Front brake shakes - please help me!

[gringostd]

Ok, I've got this chronic problem that has stumped me and my best mechanics and friends and mechanic-friends for years now. Maybe someone here can help.

I bought a 2001 Bandit 1200 about a year and a half ago. It was in excellent shape except for a fairly bad shaking that occurred when the front brakes are applied. The bike still brakes, but there is a heavy modulation as it's slowing down - more noticeable as it gets slower. Assured that it was probably warped front rotors, I bought the bike and had it immediately checked out by my mechanic. He confirmed the warped rotor situation, so we replaced them. Immediately after replacing, the shaking decreased, but wasn't totally gone. Now, 10k miles and almost a year later, it's back to as bad as it was when I bought the bike.

I recently got together with a mechanic friend and checked the run-out in the front brake rotors. Both the left and right sides were showing about 6/1000 run-out with the brake calipers in place - didn't seem too bad to me. We marked the high and low spots and removed the rotors - surfaced them with scotch brite, cleaned all the contact points on the wheel, chased all the bolt threads to clear out the excessive lock-tite, and replaced the rotors [not paying attention to put them back on in the right orientation to the wheel]. The right side rotor read about the same run-out, but the left side was then showing like 30/1000. We hammered the rotors to compensate for the run-out as best we could - getting the right side to within 2/1000 and the left side to within 5/1000. But, the pulsing persisted - no noticeable change.

My hypothesis at this point is that the surface of the wheel where the rotors contact - at least on the left side - is machined out of true. That would explain why perfectly good rotors are warping, etc. Does this hold up in your minds? How would I go about proofing that that's the case, and then what can I do about it? I'm headed cross-country on the bike next week, and it'd be great if I could get the problem addressed by then.

 

[DaveG]

Don't rule out the front tire.

have you tried swapping a front wheel with someone?

[gringostd]

Thanks DaveG,

No, I haven't tried a swap yet - don't know any Bandit riders in the Bay Area. That'd be a good idea. I've got feelers out for a complete front wheel, if anyone has one they want to sell.

I'm fairly sure it's not the tire - it's been changed / balanced twice since I bought the bike, and the problem has always been the same.

[bngboyd]

I'm fairly sure it's not the tire - it's been changed / balanced twice since I bought the bike, and the problem has always been the same.

What about the rim? Being that the rotors mount to the rim maybe the rim was machined wrong?  Kind of a long shot but sounds like your issue is one that is hard to troublshoot.

[gringostd]

Right. As I said... "My hypothesis at this point is that the surface of the wheel where the rotors contact - at least on the left side - is machined out of true."

[pawnee]

Have you checked your stem bearings? Just asking.

[gringostd]

No, I haven't check the stem bearing. I don't know how that would cause this pulsing that seems directly related to the rate of rotation of the wheel.

[pawnee]

Just saying if the stem bearing had slop in them, it may cause more shake. Don't know, thought I would mention it though. What about the wheel bearings?

Know any good machine shops that might help you out? Not talking brake shops here.

[gringostd]

Yeah, I hear you. I have no reason to believe the stem bearing has issues, and the wheel bearings are definitely fine - we checked and double-checked that possibility. I have been in contact with the best machine shops in the area - everyone is scratching their heads on this one.

So far, my hypothesis seems to be holding up. Tomorrow we'll strip the rotors off again, take a feeler gauge to the surfaces where the rotors mount [with the wheel remounted on the forks] and see once and for all if those surfaces are out of true to the rotation of the wheel at large. If they are, I either pay in the neighborhood of $150 to get them resurfaced, try to shim out the rotors appropriately [very thin shims... I guess], or try to get a new wheel.

Used wheel... anyone? Preferably with rotors.
Anyone?

 

[gringostd]

Ok, I am totally stumped!   

Today, we pulled the rotors again and checked them on a granite reference block - less than .007 on each, and that's conservative. I think they were probably less than .005 each. We also checked the run-out on the rotor mounting surfaces on the hub - less than .001. As far as I'm concerned, that is withing tolerance for floating rotors. We re-mounted the rotors, making sure to match the 'high' spots on the wheel with the 'low' spots of the rotors ---- NOTHING!

We triple-checked the wheel bearings, they're fine. Checked the head bearings, which did need a tightening, but they're fine now. Still got the pulse.

I just don't believe that this is a wheel or tire issue, or I'd be feeling this at high speeds as well. The tire is in great shape, was professionally mounted, etc.

WHAT IS GOING ON??? 

[B6mick]

Front tyre, yes.... My zrx suffered the exact same thing, don't discount front wheel bearings either.

[dhcolesj]

The funny thing about this is that my Chain was loose, and when I would let off the throttle the bike would shake, and when I stopped, if I didn't stop hard, the bike would shake.  I tightened the chain, and now it's as smooth as a baby.  The thing is the shaking all felt like it was coming from the front wheel.  the last stealer to mount the back wheel rounded off the lock nut on the right side, and I had to use a pair of vice grips to get it loose.  Another story. 

Anyway, my point is don't discount anything until you've tested it, and are sure.  Even a "professionally mounted" tire can be badly mounted (mech having a bad day), or just be a bad tire (maker having a bad run).
Oh, meant to add I used to "professionaly" mount tires, and do brakes on cages at a local garage.  I balanced the wheels manually, and hand turned rotors and drums.  Every now and then I'd have an off day, and would have to do a set of tires twice.

[gringostd]

Cool - thanks for all the feedback folks.
I will take and have the tire re-mounted and balanced. God, that'd be awesome if that was the problem. I'll check the chain tension too - can't hurt. I'd just be surprised if that was it considering the problem has been there for 10k miles now - through several chains and chain adjusts, tires, etc. Anyways... we'll see.

[smooth operator]

  Not sure the name of the devise,but I use it to balance my wheels/tires. Its used for lacing spoked wheels.While you have your front wheel off,if you know someone or a shop with one,check your rim and make sure its true.

[gringostd]

For those following this thread and who offered advice, I thought I'd follow up. I bought a used wheel off eBay and picked it up mid-way through cross-country ride last month. The problem disappeared. So, despite measuring no run-out in the wheel at the rotor pad, it appears to have been the rim.   

I can't tell you how happy I was to finally fix it. The idea of a cross-country ride with poor front brakes was not appealing. I'm home now - 9500 miles later [it wasn't exactly a direct route].   

Notable points along the way:

Oakland, Ca [home]
Portland, Or
Billings, Mt [where I picked up the wheel]
Black Hills, ND
Chicago, Il
Detroit, Mi
Toronto, Ontario, Canada
New Haven, Conn.
New York, NY
Washington D.C.
Upper peninsula, Mi
Winnipeg, Manitoba, Canada
Regina, Saskatchewan, Canada
Lethbridge, Alberta, Canada
Glacier National Park
Cour D'Alane, Idaho
Portland, Or
Oakland, Ca

How's that for a Bandit adventure?!