Reducing Unsprung Weight
09-09-2010, 10:17 AM
#1
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Reducing Unsprung Weight
Hey all, just a quick question regarding wheel weight and unsprung weight. I’ve been doing quite a bit of research while shopping for wheels recently, but haven’t been able to find a definitive answer to my question. I realize that reducing the weight of wheels, I’m inturn reducing the unsprung weight. This minimizes rotational inertia and can result in a multitude of performance enhancements.
My question though is regarding a statement made by Enkei:
“Every pound of unsprung weight savings has the effect of reducing overall vehicle weight by 20lbs.”
This seems to me to be pie in the sky type marketing hype. If this is true, then replacing my stock bullets (~27lbs) with the RPF1 (~19lbs) would affectively have the same performance effect as reducing the overall weight of my car by 640lbs.
8lbs weight difference x 20lbs = 160lbs (per corner) x 4 corners = 640lbs
That’s a HUGE claim and one I’m not sure I believe. I’m positive that I’ll gain a significant performance increase but is there a way to truly justify (or disprove) Enkei’s claims? It seems to me that the only way to do such a test would be to strategically remove 640lbs worth of “stuff” from my car, measure the performance gains (stopping distance, acceleration, lateral grip, etc) and then put all that “stuff” back into my car, swap wheels and do the same tests. Needless to say, that’s not something I can do all that easily.
Either way, I’m still very close to pulling the trigger on a set of 18x9.5 RPF1 and this really won’t factor into my decision… it’s really just based on pure curiosity.
My question though is regarding a statement made by Enkei:
“Every pound of unsprung weight savings has the effect of reducing overall vehicle weight by 20lbs.”
This seems to me to be pie in the sky type marketing hype. If this is true, then replacing my stock bullets (~27lbs) with the RPF1 (~19lbs) would affectively have the same performance effect as reducing the overall weight of my car by 640lbs.
8lbs weight difference x 20lbs = 160lbs (per corner) x 4 corners = 640lbs
That’s a HUGE claim and one I’m not sure I believe. I’m positive that I’ll gain a significant performance increase but is there a way to truly justify (or disprove) Enkei’s claims? It seems to me that the only way to do such a test would be to strategically remove 640lbs worth of “stuff” from my car, measure the performance gains (stopping distance, acceleration, lateral grip, etc) and then put all that “stuff” back into my car, swap wheels and do the same tests. Needless to say, that’s not something I can do all that easily.
Either way, I’m still very close to pulling the trigger on a set of 18x9.5 RPF1 and this really won’t factor into my decision… it’s really just based on pure curiosity.
09-09-2010, 12:19 PM
#2
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It's wildly optimistic as far as rotational inertia is concerned. High even if you say one lb/corner (times all 4) and compare the whole 4 against just 20. I'd be surprised if your 32 lb wheel weight savings would be worth any more than losing about 50 lb of sprung weight.
A ratio of 10:1 or 20:1 might be appropriate for a lb of flywheel mass on a deeply-geared car on short tires due to the far greater amount of rotational acceleration involved at the crank. But not out at the wheels.
Norm
A ratio of 10:1 or 20:1 might be appropriate for a lb of flywheel mass on a deeply-geared car on short tires due to the far greater amount of rotational acceleration involved at the crank. But not out at the wheels.
Norm
Last edited by Norm Peterson; 09-09-2010 at 12:24 PM.
09-09-2010, 12:48 PM
#3
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That's something which has been debated time and time again. Simply not true. I've seen enough testing done with different wheel sizes and weight to know that is a false statement, at least when it comes to how the vehicle performs. Didn't write this but good read anyway: When it comes to dynamic performance like your car’s handling and acceleration/deceleration, the general rule is the lighter and smaller, the better. Many people would have you believe that removing one pound of unsprung weight is equivalent to removing anywhere from 4-10 pounds from your overall vehicle. This is simply untrue; Weight is weight. However, your suspension will be able to react more quickly when there is less mass to be moved. The same goes for acceleration from a standstill. Reducing rotating mass also reduces the inertia that must be overcome to get the wheel rotating. On the other hand, adding heavier wheels will have opposite effects.
For this reason, those who want to focus solely on performance will generally seek out the absolute lightest, strongest wheels available, and more likely than not, end up paying a hell of a premium for them. This is all well and good since it is your money and not mine, but understand that on a car that spends more than half its life on city roads, there is a point of diminishing returns. Paying double the price to shave a couple of pounds is just not the most prudent decision you can make unless you are competitively tracking your car.
This is why I've stuck with the Steeda Ultralites. while not the lightest wheel at 26 lbs each, not the heaviest either for an 18x9.5. And I can replace them cheaply and easily when they get curb rashed or damaged.
For this reason, those who want to focus solely on performance will generally seek out the absolute lightest, strongest wheels available, and more likely than not, end up paying a hell of a premium for them. This is all well and good since it is your money and not mine, but understand that on a car that spends more than half its life on city roads, there is a point of diminishing returns. Paying double the price to shave a couple of pounds is just not the most prudent decision you can make unless you are competitively tracking your car.
This is why I've stuck with the Steeda Ultralites. while not the lightest wheel at 26 lbs each, not the heaviest either for an 18x9.5. And I can replace them cheaply and easily when they get curb rashed or damaged.
Last edited by Rubrignitz; 09-09-2010 at 12:51 PM.
09-09-2010, 03:43 PM
#4
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I bought a set of the new 18 x 90.5 Enkei PF01s for track only use and really appreciate the 6 or 7 pound weight saving over my Steeda Ultralites when I have to load them into the trunk and rear seat!
They also do feel more responsive on the track.
They also do feel more responsive on the track.
09-09-2010, 04:25 PM
#5
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I'll upgrade my wheels eventually but it won't be anytime soon. I may try to pick up one of those cheaper roush s/c's like yours sleeper, a vortech or something. I'll keep my wheels and just add more powa 
.
I would like to see a comparison of popular performance tire weights.
.I would like to see a comparison of popular performance tire weights.
09-09-2010, 09:13 PM
#6
6th Gear Member
I think you'd do better with an aluminum DS, slightly lighter wheels, smaller battery (are the AGM's lighter?), rear seat delete, etc.
09-09-2010, 09:27 PM
#7
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all right, this is what you need to do, seance HP is fixed and weight is known, go out and do 10 passes and average them. Then change out the lighter rims and tires, do 10 more passes and average them. Then you can simply do the calculations and see how much weight it would workout to be to make the same HP take the car across the line however faster then it was before. You can then present your finding on here and that will be our gold standard ratio for unsprung vs. sprung weight! All though you wil have to make sure the outer diameter of the drive tire remains the same or the math will get more complicated as you factor the changes in mechanical advantage.
09-10-2010, 07:33 AM
#8
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If you're going to test, you have to stick to testing. We're looking at differences that are really pretty small, and the rough correlations like 0.1 second per 100 lbs are simply too crude to be of value.
You need to either add weight back in to the "light wheels" case to match the "heavy wheels" ET/speed or remove weight from the "heavy wheels" case to match the "light wheels" results. And do this in a manner that is acceptable to whatever tech inspection requirements exist. I imagine that "loose weight" is not permitted at the dragstrip any more than it is at autocross, but secured weight like the spare perhaps is. Might be close enough to the 45 - 50 lbs I'm guessing will about match 32 lbs worth of wheel weight . . .
Norm
You need to either add weight back in to the "light wheels" case to match the "heavy wheels" ET/speed or remove weight from the "heavy wheels" case to match the "light wheels" results. And do this in a manner that is acceptable to whatever tech inspection requirements exist. I imagine that "loose weight" is not permitted at the dragstrip any more than it is at autocross, but secured weight like the spare perhaps is. Might be close enough to the 45 - 50 lbs I'm guessing will about match 32 lbs worth of wheel weight . . .
Norm
Last edited by Norm Peterson; 09-10-2010 at 07:36 AM.
09-10-2010, 10:00 PM
#9
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you sir are great at taking a simple joke and crushing it, Russian police style.
On a side note, what your suggesting to try completely goes against the trying to prove a ratio of the sprung vs unsprung. Sure adding static weight to the car is another piece of the pie, but not what we are really after. Unless he wants to use the stick on lead weight to the rims to make the wieght the same, then peel em all off. Also, the rough correlations is not what I was talking about. You have a measurement of exact feet, and the time it takes. The variables to calculate it also requires weight or HP to figure out the other. so if we used the runs with the heavy rims as a base line, to fix the HP variable, we then get what its theoretical change in weight would be. The difference between the theoretical weight that we calculated it would take to change the time, vs the actual weight removed by going lighter rims, would be the ratio I was referring to using. You way over simplified what I was hinting at, though I didn't feel like typing this paragraph last night to get all serious on my joke ;-)
On a side note, what your suggesting to try completely goes against the trying to prove a ratio of the sprung vs unsprung. Sure adding static weight to the car is another piece of the pie, but not what we are really after. Unless he wants to use the stick on lead weight to the rims to make the wieght the same, then peel em all off. Also, the rough correlations is not what I was talking about. You have a measurement of exact feet, and the time it takes. The variables to calculate it also requires weight or HP to figure out the other. so if we used the runs with the heavy rims as a base line, to fix the HP variable, we then get what its theoretical change in weight would be. The difference between the theoretical weight that we calculated it would take to change the time, vs the actual weight removed by going lighter rims, would be the ratio I was referring to using. You way over simplified what I was hinting at, though I didn't feel like typing this paragraph last night to get all serious on my joke ;-)
If you're going to test, you have to stick to testing. We're looking at differences that are really pretty small, and the rough correlations like 0.1 second per 100 lbs are simply too crude to be of value.
You need to either add weight back in to the "light wheels" case to match the "heavy wheels" ET/speed or remove weight from the "heavy wheels" case to match the "light wheels" results. And do this in a manner that is acceptable to whatever tech inspection requirements exist. I imagine that "loose weight" is not permitted at the dragstrip any more than it is at autocross, but secured weight like the spare perhaps is. Might be close enough to the 45 - 50 lbs I'm guessing will about match 32 lbs worth of wheel weight . . .
Norm
You need to either add weight back in to the "light wheels" case to match the "heavy wheels" ET/speed or remove weight from the "heavy wheels" case to match the "light wheels" results. And do this in a manner that is acceptable to whatever tech inspection requirements exist. I imagine that "loose weight" is not permitted at the dragstrip any more than it is at autocross, but secured weight like the spare perhaps is. Might be close enough to the 45 - 50 lbs I'm guessing will about match 32 lbs worth of wheel weight . . .
Norm
Last edited by JDWalton; 09-10-2010 at 10:10 PM.
09-11-2010, 09:07 AM
#10
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Norm's reasoning seems very logical to me. You set a benchmark with the heavy wheels. Then you put on the light wheels and make a run. Then you add "Sprung weight" aka the junk in the trunk until you get an equal time. Then you take the weight difference in wheels and compare that to the sprung weight which you added to the trunk. Then you have a ratio of unsprung to sprung mass.
A few other variables to consider:
1. Weight distribution - Adding all weight to the rear will provide better traction at the drag strip than adding it all to the front. To be scientific, the weight would need to be evenly distributed.
2. Moment of inertia of the wheels - I assume the moment of inertia would be mainly a factor of the wheel mass, but it also depends on how far the mass is away from the center of the wheel. A lighter but wider wheel might actually have the same moment of inertia as a stock wheel with skinny rubber because it has more mass further from the center of the wheel.
A few other variables to consider:
1. Weight distribution - Adding all weight to the rear will provide better traction at the drag strip than adding it all to the front. To be scientific, the weight would need to be evenly distributed.
2. Moment of inertia of the wheels - I assume the moment of inertia would be mainly a factor of the wheel mass, but it also depends on how far the mass is away from the center of the wheel. A lighter but wider wheel might actually have the same moment of inertia as a stock wheel with skinny rubber because it has more mass further from the center of the wheel.
