Shinkers

Quick question:

On older 3 piece crank/bb setups, what dictates your chain line? The spindle length, the crank, or both?

Basically, if an mtb is spec'd with 'xx' chain line using 'xx' bb and a mountain double crank. If I put say a road double on it, would chainline be the same or different?

Thanks.

Darth Lefty 

43.5 for doubles. Triple is usually a little wider than double to offset the rings right and put the middle ring in the best location. Mountain bike chain line is usually a little wider than road to match the wider rear. It's not perfectly precise for a square taper BB because the taper and the torque on the crank bolt might not be exactly the same from one set of parts to another. If you want to, you can move it inward with a shorter BB. You need to make sure the rings don't hit the frame but since they're smaller they may not. It's not especially crucial on a derailleur bike. Rings have stayed at about the same location over the years while the spacing at the rear has changed a lot to allow for more speeds.

Here's a good article Chainline Concepts | Park Tool

This page has a lot of info for individual crankset models though it's been headed out of date
Sheldon Brown's Bottom Bracket Size Database

fietsbob

In General, Measure... It's a line parallel to the center line of the Bike .. the rear cluster of cogs, be it a cassette a freewheel , or just 1 cog has a center line .

how far is it from the center line running thru the bike ?

Odd number of cogs, its the middle one, even number, its the space between the 2 most centered ones .

carry that line, parallel to CL, forward to the crank, it's again the middle cog on a triple , or the space between the 2 chainrings on a double .


so middle chainring of a triple is in line with the middle, say 3rd of 5 cogs on a freewheel.

chain flexes sideways an equal amount to the 1st and 5th cog .. because of that alignment.





...

bulldog1935

you always shoot for the same chainline, so changing from a triple to a road double requires a bottom bracket swap as well to maintain the same chainline.

Slash5

Both. A specific crank has a recommended BB width to achieve the normal chainline. Older cranks had quite wide BBs. Modern cranks typically use narrower BBs. You should know the model of the crank and you can lookup the BB width. Shimano has a doc, some others can be harder to find.

Shinkers

Think I have it figured out.

Thanks guys.

Bill Kapaun

The idea behind a chain line is to have every thing "centered" on average, so the chain runs straight.
Since this only occurs on 0 to 3 possible ring/cog combinations, don't obsess too much if you're a couple mm off of "perfect".
I actually run a slightly wider chain line because I tend to use the outer (smaller) cogs more than the inner cogs.

Retro Grouch 

Think about it this way:

Look at your rear wheel. The cassette is located by the locknut and the right dropout. Now direct your attention inward and measure from the center of your cassette to the centerline of your bicycle's main frame. That's your ideal chainline.

If you have a choice, choose a crankset and BB spindle length that comes close but I wouldn't obsess if it turns out to be a few mm off.

FBinNY 

[QUOTE=Shinkers;19244616]Quick question:

On older 3 piece crank/bb setups, what dictates your chain line? The spindle length, the crank, or both?

....QUOTE]

Both.

Obviously a longer spindle will push the crank out, but that's only half the story.

Over the years they've moved the hub of the crank arm inward compared to the chainring position. This was done to shorten spindles and make for a stronger, stiffer, yet lighter system.

Unfortunately, that means that if you are mixing spindles and cranks, you risk the chainring position (chainline) being more inboard or outboard than you expect.

I wish there were a simple way short of trial and error to predict chainline of non-spec combinations, but so far not. Yeas ago when this started, I cut a worn spindle in half and mounted it on a base. Then I could mount a crank and measure from chainring to table, and use that to know which spindle would put the rings where I wanted them.

Short of that, simply push the crank onto a spindle, measure chainline, then add/subtract spindle RH length to have it end up on target.