Recently picked up a folding bike with 53/39 rings up front and an Ultegra 9 speed hub on the rear. I think it's 12/26.

With the 20" wheels I quickly found that I didn't need the 39 tooth ring and I'd prefer to go with a single ring up front so that I can minimize chain drop issues when folded.

So, I removed the front derailleur and took it for a ride using only the 53t ring up front. Everything seems fine but when I go into the tallest gear on the rear the chain line seems pretty extreme. Not enough to pull off the chain, but with the angle I'm hesitant to get on it too hard.

Is there anything I can do to keep the chain line as straight as possible through all 9 of my gears? Or do you just have to accept a harsh angle on the chain with you go with a 1x setup?

If you have chainstay clearance,( quite likely not) you may try mounting the 53 tooth on the inside of the spider. That should put the chainline slightly inboard of the center of the cassette and would improve the situation.
It will never be ideal, but it should be functional.

Thought about that, but it looks like that might lead to a similar issue on the shorter gears, which I'm in more often.

Changing the ring that will be retained to one with non shift gated/lift pinned teeth profiles will help. That a modern double crankset has tooth profiles that make derailing easier to happen means that when used as a single ring set up derailing will still be more likely. Short answer- get a true single speed ring. Andy.

To some extent, yes, you need to accept a steep chain angle on a 1x setup. And those angles can be especially sharp on a bike with short chain stays. To minimize extreme angles, the only thing you can do is adjust your chainline to position your chainring closer to the centerline of the cassette -- maybe by using a different bottom bracket spindle length, maybe by moving the chainring to the opposite side of the spider, maybe by shimming/spacing the chainring.

Since there's no way to avoid running the chain at an angle in a 1x setup -- other than not using the full range of the cassette -- you may want to take other measures to keep the chain from derailing. A single-speed chainring will help. A chain keeper (aka chain guide) can be helpful, too.

First off, thank you. Helpful info.

My immediate concern was that the angle might cause an inordinate amount of wear on the chain, or snap it due to the pressure. Should I be more concerned with the chain dropping? Which has not happened....yet.

Of all of your suggestions, the new bottom bracket seems most ideal to me. But I've never really understood much about spindle lengths besides the fact that they need to match the specific crank you're planning to run. Maybe the Folder community would be a better place to discuss those options.

Chain line concerns were important in the early days of derailleur development. An Italian bike racing manual written in the middle of the 20th century included a diagram showing a drivetrain with two chainrings and four freewheel sprockets. The manual noted that the two inner sprockets were to be used with the inner chainring and the two outer sprockets with the outer chainring.

Myself, I'd check to see whether keeping both rings and simply leaving the chain on the big ring would "minimize chain drop issues when folded."

Yes.

Nominally, the center of the cassette should be aligned with the center of the chain ring(s). When you removed the smaller ring, the center moved outwards a few millimeters. There is not a "standard" spacing for front chain rings; so you'll need to measure what you had. You could purchase a BB that is twice the distance shorter to keep things like they were - for example; if you had 7mm center to center spacing on the chain rings; you might want to move the single ring inwards by 3.5mm; which means you'll need a BB with a 7mm shorter axle than your current ring. This will also move your pedals 7mm closer together.

Now that may NOT be the best idea for you. Analyze which sprockets of the cassette you use the most, for example, if you rarely get to the smallest sprockets, and are mostly on the bigger sprockets, you may want to move your chain ring further in (shorter BB axle) - OR you may want to move your chain ring to the other side, and go with a fractionally longer BB and add a chain guard if you don't have one.

So, I determine spindle length first using something similar to your 3.5/7mm logic, and then I find a crankset that's compatible with that spindle length? Isn't crank compatibility specific to spindle length?

Not sure why 1x would minimize chain drop, but since you've already removed the front mech...

Yes, as mentioned, you could try mounting the big ring on the inside. There might not be enough clearance for that though.

Maybe a 42 or 46 tooth narrow wide chainring would fit?

I was not suggesting that you change your current crankset; only that you might want to change your current BB.

What was the center to center distance of the two chain rings originally installed on your bike? = X

To recenter your single chain ring; you need a BB that is X shorter than your current BB.


Re-centering may or may not be the best plan for you, depending on which gears you use the most.

Given that he has removed the 39T ring, and likes the 53T ring; there is no way he'll like a 42 or 46 ring.

Given the wheel size; the OP might be happier with a 56T ring.

It's a Bike Friday so the rear triangle essentially folds up from underneath. Part of the issue with chain drop when folding is the angle the front derailleur took when folded since it was part of the frame that was not folding. Also, there are chain guides I can buy from the manufacturer that work for a single ring setup that will help keep the chain on the ring.

I think you indirectly answered my question. I thought crank "A" only worked with a predetermined spindle length and only that spindle length. You're indicating that understanding is not true.

Yes. A crank will work with many different BB axle (spindle) lengths. For example, if you have a 130mm OLD hub, and choose a MTB crankset intended for a 135mm OLD hub, you'd want to used a BB that is 5mm shorter than recommended - as long as everything cleared the frame. Another example which Sheldon Brown referred to; if you have your bike set up so that you use the big ring across the whole cassette, and the small ring in just the big (low) cogs at the back, you might want to center the big ring to reduce the maximum angle, with the knowledge that if you tried the small ring - smaller sprocket combinations the chain will hit the big ring for sure.

The recommended BB axle length for a given crank set is a combination that will work for 90+% of riders; but it may not be the optimal for any given rider.

So, I'm currently running a Campag Centaur Power Torque crank that I'll likely be replacing with a Shimano 7700 series using only the single large ring.

I've been looking around and can't find a spindle length for the current Power Torque crank. Guessing that's because it doesn't technically have a "spindle". I did find a chain line of 43.5mm. Can I just double the chain line to get an effective spindle length for a traditional bottom bracket cartridge - 87mm? Or is that not how it works? Seems too narrow.

Not sure if someone said it already, but if you are going to a 1x setup, you need a narrow/wide chainring (with appropriate alignment spacers) to make it work well. Otherwise, get ready for lots of dropped chains.

Don't think anyone has mentioned spacers, but we have been talking about narrower/wider bottom bracket spindles, which I believe would accomplish the same thing as spacers, or the absence thereof.

No, because the location of the chain rings is independent of location of the taper on the cranks. For example the Metropolis 48-36-26 crankset uses a 113mm BB to achieve the same chainline as a Shimano M131 48-38-28 with a 122.5mm BB.

correct.

For either a Powertorque or a Shimano 7700 series crank you cannot change "spindle" length. Chainline won't be any different changing from that Campagnolo crank to that particular Shimano crank

The 7700 takes a cartridge bb. I can't just find one that has a narrower spindle?