Singlespeed chainline question

jrab

I'm working on a bit of a parts-bin singlespeed project. Pretty much all I need to sort out now is the chainset. I started of working on the basis of using a spare Campag 53/39 and taking off the outer ring (at least until I knew better what gearing I needed).

However, even adding spacers behind the chainring doesn't really give the right chainline, and looks pretty awful, so I'm thinking about getting a dedicated singlespeed chainset now.

However, how do I know in advance where the chainring would sit relative to the bike centreline?

The sprocket is 40mm offset from the bike centreline, and the BB shell is standard 68mm, so offset 34mm from centreline. This means the BB spindle length and chainset geometry need to conspire to put the chainring 6mm outside the end of the BB shell.

How do I get that measurement for available chainsets, without fitting umpteen BB / chainset combinations before I hit the right one?

Cheers,
Richard.

huuregeil

The standard track chainline is ~42mm. What's your hub configuration that gets you 40mm? (Not that that's bad, just check you've got what you think you've got!) Different combos of hub/sproket can give subtly different chainlines, so something between 41-43 might come out

With that in mind, a specific track crankset used with it's recommended bottom bracket will give you a 42mm chainline. If you genuinely have a 40mm sprocket, then you need to get a BB that is 2mm shorter on the drive side. (Careful here: e.g. shimano 110mm and 107mm BBs are the same length on the driveside).

Alternatively, you can use the fact that typically the outer chainring is 46mm on a standard road double (depends slightly on the crank), thus you need to get a bb that is shorter on the driveside by the required amount.

Clearly, lots of people have been through this pain before! If you've got a particular crank in mind, then have a good google for info and required bb lengths. Campagnolo road cranks are bit of a PITA for ss usage because the BBs are short (e.g. 103mm for record/chorus) and knocking 3mm off that gets you a bb that is hard to find! And, you might find the crank itself fouls the bottom bracket shell because campag puts quite a bit of metal there.

Older square taper shimano double cranks make good ss conversions. Or stronglight/TA doubles.

jrab

Thanks for the reply.

The wheels are from what used to be (a while ago...) a 12-speed road bike, screw-on freewheel type. I've fitted a singlespeed freewheel and the wheel has been re-dished with the axle spacers re-arranged to centre the hub.

I also have a fixed sprocket and BB lockring I may use to try it as a fixie (being gentle with the back-pedalling of course!)

Maybe I should check the measurements, the difference between 40mm and 42mm is not that much. I did measure it fairly crudely with a tape, 130mm across the dropouts and sprocket 105mm from non-drive side.

Either way, I wouldn't have thought the 2mm difference would be critical to smooth running over the length of a chainstay? This bike has fairly long chainstays anyway, which will help?

The Campag 53/39 (Mirage 9-speed) uses a 111mm BB, was hoping to be able to re-use it but sounds unlikely?

Thanks.
Richard

huuregeil

Measure it well - might save you trial-and-error hassle with the bottom bracket.

2mm. Hmmm.... I think that's on the limit, if not over. I had a nasty FSA track crank that had a nasty, pressed and non-round chainring. So I decided to change the chainring, not having appreciated that the FSA chainring was slightly recessed (i.e. when laid flat on a table, the teeth were raised slightly. Can only have been a couple of mm. The new chainring didn't work and I ditched it pronto- the chain was super-noisy and I was paranoid about throwing a chain, an issue if running fixed for obvious reasons! (And my fixed has long chainstays too). Anyway, replaced the whole chainset in the end!

As regards the Mirage. Possible but unlikely. The major issue is that campag bbs only come in a few lengths, unless you buy an expensive bb e.g. royce. And campag and shimano + virtually all other manufacturer's are different: campag use ISO standard, shimano et al JIS. The both fit on each other but sit differently and I think that ISO crank on JIS bb doesn't fully seat. Finally, if you're going down to 40mm, you compound the problem. Have a google and remember, ebay is your friend! As I said, if not using a dedicated track chainset, other manufacturer's doubles work far easier.

Monty Dog

I have successfully used a Record road chainset with a Miche track chainring on the inside and used it for a TT bike with a rear track hub - it was 2mm out, but never had any problems - it was noisy, but that was down to the frame - Scandium alloy, as much as the alignment.

ince

Have a look at,

http://www.sheldonbrown.com/bbsize.html

... i used the info on this page and others on the site to work out what size bb I needed to get the 42mm chain line I wanted when I built my fixed.

I had got a stronglight single speed crank so for me it was just a case of getting a stronglight bb of the right length 107mm if memory serves to get the 42mm. Got it right first time and the bike runs silent.

stigofthedump

I would suggest that chainline is more important than centering the hub. Perhaps you should stick with standard BB and cranksets and have the rear wheel adjusted.

Alternatively if you are using the inner chainring position you could use spacers to move the chainring over.

Just a few thoughts, disreguard if i'm stating the bleeding obvious

jrab

Thanks for the replies.

As the wheel was never intended to be single-speed, there's only so far it will adjust, the axle spacers and spoke length mean I can't get it any further over to help the chainline. It is pretty much centred though so am not too worried about that.

I've finished building the bike now apart from some minor fettling (had forgotten how much of a pig those old Weinmann single-pivot calipers were to set up - the ones with the lock-nuts on the front - arghh!). Looks nice (very retro!), might see if I can work out how to post a pic.

I got a Campag 42-tooth ring my LBS had lying around and this has given me much better rear wheel location and chain tension than I was getting with the 39. Gearing at 42-16 pretty much spot on too, probably more so with the 15T track cog I have.

The chainline seems to be pretty close as far as I can see, but that is with big spacers between the spider and the ring, and it just looks wierd..... I'd like to lose them if possible.

I was looking at changing the chainset, but all the reasonable value "track" / singlespeed sets seem to be effectively just a road set with the outer ring dropped off, which is basically what I have......

As Huuregeil says, Campag BBs seem to only come in 115mm and 111mm (111mm fitted) unless you get a Record or other branded £££££ one. None of the easily available BBs will fit the Campag ISO taper. However, I have come across a Miche Primato track BB at 107mm which is £15 or so and I think is Campag fit - anyone confirm this? If that knocks 2mm off the spacers that might make me happy!

Cheers,
Richard.

huuregeil

One final thought - you can run ISO cranks on a JIS taper BB. They don't sit fully on, but apparently it's not as fatal as it sounds :-) I can't remember the exact difference, but it's about +2mm from memory. So 111-3+2 might be something like a 110mm JIS BB, whcih is a common size. Maybe. Google. Don't take my word for it!

jrab

I was looking late last night on Sheldon Brown (what a legacy - RIP).

It seems hardly "recommended" but if you know what you're doing......

He suggests ISO crank on JIS BB would sit 4.5mm further out than it would on the same length ISO BB. So I'd need a 106.5mm spindle to put the chainset where it is now, or something like a 102-104 to let me get rid of the spacers. I had a quick look at the Shimano UN series, also Stronglight JP400, but haven't had time to find a suitable option yet.

I did come across some dodgy reviews of the Miche BB though - some suggest the cups have a habit of working loose and needing regular fettling. Will have to check that out.

Thanks for input.

All I need now is a chainring bolt tool (can't get the funny slotted nuts tight enough without one...).

Oh, and weather like today, except when I'm not working......

If you're interested, I stuck a picture here :- http://s553.photobucket.com/albums/jj39 ... =slideshow

stigofthedump

Good looking stable

Isn't everyone interested in other peoples bikes?

Hope it all works out, the sun will shine soon.