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XTR pads touching rotor


Titanium Rocket

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Are both pads touching? You can see by shining a tourch from the side, and looking at the area of contact. 

 

If only one pad touching, one solution is to center the caliper, either with thin washer-shims with Post mount, or by adjusting the caliper sideways in the International Standard type. Some of these calipers have elongated slots, but its tricky to hold in place when tightning screw and the caliper not moving, use a few washers to solve this.

 

Good luck!
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The Retracting power comes from the elasticity of the O-ring on the piston, the spring holds the pads against the piston so by tweaking the spring a bit it may help, also take the pads out, pump the lever a few times to push the pistons ALMOST all the way out then clean and lube them, also using compressed air to clear out the gunk, then push them back in and try that.

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Thanks for your comments everybody. Relieved to learn that I am not unique in this problem. Was beginning to think I am a pedantic young fart.

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As JB says, If the rotors are warped, you're stuffed.

What did the trick for me: Loosen the calipers, take the wheel off. Fold a business card over the rotor, put the wheel back with the business card inbetween the pads. Wiggle the wheel a bit to even further push back the pistons. Fasten the skewer, hold the brakelever and tighten the caliper bolts again. Turn the wheel to remove the card. If this doesn't work - buy new rotors!
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Could be that the caliper o-rings are worn and not retracting the piston far enough. O-rings do harden over time.

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Could be that the caliper o-rings are worn and not retracting the piston far enough. O-rings do harden over time.

 

This would be my guess, from my experience with race cars having this problem you should replace the seals. The reason why is that after time the seals wear unevenly on the surface of the piston and with this "loss of shape" they can't return properly. If you want to try a quick fix you can remove the seals and turn them around. This may make the pistons retract a little too much making for a "longer lever".  
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I have never come across this in bikes, I agree with cars as the system gets hotter, but even a 5 year old system that I have still works perfectly. My theory is if the seals harden/go pap then the caliper will start leaking (not always though) a bit contradictory but thats me...

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Spring is there really just to keep the pads on the pistons' date=' the retracting power comes from the hydraulic fluid.[/quote']

 

Please explain.

 

Please ignore the part about the fluid powering the retraction smiley5.gif I have learnt something new about disc brakes this morning.

 

 

 

 

There is no mechanism in the system to retract the pads and the only reason they do seem to retract a bit is thanks to some stiction and elasticity in the piston seal and fluid.

 

 

 

On cars' date=' motorbikes and aeroplanes they constantly touch and there is no reason why people should expect them to behave differently on bicycles.[/quote']

 

 

 

But cars, motorbikes and aeroplanes are noisy pieces of machinery with large powerful engines, where a rotor rubbing poses no real problem. They also use floating calipers where it is near impossible to get the rotors not to rub for any extended period of time.

 

 

 

I don't think its unreasonable to expect your discs to not rub, once correctly setup and as long as the wheels are not removed, a disc brake caliper should not rub on the rotor unless you manage to bend the rotor. A mild warping of the rotor often happens while out riding and usually straightens itself out after some heavy braking.

 

 

 

On a bike a disc rubbing can cause much annoyance to the rider and those around him at the time. Over a long ride I reckon there would be a considerable amount of energy lost as friction, but the high pitched scraping noise is truly unpleasant.

 

 

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Maybe this is a totally dumb question, but why the hell hasn't someone designed a way to retract the pads properly, and end this source of anguish?

 

Whenever I hear this rubbing, I develop a panic attack about that friction sucking away valuable energy....may as well ride a 29er.
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Maybe this is a totally dumb question' date=' but why the hell hasn't someone designed a way to retract the pads properly, and end this source of anguish?

 

?

 

Whenever I hear this rubbing, I develop a panic attack about that friction sucking away valuable energy....may as well ride a 29er.
[/quote']

 

 

 

One problem I can see is that as you pull the lever you would also have to overcome the retracting force, meaning more force is required to brake.

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He he....all this angst about a bit of rubbing. Welcome to the world of disc brakes. That's what they do, they rub.

 

As for the friction and the wasted energy. It is very, very, very little energy wasted. Dont waste more energy thinking about it.

 

As for the retracting mechanism. I agree. We need it on bicycles, if not on larger vehicles. The little bit of retracting that does exist is there by fluke only, not by design. We'll need a dedicated device that acts on the slave cyclinder.

 

I haven't applied my mind to it more than just casually, but there are some...how do the motivational speakers put it?...Obstacles or challenges to overcome. Ideally we would like enough clearance to clear typical disc runout which is easily a mm or even two. To close that type of gap once you need to brake, would require about five or six pumping strokes on the lever. That is obviously not desirable.  We therefore need some of quick-uptake mechanism that quickly closes the gap and then moves into the background so that the hydraulics can do their work. Once the lever is released, the mechanism kicks in and retracts the disck. We want this to happen in milliseconds and obviously without any disruptive feedback.

 

Don't think powerful spring working on the pads, that brings us back to the lever pump problem.

 

We could just use V-brakes and get on with it but real MTBers would consider that naff.
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He he....all this angst about a bit of rubbing. Welcome to the world of disc brakes. That's what they do' date=' they rub.

 

As for the friction and the wasted energy. It is very, very, very little energy wasted. Dont waste more energy thinking about it.

 

As for the retracting mechanism. I agree. We need it on bicycles, if not on larger vehicles. The little bit of retracting that does exist is there by fluke only, not by design. We'll need a dedicated device that acts on the slave cyclinder.

 

I haven't applied my mind to it more than just casually, but there are some...how do the motivational speakers put it?...Obstacles or challenges to overcome. Ideally we would like enough clearance to clear typical disc runout which is easily a mm or even two. To close that type of gap once you need to brake, would require about five or six pumping strokes on the lever. That is obviously not desirable.  We therefore need some of quick-uptake mechanism that quickly closes the gap and then moves into the background so that the hydraulics can do their work. Once the lever is released, the mechanism kicks in and retracts the disck. We want this to happen in milliseconds and obviously without any disruptive feedback.

 

Don't think powerful spring working on the pads, that brings us back to the lever pump problem.

 

We could just use V-brakes and get on with it but real MTBers would consider that naff.
[/quote'] I beg to differ. The o ring sits in a groove. In a neutral position with no pressure on the brake lever the o ring will maintain its natural shape. Once pressure is applied and the piston moves forward it causes the o ring to distort to allow for this movement. The o-ring will always want to return to its natural shape therefore when the pressure in the master cylinder is released it will return to its original shape thereby pulling the piston back. So after a while two things may happen. The o ring may lose some of its elasticity (not common unless exposed to extreme heat) or the o ring may start to wear from the obvious constant movement. This wear will mean that the shape of the o ring will change slightly so that the natural position that it returns to when the brake lever is released is closer to the point that it goes to when pressure is applied to the brake lever. (The piston won't return as much)On a side note - When the pads wear down the piston needs to move more to make up for the space left by the used brake pad material. In this case the piston will slide past the o rings original seating point on its surface. Once the piston has moved enough to make up for the brake pad wear the o ring and piston will operate as normal.  This probably makes no sense to anyone but from my experience with things that have wheels it is MHO.  

  
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I beg to differ. The o ring sits in a groove. In a neutral position with no pressure on the brake lever the o ring will maintain its natural shape. Once pressure is applied and the piston moves forward it causes the o ring to distort to allow for this movement. The o-ring will always want to return to its natural shape therefore when the pressure in the master cylinder is released it will return to its original shape thereby pulling the piston back. So after a while two things may happen. The o ring may lose some of its elasticity (not common unless exposed to extreme heat) or the o ring may start to wear from the obvious constant movement. This wear will mean that the shape of the o ring will change slightly so that the natural position that it returns to when the brake lever is released is closer to the point that it goes to when pressure is applied to the brake lever. (The piston won't return as much)On a side note - When the pads wear down the piston needs to move more to make up for the space left by the used brake pad material. In this case the piston will slide past the o rings original seating point on its surface. Once the piston has moved enough to make up for the brake pad wear the o ring and piston will operate as normal.  This probably makes no sense to anyone but from my experience with things that have wheels it is MHO.  

  

 

I agree with you on how it works, but I say it is a fluke and you say it was designed that way. I'm not so sure but I'lll look on USPatent.gov to see if it was claimed by anyone.

 

The groove is a standard gland with the O-ring's half shape. The O-ring is standard and round in section. Where's the design in that?

 

Further, the retractability changes with lubrication between the piston and O-ring. A new, lubricated piston will NOT retract as well as an older sticky one.  How's that for reverse fluke?

 

O-ring and fluid elasticity combined gives us some (poor) merasure of retractibility but clearly nothing that satisfies the consumer.

 
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OK there are obviously two schools of thought here. Contrary to what some may believe the piston does not SLIDE in and out on the surface of the O-ring during braking. The piston and the Oring surfaces stay in the same position relative to each other. Only the FLEX in the rubber seal (o ring) allows the piston to move forward and back. The only time the piston slides along the O-ring is when it needs to move forward to make up for brake pad wear. This happens because the amount of outward movement the piston needs to make up for brake pad wear exceeds the amount of flex the o ring allows for and therefore the piston will slide past its previous seating point on the o ring. As for the "elasticity of fluid" the whole point of brake fluid is that it cannot be compressed therefore it has zero elasticity. We could use water in our braking systems if it didn't have such a low boiling point and the nasty tendency to rust things.

If anyone out there is checking the patent on the flex of rubber see if there is a patent on the cushioning effect of air in tyres while you're at it. We may all find ourselves in court soon. As I said before this is my humble opinion but feel free to pm me if you would like to ask me with what experience I allow myself to make these deductions.
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OPC - can't believe that pissy little spring really forces the pads apart....??

It takes considerable effort with a strong plastic goodie like a tyre lever to do this. Those little springs are really just to position the pad firmly aginst the piston surface. Anyway thanks' date=' but I have already tried new springs. Alas, no difference.

 
[/quote']

Off course it pushes the pads apart! Thats why they dont rattle....

I however NEVER said that they push the pistons back?! Confused

 

If the plates are misaligned, bent etc they might not do their job properly and not push the pads back squarely (against the piston!) causing the pads to actually sit skew in the caliper. The spring pushes the pad on its two outer edges, if it only pushes on one side the pad could actually pivot  on the piston making the opposite side rub. I imagine that this rubbing will be more aggressive on the leading edge of the pad. 

 

Noisy brakes is something I encounter daily in a motorsport environment and in 99% of the cases the noise is not generated on the braking surface but between the pad and the caliper as it vibrates at a high frequency, this will be amplified by the brake/hub assembly with stiffness and weight and design playing a big role. A harder brake compound tend to be more noisy especially at lower brake temps... my MTB wakes up my neighbours on every early ride till I get to the my complex's gate for this reason LOL 

 

Disc brakes are great when they work properly, if Johan thinks V brakes are better then he should put wooden Ox wagon wheels on his bike as they wont get punctures and must surely be better then? WinkLOL 
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