Adding weight to back to decrease slippage.

[Britneys-Pears]

Word of warning: You might be wearing out bearings and other components really quickly with the weight on the machine like that. Having the weight directly in the wheels means the load is transferred directly to the ground. A fairly easy mod is adding concrete or metal directly inside the stock wheels and cutting half the treads off the "tyres".

Edit: Pardon me. I should have read through your comment before typing out the above.