Wheel Weight and Rotational Inertia

The more mass an object has, the more energy it takes to accelerate it.  To accelerate a rolling object such as a wheel, you must both accelerate its mass plus overcome its rotational inertia.

Because of this, the weight of rotating mass such as wheels and tires on a car have a bigger effect on acceleration than static weight such as on the chassis on a car.   When purchasing new wheels and tires for a performance car, it can be useful to compare the effects of different wheel and tire combinations.  This is especially true when considering upgrading to larger wheels or tires on a car.  

In many cases, car handling can be improved by increasing the diameter of your car's wheels matched with a lower aspect ratio tire.  The drawback of this is that most plus sized wheel and tire combinations are heavier than the stock smaller wheel size with a taller tire sidewall.  This can make for slower 0-60mph  and 1/4 mile times.

In order to help make this tradeoff decision, I have put together a spread sheet that calculates the amount of torque needed to accelerate two different wheel and tire combinations at a given rate.  This rate is determined by the 0-60mph acceleration time of the car in question.

In order to use this spreadsheet, you will need to know the diameter of the two wheels and size of the two tires in question, plus you will need to know the rough 0-60mph time, torque, and weight of the car you are dealing with.  The spreadsheet will then calculate the amount of torque needed to accelerate the wheel and tire combination at the same rate that the car accelerates at.  The spreadsheet will also compare the effects of wheel weight to chassis weight by calculating the number of pounds of car weight per foot pound of engine torque on the car.  You can compare two different wheels and tires of the same or different sizes.

I am not an engineer, and it has been over 15 years since I took my last physics class, so if you spot any errors in my spreadsheet, please drop me a line.  (rocksandracing @ the-welters.com)  I used the web site listed below to refresh my memory of physics.  It explains things in a pretty straight forward manner, and is a good resource.

My spreadsheet does not use any macros, but you should always virus scan things like spreadsheets and word docs when you download them.

My spreadsheet contains three different models for calculating the rotational inertia of a wheel and tire combination.  I believe that "model 3" is the most accurate, but the other two are included as a way of showing how I arrived at model 3.

"Unsprung" weight of wheels and suspension components has effects on handling in addition to the effects of rotating mass on acceleration.  This spreadsheet does not address any of these issues.  But increased weight on unsprung components makes shocks and springs have to work harder, and also diminishes car handling with other things being equal.  The trade off of lower aspect ratio or wider tires verses their increased weight is beyond the scope of this page.

Copyright 2002 Andy Welter. All Rights Reserved
Last Updated March 17, 2002