Quote:
Originally posted by LittleDave
The formula for the force of Friction is F = u N NOTICE THAT THERE IS ABSOLUTLY NO CONCIDERATION FOR SURFACE AREA!! The thing thats going to make the biggest difference in traction is the material you are using and the coefecent of friction it provides with the carpet. Timng belts and other track materials are made of hard rubber and that means they have a low coeffecent of friction! I suggest that some of you re-think your driving mechs.
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You are not quite correct in your assumptions.
Actually, the friction formula does not account for surface area, so you are right in that regard. However, what people are forgetting is that we are not dealing with pure friction here.
The carpet acts more like snow or sand rather than a smooth floor surface. When travelling on snow or sand, most of the pushing force of a good wheel design does not come from friction - it comes from a normal (normal meaning orthogonal) pushing force. That is why there are treads on snow tires (or paddle ribs on dune buggy sand tires). The treads or ribs sink into the snow or sand and push off of the snow or sand much like you pushing off a wall using your arms.
A simliar idea is the track design on a snowmobile. There are paddle ribs that protrude from the track (now most snowmobiles come with cleats for ice, but if you've seen an older snowmobile the track was made of formed rubber). Once again, the ribs sink into the snow to push off of the snow. The friction between the rubber and the snow accounts for a very small fraction of the total motive force. The snow can only hold so much stress before breaking away, so more track area means more paddle area, which means less stress on on the snow, which means more traction.
The carpet acts quite a lot like the snow or sand since it is quite deformable. Good tank treads act like the paddle ribs on the snowmobile track. The teeth of the treads sink into the carpet between the fibers and push off of the carpet with a normal force. Friction plays a much smaller role in this design.
If you want proof of this, look at either our wheels of the past few years or the treads of the TechnoKats from last year. The coefficient of friction between the carpet and lexan (our wheels) or smooth metal (the TechnoKats treads) is quite low compared to any rubber. However, we both got traction that was equal to or superior to rubber because we dug into the carpet and used "normal force" rather than friction.
Just like the snow, if you put too much stress on the carpet fibers, they will deflect enough to let go of your tread and you will begin to slip. Just like the snowmobile track, in this design the more cleats you can dig into the carpet, the less stress you put on the carpet, which means you are less likely to have the carpet deflect enough to cause slippage, which means the more traction you'll get.