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Bike shifts when putting watts down

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Bike shifts when putting watts down

Old 02-14-20, 02:38 PM
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Bike shifts when putting watts down

Hi all,

I have a bike im about to give to my brother but whenever you are out of the saddle, the bike shifts on its own. This normally happens when I am on the trainer, but it ended up being so bad I bought a new bike with better components to work on fixing the bikesdirect model that is messing up. Is the rear derailer dying? Thanks again in advance!

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Old 02-14-20, 03:03 PM
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I would guess a crappy RDER adjustment, assuming it's shifting on the rear. Which way does it shift? Up or down?

Else, look for frame flex, like a broken weld or something.
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Old 02-14-20, 07:49 PM
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With the popularity of stiffer frames and the lever holding designs of modern indexed systems auto shifting with an otherwise well set up and adjusted system is far less then back in the day. Frame flex and lacking lever friction (with lots of cable friction sometimes) can cause auto shifting.

With no ID of the bike or the components we really have little to work with other then the usual possibilities. Bad adjustment, slipping lever, broken axle, twisted/bent der/cage/chain/cog teeth and a few others. Andy
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Old 02-14-20, 10:03 PM
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In my case, it was a combination of flexing frame, and worn out chain / cluster. but replacing the latter fixed it.
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Old 02-15-20, 07:37 AM
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It seems to me that standing and "hammering" on a bike whose rear triangle is locked into a trainer would create the most frame flex possible. This would accentuate any minor adjustment issue(s). In a way, it's the perfect storm for ghost shifting. More info is needed. Does it ghost shift in only one direction? Only certain cogs? How many miles on the components? Any chain elongation? Has it always done this? Any crashes in the bike's history?
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Old 02-15-20, 07:48 AM
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Its likely frame flexing causing it. You can fix it by running a full outer, brifter to rdr, thus separating frame flex from cable tension.
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Old 02-15-20, 08:10 AM
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Is it actually changing gears or perhaps just skipping from drivetrain wear, measure the chain and check the tooth profile and see what that looks like. Any time I had a bike that skipped it only showed up when standing and felt more like a mis shift.
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Old 02-15-20, 08:14 PM
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Thanks for all the info everyone! as far as I see, I don't see any issues with welds but I will check the chain wear.

It is for sure ghost shifting down, but I will confirm where it is ghost shifting.

These are the specs on the bike,

Frame Gravity CXSeries Aluminum, Engineered Top tube, Bi-Oval downtube, S-Bend seatstays, 2xH2O mounts, replaceable rear derailleur hanger, rear rack mounts. Disc Brake tabs
Fork Disc Brake Gravity Carbon Fiber Straightblade, 1.125 inch steerer, Front Rack mounts
Crankset Aluminum arms, triple ring 52/42/30T
Bottom Bracket Sealed cartridge, square taper
Front Derailleur SHIMANO Claris DOWN PULL for 8 Speed
Rear Derailleur SHIMANO SORA RD-3400 Long Cage for 8 Speed (24 gears total)
Shifters SHIMANO CLARIS STI for 8 Speed (24 gears total)
Cassette/Freewheel SunRace Cassette 12-25T for 8 Speed (24 gears total)
Chain SUNRACE for 8 Speed
HubsFORMULA Disc Brake aluminum, Black finish with Quick Release
SpokesStainless Steel
Rims Alex R450, Double Wall aluminum with machined brake track
Tires 700x32C Kenda KwickTrax, presta valve tubes (Wide Tires FIT with Room for up to 1.7" x 29er Tires)
Brakes Tektro Lyra, 160mm Rotors

I have only ridden around 300 miles on those parts, no wrecks.
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Old 02-17-20, 09:53 AM
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It could need a simple adjustment, if the rear derailleur is a bit off center of the cogs towards the smaller cog.
Is the bike new, with 300 miles ? If so, the rear cable has probably "stretched" a bit, and unscrewing the rear adjuster 1/2 a turn or so (counter clockwise).

This effectively lengthens the cable housing, and pulls the rear derailleur inboard a little bit.

Always make adjustments in small amounts, and be prepared to undo them the same amount if they make things worse.
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Old 02-18-20, 09:49 AM
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Something is either out of alignment (derailleur hanger, indexing), or something is either worn out (chain, cassette).
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Old 02-18-20, 12:34 PM
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A twisting shell of a BB or crank gears deflecting can cause odd chain action. Try to repeat the condition to see if it's only a gear tooth being the cause or a driveline issue..
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