OZ Alleggerita
05-20-2009, 03:23 PM
#27
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05-20-2009, 05:19 PM
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The marginal improvement, both in rotational mass/acceleration and reduction in unsprung weight, is in the realm of noise when going from 18.4# to 17.2#...not only that, but even the true "experts" don't seem to agree on how to really quantify it...
As far as acceleration, the extra rotational inertia of a heavier wheel or tire contributes to the effective linear inertia of the car, just like extra weight in the cabin would, but, the theoretical absolute upper limit is 1 pound more at the surface of the tire tread contributes to inertia as if it were 2 pounds in the cabin, contrary to the old racer's myth. If you're talking about the wheel itself, the farthest out the weight can be added is the rim. So, for an 18" rim with a 25" tire, the "bonus" weight drops from a maximum of 1 pound per pound to about 1/2 pound per pound. The bonus weight is less than that if it's not all concentrated at the lip. So a realistic estimate of how much "total weight savings" you're going to realize for your money, multiply the weight loss from wheels by about 1.3-1.4. So going from an 18.4# wheel to a 17.2# wheel has the benefit of dropping less than 7lbs from the car, in terms of accleration. This is based on data that I've been given by true propeller heads. btw, the only other really credible estimate I've ever seen regarding reduction in rotational mass vs. "cabin weight" is in Herb Adams' book, where he gives a 1:3 ratio, at the axle...which needs to be corrected for rim diameter and distance of total mass from axis, I believe.
No doubt, there are significant benefits in wheel control to be had in lowering unsprung weight...but even that benefit is muted on the super smooth surfaces of many autocross courses. I haven't been able to find any kind of credible estimate of a "number" derived from reducing unsprung weight.
Add in the inconsistency of the driver and, for most of us, we'll never be able to substantiate any value in going from an Ultraleggera to a Allegrita (how ghey are OZ wheel names, anyhow?
)
And, please, no one come back with the old "if you lost a jacket by a couple of hundreths, you'd wish you had the lighter wheels" or something similar, 'cause the "perfect" autocross run is probably only 80-85% of perfect
As far as acceleration, the extra rotational inertia of a heavier wheel or tire contributes to the effective linear inertia of the car, just like extra weight in the cabin would, but, the theoretical absolute upper limit is 1 pound more at the surface of the tire tread contributes to inertia as if it were 2 pounds in the cabin, contrary to the old racer's myth. If you're talking about the wheel itself, the farthest out the weight can be added is the rim. So, for an 18" rim with a 25" tire, the "bonus" weight drops from a maximum of 1 pound per pound to about 1/2 pound per pound. The bonus weight is less than that if it's not all concentrated at the lip. So a realistic estimate of how much "total weight savings" you're going to realize for your money, multiply the weight loss from wheels by about 1.3-1.4. So going from an 18.4# wheel to a 17.2# wheel has the benefit of dropping less than 7lbs from the car, in terms of accleration. This is based on data that I've been given by true propeller heads. btw, the only other really credible estimate I've ever seen regarding reduction in rotational mass vs. "cabin weight" is in Herb Adams' book, where he gives a 1:3 ratio, at the axle...which needs to be corrected for rim diameter and distance of total mass from axis, I believe.
No doubt, there are significant benefits in wheel control to be had in lowering unsprung weight...but even that benefit is muted on the super smooth surfaces of many autocross courses. I haven't been able to find any kind of credible estimate of a "number" derived from reducing unsprung weight.
Add in the inconsistency of the driver and, for most of us, we'll never be able to substantiate any value in going from an Ultraleggera to a Allegrita (how ghey are OZ wheel names, anyhow?
)And, please, no one come back with the old "if you lost a jacket by a couple of hundreths, you'd wish you had the lighter wheels" or something similar, 'cause the "perfect" autocross run is probably only 80-85% of perfect
05-21-2009, 09:29 AM
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Where is the bang for the buck analysis?
With Mike's analysis I would say the Rota P1 is the easy winner.
If you are going to go with a more expensive wheel why not get the lightest that your budget can afford. Between the two OZs an extra 200 bucks (less than one race tire) for wheels that are a pound lighter each is a no brainer, if you got the cash. I look at many things now in tire dollars.( i.e.$300 increments)
At this point in time please make the sound of beating of dead horse
With Mike's analysis I would say the Rota P1 is the easy winner.
If you are going to go with a more expensive wheel why not get the lightest that your budget can afford. Between the two OZs an extra 200 bucks (less than one race tire) for wheels that are a pound lighter each is a no brainer, if you got the cash. I look at many things now in tire dollars.( i.e.$300 increments)
At this point in time please make the sound of beating of dead horse
05-21-2009, 10:59 AM
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Where is the bang for the buck analysis?
With Mike's analysis I would say the Rota P1 is the easy winner.
If you are going to go with a more expensive wheel why not get the lightest that your budget can afford. Between the two OZs an extra 200 bucks (less than one race tire) for wheels that are a pound lighter each is a no brainer, if you got the cash. I look at many things now in tire dollars.( i.e.$300 increments)
At this point in time please make the sound of beating of dead horse
With Mike's analysis I would say the Rota P1 is the easy winner.
If you are going to go with a more expensive wheel why not get the lightest that your budget can afford. Between the two OZs an extra 200 bucks (less than one race tire) for wheels that are a pound lighter each is a no brainer, if you got the cash. I look at many things now in tire dollars.( i.e.$300 increments)
At this point in time please make the sound of beating of dead horse
Chris, I'm looking forward to seeing everyone at Blytheville. It'll be a strong class, I'm destined to finish in the lower half, but at least edj will have warm tires...
05-21-2009, 01:11 PM
#33
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05-21-2009, 01:57 PM
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(My best attempt at smack talk... weak sauce, I know...)
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