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Front wheel chatter.... Any tips to eliminate it?

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Loz, Oct 7, 2005.

  1. It's not something I often notice, but the front wheel of the Hornet seems to chatter up and down at the limits of hard (but progressive) braking - which increases my stopping time and totally scuppers any endo attempts, as well as being something that's at the forefront of my mind in emergency braking situations. I'd like to reduce it or get rid of it.

    The forks are non-adjustable with fresh oil (thanks Glitch :)) and the springs are the stock progressives.

    I've maxed out the rear preload as I am a fat bastard, but the rear shock is stock too. I'm on BT-020s as well woth good tread left on the front, in case that gives any of you setup gurus any more information.

    Anyone got any ideas?

  2. Is this a new problem or pre-existing?

    If pre-existing you could try a lighter weight oil.

    [Edit:] Just to explain, it sounds like it could be too much compression damping. Of course, changing your oil weight may also introduce other suspension problems. Have you tried different seating positions (try to take weight off the bars whilst braking, for instance)?
  3. If the lighter weight oil doesn't help then you might consider a set of Gold Valves for the front end.

    They can make a major difference to standard front ends (although the higher quality upside down front ends being already very good benifit less).
  4. I am certainly no guru but I noticed on my CBR600 that I had a shuddering feeling like you're describing happening when I was under hard braking and then eventually just under firm braking. Felt like the front of the bike was jittery...turns out the front rotor was warped and needing machining. Bike was absolutely fine after that.
  5. *Lighter* weight you reckon? Why do you suggest that - the more viscous oil might be making it too hard for the forks to compress and decompress in time to deal with pavement irregularities as the spring's reaching its tightest point?

    What effects are brought about by changing fork oil viscosity?
  6. Modern bikes have sealed folk units. When you brake it sompresses the air in the tube. This compressed air force the oil through the dampening holes quicker.

    so under braking your bike has less damping. Lack of rebound dampening is what is really getting you into trouble.

    You can do one of two things:

    1. increase the weight of the oil in the front folk legs. this will effect damping in all circumstances. How does it ride normally?

    2. Decrease the amount of oil in the folks. When you are compressing more air (because the oil is now where the air was), your rise in pressure in the air is less. e.g. for 100mm of dive you may have halved the volume of air (pressure doubles). If have 250mm to start with as apposed to 200, the rise in pressure at 100mm of dive is less (1.67 times more pressure)

    The result is the dampening is greater.
  7. another question
    what tyre pressures are you running?
  8. There's yer problem.
    Max the rear... stands her on the nose.
    Loosen the rear to standard settings, then go from there.
    new fork springs also might help, but I'd go back to standard settings, (with a bit more for the weight), then try.

  9. Also reduces that starting volume of air. The more I think about it the more I think you need to drain 50ml out of each folk leg.
  10. This is gonna take some time to digest before I understand what's going on and what to do about it, thanks for the suggestions guys.

    CBC, I'd have thought a warped disc would be a little easier to detect through the levers at more moderate braking pressures, did you find that to be the case or did it only become noticeable under had braking?

    Smee, I've been a bad boy and not checked lately, but I last set it at the recommended pressures so I'd guess it'd be that or slightly lower. When I had the tyres put on, the guy had them lower than the recommended, which he said would help wear them in without being too treacherous - I put them back up to recommended pressure after a few days.
  11. Pete, your idea sounds like a cheap and easy one to try first! I'll back the rear shock off to 4 or 5 instead of 7 (or whatever it's on) and give her a few test squeezes tonight, see how that affects it.
  12. On that note, is there such a thing as a long-handled c-spanner... Or are skinned knuckles just a part of the suspension adjustment process? :p
  13. I noticed the chatter first only under hard braking... at the very end of pulling the brakes on hard. After a while it was under moderate braking. You should be able to see the disc not straight by having a look. I had no clue what it was until someone test rode my bike to trouble shoot for me. :?
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  15. What pete said
    back the rear preload off and check yer tyres!
  16. I vote for brake disks warped.

    This is exactly what you would get if your front disk was warped.
  17. Yup .. this is the first, and easiest, thing to look at and eliminate. I'm pitching this will be your problem - your almost exactly describing it.
  18. I'm not convinced on the warped discs idea - it chatters very quickly, and usually just when I'm coming to a stop. I wouldn't have thought the disc was spinning quickly enough for any sort of irregularity to set off a quick clattering chatter like that.

    Course I could be wrong... I'll try backing off the rear tonight as well as checknig those discs, fart about with it over the weekend and see what I can learn.
  19. Even at 20km/h, that's a complete revolution of most disc's in around 1 sec or so. Your unlikely to notice the effects/hear of slightly warped discs at higher speeds due to road vibration and wind noise. It could also be non-symmetrically worn disc pads .. but I'm still tipping warped discs :)
  20. Nah, no way. A warped disc manifests itself first and foremost as a pulsing through the brake lever. Loz hasn't reported that.

    I vote simply for squishy forks. When he grabs a handful and sets the bike on its nose, he uses up all the travel in the springs and starts rolling along on the bumpstops. These have a much higher spring constant than the fork springs themselves. Thus, when he's bouncing along over road irregularities and it's the bumpstops moving to absorb them, the damping, which is soft on a Hornet, is completely overwhelmed. Result; fork judder.