Author Topic: Getting the right chainline  (Read 1588 times)

Getting the right chainline
« on: September 13, 2017, 09:14:57 pm »
Having done this on more bikes than I can remember, fixed, singlespeed and hub-geared, here are a few methods and common pitfalls that might be useful.

How accurate?

Chainline should be very accurate for a fixed gear and very, very accurate if you intend to run a traditional 1/8" chain with bushings.  By "accurate" I mean within 2mm and by "very, very accurate" I mean within 1mm.  You can only properly check chainline with a long straight edge held flat against the chainring (being careful to avoid any pins or bolt heads that will tilt it at an angle).  A 60cm steel rule is ideal - use the edge of the rule, not the flat of it.  Singlespeed, or hub gears, should be just as accurate for best results but Bad Things are less likely to happen if the chain unships or breaks, so you can tolerate another couple of millimetres if you're not fussy.

Selecting parts (with a minimum of trial and error)

The easiest way to get correct chainline is to buy a track groupset.  Most of us are on a considerably lower budget and like at least some weather sealing on our hubs and bottom brackets, so we'll be finding parts ourselves.

Track hubs will usually be sold with some indication of chainline - check whether this includes a sprocket, and, if so, what the manufacturer has assumed for the sprocket.  7mm addition is typical for a sprocket (from the shoulder that contacts the hub to the centre of the teeth).  "Real" track equipment is usually 42mm including the sprocket.  However, Goldtec track hubs are 46mm, On-One track hubs are 50mm and most home conversions will run between these two figures.  The actual figure is not important unless there are clearance issues; the thing is getting it the same at the chainring and the sprocket.

Conversions with an existing front setup

If you were previously running a road double, the outer ring should be 46mm, so a Goldtec hub and most fixed sprockets will give straight chainline.  Likewise, an MTB triple should have the outer ring at 52-55mm.  You are highly unlikely to find a hub and sprocket that will give that sort of chainline, so you are more likely to have to put the chainring in the middle ring position on the inside of the spider, which will give somewhere between 47.5 and 50mm.  The On-One hub is designed for Hollowtech 2 cranks up front, with a 50mm chainline.

The problem is that bikes don't always meet the above standards, since a chainline that is 2-3mm off Shimano spec isn't noticeable when you have a derailleur and 10-speed cassette.  Also, manufacturer specs and retailer listings are often hopelessly wrong.  You will need to measure and see; the best way is to use calipers to measure from the centreline of the chainring teeth to the far side of the seat tube, then subtract half the diameter of the seat tube.

There is a database of chainlines for various cranks, bottom brackets and hubs on the LGFSS website.  Try it, but treat it with caution; a slightly misaligned rear triangle may mean it works for someone else but not for you.

Adjusting chainline when it's wrong

Messing about at the front

Typically you will find the chainring is too far out and the sprocket is too far in; that's just the way it seems to turn out most of the time.  The best place to start is a shorter bottom bracket, assuming you're using a square taper BB - splined BBs are often only available in one length.  Shimano square taper BBs only go down to 107mm but Tange are also JIS and go down to 103mm.  To move the chainring inboard by 2mm you need a 4mm shorter BB, because the length is reduced on each side.  To be on the safe side, take the existing BB to the shop and measure the protrusion on the RH side of the old and new BBs, to ensure you're getting the correct reduction or increase.

Moving the chainring from the outside to the inside of the crank spider may be infra dig in fixie circles but is often necessary, and makes aboiut a 5mm difference.  It is just as well attached on the inside as on the outside, although visually not as smooth.

Small spacers under the chainring bolts between the chainring and spider are another idea for fine tuning.  Check the bolts are really tight, as there is a lot less friction keeping the ring from moving.

Messing about at the rear

Respacing and redishing the rear wheel is free and, in the case of a derailleur hub, will reduce the dish of the wheel and make it much stronger.  Not all hubs can be redished and you have to be able to re-centre and true the wheel afterwards.  For example, a Campag Record freewheel hub can be respaced so there are no spacers or washers left on the RH side, moving them all to the LH side.  This brings the hub into the range for fixed or singlespeed chainline, something Campagnolo would no doubt be horrified by.

A different make of sprocket may have a different chainline.  They vary slightly but few are over 7mm.  However, if you only want to lose 1mm at the back, a different sprocket may do it with no need for spacers or other kludges.

Many sprockets are asymmetrical and putting them on the "wrong" way will reduce chainline.  This rarely looks good and you need to be sure the sprocket is making enough contact with the shoulder of the hub, but is occasionally useful.

Putting a spacer behind the sprocket is tempting to move it out a bit, but only do it on a freewheel hub; fixed hubs have a minimal number of threads for the sprocket already.  I wouldn't go over 2mm spacing, as you could strip the sprocket from the hub if it doesn't have 5 or 6 threads engaged.  VeloSolo sell 1mm spacers, and Sturmey Archer sell them in 1/16" (1.6mm).

The most drastic way to correct chainline is to offset the rear triangle of the frame.  This can give you as much adjustment as you like, although you must redish the wheel so it is still centred on the front triangle (your normal dishing tool won't be of any use here).  One for hardcore fettlers only.

Common problems

On MTB frames the chainstays are widely spaced and a chainline narrower than 50mm sees the chainring running very close to the chainstay.  This limits maximum chainring size.  You can run it an absolute hair's breadth from the stay if you need to; pedalling won't make it touch the stay unless the BB is really worn out.

Square-taper BBs give only an approximation of "book" chainline depending on how well they're made, how worn the taper is and how hard you muller the crankbolts.  This can work to your advantage if the chainring is less than 1mm too wide; try really tightening the RH crankbolt and it may just be enough.

On small-wheeled bikes, the seatstays come into the seat cluster at a much sharper angle and you will have to run a narrower chainline and/or smaller sprocket to avoid the chain rubbing on the RH seatstay.

Never tell me the odds.

Re: Getting the right chainline
« Reply #1 on: July 27, 2018, 09:09:29 pm »
Small addition and good advice I found on LFGSS: if you buy a complete track groupset, it does not guarantee perfect chainline.  With a modern high quality Taiwanese factory frame, probably.  With a lesser frame, or an old steel one, maybe not.  We're talking about 1mm here, and rear triangles are often up to 4mm out relative to the centreline of the bike.  A steel frame can be re-tracked for about £40 if you strip it down and get it to a framebuilder.  An alu or carbon frame is what it is, and you have to use the techniques above to improve chainline if the frame is a bit out of alignment.

There is no substitute for checking actual alignment with a 2ft steel rule, which does not lie.

I have this situation at the moment with an old frame.  Front chainline and rear chainline both measure as exactly 42mm with calipers when installed (I chose the components carefully) but the straight edge says the chainline is 4mm out, meaning the rear triangle is 4mm to the left of the centreline.  This is within normal manufacturing tolerances and you'd never notice on a geared bike, but it's no good at all for a fixie.  The frameset is going to need retracking.  Fortunately this needed doing anyway because the fork is a little off.
Never tell me the odds.