Rims & Performance
12-21-06, 08:29 AM
#32
Keep a couple things in mind -
1) how important is 1/10th of a second in the 1/4 mile? Unless going with a wildly lighter tire/wheel combo, I've never seen wheels/tires make more than a 1/10th or two difference in the 1/4 mile...
2) how rugged do you really need? It's been my experience, a Volk or other lightweight wheel is just as fragile as any quality cast wheel... There are definitely CHEAP wheels out there - beware of these, but the stock wheels are most definitely the strongest (and heaviest)
If cornering is what you are looking for, tire selection is going to matter moreso than wheel diameter...
1) how important is 1/10th of a second in the 1/4 mile? Unless going with a wildly lighter tire/wheel combo, I've never seen wheels/tires make more than a 1/10th or two difference in the 1/4 mile...
2) how rugged do you really need? It's been my experience, a Volk or other lightweight wheel is just as fragile as any quality cast wheel... There are definitely CHEAP wheels out there - beware of these, but the stock wheels are most definitely the strongest (and heaviest)
If cornering is what you are looking for, tire selection is going to matter moreso than wheel diameter...
The ruggedness comments are making me think that once I buy rims, I will keep my stock tire assembly in the event I would ever have an issue.
Tire selection is definitely sounding significantly more key than I orginialy anticipated. I am discovering that tire weight varies like rim weight, and tire traction is vastly different from model to model.
I am loving the learning process...
Last edited by Blizzy; 12-21-06 at 08:39 AM.
12-21-06, 10:26 AM
#33
0.1s is not a big deal to me, but hearing accounts from forum members claiming a noticeable difference in "feel" at take-off is most disconcerting. I am trying very hard to only enhance the feel of my car.
The ruggedness comments are making me think that once I buy rims, I will keep my stock tire assembly in the event I would ever have an issue.
Tire selection is definitely sounding significantly more key than I orginialy anticipated. I am discovering that tire weight varies like rim weight, and tire traction is vastly different from model to model.
I am loving the learning process...
The ruggedness comments are making me think that once I buy rims, I will keep my stock tire assembly in the event I would ever have an issue.
Tire selection is definitely sounding significantly more key than I orginialy anticipated. I am discovering that tire weight varies like rim weight, and tire traction is vastly different from model to model.
I am loving the learning process...
So long as you go with a decent quality wheel that doesn't weigh a ton, you should be ok.
tires, oh my how tires affect EVERYTHING here... My favorite (and I have driven almost all of them out there) are the BFG KDW's (the new tread, or "2")
They are awesome, tire weight is reasonable for the size, and traction is amazing. They go in the wet too, they're actually surprising in wet weather (good).
There are better traction tires out there, BFG KD's, and maybe a couple others, but these are the best I've found for a high performance daily driven car...
12-21-06, 09:22 PM
#34
I am making a few basic assumptions:
(1) The stock 16" chrome rims weigh 22lbs. This is the weight of more recent, but similar, OEM IS300 wheels. I am assuming the 96 SC wheel is the same.
(2) If a small slice of the OEM rim was made, the weight would be balanced by a fulcrum 75% of the way from the center of the rim to the outer edge of the rim.
(3) If a small slice of the aftermarket rim was made, the weight would be balanced by a fulcrum 85% of the way from the center of the rim to the outer edge of the rim. This number is higher is due to the more extreme perimeter-weighting of the lighter rim.
(4) If a small slice of the tire was made, the weight would be balanced by a fulcrum 80% of the way from the inside of the tire to the outside of the tire.
Now, the math:
(1) The center of mass of a small slice of the entire tire assembly needs to be determined. This number is critical to the equation for rotational inertia. A weighted average must be calculated.
Add the following...
[(radius of rim)*(wheel % perimeter weighted)*(rim weight)] and
{[(radius of tire - radius of rim)*(0.8)]+(radius of rim)}*(tire weight)
Then, divide by the total weight of the tire and rim.
The result is the radius from the center of the tire assembly to the center of mass of a small slice of the entire tire assembly.
(2) Rotational Inertia = (mass of tire and rim)*(radius of center of mass)*(radius of center of mass)
-------------
The Test
-------------
(1) OEM assembly:
- 16"x7" chrome Lexus rims (22lbs)
- Bridgestone Potenza RE93 225/55/16 (26lbs)
[(8in)*(.75)*(22lbs) + [(4.85in)*(.8) + (8in)]*(26lbs)] / [48 lbs] = 9.17 in
I = mr^2 = (48lbs)*(9.17in)*(9.17in) = 4036 lb*in^2
(2) Aftermarket Performance assembly:
- 18"x8" Volk CE28N rims (14.7lbs)
- Continental ContiSportContact-3 245/40/18 (21 lbs)
[(9in)*(.85)*(14.7lbs) + [(3.85in)*(.8) + (9in)]*(21lbs)] / [35.7 lbs] = 10.25in
I = mr^2 = (35.7lbs)*(10.25in)*(10.25in) = 3750 lb*in^2
(3) Reduction in Rotational inertia from OEM to aftermarket:
(4036 - 3750) / (4036) = 7%
----------------------------------
Please note that these numbers are all based on some assumptions that may be terribly off. I am certainly willing to adjust the calculations if anyone can make claim to my numbers being way off (75%, 85%, 80%, 22lbs, etc). My weights were acquired from major retail websites. The Continental tires were not chosen specifically for their performance, but rather for their low weight. I am interested to see how a better performing, slightly heavier tire will affect the numbers. I am also interested to plug in wider rims, smaller rims, and everything in between. It should be interesting. For now, it is 12:15 and I have math finals to administer and grade all day tomorrow.
Conclusion:
A careful rim/tire selection can not only improve the look, traction, and handling of your car but also decrease the rotational inertia of the wheel assembly, effectively giving off-the-line performance a slight enhancement. Class dismissed.
Last edited by Blizzy; 12-21-06 at 09:26 PM.
12-21-06, 10:53 PM
#35
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12-22-06, 08:29 AM
#36
Fluctuations in tire weight have more an affect on rotational inertia than rimweight because the tire is further away from the center of the tire.
Some calculations:
Stock 16's w/stock Potenza RE93 - 4049 lb*in^2
Volk CE28N w/ContiSport 3 - 3755 lb*in^2 (-7.3%)
Volk CE28N w/BFG KD - 4613 lb*in^2 (+14%)
Volk TE37 Magnesium w/Contisport 3 - 3662lb*in^2 (-9.5%)
Volk TE37 Magnesium w/BFG KD - 4523 lb*in^2 (+11.7%)
The Magnesium wheels weigh only 12.9lbs at 18"x8.5", but cost about $1k more. The gain in rotational inertia is a mere 2.2%. Personally, the added cost does not justify the utility.
The BFG KD tires weigh 27lbs. A good handful of less impressive, but still decently-performing performing tires are in the 23lb range with yields very similar numbers to the stock assembly in conjunction with a lightweight rim.
A heavy, 20-25lb 18-19" rim will raise the rotational inertia stats over 5000 lb*in^2. There are accounts of these rims making a noticeable difference in feel of the car. One could assume that a 20% or greater difference in rotational inertia at the wheels has a perceptible difference. That is to say that combinations within the 20% range of stock likely have little -if any- perceptible difference in feel.
This is all consistent with the account in this post:
- lighter, forged rims with decent performance tires may offer slight variance in rotational inertia (+/- 15%), but the difference is largely imperceptible.
- heavier, larger, cast rims will offer variances to rotational inertia (>20%) significant enough to offer a noticeable difference.
Some calculations:
Stock 16's w/stock Potenza RE93 - 4049 lb*in^2
Volk CE28N w/ContiSport 3 - 3755 lb*in^2 (-7.3%)
Volk CE28N w/BFG KD - 4613 lb*in^2 (+14%)
Volk TE37 Magnesium w/Contisport 3 - 3662lb*in^2 (-9.5%)
Volk TE37 Magnesium w/BFG KD - 4523 lb*in^2 (+11.7%)
The Magnesium wheels weigh only 12.9lbs at 18"x8.5", but cost about $1k more. The gain in rotational inertia is a mere 2.2%. Personally, the added cost does not justify the utility.
The BFG KD tires weigh 27lbs. A good handful of less impressive, but still decently-performing performing tires are in the 23lb range with yields very similar numbers to the stock assembly in conjunction with a lightweight rim.
A heavy, 20-25lb 18-19" rim will raise the rotational inertia stats over 5000 lb*in^2. There are accounts of these rims making a noticeable difference in feel of the car. One could assume that a 20% or greater difference in rotational inertia at the wheels has a perceptible difference. That is to say that combinations within the 20% range of stock likely have little -if any- perceptible difference in feel.
This is all consistent with the account in this post:
- lighter, forged rims with decent performance tires may offer slight variance in rotational inertia (+/- 15%), but the difference is largely imperceptible.
- heavier, larger, cast rims will offer variances to rotational inertia (>20%) significant enough to offer a noticeable difference.
12-22-06, 08:56 AM
#38
On the contrary, a lightweight 15" rim with a high performance tire can cut rotational inertia over 25%. I assume the added snappiness of the car off the line could be felt. I also presume the loss of traction in cornering would be quite easy to notice.
12-22-06, 10:03 AM
#39
I can't argue with this. I think the argument made above is essentially that a lightweight 18"rim can keep you close enough to stock inertia that the performance losses will not be perceptible.
On the contrary, a lightweight 15" rim with a high performance tire can cut rotational inertia over 25%. I assume the added snappiness of the car off the line could be felt. I also presume the loss of traction in cornering would be quite easy to notice.
On the contrary, a lightweight 15" rim with a high performance tire can cut rotational inertia over 25%. I assume the added snappiness of the car off the line could be felt. I also presume the loss of traction in cornering would be quite easy to notice.
I really like one of the first couple posts in this thread "what type of performance are you looking for - straight line or cornering" because it's so true, you can't do the best of one without sacrificing something of the other...
12-23-06, 08:08 PM
#40
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Gret post..Im looking at 18s for my sc300(soon to have a 2jz gte in it)...would aftermarket brake clearance be an issue..how would I figure the correct offset to use
12-24-06, 06:31 AM
#41
Lexus Test Driver
There are wheels out there that have a +45 offset (like mine) and are HIGH DISK (will fit big brakes) just like there are wheels out there with a +30 offset that are LOW DISK (won't fit big brakes)
04-04-08, 04:54 PM
#42
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Fluctuations in tire weight have more an affect on rotational inertia than rimweight because the tire is further away from the center of the tire.
Some calculations:
Stock 16's w/stock Potenza RE93 - 4049 lb*in^2
Volk CE28N w/ContiSport 3 - 3755 lb*in^2 (-7.3%)
Volk CE28N w/BFG KD - 4613 lb*in^2 (+14%)
Volk TE37 Magnesium w/Contisport 3 - 3662lb*in^2 (-9.5%)
Volk TE37 Magnesium w/BFG KD - 4523 lb*in^2 (+11.7%)
The Magnesium wheels weigh only 12.9lbs at 18"x8.5", but cost about $1k more. The gain in rotational inertia is a mere 2.2%. Personally, the added cost does not justify the utility.
The BFG KD tires weigh 27lbs. A good handful of less impressive, but still decently-performing performing tires are in the 23lb range with yields very similar numbers to the stock assembly in conjunction with a lightweight rim.
A heavy, 20-25lb 18-19" rim will raise the rotational inertia stats over 5000 lb*in^2. There are accounts of these rims making a noticeable difference in feel of the car. One could assume that a 20% or greater difference in rotational inertia at the wheels has a perceptible difference. That is to say that combinations within the 20% range of stock likely have little -if any- perceptible difference in feel.
This is all consistent with the account in this post:
- lighter, forged rims with decent performance tires may offer slight variance in rotational inertia (+/- 15%), but the difference is largely imperceptible.
- heavier, larger, cast rims will offer variances to rotational inertia (>20%) significant enough to offer a noticeable difference.
Some calculations:
Stock 16's w/stock Potenza RE93 - 4049 lb*in^2
Volk CE28N w/ContiSport 3 - 3755 lb*in^2 (-7.3%)
Volk CE28N w/BFG KD - 4613 lb*in^2 (+14%)
Volk TE37 Magnesium w/Contisport 3 - 3662lb*in^2 (-9.5%)
Volk TE37 Magnesium w/BFG KD - 4523 lb*in^2 (+11.7%)
The Magnesium wheels weigh only 12.9lbs at 18"x8.5", but cost about $1k more. The gain in rotational inertia is a mere 2.2%. Personally, the added cost does not justify the utility.
The BFG KD tires weigh 27lbs. A good handful of less impressive, but still decently-performing performing tires are in the 23lb range with yields very similar numbers to the stock assembly in conjunction with a lightweight rim.
A heavy, 20-25lb 18-19" rim will raise the rotational inertia stats over 5000 lb*in^2. There are accounts of these rims making a noticeable difference in feel of the car. One could assume that a 20% or greater difference in rotational inertia at the wheels has a perceptible difference. That is to say that combinations within the 20% range of stock likely have little -if any- perceptible difference in feel.
This is all consistent with the account in this post:
- lighter, forged rims with decent performance tires may offer slight variance in rotational inertia (+/- 15%), but the difference is largely imperceptible.
- heavier, larger, cast rims will offer variances to rotational inertia (>20%) significant enough to offer a noticeable difference.
I figured it might have to do with the weight being pushed out to the edges, so I decided to test a "control" senario. I kept the stock tires, but I applied some 16x7" Enkei RS+M rims to the equation which weigh only 14.8 lbs. Again, it comes out to be a higher rotational inertia. How can this be if the tire is the exact same and the rim weighs over 7 lbs less? I'm not sure if Blizzy is still on these forums, but I'd like his input since it seems he did a lot of reseach on this topic. I'm not trying to criticize him, but I'm just very interested in the subject. If these equations are true, it would seem that aftermarket lightweight rims, no matter how light, are actually bad.
