[Clark Rice]

I have a 1990 PC800 that I have been working on myself. I have had an issue with the ride since the beginning. I bought the bike with an out of round tire which I replaced because it caused the front end to jiggle up and down. Further, it had bent forks and so there was severe binding, which likely contributed to the tire's out of round wear pattern. I replaced the forks with used ones that I rebuilt using All Balls bushings and seals and Maxima 10W fork oil. They work reasonably well but have about 15mm of stiction which I have tried to address by polishing the tubes using chrome polish and a towel. I verified the straightness of the tubes on a glass table and have reset the alignment many times using different methods. The up and down jiggle persists, but only shows up on smooth pavement when the bike hits a dip or a ripple in the otherwise smooth surface. This bobbing goes away when the wheel encounters a real bump, and also does not show up above about 45 mph. The frequency of the jiggle is independent of speed and seems to be about 5 to 10 Hz, as if the tire and bike are acting as an undamped spring and mass system excited at its natural frequency. It usually only continues for a second or two, but can maintain continuous oscillation on a very smooth road between 20 and 35 mph.

I turned my attention to the tire and wanted to check if it had balance, out of round or out of true issues. It has maybe half a millimeter of out of round and perhaps 1-2mm out of true. The wheel itself is very close to perfect. I checked this by inserting the axle into the wheel and supporting it with jack stands then using a vice and a small rod as a makeshift indicator. The balance seemed perfect while it was on this stand because it had no tendency to rotate at any angle. It is inflated to 38 psi to accommodate the weight of the bike and the rider's 230 lb mass. Still the jiggle persists.

I noticed a bit of play at the ends of the forks with the wheel lifted, and because of the bike's age I decided to upgrade the steering bearings. I installed All Balls tapered roller bearings and dialed in the correct preload through trial and error. I hoped this would stop the jiggle finally, but it is still there.

If I grease the forks, stiction reduces temporarily to a more acceptable figure of about 5 to 10mm, and the jiggling is less pronounced. However, with stiction in this range I would not expect appreciable issues with the ride, and fork travel feels smooth and reverses without a hitch. My guess is that the forks are better able to absorb the jiggling which mutes it somewhat.

My question is this: is there any other conceivable mechanical issue that I can eliminate before trying to replace the tire which has less than 1500 miles on it? I have been on a PC800-specific Facebook group and discussed it with them and eventually the consensus pointed toward the tire being the culprit, but after all I have invested, getting a new tire and still having this jiggle might put me over the edge. :wink2: Any words of wisdom are appreciated.

 

[Eye_m_no_angel]

I'm not familiar with the suspension in those bikes, (pretty cool looking machines though,) but the first thing that jumps out at me is to make sure you have the correct amount of fork oil in the tubes and didn't get any air bubbles in when filling them. Also, does that bike use an emulator valve or some other type of dampener inside the fork? It sounds like you've been pretty complete in what you've done, but that for some reason there is too much rebound being allowed.

 

[Critter]

If you have the right amount of oil and no air bubbles, perhaps a heavier fork oil? Say 15w

 

[4getful2]

You could remove all the weights and rebalance the tire but I think you have other issues.
A weak dampener or weak springs . You kinda need a manual with some specs. for proper torquing and reassembly.

 

[Clark Rice]

Compression and rebound damping in PC800 forks

When I assembled the forks I analyzed the damping mechanisms in each of these damper rod style forks. The right damper is designed with an oil lock piece. The damper rod/piston in the right fork has 2 large orifices on the bottom and 4 large orifices in the top which allow free flow and thus reduced compression damping for the first ~80% of travel. Beyond that displacement, the inner slider covers these 4 holes and the oil lock seals against the end of the slider for the last 20% or so of travel. This seals the bottom 2 orifices as well and causes the oil to have no where to go but through the small passages in the bottom of the inner slider. Thus, in the right damper's compression damping increases dramatically at the bottom of the stroke as the inner slider covers the holes in the damping rod and forces oil through these choked orifices. The oil lock also has the freedom to move up on rebound. Oil pressure on rebound moves the oil lock to the up position, uncovering two large orifices at the bottom of the damping rod. It is unsprung, so it remains open until the inner slider travels down enough to push it back into position, again covering these orifices. Rebound damping is restricted by a check valve in the bottom of the inner slider on both tubes.
As for the anti-dive - the left damper has much harder compression damping than the right damper overall, due to using just 2 large orifices compared to the right damper's 6. When you apply the brakes, the reaction of the brake caliper closes a circuit at the bottom that normally allows oil to go through the damping rod. A 1-way valve (the little spring and washer setup on the damping rod) makes it so that rebound damping is unaffected by this system during braking. When this circuit is closed it seals all but the very restrictive orifices on the bottom of the inner slider, making the left damper very stiff in compression. This is also why the damping rod piston has 2 seals as opposed to the 1 on the right damper - very high pressure difference between the upper and lower chambers.
There are 4 orifices on the bottoms of each slider that work as damping orifices by communicating pressure to the 3rd chamber between the piston seal and the slider insert. These are more restrictive on rebound when the check valve closes and limits flow to the 4 choked orifices instead of allowing it to flow freely into the 3rd chamber.

 

[Clark Rice]

I pledge allegiance to the manual

I have tried using 15W fork oil and it made no difference. When I poured the oil into the forks I moved the inner tube through its travel until no more bubbles appeared and felt the damping working. The rebound damping is a little stiffer on the right damper than the left, and compression damping is equal to the rebound on the left and lesser than the rebound on the right.

 

[Clark Rice]

The oil level is good, used the stock spec of 163mm (6.42in).

 

[Critter]

I'm I reading your post correctly, the two sides are different construction? and the dampening is different one side to the other?

It was designed this way?

 

[Clark Rice]

Yes, Critter. I thought it was a bit weird too but that's how it is designed. The diagram above shows the differences between the two sides. Seems like it would promote binding to me due to the moment created by the differential forces in the forks, but any bike with an anti-dive system will have this difference between the forks, at the very least during the anti-dive actuation.

 

[Eye_m_no_angel]

I have tried using 15W fork oil and it made no difference. When I poured the oil into the forks I moved the inner tube through its travel until no more bubbles appeared and felt the damping working. The rebound damping is a little stiffer on the right damper than the left, and compression damping is equal to the rebound on the left and lesser than the rebound on the right.

How did you measure that?

 

[Eye_m_no_angel]

I'm I reading your post correctly, the two sides are different construction? and the dampening is different one side to the other?

It was designed this way?

That's actually pretty normal for a lot of bikes, to have one side different then the other. Honda's sometimes seem to over-complicate things on top of that, but it usually works fairly well, even if it makes it hard to diagnose problems sometimes.

 

[Clark Rice]

I only measured it by feel. This observation was made with the tubes full of oil and the caps off, as I was pumping them to get the air bubbles out. I spent a little time comparing the tubes and feeling the resistance as I moved the slider in and out by hand.

 

[Eye_m_no_angel]

I don't know enough about Honda suspensions to offer anything other then general advice here. (I know on a Harley front with an emulator valve on one side they certainly react differently as you move them up and down by hand, but that doesn't help us any here.)

I think I'd go back to the bare basics and step-by-step everything again. Wheel balancing. Bearings. Steering head bearings and pre-load. And so on. I know that's not any help, but at this point I'd be floundering on a Pacific Coast and doing just that.

What you describe, almost sounds like there should be some kind of dampening function that is failing, of some kind. Hmmm..

 

[Clark Rice]

Well I don't think the damping is failing because it feels good when you stand next to it and push the front end down, and it absorbs bumps pretty good. Based on what I saw inside the forks the damping acts just how it ought to. Compression damping is weaker on the right fork than the left fork and rebound damping is about the same between the two. This makes sense based on the design because the right fork has 4 extra holes for compression, and they both have a basic check valve rebound damper between the inner slider and the damping rod. The only complications in the damping are the oil lock piece on the right and the anti-dive on the left and both seem to work as expected.

I'm guessing its the tire. I looked over the edge of the bike while riding it in my neighborhood and found some smooth pavement to make it act up. It seems like the tire is just bouncing, so its probably some awful defect of the tire. I have an extra tire sitting around so I will have someone mount it and see if it still gives me problems. If it still bounces then that tells me it might not be a tire issue. Only problem with that is that the other tire is also a Dunlop, but its pretty much brand new. It is the wrong size but will still fit the wheel and forks as long as I leave off the fender. I'll let you guys know what happens.

 

[Critter]

That is what I love about this site you learn something almost everyday. While I don't understand in my mind how this works,to me it doesn't make sense that it would, but then again a lot of things don't make sense to me

 

[BEARR045]

Its not the forks, its not the tire, its not the dampers. I went thru something similar with a 175 Honda back in the 70s . turned ou to be a hairline crack at the base of the frame where it meets the three. Just my two cents.

Sent from my LG-D851 using Tapatalk

 

[Clark Rice]

Problem Resolved

Hey guys, I had replacement forks come in the mail that were alleged to be straight but they were bent. I sent them back for a full refund and got another set, also bent. I got frustrated for a while by the apparent lack of availability of straight forks and didn't have the cash to justify having a pro shop straighten the tubes on a press. I talked to multiple used parts sellers on eBay and inquired about how they figure out if they're straight. It turned out that most were doing it by eye, which is useless. Even the tiniest bend in the forks will cause binding which creates a rough, bouncy ride. Only one company that I talked to measured the forks to check runout using a truing stand and an indicator. The forks I got from them are straight, and when I mounted them and rode the bike - bam! Problem solved. The issue with them now is that they have pitting, but I can fix that temporarily with epoxy and careful shaving and sanding.

Also, stiction that exceeds 10mm can cause the bike to bounce on the front tire over low impulse bumps. The impulse is not great enough to break the stiction and allow fork travel, so it is essentially a rigid member, and the bike can go into an underdamped, steady state vibration with the front tire as the spring. This explains the jiggling phenomenon that I was grappling with.

If you encounter this symptom then you should check the straightness of the forks. Lift the front wheel, loosen the axle nut, loosen the right side triple clamps, loosen the right side pinch nuts, and rotate the inner fork tube in the triple clamps. If that tube is straight, then the bottom of the fork leg will stay still. If it is bent, it will slide left and right on the axle with each rotation and it might be hard to rotate the tube. There should also be a very minimal gap between the fork leg and the wheel spacer, just enough to clear the wheel. Tighten up the triple clamps and pinch nuts then repeat for the left side. If the tubes are straight and well aligned, a healthy stiction measurement should only be about 5-7mm maximum. Better seals and fork tube coatings may give a lesser stiction value, and I have even seen a BMW rider on YouTube that demonstrated 0 stiction with a real nice set of forks in perfect alignment.

Thank you guys for helping me with my bike! I appreciate your input, and hopefully someone in the future can read this post and solve a similar problem or help diagnose their true problem.

 

[Clark Rice]

Hey guys, I never actually did fix this. I somehow convinced myself that it was fixed and wrote that conclusion post, but it still did it after that.

I have bought a few motorcycles between then and now. A Helix 250, 5 PC800s, a VTX1300, an ST1100 and a Rebel 250. All but 1 of the PCs had the shaking and a lot of sticky travel. ST1100 shakes worst of all. But the Rebel does not exhibit this behavior. On the Rebel, I can see the fork easily while riding and I can tell they respond to every little imperfection in the road. Paradoxically, the forks still show quite a bit of stiction with the "zip tie test." When I look down at the forks on the ST while riding, I see that they ignore the small imperfections, only moving on larger bumps, and it has a little more stiction than the Rebel. Can anyone help me make sense of this? It seems like the forks are just sticking in the rest position rather than being responsive.

On the ST, I guess I need to figure out a way to rebuild each fork leg that reduces friction and also align the fork better. I already rebuilt them, but I noticed the seal felt sticky when compressing the fork legs individually. I figured it would be ok since I did everything by the book, but alas it still shakes on smooth pavement.

I am surprised I am the only one complaining about this because it is so pervasive in my collection. I would think others guys suffer from this, at least with other Hondas.

Any ideas are appreciated.

 

[bpe]

A little front end jiggle never hurt anyone.

 

[Clark Rice]

That's undamped vibration there