unsprung wheel weight
12-08-2004, 07:34 PM
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unsprung wheel weight
does anyone know how much each wheel pound adds to the total rotation weight. I heard each unsprung pound is multiplied by 4?? so your stock alloys on a 5th gen weighs 19lb's, if your going to get rims that weigh 22lb then thats 3 x 4 = 12 x 4 = 48 extra pounds. anyone know much about this?
also would it only go for the 2 wheels that are being driven by the motor? thanks
damon
also would it only go for the 2 wheels that are being driven by the motor? thanks
damon
12-08-2004, 08:18 PM
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Re: unsprung wheel weight (damonk)
This is gonna take a little explaining. Bear with me. But I know what I'm talkin' bout here. 
First of all the unsprung weight. It only affects handling. It DOES NOT affect acceleration. one unsprung pound = one sprung pound as far as accelerating your car goes. Unsprung weight consists of your wheel, the suspension arms, upright, whichever half of your shock is attached to the suspension (and not to the body), and about half the weight of your spring (although I've seen some recent research that suggests it's actually closer to about .45 times the spring weight).
How unsprung weight affects handling:
Imagine for a moment that when you go over a bump, your car doesn't move up at all. What happens? The wheel moves up and over the bump. On the front half of the bump, the bump accelerates the wheel upwards. On the back half, the spring on the car pushes the wheel back down to the ground. Now lets compare to different wheels that weigh different. The heavier wheel has more inertia. That means that it takes more force to change it's direction. If we have a certain spring, then that spring can only push it back down into contact with the ground so quickly. But if we use the lighter wheel (which has less inertia, and takes less force to change it's direction) then that same spring can push it back into contact with the ground much more quickly. The wheel only creates grip when it's contact with the ground, so the difference in the amount of time they are in contact with the ground is the difference in the amount of grip they provide. To put it differently, the lighter wheel contacts the ground more than the heavier wheel, so it creates more grip.
That's really really simplified, but that's basically what the big deal is with unsprung weight.
Rotational mass is something totally different. Basically, that's just the fact that when you are moving your wheel, you're not only moving it forward (like the rest of the car), but you're also spinning it at the same time. So it requires much more energy to accelerate. As for the conversion for THAT, I believe it comes out to be roughly 2x a non rotating mass. That is to say, that accelerating you 20lb spinning wheel is like accelating a 40lb mass straight forward. It's also not linear, which means that the effect becomes worse the faster you spin it.
There. I'm done. I'll try and clarify if it needs it.
First of all the unsprung weight. It only affects handling. It DOES NOT affect acceleration. one unsprung pound = one sprung pound as far as accelerating your car goes. Unsprung weight consists of your wheel, the suspension arms, upright, whichever half of your shock is attached to the suspension (and not to the body), and about half the weight of your spring (although I've seen some recent research that suggests it's actually closer to about .45 times the spring weight).
How unsprung weight affects handling:
Imagine for a moment that when you go over a bump, your car doesn't move up at all. What happens? The wheel moves up and over the bump. On the front half of the bump, the bump accelerates the wheel upwards. On the back half, the spring on the car pushes the wheel back down to the ground. Now lets compare to different wheels that weigh different. The heavier wheel has more inertia. That means that it takes more force to change it's direction. If we have a certain spring, then that spring can only push it back down into contact with the ground so quickly. But if we use the lighter wheel (which has less inertia, and takes less force to change it's direction) then that same spring can push it back into contact with the ground much more quickly. The wheel only creates grip when it's contact with the ground, so the difference in the amount of time they are in contact with the ground is the difference in the amount of grip they provide. To put it differently, the lighter wheel contacts the ground more than the heavier wheel, so it creates more grip.
That's really really simplified, but that's basically what the big deal is with unsprung weight.
Rotational mass is something totally different. Basically, that's just the fact that when you are moving your wheel, you're not only moving it forward (like the rest of the car), but you're also spinning it at the same time. So it requires much more energy to accelerate. As for the conversion for THAT, I believe it comes out to be roughly 2x a non rotating mass. That is to say, that accelerating you 20lb spinning wheel is like accelating a 40lb mass straight forward. It's also not linear, which means that the effect becomes worse the faster you spin it.
There. I'm done. I'll try and clarify if it needs it.
12-08-2004, 08:31 PM
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Re: unsprung wheel weight (BA5)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BA5 »</TD></TR><TR><TD CLASS="quote">Mechanical Engineering Undergraduate
GT Motorsports FSAE</TD></TR></TABLE>
well.. sounds like someone passed ME2202
GT Motorsports FSAE</TD></TR></TABLE>
well.. sounds like someone passed ME2202
12-08-2004, 09:09 PM
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Re: unsprung wheel weight (BA5)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BA5 »</TD></TR><TR><TD CLASS="quote">This is gonna take a little explaining. Bear with me. But I know what I'm talkin' bout here.
First of all the unsprung weight. It only affects handling. It DOES NOT affect acceleration.</TD></TR></TABLE>
are you sure you know what your talking about?
First of all the unsprung weight. It only affects handling. It DOES NOT affect acceleration.</TD></TR></TABLE>
are you sure you know what your talking about?
12-08-2004, 10:30 PM
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Re: unsprung wheel weight (lakerschamp2000)
soooo....those 3 extra pounds for my 17's wont effect me that much. Thanks for that write up it was very helpful. This could be a dedicated thread to unsprung weight so i guess people can add to it, it would help, but i think Mr. Engineer already covered all the basis'
damon
"film student"
(i can help with film)
damon
"film student"
(i can help with film)
12-09-2004, 05:26 AM
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He's right. I went from 93 SI wheels (22lbs) to 15" TE37 (9lbs). I went to the drag strip with the Volk's 2 weeks after running with my SI wheels. There was no difference in my times.
There is huge difference in the handling and control of the car in corners (road course and auto cross). Bumps on the street seem much smaller and less dramatic. Oh, and unproductive burnouts are more easily obtainable.
There is huge difference in the handling and control of the car in corners (road course and auto cross). Bumps on the street seem much smaller and less dramatic. Oh, and unproductive burnouts are more easily obtainable.
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