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Phantom shifting under load

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Phantom shifting under load

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Old 12-27-18, 06:18 AM
  #1  
thehammerdog
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Phantom shifting under load

Went on xmas day ride and suffered from the
phantom shift issue. It is sn old bike dura ace
I never used before, 9 speed shifts feel different than my 105 10 speed.
So why is it shifting under pressure and what’s the fix
thanks
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Old 12-27-18, 06:45 AM
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masi61
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Originally Posted by thehammerdog View Post
Went on xmas day ride and suffered from the
phantom shift issue. It is sn old bike dura ace
I never used before, 9 speed shifts feel different than my 105 10 speed.
So why is it shifting under pressure and what’s the fix
thanks
If these are down tube shift levers the obvious question I would as is: is the screw holding it onto the braze on boss tight enough?

If if they are integrated shift levers I would be curious the mileage on them. Lately here I have encountered quite a few 9 speed STI brifters that just slip completely and flushing them out doesn’t help either. Apparently some of these failing high mileage shifters are toast and need replacement.
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Old 12-27-18, 07:16 AM
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What's the frame? Some older frames were laterally flexible enough that the rear triangle would move sideways under chain tension enough to cause a shift. Closely spaced cogs like 9 or 10 speed made this more of a problem.
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Old 12-27-18, 08:32 AM
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Originally Posted by masi61 View Post


If these are down tube shift levers the obvious question I would as is: is the screw holding it onto the braze on boss tight enough?

If that doesn't fix it for me, I know I need a new chain.
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Old 12-27-18, 09:16 AM
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First thing I ask is if the chain is really moving to another cog/ring and if so which. If not then worn chain/tooth skip is the likely issue. If moving to another cog then I'd look at the cable first. If the cable's movement is hanging up for some reason (internal to housing friction, kinking, fraying strands in shifter....) If the cable is hanging up and not fully moving then the added forces (that flex the frame) can cause the hang up can let go and the der can move. Other reasons can cause this too but that's where I would start. Andy
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Old 12-27-18, 10:40 AM
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My observation (sight unseen)

All the engineering to make a faster shifting cassette has that side effect, if everything is not in adjustment, and derailleur hanger alignment is part of that...
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Old 12-27-18, 03:07 PM
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My bike began to have poor shifting at one point after about 15,000 miles. No amount of messing with it cured the problem. One day while scratching my head and playing with components. I discovered the rear derailer had considerable lateral play, presumably due to eccessive wear. A new derailer, an easy fix, cured the problem.
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Old 12-27-18, 11:16 PM
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I guess frame flex. It was common back in the days of steel. That is , if the frame is steel
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Old 12-28-18, 07:42 AM
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Originally Posted by MarcusT View Post
I guess frame flex. It was common back in the days of steel. That is , if the frame is steel
Why guess.
Start at the beginning.
Has the bike been siiting idle for a while.
If so and it was correctly adjusted first it didn't go out of adjustment while it sat there. Pinch bolts don't loosen cables don't fray etc.
Wd40 all cables shifter and derailleurs make sure.they move freely.
Then look for adjustment issues.
Then chain and cassette.
While your at it look at the brakes.
Dura ace equipped bike ghost shifting because of frame flex, really.
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Old 12-28-18, 08:02 AM
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Originally Posted by blamester View Post
Why guess.

Dura ace equipped bike ghost shifting because of frame flex, really.
Yes, this urban myth and others existed waaaay back in the 20th century. Another is; bikes only had one mount for a water bottle. You had to actually remove your hands from the handlebars to change gears and then adjust the derailleur until it was in the right position. Don't even get me started about the box wrenches you needed just to adjust a seat.
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Old 12-31-18, 09:20 PM
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chain s new but drive train is not 2001 cables were gross gunked up I replaced and tighten up stuff felt good on stand but need to ride it it may be an old cassette I hope not because they aint cheap
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Old 12-31-18, 10:15 PM
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Originally Posted by MarcusT View Post
Yes, this urban myth and others existed waaaay back in the 20th century. Another is; bikes only had one mount for a water bottle. You had to actually remove your hands from the handlebars to change gears and then adjust the derailleur until it was in the right position. Don't even get me started about the box wrenches you needed just to adjust a seat.
Not sure if Marcus has actually experienced the frame flex shifting, I and some of my customers have. There's an easy experiment to try to see if flex really is the cause. Did it a number of times for non believing customers. Indexing pretty much stopped it though. I'll be happy to repeat what I've posted here before for those who doubt this. Andy
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Old 12-31-18, 11:01 PM
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I have a Giant NRS MTB that suffered from "ghost shifting" under load since new. Apparently it was a somewhat common problem as I found a few others who had the same issue with the same bike and the theory was that it was flexing in the rear triangle. In any event, the cure was to run cable housing from the last main triangle cable stop all the way down to the RD. Problem solved.

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Old 01-01-19, 12:02 AM
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Yeah, I had a lot of ghost shifting with my '89 Centurion Ironman when I tried using the Suntour GPX Accushift stuff in friction mode. It was a combination of problems:
  • A frame that's comfortable for seated riding on rough pavement and chipseal, but noodly when I stand to stomp the pedals.
  • The GPX Accushift downtube shifters were designed for index shifting, with friction mode an afterthought. The module isn't really user accessible to make any adjustments. All you can do is crank down the tension until ghost shifting stops, but the friction shifting feel is terrible; or adjust for good shifting feel and put up with ghost shifting when you stand to sprint or climb.

The only way to fix the problem was to get the Accushift index mode working satisfactorily, or replace the downtube shifters with something that would hold without excessive tension.
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Old 01-09-19, 10:26 AM
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Originally Posted by thehammerdog View Post
Went on xmas day ride and suffered from the
phantom shift issue. It is sn old bike dura ace
I never used before, 9 speed shifts feel different than my 105 10 speed.
So why is it shifting under pressure and what’s the fix
thanks
I went through this just recently. I changed everything out- chain, cassette, cables, housing, etc. It turned out to be the rear wheel bearing were shot. Just something else to look at.
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Old 01-09-19, 09:05 PM
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My ghost shifting is definitely frame flex. It doesn't bother me because it always downshifts just when I need it like an automatic transmission,
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Old 01-10-19, 10:43 PM
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Originally Posted by HillRider View Post
What's the frame? Some older frames were laterally flexible enough that the rear triangle would move sideways under chain tension enough to cause a shift. Closely spaced cogs like 9 or 10 speed made this more of a problem.

I can verify this from experience. I have an old Bridgestone Radac (basically a Japanese Vitus, bonded aluminum). 8sp worked fine on it but I get ghost shifts with 9sp.
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