garkoi said:
how does large vs small diameter effect the tires ability to go over sand ?and is there no way to climb the lee of a dune at all ? no matter how big or wide the tires are ?
Surprised no one has mentioned it but this is a thinly disguised age old question/argument in the off road world, to float or not to float.
Basically the same issues arise in any soft, deformable material, be it snow, sand, gravel, mud etc.
To float or sink: Basically this comes down to whether or not there something harder with more support underneath the soft stuff, if so, "pizza cutter" type tires work well ie they cut through the soft stuff to find hard material to get traction, driving wheel of a tractor would be an example of this.
If there is no bottom in sight, then "high flotation" tires are key. These aim for largest contact patch within a wheel size. They also have sidewalls designed to allow the tires to be aired down significantly, at the extreme end of this are the rock crawling guys that run tires so low they need to bolt the tire bead to the rim to stop it slipping or loosing all the air (normally tire pressure does this), google "bead lockers".
This pressure drop does two things, increases the contact patch by approximately the ratio of pressure drop, ie if at 30psi you have x contact area, reduce the pressure by half and the area will ~ double. The result is that the contact patch pressure is reduced by half. As an example, I nearly got my old landcruiser stuck on a beach, fortunately have off roaded long enough to know what it feels like when the wheels move but truck doesn't, where even a fraction of a wheel turn is enough to result in you being so augered in that you may as well start the walk home. Dropped the tires to maybe 5psi, low ratio 1st, loads of drift wood and grass in front and very very carefully drove out.
Then secondly it allows the tread to deform around the surface, and this is important both in soft and hard materials, if the tire wraps around the sand and compresses it from more sides, its less likely to move, giving you more support.
Last but not least is velocity.
As others have mentioned, sand and other fine materials exhibit fluid like properties, so as you drive faster the tire tends to float over the material rather than sinking in, much like the difference between planing hulls and displacement hulls (boats).
Then if you look at the rear tires of that bike berkeman posted, you might notice that they have paddles, not unlike an old river boat. This is because forward motion here is less about friction and more about thrust from the mass of stuff thrown out the back. With sufficient speed and sand rail tires, cars can drive on water:
So on the lee side, to get up the method would be "what ever you do, do NOT let off the gas!"
