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Forks jump when braking

chuck hahn

chuck hahn

Forum LongTimer
Past Site Supporter
I have cleaned the rotors ( both front ones ) with brake cleaner spray and new paper towels, spun the wheel off the ground and observed each rotor and not a wobble to be seen, pads are new. When I brake it feels like a warped rotor on a car feels but i can see the fork lowers slightly jumping up and down as I brake. Never have had this before so I am at a loss. Any pointers???

I was thinking maybe bearings but then why would wheel bearings make the lowers twitch and there isnt any signs of bearing wear or slop when i spin and twist on the rim???
 
Any pitting in the fork tubes? Maybe too much compressed air? When was the last time you changed the fork oil?


Ed
 
My dads 850L did this and it was the caliper axle/ bolts were stuck with old grease and wouldn't move, I took them out cleaned the 30 year old grease and it fixed the wobble. That would explain the braking and no wobble visually when rim is spun
 
New seals and new oil. No pitting in the tubes. Bike is an 81 750L and I did notice the calipers do move on the pins when i squeeze the lever. Rebuild kit in the master and its the round resivoir style master cylinder.

I will reinspect the carriers for sure. I can litteraly look down and watch the lowers go up and down on the tubes..much like its a jack hammering action going on down there. harder i squeeze the lever the more pronounced it becomes too.
 
Chuck, can you lay hands on a clamp on dial guage? {magnetic won't do you any good}
Or, is there a parts supply place you can borrow one from {advance auto/napa....} It's hard to see a wobble/warp in the rotors....obviously if you actually see it - it's bad!...but it's the stuff you can't easily see that can be a problem too.
The only way "I" can see the lower fork legs moving on you like that is due to a brake pulse...explains why the harder you grab the brake, the worse it gets.
I'd suggest putting the bike on the center stand and try moving the front wheel side to side {one hand on top of the wheel and the other on the bottom}....looking for bearing slop. If the bearings are bad that could cause the brakes to grab unevenly...had a bad set on my FJ1100...couldn't feel it in the brakes, but I could feel it at low speed and by using my hands {side to side}.
 
I warped 3 sets on my EZ. When my brake lever started pulsating I never thought past the rotors being warped. Thats what it always was.
 
Set up an indicator and measure the runout on the wheels and rotors. I bet they're warped. If not, it's wheel bearings.
 
chuck, harbor fright has a passable clamp on one for like 15 dollars or something.
 
I've use HF jack stands and HF magnetic dial indicator to measure wheel runout, so I doubt much harder to do rotors
 
Maybe it's from the suspension? I just learned that a bottomed out fork will bounce when braking. Some out-loud thinking: Check that your fork legs are parallel, wheel is square with forks, and check your fork oil levels and preload shims for consistency....

But wait for confirmation from others before doing any of the above since I'm just learning this myself.
 
The HF indicator is awesome. It's probably my favorite all-time purchase from there. If I was doing it again I'd get the one with the flexible clamp, assuming it's the same indicator. The magnetic stand is crap.
 
UPDATE: Had time to actually put MY bike on the rack for a change and see whats going on.

Calipers move freely on the carriers and I did remember to grease them.
New seal in correctly and the pistons move and relax as they should.
Return port on the master is clear.
Rotors are not warped in the slightest.

Only thing I found was the wheel bearing felt just a tad notchy ( just enough to actually feel it ) when my pinkey finger rolled them round. Next I stuck the axle in and moved up and down and could actually visually observe movement of the inner race in relation to the rubber dust seals.

So, My conclusion is that when i apply the brake the jitterings because of wheel bearing headed south. Sound about right???
 
Oh yeah...shaft oscillation will cause all sorts of problems. Off subject work issue but totally pertains - I just took a pump motor out due to front shaft looseness {bushing type}. The motor wouldn't run and the shaft was gummed up due to excess clearance...pulled it apart and cleaned the crud out of the drive end bell and now it'll spin up fine....but, to put it back in would just be a matter of time before it did it again {I need a bell end or bushing - bell and gossett water pump}.
Your axle looseness will just add oscillation even if you clean it up now...it's just a matter of time before it returns - you need to see if the inner bearing race is worn/opened up or if the axle is worn smaller...or both. Mic the axle surface where the bearings go and compare to other areas to look for a worn area. If it's all good, then the bearing inner ID might be suspect. When I swapped out oem open bearings on my old FJ1100 the new bearings were sealed...{and cheaper then oem!}.
 
Joe Garfield said:
The HF indicator is awesome. It's probably my favorite all-time purchase from there. If I was doing it again I'd get the one with the flexible clamp, assuming it's the same indicator. The magnetic stand is crap.
Click to expand...

I've got the mag stand type with the movable rod arms...the magnet works well enough...on steel.
 
I didn't check to see if they even have what you need but I like dealing with bearings direct for works pillow block shaft bearings....Carrier replacements were $300.00 for the pair....exact same thing but not going thru Carrier was $42 and some change shipped...3 years running on those and they're fine. Food for thought - shop around the web!
 
Bob......So im taking that as a yes on the bearings in the link I posted fitting the rim???
 
chuck hahn said:
Bob......So im taking that as a yes on the bearings in the link I posted fitting the rim???
Click to expand...

Yeah...my two cents was for folks who think that the dealer was the only source out there for certain things just doesn't have to be true...and you outsourced as you should!
 
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