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Why does my bike make a sound like a knife being sharpened??

Posted: 26 Jul 2020, 9:35pm
by KromHill
Recently purchased a Gravel bike with disk brakes. Last week it started to make a noise (similar to the sound of a knife being sharpened) to me it sounds like the rear disk is rubbing but when I turn the bike upside down and spin the rear wheel its fine and not rubbing. It only seems to make this noise when I'm putting in effort either uphill or accelerating. I've checked for loose items bolts etc and everything seems tight?? The pedals are around 6 years old as I swapped them from my other bike but it doesn't seem like a pedal noise??

Any ideas would be appreciated.

Many Thanks.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 26 Jul 2020, 9:46pm
by geomannie
It could be so many things. Are your wheels in straight and are centred?

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 26 Jul 2020, 9:48pm
by mjr
Front derailleur misadjusted or chainring moving off axis?

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 26 Jul 2020, 9:57pm
by alexnharvey
My guess is one of the discs hitting the pads, specially if it has cable discs.

If it does follow the centring procedure for a cable disc brake.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 26 Jul 2020, 10:24pm
by Brucey
sounds like a disc rub, but for it to only make the noise under load means the wheel/disc is moving around, so bearings with free play, excessive frame flex, disc out of true, QR not tight.... something like that.

cheers

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 5:13am
by MarcusT
What Brucey said.
I get the same noise when doing some curvy downhills. The wheel flexing causes the disc to rub. Also happens on long downhills as the disc and pads heat up.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 8:59am
by Brucey
FWIW the fact that you describe it as 'like a knife being sharpened' make it sound like a hard rub rather than just a pad-to-disc rub, i.e. the disc may be hitting the caliper body or the adaptor (if there is one)/disc mount. So especially If the pads are soft (organic ) types look for signs that the disc has been hitting something harder.

Because with a new frame the locknuts 'bite' through the paint on the dropouts, the alignment can change quickly when the bike is still quite new and hard rubs between the LH side of the disc and the RHS of the hardware on the frame are most common. Also be aware that a disc can rub radially too, so check for clearance that way too.

cheers

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 9:19am
by reohn2
Much as Brucey posts.
First I'd remove the caliper and check if there's sign of scraping of rotor on caliper either on the inside of the caliper or the outer edge of the rotor.
I'd then check the rotor for true straightening if needed and go through the complete caliper set up procedure as per manufacturer's instructions.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 3:46pm
by peetee
Might be worth checking by eye if the calliper body is fouling the disc where it mounts to the frame.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 4:33pm
by seph
2031A1DA-46F5-48E5-848C-7C058260C978.jpeg


Does it look like this? :)

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 5:06pm
by KromHill
Took the caliper off and its been rubbing on the inside. Weird how it only does it under exertion?? Re centred it and see what happens.

Thanks for your help.

Re: Why does my bike make a sound like a knife being sharpened??

Posted: 27 Jul 2020, 7:22pm
by ElCani
KromHill wrote:Took the caliper off and its been rubbing on the inside. Weird how it only does it under exertion?? Re centred it and see what happens.

Thanks for your help.

Lots of stuff on a bike flexes and moves when the rider is pushing hard.