Bicycle Mechanics - Is This Frame Modification Possible???

Hey there!

I am new to the forum and I have a very Perplexing (to me anyways) question/problem.

I currently ride my steel single speed with a chainring that is 42t. It came to me with this size ring and has not changed. This is in part because the dimple along the crank side of the frame is extremely small as you all can see.

Is it possible to extend the dimple further back to fit a larger ring or would it make the frame structurally unsound? I cannot go to a smaller rear cog and there is no room to move the chain line further out.

Thank you! :)

If it's a steel frame it's usually possible to extend the chainstay crimp. But to do it right requires a certain amount of hand skill, and some frame materials don't take to it as well as others. In most cases the simpler solution is to move the right crank outboard a bit rather than work on the frame.

So, yes it's probably possible, but may not be the best solution.

Welcome to the BikeForums L. George,

People have been "extending the dimple further back" on their frame chainstays for many years. The good thing is that your frame is steel (which makes this process much safer imo). Doing this is a "last resort" kind of thing imo.

I'll let FBnNY and/or others in here with experience and/or more knowledge on this help you (with the details on how to do this and whether or not they think it'll be OK).

Good luck!

From the top view (photo 4) it doesn't look like the chainring teeth are dreadfully close to the chainstay so I'd borrow a larger ring and give it a careful try fit. Also, you may be able to get a bit of clearance by installing thin washers under each chainring bolt to space the ring out slightly without having to replace the bottom bracket. Finally, depending on the type of bottom bracket, you may be able to install a spacer under the driveside cup's flange to move the chainline out.

I believe any of these fixes is preferable to further crimping the chainstay.

yeah, I'd go the spacer-under-the-chainring route if you can't move the BB over a mm or so (or, if its a cartridge BB, replace it with one that has a 2mm longer axle to move the ring out 1mm), AND see if you can't add the same amount of spacer under the rear cog so the chainline stays straight.

To my mind trying the above suggestions about moving the chain ring and (perhaps) cog are preferable to denting the chainstay or even just trying a larger chain ring as-is. You can always take them off and try something else but once you modify the frame there is no going back.

The new dent will chip the paint,, you want a Bigger Gear , get a smaller rear cog.

having always lived where hills were a fact of life, I really don't get the whole Fixie thing unless its for velodrome track racing.

If you went to 50 from a 42, that'd be 8 teeth or 4" in circumference, or ~5/8" in radius. Can't say for sure from the pix but it looks like it'd fit.

thats a rather funky looking crankset, those arms look really thin, are they stamped steel rather than alloy?

that sprocket seems like its really rather too close to the frame. IS there room on the rear end for the cog to be farther out on the hub ?

thats a rather funky looking crankset, those arms look really thin, are they stamped steel rather than alloy?

that sprocket seems like its really rather too close to the frame. IS there room on the rear end for the cog to be farther out on the hub ?


They look stamped steel to me and likely swedged onto the arm. I would recomend a better quality cranks and wider BB spindle. A smaller cog in the rear is a good option too.

http://www.bikeforums.net/attachment.php?attachmentid=293891&d=1358363348

Check the chainline and see if you get a bb with wider spindle

You've got room for at least a couple more teeth. How big do you need/want to go? Some people are perfectly happy running a 42T chainring.

I do want to get a better/bigger crank in the 48t or 50t. (if I can find a cheap one)
After reading what you guys posted I see now that I should probably go to my LBS and see if they can move my chainline.
Getting a new BB and crankset is a must it seems.

well, a different square post crank could quite easily have a different natural chain line on the same BB.

the key is, can the sprocket on the rear wheel be moved farther out, that hinges on whether it has spacers now, or could be used with spacers. is this a fixed gear, or a threaded on freewheel single, or a freehub single ?

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It is a flipflop hub...

It looks like it has a few spacers.

I take it back. No spacers.

you can get spacers, as already mentioned, for the Bottom bracket, and i'd recommend a spacer for the single speed freewheel too (they are the same for both on most of my SS's), if only to keep the same chainline. it looks like you have enough room to do so. i think harris cylery carries them.

then you can get a bigger chainring. how much bigger? you'll have to borrow one (with assurance of eventual purchase) from an obliging LBS. as already mentioned, spacing out the new chainring with washers may also prove beneficial.

As said above, try a bigger chainring as-is. If it hits the chainstay, put some spacers both under the chainring and the freewheel. Don't forget to trim that bolt.

I do want to get a better/bigger crank in the 48t or 50t. (if I can find a cheap one)
After reading what you guys posted I see now that I should probably go to my LBS and see if they can move my chainline.
Getting a new BB and crankset is a must it seems. I really don't see any need for a new crank or BB. What you have will certainly work for you just fine. As far as the BB, you should be able to put a 5mm spacer behind the lip on the drive side that will move the chainring out 5mm, which I think is a more elegant solution than the chainring spacers. It also looks like you have a 16T freewheel, which means you could go one smaller; the ACS 888 comes in a 15T. If there is an issue with the chainline you can probably find a thinner cone and lockring for the drive side axle.

I'm pretty sure you could fit a 44t on there no worries, and I'd be surprised if you're running the smallest available cog already.

If you need to run a bigger ring than will currently fit, here's what I'd try first: simply bending your cheap steel spider out a bit, no need to faff about with the BB.

Ok, so my LBS says that there is no real space to move out the chainline without messing up my wheel alignment a whole lot.

My new thought... is there a BMX hub that I can fit a 14t freewheel and still put into my current dropouts?

The goal is to be able to ride fixed and free without going from a 14t fixed up to a 16t freewheel every time I flip.

My new thought... is there a BMX hub that I can fit a 14t freewheel and still put into my current dropouts?

The goal is to be able to ride fixed and free without going from a 14t fixed up to a 16t freewheel every time I flip.

There almost certainly is a BMX hub that'll do that, but it's threaded for a conventional freewheel on one side and one of those special 14-tooth BMX freewheels on the other. Put the fixed cog on the freewheel-threaded side, and retrofix it on (google retrofix fixed gear cog) and you've got a fixed-wheel flip-flop hub. Obviously you haven't got the lockring for the fixed cog, so you can't go crazy and do huge skid stops, but it'll hold up for normal riding.

The only issue would be the fact that you're running a 42-14 gear, which is not likely to do your knees any good in the long term unless you've got a 20" rear wheel or something. I'd recommend you use a 16- or 18-tooth cog and learn to spin faster.

Ok, so my LBS says that there is no real space to move out the chainline without messing up my wheel alignment a whole lot.

My new thought... is there a BMX hub that I can fit a 14t freewheel and still put into my current dropouts?

The goal is to be able to ride fixed and free without going from a 14t fixed up to a 16t freewheel every time I flip.

measure the chainline very carefully. Odds are that there's room to move the chinring out without moving the rear sprocket. While not ideal, a chainline error of up to 1/8" is tolerable.

You're not likely to find a double sided hub that will have the sprockets more outboard. If you diodn't want the flip[flop feature, you could move the entire hub to the right (and redish the wheel), but that's not of use for flip flop.

Explore the chainring spacing and sprocket size options, but if none of these are good for your needs, you can go back to plan A and extend the chainring crimp. Any decent mechanic should either be able to do it, or confirm that your chainstay's material makes it a poor option.

I'll reiterate that you shouldn't need to move the chainline much at all to fit a larger ring, say 46-48t, and that should be possible by bending your spider rather than stuffing around with the BB.

If you end up with an unsatisfactory chainline, you could always just fit a narrow cog and ring, and use multispeed chain.