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Front end Wobble

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I just recently bought a 2009 KTM 150sx. I was loving the bike,but I quickly realized that the front end likes to wobble. Now I don't mean just a little bit, I mean almost tank slap, but not quite that bad. It does it anywhere at anytime, and there's no way I can control it. I have to ride third gear while my buddies fly past me because I don't want to crash my brains out. Now before you criticize my riding and say I just need to learn to control my bike, I am a very experienced rider (A class) and I have had 3 other bikes that do not do this. I know this is ridiculous, but I'm wondering if there's anything you can do to fix it. Would moving the handlebar mounts back to the 18mm holes on the triple clamps help. Or is it something that's un fixable. Really love the bike, but I'm gonna sell it if I can't fix this issue cause it scares me A LOT!

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23 minutes ago, zach_507 said:

 

I just recently bought a 2009 KTM 150sx. I was loving the bike,but I quickly realized that the front end likes to wobble. Now I don't mean just a little bit, I mean almost tank slap, but not quite that bad. It does it anywhere at anytime, and there's no way I can control it. I have to ride third gear while my buddies fly past me because I don't want to crash my brains out. Now before you criticize my riding and say I just need to learn to control my bike, I am a very experienced rider (A class) and I have had 3 other bikes that do not do this. I know this is ridiculous, but I'm wondering if there's anything you can do to fix it. Would moving the handlebar mounts back to the 18mm holes on the triple clamps help. Or is it something that's un fixable. Really love the bike, but I'm gonna sell it if I can't fix this issue cause it scares me A LOT!

 

How is the bike setup? ie: 

  1. Spring rates?
  2. Sag?
  3. Forks flushed or protruding a long way?
  4. How far back is the rear wheel in its dropouts?

I have the same bike and it is very busy for sure, particularly on small chop, but it doesnt do as you describe.

I am not A grade. Midpack to rear B grade as an estimate, but definitely like to play with setup.

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How are measurements center to three spots on the rim? Is there a lot of run flat goop in the tube? Old bulging tube? 

Edited by theBIGnaud
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It may have been crashed b4 you bought it

had the same thing happen to me on an older ktm - tank slap at high speed after a collision - turned out the forks were slightly bent plus the steering stem was bent

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How is the bike setup? ie: 
  1. Spring rates?
  2. Sag?
  3. Forks flushed or protruding a long way?
  4. How far back is the rear wheel in its dropouts?
I have the same bike and it is very busy for sure, particularly on small chop, but it doesnt do as you describe.
I am not A grade. Midpack to rear B grade as an estimate, but definitely like to play with setup.

I am a good rider and do all the mechanical work myself, but when it comes to suspension I am completely clueless. The forks look like they are flush triple clamps. The rear wheel is about half way back in its dropouts. I am a light rider, and the spring in the rear looks like it is compressed down a lot. I was thinking it might be Becuade the rear end is too high in the air resulting in the front end pointing down slightly causing it do the tank slap. Just a thought. Let me know if any of these could be an issue.

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How are measurements center to three spots on the rim? Is there a lot of run flat goop in the tube? Old bulging tube? 

I haven't had the front wheel off yet, so I don't know if it has any of that stuff in it. What do you mean by measurements center to 3 spots on the rim? Would you measure from the center of the hub to the rim or a different point of measurement. Just want to get this fixed because I love the bike

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47 minutes ago, zach_507 said:


I am a good rider and do all the mechanical work myself, but when it comes to suspension I am completely clueless. The forks look like they are flush triple clamps. The rear wheel is about half way back in its dropouts. I am a light rider, and the spring in the rear looks like it is compressed down a lot. I was thinking it might be Becuade the rear end is too high in the air resulting in the front end pointing down slightly causing it do the tank slap. Just a thought. Let me know if any of these could be an issue.

What sag measurement does it give you, I was able to get close sag numbers on my 150 and I weigh about 130.

It sounds like you arent running enough sag, hence the rear riding high, forks too straight feeling which I hate hate hate.

Was riding a YZ around today, and it was so stinkbug and stiff in the arse it wanted to kick sideways on square edges.

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What sag measurement does it give you, I was able to get close sag numbers on my 150 and I weigh about 130.
It sounds like you arent running enough sag, hence the rear riding high, forks too straight feeling which I hate hate hate.
Was riding a YZ around today, and it was so stinkbug and stiff in the arse it wanted to kick sideways on square edges.

I have not set the sag yet. I need to get a softer spring in order to set the sag because I'm so light. Hoping when I get this it will fix it

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1 minute ago, zach_507 said:


I have not set the sag yet. I need to get a softer spring in order to set the sag because I'm so light. Hoping when I get this it will fix it

What do you weigh?

You can still get the geometry better with too stiff a spring. My 200 used to wobble the whole time I rode it before I set sag, and occasionally tank slap.

I still ride with springs KTM says are too stiff with acceptable performance.

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2 hours ago, BushPig said:

What do you weigh?

You can still get the geometry better with too stiff a spring. My 200 used to wobble the whole time I rode it before I set sag, and occasionally tank slap.

I still ride with springs KTM says are too stiff with acceptable performance.

Just like bush pig, I came here to say that before I set proper race sag I had the same thing going on with my 2013 250 xc where the front end would wander.

 

Setting proper sag removed this problem for me.

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you also mentioned that you haven't had the front wheel off.  

 

I have seen this a few times where some gaper doesn't know the proper procedure to tighten down the pinch bolts on the forks.  As a matter of fact i have done this by accident and only noticed when my front end started tracking like shit. 

 

Pull of front wheel and make sure it is installed correctly.

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Something is bent or twisted or installed incorrectly causing a "bad balance" condition.  I'd start by simply taking it apart and inspecting things,  including the tire.   It's most likely an easy fix unless it's a bent fork tube.  That'd suck...

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4 hours ago, BushPig said:

What do you weigh?

You can still get the geometry better with too stiff a spring. My 200 used to wobble the whole time I rode it before I set sag, and occasionally tank slap.

I still ride with springs KTM says are too stiff with acceptable performance.

Ok Ill still try to set the sag. I am 125lbs. The rear shock is about as stiff as it can get right now. That big nut or whatever it is (i dont know what its actually called) is going to take a long time to set the sag because it is screwed down all the way and I do not have one of the special wrenches to go on there lol. Looks like ill be using the flat end on a chisel and a hammer. 

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Hoping nothing is bent, just something installed incorrectly. I am going to take off the front wheel and reinstall it in case something was done incorrectly. Is there a way to check for bent fork tubes or something else bent in the front end? Ive never had anything like this before, so im kind of lost. Really want to get this fixed before I hurt myself lol. 

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Just out of curiosity, what happens if you put it up on a stand and spin the front wheel?  Does it spin true or does it have a wobble?  Could be as simple as your wheel being out of shape.

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6 hours ago, zach_507 said:

Ok Ill still try to set the sag. I am 125lbs. The rear shock is about as stiff as it can get right now. That big nut or whatever it is (i dont know what its actually called) is going to take a long time to set the sag because it is screwed down all the way and I do not have one of the special wrenches to go on there lol. Looks like ill be using the flat end on a chisel and a hammer. 

Yeah I can only imagine how bad that would be.

Before you loosen the big nut, clean the threaded shock shaft as best you can. Spray into the nut and on the threads with a good penetrating oil to prevent ally on ally galling. Lift back wheel off ground to minimise the load on the threads. Dont forget to loosen pinch bolt.

If the nut isnt starting easily, find the right tool and ride another bike for now.

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Just out of curiosity, what happens if you put it up on a stand and spin the front wheel?  Does it spin true or does it have a wobble?  Could be as simple as your wheel being out of shape.

I don't know, I haven't done that yet. I'll have to wash it up and do that to see if it does wobble a little bit. I did tighten the spokes on it because a few were loose, but didn't know about the issue because I did the spokes before I rode it for the first time. If it does wobble does that mean the rim is a little bent somewhere?

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If it is bent, it will be pretty obvious. Messing with the spokes can cause all kinds of rim problems and a wobbly rim could be your issue. I'm sure there is a thread in here somewhere about "how to true a spoke wheel". I sure hope this fixes your issue! Headshake can be damned scary!

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If it is bent, it will be pretty obvious. Messing with the spokes can cause all kinds of rim problems and a wobbly rim could be your issue. I'm sure there is a thread in here somewhere about "how to true a spoke wheel". I sure hope this fixes your issue! Headshake can be damned scary!

I've been working on bikes for a long time, so I don't think it could be my issue but I'm not perfect. The first time I rode it I clicked into 5th gear and when I dropped the front end back down the front end went in to the speed wobble. Thought I was never gonna get to ride that bike again lol. Thanks for all the help and advice

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