Loose Chain
 

Any idea on a way to to tighten the chain? Our chain hangs at the bottom.

Re: Loose Chain
 

It depends on how your robot is built. If it has slots to move the wheels or transmissions front/back, then you can loosen the bolts, slide them till the chain is tight, and tighten the bolts again. If there are tensioners, you can adjust them. If not, you might be able to add some. Or if the chain is too far worn to adjust it, you could remove a link and add a half link to make the chain shorter, then adjust it.

Pictures are very helpful for explaining your problems so others can see what you're trying to fix.

Re: Loose Chain
 

http://www.chiefdelphi.com/media/photos/31899
or
PVC/Delrin blocks in the middle
or
floating idler tensioners (AKA sprockets)
or
sliding the wheels out or the transmissions up and down
or
come up with something better if you don't like these suggestions.

Re: Loose Chain
 

Pictures or sketches would be very helpful.

Re: Loose Chain
 

If you upload some pictures, we may be able to help you more because different types of tensioners work better for different situations. There quite a few different tensioning systems that you could use.

-Vivek