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<p>Went out on my Cervelo c3 the other day. After riding down Kirkstone Pass every time I pedal hard there’s a rubbing noise from the rear disc when my left leg’s near the bottom of its stroke. It’s never done this in 18 months. Been back to the shop to be fettled but it’s still doing it. They tightened the through axle and trued&nbsp;the wheel slightly. Anyone out there with a theory?</p>

<p>Mike &nbsp;</p>

<p>&nbsp;</p>

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14 comments

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Drinfinity [264 posts] 5 months ago
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<straw clutch> Pad wear leading to pistons out further than usual, and getting stuck out, so less clearance? Then pad is detecting slight flex in frame.

Callipers moved slightly on mounts? </straw clutch>

 

 

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hawkinspeter [4079 posts] 5 months ago
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I'd be thinking of pads/disc alignment. Check the disc isn't warped and maybe re-align the caliper.

If you put the bike on a stand (or even *THE HORROR* turn it upside down), then when spinning the back wheel you should be able to get your head in a position to eyeball the gap between the disc and the pads. A light surface or piece of paper put the other side from your head will sometimes make it easier to see. If the gap is nicely consistent and about 1mm or so either side of the disc, then the alignment is spot on.

If the alignment is good, then I'd be looking for anything that could shift the position whilst pedalling. Rear wheel bearings could be a suspect.

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Mikehudson [9 posts] 5 months ago
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Pad/disc alignment seems spot on. Bearings were checked at the shop and found to be ok. It only manifests when putting the power down, ie when introducing a twisting force into the frame. It was absolutely fine until the first time I stood on the pedals after a fairly fast descent. Looks like a back to the shop job. I don’t suppose you can break a thru ankle? They seem pretty substantial to me. 

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hawkinspeter [4079 posts] 5 months ago
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Mikehudson wrote:

Pad/disc alignment seems spot on. Bearings were checked at the shop and found to be ok. It only manifests when putting the power down, ie when introducing a twisting force into the frame. It was absolutely fine until the first time I stood on the pedals after a fairly fast descent. Looks like a back to the shop job. I don’t suppose you can break a thru ankle? They seem pretty substantial to me. 

A thru-axle should be pretty tough. You'd certainly not snap it in half, but the thread on the end could get fouled up, I suppose. You'd notice if the thru-axle was faulty when removing it and then putting it back in place. Also, I wouldn't expect a thru-axle to be able to slip/move around if it's tightened correctly.

If the wheel bearings are all fine, then maybe there's an issue with the frame?

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Team EPO [219 posts] 5 months ago
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bit of cleaner often does the trick next stop change the pads 

https://www.amazon.co.uk/Muc-Off-Disc-Brake-Cleaner-400ml/dp/B002MXKGZC/...

 

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ktache [2122 posts] 5 months ago
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I noticed yesterday that Decathlon had it for £8.99.

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CXR94Di2 [2726 posts] 5 months ago
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grab the wheel at the rim with boths hands on opposite sides. Push and pull to see if you can hear or feel flex in the hub. If no flex then its alignment of caliper/pads or rotor bent. Last resort, extremely unlikely there is something wrong with the frame-crack or weakness in stays

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huntswheelers [185 posts] 5 months ago
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R8000 per chance...

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Swiss Tony [5 posts] 5 months ago
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huntswheelers wrote:

R8000 per chance...

is there a known problem with R8000 then?

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Mikehudson [9 posts] 4 months ago
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Update time. Went for a ride the other day. Bike’s still doing the same thing, the noise is actually coming from the front disc. I was told the first time “oh it’ll be the rear disc” so thought I’d trust the experts! Went back to the shop with it.  Got a phone call to say they can’t find anything wrong and “they all do that to a certain extent”. Surely if that’s the case it would have done it since day one, but something must have changed between the top of the hill and the bottom. If “they all do that” then surely we’ll be able to take any disc road bike out of the showroom and replicate that noise? Any more theories from you guys? 

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StraelGuy [1746 posts] 4 months ago
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Team EPO wrote:

bit of cleaner often does the trick next stop change the pads 

https://www.amazon.co.uk/Muc-Off-Disc-Brake-Cleaner-400ml/dp/B002MXKGZC/...

Or a massive can of normal brake cleaner for £3-4 from Screwfix or Toolstation... Oh, and don't believe the 'brake cleaner has oil in it' propoganda. It doesn't. Brake cleaner evaporates to leave NOTHING behind. Brake cleaner with oil in it is carburettor cleaner.

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Mikehudson [9 posts] 4 months ago
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Cleaning the disc won’t make a bit of difference given that most of the time the disc and pads aren’t in contact with each other. It only happens when applying force through the pedals. I can also make it happen if I push down quite hard on the right handlebar while pushing the saddle into my right thigh while freewheeling. If anyone else out there has come across the same phenomenon I’d be interested to hear because, as I’ve been told, “they all do it “. 

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IanEdward [340 posts] 4 months ago
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If I'm imagining your position on the bike right, it does sound like the position most likely to flex the wheel in the fork, i.e. your wheel is now tilted to the right with your weight on the left pedal? This would make the fork want to flex to the left, so I guess it could well move the calliper a tiny bit closer to the disc.

If I was ever to make my rim brakes rub it would be in a similar position whilst pedalling hard.

If you are positive there is no additional movement in the wheel (is the QR/thru-axle as tight as it can be? Are the bearings in good condition with any play adjusted out if possible?) then the only thing that could have 'changed' is the pads self adjusting closer to the disc on one side. You could push them back in with a tyre lever (or better yet buy the Hayes alignment guage) then go through the process of re-aligning calliper.

Also, now I think about it, I once mananged to create a burr on the side of a pad by re-installing the wheel clumsily, the disc caught the edge of the pad and left a 'tuft' sticking out which hit the disc when I put any power through pedals.

My local shop also gave up with some noisy discs of mine, basically admitting they spent about 50% of their time trying to resolve noisy disc brake issues...  

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Pilot Pete [226 posts] 4 months ago
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Try removing the pads and giving them a rub on some emery paper (wet and dry paper will do). Pads can get a little uneven wear if a caliper has not been perfectly aligned and ridden like this for many miles. Don’t go mad, just take the very surface off (any glazing you can see) and any edge lip which sometimes occurs.

Re-install after cleaning the pistons with isopropyl alcohol and cotton buds and exercising them gently in and out without the pads in (don’t pull the brake hard and fast or you may dislodge a piston completely). Try cleaning the edge of the piston around the seal whilst it is ‘out’. Pry back in with a soft lever (plastic rather than metal) before re-fitting the pads.

Check the alignment of the caliper and also check the rotor is running true (you could so this whilst the pads are out). You can check this by holding something close enough to just touch the rotor surface whilst gently spinning the wheel. A more accurate way is to set up a dial gauge. If the rotor is not running true gently bend it with an alignment tool or and adjustable spanner so that it does spin without any wobble.

With that complete, if you are still getting the noise under load then it sounds like flex between axle and frame/ fork. If it is after hard or extensive braking it could be something else;

Under braking the rotor heats up as do the pads  and the caliper. If there is any air in the system (a tiny amount that doesn’t really affect braking even) then the air heats up and expands and prevents the pistons from going all the way back until the caliper/ fluid has cooled sufficiently. This can take quite a few seconds to achieve and you can get rotor rub which reduces as the fluid cools. It is worth just re-bleeding if you get this to ensure you have eliminated all air, even the last tiny bubbles. It could be the cause if you only get it under load after hard or prolonged braking as t(e piston(s) have just stayed out a bit far and any lack of trueness in the rotor or very slight flex could exacerbate it.

Hope this helps.

PP