A discussion of unsprung weight w/ regard to wheels...
02-27-2003, 01:00 PM
#21
There's no question there'll be a difference-> [edit]
I'd guess that the average spirited driver would start noticing a difference at around 7lbs+.
My frame of reference for this is my old Accord (yeah baby!): I upgraded from 13" steelies to 15" Dial wheels, and with tires, they weighed about 7-9lbs more than the steelies. The difference was definitely noticeable:
- throttle response slightly softer
- handling response less quick
- braking required slightly more effort
Part of our difficulty is that it's so hard to quantify things like handling response - it may be subjective and discretionary. I offer that this was <i>not</i> a placebo effect, since at first, I couldn't figure out why my car was so much more responsive when I put the steelies back on (it took me a bit to suppose that it was the reduced unsprung weight and rotational mass).
Granted, this was for a given suspension setup (sport Tokico dampers and coils), but the throttle response change was undeniable. The net effect, however, was that while it felt more responsive with lighter wheels, the handling wasn't necessarily bad with the heavy wheels. I liked the way the car ran up the revs easier with the steelies, but the tires made the handling in general ****-poor with them on.
I don't know what the +/-lbs threshold of noticability on the TT is, but I'd suspect it's similar, or maybe even less for quattro.
Question for the more engineering-astute: Doesn't a small change in weight amplify when the wheel is rotating?
My frame of reference for this is my old Accord (yeah baby!): I upgraded from 13" steelies to 15" Dial wheels, and with tires, they weighed about 7-9lbs more than the steelies. The difference was definitely noticeable:
- throttle response slightly softer
- handling response less quick
- braking required slightly more effort
Part of our difficulty is that it's so hard to quantify things like handling response - it may be subjective and discretionary. I offer that this was <i>not</i> a placebo effect, since at first, I couldn't figure out why my car was so much more responsive when I put the steelies back on (it took me a bit to suppose that it was the reduced unsprung weight and rotational mass).
Granted, this was for a given suspension setup (sport Tokico dampers and coils), but the throttle response change was undeniable. The net effect, however, was that while it felt more responsive with lighter wheels, the handling wasn't necessarily bad with the heavy wheels. I liked the way the car ran up the revs easier with the steelies, but the tires made the handling in general ****-poor with them on.
I don't know what the +/-lbs threshold of noticability on the TT is, but I'd suspect it's similar, or maybe even less for quattro.
Question for the more engineering-astute: Doesn't a small change in weight amplify when the wheel is rotating?
02-27-2003, 01:05 PM
#22
AudiWorld Senior Member
Join Date: Jul 2002
Posts: 1,067
Likes: 0
Received 0 Likes
on
0 Posts
Right, there is a distinct difference, physically, as well as mathmatically....but don't ask me what
it is, I'm not an ME. Has to do with moment of inertia and all that. Less = better, with respect to acceleration.
02-27-2003, 01:24 PM
#26
AudiWorld Senior Member
Join Date: Nov 2001
Posts: 1,314
Likes: 0
Received 0 Likes
on
0 Posts
OK, I went looking on the web, found this uk study. Net effect was wheels added mass plus...
rotational inertia equivalent to 35 kg or 77 lbs. Taking some liberties by assuming this is the effect for our 21 lb wheels then the effect of wheel weight is:
(21 x 4) mass plus 77 lbs inertia equivalent = 161 lbs.
For a 26 lb wheel:
(26 x 4) mass plus 101 lbs inertia equivalent = 205 lbs.
Total difference is equivalent to 44 lbs of extra weight. Now I have assumed that the inertia is linear when compared to the wheel weight which probably isn't true. In fact inertia is dependent on the distribution of the weight from the hub. So potentially it could be on the order of maybe a 50-60 lb. equivalent weight gain as far as acceleration is concerned. It should be noted that the inertia effect is most pronounced in 1st gear and drops off as the car gains speed and the rotational acceleration slows.
Now for cornering on unsmooth surfaces it is extremely important.
Of course this is based on someone's homework assignment. Maybe I should email and see what his grade was.<ul><li><a href="http://www.tomstickland.co.uk/astra/homework/acceleration_model/model1.htm">Paper on acceleration factors/model</a></li></ul>
(21 x 4) mass plus 77 lbs inertia equivalent = 161 lbs.
For a 26 lb wheel:
(26 x 4) mass plus 101 lbs inertia equivalent = 205 lbs.
Total difference is equivalent to 44 lbs of extra weight. Now I have assumed that the inertia is linear when compared to the wheel weight which probably isn't true. In fact inertia is dependent on the distribution of the weight from the hub. So potentially it could be on the order of maybe a 50-60 lb. equivalent weight gain as far as acceleration is concerned. It should be noted that the inertia effect is most pronounced in 1st gear and drops off as the car gains speed and the rotational acceleration slows.
Now for cornering on unsmooth surfaces it is extremely important.
Of course this is based on someone's homework assignment. Maybe I should email and see what his grade was.<ul><li><a href="http://www.tomstickland.co.uk/astra/homework/acceleration_model/model1.htm">Paper on acceleration factors/model</a></li></ul>
02-27-2003, 01:26 PM
#27
New Member
Thread Starter
Join Date: Nov 2002
Posts: 498
Likes: 0
Received 0 Likes
on
0 Posts
I do like the look & the price... guess I've got some careful thinking to do about what I want.
I've got the 17" OEM forged 5-spokes which weigh 21.5 lbs... They're O.K., but I want a more aggressive (not to mention custom) look.
I REALLY like the look and price of the 17x7.5 Moda R6... better than anything else out there. But what's slowing me down (literally) is the fact that they weigh 26 lbs.
On the other hand, the looks & price of the OZ Superleggera 17x8 are a good second to the Moda, but it might be a nice compromise given that they only weigh 17 lbs.!
And hey, if this is the biggest decision I'm trying to make in my life right now, things must be good right? ;-)
I REALLY like the look and price of the 17x7.5 Moda R6... better than anything else out there. But what's slowing me down (literally) is the fact that they weigh 26 lbs.
On the other hand, the looks & price of the OZ Superleggera 17x8 are a good second to the Moda, but it might be a nice compromise given that they only weigh 17 lbs.!
And hey, if this is the biggest decision I'm trying to make in my life right now, things must be good right? ;-)
02-27-2003, 01:30 PM
#28
New Member
Thread Starter
Join Date: Nov 2002
Posts: 498
Likes: 0
Received 0 Likes
on
0 Posts
Cool. That makes perfect sense to me as I'm a mathematical type of guy.
Now I just have to decide what's more important to me... Looks and a little $$$ saved, or slightly compromised looks w/ better performance.
02-27-2003, 01:42 PM
#29
AudiWorld Super User
Join Date: Mar 2000
Posts: 6,175
Likes: 0
Received 0 Likes
on
0 Posts
I like the OZ wheels you mention.
link to wheel weights...<ul><li><a href="http://www.jimsnodgrass.com/wheels[1].html">http://www.jimsnodgrass.com/wheels[1].html</a</li></ul>
02-27-2003, 01:46 PM
#30
But, balance out the weight gain vs the lower profile and wider tire
These are both good things (IMO) when it comes to handling so even tho you gain a few pounds the handling will probably be better.
When I went from stock 5 spokes to my current 18x8s which weighed about 4 lbs more a piece I noticed better steering response and more stable handling. Do they have all the downsides associated with a heavier wheel? Of course, but not so much that it negates the other benefits.
When I went from stock 5 spokes to my current 18x8s which weighed about 4 lbs more a piece I noticed better steering response and more stable handling. Do they have all the downsides associated with a heavier wheel? Of course, but not so much that it negates the other benefits.