fun with 4 wheel drive bases...

I’d like your opinions and observations on something…

I used to have a six wheel drive, and then I switched to four wheels to get over the bump easier. I still had a bunch of idler sprockets keeping the chain wrapped nice and tight. These are present all the way down the drive train. Our drive (four motors, 1:1 ratio on a 16 pound [7.3kg] robot) was starting to die a bit too much for my tastes, so I figured I’d eliminate some non-conservative forces by only powering the back wheels.

The question is thus:

In general, is it better to power all four wheels? I know some teams only powered the back wheels when using planetary transmissions last year, but I was wondering what everybody thought on this? How will it affect driving? turning? going over the bump? etc…

Your help is much appreciated.

Without powering both wheels we find it almost impossible to get over the bump in both directions. I also find that it makes acceleration quicker and makes the robot in general less risk averse. Powering just the back wheels was definitely not feasible in sack attack for example because of the potential of getting caught on the sacks.

You should definitely direct drive all four wheels. If your chain snaps or something, you aren’t left with two powered wheels and two idler wheels, which would be very bad if you wished to cross the bump or were tipped, but instead you still have all four wheels powered.

You have a problem if you’re killing your 4 wheel drive at 1:1. We were at 2:1 on one draft and it worked perfectly fine. Find that before anything else. There’s a ton of friction somewhere you need to eliminate.

The two wheel drive isn’t working terribly bad. It gets over the bump quite well, and didn’t die for the entire hour and a half we drove it. I think the problem with our four wheeled approach was our chain loop, which had 6 or 7 idlers on it to keep the chain wrapped in the shape needed to get over the bump. The energy lost through so much chain was a problem, and a shorter loop worked better. We even geared our drive up after we just powered the back.

That being said, I will probably go to direct drive in a few weeks, given the advantages of doing so. We have a few tournaments we need to survive first before we alter the drive any more.

With a 4-wheel 1:1 and direct drive, you shouldn’t need 7 idlers. Our 6-motor 4-wheel drive has 2 idlers on each side. With 4-motor 4-wheel direct drive you would probably only need 1 at most if you chain the wheels together.

You should try to build your drive base so you don’t have any idlers at all.
I had no idlers on my 4 wheel 6 motor drive.