Question on rotational mass
Thread Starter
Team Owner
Joined: Jan 2007
Posts: 22,136
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From: In the woods looking for my thrown putter.
St. Jude Donor '13, '15
Question on rotational mass
There have been lots of threads on unsprung weight and rotational mass and how it affects handling and acceleration. I'm not a physics major but maybe some of you guys could help me out. My question is how or if rotational mass differs on the rear wheels(in wheels i mean any rotational part, rotors, rims, tires...etc.) compared to the front wheels in terms of acceleration. I would think that since the motor is having to turn the rear wheels and the front wheels simply "act" as a result of the rear wheels driving them, that it would be more important to keep the rear rotational mass low. In other words, adding 5 pounds to the rear wheels would be more harmful to acceleration than adding 5 pounds to the front wheels. Am I off base here? Or am I completely off base in thinking rotational mass of the wheels has much if any effect on acceleration? I was told, and have read that rotational mass, is 3 times as important as non-rotational, or "sprung" weight in terms of acceleration. Share your thoughts.
Race Director
Joined: Dec 2005
Posts: 11,344
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From: Carlsbad Ca
Regarding rotational mass, the farther away the mass is from the axis, the more power required to change speed.
Since the front wheels are connected to the rear wheels via the pavement, it makes no difference whether it's front wheels or rear wheels regarding the effect of torque. (Unless the rear wheels are over-coming the friction of the pavement.)
Think of your wheels and brake rotors as 8 more flywheels that the engine has to overcome.
Whether rotational mass has 3X the effect as static mass is a hypothesis. It does have some effect certainly, though.
You could also look at all the gears,shafts,bearings in the total driveline as "rotational mass."
Since the front wheels are connected to the rear wheels via the pavement, it makes no difference whether it's front wheels or rear wheels regarding the effect of torque. (Unless the rear wheels are over-coming the friction of the pavement.)
Think of your wheels and brake rotors as 8 more flywheels that the engine has to overcome.
Whether rotational mass has 3X the effect as static mass is a hypothesis. It does have some effect certainly, though.
You could also look at all the gears,shafts,bearings in the total driveline as "rotational mass."
Drifting
Joined: Jun 2005
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Well, its really the inertia that counts, and that depends on the mass and distance. And, in the case of the drivetrain, the relative speed.
As long as you aren't pulling the front wheels off the ground 5 lbs on the front or rear wheels are the same. But 5 lbs right at the hub is less than 5 lbs at the rim. Simple to demonstrate this with barbells - put two 5 lb weight on a bar, as far in as they'll go (no spacer) then spin it like a baton. Now move the weights out as far as they'll go and try. Same weight, but a lot harder to move.
Now as far as the drivetrain, when you're in first gear the flywheel is spinning around 10 times as fast as the rear wheels, so 5 lbs on the flywheel count for a lot more than those 5 lbs on the wheels. Or the car in general.
This is also what accounts for the loss seen on a dyno, and why you'll see more loss for a car with, say, 4.10 gears than one with 2.73s - a greater percentage of the effort goes into spinning up the drive train. Its moving faster relative to the major load (the dyno rollers).
As long as you aren't pulling the front wheels off the ground 5 lbs on the front or rear wheels are the same. But 5 lbs right at the hub is less than 5 lbs at the rim. Simple to demonstrate this with barbells - put two 5 lb weight on a bar, as far in as they'll go (no spacer) then spin it like a baton. Now move the weights out as far as they'll go and try. Same weight, but a lot harder to move.
Now as far as the drivetrain, when you're in first gear the flywheel is spinning around 10 times as fast as the rear wheels, so 5 lbs on the flywheel count for a lot more than those 5 lbs on the wheels. Or the car in general.
This is also what accounts for the loss seen on a dyno, and why you'll see more loss for a car with, say, 4.10 gears than one with 2.73s - a greater percentage of the effort goes into spinning up the drive train. Its moving faster relative to the major load (the dyno rollers).
Thread Starter
Team Owner
Joined: Jan 2007
Posts: 22,136
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From: In the woods looking for my thrown putter.
St. Jude Donor '13, '15
My entire reason for posting this was because I just bought some new rotors. (Baer Eradispeed's) and the rear rotor's are approx 2" (14") bigger in the rear than the stock rotors. They weigh slightly more than the stock rotors(don't know exact weight but about 2 pounds more each) and they also have an aluminum hat(which places more mass out farther). The front rotors are slightly lighter in total weight(2 pounds each), but once again since the aluminum hat places more mass out farther.....you see where I'm getting. I'm wondering if I am going to see any losses in acceleration due to this. Based on the above responses, I'd say the answer is yes, now the question is......how much? Any thought?
Le Mans Master
Joined: Sep 2004
Posts: 5,707
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From: Los Angeles
My entire reason for posting this was because I just bought some new rotors. (Baer Eradispeed's) and the rear rotor's are approx 2" (14") bigger in the rear than the stock rotors. They weigh slightly more than the stock rotors(don't know exact weight but about 2 pounds more each) and they also have an aluminum hat(which places more mass out farther). The front rotors are slightly lighter in total weight(2 pounds each), but once again since the aluminum hat places more mass out farther.....you see where I'm getting. I'm wondering if I am going to see any losses in acceleration due to this. Based on the above responses, I'd say the answer is yes, now the question is......how much? Any thought?
Last edited by robvuk; Apr 26, 2007 at 06:00 PM.
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
The bottom line is, while there may be a difference, there's no way you'll ever tell the difference by the seat of your pants. You won't see the difference with a stopwatch either. It would take a very accurate and sensitive timer with a very consistant driver to measure the difference. If you'd like to read more on the subject, take a look at this post:
http://forums.corvetteforum.com/show...9&postcount=21
I haven't run the numbers, but compared to the ~50 Lb wheel/tire, a couple of extra pounds at the radius of the rotor would take less than 1 HP extra in a 0-60 run and even less as the speeds increase. Stop worrying over trivial things and go enjoy your 'Vette with those new big brakes.
http://forums.corvetteforum.com/show...9&postcount=21
I haven't run the numbers, but compared to the ~50 Lb wheel/tire, a couple of extra pounds at the radius of the rotor would take less than 1 HP extra in a 0-60 run and even less as the speeds increase. Stop worrying over trivial things and go enjoy your 'Vette with those new big brakes.
Thread Starter
Team Owner
Joined: Jan 2007
Posts: 22,136
Likes: 23
From: In the woods looking for my thrown putter.
St. Jude Donor '13, '15
Interesting thread. I'm not a big "racer" anyway, but was just curious to learn more about the effects of rotational mass and unsprung weight. You know the saying.......you learn something new everyday!!!
I mainly bought the rotors for looks..........and I must say they do look good. Thanks for the responses guys. 
I mainly bought the rotors for looks..........and I must say they do look good. Thanks for the responses guys.
Instructor
Joined: Jun 2005
Posts: 223
Likes: 0
From: Peoria Arizona
i have 2 things to add
look at the old days when dragsters had 2 real small diameter wheels in front instead of the larger thin bicycle wheels. this is directly because you want to keep the wheels as light as you can and also keep any wieght as near the axis point as possible.
another way that may make sence is to swing a baseball bat from the correct (skinny) end, then swing it from the incorrect (fat) end, you will see the difference. alot of aftermarket wheel manufactures will make wheels with most of the metal mass in a wheel on the outer side (for looks) while GM, (Z06 specifically) puts most of the metal mass on the inside near the hub. the 10 spokes are fatter and wider near the hub and get skinnier as they extend outwards this moves the rotational mass towards the center
the equation is something like Energy=.5 (Mass*Velocity²) meaning as you apply the same energy to a tire as you move the mass down or towards the center you can increase the Velocity in a squared amount. but since you cant change velocity of the wheels given a certain rpm, and the mass doesnt change for a certain wheel this means the energy required to rotate those wheels will be alot less, meaning more energy can be converted to speed of the vehicle. blah blah.
another way that may make sence is to swing a baseball bat from the correct (skinny) end, then swing it from the incorrect (fat) end, you will see the difference. alot of aftermarket wheel manufactures will make wheels with most of the metal mass in a wheel on the outer side (for looks) while GM, (Z06 specifically) puts most of the metal mass on the inside near the hub. the 10 spokes are fatter and wider near the hub and get skinnier as they extend outwards this moves the rotational mass towards the center
the equation is something like Energy=.5 (Mass*Velocity²) meaning as you apply the same energy to a tire as you move the mass down or towards the center you can increase the Velocity in a squared amount. but since you cant change velocity of the wheels given a certain rpm, and the mass doesnt change for a certain wheel this means the energy required to rotate those wheels will be alot less, meaning more energy can be converted to speed of the vehicle. blah blah.
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