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Down Tube Friction Shifting Issue

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Down Tube Friction Shifting Issue

Old 10-29-22, 05:51 PM
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cpsqlrwn
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Down Tube Friction Shifting Issue

I have three friction bikes that were recently tuned at my local bike shop. All three came back with the same shifting characteristics and I would like to know what the problem might be and how best to correct it. These bikes are all 7 speed, 2 with Sachs 12-18 freewheels and one with a Sachs 12-19. One bike is complete Nouvo Record, one is complete Suntour Superbe/Superbe Pro and one has Simplex retrofriction shifters with first generation 7 speed Chorus drive train. All shifters are down tube friction shifters and all components are in excellent condition. The Chorus rear derailleur has a barrel adjuster, but there is no barrel adjuster on the Superbe Pro or Nuovo Record rear derailleurs. The shifting on all the bikes is very smooth with one exception. With the chain sitting on the large chain ring (52 teeth), it is extremely difficult to move from the 12 cog (smallest cog) in the back up to the 13 cog without it traveling all the way to the 14 cog. I can manage a fairly clean shift about 1 in 5 tries. Otherwise it moves two cogs. There is very little distance on the shifter mechanism between nestling into the 13th cog and going past it into the 14th cog. All other shifting both up and down is fine. Moving down the freewheel (upshifting) is no problem. Every gear is easily accessible step by step from the larger rings right down to the smallest. It is only downshifting from the smallest ring where the problem occurs. I have been riding friction bikes for many years and this is the first time I have encountered this problem. Being that all three are exhibiting the same tendency, it seems clear that there is an adjustment issue. My guess is that it might be a tension issue but I would like to know what the possible causes are and what the solution might be. Thanks for any info.
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Old 10-29-22, 07:21 PM
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Too much slack tension. Give it a little more slack when in the Big/small position.
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Old 10-29-22, 11:19 PM
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Off the cuff, and without knowing the details, you might do better with a longer chain.

On these derailleurs the jockey wheel arcs backward as the chain shortens. This can put it too far back to shift smoothly.

Keep in mind that I'm a "long chain" guy, and generally cut chains based on looping small/small without droop.

I suspect that with the smaller freewheel, you couldn't resist going with a shorter chain, and that's the problem.
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Old 10-30-22, 12:37 PM
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So basically, to move the chain from the 12 to the 13 requires some overshifting, but when you do this the chain skips over to the 14. And it's worse with the chain on the big ring. FBinNY might be correct in suggesting a longer chain. But here's what's behind it.

There's mechanical "slop" where the chain exits the jockey pulley and enters the rear cog. The slop increases with the length of this span of chain, the flexibility of the chain, and the sideways motion of the chain on the freewheel cog and the jockey pulley. You can't do much about the last two. As for chain flexibility, there isn't much choice in 7-8 speed chains anymore. Back in the day I preferred Sedis chains because they were stiffer than the more expensive brands like Regina. Nowadays I just throw on a SRAM PC-850 and forget about it. So that leaves the span between the cog and the jockey wheel. You can change this by lengthening or shortening the chain, repositioning the wheel in the horizontal dropouts (if you have them), or adjusting the B-screw if your derailleur has one. Generally, loosening brings the jockey wheel in closer. My old-school Campagnolo Super Record doesn't have one. Shorter distance means more precise shifting. On most road setubs you want as short as you can get without causing chain rumble, which usually occurs on your lowest combination (small-big). You'll know chain rumble when you hear it.

These old 5-7 speed cogs and chains are pretty primitive next to today's indexed systems. There's no ramping on the cogs and the chain bevels were designed more for quiet operation than precise shifting. Overshifting with finesse was pretty much part of the art of riding a bike in those days, and it got harder when the space between the cogs decreased.

Last edited by oldbobcat; 10-30-22 at 12:44 PM.
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Old 10-31-22, 12:22 AM
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What I do with my double chainring vintage bikes is remove every single mm of slack out of the cable and make the chain as long as possible.

The latter is usually found with the chain on the small ring and largest cog with a length that will just barely avoid the derailleur's upper/jockey wheel interfering with that big cog.
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