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Old 05-21-2012, 10:38 PM   #1
mfp4073 OP
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tires, fork oil, road, old tech? Whats causing my bouncing front forks

1974 r90 /6 bmw.

Bridgestone spitfires, front tire with 12k miles, fork oil who knows.

Seems like lately the front end is a little bouncy. Almost feels like at times its got a bounce to it, but will run smooth others when the road is smooth. Its NOT a high speed vibration. Tire does show a small amount of cupping that can be seen in when the shadows are right (like at certain angels with the sun).

As I am new to this, (only 20k miles under my belt) and this only my second front tire. Never changed fork oil, and its on my list (probably need to do a full rebuild too).

Lets hear your thoughts
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Old 05-22-2012, 09:40 AM   #2
DoktorT
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New tire, then spec the forks.

http://w6rec.com/duane/bmw/fork/title.html
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Old 05-22-2012, 12:21 PM   #3
hardwaregrrl
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Could be an out of round tire. Happened to me and it was the rear tire. Made the front pogo a bit just as you describe. Put it on the center stand and have someone spinnit as you hold a straight edge to it
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Old 05-22-2012, 01:38 PM   #4
Tosh Togo
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Originally Posted by mfp4073 View Post
1974 r90 /6 bmw...., fork oil who knows.

...Lets hear your thoughts
Fix that bit first.
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Old 05-22-2012, 02:23 PM   #5
photorider
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Hit any curbs lately?
I had the same situation. Right around 40mph it would start bouncing like a pogo stick.
Turned out the front wheel had a "hop" in it. Trued wheel, problem solved. Or as hardwaregrrl suggested, it could be an out-of-round tire. If you watch it closely as you spin it, you should be able to tell.
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Old 05-22-2012, 02:26 PM   #6
mfp4073 OP
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Hit any curbs lately?
I had the same situation. Right around 40mph it would start bouncing like a pogo stick.
Turned out the front wheel had a "hop" in it. Trued wheel, problem solved. Or as hardwaregrrl suggested, it could be an out-of-round tire. If you watch it closely as you spin it, you should be able to tell.

Know I need to get the fork oil changed. Its been on my list....as well as many other things

Hit nothing. Will the "out of round" issue be present all the time? Would think it would be. The is seems like a rebounding issue. But as this is the only street bike I have ever ridden, I have no frame of reference for whats normal.
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Quote:
Originally Posted by Martin Blank
loner, lonegunman, get it. Thatís the whole point. I like the lifestyle, the image. Look a the way I dress.
Quote:
Originally Posted by bgoodsoil View Post
Cavemen must've designed them shortly after inventing the wheel.
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Old 05-22-2012, 02:40 PM   #7
rufusswan
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You wouldn't necessarily feel a "constant" effect if the tire were out of round.
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Old 05-22-2012, 05:38 PM   #8
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On two airheads and a Honda Hawk GT 650 i followed Race Tech's instruction when installing springs, oil and their emulators.
Some of the best money i spent on all three bikes.

A few mc web sites have it posted. Google mcallister suspension 101 for mind boggling detail on suspension set-up.
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Old 05-22-2012, 06:03 PM   #9
mfp4073 OP
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I dont need complicated and crazy detail, I need simple for stupid!

The link posted early in this thread didnt work. Is it just as simple as support the forks, draining fluid, out the bottom, take off caps and replace measured amount in the top?
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Quote:
Originally Posted by Martin Blank
loner, lonegunman, get it. Thatís the whole point. I like the lifestyle, the image. Look a the way I dress.
Quote:
Originally Posted by bgoodsoil View Post
Cavemen must've designed them shortly after inventing the wheel.
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Old 05-22-2012, 06:22 PM   #10
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That's how I'd do it, but I would remove the caps to make sure that a vacuum did not occur to hinder draining. That will get you back to a baseline so that you can then begin picking off the pogo issue.
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Old 05-22-2012, 06:40 PM   #11
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Sorry, i should have known better than post in an airhead thread.
Most want a quick, cheap, dirty fix or a 15 second sound byte that tells or fixes all.

Read over at Race Tech's site about suspension with simple words.

Remove and fully service the forks as new oil will just stir-up old crud inside'em.
Seriously consider new spring and emulator along with new seals. Sheesh!

Again sorry and i'll never post in an airhead thread again even though i owned two of'em. I found them to be boring bikes BTW.
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Old 05-22-2012, 07:04 PM   #12
mfp4073 OP
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Originally Posted by OldPete View Post
Sorry, i should have known better than post in an airhead thread.
Most want a quick, cheap, dirty fix or a 15 second sound byte that tells or fixes all.

Read over at Race Tech's site about suspension with simple words.

Remove and fully service the forks as new oil will just stir-up old crud inside'em.
Seriously consider new spring and emulator along with new seals. Sheesh!

Again sorry and i'll never post in an airhead thread again even though i owned two of'em. I found them to be boring bikes BTW.
I have had a few older cars that when I first got them did a "double oil change" for that reason. Not ready and have time for the full project of rebuilding the forks. It is on my list. Baby steps!
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Quote:
Originally Posted by Martin Blank
loner, lonegunman, get it. Thatís the whole point. I like the lifestyle, the image. Look a the way I dress.
Quote:
Originally Posted by bgoodsoil View Post
Cavemen must've designed them shortly after inventing the wheel.
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Old 05-22-2012, 08:40 PM   #13
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It sometimes helps to pump the forks after the initial drain, to make sure you get all the old fluid out. Then maybe fill 'em, ride a while, and repeat? If the first fill fixes the problem, though, you have my blessing to stop there!

And maybe point it away from anything you care about when pumping them. My mountainbike fork filled my eye with old fluid. Bastard.
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Old 05-22-2012, 10:56 PM   #14
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Quote:
Originally Posted by mfp4073 View Post
Bridgestone spitfires, front tire with 12k miles,

thats a lot of miles on a front tire. Time for a new one I reckon. Get the wheel balanced when new tire is fitted.

And you need a basic Haynes workshop manual, so will see changing fork oil is very simple, eg put on centrestand, drain oil out the bottom, fill in the top. Be sure you can get the fork top nuts undone before draining the oil, they can be a bitch.

While you have it on the centrestand, spin the front wheel and make sure it runs true with about 1mm, maybe 2mm max. Tighten spokes if necessary to true rim. If you have snowflake mags, they always run out out so tighten steering head bearings to take up the shimmy if needed.

And whiel on centrestand check steering head bearings and tighten if needed.

All real simple stuff and no need to dig into any high tech solutions until you have eliminated the simple basics.
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Old 05-23-2012, 09:32 AM   #15
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How does the fork feel when you dive it standing still with front brake on? Does it return as fast as it compresses?
The Racetech stuff is great and a worthy investment, but may be overkill for your riding style. If it were my bike I would do this:
  1. Drain and replace fork oil to see if there is any change. No real change for the better? Then...
  2. Check your static sag. If it sags too much under its own weight then new springs are in order, but probably not the cause of the pogo.
  3. disassemble forks. There are rubber bits in there that break down and can cause a pogo effect. Don't mess with the wiper rings unless they aren't there. The new ones are too big and require filing.
  4. Carefully reassemble, re-align and refill.
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