Cycling UK Forum

I've just built up a fixed wheel bike from an old 501 frame I had knocking about and a pair of wheels I bought. Rear has a 17 teeth sprocket and lockring and I'm using the front 42 teeth chainwheel.

I've set it up and ridden it but the chainline is about 4mm out -i.e. the 42 tooth chainwheel is 4mm further from the centreline than the rear sprocket.

No experience with fixed wheel but I've heard that this is too much of a difference. I think that's probably right and I'd like to get the chainline " straight" if possible.

I can see two solutions. Move the chainwhhel inboard or move the sprocket outboard. The latter seems the easier but how would I do that ? There doesn't seem to be any room for a spacer as the sprocket is screwed up tight against the hub flange.

As always I'd be most grateful for some advice.

If this is a normal back wheel with a thread on hub for a 5 or 6 speed block, you will need to respace the axle and re-dish the wheel.

On the driveside, there will be a spacer between the bearing cone and the locknut. Replace this spacer with 10mm washers to make up the same thickness. Then start moving the washers to the other side, one at a time until you have the correct chainline. Put the washers between the locknut and bearing cone on each side, adjust bearing as normal and put the wheel in the bike. Re-dish the wheel by adjusting the spokes so that that the rim is central in the frame.

If you are using a flip-flop hub with a freewheel on one side and a sprocket on the other, you will need to keep it central between the drop outs and make changes to the bottom bracket width instead.

Is the chainset you are using specifically a single speed or an old double or triple? If it's a single speed chainset then you can change the bottom bracket for one with a shorter axle length, to reduce the chainline by 4mm you need a bottom bracket with an 8mm shorter axle, if its a symmetric one.
If the chainset is a double or triple and you've got the chainwheel on the outer position, move it in one position and you will probably get the chainline pretty close to where you want it, you can then fit a blank chainwheel type chainguard to the outer position if you want.

rualexander wrote:Is the chainset you are using specifically a single speed or an old double or triple? If it's a single speed chainset then you can change the bottom bracket for one with a shorter axle length, to reduce the chainline by 4mm you need a bottom bracket with an 8mm shorter axle, if its a symmetric one.
If the chainset is a double or triple and you've got the chainwheel on the outer position, move it in one position and you will probably get the chainline pretty close to where you want it, you can then fit a blank chainwheel type chainguard to the outer position if you want.

Yebbut ... the chainring teeth may foul the rear forks if you shorten the BB too far ... you may need to fit a smaller chainring and different rear sprocket to get over the fork-fouling problem yet retain the same gear ratio.

Rob

Maybe I'm missing something but I’ve never been too convinced of the need for a spot-on chainline for a fixie. I quite happily ride my derailleur geared bike without thinking about chainline, apart from avoiding the most extreme combinations, so why would I be too concerned if my fixie had a chainline a few mm adrift? A poor chainline might increase the chances of unshipping the chain on a fixie but then I don't ride with a slack chain.

In Bicycling Science (David Gordon Wilson) research is quoted saying that efficiency losses because of chain offset are negligible but that transmission efficiency decreases as the size of the rear sprocket is reduced (due to the higher angle of articulation between links as they go around the sprocket) and that unusually low levels of efficiency were found on 12 tooth sprockets. So I have a 20T sprocket on my fixie and a chainline that eye-balls OK but that I have never tried to measure accurately.

Thanks for all the replies.

I should have explained that it's a track hub and I'm using the original double chainring.

I've got to change the bottom bracket bearings anyway so I'll look at getting a slightly shorter axle. A not very good picture here:

I have heard of an adjustable BB that allows you to alter the position of the chainset. Not sure if they were any good or are still available but I can recall they were a bit fiddly to set up.

It looks like you are using the inner chain ring. If it is removable another option may be to alter its position with washers.

My chainline is a couple of mill out, about three i would say, been riding mine like that for three years with no problems.

Mine was out much further before but I bodged a repair to it by cutting off the larger chainring and fitting the smaller 42 tooth on what was effectively now just a spider. It would normally would have been easier to change the entire crankset but I have BB issues and need to retain the original set up to easily use the bike.

Ash28 wrote:I have heard of an adjustable BB that allows you to alter the position of the chainset. Not sure if they were any good or are still available but I can recall they were a bit fiddly to set up.

I think you might be referring to an "eccentric bottom bracket" - that is adjustable for fore and aft movement (i.e. it moves the axle in relation to the wheel hub) - used for frames that have vertical dropouts but have a single cog at the back (fixed or singlespeed .. or a hub gear) to tension the chain

Rob

"track" hubs are 120mm OLN. That frame is 126 or 130mm OLN?

Surly do singlespeed hubs for various dropout widths...look here diligently and you will find the distance from centre to flange varies with the OLN..http://surlybikes.com/info_hole/spew/spew_spoke_calculation_info

Somebody, somewhere, will know the centreline to chainline dimension of track/road/mountain/1960's/1970's/1980's/etc. bikes, but it isn't me, and I can't possibly know if your bike has a "standard" chainline anyway..

The wheelbuilder can't see your bike when he builds your wheels...the observation "I can't quite see it from here, can you hold it a bit nearer the phone"....is not always received in the spirit in which it is offered!

Bottom line....I suggest if you are wanting your chainline "spot on" you buy the hub and sort out the chainline before you get the wheel built.

robgul wrote:

Ash28 wrote:I have heard of an adjustable BB that allows you to alter the position of the chainset. Not sure if they were any good or are still available but I can recall they were a bit fiddly to set up.

I think you might be referring to an "eccentric bottom bracket" - that is adjustable for fore and aft movement (i.e. it moves the axle in relation to the wheel hub) - used for frames that have vertical dropouts but have a single cog at the back (fixed or singlespeed .. or a hub gear) to tension the chain

Rob

I have a BB which takes loose cartridge bearings (from the bearing shop) ....BOTH cups have lockrings, so I can adjust the chainline a mm. or two.

JonMcD wrote:A poor chainline might increase the chances of unshipping the chain on a fixie but then I don't ride with a slack chain.

Thinking about this further I have ordered some chain tugs to fit to my sloping dropouts. If the chain-side wheelnut slips, which I have had happen, you simultaneously get slack in the chain and a poor a chainline which might unship the chain/lock the rear wheel.

Do chain tugs work on conventional sloping dropouts? I thought they only worked on rear facing ones.