We have been doing a bit of investigation into the "topping out" clunk that comes from the forks when returning to full extension with the front wheel off the floor.
We noticed this occurs on both the OEM bikes with standard springs and fork caps, and only seems to occur when riding the bike over whooped out trails where the front wheel is skipping across the tops of the ridges, so for most riders it would never become apparent, but now we have raced and ridden the G310GS extensively, and with feedback from Jenny's trip in the US, plus Amy on her Mongolia to UK ride, it is time to look into the workings of the BMW fork today in the workshop.
As most of you know, the right fork leg has an open cartridge with a small top-out spring inside the cartridge, plus a fork spring on top of the cartridge. The left leg has another fork compression spring in a chamber with a piston rod and another top-out spring above the piston, held into the tube by a threaded cap.
We made a tool to unscrew the internal cap to gain access to the top-out spring and compression spring in the left leg, and it was very heavily loctited into the inner stanchion, so heat is needed to soften this before attempting to unscrew the dap.
We did some measurements and found that we had a slightly longer and stiffer top out spring in stock, so fitted that into the left leg, re-assembled it with 450cc of fork oil, the oil weight is not important as the left leg has no damping inside, the oil only acts as a lubricant and to set the air chamber height.
We decided to leave the top out spring inside the right leg, as when the open cartridge was removed it became clear that the tube that holds the damping piston and top out spring is sealed t the factory by crimping over the metal cap, so we just refilled the leg with approx 450cc of 15W fork oil, to give an air chamber height of 100mm, without spring and the fork closed.
After a long ride this afternoon the "clunking" noise seems to have virtually gone, and the reduced air gap, plus the change from 10W to a slightly thicker 15W oil has made the fork action much plusher and less inclined to bottom out.
I must point out that we are riding the bike VERY hard on our Enduro test track, and I am not a lightweight rider, so for most owners, with or without our Fork Kit fitted, the clunking noise is not a problem, but as we are continually striving to improve our products and the performance of the G310GS, we will look to add a new top out spring, plus the special tool needed to access the spring so that owners can get the best performance from the fork, to compliment the superb Tractive shock on the rear.