Vibration/shimmy at 55

[section8joe]

I have a david bird hardtail with 4 inch stretch. I've been getting a weird shimmy/vibration at around 55MPH. If I go a bit faster it goes away and I can't feel anything below 55mph. I don't think its an out of balance tire because it's not a constant vibration. It seem to be more along the line of the chain rotation vibration. I've checked my valves, synced up the carbs, re-adjusted the clutch, check both wheels bearing, Steering head and brake drag. I think the problem may lie with my monster craftsman chain tensioner. I have the chain tensioner mounted kind of low and the skate wheel tension kept flipping over. I made a new swing arm part of the chain tensioner that's about 1.5 inches longer then the stock one. I'm thinking maybe it's not getting the snap that it should to keep the chain tight. I hear they sell a beefier spring. I also have been reading about "harmonics" and that this may be something I may just have to deal with. Can someone suggest a solution?

 

[TwoManyXS1Bs]

Hey, Joe. Yeah, I developed a shimmy when I turned 55 too. Just walk in circles now...

Can you post a pic of your chain/tensioner setup?
Couple things to check: wheel/rim trueness and fully seated tire bead, rear sprocket trueness and chain tight spots.

 

[section8joe]

Thats my chain tensioner. The wheel seems true and I'll have a look at the tire bead. How can I check the trueness of the sprocket? Should I take it off and lay it on a flat surface? The chain is a brand new ORing chain but it does have the usual tight spot.

 

[jd750ace]

Can you re-position the tensioner? If a resonant frequency is developing in the chain at a given speed, moving the tensioner will change the frequency.

 

[section8joe]

Steering is stock. I'm almost positive it's rear end. Steering head is tight. I'm going to try to increase the snap on the tensioner spring.

 

[TwoManyXS1Bs]

To check the rear sproket, just lift the rear, spin rear wheel, look at sproket from behind, sighting down the chain path, look for runout/wobble.

You can also try: Lash-down or remove tensioner, adjust chain for 1/2" total up/down slack at tightest spot, ride and see if it's different...

 

[angus67]

how tight is your chain? I have the same tensioner, and from the drive sprocket back, the upper and lower parts of the chain is parallel, then at the powell wheel, it angles down to the rear sprocket. your shimmy prolly has something to do with that tensioner, as you predict. I cant find a foto of my chain rite now, but if I had my tensioner that low, the chain is way way tite.

 

[angus67]

found this.....
http://www.xs650.com/forum/showthread.php?t=26505
My chain runs like this one.

 

[section8joe]

Ok. I checked the rear sprocket for wobble and it seems pretty damn straight. I added more tension to the tensioner and it seemed to do the trick. I can still feel a little shimmy but its pretty much barely noticeable. There another notch on the tensioner to add even more tension and I may give it a try. You can see in the first picture where my spring was set and in the second picture how I added more tension by moving the spring to the next notch. Thanks for the help guys!!!

 

[section8joe]

how tight is your chain? I have the same tensioner, and from the drive sprocket back, the upper and lower parts of the chain is parallel, then at the powell wheel, it angles down to the rear sprocket. your shimmy prolly has something to do with that tensioner, as you predict. I cant find a foto of my chain rite now, but if I had my tensioner that low, the chain is way way tite.

How did you adjust your chain slack? I flipped the chain tensioner so there was no tension on the chain. I then adjusted the chain for 1/2 play in the tightest spot. Then I flipped the swing arm part of the tensioner over and bolted on the skate wheel.. Maybe my chain is too tight....

 

[TwoManyXS1Bs]

What you've done is successfully isolated the problem to the chain. Ideally, a new chain has no tight spots, but it happens. Now, how to handle chain tight/loose oscillations? Try Angus's setup, increase slack, let your tensioner do the work, spring tension your choice.

 

[section8joe]

Im gonna give it a shot and loosen it up a bit. The chain's a brand new $120 Oring chain. I also have a newer rear sprocket. I know I should have bought all new sprockets and a chain at the same time but I didn't. I'm just glad I think I've pin pointed the problem. I found this video on the monster craftsman site setting up the tension on the chain. Hope this helps someone else with a similar problem.

 

[section8joe]

Well guys, I thought I fixed my problem but I guess I'm wrong. I was on the highway today and I still felt the vibrations. I think I'm gonna order all new sprockets. I looked at them and they look alright but I know the front one is the Original sprocket on my xs. Maybe one of these sprockets are "out of round" or worn out. I called Mike from 650 central and he suggested the sprockets swap. He also said that the vibration just may be a part of the bike and that there may be nothing I can do about it.

 

[TwoManyXS1Bs]

Hard to imagine an expensive new chain having a tight spot, enough to notice. Could try an el-cheapo chain (keep as backup), and see if you still get 'tight' spots...

 

[Pedal2metal]

Did your new sprockets help out?

 

[jd750ace]

What rear tire are you running? Did you balance it? Have you checked it for lateral variations and roundness? Can't really buy that it's all on a new chain and new rear sprocket. Have never had any issues with new chains and sprockets, but have also never ran a hard tail and tensioner. I always look to the tire first, as it's the biggest variable, front or rear.