Fork cartridge bush leak

Hello all! I'm having trouble with bottoming out on my '99 VFR800.



I've installed new compression valves and revalved the rebound valves (parts sent from one of the members here). Springs are 0.95 kg/mm linear rate. Oil is equal mixture of Motul Factory 5w, Factory 2.5w, and Expert 5w to get as close as possible to the viscosity of Honda Pro 5w. I ride fairly hard in the twisties, getting to the edge of my rear tyre, and 5mm chicken strip on the front (Bridgestone S20 at both ends). I weigh 80kg, plus 10kg riding gear, and usually carry about 7kg of stuff in my gear pack. The oil level is set at 120mm.



I had an incident where I was lightly trail braking toward the apex of a corner (at a lower speed than I normally take the corner because it was at night, and half an hour earlier I had nearly been hit by a suicidal kangaroo), then I hit a bump about 1.5" high, which caused a hard bottom out (loud bang and shock through the frame). The front tyre left the ground, and when it touched down it screached then let go.



I've re-faced all valves and checked shim flatness, and they look ok on a flat plate of glass, but the bike still bottoms out when I do a quick-stop test (not emergency braking), and bounce testing overshoots at least once, sometimes twice, before settling. I've pulled one of the cartridges out again, and checked it for bush leakage, and there is some, but I don't know what amount of leakage is ok. The video I took is at this link:



http://m.youtube.com/watch?v=tX7-Iv...top_uri=/watch?v=tX7-IvHlowI&feature=youtu.be



Is the amount of leakage from the top bush ok? This is a standard Showa cartridge (right-way-up fork) from the VFR, but with the valving changes. Any help that people can give is greatly appreciated!

 

The behavior is completely normal. the flow past the bushing is non-linear and during operation it would gush out like that. Rebound speeds are much faster than you are actuating the rod. Your cartridge looks fine to me!

 

You might want to make sure that the check valves on the top of the compression stack are closing properly and not hung up on the lip of the bolt. If this happens the compression stack won't be doing much. The spring rate and oil level sound correct (I looked at Racetech's website).

 

Something's not right if he bottoms out, right ??

 

I've bought a funky little 50x magnifier attachment for my phone's camera. Even though the shims and check-valve on the compression valve looked (to the naked eye) like they would seal, the magnifier showed that there would be some leakage. I've been gradually working at re-seating the compression valves, using a piece of plate glass and 1200 grit sandpaper, then cleaning, drying, lightly assembling and checking the valve/shim/checkplate for gaps. I'll see how well it works a bit later today.

 

If you have to look through a 50x magnifier to see it - that's not your problem! Nothing seals perfectly, nor does it have to. In fact, you want to have (dare I say, must have) some leakage. That's called "free bleed". When those shims open the flow will be orders of magnitude higher than any free bleed will provide. Flow through an orifice is also very nonlinear so the higher the shaft speed the less effect it has. Under hard braking you are way beyond the free bleed-only range.

 

This is from my copy of the "Race Tech Motorcycle Suspension Bible"
Bottoms-Too Soft, Mushy
Ask questions:
Ask questions:
1. What kind of conditions? (G-outs, landing on jumps, face of jumps, etc.) We are determining whether the <fork> velocity is low or high.
2. Does it feel too soft everywhere? (If yes skip A, go to B and C)
A. Oil level low – raise oil level – affects mostly the last 1/3 of travel
B. Not enough low-speed compression damping
C. Not enough high-speed compression damping
D. Spring rate too soft
E. Not enough preload
F. Dirt in valving, broken valve, bent shim, burr on the piston/shim
G. Cartridge rod bushing worn out (typical problem with pre-1996 KYB)
H. Compression valve o-ring broken, especially if just rebuilt
I. Cartridge rod not attached to cap – broken or unscrewed – oops”
“Upon compression, the damping rod goes into the cartridge and displaces fluid. This volume of oil (the volume the damping rod displaces) must exit the cartridge. In a standard cartridge fork, it is this volume of fluid that controls compression damping.”

Sounds to me that if the compression cartridge is OK, then the shaft seal could be a cause. You do need to ask yourself if aside from this one instance, the forks were working OK. Hitting a big bump, leaned over, on the brakes, does sound like a likely cause to break traction to me.

 

Here's a photo of a typical interface between the shims/check plate and the valve that I've achieved:



Unless Tapatalk changes the size of the photo, it should be 50x actual size at 100% on a computer screen. Also bear in mind that the gap looks bigger than actual because of the reflection on the check plate. Unfortunately, it doesn't seem that this has fixed the issue. I intend to test ride the bike in the morning, but the initial indications (tested rebuilt cartridges in a bucket of fork oil) look like the bike will dive just as much as it has been all along.