Author Topic: Rohloff Chainline  (Read 6623 times)

Matt2matt2002

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Re: Rohloff Chainline
« Reply #15 on: July 18, 2023, 07:46:57 pm »
Interesting subject.

May I ask how folks are gauging the chain line measurement?

A straight metal ruler?

Best

Matt
Never drink and drive. You may hit a bump  and spill your drink

PH

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Re: Rohloff Chainline
« Reply #16 on: July 18, 2023, 09:44:34 pm »
I have a length of angle aluminium I use for all sorts of straight line tasks including verifying chain line.  But I'll set it by measuring the centre of the chainring from the seat tube, I know what it is at the hub, depending in the carrier I'm using, I just need to match that.  It's a good idea to check frame alignment first and that the chainring is completely flat, or you can be chasing your tail. 
Easiest cheat with an EBB bike if it's a couple of mm out is to simply run the insert slightly offset in the shell. There are people who say they can notice this at the pedals, I'm not one of them.

martinf

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Re: Rohloff Chainline
« Reply #17 on: July 19, 2023, 05:27:04 am »
May I ask how folks are gauging the chain line measurement?

I don't bother measuring chainline, I just try and align the chainring and sprocket as accurately as I can.

To do this I generally use a 1-metre builders straight edge in rectangular section aluminium.

Placed on the outside of the chainring before putting the chain on. Then I allow for any difference in the thickness of the chainring compared to the sprocket.

Eyeball method without the straight edge is slightly less accurate.

To align sprocket and chainline, several methods:

- changing the order of spacers at the sprocket end (not with Rohloff hubs)
- dished sprocket (not with Rohloff hubs)
- spacers or washers on the rear hub axle (not with Rohloff hubs)

- spacers between chainring and crank
- sliding the bottom bracket eccentric sideways (small adustments on Thorn frames)
- spacers between right hand bottom bracket cup and frame
- different bottom bracket unit if necessary


Andre Jute

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Re: Rohloff Chainline
« Reply #18 on: July 19, 2023, 10:48:13 am »
...Shimano rate their well-regarded Nexus Premium HGB at 50,000km MTBF, and that I managed to wreck two of their boxes within a total of only about 5000km.
I've got nowhere near 50,000 km on the family Nexus 8 Premium hubs.

I'm still an admirer of Shimano and even of the Nexus hubs, despite that experience. There are millions of Nexus hubs running around The Netherlands on commuting and utility service in advanced stages of total neglect. Not many people know this, but you're supposed to soak the gubbins of the Nexus in an oil bath every so often, using a kit that costs almost as much as a complete new Nexus HGB strung into a reasonable quality wheel on Ebay. I can't remember anyone except you who even knew about the requirement and the service kit... Shimano makes good gear and sells it at a reasonable price with a range of qualities and prices offered to suit almost every pocket; I've never seen the point of going beyond Deore except bragging rights.

Andre Jute

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Re: Rohloff Chainline
« Reply #19 on: July 19, 2023, 12:02:41 pm »
May I ask how folks are gauging the chain line measurement?

The boy at the back there will instantly cease asking difficult questions!
 
The first thing is not to tie yourself up in knots trying to calculate everything. There's an example on this board where several people tried to help me in my aspiration of achieving a perfect chainline, with a permanent record of all the blind alleys.

In fact, it is real simple. From Thorn you know the diameter of your seat post. Take half of that and write it down. Don't try to measure from some notional centre of the seat post. The maker of your chainring tells you how thick it is. Write down half that thickness. From Rohloff you know the centre-line of your sprocket, which is the chainline you're trying to achieve. Now with a builder's steel rule, the kind that measures right to the edge of the metal, measure from the drive side edge of the seat post to the outside edge of the chainring. This measurement plus half the seat tube width minus half the chainring thickness is your starting point. the difference between this measure and the sprocket distance from the centre of the frame aka Rohloff's perfect chainline is the amount you want to move your chainring.

If the existing chainline is within 1mm, congratulations, the job is done. Unless you're a time-served mechanic, you're not likely to do better and the possibility of doing worse must be considered.

Under Rohloff's latest advice, even a few millimetres off will do the gearbox no harm, but may wear your sprocket and chainring and chain a bit faster than a perfect chainline.

If you have to adjust it, or want to, to make your components last a bit longer, read Martin's post for ways and means. All that I do differently is the order in which I approach the job, merely for convenience and the least amount of spannering (American: wrenching) for fear of upsetting some other relationship that I may not even know about.

First off, if you have one of the common so-called "compact" cranksets, there is no law that says you need to bolt your single chainring onto a particular set of threaded holes; you can move the chainring in or out a quite big chunk if you need a quite big chunk of realignment. Second adjustment, still fairly big: Rohloff supplies or at least supplied when I bought my Rohloff bike, with every bike a set of chainring spacers. I don't know if Thorn supplied these "free" spacers with the bikes or whether they took the view that they didn't want amateurs messing with a properly set-up bike, but you can buy them. Now you should be two or three millimeters away from a straight chainline, and can call it a day unless you're obsessive about perfection.

If you want to continue, you will need to remove at least the cranks and possibly the bottom bracket. First off, in theory, though I've never seen such a thing, you can buy an asymmetrical bottom bracket with unequal arm lengths to arrange the correct chainline without disturbing the tread-width, what the smart cyclists now call the Q Factor. You may also arrange your bottom bracket asymmetrically in the BB shell on the bike, in the case of your bike inside the eccentric bottom bracket carrier, to arrange a little more or less length of axle showing on the drive side; this would ba a small amount and would unbalance your tread width, which old roadies claim to be sensitive to. (I believe them, not because I'm that sensitive about where I put my number twelves, but because I'm extremely sensitive to putting my hands and arms in positions critical to 1mm in all directions to avoid buying my physio's teenage children a BMW convertible each.) Third, you can use spacers on the axle for fine-tuning. These are just washers of warning thicknesses, technically called shims. I used, from memory, a 1.6mm (what I had v. what was required for perfection) to make my final adjustment, plus a 2mm on the other side to obtain clearance from the frame for said size twelves, necessary because I cycle in street clothes including sturdy street shoes, and don't want to damage the irreplaceable heritage paintwork on my bike. I happened to have a can of spray paint that matched my bike colour (non-RAL "forest green", a custom colour mixed in The Netherlands, so I used that on the spacers to make them invisible, but plain matt black will also do the job.

That brought my chainline to less than 1mm from perfectly straight -- or even zero offset, actually perfectly straight, because 1mm is probably within the margin of measuring error. You'd need sophisticated fixtures bolted to the bike frame to make manual measurements down to say half a millimetre on a bike, and I'm not that obsessive. After that, you're into lasers and diminishing returns.

It's the sort of job you do only once unless you churn components all the time. The process last happened when I converted the bike to a bottom bracket shell-mounted central electric motor, probably a decade ago, so E&OE.



mickeg

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Re: Rohloff Chainline
« Reply #20 on: July 19, 2023, 02:39:58 pm »
...
May I ask how folks are gauging the chain line measurement?

A straight metal ruler?
...

When I built up my Nomad, Mk II, used a calculator to calculate what the chainline would be based on the crankset manufacturer for their recommendation for bottom bracket spindle length (square taper crank) and based on Rohloff data for hub sprocket position.

I changed the bottom bracket spindle length from recommended to change (reduce) Q factor, and included that in the calculations.

And once built up, I think I put a short straight edge on the sprocket that was not long enough to reach the chainring, but a short straight edge increased my ability to sight the chainline better than simply looking at a sprocket.

Since I knew I would have some offset from the ideal chainline, I was only trying to estimate how much that offset is, and sighting it pretty much confirmed what my calculator said.

In the first two photos in the second post in this thread, I am showing the difference in wear on the sides of the sprocket teeth from my chainline offset which I think is about 5mm.  It is clear that the side of the sprocket tooth has additional wear from the chainline offset.

And if the crankset manufacturer used a common spindle length based on availability for their suggestion instead of the perfect chain line spindle length, there could be some additional error in my calculations.

With this much discussion on this topic, perhaps next time I have the chain off, I should use a longer straight edge?  Since I am using a double crank, I made some assumptions in my calculations for spindle length, I am using the inner position on the crankset for the chain.

In this post, I used the word offset for how far my chainline is different from ideal, but previously I used the word error.  Reason for my wording change is the word error could also mean I made a calculation error or some other unintended mistake.  Thus, I chose to reduce the ambiguity here.

JohnR

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Re: Rohloff Chainline
« Reply #21 on: July 19, 2023, 05:31:22 pm »
There's a useful table at https://www.firstcomponents.com/wp-content/uploads/2018/10/DX30-assembly.pdf which shows how dimension C1 (the drive side) varies in length with different bottom bracket lengths. They aren't quite symmetrical. This information is useful if wanting a different chainline.

As a starting place, the middle ring of a triple crankset is about 47.5mm chainline https://www.sheldonbrown.com/chainline.html and the outer ring is around 7mm further out. In my own case, my right foot is comfortable further out than the left foot so I bought the longest bottom bracket I could find (131.5mm) and put the chainring for the Rohloff on the inside of the spider. However, this resulted in the ring being about 3mm too far in until I used some spacers on the bottom bracket (placed between the frame and the drive side bearing cup) to move the BB across. One can't use spacers to move the chainline inwards.