Chain falling between gears?
05-25-15, 09:58 PM
#1
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Chain falling between gears?
OK, first off, while I am rather mechanically inclined, I am not the most knowledgeable on exact bike terminology. Please bear with me!
I've been rebuilding a 1985 Peugeot PH10. The bike, for information, has a Maillard Helicomatic 6-spd rear setup, a Nervar 52/42 front crank, and Sachs-Huret derailleurs front and back (Rival model on the back, specifically). The rebuild included a new KMC Z50 6-spd chain.
After taking it out on the first ride yesterday for about 21 miles, I had loads of trouble with gear shifting from little rear gears to big (fewest teeth to most teeth, again, pardon specific terminology). Going from big to little on the rear, absolutely no problems, shifted smooth and as on cue as I can expect a non-indexed system can, as I've never used one before. Opposite direction, though, and almost all the time, the chain would drop between gears and I'd just spin the pedals. This happened mostly on the rear derailleur, although as you can see in the attached picture, it also happened once on the front, where it was much easier to get a decent picture illustrating the problem. The difference being on the front, it happened going from big to little, while going the opposite direction was never an issue. I ended up spending most of the trip in the big front and littlest back gearing.
As it has happened on both cranks, I think I can rule out derailleur issues, although if I am wrong correct me. Is this an issue of an old French setup not having the same dimensions as other bikes, and a 6-spd chain not being appropriate? Is this possibly just an issue with me not knowing how to properly shift a non-indexed downtube system? Is there any thing else it may be?
I've been rebuilding a 1985 Peugeot PH10. The bike, for information, has a Maillard Helicomatic 6-spd rear setup, a Nervar 52/42 front crank, and Sachs-Huret derailleurs front and back (Rival model on the back, specifically). The rebuild included a new KMC Z50 6-spd chain.
After taking it out on the first ride yesterday for about 21 miles, I had loads of trouble with gear shifting from little rear gears to big (fewest teeth to most teeth, again, pardon specific terminology). Going from big to little on the rear, absolutely no problems, shifted smooth and as on cue as I can expect a non-indexed system can, as I've never used one before. Opposite direction, though, and almost all the time, the chain would drop between gears and I'd just spin the pedals. This happened mostly on the rear derailleur, although as you can see in the attached picture, it also happened once on the front, where it was much easier to get a decent picture illustrating the problem. The difference being on the front, it happened going from big to little, while going the opposite direction was never an issue. I ended up spending most of the trip in the big front and littlest back gearing.
As it has happened on both cranks, I think I can rule out derailleur issues, although if I am wrong correct me. Is this an issue of an old French setup not having the same dimensions as other bikes, and a 6-spd chain not being appropriate? Is this possibly just an issue with me not knowing how to properly shift a non-indexed downtube system? Is there any thing else it may be?
05-25-15, 10:19 PM
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This is a fairly common issue, and the result of the chain being too narrow for the gap between the chainring.
Even though you're using a 6s chain, the chain is still too narrow, because over the intervening years chain design went from pins that extended beyond the outer plates to chains peened down flush. It was the protruding pins of the vintage chain that would push it over far enough not to jam.
The fix is to either find a vintage chain with the protruding pins, or to see if you can narrow the gap between the rings. It's that simple, but may not be that simple in practice.
Even though you're using a 6s chain, the chain is still too narrow, because over the intervening years chain design went from pins that extended beyond the outer plates to chains peened down flush. It was the protruding pins of the vintage chain that would push it over far enough not to jam.
The fix is to either find a vintage chain with the protruding pins, or to see if you can narrow the gap between the rings. It's that simple, but may not be that simple in practice.
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An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
05-26-15, 02:25 AM
#5
Bicycle Repair Man !!!
Your 6-7 speed KMC chain is 7.3mm wide and your chainwheels are spaced for the old 5-6 speed chain which was wider and did not have smooth side plates... as such it skates between the chainwheels.
Your downshifts might just need practice, the 6 speed Helicomatic was designed for narrower chain so this should not be a problem and rarely is... most older bicycles with 6-7 speed freewheels / cassettes run very well with 8 speed chain which is 7.1 mm but the spacing between the rear cogs is not enough to cause the chain to skate between them.
Your downshifts might just need practice, the 6 speed Helicomatic was designed for narrower chain so this should not be a problem and rarely is... most older bicycles with 6-7 speed freewheels / cassettes run very well with 8 speed chain which is 7.1 mm but the spacing between the rear cogs is not enough to cause the chain to skate between them.
05-26-15, 07:38 AM
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OK, thanks! I'll keep the quick shifting in mind, trailangel, like I said I only had one or two issues up front.
Would it be worth spending $5 the next time I put in a Niagra order (I also found out the hard way that 1 1/4 tires from the 80's were much smaller than actual 1 1/4 tires of today...) to try out something a bit wider, like a 5/6spd KMC Z33? Or is this just going to cause problems in the opposite direction, if the Helicomatic was designed for a bit narrower chain?
Would it be worth spending $5 the next time I put in a Niagra order (I also found out the hard way that 1 1/4 tires from the 80's were much smaller than actual 1 1/4 tires of today...) to try out something a bit wider, like a 5/6spd KMC Z33? Or is this just going to cause problems in the opposite direction, if the Helicomatic was designed for a bit narrower chain?
06-05-15, 11:37 AM
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For anyone who searches this subject in the future, since I absolutely hate finding the exact problem I am having from a years old post and no solution:
I managed to mostly sort out the issue. I tightened down the screw holding in the downtube shifters, and rode a couple more rides on the bike concentrating solely on my shifting. I feel that a majority of the problem was transitioning from indexed to friction, with the loose shifter contributing some as well. That said, in the future, I will probably go with a PC830 chain with the bit more bowed out sides on a vintage bike, just to hopefully eliminate the last tiny bit of issues I am having.
I managed to mostly sort out the issue. I tightened down the screw holding in the downtube shifters, and rode a couple more rides on the bike concentrating solely on my shifting. I feel that a majority of the problem was transitioning from indexed to friction, with the loose shifter contributing some as well. That said, in the future, I will probably go with a PC830 chain with the bit more bowed out sides on a vintage bike, just to hopefully eliminate the last tiny bit of issues I am having.
06-05-15, 01:02 PM
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For anyone who searches this subject in the future, since I absolutely hate finding the exact problem I am having from a years old post and no solution:
I managed to mostly sort out the issue. I tightened down the screw holding in the downtube shifters, and rode a couple more rides on the bike concentrating solely on my shifting. I feel that a majority of the problem was transitioning from indexed to friction, with the loose shifter contributing some as well. That said, in the future, I will probably go with a PC830 chain with the bit more bowed out sides on a vintage bike, just to hopefully eliminate the last tiny bit of issues I am having.
I managed to mostly sort out the issue. I tightened down the screw holding in the downtube shifters, and rode a couple more rides on the bike concentrating solely on my shifting. I feel that a majority of the problem was transitioning from indexed to friction, with the loose shifter contributing some as well. That said, in the future, I will probably go with a PC830 chain with the bit more bowed out sides on a vintage bike, just to hopefully eliminate the last tiny bit of issues I am having.
