of heavier rims, you save about 5 kg per each wheel and 70 to 100 kg virtually. Is this right?!?

I take it that the rolling radius of wheel and tyre is the same in both cases? You can check that using the converter here if you are not sure.

The rolling radious is nearly the same in my case (235/37/19 & 225/45/17). But on 19" mass is located farther off the rotation axle. That makes the moment of inertia bigger. bigger inertia recuires more power (and momentum) to get accelerated.

The guestion is how much does heavier rims effect? i mentioned earlier what ive heard on effect and im curious to get more opinnions.

thanks.

The majority of the car's weight is supported by the car's springs and this is known as sprung weight. The weight of all the components not supported by the springs (including the springs themselves) is known as "unsprung weight". The unsprung weight therefore typically includes: wheels, tyres, brakes, springs and half the weight of the shock absorber and the suspension links. Note that those parts that connect the vehicle body to the suspension (or form part of it) only count as half weight because one end is part of the sprung weight of tnhe car, while the other end is unsprung.

Increasing the ratio of unsprung to sprung weight, especially when it is achieved by reucing the unsprung weight, will improve not only the general handling of the car but also traction (grip) in acceleration and braking, particularly on bumpy road surfaces. The reason being that weight creates inertia and the less inertia the unsprung components have, the more easily will the springs keep the tyres in contact with the road.

To answer your question. 5kg saved = 1 bhp gained. I thought unsprung weight came in at x2 (ie 2.5kg saved = 1 bhp), although I stand ready to be corrected.

Oh and sorry for rambling on.

I think 2,5 saved could be close to 1 bhp. In my case the f.. heavy 19" inch rims were 23 kg with tires. So putting back the orginal 17kg rims saved 4 times 6 kg. that makes 24kg total weight. divide it with 2,5 makes nearly 10 bhp powerloss while running 19". My "ass-dyno" told me about the same. And the effect was like having extra 84 to 120 kg on car (F=MA) . I wonder when some car mag tests this matter? it would be very interesting.

They won't bother. Its a fairly obscure area and, lets face it, who'd be interested. Most car mags will test standard cars which are what they are, it's very rare that they will do a back to back comparison between 2 otherwise identically specced cars. The only comments I have seen have been about how bigger alloys ruin ride quality. The modified car mags will on the whole avoid performance variations with different wheels, they will be more intersted in what they look like.

To most people it's the looks that are important not the wieght savings. Me, I'm just wierd

Ill join your clan...

You're welcome

Your thread has inspired me. By my calculation if I take a chainsaw to my 68bhp Fabia and lose 480kg I should be able to keep up with you lucky people in your vRSes.

I'll start with the doors, tailgate windows and the other three seats...

I have actually read an article on cuttin a nissan into pieces and running 1/4 mile times. First one that they did was get rid of 19" to 15". In the end there was only the floor of the car motor and wheels....

I've read it as well and have been trying to find it all afternoon. but couldn't find the link.

Different forum to when I first saw it but....here it is:

http://www.automotiveforums.com/vbulletin/t94570.html

Excellent, thanks

So saving 13lbs (=5,9kg?!?) a corner gets you 0,5 sec faster 0-60mph. interesting!