MTB Conversion - Chainline

Hello all,

Tired of having my nice bikes pinched, i'm building up a cheapish SS but have run into (probably schoolboy) problems.

I'd really appreciate any advice you could offer.

I've all but built up a single speed conversion. Had an old Marin frame and my brother let me have his wheels, so I've done the whole conversion thing with spacers etc... All went smoothly until I got to the drive train. I left the big ring on some octalink LX cranks using slim bolts and put the sprocket on the back right next to the frame. The shortest bb I can find is 113mm but the chainline was visibly out. I stuck in a 107mm tapered bb with some old cranks and left the big ring on (as above). I measured the distance between the ring and the centre of the seat tube - about 51mm (Sheldon reckons on 52). But it's still not right. I now think that

1. I'm going mad
   
2. The big ring must go in the middle position - a pain in the arse
   
3. The chainline will be alright as it is (?)
   
4. I don't get it
   
   

If you could offer me some help I'd be truly grateful.

what is your rear hub? freewheel or cassette?

What Tommy asked.
It took me about an hour to sort the chainline for my fixed / SS mountain bike.

Realistically that was about 10 minutes actual work and 50 minutes of checking and re-checking all of my measurements.

The cog on the back should be roughly in the middle of the cassette splines - can you adjust it's position by moving/swapping spacers?

provenrad The cog on the back should be roughly in the middle of the cassette splines - can you adjust it's position by moving/swapping spacers?

Assuming it's a cassette!

wisenedcat Hello all,.... Had an old Marin frame and my brother let me have his wheels, so I've done the whole conversion thing with spacers etc...

I am not a smart-arse! ;-)

wow, that was fast.

Yeah, It's an xt hub made for a 9 speed cassette. The sprocket, at the moment, is as close to the frame as it'll go. If I moved it in, the chainline would be even worse. What are the measurements I should be dealing with exactly? At least if deal with measurements, I can rule out the madness. Thanks

rule out the madness

hehe - I used a similar setup with a chainring in the middle position on some cheapo shimano cranks.. only problem was that I couldn't go bigger than 40/42T else it fouled the chainstay. Do you have the same problem?
What else is hassle with putting the chainring in the middle? It looks a bit ghetto and you may need to do some filing of the crank spider I guess, but from the sound of it thats the solution you need?
I never measured my chainline as it goes.. :-)
What chain are you using, a 5/6 speed one? Singlespeed can cope with poor chainline better than a fixed-gear - it it riding ok?

@ Prav - that's what I get for not reading things properly :]

@ wisenedcat - What size SS sprocket are you using?

I used the middle ring on the GT but that was always setup for mountain biking with a 32/15, 32/16, 32/17 and now 32/14 gearing.

With the cranks I've got, I'm going to have to take a file to the cranks to get the chainring on the inside. Plus, the chainring makes a nice 'all in one pattern' with the cranks when it goes on the outside - haha. I haven't ridden it yet. I wanted to get the chainline right, at least by eye, before taking it out. Also, didn't want to start hacking things up and then find out it could have been a whole lot easier and i'm just not getting something - I'm not that gifted a mechanic.

I've got a 16 sprocket on the back. Just what came with the DMR spacer kit but it looks about as tough soup. That may need changing. The chainring's a 44 so I reckon that's an alright ratio.

I may be being naiive/simple but surely measurements are measurements. If one person can do it, surely we all can. It certainly feels naiive to say it.

Standard chainline measurements are as follows but in practise can (and normally do) vary a fair bit:

Track and singlespeed road = 42mm
Road double inner ring /outer ring = 41mm/46mm
Singlespeed mountain bike = 54mm
Mountain bike triple; middle ring/**outer ring = 48-50mm/52-54mm **

If using a normal cassette MTB hub to make a singlespeed then usually the front single ring goes on the outer position and the rear cog is spaced with thin spacers to match. If your front chainline is 51mm you will not want the cog all the way over by the inside of the dropout.

To be exact it will need to be 16.5mm from the inner face of the driveside dropout.

Within a couple of mm is ok though.

You will need about 5mm of spacers between the cog lockring and cog.

Thanks for that. If that's the case then I must just be wrong. I'd take a photo of it but my girlfriend's taken my camera to Uruguay. I keep looking at it and the chainring just seems further out than the sprocket even when it's right next to the drop out. If I've measured this correctly, how can that be.

I may be interested in one of those spacer kits, thanks. as they say, 8 spacers is better than 4.

Check with a straight edge ruler from centre of seat tube to centre of chainring = front chainline.

For the rear presuming a 135mm rear hub then measure your current set up - again with a straight edge but from inner face of RH dropout to centre of cog.

Subtract this from 67.5mm to give rear chainline.

okay, great, thanks so much. With the madness ruled out, I'm just going to do it all up, with the numbers on my side, give it a ride and then come back and moan. Cheers everyone.

Just to update you, if you're interested...

I used a 107mm bb with some deore cranks (the others were fooked and its teeth looked like steptoe's) and left the 44t ring on (the outside) - The chainline was 56mm.
I sorted the sprocket at the back to match. Using my eye was rubbish, it depended on which eye I used, time of day, type of light, desire for it to be straight/resignation that it wasn't/paranoia.

put the singulator on(pushing up - just managed it) and just took it for a spin. It was beautifully smooth which was nice. Heard one almighty crrrrack as I went over a sleeping policeman but I reckon that was just the saddle 'bedding in'.

thanks again all

Job's a good'un! :-)
You are now qualified to dispense advice.