Fork mods

[Bren_chch]

Im not trying to reduce the fork travel, just the extra length sticking up above the top triple clamp. The other thing Im hoping to achieve is firm up the forks as they are hopelessly soft. Under hard brakes into the hairpin the forks are fully compressed and mechanicly solid (really compressed) so the handling is a serious problem. by chopping 70 mm off the top of the fork leg and a similar but to be finally worked out amount from the spring, I hope to shorten the fork and increase the spring rate all in one go

And then you can do my set!

[F5 Dave]

Well treat them separately. So you cut off 70mm & then have to remachine the fork tops. That's ok. If you cut 70mm off the springs they would doubtlessly coilbind + be way too stiff.

As I said there will be a spacer in there probably about 100mm long. Then cut off say 10% or maybe 15% of the active coils from the springs (after carefully checking that you won't coil bind whilst preloaded & compressed). I cut off starting with any close wound coils first.

Then measure how much shorter the spring is, lets say 30mm.
Then that 100mm spacer - 70(from the fork tube)
+ 30 (removed from spring)
should be cut to 60mm as a good starting point.

Then try it. I'd stick to small changes as you will also get the increase in air spring from the relatively higher oil level. To that end one will need to do a sensibility check to see that the oil level will not be way too small. You may get away with lowering the level slightly, or you may decide to only cut 50mm off.

[Yow Ling]

Ok made some progress, my engineer guy has cut to top 70mm off and recut the threads inside the forktubes. Now I need to collect the data here and reassemble forks and figure out the perfect springs. Not much eh?

Here is the BSB guy, hes sorting the MotoFXRs for this season, beats endless BSB testing I guess !!

[Yow Ling]

Time for some action on this job, we got racing in 2 weeks !
After shortening the fork tubes I wanted to start with about a 10% increase in stiffness, before I cut anything I put a full spring (unmolested ) into a fork assembly and measured it on the test rig , pic 1. I made about 6 measurments and they averaged out at .50kg/mm. I just applied some weight measured the peak then measured the forktravel with a zip tie.

Ok now calculate the number of turns to remove about 2 1/2, just used an angle grinder with a skinny cut off blade. Heat the ends to reshape the flat end , linish it kinda flat , then back to the test rig.

I made 3 measurments and the average was .54kg/mm so I got around 8% increase, close enough to try out, now i need to mess with the oil levels and fit to my bike

[F5 Dave]

So presumably you ground the ends flat so the spring is as flat as it was before. Then you will check preload is ~ 25-30% rider on board. Sounds promising.