MadSci Network: Physics

Re: What material when wet is good to use on tires to drive up a slanting mirror?

Date: Tue Oct 17 18:12:53 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 971362990.Ph

You have quite a project to accomplish!  It is not my intent to do your 
homework, but I can help.  You probably can figure out what materials 
would work from your own experience.  What has good traction when wet?  If 
your answer was a rubber tire--you are correct.  Now think about what kind 
of tire treads are used on wet roads, and what types on dry.  Drag racers 
only operate on dry road surfaces, and have no tread, and very high 
surface area contacts.  If you have seen the tires used on sports cars for 
wet roads, you may see that the treads have many sipes, cuts that cross 
the rubbing path.  Wet road tires also have higher contact pressures than 
dry road tires.  If you think about it, the higher contact pressure helps 
force water away from the contact.  This increases friction by decreasing 
the lubricating effect of the water.   

You did not indicate the size of the vehicle you are building.  Model 
shops carry tires of different sizes for model cars and aircraft.   If 
building a larger vehicle, you might want to try a lawnmower tire, with 
added saw cuts across the tread to increase wet traction.  If larger yet, 
a bicycle tire may be used.  Remember to keep the contact pressure high 
enough to remove the water. Avoid foam rubber tires.

Another point to consider.  Cleanliness affects friction--by a large 
amount.  You would do well to consider a careful cleaning of the tire 
rubbing surfaces.  Oil and solid particles must be removed.  Laundry or 
dishwashing powder detergents are good.  Rinse well, and then rinse again 
with some alcohol.  Then do not touch the tires, as your fingers will 
likely contaminate the tires with oil.

Finally, run an experiment before the final test.  This is how you learn 
about friction, and the variables that affect it.  You might try different 
solvents on the tires to see how they affect traction.  You also may want 
to try narrowing the tire tread width.  Higher contact pressures usually 
help wet friction, as long as solid (not foam) rubber is used.

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