I'm considering purchasing a crankset (edit) . The model I have in mind is available in BCDs of 110mm, 130mm and the 4 arm Shimano Ultegra size.

It is my intention to perhaps experiment with a shorter crank length in future. This is for my recumbent bike so crank arm length tweaking helps.

What is a good BCD size to choose if crank arm variety is a factor?

Also are there crank arms out which can be natively adjusted to different lengths - sort of like those crank arm shortening mods?

https://www.google.com/#q=adjustable+crank+arm+bicycle

The chainring size you'd want is determined by your crankset. Your crankset may have a bolt circle diameter of 110, 130, or something else. Pick the size/type that fits your crankset.

Crank arm length isn't a significant factor when choosing the BCD of a crankset. What is significant is how low you need your gearing to go. A 130 mm BCD can't take a ring smaller than 38 teeth. If you want lower gearing than that, 110 is a common BCD that allows for lower gearing.

you don't have much choice here. If you want to use the ring on your current cranks, it has to match the BCD. hopefully, you can find cranks in the future wit the same BCD, but shoud be a secondary consideration, since the price difference for new cranks with and without rings is very narrow.

The best advice is either buy the ring to use now, and worry about new cranks if/when you buy them. Or, if the plan is yo buy cranks very soon, pass on the ring and move up the crank buy. Sometimes, when you try to kill two birds with one stone, you end up killing neither.

I'm afraid that in my haste to post I've not been very clear. It is is my intention to purchase a crankset (not chainring). Original post corrected now.

At present the bike has a single chainring with crank arms 170mm riveted on. I can't remove the crank arms so they come off as well. All that will remain is the square taper bottom bracket.

It is the replacement crankset that comes with a choice of BCD 110, 130 or 4 arm Shimano Ultegra.

I have not yet chosen the crank arms that will go with it. But for future consideration which BCD would be best if crank arm experimentation is to be done?

PS. I'm the eyeing the Redline Microline 110mm BCD 155mm crank arms as they seem reasonably priced for shorter arms.

You seem to be getting the bolt-circle diameter confused with the crank length. They have nothing to do with each other. Nothing!

The BCD determines the smallest chainrings you can run, and that is a factor in the gearing you want on your bike. Larger BCDs are used for more crank/chainring stiffness.

If the future availability of chainrings in many sizes is the issue, I'd go with 110 which is probably the nearest to a universal pattern with many vendors making rings to fit. The smallest ring that will fit is a 34t. If it's a triple, they generally use a 74mm bcd for the granny.

Origin8 (J&B) has 110 BCD cranks in a variety of crank arm lengths . affordably Priced.. want to experiment with several crank arm lengths , get them in those lengths

Its in the BMX parts sector. . square taper BB style..

I'm not confused but perhaps my original post was not clear.

Basically the same crankset comes in a variety of BCDs. I just need to pick the BCD that matches my crank arms. But I don't have crank arms so I need to buy that as well.

Now I also want to try out a shorter crank arm length. I found a 155mm length model that comes in the 110mm BCD - this would go with the 110mm BCD crankset.

But what if I don't like the 155mm length? Let's say I want to try 160mm. Would I be able to find 160mm crank arms in the 110mm BCD
So my question is really, in which BCD would I find the widest variety of crank arms available.

Hope that makes things clear.

Thanks. Looks interesting. Couldn't find prices. Are they expensive?

Darn those things are expensive. It seems cheaper to simply buy different crank arm lengths and switch them around.

That's better, but I think what you "need" to do still needs work.

Cranksets generally come with chainrings, and are often cheaper that way! So you don't need to be married to a particular BCD. Start your search with cranks in the length you want, 155 and 160mm. That will narrow things down quickly. What is your current gearing, and do you want to maintain it?

You're going about this backwards. First find the cranks you like in the length you like, and then buy the rings to fit it.

A few cranks are made with the chainring arms separate and interchangeable, but the majority are made in one piece with the chainring and crank arm either of a single piece of metal, or permanently attached.

As I said, 110 rings are probably the most common these days, so if you find a crank you like and if you then have an option about the BCD, then opt for 110. But don't let that limit your choices of cranks, which is the more important decision, especially if you need special lengths,

Thanks for the advice. I shall look at the crank carefully then.

Thanks and noted.

My planned gearing is 26 to 85 gear inches. This is alright for the commuting purpose.