# Rocky Mountain Microdrive 28T cranks BCD?



## nobody special (Jun 21, 2019)

We recently bought a 2020 Growler Jr 24 for my daughter which has the 155mm cranks and 28T front chain ring. My daughter is short, and I need to swap out the cranks for shorter, probably 130mm or 127mm cranks depending on what I can find.

What is the BCD of the Microdrive cranks? I measured with calipers and they look to be 76mm BCD but I want to make sure I'm not missing something.

Anyone know if it ISO taper or JIS taper? It is a seal cartridge bearing bottom bracket.

Thanks.


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## svinyard (Aug 14, 2017)

nobody special said:


> We recently bought a 2020 Growler Jr 24 for my daughter which has the 155mm cranks and 28T front chain ring. My daughter is short, and I need to swap out the cranks for shorter, probably 130mm or 127mm cranks depending on what I can find.
> 
> What is the BCD of the Microdrive cranks? I measured with calipers and they look to be 76mm BCD but I want to make sure I'm not missing something.
> 
> ...


Just email rocky at [email protected]

Matt or someone else there will dial you in. I traded a zillion emails with them when building my RM Instinct and they were fairly quick to get back to me (usually a day or so).


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## nobody special (Jun 21, 2019)

I actually did contact them this morning but last time I contacted them (couple weeks ago) it took a while for them to get back. I think they are just under staffed right now.

I'm pretty sure it is 76mm and I'm pretty sure I could use a GPX spider to Brood DM cranks and make it work with our existing chain ring. Not sure if the Trailcraft cranks will have the correct offset at 6mm. I'd really like to just use the BMX cranks but the 104 BCD is going to be too large from what I've measured. I ordered some longer bolts and 5mm chainring spacers this afternoon to empirically fix the backpedal drops before I order the expensive cranks though. Now I guess I'm just rambling.

Thanks


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## nobody special (Jun 21, 2019)

RM replied quickly this time. Maybe they read this forum?  Anyway it is JIS taper and 76 BCD but they said it is not the SRAM 76 BCD found in the GXP spiders.

Thanks


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## svinyard (Aug 14, 2017)

Nice, I always find the BB stuff overly complex so its certainly not a bad idea to figure out the details. What are you going to install on there?


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## nobody special (Jun 21, 2019)

Taken a little while but I ordered the square taper 127mm Brood cranks from Spawn. They are a 104 BCD and come with a 32T chainring. I will install and see how far I can inset with spacers. I will try to run this setup for a bit and see if the backpedal issues continue or improve. The cranks are supposed to arrive today.

I'd like to keep the 32T instead of the 28T because my daughter is never using the 42T rear cog, and she is slow on flat ground. I'm planning on going to a 1x10 later anyway because the Sunrace shifter that came with the bike is pretty lousy and the Altus that the bike came with is only rated to 36T but RM installed a 42T cassette, and it was hitting the idler on the 42T cog.


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