Saturday, 26 November 2016

Chainline Issues

Preface for this blog
This is a shared post with my other site. I am reposting here because it involves some technical issues that may affect the "practical" cyclist, as well as the "sporting" rider. Indeed, If one is the sort of "sporting" rider that buys a new, ever fancier, bike every couple of years, one might never need to know this.
Bit if one is trying to run a bike cheaply and efficiently for an extended period of time, then it is good to be aware of the issues.

Where it all started - looks like the Incredible Hunk broke my cranks ...

My bike is a bit slow changing into the smallest chainring.
When I have a triple chainring mounted, to get into the smallest ring at the front may require me to get into one of the smallest two rings at the back first.
Changing to the larger rings is fine, it is just trying to get into that small one.

As regular readers will know, my bike is a mix of parts, sourced at a modest cost, usually to replace worn out components.
So I will use my bike as an example of some issues that can occur.

A few years ago, I broke the cranks arms on my chainset. Possibly I have legs like an Olympic sprinter (yeah, right!). More likely, the cheap cranks that came with the cheap bike I was riding were made of low quality materials, and combined with metal fatigue, they simply "failed".

"Emergency" crankset, from my toolbox. A heavy 152mm steel "double" crankset just happened to be in my tool box.
You can see hwo short it is, compared to the busted 170 mm crank shown.
The 152mm set was scavanged off a worn-out cheap "teen" "mountain bike".
May not be classy, but it was good enough to get me to work and back while I sorted out a better solution.
Note the lack of front derailleur, so I am really running this as a 38t single.
 Anyway, time to get a "proper" replacement crankset. Nothing too fancy.
Don't mention the dirt ...
I got a forged alloy triple with 175mm crank arms.
Pinned middle and outer rings,
Changed pretty well, but a bit slow into the smallest ring (48/38/28t rings shown)
After taking three teeth off the middle ring, I fitted a steel middle (from my parts box) and a slightly smaller inner (again steel).
It actually worked OK, but losing three teeth, all at "pin" locations, didn't bode well for the future.

The original alloy 48t, with a steel 34t and a steel 26t. Note the lack of pins on the middle ring.
The arms are the same forged alloy 175mm set.
So now my "triple" is more of a "double plus granny".
Very slow changing into the smallest ring, and sometimes a bit "touch and go".
Changes under load aren't happening at all!
(One learns to change down a bit earlier")
So, the changing is now a bit suspect.
Yes, I have lost the "pins" on the middle ring, but is there more to it than that?
Remember the change into the small ring was a bit slow, even with the "pinned" 38t ring.

Which bring me to chainline.
The triple with the 175mm forged arms is a "road" chainset.
The recommended BB has a 110mm width.
The original set up had a 127mm BB.

Of course, because some crank arms are fairly "flat", and don't stick out much from the cahinset, and some crank arms curve outwards somwhat, the BB width required for a bike varies depending on what crankset is fitted.

But there is also "chainline" - how far out from the mid-line of the bike should the chain be?

A typical modern "road" bike has a width of 130mm between the rear dropouts, while a typical modern "hybrid" or "mountain bike" has a 135mm width. Of course, new mountain bikes and things like tandems are often even wider, while single speed bikes and older bikes built for just a 5-speed rear derailleur are often narrower.

The forged alloy chainset shown in the pictures is a "road" chainset, and I have a "hybrid" bike ...

A quick look at a Shimano spec sheet shows a typical "road" chainline for a "triple" chainset is 45mm (for a 130mm rear wheel width), and a typical "hybrid/MTB" (135mm rear wheel) chainline is 47.5mm.
This is logical, because the rear wheel is 2.5mm wider on each side of the centre, so the chain should be that much further out, to.

The obvious conclusion is that the same 2.5mm needs to be aded to the width of the bottom bracket on both sides.
So I actually should have bought a 115mmwidth bottom bracket, not the recommended 110mm.

If I instead swap in the 127mm BB that I have, the bike changes into the smallest chainring very well, but won't change into the biggest (!)

The root cause of the poor shifting into the smallest ring is thus not likely to be the lack of pins on my middle chainring, but instead that the front derailleur is having trouble moving far enough to the left for a proper gear change.

When I fit the wider (127mm) BB, the root cause of the poor shifting into the biggest ring is that the front derailleur is having trouble moving far enough to the right.

By the way, I did also look up the Shimano spec sheey for my model of front derailleur - it is specified for use with a chainline of 47.5mm, so it is a "hybrid/MTB" item as well.

No doubt there are a range of values for BB width that will work, but I know 110 is to narrow, and 127 is too wide. Of course, if I buy a different model of crank arms, then the "correct" range in which the front derailleur will function properly will be different again.

The moral of the story is to match your BB width to the crankset AND THE FRAME you are using, whatever the manufacturer's recommendations are. Non-standard installations require non-standard answers!

Time to be saving up for a new Bottom Bracket for 2017, methinks ;-)

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