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Old 05-24-2005, 09:36 AM   #1
meat popsicle OP
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Front End Wag?

My 03 KTM 640a seems to have developed the tendancy to become unstable at highway speeds recently. There are a couple of things that may contribute to this:

1) The front end was a bit out of sorts - the fork tubes were not even. I straightened this out (with the torque wrench to boot ) and this seems to have helped.

2) The front tire is a MT17 DOT knobby. The tire pressure seems to alter its handling drastically (steep profile = bike DROPPED into turns!), so much so that at first 30psi almost made me remove the tire But I found ~25psi to be fine and actually like the tire. Recently I may have snuck 30psi back into it (hurrying, sloppy...). But I don't think that is a major factor in the wag.

3) Front tire install was 10 bucks! But the buggas were purdy sloppy (had to take the rear back to have it turned around ) and the front weight is one big fat weight that they screwed up so it just flops up and down on the spoke while walking the bike (slow speed).

So my questions are this: can a crappy tire balance contribute to front end wag? What's the deal with static vs. dynamic tire balance? Any other ideas appreciated because its an uncomfortable feeling to have the front end start to ossicilate at speed
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Old 05-24-2005, 10:22 AM   #2
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is your wheel true? are yer spokes torqued?

only time my wheel has ever shaken a spoke was about to break.
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Old 05-24-2005, 10:23 AM   #3
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Come on Loaded....wag your front end a little....
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Old 05-24-2005, 10:23 AM   #4
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one more... tcoh had major head shake on his 950 and it tossed him off the bike on the freeway. i believe his problem was due to a loose headshaft. is yours tight?
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Old 05-24-2005, 10:39 AM   #5
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Quote:
Originally Posted by meat popsicle

So my questions are this: can a crappy tire balance contribute to front end wag?
Yes.

Mine inproved a lot after balancing, but it was unbalanced with a rimlock so I doubt yours is as far off.
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Old 05-24-2005, 10:53 AM   #6
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Quote:
Originally Posted by meat popsicle
My 03 KTM 640a seems to have developed the tendancy to become unstable at highway speeds recently. There are a couple of things that may contribute to this:
Unstable? a bit of an open a discription, my wife calls me unstable but I don't think I have a front end problem, maybe..
By oscillation you mean - Moving up and down or left and right?

Quote:
Originally Posted by meat popsicle
1) The front end was a bit out of sorts - the fork tubes were not even. I straightened this out (with the torque wrench to boot ) and this seems to have helped.
Yep, that'll do it.

Quote:
Originally Posted by meat popsicle
2) The front tire is a MT17 DOT knobby. The tire pressure seems to alter its handling drastically (steep profile = bike DROPPED into turns!), so much so that at first 30psi almost made me remove the tire But I found ~25psi to be fine and actually like the tire. Recently I may have snuck 30psi back into it (hurrying, sloppy...). But I don't think that is a major factor in the wag.
Yeah, tire preasure is a contributor.

Quote:
Originally Posted by meat popsicle
3) Front tire install was 10 bucks! But the buggas were purdy sloppy (had to take the rear back to have it turned around ) and the front weight is one big fat weight that they screwed up so it just flops up and down on the spoke while walking the bike (slow speed).
Not great, I'd crimp that sucker on so it don't move. Anyway, if this was wrong you'd get more a vibration (front moving up and down) that typically gets worst and better at different speeds. Is that what you mean by oscillation? Moving up and down or left and right?


Quote:
Originally Posted by meat popsicle
So my questions are this: can a crappy tire balance contribute to front end wag? What's the deal with static vs. dynamic tire balance? Any other ideas appreciated because its an uncomfortable feeling to have the front end start to ossicilate at speed
Like Loaded said, check the headshaft. That can lead to very bad stuff at speed..
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Old 05-24-2005, 10:57 AM   #7
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thank you for the replies. i will check the spokes - not sure how to check the wheel true 'cept for spinning it and takin a gander. loose headshaft... um, whazzit? can't be the axle; must be the steering head, between the two forks right? I just retorqued it when realigning the front end; will recheck. thrown off!

so aboot this "Dynamic" vs. "Static" wheel balancing...

I hear dynamic means "two plane" wheel balance. nice linkey:

http://www.toyo.com.au/tech_info6.html

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Old 05-24-2005, 11:01 AM   #8
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Quote:
Originally Posted by AusieRob
Unstable? a bit of an open a discription, my wife calls me unstable but I don't think I have a front end problem, maybe..
By oscillation you mean - Moving up and down or left and right?
oscillate, or wag, meaning left-right, like when you turn the handlebars left right. it just starts as a light oscillation that grew in amplitude until I would say "woah!" and back off. now I back off and have started avoiding situations that seemed to spawn it (like hitting some groves or bumps at speed while lane changing).
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Old 05-24-2005, 11:07 AM   #9
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I had the same problem, it was caused by the front tire bead not being properly seated. Make sure the tire bead is centered on rim on both sides of the tire.
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Old 05-24-2005, 11:13 AM   #10
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Quote:
Originally Posted by tomc
I had the same problem, it was caused by the front tire bead not being properly seated. Make sure the tire bead is centered on rim on both sides of the tire.
holy hand grenades batman! that would be a bad install job. might have to go back for "reducation" purposes...

although I have done about 1000 miles since the install so I guess its purdy stable.
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Old 05-24-2005, 11:42 AM   #11
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Quote:
Originally Posted by meat popsicle
oscillate, or wag, meaning left-right, like when you turn the handlebars left right. it just starts as a light oscillation that grew in amplitude until I would say "woah!" and back off. now I back off and have started avoiding situations that seemed to spawn it (like hitting some groves or bumps at speed while lane changing).
Would the term "headshake" work? ie is it a high or low frequency oscillation?

Have you tried lifting the front end (centerstand, have your SO siting on the back )
- Spin the front wheel to see if spins true. A pointer held against a fork can give a good guide. Check the tire is true as well.
- holding the bottom of the forks (at the axle) Try to move the wheel/forks forward and back.
- also check for left and right movement.

-also check the wheel bearings are good. Hold top and bottom of the wheel and try to move it.
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Old 05-24-2005, 11:59 AM   #12
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good point rob....

how are yer wheel bearings meat? how are your head bearings also? how often have you re-packed them? manual says once a year. done that?
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Old 05-24-2005, 12:23 PM   #13
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Check:
swingarm pivot
rear axle and bearings
rear wheel
rear spring and damper

Is that new rear tire real wide? Does it plant a real wide foot print? Does it have tall soft sidewalls?

Everything is connected to everything else. The Buddha said it, Lao Tsu said it, my chiropractor said it, and some dirt racing guy I ran into once said it.
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Old 05-24-2005, 12:24 PM   #14
meat popsicle OP
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Quote:
Originally Posted by AusieRob
Would the term "headshake" work? ie is it a high or low frequency oscillation?

Have you tried lifting the front end (centerstand, have your SO siting on the back )
- Spin the front wheel to see if spins true. A pointer held against a fork can give a good guide. Check the tire is true as well.
- holding the bottom of the forks (at the axle) Try to move the wheel/forks forward and back.
- also check for left and right movement.

-also check the wheel bearings are good. Hold top and bottom of the wheel and try to move it.
headshake sounds good; its like a wobble. well its more towards low frequency oscillation - definately not in the direction of a vibration.

great suggestions Rob, I will look into it.

I just QA/QCed the front and torqued the head nut to spec but I can check it again. Part of that process was lifting the front end and pulling on it to check its play. Seemed ok (is it pretty obvious when its loose? ).
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Old 05-24-2005, 12:25 PM   #15
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Quote:
Originally Posted by Loadedagain
good point rob....

how are yer wheel bearings meat? how are your head bearings also? how often have you re-packed them? manual says once a year. done that?
'taint been a year - well right abouts now! Guess that's on the list now! thanks for the heads up!
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