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I suppose most of the people have noticed it. While adjusting both front and rear derailleurs, it will shift accordingly, when you have adjusted or are adjusting it, but when you test it by riding, the adjustment fail to perform the same way, Why does this happen?

How does the added weight affect the shifters?

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What I have always done, take a few tools with me on the ride, make small adjustments during the test ride until it is tweaked as best as I can get it. –  Moab Jul 1 '11 at 17:20
    
Lots of bikes have cable adjusters you can adjust by hand while riding. The trick is knowing which way to turn them. –  Daniel R Hicks Jul 2 '11 at 17:25
    
@Daniel R Hicks, Do you have any good guide for those tricks? –  Starx Jul 3 '11 at 8:59
    
Not really a trick. You have to have a setup with adjusters on the brake levers or the downtube bosses, then you have to study, in advance of riding, which way loosens the cable and which way tightens it, and how that relates to moving the derailer to a higher or lower gear. Then, if, say, you find while riding that the derailer does not upshift as reliably as it should, you twist the adjuster appropriately. –  Daniel R Hicks Jul 3 '11 at 12:09
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3 Answers 3

up vote 4 down vote accepted

On a good bike with a good chain and cogs and properly lubricated cables and derailers there should be no significant difference between shifting on a shop stand and shifting while riding, in terms of the indexing adjustments.

Where you may encounter a problem is when shifting under load while riding, especially with the front derailer. The tension on the chain will make shifting more difficult, requiring a bit of "overshift", especially when moving to a larger cog.

Chain wear can also be a problem. The chain is the first part to wear out on a bike, and failing to replace the chain when worn (after 1000-2000 miles) will cause uneven wear on the cogs, resulting in increased difficulty when shifting, especially under load. And even when the chain is replaced at regular intervals, the rear cluster will wear out after about 5 thousand miles and the front chainrings after 10 thousand or so, making shifting more difficult (and sometimes a bit more of an "adventure").

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5K miles/10K miles - true but terrible! –  ʍǝɥʇɐɯ Jul 1 '11 at 11:29
    
+1 I was going to answer. But you beat me to it with the same info. BTW, Shimano's rep at their tech event in Seattle 2 years ago quoted me 800 miles as the expected life of a 10 speed (current road) chain. I expect there is some CYA in there, and that there is some "in a perfect world" in there. But that is what they told all the Pacific Northwest dealers. –  zenbike Jul 2 '11 at 12:01
    
Along with the recommendation that changing the chain in that time frame would significantly extend the life of the cassette and chainrings. –  zenbike Jul 2 '11 at 12:07
    
Yep, running a chain beyond its end of life causes uneven wear on the cluster and rings and can trash them in 3K miles or so. –  Daniel R Hicks Jul 2 '11 at 17:23
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Frame flex under the added weight affects the relative position of derailleurs and shifters, which is reflected in the cable length. Movements of the cable when changing gears are on the millimeter scale, so even lengthening of the cable by a fraction of a mm due to frame flex can have an effect.

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Derailleur adjustment on your build stand is just the starting point. I then move the bike on to a trainer for further tweaking of the adjustment. Frame flex can be significant on Al frame bikes. Trainer isn't a perfect representation of riding on the road, but it's close enough to get the derailleur adjustment set to something that will be reliable on the road too. –  Brian Knoblauch Jul 1 '11 at 13:43
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Normally if everything is going right: you shouldn't have this problem.

I was having this problem: The bike shifted great on the stand but performed horribly under load.

The solution was the frame was failing near the bottom bracket, causing the frame to flex when I was pedaling.

I've also seen posts where a rear hub failure was the culprit.

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