chatter

twinweb

twinweb

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Location
oz
getting front end chatter on hard downhill braking.mmm front discs?no didnt do it before fork oil change -have 300 mm of air space and 15 wt oil-back to ten?when i changed oil for first time only got roughly 150 mm of black shit from both forks-she didnt chatter but occasionally bottomed -have ruled out discs-only does this when wringing the shit out of it downhill hard cornering
when i say chatter i mean the whole front shakes like fuck-advise?
 
Paul, have you only just changed your fork oil? I ask because "they" always say to pump the fork without springs to get air bubbles out. Buggared if I can see where air bubbles can get trapped in a damper rod fork but a big compressible air space could be causing the problem. It should get better quickly if this is the problem (or you could practice your front wheelstands. Send pictures if you do).

Otherwise, maybe crud from the bottom of the fork in the sliding/stanchion bearing surface? Maybe not enough to cause noticeable binding but still considerable stiction.

Everything tight? 2mm spacer present between right slider and wheel?

Ken
 
Might be hydraulic lock.  Do you have the stock damper rods?  Braking hard on downhill slightly bumpy roads all that oil has to go through a single small hole, at a certain point the hole cannot flow wnough and you get a virtual hydraulic lock.  15wt is way too heavy, go back to at least 10wt.  I'm not sure how you're measuring oil height, my reference is with the forks fully compressed and springs removed, 150mm from the top (are you talking mm or ml in your mail?).

Racetech gold valves (or similar) produce a huge improvement for not much cost or work, I have run them on my RGA since 1996.  They create different oil paths for high and low speed damping, a nice small hole for low speed  (brake dive etc), then a plate that rises against a spring to allow large amounts through for high speed damping (bumps etc).  There is also a check valve to let the oil freely back through for rebound (done by your original rebound damping system) so the oil does not cavitate.

I have some suspension info on my website:
http://www.breganzane.com/workshop/suspension.php

Cheers
Steve B
 
mate i didnt pump them much but changed oil with forks in situ(had help to hold her while compressing forks to check air space)not up to much this arv-will pull them and try this-oh flushed forks with kero to get rid of crud-might drop down to 10 wt as well-only seems to chatter when really hot downhill-hard braking from ton up its fine

cheers paul
 
thanks steve-150 mm airspace each leg-shes got progressive springs-not sure of valving-didnt do this with bugger all shit oil-wierd
 
The easiest way I have found is with a small car jack under the wheel. After pulling the caps and springs I jack the wheel up till it bottoms and then muck about with a Turkey Baster,bit like a big syringe to pull the oil out at the height required.Dont go jacking it off the stand though.
 
Re the wheel weight,I lost one off the back and now put a peice of grey waterproof duct tape over it.You hardly see it on the Borrani rim,maybe they make other colours for the gold type rimes.
 
I'm with the 10wt brigade, and I wonder too about brake fluid.. are your calipers cleaned and good brake fluid.. like SRF Castrol. Not cheap, but excellent feel..

8)
 
Just a though - wouldn't a lost balance weight mean that the front wobbles at around 80km/h and not cause the chatter under hard braking?

I go with the to high weight oil. If you have 300ml each leg and are at 150mm from top, fork fully compressed and springs removed, drain 50% of that off your fork legs and fill it up with 5W oil. The mix is roughly 10W then. Make sure to SLOWLY compress the fork several times to remove air bubbles.

Do you have Ceriani or Marzocchi forks, please? On Ceriani, check the air-valve in the top nuts. Remove the screw, check the spring and ball underneath are moving freely and the hole is not blocked. Not sure if the Marzocchis have the same, but I think they have from memory...

Make sure you "relax" the fork completely: loosen all screws and nuts (axel, the one or two screws holding the axle, the mudguard, lower triple clamp) EXCEPT the ones from the top triple clamp. make sure both legs are at the same hight above or in the top triples clamp. With the springs back in and top nuts installed, put the bike off the center stand, engage front brake and push the bike down to compress the fork several times. Now fasten the 2 screws on the lower triple clamp, push the bike down again several times, fasten the screws on the mudguard, push down again several times and then fasten the axel and screws as usual.

Make also sure your caliper screws are fastened well (The two bikes I bought last year both had at least 1 screw not tightened after a wheel change...  :o ).

HTH
 
bevelman said:
when i say chatter i mean the whole front shakes like fuck-advise?
Click to expand...

Hi Paul,

If it's a serious as your last sentence then I wouldn't be worrying about 10 or 15 wt oil, I'd go straight for the steering head bearings. What sort of bearings do you have, even if they are new, if they aren't seated and shimmed correctly they can move with a bit of aggressive riding. A loose steering head will make the whole front shake like fuck ... Check it out post haste.

Cheers Crispin
 
I'm with Crispin, check the headstock bearings. Had it happen to me, a tad extra tension, problem solved. Any extra movement, has to be felt somewhere.
Fork leverage is huge, when you consider the bearing size v fork length.

Old oil may have been masking the problem. new oil @ heavier weight just extenuates it.

Nip up, and take that out of the equation, if not solved after a nip or two, look elsewhere.

Chris
 
Have you removed/refitted the front tyre, new one perhaps?, if yes, then tyre or rim related, but........
my money is on the brakes, maybe a warped disc, or a single caliper piston seizing, which will then cause the same, even if it has not yet.Floating discs or solid?, perhaps bobbin related?
CLEM
 
Agree, also, with Crispin & Chris.
Had the same chatter problem recently on downhill braking.
Tried oil weight & volume changes, to no avail.
Then noticed slight play in the lower head bearing.
Tightened it up and no more problems.
 
fixed-sterr head brgs new and spot on-oil the culprit-as i originally changed oil without removing forks, only compressed forks the once before replacing caps-i had a 220 mm air gap. thanks for advice guys-pumped forks heaps and set up with 10 wt and proper air gap(used 330 odd ml though)-dont have a 2mm washer on speedo side though-its shown in green book-put back together without and its fine-will chase one up though
paul
 
Cosi said:
maybe a little dent in the rim from chasing the stunt man through washouts?
Click to expand...

Perhaps the stunt man should listen to Bart more..... :D :D :D

bart_dude.jpg
 
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