Login



KEEP M3POST ALIVE BY DOING YOUR TIRERACK SHOPPING FROM THIS BANNER LINK! 
Post Reply 

Thread Tools  Search this Thread 
10182011, 07:05 PM  #23 
Brigadier General
43
Rep 3,245
Posts 
Now we're having an intelligent discussion. Thanks for all the info on the subject... Keep discussing
__________________
mods: akra evo, dinan 3.45 diff, ess akra tune, dinan stg.3, Alcon bbk, HRE P40

Appreciate
0

10182011, 07:29 PM  #24 
Banned
22
Rep 1,109
Posts 
People fail to account for one major factor when comparing 18's to 19's. I think it has been agreed upon and the truth is that the mass furthest from the hub/center is the most detrimental mass on the wheel as it effects moment of inertia and thus acceleration/stopping of the wheel.
18s would clearly be better given the same mass if they ran on no tires, however keeping total diameter the same, 18's will have thicker sidewall tires that have a mass of 24 pounds heavier per tire and this mass is at the very furthest point from the center, making the wheel mass itself the much less detrimental aspect of the rotational mass. All being equal if you found a tire that weighed the same in 18 as 19 than yes lighter 18inch wheels would be better. To make up for the extra pounds in sidewall at the very furthest distance from the center, you have to makeup that weight in the 18's so that they cannot just be lighter but needs to be signifigantly lighter per wheel to make up for the difference and start to show any benefit. 
Appreciate
0

10182011, 07:44 PM  #25  
GODXSPD
2
Rep 155
Posts 
Quote:
You underestimate the impact of the MoI compared to the impact of unsprung weight. The MoI impacts the vehicle in far greater proportion than the actual weight of the wheel / tire combination. Any reduction of wheel weight is comparatively reduced by the action of the spring and damper according to the standard equation of motion (vibrations simplified: F(t) = mass*accel + velocity*damping + susp_travel*springrate). By comparison, the MoI impacts the amount of work required to accelerate as follows: KE_rot = 1/2*MoI*w^2, where w is, essentially, the RPM of the wheel. All this isn't to say that lighter isn't better than not. But, just because you see 'lighter,' don't automatically assume better.
__________________
e90m / ssii / foxred / wedssport sa55m 19x9.5 et24 + 265/35r19 square / 15mm rear spacer / no chrome Last edited by CT; 10182011 at 07:52 PM. 

Appreciate
0

10182011, 08:30 PM  #26 
Major General
109
Rep 6,009
Posts 
assuming both rims are the same style. which on this car they are not.
__________________

Appreciate
0

10182011, 09:23 PM  #27  
Colonel
117
Rep 2,842
Posts 
Quote:
__________________
2014 Audi S6 * 2015 Audi S3 * 2015 Porsche Cayman GTS
Former 2011 BMW M3 owner 

Appreciate
0

10192011, 06:53 AM  #28 
GODXSPD
2
Rep 155
Posts 
This is all a game of assuming things to simplify the equation. Nobody here seems to care about getting precise, fullyderived answers  they just want a Yes or a No or a generalized statement to feel better. So I ballparked it. Of course there are assumptions there that are either wrong or incomplete.
__________________
e90m / ssii / foxred / wedssport sa55m 19x9.5 et24 + 265/35r19 square / 15mm rear spacer / no chrome 
Appreciate
0

10192011, 08:03 AM  #29  
Major General
109
Rep 6,009
Posts 
Quote:
__________________


Appreciate
0

10192011, 08:07 AM  #30 
Banned
41
Rep 1,680
Posts 
because there is better selection of tire and size for 18" track wheels.
unsprung weight and all that other crap is minimal at best. 18" for size availability and also the tires are cheaper than 19. 
Appreciate
0

10192011, 08:20 AM  #31 
Brigadier General
43
Rep 3,245
Posts 
That's what I think, it's more about price and selection, the performance criteria is minimal at best, but then again, I'll be trying to shave a few seconds off of my laptimes so this might be something to consider. Once again, the better driver trumps all these "mods"
__________________
mods: akra evo, dinan 3.45 diff, ess akra tune, dinan stg.3, Alcon bbk, HRE P40

Appreciate
0

10192011, 08:21 AM  #32 
GODXSPD
2
Rep 155
Posts 
That may be the case  I don't know for sure. But if we're talking track, like the title implied, I was thinking he'd be going aftermarket to really get into the big weight loss numbers.
__________________
e90m / ssii / foxred / wedssport sa55m 19x9.5 et24 + 265/35r19 square / 15mm rear spacer / no chrome 
Appreciate
0

Post Reply 
Bookmarks 
Thread Tools  Search this Thread 

