rear disc makes noise, not bearing right?

[not_scooter_Scura!]

so pushing the bike backwards when parking , brings out a sqeal from the rear, I take it this the disc & not the wheel bearing?

 

happens at same spot pretty much.

 

 

n_s_S!

[jrt]

yep, its the brake. There's probably a whole bunch of old brake dust built up in there; it should be cleaned periodically.

Mine does it as well.

[Guest ratchethack]

A couple of things appear to contribute to the rear brake dragging enough to cause excessive pad and disk wear. I had noticed that mine seemed a lot hotter to the touch than the fronts after a ride. Until I fixed mine (see below), the pads and disk were wearing excessively compared to the front brakes.

 

Heat buildup from the pads dragging seems to cause expansion of trapped air in the caliper, which starts a vicious cycle. One symptom of this is the squeal or groan noise when backing up. The bleed nipple being upside down on the bike (can't bleed it properly this way) appears to be a significant factor, and buildup of caked-on brake dust in the caliper, preventing retraction of the pistons, makes the problem worse. We just did a procedure on my Pal's '04 LeMans on Saturday that worked just as well for him as it has on my '00 Sport:

 

With thanks (again!) to Rich Maund: Remove the rear brake caliper from the brake hanger and take out the pads. Don't touch the "working surfaces" of the pads, and be careful to note their right & left-hand side orientation for re-assembly. You can do this fairly easily without taking off the rear wheel, and without disturbing the hydraulic line. Dunk the caliper in your favorite cleaning cocktail (I use 50/50 Simple Green/H20). I like this best because afterward my caliper has that "Spring Fresh" scent. Brush out the inside of the caliper thoroughly with a toothbrush, paying special attention to get out ALL the caked-on brake dust in the circular groove between the pistons and the caliper next to the seals, where you can't see. Rinse carefully with clear water. Blast out all the H20 with compressed air if you've got it. You can also use a heat gun or even a hair dryer, but it takes a lot longer. Before re-assembly, bleed the brake carefully with the bleeder nipple pointing UP so that you get ALL the trapped bubbles in the caliper out. It helps to rap on the caliper with a soft-faced mallet while you do this, tilting the caliper slightly from side to side, which seems to release a significant amount of trapped air. It also helps to have an assistant, but you can do it on your own if you're careful. Caution! - wedge a block of wood between the caliper pistons to prevent them from falling out when you pump the brake while you're bleeding it. YOU REALLY DON'T WANT THIS TO HAPPEN! Clean the caliper bolts properly and Locktite the caliper back in place with pads re-installed the way they came out, and your "backup groan" is GONE. Make sure to adjust the threaded rod and/or cam stop on the brake lever so you can feel some clearance to allow for a tiny bit of heat expansion in the system, to ensure you don't start the problem back up again.

 

Rgds.,

 

Ratchethack

[staedtler]

yes, i had the same issue....

 

i just spray some of this

and, since then, not a sound!!

[Guest ratchethack]

...uhhhh.......The last thing I ever want anywhere near my brake pads and disk is a big sloppy shot of Stoddard solvent-based penetrating oil. But that's just me.

[txrider]

Ratchet, some of the Brit Aprilia Futura riders who use their bikes as commuters rack up a lot of miles and follow this same regimen with additional recommendations. One, carefully extend the pistons (short of coming out) so they are more exposed to cleaning. Two, some have used, sparingly, a brake cylinder lube around the piston seals to keep them soft and pliable and extending their service life. I believe the item mentioned was Sylgard, silicone based, available at NAPA stores.

[Guest ratchethack]

Sounds like a reasonable tip, Txrider. Thanks. I haven't run into a seal-longevity problem yet (22K mostly back-country road miles). For heavy stop-n-go commuter work, I'd tend to expect piston seals to go away much sooner. I'm just pleased at this point to have apparently stopped eating rear pads and disk. But I'll give your suggestion a try next pad replacement.

[guido]

Just shortly. The adjustment of the (foot) brake pedal is often incorrect. That is, there is no free play. Result is the brake drags lightly at first, heats up, heats the fluid, which expands, putting more pressure on brake more heat etc. It can get to the point that the brake will lock up.

 

The cure is simple, adjust the pedal to have a couple of mm free play.

 

My V11 has a noticable band towards the center of the rear disc brake surface, where the plate is rough on both sides. Why it has not kept the whole brake surface nice and shiny I don't know, since the outer half of the brake surface is like the front brakes, nice and shiny.

[Guest ratchethack]

Guido, what you've noted about brake pedal adjustment was mentioned above in this thread (by me ).

 

Re: expansion of brake fluid, I'd offer one important correction to what you've stated - it's not expanding brake fluid that causes the brakes to drag. The coefficient of heat expansion of brake fluid is imperceptably small, by careful design - for the exact purpose of preventing heat-induced drag in hydraulic brakes. Brake fluid is also designed to have a boiling point that's as high as possible for the same reason. Ever notice how corrosive brake fluid is? It'll zip right through unprotected paint (ask me how I know ).

 

It's nasty because less nasty stuff (hydraulic oil, for example, whch is universally used in similar, but non-heat-sensitive applications) has a higher coefficient of heat expansion and a lower boiling point, though I'd have to check some SAE standards list somewhere to find the exact numbers... brake fluid is also powerfully hygroscopic, meaning that it attracts water so much that given any opportunity, it'll actually extract water vapor out of the open air and put it into solution in the brakesystem. Too much H20 in there (it doesn't take much) will wreak havoc. That's why it's so important not to leave your new Dot 4 BF bottle, or your clutch and brake fluid reserviors on your bike uncapped for any longer than necessary.

 

The point is, you don't want either any fluid expansion, or boiling of fluid at all in a brake system, and the stuff is engineered not to expand AT ALL with heat. As it happens, the lowest possible coefficient of heat expansion and highest boiling point in a working fluid can be achieved only with a fairly noxious combination of chemicals. Therefore, brake fluid. Strange stuff.

 

The culprit is trapped air in the caliper (again, as mentioned above), or other contaminants that get in there (water, typically) that can vaporize (boil off) at relatively low temperatures, introducing gases into the system. Air, being a mixed gas, has physical properties in entirely different ranges from that of most fluids. Air and other gases expand very significantly in direct proportion to an increase in heat. Trapped air in a caliper that absorbs any any heat at all above ambient will instantly increase it's volume very dramatically, displacing brake fluid, and put a relatively steady, relatively low pressure on the pads, causing them to drag - think of it as an air spring pushing on the pads. Enough expanded air in the caliper and you get a "spongy" feel at the lever. Racing on a tight track can get your calipers hot enough to boil the brake fluid, despite the high boiling point. This achieves the same thing, even with NO air in the system. Air, water vapor, or brake fluid in a gaseous (boiled off) state will initiate the vicious cycle mentioned above, where the dragging brake creates more heat, which makes the pads drag more, etc.

 

This understood - and just as you pointed out (though for a different reason) - If you don't have at least a bit of adjustment clearance at the brake lever cam - well, you're just inviting the problem to start, unless you've somehow determined that there's absolutely NO trapped air whatsoever in the caliper. You can't ever be absolutely sure there isn't any air or water in there, but for all practical purposes, you can sure decrease the amount and the probability by properly bleeding, per my prev. post above.

 

On another note - the rough band on the inside surface of your disk may be due to contamination of the pad on that side. It happens. You might consider replacing the pads with a new set if you're concerned about it.

 

Rgds.,

 

Ratchethack

[guido]

Sorry ratchet, did not catch the adjustment bit in your writing.

As for water/air in the system, that will not cause the brakes to lock up.

You are right that the gas will expand, but the problem is, it is compressible, acting as a spring in the system, preventing you from transferring full pressure from the pedal to the brakes.

Fluid, on the other hand, is well neigh incompressible, not changing volume with pressure, and thus able to transfer pressure from the master cylinder to the slave cylinder.

If then, as has been the case on one V11 I took for a test ride, the brakes have no free play at rest, you will get the brake lock as the fluid expands with increased friction and temperature, to the point where it exceedes the engine output and forces the bike to a halt!

Kind regards

 

guido

[Guest ratchethack]

Well Guido, I beg to differ. As my ol' man used to say, "You can lead a horse to water..."

[dlaing]

You guys are confusing me.

As I understand it, to be safe, you want sufficient freeplay, and minimal air and water in the lines.

Are either of you saying this is not true?

Regarding lubricating the pistons; I think it is fine to do as long as you are careful and clean the excess so that it will not contact the the pads or rotors. Keep in mind that lubricants will creep and move to places that you might not have predicted.

[Guest ratchethack]

OK, I'm gonna take another run at this because unless it's understood properly, somebody's likely to get into trouble for the first time, and Guido will get into trouble again. I've argued the details and the supporting facts above. If there's anything you disagree with in what I've posted, ask a good brake man or a race mechanic:

 

1. Pure brake fluid, for all practical purposes, DOES NOT expand with heat (extreme racing conditions to the point of boiling notwithstanding).

 

2. Air and/or water in your brake caliper and/or lines CAN cause your brakes to lock up.

 

So what, who cares, and why is this important? If you believe that your troublesome brake, which makes a funny noise when you back up your bike, and also seems to heat up and drag, can be "cured" by putting more free-play at the lever, you might be headed for the ditch (or worse) at a time and place not of your own choosing. What you need to do is to get the air and/or water out of your brake caliper and/or lines and/or fluid reservoir. Bleed it properly (see my post above).

 

"Hell, there ain't no air in my brakes! My shop just bled 'em!" - (see upside-down bleeder nipple, above).

 

"Hell, there ain't no water in my brakes! It's never even been in the rain!" - (See hygroscopic, above).

 

In summary: A very little amount of H20 will create a very big problem. Water in brake fluid quickly becomes water vapor with heat from normal braking. Water vapor in the caliper acts (for all practical purposes) just like trapped air, with the main difference being that WV only comes to the party when the brakes warm up. Now air loves ANY brake caliper party. Air will show up whether it's hot OR cold. The volume of each increases directly with temperature, and their initial low "spring pressure" on the brake pads will continue to increase with a rise in temperature until, when hot enough, either one of these Party Criminals alone is powerful enough to lock the brake, overcoming momentum and engine torque. (See vicious cycle, above).

 

Always bleed 'em with NEW brake fluid (see hygroscopic, above, Part 2). Unless you live in Death Valley, where the humidity's low enough to turn a gila monster into a hockey puck, your old can of BF has sucked up water out of the air every time you've had the cap off. If you put this in your brake system, you could be in danger of experiencing the same kind of brakes that Guido experienced on his test ride - or worse.

 

Ride safe.

 

Ratchethack

[dlaing]

Unless you live in Death Valley, where the humidity's low enough to turn a gila monster into a hockey puck,

50805[/snapback]

OK, I hear yah.

Thanks for the clarification.

I often wait for Santa Ana's to bleed my brakes. But, I should not wait too, long. They are a little spongy right now.

Maybe I'll go open up a motorcycle tire and brake specialty shop in Borrego Desert.

I'll be pumping the air full of drier air into the tires and I could bleed brakes with lower risk of humidity getting in.

I'd be near the best roads.

The rent is cheap.

Dry air for sale! What a marketing strategy!

[Guest ratchethack]

Dry air for sale! What a marketing strategy!

50825[/snapback]

The real estate knuckleheads in Zonyland have been using it to sell the idea of living in oven conditions for years...