Wheel/Tire weight and rotational inertia

Shift518 Automotive
1

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.

spread sheet:
http://www.the-welters.com/racing/rotational.xls

NOTES

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.

2

Good link, however one thing I always pondered on is that nobody considered the flywheel effect of heavier wheels.

Heavier wheels will be harder to slow down in stopping but they could also maintain the cars travel automatically possibly helping factors like highway mpg. :ponder

3

That’s a great thought, sounds possible.

4

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.

My 2 cents.

The improvements in the car’s handling is small by increasing “rim” sizes with a lower aspect ratio tire to keep the “wheel” size the same. While the lower aspect ratio tire will help improve tire response, but at a cost of more thread deformation and therefore higher tire temperature. Depends on what compound and what tire temperature you want to achieve, this can be use to improve a certain aspect of the car’s handling in certain competition environment.

However, the one major advantage of going with larger rim sizes is to facilitate the install of a larger brake system which can dramatically increase braking performance, both in terms of stopping power and/or better cooling/wear on a vehicle with the same weight and traction.

The drawback of fitting a larger rim (same rim design) not only as the mentioned increase in rotating mass, but the increase in unsprung weight can be devastating in vehicle design and set-up for circuit racing, as suspension setting will have to be further compromised to compensate for the effects of the unsprung mass acting on the sprung mass. As the sprung mass of a vehicle decreases the more dramatic the negative affect will have on the vehicle as the unsprung mass increases.