So both mine and my son's Tamiya DF02 Chassis's broke, in the exact same place... I've read of other accounts of it happening, so clearly a design flaw... here's where they broke, right where the lower suspension arm meets the chassis:



So we ordered new chassis's, and decided to reinforce them such that they don't break again.

It seems that the problem is that the bent steel forks that reinforce the lower suspension arms are not supported by the chassis at the open end of the forks, so they really aren't very effective.

So the plan was to make the forks longer, and have the open ends of the forks extend into the chassis... thereby the chassis will support the open ends of the forks, and the whole assembly will be a heck of a lot stronger.

So first I got some 1/8" steel rod, and bent them into new forks. This took some trial and error to get the width correct, but wasn't too bad. To make the bends, I clamped them in a vice, and pounded them over with a hammer. This pic shows after the first bend was made, about to make the second bend.



Here you can see the forks for the front and rear:



Next to drill the new holes in the chassis. Using a drill bit slightly bigger than 1/8", I bored out the existing holes that support the suspension arms (they were slightly smaller than 1/8"), then continued the drill bit in until it drilled right into the chassis. Here you can see drilling into the rear of the chassis, we also did the same on the front of the chassis:



Here you can see the rear fork extending into the chassis:



Then it was just a matter of reassembling the cars. Here the car is partially reassembled, you can see the rear fork extending into the spur gear box:



So we haven't fired up the cars yet, but I feel confident that the connection between the lower suspension arms and the chassis should be WAY stronger now.