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Discussion Starter · #1 ·
Not sure what is going on with this bike, maybe someone here has a clue.

I've had this Trek Rail 7 a couple months now and I really like it, but I've noticed that when I am cranking on it with some power assist, there is sometimes (not always) a loud screeching sound like metal-on-metal. I can't reproduce the issue while I'm off the bike, which is frustrating, because I can't pinpoint where the noise is coming from. If I put very slight pressure on the rear brake lever, it goes away... but it really doesn't seem to be the brake alignment, because as I said I can't reproduce it while off the bike, and it only happens when using power assist and cranking uphill. Anyway it isn't similar in sound or symptom to brake rub on the rotor. Totally confusing. The brakes seem totally fine, so if it were the motor or other part of the bottom bracket (which is where the assist is located) as it seems it should be based on when it happens, why does slight pressure on the brake lever have any influence over this noise?

I've been pretty adept at mountain bike maintenance in the past but this is my first eMTB and I have no idea how to diagnose or resolve any issues with the power assist.
 

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I would take it back to your shop and have them check it out. It sounds like something may be wrong, but since you are new to eMTB, it could be normal motor sounds.

(I have a new Trek Powerfly with that Bosch CX motor and it does seem a little louder than my previous Shimano and Yamaha motors)
 

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Discussion Starter · #3 ·
I would take it back to your shop and have them check it out. It sounds like something may be wrong, but since you are new to eMTB, it could be normal motor sounds.

(I have a new Trek Powerfly with that Bosch CX motor and it does seem a little louder than my previous Shimano and Yamaha motors)
Yeah I plan to take it back to the shop. It's definitely not normal. It is a loud piercing metal-on-metal shrieking sound. I suspect some bearing in the motor, but I just think it's bizarre that the right brake lever has any impact.
 

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Listening to your story my gut instincts says rear brake issue. You probably have four piston brakes and one of the pistons may be hanging up a little and creating intermittent drag and noise or pads are toast. So disassemble rear brakes, inspect the pads and rotor for weird wear, inspect pistons and notice if they are equal, then if pads are in good shape, sand them flush, clean everything and re assemble. EMTB's eat rear brakes so don't be surprised if they wear out after a short time riding as compared..................IMO
 

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I think the brakes and the motor work independently but anyway look at the easy things first like brakes and suspension. I had a friend who had his rear axle come loose and created a big noise and my wife's Orbea had a noticeable squeak in the rear bearings of a brand new hub.
 

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Discussion Starter · #10 ·
Rear suspension pivots may be loose, allowing the wheel to get cocked sideways when under E-power, causing the brake to rub/screech.

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I dropped the bike off today at the shop, and so far the above seems like the most likely diagnosis, or at least something similar. The guy said it's a common issue with the large rotors on the Trek Rail bikes when under the kind of large torque that can be provided by the motor in certain circumstances. Perhaps pulling on the brakes (even slightly) helps center the rotor so that it's not scraping on the caliper housing? Overall I can't really make sense of what is going on. I didn't notice any unusual wear patterns.

Definitely the motor is not disengaged with the brake pulled otherwise I wouldn't be able to climb these grades this fast :) And the brakes make no noise when used normally on downhills so it's not the pads. And I don't think it's solely a brake issue because it only happens when I'm really cranking the bike with the motor to jam up some steep grades.
 

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Discussion Starter · #11 ·
Well, they couldn't reproduce the issue so I might be back to square one. I will try to repro it with them when I come back to the shop on a steep grade somewhere.

If anyone can give a good explanation of why something like this might happen I'd love to hear it.
 
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