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My bike has a 175mm crank arm (https://www.sram.com/en/truvativ/models/fc-styl-6k-b1) on a Specialized Epic HT (https://www.vitalmtb.com/product/guide/Bikes,3/Specialized/Epic-Hardtail-Expert,27186)

I think the important words are (don't know their meaning): boost, threaded, dub,

Now I want to try shorter arms 170 and even 165. Since I don't know exact length I'd like, I'd like to try cheaper ones if I can, try them first and buy a more expensive one once I decide the arm length.

I read DUB cranks are compatible with any existing BB (https://www.mbr.co.uk/news/product_news/sram-dub-370517). What I want to know is if dub BB is compatible with non-dub cranks? Otherwise my options are pretty limited to SRAM cranks it seems.

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  • Thorn do an infinitely adjustable crank shortener that needs a square taper BB. You may want to consider using a cheap set of ST cranks and BB for trialing lengths. Alternately you can get crank shorteners that work on just the pedal bolt that are less adjustable.
    – mattnz
    Dec 21 '20 at 22:00
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No, DUB cranks are not compatible with any existing bottom bracket.

DUB is the deign of the crank spindle and the bearings in the BB. A dub crank only fits in a dub bottom bracket. It’s possible to get DUB bottom brackets that fit in many different frame bottom bracket shell standards, that may be where the confusion originates.

If you want different cranks you either have to get DUB compatible ones, or replace the bottom bracket along with the crank. There are many different crank and bottom brackets that fit in a threaded bottom bracket frame shell. If you choose to go with non-DUB cranks, you will have to deal with other compatibility issues such as getting the proper crank chainline. You’ll also need some special tools or a repair shop’s help to un and re-install the bottom brackets.

In your position, I would look for a used SRAM DUB crank that is a drop-in replacement for your current one; possibly one in rough condition or missing the chainring that is relatively inexpensive. If you do not like the shorter arms you will be able to sell it for approximately what you purchased it for.

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One aspect of the question that @Argenti didn't cover is Boost. Boost increases the spacing at the front fork (from 100mm to 110mm) and at the rear dropouts (from 142mm to 148mm). This is a mountain bike design standard, which increases wheel strength and allows for bigger tires. It does also move the chainline outward (i.e. away from the bottom bracket). Some discussion of the technical aspects is here.

Admittedly, I primarily ride road, and I am less familiar with the standard. However, it is commonplace on new mountain bikes. I suspect most of the cranks you encounter will be Boost compatible. Several people on this thread on the Mountain Bike Review forum contend that you can run a non-Boost crank on a Boost frame, although there might be some compromises. In any case, you do need to search for DUB Boost cranks, DUB being the spindle diameter (a SRAM standard, with a 28.99mm spindle, the rationale for which is a long story).

The question also asked the meaning of threaded. You do not need to change your bottom bracket if you buy a Boost DUB crank. However, you did ask. Threaded means, in this case, that the BB shell has threads in it. In Specialized's case, they correspond to the BSA thread standard - alternative threaded standards include the T47 oversized threaded standard, Italian threading, and some other niche ones like French that you'd only see on vintage bikes. Italian threading only exists on a very small number of Italian road bikes. I'm not certain that it's ever been offered on MTBs. It persists because some Italian manufacturers make very nice bikes, and they're Italian. An alternative to threaded BBs is press fit BBs (i.e. the cups are pressed into an unthreaded shell), but the shell has to be designed for press fit, and much ink has been spilled on the disadvantages of press fit. Anyway, if you acquired a crank with a different spindle type, e.g. a BB30 or 30mm spindle, or a Shimano 24mm spindle, you'd just change the bottom bracket to something with BSA threads that accepts that spindle type (but this could render your experiment uneconomical). Basically, stick to DUB cranks.

A side note on crank length itself. Humans are adaptable. There is some experimental evidence that our power production is pretty insensitive to crank length. With longer cranks, you may think you have more torque, but your cadence is lower. The reverse is true with shorter cranks. In road and triathlon, shorter cranks may be more conducive to aero positions because they open up your hip angle. However, I suspect most riders don't need to immediately go and change crank lengths. You could put the question to a bike fitter, which is always a worthwhile investment in making you more comfortable and more powerful on the bike. Experimenting with crank length can be expensive, after all.

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