CATAGORY: Mechanics QUESTION: Can you explain to my husband and my brother why thin tires produce more traction than wide tires and why race car drivers prefer wide tires with no tread over thin tires with deep treads. I have tried many times with the typical male response; "you don't understand racing." They seem to think that race drivers like the wide, treadless tires because they produce more traction. Duh!! Can you explain in terms that they would understand? I give up.
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There is a wonderful essay on the physics of racing in a UNIVERSITY PHYSICS textbook by William P. Crummet and Authur B.Westin, Wm. C. Brown Publishers, 1994. The essay is on page 160. The author of the essay in this text is Ruth Howes and the title of the essay she wrote is `Physics and racing at the Indianapolis 500'. It addresses most of the issues you would like to discuss with your son and husband. If you are considering car tires for normal driving on roads that may at times be wet or snowy or icy then without a doubt you need cars with a good set of deep treads. The treads channel water away from the tire so that the tire can be in contact with the road. The coefficient of sliding friction between the tire and the road (needed to propel the car forward and to brake the car) will drop to nearly zero if a layer of water gets between the tire and the road (this is known as hydroplaning). This dangerous situation easily happens to treadless tires because there are no grooves to channel the water away from under the tire. Races are delayed or canceled if there is water on the track for this very reason. Unfortunatly we cant cancel driving to work every morning because of some rain or snow so for normal driving you are far better off with tires that have treads than ones that do not. This begs the question however as to why race cars have wide tires with no treads. At first glance it does not make any sense because in PERFECTLY DRY CONDITIONS (to a first approximation) there is NO difference in traction between wide tires and narrow tires. The reason has to do with the fact that the traction depends on the number of contact points between the tires and the road. If you could look between the tire and road you would see that there are gaps between the road and tire due to the roughness of both tire and road so within the area of contact not all parts of the tire are actual points of contact with the road. Narrow tires provide less opportunity for there to be contact points but because the weight of the car is distributed over a smaller region there is greater pressure on the road under the tire which helps to create more contact points per unit area than with a wider tire. Conversely wider tires provide more opportunity for there to be contact points but because the weight of the car is distributed over a larger region there is less pressure on the road under the tires so there are fewer contact points per unit area than with a narrower tire. As a result the number of contact points, hence traction, remains about the same. Now if there were a way to increase the contact points by increasing the pressure of the car tire on the ground or by increasing the contact points without regard to the pressure or area then one would have more traction. In todays race cars this is done by putting fins on the car which use air pressure to push the car against the ground to increase the tire-to-road pressure. Next they use special tires made of sticky rubber. According to the essay I read the tires cost about $1000 a set and Indy drivers go through 12 sets! The sticky tires actualy act like glue. During the race the rubber melts a bit and sticks to the road. the number of contact points then does not depend so much on the pressure but just how much sticky area there is. So in this case wider IS better for traction. OK in summary: for normal, typical, driving conditions in all sorts of weather [ or for situations where it is perfectly dry but you dont want to spend thousands of dollars per 500 miles on a set of tires! :-) ] use tires with treads PLEASE! both for your saftey and every one elses too. On the other hand if you are racing on a groomed, dry, race track then wide special sticky tires will give you more traction. P.S. It is unfortunate that they (your husband and brother) said "you don't understand racing." That kind of a response usualy happens when, even if they do happen to be correct, do not understand themselves! Othewise they would explain rather than belittle. It is also unfortunate however that you refer to this as "the typical male response". Please do not stereotype! I have found that everybody who uses stereotypes (both men and women) hurt themselves and the society they live in. The results of physics, math and logic is independent of ones sex which is one of the great things about it. Perhaps with truth and understanding in hand we can all get along a little better in the world. Good Luck.
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