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Ok I have a number of variables at play here and I'm trying to obtain the best chain line possible this is on my polo bike so not super important but if there is a way I can do it I'm willing to put the effort into it.

Being a polo bike it is treated as a single speed and with my setup I've strived to created a more functional polo bike with a 2 cog freewheel. Hence I can ride my bike to the court on my commuter gear, take a 15mm wrench out of my bag slam the wheel and use my polo gear on the same freewheel.

rear hub is a Primo son of a bomb which I believe is 120mm spaced with a 2 speed suntour freewheel.

My rear track ends are 135mm.

Up front is a deore triple crank with a rageface 32tooth (74BCD) chain ring on the utmost inner ring. I had to space this out away from the crank spider with 2mm chain bolt spacers to stop the master link rubbing. The chainring as about 5-7mm of space between it and the chainstay.

Current BB (unsure)

So with all these metrics is it possible for me to workout a BB length and rear spacer set to get a straight chainline?

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`Taint no such thing as an "optimum" chainline for any derailleur-style setup. You just strive for it to be not as bad is it might be. –  Daniel R Hicks Apr 7 at 17:13
    
Ah Daniel this is a single speed setup. No derailleur or tensioner. I will edit the question to reflect it. –  Will Apr 7 at 17:15
    
OK, I didn't pick that up. Then "optimum" is obviously where front and rear cogs are exactly in line. But note that the setup can easily handle 5mm or so of offset without noticeable problems. –  Daniel R Hicks Apr 7 at 17:18
    
Yeah that's pretty much it. I wondering if taking all these variables there is a mathematical means of calculating a straight chain-line –  Will Apr 7 at 17:44
    
I think it's too hard to figure out all the variables, especially just based on description. But note that if you have adjustable cups you can usually offset the cups about 2mm one direction or the other. –  Daniel R Hicks Apr 7 at 17:45

1 Answer 1

up vote 2 down vote accepted

We can make this easy... Lets assume your frame is totally straight and not bent in anyway.

Measure front chainline. Chainline is the distance the chain is from the bike's centerline. You will need an inexpensive machinist's scale . A machinist scale is just a small 6" long ruler. On a machinist's scale the zero mark is exactly at the edge of the scale.

Front Chainline

Measure the front chainline from the edge of the seat tube. To that number add the radius of the seat tube. My seat tube is 28mm in diameter, half of that is the radius. My front chainline is at 14 + 34 = 48mm

Rear Chainline

For the rear chainline you measure from the inner edge of the dropout to the gear. You know the width of your rear hub (135mm) and you subtract the measurement. (oops.. my rear hubs are 135mm yours are 120mm, right? Obviously you should use your measurements! ) In my case, the chainline to that center gear is 135/2 - 24.5mm (from photo) = 43mm Remember the chainline is the distance between the chain and the CENTER of the bicycle.

Obviously my bike is using a derailleur so I'm not freaking out about a slight mismatch.

Does that help? If you know where you are, you can then make changes to the front crankset with shims to adjust chainline a bit. Depending on which hubs you use, you can do some spacing work at the rear as well. Easiest combo is a multigear hub used for a single speed with a Surly hub spacer kit. That makes it all easy.

That cheap machinist scale takes all the guesswork out of the conversation. Do note, because of how cranksets mate to the crankarm and bottom bracket, its almost impossible to pre-determine chainline from parts sitting on a table. You have to install them on a bike to the correct installation torque, then do your measurements, then take it all apart to make adjustments as necessary.

And Sheldon Brown, although he has passed away now (sigh), is still your friend.

http://sheldonbrown.com/chainline.html

One more comment: Are you really trying to use a 120mm wide rear hub on a 135mm frame? I don't think that's a clever idea. 5mm off, okay. That far off, you risk bending the frame. Yes, you could convert that 120mm wide rear hub to an effective 135 with spacers and a long axle. If you are going to do that please purchase SOLID axles intended for a 135 hub. With the loading from a 120mm wide hub a hollow skewer axle probably won't cut it.

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Good answer! And nice to see photos. –  Mσᶎ Apr 8 at 22:22
    
This is what I was after, it's a solid long axle hub (BMX) which I have a few spacers on there already. Bare in mind this is a polo bike so will no doubt break at some point. Although I'm sure the Alu can do without the additional stress. –  Will Apr 9 at 16:24

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