Front End Vibration/Damping - Causes?

[mschirmer]

Newbie CBX owner here - so apologies if this question has been answered before.

Have an '81 that I purchased and brought back to life after about a 10 year unsympathetic storage (rebuilt carbs, brakes, lots of new gaskets, new tires, etc.) - now showing 8900 miles.

Problem is that on anything less than perfectly smooth pavement, I sometimes sense an small amplitude, rapid up-down oscillation vibration in the front end - most obvious between about 30 and 50 mph. Feels for all the world like an out-of-round rim or tire on a bike w/ really hard shocks. Checked the static balance of the front wheel (OK), fork pressure at 9 - 10 psi, no fork seal leaks, tire & wheel runs true, no rim damage. The hard thing to figure out is that it doesn't happen all the time. Otherwise the bike handles incredibly.

Any ideas on what could be going on? Could the fork oil need changing (may be the original 30 ear old stuff)? Perhaps these bike do better w/ a dynamic wheel balance? Perhaps a front wheel bearing? Does a fork brace help this type of problem?

Or maybe I'm just expecting to much from a 30+ year old design? (Lots of miles on newer BMW K bikes).

[steve murdoch icoa #5322]

Welcome to the site.
An '81 and new BMW, you will be talking with EMS shortly.
I would certainly change the fork oil but i wonder if the front end vibrations could be coming from the steering stem.
Maybe the bearings have been damaged or worn out. Retorque everything around the front end?

[NobleHops]

I've had a few older bikes exhibit this very thing - my 91 ST1100 did it and my 2003 VStrom still does. The bike 'pogos' as a certain speed.

I never did get to the bottom of it, so I will be ver interested in what you learn.

Oh and BTW WELCOME to the site. Sounds like a great bike you've got there.

N.

[mschirmer]

Here's an update on the symptoms.

First checked the head bearings - all seems OK there - no play either axial or runout.

What appears to be the problem is stiction of the sliders. When there's a moderate impact, everything works as it should. But for light bumps, the forks do not compress and the only compliance I'm getting is from the tire. When the fork does absorb a bump, it slides much easier that what is required to get the first compression - you can sense this by bouncing the front end up & down with the bike stationary (and front brake held on) and noting how hard you have to push to get the initial compression vs. keeping the bike bouncing. The fork air pressure seems to have no effect on the stiction.

Maybe this gives a clue to some CBX experts out there!

Also , can the fork oil be changed w/o taking the fork tubes apart? iT looks like there's a drain plug on the slider, and the forks caps can be removed easily - so I can see no reason why this can't be done. But the factory service manual says nothing about it!

[steve murdoch icoa #5322]

You could do it without removing the forks but i was taught to remove them to get all the oil out, as well as the crud and debris that have settled in the bottom.
10 weight oil for me.
About half way down in this thread there is some great info from Dave.
viewtopic.php?f=24&t=6701&p=50247&hilit=fork+oil+changing#p50247

[daves79x]

A couple of the early models that I put Pro-Link forks on exhibited this annoying trait also. Your observation about stiction may be indeed be some of the problem. Honda recommends 5wt oil (or ATF in the old days). That's lighter than most guys like to use, but may be worth a try to minimize stiction. I use 7wt if I can find it, 10wt if not. 10wt is a little heavy for me even. You also need to know that Honda's Comstar wheels are about as round as an egg. Very few I've seen are perfectly true. It is amazing to ride a bike with perfectly true wheels. A company in Tennessee (Frame Straight Systems, I think) trued a couple of wheels for me (not Comstars) and the difference in smoothness is amazing. It is well worth having it done. They have a video on their website about how they do the work.

However, I installed different 1100F wheels that were pretty true on most of my Pro-Link fork conversions and the 'bounce' was still present at certain speeds on certain roads. Best I can tell you is to rebuild the forks completely (off the bike), cleaning everything well - you will find a lot of crud in them. I'd try the recommended 5 wt oil set to 6 inches from the tube top with the spring out and fork collapsed (a lot more than Honda calls for). While you are that far, carefully check the steering head bearings for roughness and a straight-ahead detent. If any of this is detected, replace them both. Some bikes will behave pretty well after this and some need more tweaking. As has been mentioned, tires have a lot to do with this. Brand new matched tires will make the bike feel completely different than with old or partially worn ones also.

Dave

[mschirmer]

Thought I'd update folks on what I've found thus far.

I drained the forks and removed the springs. The oil in the forks was very viscous purple stuff - maybe ATF? The springs were about 1.25" shorter than stock w/ the difference made up with 1" PVC pipe - probably not original spring but probably Progressive replacement ones, as they commonly use PVC spacers. The springs were definitely not linear, nor the usual progressive style (only one end w/ tighter coils). Instead, both ends had tighter coils and they were VERY stiff, heavy duty springs. The fork tubes looked pristine - no signs of corrosion, pitting, or gunk from deteriorated seals.

I replaced the springs I took out with new Progressive springs, per their cross reference for a 1981 CBX. The new springs had tighter coils only on one end, and were much softer than those I took out. They also used 1.25" PVC pipe spacers. Flushed and refilled the forks with Bel-Ray 5W oil, filling them to about 5.5" below the top of the tubes with the fork compressed to the stops (springs removed). Reassembled with zero air pressure (per the Progressive recommendation).

Test rode the bike, and things were noticeably better, but there was still a substantial amount of stiction resulting in a harsh ride.

I then loosed the front axle clamps, and bounced the suspension several times, and then gradually re-torqued the axle clamps in small increments.

Test rode again and things were much, much better. Not yet perfect, but quite acceptable for a 30 year old ride with bias ply tires.

So...it looks like I had at least three problems. (1) The oil was probably too high a viscosity, (2) The non-OEM springs were too stiff for the bike, and perhaps not the proper replacements, and (3) The fork tubes were probably not perfectly parallel, likely being misaligned after a tire replacement.

I have not yet replaced the fork seals (they seem fine) - maybe a project for next winter.

[steve murdoch icoa #5322]

Man, with a follow-up like that you did not need our help. lol.
Great to hear you got it sorted out.

[NobleHops]

+1, nice work, and thank you very much for following up. It's nice to know what was tried, and what worked .

N.

[EMS]

So...it looks like I had at least three problems. (1) The oil was probably too high a viscosity, (2) The non-OEM springs were too stiff for the bike, and perhaps not the proper replacements, and (3) The fork tubes were probably not perfectly parallel, likely being misaligned after a tire replacement.

.

Cool. Great report on how to address a common issue!

Just to clarify one thing, I am not sure whether you were saying the same thing: "High viscosity" means the oil is "thinner", low viscosity means it is thicker, it "flows not as good", is less viscous. Fork oils are usuall in the range of 5 to 15 weight, with 15 weight really being the max and used rarely. This actually means, fork oils have high viscosity.

[daves79x]

Good job and great info!

Dave

[Jeff Bennetts]

Well now it looks like the front is pretty much sorted now its time to take a good look at the rear. Get the rear off the ground and spin the rear tire making sure it spins freely and there is no slop anywhere, check the swing arm for smooth travel up and down and make sure there is no play side to side, check that the rear tire is parallel to the swingarm and not cocked to one side, is the tire wearing evenly.

People sometimes neglect to realize problems out back can transfer handling issues to the front of the bike.

[cbx6ss]

Maybe I missed it, but I did not read a check for fork tube straightness. It would seem this might effect the stiction.

I believe this is a really informative write up of the principles of fork suspension, and how the gold valve improves suspension.
I had to read a few times for some of it to sink in.
http://racetech.com/html_files/DampingRodForks.HTML

I wound up buying the Race Tech "Motorcycle Suspension Bible". Wow some great info!! Well spent $35 for those wanting to improve their suspension knowledge.

Regarding the previous pix, after re-evaluating my bushings, the wear surface of the wide bushing is the outside surface. It really was not that bad. The wear surface of the narrow(er) bushing is the inside surface. Again it was not that bad. My thought was, if I was gonna rebuild to a race tech suspension, I wanted to start with a clean baseline, thus new bushings, seals, etc. to eliminate all variables.

In assembling the forks, the left assembly had a bind to it I had a hard time tracking down. At one point I thought the tube was bent. I tracked it down to the narrow, top bushing. The last time the old narrow bushing was pressed in (um, by me...), I'm guessing it got cocked, and the edge scratched the surface of the area it is installed in as I found a glob of soft aluminum protruding from the bottom of the scratch. Not noticing this, when I installed the new bushing, the gob, forced the new bushing to protrude in that area. This caused the tube to bind just a bit. i disassembled and found the issue. I used some 500 grit sand paper to smooth out the scratch. I followed that with a gob of mothers polishing paste to polish out the sanding marks. Lastly of course I thoroughly cleaned all surfaces to ensure no grit remained.

I found a 8" length of 2.5" PVC tubing works great as a slide hammer to install the new seals. Be sure to install the backer plate , then the seal, then the backup plate before hammering the seal in... (I now know the correct order from memory...). for fork oil level, (before installing the spring) collapse the fork tube. Fill with fork oil (I used 10W) to 100MM from the top. I crafted a dipstick and marked with 100MM and 130MM levels. Pull the tube all the way out before installing the spring. Otherwise the oil will overflow... I found this out too...

I took a short ride. First impression? WOW!! You should consider the Race Tech Fork Emulator upgrade. Significantly different feel! Much tighter (not harsher).

Here's a link to my 81 fork rebuild w/pics.
viewtopic.php?f=24&t=7496

Welcome and Best
/r
Brad

PS - 2 rallies coming up

Newburgh NY 6/7-9
viewtopic.php?f=106&t=7565

St Clairesville OH 6/15-17
viewtopic.php?f=106&t=7769

Hope to see you there!

[NobleHops]

Nice post Brad, good stuff.

N.

[cbx6ss]

Looks like you two could meet half way in 30 minutes for a cold one and a quick ride!