My Rocker is lowered 2 inches in the front and 1,5 inches in the back? Now I would like to mount a 21 inch front wheel. Will I get a trail problem? I think the stock trail is 6,2 inches.

 

The diameter of the 21 compared to the 19 isn't going to be much different. You've decreased your trail with the lowering you've done, so it is a little less than the stock 6+ inches. The 21 may raise the front a wee bit, which would increase trail. You'll probably end up around 6 inches, which is fine. 6 inches is on the high side for an easily handling bike at low speeds. You'll get more low speed front end flop compared to a bike with less trail, but at least you won't come near to the high speed tank slapping range caused by too little trail.

 

Thanks for your answer. I used RB Racing Rake and Trail Calculator and it says 6,51 inches. Ok?

 

6.51 will be very stable. You might notice a little less nimble-ness at very slow speeds.