Weird wheel theories

[JAXRS6]

I'm still shopping for wheels & came across two stories that are hard to believe.

One came from a fellow Audiphile who said light weight wheels caused the RS6 to have problems "hooking up" (traction) at launch. He didn't know if the car in question was chipped.

The other came from a wheel pro at a prestigious tuner. He said lighter wheels caused the RS6 to have some vibration at 60-70 mph. Something to do with calipers but I don't recall exactly. I mentioned that many A6 owners have complained about a similar vibration, and he thought lightweight wheels might be the culprit there too.

Do either of these make sense?

[Nordschleife]

I think they are blowing smoke - ever picked up a race car wheel? You are far too polite with these people!

R+C

[Nordschleife]

Still shopping?

Those Sportec rims are way cool, as people a lot younger than me say.

or be outrageous and fit the Gallardo rims, actually pretty good looking.

R+C

[JAXRS6]

Originally posted by Nordschleife
I think they are blowing smoke - ever picked up a race car wheel? You are far too polite with these people!

R+C

Maybe, but the tuner guy lost a sale to me by making the comments that he did. Why would he blow that kind of smoke if it wasn't true?

He's with Champion Motorsport & commented that the RS6 was "designed" to accommodate "heavier wheels." He estimated the 18" OEM wheels weigh 30-32 lbs each & I was considering another 18" that weighs 21 lbs. He thought 24-25 lbs might work OK -- light enough to improve handling a bit, but heavy enough to avoid too much vibration.

[Nordschleife]

Champion Motorsport is the company that did NOT baseline measure their R8 when taking delivery from Audi. So they wasted an entire season. Absolutely lovely guys, "Diamond Geezers' as we say on this side of the pond, but...

Steam engines have heavy wheels...... er maybe I should not have said that

R+C

[JAXRS6]

R + C, are you saying that, assuming adequate strength & proper fitment, lighter wheels are always better, no matter what?

[rs6_newyork]

I had vibration problems with BBS RGRs, which are pretty close to the lightest you can get, despite multiple rebalance/road force of all 4 tires. The vibration was a combination of a steering shimmy that peaked at ~50mph, and a whole chassis/car vibration (the kind that shakes anything in the cupholder) that had various peaks, but an annoying one at ~80mph.

In the end I eliminated 95% of the problem by running lower pressure than I wanted to... from the moment I had these wheels with PS2s i was running 40psi cold - to lower risk of pothole edges busting a sidewall. The fronts would get pretty hot on the highway (from the engine) so pressure would rise. I took them down to 36psi when hot, probably 32psi cold, and the problem is all but eliminated.

It wasn't much fun while it lasted though. I was ready to go back to stock heavier rims.

[SpinEcho]

Originally posted by rs6_newyork
I had vibration problems with BBS RGRs

Were they 18's or 19's?

[rs6_newyork]

I went from stock (18") to 19s.. they were still much lighter than stock though.

[SpinEcho]

The lower the tire profile and the larger the rim, the tougher to keep in balance...

[rs6_newyork]

maybe but it wasn't 20"s, RS6 in euro has 19s as an option, nobody has problems with them i think.
Plus, I paid extra to road-force *and that didn't help*. The machine said the rims were all true, and after only one tire required shuffling round a bit, the variability force was within limits as well (~20lbs if i remember, whatever that means).
Yet it shook this heavy car like crazy at high PSI.
or maybe i have a glass ass.

[JAXRS6]

RE bigger rims causing balance problems:


Originally posted by rs6_newyork
maybe but it wasn't 20"s, RS6 in euro has 19s as an option, nobody has problems with them i think.

Good point. However, Audi OEM wheels always are heavy IMO, so that doesn't really answer the weight question about the impact of lighter wheels.:confused:

I don't think the tuner tire guy was saying the vibration was caused by imbalance. He mentioned something about the brake calipers, I think, but as said before I don't recall exactly what.

However, I think he did say the vibration was more in the steering wheel than the entire car.

[Aronis]

Very interesting story.

The ligher rims have less rotational enertia.

What keeps a bicycle upright and going straight is the rotational enertia, so it definely has an effect on the stability of a two wheel vehicle.

The same must be true for a four wheel vehicle, so a lighter tire/rim combo with less rotational enertia could lead to less 'stability' and make vibrations more prominant at normal driving speeds, where the benefit of the lighter tire/wheel would perhaps be seen/fealt at higher than normal speeds, ie race speeds.

Vibrations are the result of a 'resonance' in the moving parts of a car. If you change the resonance frequency of the entire motor vehicle you may produce the unwanted effect of a 'new vibration' without there needing to be a problem with a wheel/tire balance.

That is why such vibrations become noticible at certain speeds, and infact can disappear above that speed.

In my experience an out of balance wheel would produce a noticible vibration at any speed, rather than one resonance speed.

Just my two cents worth......

Mike

[Bauer]

I have BBS CH's in 19's with PS2's and have noticed that the ride is not as smooth as the stock set up.....it is not enough to bother me though.

[audirs6sport]

Originally posted by Aronis
Very interesting story.

The ligher rims have less rotational enertia.

What keeps a bicycle upright and going straight is the rotational enertia, so it definely has an effect on the stability of a two wheel vehicle.

The same must be true for a four wheel vehicle, so a lighter tire/rim combo with less rotational enertia could lead to less 'stability' and make vibrations more prominant at normal driving speeds, where the benefit of the lighter tire/wheel would perhaps be seen/fealt at higher than normal speeds, ie race speeds.

Vibrations are the result of a 'resonance' in the moving parts of a car. If you change the resonance frequency of the entire motor vehicle you may produce the unwanted effect of a 'new vibration' without there needing to be a problem with a wheel/tire balance.

That is why such vibrations become noticible at certain speeds, and infact can disappear above that speed.

In my experience an out of balance wheel would produce a noticible vibration at any speed, rather than one resonance speed.

Just my two cents worth......

Mike


agreed. btw, enertia => inertia =) Thanks for your bentley manual pics Mike. Def + & appreciations =)

[clam]

What keeps a bicycle upright and going straight is the rotational enertia, so it definely has an effect on the stability of a two wheel vehicle.


What keeps a bycycle going straight is the angle of the fork in front. Same as a car really. B/c of the angle, the front lifts up when you turn, and since it will always seek the lowest point, it will return to it's straight postion when there's no steering input.

But you are right about the 'upright effect'. A heavy wheel will act like a flywheel, smoothing out the vibrations. An AWD car, with a lot of extra rotational mass, will suffer from more vibration.
So just like a lightweight flywheel, lightweight wheels will have a negative effect on vibrations. But they will make the car more responsive.

I don't thing there's any way of fixing the problem, aside from increasing the rotational mass (like a heavier flywheel), but that's counter-productive. Balancing the whole drivetrain is out of the question.
Like said before, it should increase high-speed performance, but make the car 'rough' in everyday driving.

[Nordschleife]

I think this is a field where there is a huge difference between theory and practice.

For example, in practice, drive trains are balanced, the McLaren F1 engine and drivetrain is so perfectly balanced that it has no flywheel. I'm not sure but the Pagani Zonta may be the same.

Notice how F1 cars have very light, very small rims, the benefits gained from weight saving outweigh all the other disadvantages.

When people switch from 18" to 19" rims, they usually sacrifice some tyre sidewall depth, i.e. they have less rubber to act as a shock absorber when going over uneven surfaces. This should not be confused with changes induced by switching from heavy to light rims.

It is interesting that the Porsche GT3 Cup race car, which is designed to be comparatively inexpensive, does use expensive BBS Magnesium race wheels, in its homeland (not at Daytona, but thats a 'US regulations level playing field issue' rather than a practical reason), the need for reduced unsprung weight offsetting the added cost.

R+C

[Aronis]

I wonder how many rattles and vibrations a race car has????? Never drove one myself, but I suspect they don't give a quiet Cadillac Drive!

So lighter wheels on those cars produce the desired racing advantage, but they perhaps don't care about a vibration or squeek.

LOL

Mike

PS, sorry about the spelling error LOL ....

Inertia.....that's what happens when I am rushed :360:

[clam]

Notice how F1 cars have very light, very small rims, the benefits gained from weight saving outweigh all the other disadvantages.


The big tyres on an F1 car are actually about sidewall stiffness. B/c an F1 car produces so much downforce and lateral acceleration, coupled with a very hard suspension, the sidewalls need to be flexible.
A sidewall that is too stiff will pass all these forces onto the contact patch. Especially on very wide tyres.
There are also advantages in durability, handling (gradual grip loss @ breakaway point), more room for the engineers to play,...

The rim is actually regulated to max. 13inch to controle the size of the brakes in F1. Otherwise the rim might be bigger. But not much.

The only advantage of low profile tyres is change of direction. And they do carry disadvantages. So it's not 'the bigger the better'. You'll notice that even unrestricted race tyres have a couple of inches of playing room.

[nene]

My 19" iForged do vibrate a little at around the same speed previously mentioned, and they are lightweight, however I believe this to be because of the 5mm required spacer. This was true on the S4 as well, and the vibration doesn't seem to be any different on a much heavier car.

[LU-RS6]

I just bumped into this thread via steve , very interesting as I'm looking to buy MTM bimoto's and many are telling me they are too heavy.

This thread seems to prove them wrong :nono:

http://www.mtm-online.de/imgneu/felgen/16_bildg.jpg

[Mr Balsen]

The Formula 1 cars have some rims because this is the rule. nothing else ! ! !

The lighter, the wheel, the better it is. And trust me, some companies are spending a lot of money for lighter wheels, in the race world for example. Not to mention carbon wheels which are very very light !!!!

Frederic

[Mr Balsen]

I just bumped into this thread via steve , very interesting as I'm looking to buy MTM bimoto's and many are telling me they are too heavy.

This thread seems to prove them wrong :nono:

http://www.mtm-online.de/imgneu/felgen/16_bildg.jpg

MTM wheel (in 19") are 14 kilos a piece. this is much lighter than an Audi Wheel (in 18").

Cheers,
Frederic

[LU-RS6]

MTM wheel (in 19") are 14 kilos a piece. this is much lighter than an Audi Wheel (in 18").

Cheers,
Frederic

Hi Frederic,


I phoned up Ingolstadt today and they said 17kg.


Anyway, what do you consider to be the best tire on these wheels for 80% track, 20% road. I'm only a bit afraid PS cups won't take the heavy weight of the RS6.

[paulhiggs]

I have a 2004 A6 2.7T S-Line which I have been told has vsame axle, caliper and brake assemblies as the RS6. I had 19" BBS LM wheels (19x8.5) with 245/35 tires. I think they are a 32mm offset, whatever, they are the exact BBS specification for the A6 2.7T.

Always had a steering vibration between 50 and 60mph which was still there (much lighter) above and just below. Audi said it was not the steering rack (although there was some service note - I had the right part).
Talked with BBS and made sure I had the right color alignment rings (green for 2.7T and RS6, silver for A6 2.8, 3.0 and 4.2)

I had the wheels balanced to 100mph and they were spot on, alignment was checked/rechecked etc etc and as good as the best alignment machine could get it. It was upsetting my driving so much I forked out $4000 for OEM wheels and tires from the local dealer. And now

No vibration, drives straight and smooth as an arrow.

I'll pull one off sometime soon and do a weight comparison (wheel + tire) between the BBS and the OEM wheel. I know the BBS wheels are lighter and stronger than anything OEM (I used to race cars and only used BBS)

[Mr Balsen]

Hi Frederic,


I phoned up Ingolstadt today and they said 17kg.


Anyway, what do you consider to be the best tire on these wheels for 80% track, 20% road. I'm only a bit afraid PS cups won't take the heavy weight of the RS6.

I will be able to check the weight of the MTM Wheel very soon at Ingolstadt. And will let you know the precise figure. And apologies in advance if I was not correct.

Frederic

[KK265]

Hi Frederic,


I phoned up Ingolstadt today and they said 17kg.


Anyway, what do you consider to be the best tire on these wheels for 80% track, 20% road. I'm only a bit afraid PS cups won't take the heavy weight of the RS6.

RS4 V8 wheels:
18" 12,3 kg
19" 14,5 kg
Official information

[hahnmgh63]

The BBS RS-GT wheels in 19x8.5 are only 22lbs (10Kg's.). And I do agree that on any performance car, a lighter wheel is better, that is if the lighter wheel is forged. A lighter cast wheel may mean catastrophic failure. I have the RS-Gt's in the black diamond finish and not only are they lighter and stronger, I get compliments on the good looks of them all the time. MGH

[Buff-Daddy]

The BBS RS-GT wheels in 19x8.5 are only 22lbs (10Kg's.). I have the RS-Gt's in the black diamond finish and I get compliments on the good looks of them all the time. MGH

That tends to happen with $4,000 rims...

[hahnmgh63]

Yea, but I got lucky, had been looking for a forged wheels for a while and found these used on Ebay and played the game and got them with decent S03's for $2800. Need to replace the S03's soon as they are not XL rated and very noisy but good grip. MGH

[LU-RS6]

I will be able to check the weight of the MTM Wheel very soon at Ingolstadt. And will let you know the precise figure. And apologies in advance if I was not correct.

Frederic


Allright thanks. I mean to use them on the Nordschleife only so the weight factor is of importance too.