Rear Disc Break Issues

[Drunkskunk]

I have a weird problem with my rear breaks. Under full load, the disk rubs loudly. It doesn't do it at part load or slow speeds. No idea why.

WHat I have is a Avid 7 calliper on a Clyte disc, fitted to a 4012/408 motor with a Clyte adaptor. the disk has been trued, and runs smooth. I have the padds adjusted so I can see daylight on both sides. the motor has no measurable side to side movement.

the disk is only 10mm from the motor, and I had to cut off the adjustment wheel from the Avid to fit, but it works other than the noise at full power.

My Very rough guess is maybe the magnetic force of the motor is sucking the pad against the disk??? seems far fetched, but I can't think of anything else. I can't make it do it unless I'm riding above 15mph, and It won't do it in garage testing.

any ideas?

 

[fechter]

I don't think the magnets would affect the disk.

The disk might be warping with high load.

 

[Drunkskunk]

been looking this over. not sure how motor load could warp the disk when there is no load being appied to a disk untill the breaks are pulled. My theory is the magnetic pull of the motor under high load was effecting the pads them selves, pulling the outer pad in toward the motor, and against the disk.

Just a wild theory though.

I'm about to give up. anyone know of a source for those side mounted band brakes?

 

[Ypedal]

I have had this problem.....

Pretty sure it comes from the vibration caused by the current limitter or in my case both this and some looseness in the bearings once the motor heats up enough..

Either way, i fixed it by adding some plastic pieces with hot glue between the motor cover and the disk ( below the caliper of course) .. voila. no more zing. :wink:

 

[Zoot Katz]

I have had this problem.....

Pretty sure it comes from the vibration caused by the current limitter or in my case both this and some looseness in the bearings once the motor heats up enough..

Either way, i fixed it by adding some plastic pieces with hot glue between the motor cover and the disk ( below the caliper of course) .. voila. no more zing. :wink:

I'm having a hard time visualising this.
The plastic bits and glue are shims where the rotor bolts to the hub motor cover?
The plastic bits and glue are just attatched to the rotor where it doesn't interfere with the caliper?
What happens if the rotor gets hot enough to melt the glue?
Do the plastic bits reduce vibration by simply being glued to the rotor?
They certainly have no effect the bearings.

 

[Ypedal]

Ok.. it's ugly.... but it works !! ( There is plastic in there.. but mostly hot glue ! )

 

[John in CR]

Ypedal,

Good to see you not only about beautiful builds with bike pictures in classy nature scenes. This was an interesting problem that must have been driving you crazy until you got to the stage where you would just slather plastic and hot glue in there until it went away. It must have been some kind of vibration, and not a misalignment once the bearing heated up, since that solution would have been unaffected by a misalignment. Did you try a different rotor too? I can't imagine the magnetic pulses from the motor being strong enough at that distance, but small forces stimulating a resonance is quite possible, and your glue and plastic damped it away quite effectively. If it is a resonance, a lot less of something less permanent could be tested first, like maybe my old pal duct tape around pairs of the spokes of the rotor could be a quick and easily reversible test.

John

 

[Zoot Katz]

Ok.. it's ugly.... but it works !! ( There is plastic in there.. but mostly hot glue ! )

The plastic appears to be held away from the rotor by the hot glue.
It's just a damper.
I was stumped for why it works until John in CR nailed it.. . maybe.

 

[Drunkskunk]

Awesome. I'll try some hot glue and Plastic shims tonight.

If it doesn't cure my scraping noise under acceleration, then I've managed to pick up one of these:

Its an Arai Drum brake for a tandum. There out of production, but the LBS had one.
Its actualy a drag brake ment for a tandum for coming down mountians, but it mounts the same way as the Clyte disc. I figure it ought to be beefy ennough to haul a 100 pound bike down from 35mph to stop with.

And there's the Cool factor.

 

[voicecoils]

Have you thought about frame flex?

When you say 'under load' are you talking about the bike being fully loaded up or when your pedaling hard / accelerating?
The frame or swingarm would only need to flex a small amount for the caliper to go out of alignment.

Any way to get a view of the caliper when you experience the rubbing? Camera mounted back there while you ride perhaps?

The magnetic theory I find very unlikely.

 

[Drunkskunk]

under load in this case is running 74 volts, 20 amps, full throttle, up steap hills. it doesn't do it from a dead stop, but above about 15mph it starts, and quits if I back off the power.

Frame flex is posable I suppose, But its a Kona Down Hiller, and I've beefed up the dropouts with 1/4 inch forged steel. its also 5 pounds of billet aluminum. if it's flexing, I'm in trouble.

Good idea about the camera. I don't have one, but maybe i can borrow one. The hot glue idea is dead for tonight. apparently the GF decided she needed it more, at her place.

 

[markcycle]

See if this helps to figure out whats going on.

Turn the bike upside down now grab the tire with one hand and the frame with the other and see if you can rock the wheel back and forth. I had a poorly machined shaft on one motor such that the bearing had play on the shaft and it was on the disc side. I had to build up the shaft with epoxy and shim stock.

Mark

 

[John in CR]

With the symptom being so specific, it almost has to be some kind of harmonic resonance being stimulated in the rotor making it vibrate enough to rub the pad(s). Regardless of the cause of the vibration stimulating it, if it is a resonance, it shouldn't require such drastic measures to get rid of it. Just for grins wrap a piece of tape that effectively ties a pair of the rotor spokes together, and see if it changes the sound.

John

 

[recumbent]

With the symptom being so specific, it almost has to be some kind of harmonic resonance being stimulated in the rotor making it vibrate enough to rub the pad(s).

Yep, that's the first thing i thought of was some sort of freak harmonic oscolation (sp), but the first easy fix is clean the disc with iso alcohol, this gets rid of brake noise for me always. You just have to touch the disc once to have a grease spot that grabs, then lets go rapily to vibrate the pads, causing them to squeal.

 

[Drunkskunk]

The problem was never a squeal, it was a grinding, crunching scrape, like my pads were contacting the disk at an odd angle but not under pressure. it only did it at full load, but letting off the throttle cured it. I think the best way to describe it is the sound a thick chunk of steel makes as it stalls out a grinding wheel.

I took the caliper off and ran it that way to make sure it wasn't a bearing failure or a magnet loose in the motor. its fine as long as the caliper is off. Still no hot glue gun yet.