Loss of power from bent derailleur hangar?

My wife’s Emonda SL5 has been making all kinds of drivetrain noise and experiencing poor shifting for quite awhile. I tried re-indexing it, resetting limits etc to no avail, I’m a newbie at this stuff but somewhat competent. We had velofix come out and give it a full tune up (amazing service btw) and repaired a severely bent front derailleur hangar and gave it a full tune up. The velofix guy said that the derailleur was so misaligned it was like she was severely cross chaining all of the time due to all of the friction and un-optimal chainline…

He noticed her 4iii left side crank power meter and said he wouldn’t be surprised if she immediately gained some watts due to the drivetrain issues.
Her coach had her do a tempo test on the weekend and based on that he said he could problably bump her ftp up to 235 or 240 from 230.
Could a 10 watt increase be at all drivetrain related?

Yeah, a dirty chain can cost more then 10W, so wouldn’t be surprised if a twisted, rubbing derailleur does the same.

You don’t lose power in the cranks because of a dodgy chain line.


If the hanger was that badly bent I’d recommend buying a new one and replacing that existing one. To me it sounds like a ticking time bomb for failure and the consequences of such aren’t something I’d want to experience (i.e. derallluer into the spokes destroying it, the wheel spokes and potentially causing a crash). They are cheap enough relatively speaking I’d do so as preventative maintenance.

On to the power portion. I don’t feel there will be a magical increase in total power. But an potential decrease in drivetrain friction losses will translate to a bit more speed. To put it frankly I don’t think fixing the alignment issues will magically bump up an ftp 10w but can increase power translated to the wheels up to 10w or so.

I’ve ridden a couple of times where the brake was rubbing and while outputting a decent amount of power was going 20% slower than I normally would for that effort. That’s when I noticed the front brake was rubbing slightly (it happened when I had a minor crash on rollers).

Ultimately how fast you go depends on how much power is translated from the tire to the road. Crappy tires and road surfaces make a difference. Drivetrain losses do too and can rob you of a few watts or so to the wheels.

Thats a good point actually, you would only lose speed at the same power. IE it would matter for a race, but not a power test.

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Power is power, a dirty drivetrain will result in a loss of speed / efficiency but she will still be putting out the same watts regardless. I would be a little concerned if her coach is looking to up her FTP based on a service and not from data.

Ok thanks, that’s kinda what I thought. Her coach was considering upping her numbers based on some kind of tempo test that showed she could hold higher power than previously at lower heart rate or something to that effect. The repair had nothing to do with him upping her number.

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