Fierce Bonneville forks
Tony Wood asks if I have any idea why the forks of his Us-spec ’68 Bonneville are so harsh. He’s rebuilt them with all new parts, correct springs, along with the right oil grade and quantity, but they are too hard and don’t dive at all under braking. “There’s conflicting advice about the plastic damper sleeves – I fitted them with the tight end downward, is that right? Should I try more oil or a different grade?”
I’ve never had this problem with the same forks on my Triumph – but I have noticed with forks generally that replacing both stanchions and bushes together can lead to sticky action. That shouldn’t be the case, and pattern parts may be to blame – but I bet that the forks at Meriden were selectively assembled from piles of parts by factory fitters who did the job every day and could feel exactly how free they should be before signing them off.
The Triumph manual confirms that the internally-stepped plastic damper sleeves have their loose end up and tight down, as Tony has done, so that’s not the issue.
Fork oil? I don’t know. Grades relate to new forks; 50 years on they may need something thicker. The bottom sliders wear and corrode internally, making fitting new bushes like putting a new piston in a worn cylinder bore – but that affects damping not stiffness. I wouldn’t recommend altering the capacity, that’s part of the design, but a thinner oil might help. I’d try draining all the oil out and pumping the fork – that will reveal whether oil is part of the problem.
If not, it’s worth pumping again with the wheel spindle caps slackened. If the forks are not perfectly parallel, they will be stiff – particularly with all slack taken out by new parts, as before.
Wider yokes were fitted from 1969 along with a longer spindle and it’s just possible the long spindle could be wedged into the narrow yokes, splaying the legs. Tony’s going to take a closer look and let me know his findings...