Chain line; a starter for ten

[yello]

I've been reading a bit about chain line in the last few days (as I'm messing around building up a frame). I didn't realise that frame manufacturers specify a specific chain line for their frames; I thought you could whack any chainset on a frame and make it work. Which you can after a fashion but not optimally.... not explaining that very well...

... anyway...

I was reading about calculating forward chain line and rear chain line and how, in an ideal world they'd be equal but practicalities (like gears!) prevent this. What I haven't discovered is whether you can retrospectively calculate what the frame manufacturers 'suggested' chain line would be. I didn't realise that it's pretty much trial and error with axle lengths depending on the chainset / front mech.

Sorry, not so much a 'Know How' question as a discussion piece. What's your understanding of getting the best/optimal chain line?

 

[Gerry Attrick]

Have a look at the master's words:

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

 

[yello]

That was one of things that I've been reading!

 

[Gerry Attrick]

Try this then?

http://www.bikerepaironline.com/chain line.htm

 

[yello]

Thanks for the links Gerry. I don't really have specific questions, I think I was interested more in other peoples experiences of fitting new chainsets and bottom brackets. Like many a subject, when you scratch the surface there's often more to it than it seems.

I've got an mtb frame that I'm building up (just for fun) with an old Shimano XT triple, but no bottom bracket as yet. I've read that, as it was an mtb frame, it probably was designed to have a 47.5 mm chain line. So I've calculated that I'll need around a 122mm axle.... but I can't know if the front mech will actually work okay until I try it out! I'd love to have a stock of bb's just to see the effect a couple of mm makes!

 

[tdr1nka]

So setting up to the frame manufacturers optimum chain line will help get the best out of the drive train and a stretched or skipping chain might be caused by having the wrong chain line?

 

[yello]

I don't know if it'd cause stretched or slipping chains but the 'wrong' chain line could effect chain wear. In the same way that riding with the chain across the blocks angles the chain, a poor chain line has the chain angle across that 'perfect plain' whereby the chain runs true from front to rear.

From my reading, frames are designed for a purpose, mtb or road, and a chain line calculated accordingly. An mtb will use the granny ring more than the road bike, so the chain line is usually longer (longer axle) to move the granny closer to the 'perfect' line (i.e. exactly the same plain as the middle of the cassette) but all chain lines are compromise! Road bikes are bias to a shorter axle length for the opposite reason. But we're talking only a few mm.

Then you stick a front mech on; that mech has to have the swing to service all rings. If the big ring is too far out then the mech won't reach, conversely if the granny is too close then it can't drop the chain onto it. That few mm becomes important.

Equally, that's why mechs can only service certain tooth spans; they just haven't got the reach to do more. So the optimal chain line also has to take into account the front mech! As I say, the more you read the more you realise there is to know. Front and rear mechs are surprisingly precision bits of kit!

 

[tdr1nka]

That might explain why, when I recently built an MTB out of bits for a friend, the front mech lacked that extra bit of play to get the chain onto the big chainring!

Thanks for bringing the subject up as I have been puzzling over the matter for a while and if all I need do is put spacers on the spider we should be laughing.

All my mechanic skills have come from trial and error, ham fisted bungling and for the most part only practiced on clarty hacks and utility bikes so 'optimum performance' per say was always secondary to getting the damn things back out on the road.

I'm not sure I'm up to playing with BB axles yet but it's very handy to have the numbers to hand as a rule of thumb and diagnostics.


You learn summink new everyday!

 

[Destry]

I came across this problem when building up the frame that soon afterwards got nicked (see stolen bikes section if you fancy a quick weep...)

The easiest way to find out the chainline of the frame is to email the techies via the manufacturer's web site... Failing that, most cranksets quote a chainline, so if you know what was fitted to your frame originally, that will give you your answer.

I was running a 48t single chainring up front and when I fitted a nice old 112.5mm octalink bb with spacers either side so it was exactly centered on the shell, I got a bad chainline -- which is to say that the chain was straight when it was on the third smallest cog and heavily slanted when on the largest cog. It worked, it didn't make any noise, but it didn't look pretty and I guess it would have been heavy on chain and cogs.

I then re-installed the BB to locate it off-centre, with the drive train side about 4mm closer to the frame and the chainline was pretty much perfect -- the chain was angled when on the top or bottom cogs, but not too badly and it was fine across the rest of the cassette. The teeth of the ring were perilously close to the chainstays, but just room for a bit of flexing... I reckoned this was the best I would get with a single ring up front.

However, I did read somewhere that offsetting the BB is not so great for your hips. Since I am one of those riders that people tut-tut at because I can't stand anything that ties my feet to the pedals, I thought I could probably get away with it.

I think it would be very hard to accurately measure the chainline of a frame... An adjustable BB would be nice, but we seem to be going the other way with the new(-ish) external BBs -- I think the Raceface MTB ones can be adjusted from 149-151 mm, but that isn't much.

Happy tinkering!

Tom

 

[yello]

Top post Tom! It was exactly the kind of post I was hoping for; just some additional thoughts for the topic.

You've made me think of 'Q factor' (I think that's what it's called). The distance the pedals (therefore, feet) are from the bb/chain rings. I think this impacts on hips too. I guess when bikes are being built for a specific cyclist, the 'q factor' will be taken into account - so the cyclist is sat in the best possible position for producing the power.

tdr1nka, I'm a tinkerer/bodger too. I have to play with stuff first before I do any reading on the subject, otherwise the reading is just too abstract for me.

I reckon I'd try some spacers (washers!) on the spider too, just to see the effect it has on the front mech swing. It may just buy you that couple of mm you need. Whether you risk the chain jamming between chain rings is something I'd keep an eye on - but, again, trial and error - suck it and see!

 

[Saddle bum]

Unless you don't accept what the manufacturers give you, it can be worked out quite simply.

Assuming your frame has 130 mm OLD, measure up the hub with a cassette fitted. The position of the centre of the cassette relative to the centre line of the frame/hub can be calculated. That is the chainline.

If it differs from that set up by the manufacturers, there must be a reason.

 

[yello]

Again, from my reading, that's the rear chain line.

The front chain line can never be the same as the rear on a geared bike, for obvious reasons. So it's going to be a compromise to get the front chain line as near as possible to the rear for the most used gears whilst retaining slick gear shifting. It seems like a real balancing act to me!

 

[tdr1nka]

suck it and see!

I tried this and ended up having to brush my teeth with Swarfega!

 

[yello]

 

[Destry]

Adjustable bottom bracket?

Found this on the MBR web site, a review of the 2006 Raceface Diabolus X-Type BB -- sounds like it might offer a few options for seekers after perfect chainlines!

Cheers,

Tom

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"This is the same bottom bracket that comes with the Race Face Diabolus cranks, so you know it's built to last, but what you might be unaware off is that the Diabolus cups have 10mm extra thread insertion. Not only does this reinforce the bottom bracket shell and reduce the risk of stripping, but it also gives you more scope for fitting chain devices to frames without ISCG mounts. Additionally, the Diabolus bottom bracket can be used for spacing out your BB shell so that you can run cranks designed for 83mm shell widths on standard 73mm bottom bracket shells. This improves the chainline with 150mm rear hubs without leaving your BB cups literally hanging on by a thread. Bearing life has proved reliable and you can't beat a splash of gold on the dirtiest area of the bike."