well the old nitro RS4's that are belt driven are great because there is very little flex in them around where the engine sits if you get the Al or graphite rear brace. Mine never had gear striping issues and if you bolt a good torque rod from the nose to the rear of the car it stiffens things up plenty. ohh and to run 1/10 truck foams on the back of the RS4 you need the NMT rear axles A549 or easier yet TM racing makes bolt on foams for TC's 30MM x 2.650" Tall. seriously put a wing on the back possibly even a small wing on the nose, aim the exhaust outlet up. aiming it down actually will push the car up into the air especially if you remove all the front drive components.

http://www.tm-rc-racingcomponents.com/FrameSet15.htm