Front Brakes err Jack-hammer!!

[tony91]

When I brake hard off a long straight the front end sort of dives and rebounds rapidly and repeatedly. I've checked everything I know to check. Is this normal. Or, more likely, have I missed something?

 

[rc331]

Sounds like a very strange problem. I could be way off base here but have you checked your front rotor to see if it's straight?. :confused:

 

[tony91]

The rotor is cool (I think). I put it flat on a piece of glass and tried to slide a thin feeler gauge under at several different points and it seemed OK. Is their a better method to check it ?

 

[rc331]

Checking it while on the bike with a dial indicator is a better method. The limit for warpage is .15mm :confused:

 

[tony91]

Will do. Thanks. Also while looking it over last night, I noticed that he brake line bulges a bit. The bike and the brake line are 10 years old with plenty of use. Think this could be contributing?

 

[rc331]

Just my opinion but i think if my brake line was bulging I would replace it. If your rotor seems good along with wheel etc. I would probably start looking at suspension settings. You could could be getting a front end shake from:

1) Too much preload on rear shock.
2) Not enough rear shock rebound damping.
3) Fork oil level too low.

Is this a new problem? or has it been a problem since you have had the bike? :confused:

 

[tony91]

It's been there I while. Just getting progressively worse. From your opinion, it seems that the line shouldn't bulge at all? I thougnt A little was normal. If not, I will replace it. Thanks for the other tips as well. I hadn't even considered supension as a contributing factor. Still learning !! :think:

 

[rc331]

Sorry I thought you meant an abnormal bulge. Like you said a little seems to be normal. Learning is a good thing, I do it all the time and have a long way to go!! ;) The brake line bulge (if it was excessive) would probably just give a mushy feel to the lever not front end shake.

 

[David Trustrum]

The problem has to be with your disc. Spin the wheel with the calliper off (& in the absence of a dial gauge) hold a edge against the fork leg, or better yet a decent felt pen closer until it just touches (this will tell you where it is warped). Basically this problem is fairly common on dirt bikes.

The discs can also cone, when disc rotors warp the metal gets too hot & expands, either they get a buckle in them or the outside gets bigger than the inside they can ‘cone’ - the outside moves to the left or right. This will flex somewhat. Check the rotor is flat with a straight edge to be sure.


On the far less likely file: There might be a contaminant on your disc & the pads may need looking at. The calliper and brake pins should be in good order. Also check your wheel bearings & axle is straight.