Changing folder chainset from 50/34 to 40/24 with braze-on FD

[horizon]

My Tern Node folder has a 50/34 chainset with a chainguard acting as the outer ring. I want to change this to 40/24 either by swapping the chain rings or a new chainset (I'm happy to buy this). Spa do the following with a similar set-up with a chainguard replacing the outer ring:

https://spacycles.co.uk/m2b0s109p3383/S ... cral-Rings

The rear is an 8 speed Claris 11-30. I'm assuming no conflict with that or with the shifters.

So the problem as I see it is that the braze-on front mech will need to be lowered. There is about 4 mm of downward adjustment available. Would this be enough?

NB I don't see another option unless braze-on front mechs come in different "sizes".

[the_twin]

You might try a front mech for a triple and set the limit screws to stop overshifting. It doesn’t matter if the cage sits a bit higher than the outer ring, the crucial thing is whether you can shift up from the inner chainring.

[horizon]

Thanks for that. I think the idea is that the chainset acts as a triple in order to get the very low chain rings and the guard then just occupies the space of the redundant outer ring. It's quite a clever idea, really. So, AFAIK, the front mech is already a triple. It is set at maximum height so the (small) amount of adjustment downwards may just be enough.

I suppose this would be the same for anyone with a brazed-on FD mount (not just on a folder) hoping to lower the front gears. It is a bit inflexible (unlike a band-on) but it might be enough. I just need to get the chain rings to try it out.

[NetworkMan]

You might try a front mech for a triple and set the limit screws to stop overshifting. It doesn’t matter if the cage sits a bit higher than the outer ring, the crucial thing is whether you can shift up from the inner chainring.

It's amazing - I was just about to raise this issue. My old Dawes Audax has a 52/42/30 triple but years ago the bikeshop, as a condition of sale, fitted a 11-32 mountain bike cassette and long cage RD to give me lower gears. The result is that I hardly ever use the big ring so I wondered about the Spa super compact double idea, keeping the 42 inch ring and the existing front FD. If the top limit screw on the FD will stop the chain coming off by mistake it can't not work though a 3 mm longer BB unit might be an idea so as to get the chain line better for a double.

I could then use a short cage RD of say 37T capacity and get rid of some chain and 5 spacers could replace the big ring. Surely Spa's chain guard is mostly cosmetic.

[NetworkMan]

You might try a front mech for a triple and set the limit screws to stop overshifting. It doesn’t matter if the cage sits a bit higher than the outer ring, the crucial thing is whether you can shift up from the inner chainring.

The inner face of a FD is usually very uncomplicated and will presumably accommodate many different sizes of little ring in terms of upshift. The outer face is more complex and has probably been optimised in some way for the size of the large and middle rings. FD height and large (previously middle) ring size may be more important. I guess that horizon will just have to suck it and see.

By the way is the use of a partially redundant triple FD what Spa envisioned I wonder?

[Brucey]

….So the problem as I see it is that the braze-on front mech will need to be lowered. There is about 4 mm of downward adjustment available. Would this be enough?

NB I don't see another option unless braze-on front mechs come in different "sizes".

The best solution here is probably to use an adaptor that allows the FD to be moved on the BO mount. Such things are commonly used (to move the FD the other way) by loony triathletes etc who want to use massive oval chainrings. Such things can be made by competent metalworkers too.

If you leave the FD well above the big ring overshifting off the chainring will likely be a constant hazard (else they would all be like that) and I would say that the inner side plate of many modern FDs is distinctly not 'uncomplicated'. Your envisaged 14T shift is not a big challenge anyway; most mechs will do that without trouble, if they can but be put in the right place to do it.

BTW BO FDs do come in different sizes but ones meant for rings that size are rare indeed. I have some that will do a 46T or 48T big ring off a standard mount (meant for 52T) but they are not at all common.

cheers

[horizon]

Thanks for that Brucey. I've had another look and the FD cage has some space below the chain (on the small ring) at present so I'm guessing that that leeway plus a .5 cm shift of the FD downwards may do the trick. I'm off to work now but after the weekend I'll do some more precise measuring to see where the 24T (and the 40T) might end up.

Note: I don't want to compromise on the 24/40 at this point as it's the least I want to achieve.

[NetworkMan]

If you leave the FD well above the big ring overshifting off the chainring will likely be a constant hazard (else they would all be like that) and I would say that the inner side plate of many modern FDs is distinctly not 'uncomplicated'. Your envisaged 14T shift is not a big challenge anyway; most mechs will do that without trouble, if they can but be put in the right place to do it.
cheers

Sorry folks I think I'd got myself confused. I'd looked at the lower part of the inner plate on mine and it looked completely flat in the region of the small ring. When I looked at the outer plate it looked more complex but of course it isn't so complex in the region of the middle ring and the big ring region is no longer of interest here (unless we drop the FD too low). Hope that makes more sense now!

Brucey, are you saying that a triple is still the best FD to use for Spa's "super compact"? Presumably most double ones are intended for shifts of around 10-12 T.

[Brucey]

...Brucey, are you saying that a triple is still the best FD to use for Spa's "super compact"? Presumably most double ones are intended for shifts of around 10-12 T.

TBH I'm not sure what the best one to use is here; my suspicion is that if it can be made to fit (i.e. be low enough) then a modern FD meant for a compact double would work OK, since these often have 14-16T intervals. Hence my suggestion of an adaptor bracket. Note that the adaptor can be made with an angle offset so that the curvature of the outer side plate is better matched to the outer chainring, even though it is a much different size to the original.

Old style double FDs (meant for 42-52 or even 39-53) will also work OK I'd have said; these have a capacity of at least 14T and (importantly on many bikes where a clash with the chainstay is likely with very small chainrings) the tail is not very deep. These mechs do more often have uncomplicated inner side plates, which adds to their versatility.

cheers

[the_twin]

If you leave the FD well above the big ring overshifting off the chainring will likely be a constant hazard (else they would all be like that) and I would say that the inner side plate of many modern FDs is distinctly not 'uncomplicated'. Your envisaged 14T shift is not a big challenge anyway; most mechs will do that without trouble, if they can but be put in the right place to do it.

cheers

There was a bit of fashion for mountain bikers to replace their outer chainring with a bash ring just bigger than the middle. I haven’t had a problem with unshipping the chain with this arrangement despite there being quite a gap between the bash and front mech

[Brucey]

If you leave the FD well above the big ring overshifting off the chainring will likely be a constant hazard (else they would all be like that) and I would say that the inner side plate of many modern FDs is distinctly not 'uncomplicated'. Your envisaged 14T shift is not a big challenge anyway; most mechs will do that without trouble, if they can but be put in the right place to do it.

cheers

There was a bit of fashion for mountain bikers to replace their outer chainring with a bash ring just bigger than the middle. I haven’t had a problem with unshipping the chain with this arrangement despite there being quite a gap between the bash and front mech

hopefully this is because your FD is not set to shift the chain onto the bash ring... But it is a good point if the OP intends to run a bash ring or chainguard.

cheers

[Tinnishill]

I changed the gearing on my Tern Node and reported on it in this thread

viewtopic.php?f=5&t=107139&p=1060751&hilit=tern+node#p1060751


Now 18 months further down the road and it's been OK.

[horizon]

I changed the gearing on my Tern Node and reported on it in this thread

viewtopic.php?f=5&t=107139&p=1060751&hilit=tern+node#p1060751

Tinnishull: thank you for that. I did do a fairly comprehensive search before posting but it didn't come up.

I think my starting point must be the double at the front because that is what I have. It should be the cheapest and easiest option - if it works.

Update: Just to say I might still want to go for a 32 rear sprocket to get the gears nice and low. This might have been an option on its own but I reckon both would be better.

[gxaustin]

I was worried about unshipping the chain because my preferred 46t 'big' ring is 6mm from the bottom of the FD outer plate.
However, the chain hasn't come off once in about 1,500 miles of the new arrangement. I have unshipped the chain on my other bike though and that has the recommended gap - hmmm.
A friend reports no trouble and his FD is a good 10mm above his 44t chainring. I should give it a try

[horizon]

Thanks for that. Brucey also talks about unshipping the chain from the large ring but which way do you and he mean: back towards ther inner, off the outer edge or either way at different times? And how does it happen or is in normal use prevented? (Sorry about the very basic question but I cannot visualise it.)

Update: yes, I can see now that the chain is guided/held between the two plates (albeit at different heights depending on the chain wheel engaged). I was just thinking about how it shifted rather than how it is held in place. But since the chain shouldn't really touch the plates while running, then no, it shouldn't matter. But it sometimes might ...?

I'm quite familiar with the workings of a front mech but sometimes you need to get your head round a different aspect of it ...