Can't get correct chain length on vintage Peugeot

[lurcher106]

Hi everyone, hope you're all doing well!

For a while now I've been working on a nice little peugeot, disassembled it, and stupidly threw away the old chain for reference.
I followed this sheldon brown guide for approximating the best chain length but am having some weird trouble.

https://www.sheldonbrown.com/derailer-adjustment.html#chain

When I set the chain from the largest chainring to the largest cog on the back, bypassing the rear derailleur, I end up with an odd number of links in sheldon's left hand diagram. this means that i'm really, really close to being able to pull the chain just a bit tighter and being able to get it on nice and tight, but the bike just won't allow it, and if I set the chain up to match the diagram, it's got loads of slack and is much too loose. The chain I have is an SRAM PC850 Powerlink. It's for an 8 speed but the guy in the shop told me it would be the same width as a 5-speed chain, which the peugeot is.

Looking at the rear wheel, there's been a strange issue with placing the wheel. Because of the way this rear mech fits, it kinda pushes the rear wheel forwards on its axle.


I'm thinking this is making the chain length difficult, but I can't find any other way to fit this rear mech. If you look at the photo, the previous placement was further back. For what it's worth, the way it's setup right now is also kinda forcing the rear triangle outwards, as if the whole rear axle is 1/2cm too wide for the frame.

The axle looking a bit wide. (sorry about my foot sneaking in there) Also see the inside of the derailleur, where there is this 'spacer' thing.

Said spacer, closer up. The outside of the derailleur shows a nut which tightens this into place. It's this thing that is effectively pushing the whole wheel forwards, but I have no idea how else it would be installed.

It's a lot like this one here:

Sorry the photos aren't great, I've been too busy to take anything apart but can happily do so if it's required to take more photos and get to the bottom of this.

Thanks so much for taking a look at this post!

Fin

 

[Ajax Bay]

When I set the chain from the largest chainring to the largest cog on the back, bypassing the rear derailleur, I end up with an odd number of links in sheldon's left hand diagram. this means that i'm really, really close to being able to pull the chain just a bit tighter and being able to get it on nice and tight, but the bike just won't allow it, and if I set the chain up to match the diagram, it's got loads of slack and is much too loose. The chain I have is an SRAM PC850 Powerlink. It's for an 8 speed but the guy in the shop told me it would be the same width as a 5-speed chain

The mech fits as you have it and it doesn't matter that it's a bit forward on the drop outs. The chain is the correct one. For assurance for you, from Sheldon: " 3/32" (0.094", 2.30 mm) chain is used on derailer equipped bicycles that have more than 3 cogs at the rear." (9sp and up needs narrower ones yet.)
The chain must be long enough to allow you to go large/large (acknowledge you'll avoid doing so). Links are one inch long - it doesn't matter whether the number (eg 57) is odd or even. Run the chain large/large (and not through the mech), go to the first number of links (1") that will manage that, and add one link. When threaded through the mech and joined, when on small/small the mech will wrap the "loads of slack" up chain on its 'return' section should not touch the chain going round the guide jockey wheel. You will rarely be in that combination, so if the mech hasn't got sufficient (wrap) capacity then when riding along you will hear the noise and that will prompt you to shift to the/a large(r) chainring, with no damage done. Too short a chain and accidentally shift onto largest sprocket while still on big ring (almost ceetainly with power applied up hill) = damage done (or shift fail and stall/fall).
You don't say what size rings you have on the chainset. I assume the block is 14t-25t.

 

[Gixxerman]

My rule of thumb is to route the chain on big rear cog and big chain ring, but bypassing the rear deraileur (do not thread it through the deraileur jockey wheels). Overlap the chain by 2 links (one inner and one outer) and break the chain there. It has worked on every bike I have ever had.

 

[slowmotion]

This calculator tends to give the right chain length.

http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

 

[Sharky]

For what it's worth, the way it's setup right now is also kinda forcing the rear triangle outwards, as if the whole rear axle is 1/2cm too wide for the frame.

Regards this point - is it an original rear wheel? Your frame is likely to be 126mm width and if it's a newish wheel, the wheel is likely to be 130mm,
You might be able to remove spacers, if you can do both sides, it will keep the wheel centered.

Good luck

 

[raleighnut]

Regards this point - is it an original rear wheel? Your frame is likely to be 126mm width and if it's a newish wheel, the wheel is likely to be 130mm,
You might be able to remove spacers, if you can do both sides, it will keep the wheel centered.

Good luck

Yep it does rather look like a 5 speed freewheel on a 7 speed axle to me.

 

[Ajax Bay]

Good spot @Sharky
Might even be a 126mm hubbed (OLN) wheel fitting into a 120mm between dropouts frame. From the image, that hub really doesn't look like a 130mm hub.
Maillard Atom?

 

[TheDoctor]

IME chain length isn't so very critical on older, non-indexed setups. I've always gone big-big, through the mech with a little slack, and lived with the chain possibly rubbing on itself in small-small.
And yes - you do have the rear mech mounted right. As you say, it's hard to see how else it could go.