eduardo

Can one feel a 12lb weight difference in wheel+tire weight? I don't mean in the track
using a chronometer but under normal (spirited) driving conditions on good roads. For example,
what do you think would be the perceptual differences between 215/55x16 (39lbs, wheel+tire), 245/45x17(43lbs) and 245/45x17 (51lbs, same tire, heavier wheel) mounted on an A6 2.7T 6sp?

CraigB

I didn't notice a difference. The difference may come in suspension bits wearing out sooner.

CraigB

wmv

~8lbs. ie 40 lbs off of wheel and tire is worth ~320 lbs. Think about the effect of 2 160 lb passengers. Also suspension will be some what more compliant and able to follow irregularities in the road better. It is easy to save 10 lbs a corner, I went from S4 avus and P210 snow to BBS RK with stock BS and lost 10 lbs per corner, BBS RC would have been lighter but I like the open lug design and was willing to sacrifice a few lbs per corner for the looks. It will be very expensive to go lighter than BBS. I put Kinesis on my Carrera 17x8 frt is the same size as the RK's, they were twice as costly for a 2lb weight savings. Their advantage is custom offsets.

brianW

after rolling on my stock 15"s all winter, my car definitely "felt" quicker, and much more peppier..
now that ive got my elephant wheels back on (18x8 avant garde design S), the pep is gone.. im pretty sure that the a! wheels are much heavier than the stock wheels.. so much so that i think i may have developed a slight hernia problem while swapping them..
my next wheels are definitely going to be much much lighter.. forged perhaps.. magnesium maybe.. once i win the lottery of course

regards,
brian

Reggie3

But the performace of the response with 17" was great.

eduardo

Brian, what tire sizes are you using in the 15" and the 18"? Your problem may be composed by a diameter increase...

Pelican Pete

I've been wondering the same thing myself, and I asked a few friens who were physics or engineering majors in college. I haven't gotten any clear answers yet.

So I am especially curious how you arrived at the figure of 1lb of rotating mass being equivalent to 8lbs of "dead weight"

Thanks,
Peter
98.5 1.8tqms 1 bar

Pelican Pete

I was just chatting with a college buddy who is a high school physics teacher.

He's been telling me to save money since I met him.
Now he's throwing out physics equations to support his argument. :-)

I asked him about the wheel weight question.

And he told me that the Moment of Inertia (also called rotational intertia) is more dependent on radius than it is on mass.

The equation is:
I = 1/2 mr^2

To clarify (since I can't do superscripts online)
it is "One-half times the mass times the square of the radius"

Where I = the moment of inertia
m = the mass of the wheel (convert to kilograms)
r = the radius of the wheel (convert to centimeters)

For faster acceleration, you want a LOWER
moment of inertia.

However, I did get him to admit one thing to favor larger wheels...
This equation is based on a solid cylinder, not a spoked wheel in which the mass is variable with the distance from the center.

Of course, this friend was raised on Detriot muscle cars. So he told me to save my money on wheels and buy a '69 Dodge Charger 426 hemi instead of new wheels for my A4.
Hmmm... not a bad idea.

But he did convice me to keep my steel frame bike, and just upgrade the wheels to aluminum, instead of buying a lighter aluminum bike with steel wheels.

Enjoy !!
Peter
98.5 1.8tqms

mpc

Are you sure you're only supposed to take into account he radius of the wheel and not the radius of the wheel and tire? It's been 3 years since I've taken any physics (Chemical Engineering major) but from what I remember, you're supposed to measure the the entire radius with respect to acceleration. If this is so, the radius will be constant +- 1-3% (to account for minor differences in plus sizing). That being said, everything in the equation is constant except for the weight. So, more weight means a higher moment of inertia and less weight means a lower moment of inertia. You could get a heavier wheels/tire combo but go with a smaller radius tire to offset this but I wouldn't recommend that.

Pelican Pete

... that the bulk of the mass is concentrated in the wheel, not the tire.

I do agree with you. In fact, I was playing devil's advocate with my friend, hoping to get a scientific justification for buying larger wheels.

Like I said in my previous post, my friend was seriously trying to convince me to spend my money on an old muscle car, or a truck to pull my boat, instead of a new set of wheels for my A4. So he didn't explain it fully to me.
Plus we were chatting on AOL, so it was difficult to get all the details.

My friend did agree that the equation does not fully hold true because the mass of the tire/wheel combination varies depending on the distance from the center, and that the mass of the wheel is greater than that of the tire.

So I guess my original question still stands --
how do people figure that 1 lb of rotating mass is equivalent to 8 lb of "dead weight" ???

-- Peter