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Simplex SLJ Chain Jump

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Simplex SLJ Chain Jump

Old 04-25-19, 09:55 AM
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Simplex SLJ Chain Jump

I am running an SLJ in a bit of odd configuration with a single 46 tooth front sprocket on a Stronglight crank and with a 13-30 Shimano rear freewheel. The shifter is the single retro friction SLJ. The bike is on a St. Etienne 531 with Simplex dropouts. The problem is that sometimes when shifting to a smaller cog on the freewheel the chain jumps off on the front sprocket. It is of course on a downhill as I am upshifting, so mostly I can reach down and put the chain back on, but this is not a terribly complicated setup especially since is happens on the smaller cogs.

I think the problem is that the spring in the pivot that attaches to the drop out feels weak. I have found limited bit about winding up the springs in those pivots. To rule other issues out, the sprocket and chainline is centered on the freewheel but I cannot imagine that would create hyper sensitivity because a chain does not jump when on double or triples and moving through the range.

Teeth are good, chain is new, chain length is not sagging when in small cogs, but I can push the rear derailleur forward and back without it resetting itself. I heard the early Campy RD were unsprung in the rear pivot and I can believe that the lower spring can also pull the chain tight so perhaps there is magic to getting the balance right. But first, how does one tension these things?
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Old 04-26-19, 07:32 AM
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Two things come to mind. The bike is flexing enough at the bottom bracket to allow the chain to fall off under load or the ring is bent a bit. Other than that, don't know.

As for early Record and Nouvo Record being unsprung, I believe that is true. I have both Record and early Nouvo Record and both are unsprung, as I recall but I cannot see that impacting the crank ring function...


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Old 04-26-19, 08:27 AM
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Iím running a long-cage Simplex SX410 with a 13-30 freewheel and a 46, 42 & 30 triple. Mine works well but requires some sympathy and finesse while shifting due to weak pivot springs. Iím using a Suntour front derailleur, I donít have any issues with the chain dropping.

I ran a single chainring in the past on a different bike. That bike had all new components, including an Ultegra long cage rear derailleur. It would drop the chain, so I installed an outer and inner chain guard. A vintage rear derailleur was not designed for a single chainring. Modern 1x11 rear derailleurs, as found on some gravel bikes, feature a clutch to help keep the chain from dropping.

The front chainring is going to need some chain retention. Installing a front derailleur and fixing the position with the upper and lower adjustments is one solution.


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Old 04-26-19, 08:36 AM
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While I don't have a Simplex SLJ in my collection, I have serviced one of the gold, early versions on a friend's Falcon. The spring tension was adjustable but only to the extent that there were two spring tab mounting holes in the backing plate that interfaces with dropout.
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Old 04-26-19, 08:40 AM
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Originally Posted by Barrettscv View Post
The front chainring is going to need some chain retention. Installing a front derailleur and fixing the position with the upper and lower adjustments is one solution.
Eww! There's got to be a better way.

Long ago, I was running a 1x5 (Shimano Crane) and had this problem; dropping the chain in the middle of the pack during a criterium is not fun, and I experienced it too often. I blamed it on the Crane's spring tension which I felt was weak. Perhaps something else is the cause.

I was daydreaming about fabricating a widget made of a rear derailleur fully plus a front derailleur clamp, positioned high enough not to contact the chain unless it tried to jump the chainring.
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Old 04-26-19, 09:21 AM
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Originally Posted by tiger1964 View Post
Eww! There's got to be a better way...
I was daydreaming about fabricating a widget made of a rear derailleur fully plus a front derailleur clamp, positioned high enough not to contact the chain unless it tried to jump the chainring.
That's your idea of a better way? The O.P. could take a 52t Stronglight chainring, file or grind off the teeth and install it on the outside position. Then install a Dogs-tooth chain retainer on the seat-tube.

Last edited by Barrettscv; 04-26-19 at 09:49 AM.
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Old 04-26-19, 09:22 AM
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You could make a front chain retainer using a few old Simplex parts.

Example I use from a Campagnolo single shifter clamp and 'matchbox' Campy front derailleur cage.

Campagnolo offered right or left single shifters, so to keep the logo proper upon clamping, this DIY chain retainer uses a lefthand side shifter clamp. One could do the same with Simplex.

(Ps. Not trying to drift from the OP and Simplex but this setup works flawless and shifts rapidly with the regarded lowly steel Campagnolo Velox rear derailleur. Laughably, I have Bullseye pulleys installed and have to say, its better action than any of my other rides with Nuovo Record. The chrome steel and pivots are really good with the Velox.)



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Old 04-26-19, 10:33 AM
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Originally Posted by Barrettscv View Post
That's your idea of a better way?
Not really but it would seem to have less chance of a FD cage dragging on the chain as one shifts between rear sprockets, because there wouldn't be a cage. 92.7% of my interest in a 1x setup is not having to mess with trimming the FD (the other 7.3% is losing weight on the bike, plus aesthetically cleaner)

I don't experience chain drop on a bike with a FD, one could argue that it's specifically because the FD is there.

It would be interesting to have slow-motion film of chain drop to examine what's happening. My mind keeps going back to RD spring tension. Thus my interest in the Simplex 543.
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Old 04-26-19, 11:51 AM
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The chain tension provided by the rear derailer spring(s) has virtually no effect on the tension of the chain's upper run, which is where the chain is derailing from the chainring.

Added spring tension might however provide better chain control in the instance where the rider is "soft-pedaling", i.e. turning the cranks while the freewheel is free-wheeling.

My laterally-flexible Pedersen bike routinely dropped it's chain off of the chainrings while I was sprinting despite having a front derailer, until I added reinforcements between the "floating" pair of seat tubes and the seat stays.
So frame flex would likely play some role in chain-drop on any frame where there is no front derailer.
Plastic-pipe reinforcements shown here, near the rear brake caliper:

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Old 04-26-19, 12:17 PM
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Thanks all, these are all interesting potential solutions. It does happen more when soft pedaling, just starting to go down a hill and wanting to upshift. Very valid point about the top run of the chain. When shifting to a smaller cog it does tend to create slack for just a moment. If I keep pressure on pedals it does not occur, that was what led me to the spring not pulling the chain into the smaller cog teeth fast enough.

How did the 5 speeds with a single chain ring not have this issue, or did they?
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Old 04-26-19, 01:23 PM
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Originally Posted by Revracer View Post

How did the 5 speeds with a single chain ring not have this issue, or did they?
With the Campy example, it often happened during rapid upshift.

Without the chain ring guide, I would have to slowly shift and be mindful of NOT jumping cogs.

With the chain guide, its bulletproof. Can fly between cogs, or precisely select -zero issues.
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Old 04-26-19, 01:26 PM
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Modern chains tend to exacerbate the issue.The bushless designs and profiled links make them more flexible and facilitate shifting. However, the increased flexibility means the lateral deviation for a non-perfect chainline happens closer to the teeth that pick up the chain,causing the profiled links to catch the teeth and cause a derailment. Also, modern bicycles tend to have shorter chain stays and wider cassettes/freewheels, resulting in larger chain angles, which add further to the problem. Those old R/T five speed set-ups had quite long chain stays.
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Old 04-26-19, 05:04 PM
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Originally Posted by dddd View Post
The chain tension provided by the rear derailer spring(s) has virtually no effect on the tension of the chain's upper run, which is where the chain is derailing from the chainring.
At first glance -- VERY valid point.

Originally Posted by T-Mar View Post
Modern chains tend to exacerbate the issue.
Also interesting, perhaps time to abandon modern chains?
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