Weight of wheels
01-12-2017, 10:58 PM
#16
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01-12-2017, 11:20 PM
#17
Three Wheelin'
Thread Starter
Yea, you certainly don't want one that looks like this:
Its was the rt front tire from my friends 996 TT race car
ABS locked up on him. Good thing it didn't happen in T1 or 2
Its was the rt front tire from my friends 996 TT race car
ABS locked up on him. Good thing it didn't happen in T1 or 2
01-16-2017, 02:02 PM
#18
Rennlist Member
Since KE for rotation is 1/2Iw^2 and for Linear, its 1/2Mv^2 (or I/2M(wxR)^2, you can solve for "M" and get the relation that is the answer everyone seems to look for:
M=I/R^2
So, if you do the math, for a 15" rim, adding a lb to it is like adding 1.37lbs in the car....for a 16" rim, its 1.41lbs, for a 17" rim, its 1.47lbs........ all the way out to the tread of the tire, where that weight gets to be near 2x if it was sitting in the car. suddenly you see the effects clearly of what 5lbs per wheel savings equals as far as hp..... as mentioned that's 3hp on a 10:1 hp/weight car.
So the message here is, when you think about saving weight in the wheels , its effects are no more than about 1.5x as if that weight was sitting in the car (or being pulled out of the car)
The removing weight from the clutch, flywheel , etc, is more dependent on what gear you are in for its effects... that's another, similar, story.
01-16-2017, 05:29 PM
#19
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I dont think i mentioned it before, but its a pretty simple equation to equate rolling mass and its KE to the equivalent linear KE
Since KE for rotation is 1/2Iw^2 and for Linear, its 1/2Mv^2 (or I/2M(wxR)^2, you can solve for "M" and get the relation that is the answer everyone seems to look for:
M=I/R^2
So, if you do the math, for a 15" rim, adding a lb to it is like adding 1.37lbs in the car....for a 16" rim, its 1.41lbs, for a 17" rim, its 1.47lbs........ all the way out to the tread of the tire, where that weight gets to be near 2x if it was sitting in the car. suddenly you see the effects clearly of what 5lbs per wheel savings equals as far as hp..... as mentioned that's 3hp on a 10:1 hp/weight car.
So the message here is, when you think about saving weight in the wheels , its effects are no more than about 1.5x as if that weight was sitting in the car (or being pulled out of the car)
The removing weight from the clutch, flywheel , etc, is more dependent on what gear you are in for its effects... that's another, similar, story.
Since KE for rotation is 1/2Iw^2 and for Linear, its 1/2Mv^2 (or I/2M(wxR)^2, you can solve for "M" and get the relation that is the answer everyone seems to look for:
M=I/R^2
So, if you do the math, for a 15" rim, adding a lb to it is like adding 1.37lbs in the car....for a 16" rim, its 1.41lbs, for a 17" rim, its 1.47lbs........ all the way out to the tread of the tire, where that weight gets to be near 2x if it was sitting in the car. suddenly you see the effects clearly of what 5lbs per wheel savings equals as far as hp..... as mentioned that's 3hp on a 10:1 hp/weight car.
So the message here is, when you think about saving weight in the wheels , its effects are no more than about 1.5x as if that weight was sitting in the car (or being pulled out of the car)
The removing weight from the clutch, flywheel , etc, is more dependent on what gear you are in for its effects... that's another, similar, story.
I get the end result.What about the diameter of the wheel/rim when it comes to speed...any relationship top end, torque or otherwise?
Also, (getting real technical here) we all want more tire patch (width), but how does that affect straight speeds, hence the stickier tire.
This stuff may not make much difference in high hp cars. But lower hp and lightweight cars like SM...everything matters.
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The apex of performance, comfort & safety since 2011.
All new racewear available now→ traqgear.com
01-16-2017, 06:25 PM
#20
Rennlist Member
As far as diameter of the wheel and tire combo, and its relationship to Speed......... you mention "torque". what i think you mean is torque at the drive wheels. This gets into a "gearing" discussion, as larger diameter tires will shift the thrust curves around slightly, but there are generally trade offs that equalize out in the end.. sometimes, you can get more optimal gearing, as i did by going 1" larger diameter. again, its all about HP optimization for the speed ranges you operate in.
The effects are very close to what they are on high Hp cars as they are to lower HP cars, especiallly if the cars have the same HP.weight ratios, the effects will be the same (high hp or low hp)
However, say you want to compare the effects on a spec miata with 100rwhp and 2000lbs vs a 3000lb/300hp grand am car.
both saving 5lbs per wheel will have very close to the same effects performance-wise.
1. the 3000lb/300hp car would sacrifice 1% (3hp) of its power to driver the heavier rolling mass of the new wheels ( 5 lbs per wheel , or 20lbs which has the effect of 30lbs if siting in the car)
2. the 2000lb/100hp car would sacrifice 1.5% of its power to the 5lbs heavier wheels.
Since the wheel sizes of the lighter, lower HP car are generally smaller, you can easily see that the effect on the smaller car might actually be the same as the larger car for a proportional increase in wheel weight.
3. However, if we had a low hp 3000lb car, of 200hp, that Hp loss for the linear mass of 30lbs, would still equal 1% (2hp.)
so, basically, it hurts all cars close to the same.... it's a % of the added translational (weight in the car) weight and the overall cars weight that determines the loss (cost) of usable HP
it's really about the weight of the car. same weight, the effect of 1% weight gain, and 1% HP loss would be the same with any HP value. But , to answer your question, this exercise for a 2000lb car would be a 1.5% cost and if the HP was 100, thats 1.5hp.. if it's 200hp, thats 3hp.
The effects are very close to what they are on high Hp cars as they are to lower HP cars, especiallly if the cars have the same HP.weight ratios, the effects will be the same (high hp or low hp)
However, say you want to compare the effects on a spec miata with 100rwhp and 2000lbs vs a 3000lb/300hp grand am car.
both saving 5lbs per wheel will have very close to the same effects performance-wise.
1. the 3000lb/300hp car would sacrifice 1% (3hp) of its power to driver the heavier rolling mass of the new wheels ( 5 lbs per wheel , or 20lbs which has the effect of 30lbs if siting in the car)
2. the 2000lb/100hp car would sacrifice 1.5% of its power to the 5lbs heavier wheels.
Since the wheel sizes of the lighter, lower HP car are generally smaller, you can easily see that the effect on the smaller car might actually be the same as the larger car for a proportional increase in wheel weight.
3. However, if we had a low hp 3000lb car, of 200hp, that Hp loss for the linear mass of 30lbs, would still equal 1% (2hp.)
so, basically, it hurts all cars close to the same.... it's a % of the added translational (weight in the car) weight and the overall cars weight that determines the loss (cost) of usable HP
it's really about the weight of the car. same weight, the effect of 1% weight gain, and 1% HP loss would be the same with any HP value. But , to answer your question, this exercise for a 2000lb car would be a 1.5% cost and if the HP was 100, thats 1.5hp.. if it's 200hp, thats 3hp.
Makes sense...well sorta. I get the end result.
What about the diameter of the wheel/rim when it comes to speed...any relationship top end, torque or otherwise?
Also, (getting real technical here) we all want more tire patch (width), but how does that affect straight speeds, hence the stickier tire.
This stuff may not make much difference in high hp cars. But lower hp and lightweight cars like SM...everything matters.
I get the end result.What about the diameter of the wheel/rim when it comes to speed...any relationship top end, torque or otherwise?
Also, (getting real technical here) we all want more tire patch (width), but how does that affect straight speeds, hence the stickier tire.
This stuff may not make much difference in high hp cars. But lower hp and lightweight cars like SM...everything matters.
01-16-2017, 06:39 PM
#21
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As far as diameter of the wheel and tire combo, and its relationship to Speed......... you mention "torque". what i think you mean is torque at the drive wheels. This gets into a "gearing" discussion, as larger diameter tires will shift the thrust curves around slightly, but there are generally trade offs that equalize out in the end.. sometimes, you can get more optimal gearing, as i did by going 1" larger diameter. again, its all about HP optimization for the speed ranges you operate in.
The effects are very close to what they are on high Hp cars as they are to lower HP cars, especiallly if the cars have the same HP.weight ratios, the effects will be the same (high hp or low hp)
However, say you want to compare the effects on a spec miata with 100rwhp and 2000lbs vs a 3000lb/300hp grand am car.
both saving 5lbs per wheel will have very close to the same effects performance-wise.
1. the 3000lb/300hp car would sacrifice 1% (3hp) of its power to driver the heavier rolling mass of the new wheels ( 5 lbs per wheel , or 20lbs which has the effect of 30lbs if siting in the car)
2. the 2000lb/100hp car would sacrifice 1.5% of its power to the 5lbs heavier wheels.
Since the wheel sizes of the lighter, lower HP car are generally smaller, you can easily see that the effect on the smaller car might actually be the same as the larger car for a proportional increase in wheel weight.
3. However, if we had a low hp 3000lb car, of 200hp, that Hp loss for the linear mass of 30lbs, would still equal 1% (2hp.)
so, basically, it hurts all cars close to the same.... it's a % of the added translational (weight in the car) weight and the overall cars weight that determines the loss (cost) of usable HP
it's really about the weight of the car. same weight, the effect of 1% weight gain, and 1% HP loss would be the same with any HP value. But , to answer your question, this exercise for a 2000lb car would be a 1.5% cost and if the HP was 100, thats 1.5hp.. if it's 200hp, thats 3hp.
The effects are very close to what they are on high Hp cars as they are to lower HP cars, especiallly if the cars have the same HP.weight ratios, the effects will be the same (high hp or low hp)
However, say you want to compare the effects on a spec miata with 100rwhp and 2000lbs vs a 3000lb/300hp grand am car.
both saving 5lbs per wheel will have very close to the same effects performance-wise.
1. the 3000lb/300hp car would sacrifice 1% (3hp) of its power to driver the heavier rolling mass of the new wheels ( 5 lbs per wheel , or 20lbs which has the effect of 30lbs if siting in the car)
2. the 2000lb/100hp car would sacrifice 1.5% of its power to the 5lbs heavier wheels.
Since the wheel sizes of the lighter, lower HP car are generally smaller, you can easily see that the effect on the smaller car might actually be the same as the larger car for a proportional increase in wheel weight.
3. However, if we had a low hp 3000lb car, of 200hp, that Hp loss for the linear mass of 30lbs, would still equal 1% (2hp.)
so, basically, it hurts all cars close to the same.... it's a % of the added translational (weight in the car) weight and the overall cars weight that determines the loss (cost) of usable HP
it's really about the weight of the car. same weight, the effect of 1% weight gain, and 1% HP loss would be the same with any HP value. But , to answer your question, this exercise for a 2000lb car would be a 1.5% cost and if the HP was 100, thats 1.5hp.. if it's 200hp, thats 3hp.
01-16-2017, 07:07 PM
#22
Rennlist Member
Cool, thanks! Engineering and physics are just awesome. On the diameter, I did notice my shifts points would change with the difference in the diameter of the tires. I used to run two different brands of slicks on the 911 and their diameters were at least an inch difference, maybe more.
I noticed the same thing..... at Laguna/sears, used to redline right at start finish, then, with the 4% taller tire, just past it. same lap times, but I think i was pushing harder to hit the same RPM points with the taller tire, which actually made me a little faster. (as well as not having to shift at one prior redlne spot before braking)
cheers!
Mk