Here is a video of a dynamic pulloff between the VEX 4" wheel line and the new Mecanum wheels. The test was carried out on 2 protoBOT bases
Hmmm. Interesting. Thanks for posting this! 
Wow, well composed video. And thank you for doing this, these sort of empirical data are always valued. Looks like the mecanum wheels provide much more friction than the regular 4" wheels. You’d think that the high traction wheels would provide more traction. You should do a test in which you have the mecanum wheels running to the side, compare that with the various wheels.
Nice experiments.
I recommend trying the tests again, and in the new trials, putting extra weight (near the 4 wheels) onto the non-mecanum robot in order to keep the weights of the two machines equal. The Mecanum wheels are heavier than some (all?) of the other 4" wheels.
Also be careful to clean all the tires involved. Dusty, worn down (a bit) tires are less grippy than fresh-from the box new tires.
Blake
If the Mecanum wheels are larger than the 4" wheels, they will keep traction longer as they have less force, but the small diameter difference and the large difference in performance would indicate that this is not a factor.
Do you have the ability to test VEXplorer wheels against 2.75" double-roller omnis and single-roller omnis? I know it’s straying from the Mecanum focus, but I think it would be interesting, considering the 4" omni was equal to the Mecanum.
fyi. i believe that the mecanum wheels are mounted wrong
They’re mounted “wrong” in that the robot won’t be able to move sideways, but in this situation that could be an advantage and it won’t affect their foward pulling ability.
The results of the video are interesting, are the mecs noticeably heavier (0.41lb)? Are you planning on posting a video where the mecs are mounted for strafing? A bit curious how that configuration holds against the pull of the other wheels.
For anyone new to the use of mecanum wheels, they need to be used on a 4 motor base for holonomic motion. They will not function as they are intended, motion in 2 planes (front - back, side - side), in the configuration used in my video VEX Mecanum Faceoff. The ProtoBOT base has been the standard used for all the wheel tests at the simplerobotics.org website and the test was designed for a linear comparision.
Unfortunately, the forces were nearly equal when comparing the 4" omni wheels and the 4" mecanum wheels and they stalled the two 269 two-wire motors that power the ProtoBOT base. Nonetheless, it does show that both the 4"omni wheels and the 4" mecanum wheels are great wheels to use.
For more wheel test results:
http://simplerobotics.org/Compare_wheels.htm
Also recall that the ‘grippyness’ of the wheels depends on the surface. I’m willing to wager that we’d find completely different results if the tests were performed on carpet, cardboard, or lexan.
If you want to isolate the comparison to Coeff of Friction (traction) only,
build a long robot with
- different kinds of wheels at each
- All wheels geared to one set of motors at 5:1 for torque (slow!)
- each end pulling in opposite directions
- with most weight at center of a long wheel base robot
This setup should force the wheels with least traction to slip.
Traction is only one component of an effective wheelbase,
and needs to be balanced by other components, such as efficiency, reliability, robustness, size/space, mass, weight, center of balance, etc.