I got a 520 about a little over a year ago and want to replace the standard 50/39/30 tiagra crankset. I've looked at the shimano lx and other cranksets that have 44/32/22, but I want a inner chainring of 20. Are there any cranksets out there or can I buy a 64mm 20 tooth chainring and switch out the 22 tooth?

20? Wow!! Are you planning to climb cliffs? Combined with a 34 cog, that gives you less than 16 gear-inches.

I just did a similar conversion to my wife's touring bike (44/32/22, 11-34). Something you might want to consider when doing the conversion. I believe the Tiagra crankset uses a square-taper bottom bracket, while the Deore LX uses an Octalink type spindle. Regardless of what sytem use, you will most likely need a new bottom bracket to maintain a chainline in the 46-47mm range. I have done several conversions from road cranksets to mtn cranksets and have had to use a shorter bottom bracket spindle in all cases. I think there is more flexibility with the square taper cranks. I have put Sugino cranks on three bikes with a lot less problems mixing and matching than I have with Shimano cranks.

Back to your original question. I can't help there, but you might want to look at what you are gaining (or not) by going to a 20. a 20/34(rear cassette) combination will give you 15.7 gear inches or 3.7 mph at 80rpm. A 22/ 34 will give you 17.2 gear inches or 4.1 mph.

I find the site below extremely helpful whan I am building cassettes, and changing gearing.

https://home.earthlink.net/~mike.sherman/shift.html

I switched to deore lx using the 118 bottom bracket. Had shifting problems with the 105 front derailler. switched to deore derailler and with bar end shifter and works great. Ultegra sts shifter for rear deore was convenient and works for me

For a 20 tooth inner you need to find a 94/58 mm bcd 5 arm crankset. The 4 arm cranks commonly available now are 104/64 bcd cranks and the smallest chainwheel you can get is a 22.

Old 5 arm RaceFace Turbines are 94/58. You might be able to find one on Fleabay

thanks for all the responses. I wanted a very low low gear because I really hurt my knee when climbing 7% grades with a 25 inch gear (I was grinding on the pedals). I have done some research on this, and it's nearly impossible to find a 20th with bcd 64mm, but I did find one site (action tec I think). It sounds much easier to just get the 22 tooth low chainring though.

My tiagra crankset is hollowtech and fits the bottom bracket, and I thought the deore lx uses both octalink and hollowtech? or am I wrong?

Thanks again for all the help!

Tiagra and lx both come in a number of configurations.

This is a good site for Shimano technical info.

http://techdocs.shimano.com/techdocs/index.jsp

What are you running on the cassette? It would be much cheaper to find one with a 34 tooth 1st gear if you are not maxed out already.

I have a 42/32/22 on one bike, 44/32/22 on another and hate them both for general riding. I spend way too much time jumping between the 32 and 44/42 and spin out to early on the down hills.

The crank set up that seems to really work the best for me and piss off weekend roadies on centuries is the 48/36/26. The 36 ring is just a real joy for most of the riding I do. I rarely need to shift up or down on the front. I do want to get a little lower small ring for loaded climbs but it does well enough with the 30tooth on the rear but when I wear that cassette out I will be putting on a new one with a 32 or 34.

You're not going to get anything in Tiagra in 42/32/22 their smallest crankset is 30

Is it possible to build your own configuration in a crank?

Sure. Chain rings are available. The most common size is the 110/74 5 bolt seen on road bikes and many touring. 24 tooth is about as small as they get for the crawling gear but you run that against a 34 on the cassette and you will have a cadence of 100 and have turtles pass you.

Compacts from a mountain bike kind of top out around 44 (that's what my 95 T700 has) and have 20's for the low side. 44 just enough top end to make you mad when you run out of gear on the flats or slight down hill grades.

Places like Harris and Niagara have a good selection of rings. As always Sheldon has some good info on the subject as well. https://www.sheldonbrown.com/harris/chainrings.html


This is my next set of cranks. They are just awesome looking and come in the combination I really like.
https://www.velo-orange.com/suoldlotr.html

I've had my eye on the same crankset.