chainline

[JimK]

A month or two ago I was chasing a clicking at frequency of my cranks. Turned out to be pedals. But I was worried it might be the bottom bracket. That's probably the part of a bike I understand the least & so I have been studying up a little.

This morning I made a little measurement on my Nomad. I measured from the center of my down tube over to the chainring. I came up with 60mm. Now, what I measure was my chainline, right? The ideal chainline for the Rohloff is 54mm, yes? Surely 6mm is no big deal, but still.

I am trying to understand how all these measurements fit together. E.g. suppose I wanted to fit this crankset:

http://www.rivbike.com/product-p/cr7.htm

I imagine the chainline for a double chainring is measured to the halfway point between the rings? and the standard chainline is 47mm, i.e. the Rohloff is 7mm outside the standard?

Probably the spindle is centered. So if I want the drive side to stick out an extra 7 mm then I need a spindle that is 14 mm longer... well, if I would be using the 100 bcd ring then I wouldn't need the full 7mm... maybe just 4mm.. so maybe I want a 118 mm spindle or thereabouts?

I have no plan to swap anything at this point. But I am thinking about buying something to have it in house in case of failure. Anyway if anybody sees a flaw in my reasoning.. I would appreciate any tips!

[JimK]

I should collect the previous posts on this question!

http://www.thorncycles.co.uk/forums/index.php?topic=3898.0

[Andre Jute]

Lot of speculation in that thread, Jim, as we all stumbled (very agreeably though) our way to enlightenment. The post you want is this one: http://www.thorncycles.co.uk/forums/index.php?topic=3898.msg19603#msg19603

If you measured right and you're honestly 6mm out, you don't want components on standby, you want to act and soon, before you have to change the entire drivetrain.

A Rohloff chain is supposed to run straight to within 1mm. Mine is less than half a mil out. You make fine adjustments with thin shims behind the bottom bracket lockring.

One more point. That XD2 crank is good, and commonly available from about a sixth of the price those boys want. Spa Cycles in the UK will sell you a set of arms for about twenty pounds to which you can then fit a Surly stainless chainring and have money left over to buy several more of the same, Thorn has something with their own brand on it which looks suspiciously like it (which would make sense as Thorn's central philosophy is about excellence at realistic prices), Stronglight packages the same thing as their Impact line (try XXCycles in France for good prices and delivery). Rivbike appears to be pretty pricey.

[JimK]

Thanks, Andre. I can see that the measuring process is a bit tricky so I will have to repeat that a few times & maybe the time has come to shop for some calipers. I expect that coarse adjustment can also come from moving the chainring. Mine is now on the outside. If I just moved it to the middle ring position, seems like that'd go a long way toward straight.

[JimK]

eek! Moving the chainring from outer to inner position seems to move it 10 mm. So I go from about 60 mm to about 50 mm.

[Andre Jute]

eek! Moving the chainring from outer to inner position seems to move it 10 mm. So I go from about 60 mm to about 50 mm.

Looks like you're still committing some measurement error, though, even recalling the several I perpetrated in the same quest, it is difficult to identify. Let's concentrate on what you can do right.

1. Determine the diameter of a reference tube by looking it up or measuring. It can be either the seat tube or the down tube, both of which run down the precise centreline of the bike, parallel to the chainring. It cannot be the chainstay.

2. Many steel rulers used in printing and graphic arts, sold at art and stationery stores, are graduated from zero. Others have zero 3mm or one-eighth inch from the edge. Metal measuring tapes are also graduated from zero and stiff enough for decent accuracy over such a small distance. Measure with the blunt edge of the ruler against the reference tube perpendicularly to the middle of the thickness of the chainring.

3. To this distance add half the diameter of the reference tube. That's your chain line. Now you're not likely to be more than 1mm out.

You don't actually have to go through all this, though it is therapeutic to determine the measurement for your own peace of mind. The purpose of the exercise is to discover the length of the bottom bracket shaft you require to work with a given set of cranks and your Rohloff gearbox. In the case of these Sugino cranks the answer is 118 or 119 plus, perhaps, a collection of shims up to half a mil thick which will certainly bring you within half a mil of a perfect chainline.

[mickeg]

Mine is off by maybe 5mm, intentionally.  I made these comments on a different pair of posts, copied and pasted it here:

I have been building up my Nomad over the past couple months.  For normal use, am using 44t front and 16t rear.  With the tires I currently have on the bike, this gives me a range of 20.2 to 106.4 gear inches.  I normally use a chainguard/bashguard in the outer chainring position and the chainring on inner on a double crankset.  My chainline is off by about 5mm, I did not want my Q factor to be more than about 10 to 12mm wider than on my other bikes, thus I compromised on chainline.

I felt it was more important for me to keep the Q factor similar across bikes so I can find the pedal with my foot to get cleated in when starting from a stop.  I switch bikes often enough that my foot might remember where it was the last time I was on a bike.  In my case the crankset manufacturer recommended a 110mm (square taper) bottom bracket for a normal road bike, I used a 122.5mm.  I have not had any problem yet with my foot landing in the wrong place.

I am not using the stock Thorn crankset, so my spindle length does not compare to yours.

I really think that a small chainline error is not an issue.  Considering that no derailleur bikes will consistently have the right chainline, one with a small but consistent error is likely not any problem.

[JimK]

I got myself a simple caliper to help with the precision. My current estimate with the ring on the outside is 58mm.

Yeah it would be interesting to see how efficiency and wear vary with the chainline offset. 4mm is about 1% over the length of the chainstay. So the chain is veering off at about a half a degree bend as it leaves/joins the ring/sprocket. Certainly a derailleur bike spends a large fraction of the time with an offset above 4 mm.

How long can I expect my bottom bracket to last? How do cranks wear out, anyway? I imagine the square taper interface might get a bit funny from being removed and remounted, over the years. I figure statistically maybe I have 25 years of riding left... maybe one or two bottom brackets and probably the cranks will last the duration.

On the teenager's bike he was struggling with the chainring shifts and getting the chain jammed in there tight. I would remove the granny ring to free the chain. Those granny ring bolts were pretty stuck. I think I pretty much stripped out those holes. So those cranks are probably doomed. If I decide to get that bike running well again, probably I will get some more experience with chainline!

It's funny. Probably it is just jags drooling over the Audax, or maybe on Jan Heine's blog he was talking about the Seven Axiom SL and hey wouldn't you know but my LBS around the corner is a Seven dealer! My Nomad is plenty comfy but really not a zippy bike! So now my therapy is to take out the teenager's Giant OCR 2, 23 mm tires at 100 psi etc. Zippy, yeah. But really I prefer the Nomad!

 

[JimK]

I was reading up a little on chainline in Barnett's Manual. One method it suggests for checking chainline is to hold a straightedge against the chainring & then look at how the chain diverges one way or another along its route to the rear. My chain seems to follow the straight edge quite nicely! How that fits with my measurement of the chainring being 58 mm off the frame center?

Barnett recommends not messing with the chainline unless there are definite symptoms. Too complicated otherwise! Of course the manual is all about derailleur bikes.

[Andre Jute]

Seems likely to me that the writers of a derailleur-rooted book like Barnett's have given up on a precision chainline before they wrote word one. Precision in derailleur chainlines is a minimum of 1.5mm out, because a "straight" chainline is measured to between chaingrings... In any gear derailleur "precision" gets progressively worse.

Rohloff people by contrast start with a concept of precision at precisely 54mm, which is perfectly achievable with a single chainring, and not off by more than 1mm, which is a pretty casual measurement error for intelligent people who like nothing better than a reason to buy tools. (Heh-heh.)

It's a defining difference of outlook, I think.

[JimK]

At this point I seem to have a 4mm measurement discrepancy! I figure if I just struggle with the geometry steadily, by the time my current bottom bracket wears out, maybe I will know what size spindle to get for the next one! Same as the old one, or tweak it up a notch?

Maybe what I really need is just to move my chainring to the middle, which should put it below the 54 mm mark, then get some spacers to pull it within the magic millimeter.

Yeah, tools! Someplace I could swear I have a crank extractor from years ago, but it refuses to resurface. Then a bottom bracket tool!