Ah finally someone reported the awesome TRF501X skick pack idea

.
I own a 501 and have used 2 different kind of stick pack configs :
-The first one is to put the stick pack under the rear belt. Let me try to explain (keep in mind French is my mother tongue^^). I used 1 Losi belt tensioner and another tamiya belt roller to raise the belt to achieve clearance. This way the pack could slide under the drive belt (it's close but it works). But this is only achievable if you use the 16 teeth center pulleys instead of the 18T. Another option would be to use a longer belt. (The stock belt is 369mm)
(I actually raced 1 day in Kampenhout without any breakages with this configuration

)
-The second idea is the one mole2k explained nicely (I wish I had your shooting skills

). The only thing I did differently is using another slipper clutch spring instead of raising the whole layshaft. The height of a Trakpower 3600 lipo stick pack is 20,8mm and the problem is that the silly slipper spring is in the way...To solve this i used a tamiya differential spring (the ones that come in any trf car). These are a bit smaller than the slipper spring and allow 1 good mm clearance between the pack and the spring.
Anyway, I raced with both layouts and it worked fine for me

. Ideally, an expert should test it because im not really Hupo Honigl

. Some will problably say using a stick pack in the 501X will change weight distribution and therefore the way the car handles... (personally I couldn't even notice a difference

)