Monday, February 06, 2023

Bicycle Chain Line Nerd-Out

 Last week in my post about the upcoming Singular Gryphon Mk3 build, I received a couple of comments regarding why a triple chain ring crank set would not work on that frame if a double crank set might. The thinking there was that if there is room for two, why not three? 

That's a great question, and I tried my best in the comments to take a stab at answering that. However; there are two problems with that. One: With no visual aids, it gets difficult very quickly to communicate ideas. Two: My answer was in the comments, which means not very many people would see that discussion nor get any benefit from it. Since two folks were wondering what the deal was, I figured perhaps many more of you were as well. So, today I am going to cover this problem and I have some visual aids and extra reading sources for you to explore, should you want a further dive into this subject. 

The area around your bicycle's bottom bracket is a very complex, busy area that has a lot going on in terms of engineering, physics, and mechanically also. It is a lot more than meets the eye. Our problem that we are going to tackle today is one that is confusing for some folks and a mystery to many- That being how one determines what size chain rings work on a given frame. While there are recommendations for chain ring sizing for many bikes, not all examples have that resource. You can check this out for yourself at home with a metric ruler and, if you really want to get nerdy, a plumb bob. But you should be able to do this with a ruler and a good eye.

Illustration from Australian publication, "RideOn"

Chain Line: The big determining factor in sizing chain rings is what is called "chain line", or as it is often shown all run together as "chainline". Chainline is the distance between your bicycle frame's centerline vertically and an imaginary line which runs parallel to that through the centerline of the rear cassette and crank arm chain rings. This helps designers of bicycles and drive train gear to match up the best, optimal shifting drive train with clearances for frame members and rear tires. 

In the illustration above note how the chainline for a triple crank centers on the middle chain ring while the chainline for a double ring crank set falls in between the two rings. You can also think of this as the "space" that a triple carnkset takes up which a double crankset fits within with room to spare because both sets of rings are centered on the chainline. In effect, this double crank set's inner ring actually sits slightly further from the bicycle's centerline than a triple carnkset's granny/inner ring does. That's why a triple might not fit a frame, but a double may fit that same frame. 

Keep in mind also that the bottom bracket spindle measurement is fixed in length and is not changeable. You might  be able to offset your crank a bit using spacers, but this is likely minimal if a possibility at all these days. It used to be that chain lines were adjustable with the use of certain bottom brackets (Phil Wood) or by using different offset length spindles on the old, serviceable bottom brackets, but those days are long gone now. 

Chainline affects three critical areas on a bicycle

The first illustration I used is a bit misleading because nowadays bicycles are cramming a lot more tire into a space that has a shorter distance from the cranks to the rear axle than ever before. The second image here is a bit better look at what most designers are dealing with today. A big tire, a shorter rear/center distance, and drive train constraints which have to all work together in harmony. I've numbered three critical places that can cause issues if everything (all three things) don't work in harmony or within predetermined specs set up by the manufacturers. 

1: Chainring/chainstay clearances. Remembering that our bottom bracket spindle length is fixed, and that is pretty much not a thing we can offset much one way (side-to-side) or at all. So, we have to keep chain ring sizes to a certain range so the frame's chainstay has room to pass the chain rings and the rear tire. This, more than anything else, limits chain ring sizes, and in many cases today, limits you to one chain ring only

2: Tire Clearances: Riders today are wanting to slam in the biggest rubber that they can, and even road bike riders are looking at 30+ mm tires these days. Well, you have to allow, by dictate of testing protocol and CPSC rules, 5mm of tire clearance, at minimum, between frame embers and tires. This is a limitation any brand that wants certification and testing in the marketplace has to adhere to. Custom frame makers? Not so much, but the good ones will still hold to this standard. Keeping things to a 5mm clearance limits chain ring possibilities along with tire sizes. 

3: Crank Arm Clearances: The last thing we need to understand is that you cannot just flare out the chain stays on either side to get around tires because your cranks have to have room to spin by frame members as well. Additionally, your feet are swinging around in circles down here also. That only adds to the complexity and all of this relates back to chain line and centerline for frames.

In Conclusion: The complexities of frame/chain line/drive train/tire clearance issues is a thorny problem to solve, but amazingly, designers and engineers are working out new solutions all the time. Boost chain line, for instance, which allows MTB bikes to have bigger tires and room for chain ring, frame members, and works with that wider rear hub spacing which also strengthens rear wheels. Dropped chain stays on gravel bikes which is a ploy to keep chain stays short but allow for big tires as well.

But in terms of what drive train bits will and will not work on your bicycle, well that is something you can measure at home. This article by Sheldon Brown is a gold mine for tinkerers or anyone that wants to nerd-out on the subject. 

I hope that helps you all out there who were wondering how crank set/chain ring choices may or may not work on a particular bicycle. Thanks for reading Guitar Ted Productions!


Skidmark said...

Thanks GT, Might you get 2-3mm wiggle room from the eccentric bottom bracket on the Gryphon MK3? RIP Sheldon B. And long live the square taper spindle.

Guitar Ted said...

@Skidmark - Well, you theoretically could move the insert in the frame's shell a bit, but with a fixed crank spindle/crank arm on the left side, you end up running the risk of the crank arm's end striking the chain stay. I would imagine that there is going to be little wiggle room, if any, there on the Gryphon due to the combination of the 73mm wide shell and the capability of a claimed 29" X 3.0" tire. That chain stay is going to have to flare outward at a pretty severe angle.

You'd get greater flexibility in running a square taper set up in this regard, but even then, your gains would be minimal, in my opinion.

We'll see when the frameset gets here.

Derek said...

Your post reminded me of a nice old garage sale Gitane I rode that had an adjustable Mavic bottom bracket. Neat design, sorta "floating" in the bb shell and allowed for a bit of lateral adjustment/ placement and chainline. It required the bb shell to be chamfered 45 deg to properly seat the lockrings, ouch. I imagine you've seen one at some point?

Guitar Ted said...

@derek - I vaguely remember that BB. Probably saw it back when I first started in the trade in the 90's.

Nooge said...

The other thing about pushing the chainline outboard (relative to the designed chainline) is shifting quality will start to degrade really fast with 11 and 12 speed drivetrains. The spacing is so narrow that the extra cross chain travel can cause ghost shifts. This was the motivating factor for wider hub spacing - it allows a wider chainline design.

Guitar Ted said...

@Nooge - Agreed. Every time that Shimano or SRAM add another cog in the same space we've been using for decades, it gets more important to have EVERYTHING within a tight spec window. Chainline is just one of those spec issues. In a way, it makes keeping your chain as straight as possible even more critical than ever. Which makes 1X pretty ironic.