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Answers to Readers' Questions on Physical Science:

Rolling Friction for Tires

The following 3 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Rolling Lego car down a ramp
• What is the best configuration for wheel sizes?
• Want to know friction between tire and asphalt road

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Rolling Lego car down a ramp

Question

January 13, 2008

My son is doing a 4th grade Science project comparing the kinetic energy of a Lego car rolling down a ramp. He will be changing the tires and comparing the resultant distance travelled. Can you give me some information on how the tire size (diameter and width)as well as tread would change how far the car will roll? Does the total surface area of the tire have anything to do with it or just the weight of the tire?

Linda - USA

15120

Answer

A car with larger wheels will require longer to build up its speed when rolling down a ramp, due to the inertia of wheels. But with a Lego car, that would not be much of an issue.

Softer tires produce more friction. Also, tires with knobby treads would have more resistance than smooth tires.

The biggest problem I see is that with a Lego car, the size is so small that differences might be difficult to measure. But it is a good experiment to see what would happen.

Best wishes in his project.

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What is the best configuration for wheel sizes?

Question

December 14, 2007

I recently took a test that involves mechanical aptitute. There was a question involving pushing a cart, where each cart (with a load of bricks) was the same- no changes in load distribution or size. The question is refering to the wheels themself and which configuration of front and rear wheels would be easiest to move the load... a) f/r both small, b) front wheels small rear wheels large c) front wheels large and rear wheels small, or d) f/r wheels both large.

I appreciate any help you can give, and if you do have an answer can you please educate me on the reason why ?!(or let me know where I can look to get the info)

Thanks,

Jon - USA

14973

Answer

I know that stagecoaches have large rear wheels and smaller front wheels, but I'm not sure why. Someone jokingly said the reason many vehicles have smaller front wheels is because then it is always going down hill. Obviously, that's not possible.

I'm sorry that I can't help you beyond the fact that many vehicles have smaller front wheels.

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Want to know friction between tire and asphalt road

Question

August 2, 2007

I want to know what the coefficient of rolling & sliding friction between tire an asphault road is. I searched in different websites and books but now I'm staggered about thge exact number. In your site I find the coefficient of rolling friction equels to 0.015 & in another site (http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html) I find the coefficients of friction equels to 0.7 for dry roads and 0.4 for wet roads and in another site I found the coefficient of sliding friction equels to 0.41. please help me about it.

Iman - Iran

14149

Answer

The problem with establishing the coefficient of friction between a tire and an asphalt road is that there are many factors that can affect the value. The treads on the tire cause more rolling friction. A tire with more air will have a lower coefficient of rolling friction than one that is under-inflated. If the asphalt is smooth, it will cause less friction than if it was rough. This is also true for sliding friction.

Although it be nice to have a some numbers for the coefficients to use, they would not be accurate unless the variables were exactly the same as in your situation. The best way would be to set up experiments to measure the friction under specific conditions.

It is not an easy problem, but I hope this information helps you.

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