Post your Weird/Hard To Diagnose mechanical failures thread

As title suggests, love to hear your weird pain in the ass to diagnose mechanical issues. Probably more for ventilation albeit maybe this thread can prevent someone from forming a headache whilst trying to figure out where that noise is coming from.

I’ll start with a fun one. Had two tubes puncture after getting a pinch flat on a ride. Thought I had bad tubes since I checked the tire thoroughly (especially after the first puncture). Got home, inspected the rim and tire, changed the tube, pumped up and everything is honky dory. Two weeks later, get another flat, whip out the CO2 and bang another flat from the valve area within a few minutes. Finally figured out my rim tape slightly stretched over time around the valve hole and the sudden temperature swing from the CO2 discharging allowed things to move in a way that lead to punctures, replaced with velox and didn’t have the issue again until I retired the wheels.

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What sounded like a dead BB creaking very loudly meant we faced the shell, replaced BB (and cranks eventually), rebuilt pedals all to no avail she still creaked very loudly.

It took a random comment from someone on a ride to figure it was the headset creaking and echoing down the massive Super V downtube. Serviced the headset and it was silent again.

As an aside, that C’dale was my favourite MTB, wheelie so well. I’d love another but without C’dale’s proprietary shit.

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I had something similar, even ordered a new headset, turned out to be my thru-axle needing a bit of grease. Drove me nuts :rofl:.

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#1 Carbon rim which would blow up supersonic inner tubes after 30km, but other inner tubes would be fine. Well it took about 10 tubes to find this out and then find the problem. It was a tiny void in the rim that would cut the tube. Put tape over that and it was fine.

#2 In 1990 something, a friend had a brand new bonded aluminium framed bike. It looked great and made our steel bikes look ancient. Second or third ride however, the gears started to unexpectedly change up the block. Not down as expected when the cable slips. We looked at the bike and couldn’t see anything wrong. A few km further and we figured it out. The bb shell was separating from the tubes and lengthening the cable paths. It was replaced under guarantee.

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with punctures, I always take the time to identify the location of the puncture. if it happens again, and the location is the same, you most likely have a cause that is not penetration or the tube - but the rim or tyre, or tape.

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I’m sure this was in a CT article once. always check the skewer / through axle - for tightness and lube.

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if people paid attention to every single detail and facet of a part install/bike build this thread probably wouldn’t exist.
a healthy bike is a creak-free and trustworthy bike.

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I originally had a Litespeed Ultimate in the early 90’s with a 1” Aheadset on a Kestrel fork. I had no idea how to compress the Ti stem onto the headset and would just push down and tighten the stem up at the same time. I just couldn’t work out how to do it as I was just so used to a quill stem which I went back to on my next bike until Aheadset became the norm from then onwards.

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I had a clicking when riding out of the saddle once. Took me hours to work out that the extremely flimsy Roval QR skewers I was using would work slightly loose every few 100km and allow a fraction of play in the wheels.

A friend of mine has a weird intermittent shifting issue which has currently defeated 3 shops. Randomly, every now and again, a single downshift in the middle of the cassette will result in a 2 cog drop. The derailleur hanger is straight (checked multiple times), the mech is indexed correctly (shifts beautifully in the stand, and 98% of the time on the road), cables have been replaced. That the problem can’t be reproduced in the stand at all, and only happens occasionally on the road, makes it very hard to diagnose. He’s running mechanical, disc brake Ultegra, c.2019. The last shop suggested a worn shifter as a long shot and to try replacing it. Any guesses?

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On my new road bike, I thought my bb was creaking under hard efforts. It turned out to be loose spokes in my rear wheel rubbing / vibrating together and the sound went away after they were tightened up.

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I had a warranty Bowman that developed a creak after a few weeks of riding. Seemed to be based on saddle position. Checked seat post clamp, re packed the seatpost with carbon grip, checked the BB, relubed the handlebars and stem, lubricated under the saddle and even swapped the seatpost clamp with another one in the house to no avail. Ended up being the base of the seatpost was rubbing on the seat tube when sitting towards the back. Resolved it by sticking some friction tape along the contact area and reinserting.

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I feel like if the shifter is worn then it would be more of a consistent issue or it would be noticeable when the shifter munches the shift. My R8000 front shifter will occasionally not engage an upshift but it’s very obviously just a defective shifter and you can feel it malfunctioning. Definitely don’t make ‘‘em like they used to. My 6800 rear shifter started missing shifts at 45,000 miles of use and abuse, sprayed some of PB blaster in there and it worked perfectly for 2 more years before selling it.

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Some of these are just things that are commonly overlooked and lessons in being lazy and making assumptions (like my rim strip example) but stuff like joint separations that occur on the road likely after shock loads, equipment specific issues, or things that loosen up over 10s of thousands of miles aren’t really going to be found in the frame build up process.

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Every time a weird sound happens to me I simply look at my top tube and remind myself that it’s a feature of the Cannondale I am riding.

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Creaky derailleur hanger.

Disassembled, degreased, regreased, and reinstalled BB, cassette, pedals, etc etc. Finally noticed the rear derailleur would creak when pressed or under load. I cleaned and lightly greased between hanger and frame and its been silent since.

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A downshift? That means shifting onto a bigger sprocket?

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A classic.

No, a smaller one, as in the chain moves downwards.

One maddening intermittent chirp around the front wheel (definitely a chirp as opposed to a squeak etc) turned out to be the (tubeless) tire, promoted as an especially supple one and high volume at 43c, occasionally rotating along the rim with even light braking. This was after only 500 miles. There was no sign of burping or rim tape detaching. I put a tube in and the chirping stopped.

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B screw on my grx derailleur is working its way loose causing shift issues. Took me ages to work that out.

Need to put some loctite on it or something

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