Just made a huge noob mistake by buying a single speed road crank for my SS MTB buildup. I am converting an old rigid frame 90s MTN bike into a SS drop-bar, Townie cruiser and just bought a Sram 300 crank and Surly 18t cog. (Gain ratio 5.12)

Well, I just got schooled by the hipsters at my LBS and now I need a Mountain Bike Single speed Crank. Unfortunately they didn't know much about SS MTBs. My question is:

What MTB SS crank can I find that is most like a road crank? I see that MTB cranks tend to have longer arms and smaller chainrings than roadies so what is my best choice to get close to 48t, 170mm crank arm?
Otherwise, can I fit a 48t chain ring on a SSMTB crank spider?

Thanks,
Any help or suggestions would be very welcomed

what exactly is the problem? I use Shimano 105 cranks in my 80s mtb conversion without issue. you may need a shorter or longer spindle due to chainstay clearance or chainline issues. mtb specific cranks have a smaller bcd but you can get equivalent chainrings for it. still it's unclear what the problem is that these hipsters schooled you on.

Thanks,
I'm not excactly sure, but I just brought it to the bike shop to have it installed and the kid sworking there said that it did not fit because the chain ring was touching the chain stay. They said that the S300 was track crank and I needed a specific SS MTB crank instead.
The S300 uses a GXP bb if that matters.
Specifically, they mentioned the width between the chainstays on a MTB are wider than a road bike, so it needs to be wider to clear the chainstays.

I'm not sure what you mean by spidle, do you mean the axel that is attached to the drive side crank arm? I think that is permenantly attached.

You may be able to get it to work with a longer bottom bracket spindle. If they don't know what that is then don;t take it back to them again.
Also, you may just need a different chainring on the same crank. What's on there now? 53 tooth? Try a 42 tooth or in that range.

Thanks, I did ask about using another BB, but they didn't think it would work. The Crank is 130 BCD and has a 48t chainring. Honestly I didn't even think about a smaller chainring. I will look to see if they make a tall tooth one that will fit. The problem, is that I don't know what size to buy that would fit.
I would be more interested in a different BB if that would work. Can you just pop a longer one on for a gxp setup?

The S300 crank has an integrated spindle. You cannot install a longer one. The smallest chainring you can put on that crank is a 39. If that is still too large and the teeth contact the stays, you will need to replace the crank. Alternatively, if the frame is steel and not dear to your heart, take a hammer to the driveside chainstay to make some more clearance.

Thats what I was afraid of. I guess i could try but not sure I want to buy chainrings to see if they fit. IF not than Im stuck with a crank, bb and extra chainrings... and still need to buy a new MTB crank.
I hope you are kidding about the hammer..



Otherwise.. worst case scenario Does anyone know what would be the best replacement option for a SS MTB crank that would let me keep my new 18t cog and have a relatively close gain ratio? All the ones I am finding have monster long arms and tiny little chainrings. Honestly, after spending my time looking at track cranks, SS MTB cranks look comically small by comparison.

The problem is not the crank but the chainring. If you cannot but a spacer behind the driveside BB cup to move it out a little bit, only a smaller chainring will help. Go to a shop and get them to install the BB (with a spacer) and a 39t ring. If it fits, buy it from them. If it doesn't, offer to trade them the crank you have for one that will fit.

Also, you'll probably need to swap the 18t cog you bought for a 16t or so, to compensate.

Edit: Ask to trade your S300 for a Truvative Stylo OCT 1.1G. Keep it in the Sramily and it should fit better. Alternatively, there are a billion others to choose from that are all worth their salt.

But seriously, consider the hammer if the frame's not worth a ton of money. My city bike wouldn't fit a 39t ring. Now it does.