davester

I'm trying to set up my Raleigh "International" as a worthy touring bike. It came with an Ofmega 50-46-30 triple and Suntour Winner 14-30 6-speed cluster. So, I did a careful gear charting and decided to set up a different half-step + granny (46-42-24), which gives me what I thought would be a nice setup with an uber-low 22" hillclimbing gear (see the lower gear chart below).


half-step gear chart
Unfortunately, when on the small chainring and shifting up to the 21 or smaller cogs, the chain hits the 42T middle chainring. It works sorta but accompanied by unpleasant clanking sounds and probable fast chainring wear. Unfortunately the Ofmega crankset has a 144BCD so I'm stuck with 42T middle chainring. I'm trying to figure out the best way to fix this. The options seem to be
a) give up on the low gearing and go back to the 30T small chainring (see upper chart above) - really don't want to do that if I'm going to do any loaded touring since I'm old and usually ride very hilly country,
b) switch to the 30T chainring and buy an expensive IRD 13-32 freewheel for a tiny bit lower low gear (26") - that's almost not worth doing since it is only 2 teeth different to what I've got.
c) Try to find a 34T Winner cog to trade out my 30T for - good luck with that!...also would probably require a new derailleur
d) Buy a whole new crankset with a different BCD ($$$$) and ditch the idea of half-step.
e) Find a longer spindle and push the chainline out (which would put the middle and large chainrings out of alignment).
f) Something else I can't think of right now.

Advice would be appreciated.

Some pictures of the drivetrain below:

merziac

I would bring the middle ring tooth count down as much as you can to lessen the airgap and help it make the jump, maybe even bring the granny up a couple of teeth if you have to.

I see you already talked about the smaller middle ring and you're right, they are hard to come by and not cheap. 40 teeth can be had though. None of it will be cheap. You should be able to change to a smaller bcd crank for this, many here have plenty of spares that could be made to work, co-ops too.

thook

I ran the same chainset for a while on two different bikes and didn't have the problem you're having. Not sure what difference within your set up is causing the chain to rub the middle ring, though. i really don't think a bigger granny ring will be the fix, either. Nor will a bigger low cog/s. How wide is your bottom bracket? I ran a 113 in both instances with my set up. Plus, a seven speed cassette on a 130 OLD

bikingshearer 

Maybe a spacer or two behind the fixed BB cup to move the DS crank out a little bit? That might give you a noise-free 24x21. Maybe. Of course, it may cause problems with the FD reaching our far enough to get into the big chainwheel. Might be worth trying as it's a relatively easy and cheap fix, if it works.

Also, maybe ever-so-slightly thicker/wider spacers for the small chainwheel bolts? Maybe all of the above?

Just spit-balling here. I have no idea if any or all of this would work.

oneclick 

If the chainring teeth are offset, flip one for more spacing where you need it; check the chain does not fall in the gap.

And Alpha splined sprockets fit, I think the AG 38 will as well.

steelbikeguy

I think you've identified all of the viable options.
In my HS+G arrangement, I only use 3 of the 7 cogs in back before similar rubbing occurs, so I don't think there's any big error in your configuration.

Depending on the bottom bracket, the easiest thing might be to install a couple of mm spacer under the fixed cup, which will move the crank to the right.
There might be a chance to use spacers to move the small chainring away from the middle ring a few mm too. Not sure what spacers are available, though.

Another option might be to look at where the chain is rubbing. Could you file some of that off?? It might not be essential.

Steve in Peoria

Trakhak

The answer is in the name: half-step plus granny. Translation: shift as if you're using a two-chainring setup except when you need your one or two super-low bailout gears. (Quoting Sheldon Brown on half-step plus granny: "The idea is to use the half-step gears for general cruising, and to have the granny chainring as a "bail-out" gear for the serious hills.")

In other words, respect the "limitations" of the half-step/granny setup. You've got beautifully spaced gear choices with the bigger rings; you've also got the lowest gears you're likely to need. Expecting to be able to use your granny ring with more than two or three sprockets in that bike's gear configuration is about as reasonable as expecting to be able to shift in front or back or both while pedaling backward.

Or---a pet peeve of mine---like expecting all derailleur-equipped bikes to allow pedaling backward in all gear combinations, including large to large and small to small. It might or might not be possible with any one bike, but no derailleur system was ever designed with pedaling backward in mind, obviously.

rccardr 

On my setup, I can only use the granny in the two or three largest cogs.
Also agree that a 40 tooth middle ring would be slightly better.
Am also of the opinion that using the granny for anything but grinding up a big hill slowly is inefficient & try to remain in the largest possible chainring for as long as possible.

madpogue 

^^^^ The issue is, the OP only gets the _two_ largest cogs with the granny. Looking at the chart, that means a jump from 28 gear inches (24/24) to 39 (42/30). The 24/21 would fill that gap, but (pardon the pun) there's the rub. Maybe a 1mm spacer on the BB as mentioned above would make that possible, and be little enough not to throw off the chainline elsewhere.

Mr. 66

I would check the chain line, your setup sounds inboard. I use those cranks and I'm not having any issues but I'm running stock 6 gear.

dddd

Yeah, possibly the frame was spread, with the weaker driveside chainstay doing all of the bending(?).

A switch to Shimano 9s chain would give added clearance for sure. I say "Shimano" 9s chain because it does actually grab teeth better than all other 9s chains out there, and which might preclude the chain skating atop or between cogs.

Steelbikeguy mentioned removing superfluous protruding metal from the middle ring. I've done this in the past when I had a clearance problem, and it did cure the issue.
A spacer material and a strip of coarse belt-sander stock can be forced in between the middle ring and the chainstay, Then the cranks can be rotated to act as a machining operation. Keep the "sandpaper" moving slightly or it will load up. Have the grit facing in the right direction, lol. Best to protect the paint with thick tape as well before proceeding.

davester

OK, good advice so far. Keep it coming! My thoughts on the suggestions so far:

I don't think removing chainring metal is going to help given where the chain hits would mean removing a lot of metal probably including parts of the teeth.

Adjusting the inner ring inwards with spacers would be difficult given the unusual spacers used with the Ofmega crank, not to mention increasing the risk of dropping the chain into the rather large gap between chainrings.

I will definitely take a look at both the front and rear chainline measurements to see whether adding or moving spacers can bet me a few millimeters.

As far as getting a smaller middle ring (i.e. getting a new crankset), these cranks are so pretty (if you followed my earlier thread on this bike they appear to be pristine and essentially unused) that I'm not that keen on changing them out. That would be a last resort solution.

Interesting thought on using a 9-speed chain. I hadn't thought about that. Are there any issues with that approach? Actually, I'm thinking that the Sedisport chain that is on there may be wider than even standard modern 8-speed chains so just getting a thinner 8-speed might be a possibility (Wipperman Connex chains seem very narrow in my experience).

I don't think the frame was spread, so I'm thinking the alignment is probably fine, though it wouldn't hurt to check using Sheldon's string method.

SJX426 

@davester - IIRC the overall width of chains is what differentiates chain speed labels. The inner (tooth) dimensions are much the same. I may need to be calibrated.

zukahn1 

Not just half plus granny this is good advice for any vintage wider range triple rather road, touring or mountain.

73StellaSX76

Dave, if you’re currently using an old Sedis chain and it is stiffer side to side then I would consider switching to a more modern chain such as the 9 speed one suggested, and widening your bb spindle, following your option e). Your concern about pushing chain line becomes less critical with more modern chains. Did you mention how wide your current spindle is?

davester

I'm pretty sure that it's a NOS sedisport, which was if I recall correctly is a bushingless relatively flexible chain. Nevertheless, a 9-speed chain will definitely be skinnier and adding that difference to a 1 or 2mm spacer may get me the clearance I need. Not sure how wide my current spindle is. Time to get my caliper out and measure a few things.

jonwvara 

Eighteen teeth difference between the granny and the middle ring is a lot. The rule of thumb used to be 14 teeth of difference or less. I have several bikes with half-step and granny setups, all with the traditional 14-tooth difference. No problems with any of them. You may not want to use a bigger granny, but that would likely solve your problem.

John E

Your 42-24 drop of 18 teeth is pushing the limits of most systems. I have never heard of a 40T ring for 144mm BCD, but 41s can be hand for a price.

I used to run 48-40-28, alternating with 48-40-24, on my mountain bike. As most of the respondents here, I used the granny ring with the inboard cogs in back. I had to be very careful when performing a 24-to-40 shift, using a SunTour XCD front derailleur. The 28-40 shift went much more smoothly, with much less fuss.

I recently changed my outer rings to 46-38, and I plan to try the 24 again the next time I either overhaul the BB or convert to a cartridge. I don't like to put aluminum cranks through too many remove/replace cycles, and I can't swap granny rings without pulling the crank.

thook

davester you might want to read this. It covers a couple of points brought up here in your thread. Namely chain deflection.

https://veloapocalypse.wordpress.com...d#comment-4276

davester

Hi thook , I don't see the link you appear to be directing me to.

The previous comments about 14T rule of thumb are interesting. I've never heard such a thing but wish I had before buying and modifying the 24T ring to fit my crankset (yes, modifying...though a standard 74bcd, Ofmega in their wisdom decided to make the bolts a nonstandard larger size, requiring drilling out all of the bolt holes in the new chainring). Knowing that might have persuaded me to back off a little and only try a 16T difference (I always like to push the limits a little) instead of 18T.

As for the warnings about only being able to use the granny on the lowest few cogs, I've always been well aware of that limitation but two cogs is definitely too few.

thook

sorry...my brain left the building for a minute

thook

I'd never heard that, either...hmm
Modern compacts are 16 tooth difference, typically. And, I've seen more than a few successful wide range doubles with a 20t difference. 18t difference is kinda big, but not unheard of much less non-doable. Really, it's not the tooth difference, though, being your problem. IF it were, I would have that problem running the same set up in front and I didn't. I used a 7spd chain and, again, a 113mm bottom bracket