Think I found cause -- Pedal Reverse -- Chain Jumps off

[JanJ]

~1970 Gitane Mixte. Prior to rebuild wife could only pedal forwards, as peddling backwards would have chain occasionally jump off.. but only in larger rear gears and larger front sprocket.

To my knowledge, this bike has not been serviced.

I believe I now know why.

If I look from rear of bike forwards with eyes at axle height, I'm seeing the that the chain from what would be the middle of 2 front sprockets, would hit the 4th of 5 gears (next to smallest rear gear)...

Looking at internet posts, this would seem to want to be aimed for the 3rd (middle)...

When I did the rear bearings last night I found 2 spacer washers in between gear side cone and gear side nut... but only 1 in-between non gear cone & nut.... which would seem to shift the gearset to the left, moving 'center of chain' toward the 4th gear.

Methinks if I remove one spacer washer, this should move the gearset to the right, moving the optimum chain positioning toward gear 3, and hopefully pedaling backward, on large sprocket will keep chain on???


????

[cny-man]

Your freewheel chainline may indeed be off to the left, but it's best to measure rather than eyeball before making changes, especially because it could be off because of a misaligned rear triangle. It's also best to maintain the same OLD (over locknut distance).

(Pedaling) backwards is best done to a limited amount. One generally should be able to do so except for cross-chaining, which should be avoided anyway.

Rather than me explaining all the above concepts, please Google "chainline OLD," "cross-chaining," and "align rear triangle." Sheldonbrown results are some of the most helpful, and his site is a wealth of information generally. That way you'll learn about more than just your specific problem.

[cny-man]

p.s. I made multiple edits, so you might want to re-read the above post if you checked it when you were first emailed!

[JanJ]

Thanks! Will revisit this with multiple searches during lunch!!

[JanJ]

Comparing the measurements from the links in your post cny-man, to the Gitane measurements confirms the chainline is wrong...

I'm spending time measuring frame to front crank (Which seems wider than what it should be), axle (which bare ends are not exactly equal), and frame triangulation (which seems good).

When I re-bearing'd the rear axle, I loosened the gear side, which may have caused the issue with un-equal axle ends... little over 2mm. Tonight I adjusted axle such that bare ends are equal in spacing. This improved things a couple mm.

I'm still confused with why I'm finding 2 spacers on the gear end. This puts rear gears further to the left on Gitane than Peugeot, lining up with 4th gear, not 3rd..

looking at the two bikes, Peugeot and Gitane, the Gitane front gears are at greater distance from the center of the down tube than Peugeot.... But, then the crank & gear pieces are also thicker rough cast chromed pieces on Gitane, compared to thinner aluminum pieces on Peugeot, all of which add up.

I'm stopping with Axle fix for tonight, and going back to your links to refresh my new knowledge...

Thanks....

[cny-man]

Changing the amount of axle exposed (if that's what you mean by "bare end") and moving the cones/nuts on the axle does nothing to change chainline at all. You can't change the chainline unless you change the spacing on one or both sides. As for spacers, the amount of spacers is irrelevant - manufacturers use all sorts of combos. Spacing back then varied all over the place. What's important is front vs. rear chainline.

[JanJ]

The Gitane has quick release axles, vs. Solid axle on Peugeot, so this was my first Quick Release rear axle re-build, and:
I did not center then cones on axle when I did the re-bearing. I loosened both side's cones, and I should have left the gear side intact. My Bad!

Yes, last PM I 'centered' the the amount of exposed axle. During the re-bearing of rear wheel I had in-inadvertently offset cones on axle such that the gear side lock nut was contacting the frame (too little exposed axle on gear side), and that shifted the gear centerline further to right (over 4th gear) compared to where it was before, which is till closer to 4th gear than 3rd.
With last night's centering of cones on Rear axle improved the chainline somewhat, but it's still 'off'.

I'll post my mm chainline findings on this later tonight... Starting from front sprockets...

[DaveM]

I'll just post a reminder than once you get done fiddling with the axle, you're probably going to have to re-dish the wheel to get the rim centered in the frame. Get the hub where you want it first. But moving the hub around means you're moving the rim too. You'll have to recenter a little.

[JanJ]

Regarding chainline calculations...
If I measure 44mm on the front to center of two sprockets...

After completing the measurements and the calculations for the rear chainline.... Should the optimum number then be 44 as well, to match the front?

Measurements:
center of downtube to center in between 2 front sprockets 44mm
frame spacing 124mm
inside of rear forkend to 5th gear 11mm
cluster width 26mm

11+13=24 62-24=38mm (This is as it was last night, with chain looking to be even with gear 4)


If I remove one (of 2) 2mm spacer from in between cone and lock-nut, this changes inside of rear forkend to 5th gear to 9mm, which changes calculations to 40mm (from 38mm) (This is how it is NOW and chain is looking to be even with gear 3)

Tire is still centered in frame. Backwards pedaling is much better, but not 100% perfect.

I can put the spacer back in if necessary...

P.S. Before and after the crank bearing and lube work, the front derailleur has worked flawlessly, and has not needed any adjustment.

[cny-man]

Remember OLD and dishing? If you put the spacer on the other side and redish you will restore the OLD and have a stronger wheel.

[JanJ]

I can put it on other side easily..

I am not certain of the term redish. ???

Does the math seem to confirm what I did was correct? And is the rear calculated number supposed to match the front, or be close, or ????

(11-20-2014, 08:35 AM)cny-man Wrote:  Remember OLD and dishing? If you put the spacer on the other side and redish you will have a stronger wheel.

[cny-man]

It should be close to the same amount, but a little off is OK. If one tends to use the large chainwheel more then the preference would be for the rear chainline amount to be larger than the front, and vice versa. You should have come across the dishing concept when looking up chainline or OLD. Google "wheel dish." If you've trued wheels dishing is not a large challenge. You basically loosen slightly all the spokes on the drive side (DS) and then tighten all the ones on the non drive side.

[JanJ]

I saw a video on single speed bike with guy with straightedge checking chainline... Darn! I should have been able to figure that out myself!
OK on the large sprocket 'preference' info... Based on all this, I have a little more to adjust... based upon calculations.. and I'll double check with straightedge as well...
I'm thinking another trip to Home Depot with the removed spacer and the micrometer is in order!! My new-found knowledge would seem to indicate that I'm looking for a very thin spacer, and remove the 2nd spacer that I found...

(Will remember to put it back on 'other' side!)

Dish... OK.... once things calm down on positioning, I'll remember that.

Thanks to all your posts!!!

[cny-man]

Do keep in mind that one thing may affect many others. When removing spacers you have to maintain enough DS space so that the chain can shift down to and run on the small cog.

You don't HAVE to adjust the chainline if using the large *chainwheel* (more common usage and clearer than "sprocket"), and remember that many people overuse high gears. If you are regularly pedaling at a rate of much lower than 75rpm or so you may want to train yourself to "spin" more.

[JanJ]

Actually, wife (this is her bike), shifts only with rear gears.. on large front sprocket. Her gear levers are on down tube. I don't recall her using more than 2 & 3 and rarely 4.

I have checked small to small, and it still is fine.... I could stop now.... but I'm 'learning and it's 20 some degrees and light snow on ground, so....... !

(11-20-2014, 01:29 PM)cny-man Wrote:  Do keep in mind that one thing may affect many others. When removing spacers you have to maintain enough DS space so that the chain can shift down to and run on the small cog.

You don't HAVE to adjust the chainline if using the large *chainwheel* (more common usage and clearer than "sprocket"), and remember that many people overuse high gears. If you are regularly pedaling at a rate of much lower than 75rpm or so you may want to train yourself to "spin" more.

[JanJ]

Today's the day. After reviewing chainline docs again, measuring again, calculating what it 'should' be, and comparing clearance on small to small and guessing what results that re-positioning chainline would have on that....

I moved cone spacing based on calculations. I was aiming for 2mm, and after it was all back together, I achieved 1.3mm.

Spacer removed from cluster side last week returned to axle on non-cluster side of axle.

Overall it looks better than it did before.. Measures better.
Large to Large pedaling backward will still jump off, but less than before also.

Rear Tire is still within reason (center of frame). I think I need to check parallel of cluster with front sprocket to be certain I'm not centering tire in frame.... Which may have some effect on pedal backwards.

No question about it.... Chainline is Better than a couple weeks ago... (Now that I know what it is! ! ) I do believe I'm on a stopping point on chainline.

And I can't begin to indicate how much easier the tire and pedal action is....

One last question on the reverse pedaling chain coming off:

As I watch this happen.... I'm seeing the chain flop a bit before it comes off.

Granted, the bike is 1969-1970 vintage. Don't know if this is derailleur tension or just floppy chain that's made for side to side movement due to derailleur. Teeth on sprockets are not scalloped (I'm fluent with this term from motorcycles)

Since my wife doesn't use all gears --- If I limit access from Large rear sprocket with derailleur adjustment screw --- all is then fine! I'm considering doing that, and calling it a day..... It is a 45 year old bike with all stock parts!
but I thought I'd ask one more time.

Thank You!

[cny-man]

Large-large and small-small is cross chaining - as I referred you to earlier, and is not recommended due to excessive wear and noise. Problems backpedaling are to be expected in those combos.

The chain may flop because it's about to come off or, if it sags, because there's a bit too much resistance in the freewheel bearings or (more rarely) derailleur pulleys. Just being in the large-large combo may introduce more friction in the pulleys.

There's no reason to limit access to the large rear sprocket unless your wife does not need the lowest gear small front-large rear) at all and also wishes to backpedal to her heart's content.

Finally it would be adviseable to redish at some point (you have all winter) as if the wheel is off-dish it may make the bike veer to one side if riding no handed, will make brakes harder to center, and dishing to the left will make the wheel stronger.

[JanJ]

I showed this thread to wife... After explaining the term cross chaining, (with pics from the printouts from the chain line links) she said she understood... Leave access to low gear, she just won't back pedal! She only told me about it because it happened.

End of subject! ('Dish' to be examined later)....

[cny-man]

Sounds good - thanks for being open to my observations.