Rotational Mass & Performance
05-23-2014, 09:49 AM
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Rotational Mass & Performance
Rotational Mass (RM) is often overlooked as a method for increasing performance, so I figured I would make this thread as a discussion of the effects of decreasing RM.
I'll do my best to explain the concept, however it will be most beneficial to read the article in the link at the bottom of this post.
Rotational Mass (RM) of a vehicle includes any parts that are actively being spun by the engine & drivetrain. This includes axles, wheels, tires, brake drums, etc. It is commonly assumed that 1 pound of RM is the equivalent of ~3-4 pounds of Actual Vehicle Mass (AVM). However, in reality each rotational part has different AVM equivalents. This is due to the effects of sprung vs. unsprung weight.
With wheels and tires you can expect roughly a 1 : 2-3 ratio - meaning 1 pound of RM on the wheels/tires is the equivalent of 2-3 pounds of AVM.
I'll use BMW's 162 wheels and run-flat tires to show the effects of reducing RM in wheels and tires:
162 wheels = 24.5 pounds
Run-Flat tires = 31 pounds
With a proper set-up you could run performance wheels which would weigh in at 17 pounds each, and Michelin PSS tires which weigh 25 pounds each. This would represent a decrease of 13.5 pounds of RM in each corner.
13.5 x 4 = 54 pounds RM
54 x 2 = 108 pounds --- 54 x 3 = 162 pounds. This means decreasing 54 pounds of RM would have an equivalent effect to reducing 110-160 pounds of actual vehicle mass.
Following the general rule: 10 hp = 100 pounds = .1s (and 1mph) difference in 1/4 mile - we can assume decreasing 54 lbs of RM would decrease 1/4 mile times by .1 to .15 seconds and increase trap speeds by 1 to 1.5mph.
Lets say with the stock set-up in a 335i you trap a 13.5 second 1/4 mile @ 104 mph. With the aftermarket, much lighter wheel/tire set-up you'd be looking at a 1/4 mile of roughly 13.38 @ 105.25mph.
That's a pretty significant improvement in the 1/4 mile for simply reducing RM of the wheels and tires.
Here is a great article: The Effects of Rotational Inertia on Automotive Acceleration
Notice the article mentions reducing RM of wheels/tires has the lowest equivalent AVM. This means you could see even bigger improvements by reducing the RM of flywheels, axles, drivetrains, etc!
Rotational Mass of flywheels could be the equivalent of nearly 10x the AVM. In this case, if you could drop 5 pounds from your flywheel it would be the equivalent of roughly 50 pounds of actual vehicle mass.
What do you all think? Does anyone have experience with actively trying to reduce RM of their car and what were the effects? After I switched from the run-flats to Michelin PSS I did notice a slightly improvement in performance! Maybe it was just the placebo effect though.
I'll do my best to explain the concept, however it will be most beneficial to read the article in the link at the bottom of this post.
Rotational Mass (RM) of a vehicle includes any parts that are actively being spun by the engine & drivetrain. This includes axles, wheels, tires, brake drums, etc. It is commonly assumed that 1 pound of RM is the equivalent of ~3-4 pounds of Actual Vehicle Mass (AVM). However, in reality each rotational part has different AVM equivalents. This is due to the effects of sprung vs. unsprung weight.
With wheels and tires you can expect roughly a 1 : 2-3 ratio - meaning 1 pound of RM on the wheels/tires is the equivalent of 2-3 pounds of AVM.
I'll use BMW's 162 wheels and run-flat tires to show the effects of reducing RM in wheels and tires:
162 wheels = 24.5 pounds
Run-Flat tires = 31 pounds
With a proper set-up you could run performance wheels which would weigh in at 17 pounds each, and Michelin PSS tires which weigh 25 pounds each. This would represent a decrease of 13.5 pounds of RM in each corner.
13.5 x 4 = 54 pounds RM
54 x 2 = 108 pounds --- 54 x 3 = 162 pounds. This means decreasing 54 pounds of RM would have an equivalent effect to reducing 110-160 pounds of actual vehicle mass.
Following the general rule: 10 hp = 100 pounds = .1s (and 1mph) difference in 1/4 mile - we can assume decreasing 54 lbs of RM would decrease 1/4 mile times by .1 to .15 seconds and increase trap speeds by 1 to 1.5mph.
Lets say with the stock set-up in a 335i you trap a 13.5 second 1/4 mile @ 104 mph. With the aftermarket, much lighter wheel/tire set-up you'd be looking at a 1/4 mile of roughly 13.38 @ 105.25mph.
That's a pretty significant improvement in the 1/4 mile for simply reducing RM of the wheels and tires.
Here is a great article: The Effects of Rotational Inertia on Automotive Acceleration
Notice the article mentions reducing RM of wheels/tires has the lowest equivalent AVM. This means you could see even bigger improvements by reducing the RM of flywheels, axles, drivetrains, etc!
Rotational Mass of flywheels could be the equivalent of nearly 10x the AVM. In this case, if you could drop 5 pounds from your flywheel it would be the equivalent of roughly 50 pounds of actual vehicle mass.
What do you all think? Does anyone have experience with actively trying to reduce RM of their car and what were the effects? After I switched from the run-flats to Michelin PSS I did notice a slightly improvement in performance! Maybe it was just the placebo effect though.
Last edited by zach1328; 05-23-2014 at 09:54 AM.
07-22-2014, 01:35 PM
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Nice write up, but the RM equation is only for the drive wheels. So if you have a E60 M5 then you only multiply that 13.5lb reduction by 2.
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