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Clonk sound from front forks

Discussion in 'BNS12 Chat & Tech' started by NPS, Sep 21, 2018.

  1. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Just wondering if other owners experience a heavy clonk sound going over sudden or deep bumps? My bike is 2005 Breva 1100.

    I rebuilt my forks last year. I have noticed a clonk sound that seems to come from the headlamp cluster.
    I'm not sure if the sound is from here or is a shortwave been transmitted through the front suspension.

    Last week I removed each suspension unit and stripped to check over. I found nothing that did not seem out of the normal condition. I use standard fork oil of 10 viscosity with 400cc in each leg. The adjusters have been screwed in and recently far out with no difference.

    I have checked the steering head bearings for play and there is nothing to speak of.

    The sound seems to happen near or at the far out of fork travel. Perhaps I should try using less oil to reduce the air compression to lower the height slightly. I think this works. I certainly has the opposite affect if too much oil is added.

    Does anyone have any thoughts?
     
  2. Trout

    Trout GT Reference

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    Heavier grade fork oil.

    Also use some zip tires on the forks just above the sliders.
    When the fork compresses it will push the zip tie up.
    Now you can tell how much fork travel is happening.
    Easy way to see if the forks are bottoming out.
     
  3. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Its on extension that the clonk is happening. But I will certainly consider trying heavier oil.
     
  4. Trout

    Trout GT Reference

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    Totally missed that.
     
  5. john zibell

    john zibell Moderator Staff Member GT di Razza Pura

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    My personal experience is my forks seem to better with 15 weight fork oil. I have no strange sounds and the feel/handling is pretty good.
     
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  6. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    I drained and refilled the forks with 15 weight oil today (380cc per leg as not dry inside). Before replacing plastic pre-load tube and spring I pumped to remove the air. The feeling was as I thought it should be. Easy to push down and hard on pulling out.

    I went for a ride. The clonk is still there on hitting a pot hole either on compression or extension against the spring stop in the damper. It is hard to tell which but not full compression of the fork.

    The ride was very uncomfortable. I think I'm used to a 5 weight oil. On bumpy roads it was a lot harder to control so mine does need a lighter oil. With the wheel off and the spindle shaft clamped in place I hit it with a rubber hammer. The shock made quite a loud noise. Seemed to be amplified by the mud guard. I heard the LH spring vibrating inside the stanchion tube. Also sound from the headlamp bracket and headlamp area. I'm beginning to think this is a shock wave sound transmitted to the lighter items in the steering that is amplifying the sound. I'm not imagining it and this sound is new since I rebuilt the bike.
    The head bearings are tight with no play. The top yoak is tight against the steering head bearing adjuster rings and the large top disk like fastner (with massive hex in the middle) is also tight.

    Perhaps I will dismantle the head bearings over winter to check, but not sure what good that will do. Handling is good though (with lighter oil). It is just the clonk noise over pot holes and speed bumps that spoils my enjoyment.
     
  7. john zibell

    john zibell Moderator Staff Member GT di Razza Pura

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    What is the spring preload? On mine from full preload I back of 8 revolutions.
     
  8. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Thanks John. I have tried different settings. I had mine fully in for a number of years. Two weeks ago I wound it fully out. Does not make any difference to the clunk.
     
  9. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Changed back to 5 weight oil today. removed top yoke to inspect head bearings. All fine.
    I have found something that could cause a the clunk. Either side of the headlamp is a u section plastic tube thing. These were just on the headlamp support frame and the obviously have a use. Perhaps to prevent cables wearing on the blunt edges of the headlamp mounts.
    I decided to push them forward slightly so they fit between the headlamp and the head lamp supports. I tightened up and the sound when taping the headlamp is very muffled due to these two u section plastic things in the gap. I wonder if this could be it. I will have to have a ride and see.
     
  10. Android

    Android Cruisin' Guzzisti GT Famiglia

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    mine is doing the same clonk, I have heavier oil and hyperpro springs .....clonk is much reduced now I have tightened the steering head bearing.
     
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  11. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Edit: Made no difference.
     
  12. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    I'm going to take yet another look at my head bearings. Has anyone fitted tapper roller bearings?
     
  13. john zibell

    john zibell Moderator Staff Member GT di Razza Pura

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    Yes I have. Bit of a job taking everything down to get the old rollers out and the tapered in. Customer said he felt the bike handled better after.
     
  14. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Just stripped the head down yet again. Roller ball races are completely OK. The races are very good like new. I ordered some tapper roller bearings anyway to fit in over the weekend if they come tomorrow. I just cant see the problem being the head bearings. There was no up/down side to side play that I could feel. Certainly not enough to move to cause a clunk.

    While I had the forks off I tried pumping each. There appears to be a distance of about an inch between the compression damping and the rebound damping. Obviously rebound is the more heavy of the damping but there is still some damping on compression of the fork.
    This distance at damping change over could be the culprit. Done quickly it does generate a sound. It is roughly the same on both forks.
    I didn't have this problem before my rebuild. In the rebuild I replaced both fork stanchion tubes with MG replacement items. Also the top and bottom bushes. Springs and preload tubes are original. The plungers are the originals although I replaced the plastic sealing rings.

    I'm using 380cc of 5W oil in each leg. Oil viscosity does not make much difference to the sound, nor does the spring compression adjuster.

    If I could upload a video I would. I wonder if it is possible. I will record a youtube video and post the link.

    This clonk noise spoils all enjoyment when riding. There must be a solution.
     
    Last edited: Oct 19, 2018
  15. NPS

    NPS Cruisin' Guzzisti GT Contributor

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    Clonk solved. Well 7/8ths of the clonk gone.
    New taper roller head bearings. Filled each leg to 450ml 5w oil.
    One or both is responsible and I don't know which. I have tired greater than 400ml per leg before but found the ride to hard due to the air compression. I had a 40 mile ride today and I can cope with the firmer forks.
    Do I feel a difference with different head bearings. Unfortunately, I made two changes at the same time. The bike does feel quite different. But the clonk has largely gone too. So all very confused and subjective. I do like it though. Moved on considerably and much more enjoyable to ride compared with what I have been used to.

    I feel the clonk comes from the movement in the fork between compression and extension. There is a valve in the end of the stanchion tube that the plunger shaft passes through. This valve must let oil through on compression but not on extension. The shutting of the valve causes an immediate change in damping. It is a piece of plastic that shuts against a flat bottom of the stanchion. You would expect it to be immediate in the affect. However, I believe this created a shock wave into the fork and to the steering head. Perhaps the extra metal contact of taper roller bearings over balls somehow modifies this shock transferring it to the frame rather then headlight and instrument cluster (causing the noise). Just guessing but the difference is very positive and will allow me to enjoy the ride again.
     

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