Tjaf,
Buy the graupner 1/5 scale they are lots of fun and super stable.
A little rock will not topple your R/C bike slow speed will. Your bike is happiest with a little speed.

I started with the Graupner bikes, myself and my brother, would race them.
It's a different skill you'll learn with the R/C motorcycles which I'm sure you'll appreciate.

I have two graupner bikes, one thundertiger FMe1 and two Kyosho HOR still in the box. Buy far the best bang for the buck were the Graupners at around 100 bucks at the time.

Anderson