# 4 Wheel Drive Too Much Traction

Our robot is using a 4 wheel arcade drive using the wheels given in the kit of parts. However, we are experiencing a hard time turning the robot due to friction opposing the wheels. How should we address this problem?

Can you post a picture of your robot, or give more details. It’s hard to find a universal solution. You can search and find lots of threads on this but a picture will get you more useful advice.

6 wheel dropped center drive

Or you can switch the back or front to omins which is what we did rookie season and it worked wonderfully.

And a photo would help

Sorry, should’ve posted that earlier.

http://sphotos-b.xx.fbcdn.net/hphotos-prn1/859989_153240221498409_1413115399_o.jpg

That’s an interesting setup.
Going to 6 wheel dropped center will help but your current problem is that your drive train is geared way to high. Those look like CIMpleboxes when paired with the kit belts and pulleys don’t provide enough of a reduction for the motors. (EDIT: on second glance those aren’t the kit pulleys so you might be running 1:1 which is even faster)

I would recommend adding a center wheel that is about 3/16" lower than the current wheels and moving to sprockets and chain. 12 to 26 or 12 to 32 would probably be good. Though those are just guesses. You can use a design calculator to figure out your actual speeds; something around 8-12ft/s should be what you are aiming for with a 6 CIM drive. You would want to power the center wheel off of both of your gearboxes so that all three wheels will be linked and spinning at the same speed. That will also allow you to maintain all your driver train power to the ground even when some wheels are lifted off the ground.

AndyMark and VEX both sell sprockets and chain that would allow this fix for around \$100.

Omni wheels will work but they cure the symptom not the cause of the problem.

The problem is that the combination of the length of your drive train and the friction of your wheels on the driving surface is not allowing you to turn effectively. To solve this you can shorten the steering base. Which is what is being suggested by the recommendations for six wheel drive with a raised center wheel. You could also move your four wheels closer together, but this may give you stability problems.

Another option is to reduce the friction by changing 2 or all 4 wheels to wheels with less friction. You could replace two of your wheels with slicker wheels or Omni wheels. Obviously this option surrenders a little traction.

Changing from arcade drive to tank drive( controlling each side of the drive train with a different joy stick) would not solve the problem but it might give your driver a little more control.

If your speed controllers are set to coast, you may be able to improve control slightly by setting them to brake.

You have to evaluate what will work best for you with the time, space and material constraints your team has.

Good Luck!

There are several things wrong with this drive train setup, and there are multiple ways to remedy it. The root cause is pointed to by David, and the result is that your entire robot is current limited while turning (pulls more than 120A while turning @ 12V…which causes battery voltage to drop*). I can confirm Mr. Gregory’s gear ratios because last year that’s exactly what we ran (12:32) using higher traction 6" wheels and no issue with turning (though our wheelbase was MUCH wider than it was long…).

Start by examining the attached photo that was created with this tool (not to toot my own horn – but there are a couple of tools on CD that should come on a KOP CD or something … namely, JVN’s to start with).

In no particular order, you could do one or more of the following:

1. Use omni wheels*]Use smaller wheels*]Offset your center of gravity*]Swap Gearboxes to Toughbox Mini’s/Nano’s (hard to tell exactly what gearbox you have – looks like CIMple boxes since the CIM shafts stick out)]Make a ‘lessons learned’ so your team never designs a wheel base that is longer than it is wide]Use 12:32 or 14:38 sprocket reduction, in picture 2*]Change the cogs on the gearbox to 16-tooth cogs with the same tooth profile

*One day I’ll get around to updating my sheet to model this, now that I understand it better

I know this is a little off topic, but is your robot made out of aluminum? It doesn’t look like aluminum to me.

Find your center of mass and (looks like it is toward the right side in the picture) and shrink your wheelbase by moving either your front or back wheels closer to the center mass.

That’s pretty obviously bent and welded sheet steel. I know we’re all about the lightness in FRC, but has it actually reached the point where people can’t recognize steel when they see it?

[/li]
Can you please explain your thought on this? It’s common practice for teams to have robots longer than they are wide - 48 had a (in)famously long robot last year, and they were multiple regional winners. I agree that wider bases have less scrub, but never longer than wide?

If the team doesn’t understand the fundamentals of the forces of scrub in a skid steer drive train, then it’s probably best they steer clear of longer wheel bases (w/o dropped centers) until they do. Team 25 won Einstein last year, and they typically have the same setup (8WD, all touching the ground, all long configurations from year to year). The difference is, they understand what works, what doesn’t, and why.

I think we’re going to see alot of robots with this approximate design (chute with indexer, dropping into a linear shooter at a ~30deg angle).

This particular robot seems much heavier than it needs to be though… Why two gearboxes to a side unless you were putting mecanums on it? The whole chassis appears to be steel tube and bent sheet steel, which, might be more viable this year than ever before, with the reduced perimeter, but still.

Wheel base, not robot/frame. I believe 48 had a dropped center, so their wheel base was still wider than it was long.

I recognized it as sheet steel right away, I was just curious because I was talking to someone on 253 earlier and they said they were using aluminum.

I’m curious as to how those pneumatic wheels/hubs are fixed to the output shafts on the CIMs. Care to share the details?

Yes, under certain conditions wheelbase can be longer than trackwidth

For the robot pictured, if you make the following simplifying assumptions:

1. all four wheels identical and driven independently with the same torque

2. coefficient of friction of the wheels the same in X and Y directions

3. Center of Mass lies on the the longitudinal axis, somewhere aft of the geometric center of the 4 wheels

… then the analysis becomes straightforward: