Hi,

I just received a Dig Dummy frame, and now I have to figure out what I need to build it. I have very little experience working on my bike, so this is going to be quite a learning experience.

I would like to use Surly gear where possible, so was looking at their Mr Whirly Crankset as a good place to start my building adventure.

dwnptrl_777 said “I ordered Whirly single speed crankset and then chose a properly-sized Salsa chainring (within Rolhoff's rec's) to bolt on to the Whirly.”

I thought I would do the same thing as dwnptrl with a Surly Chainring. From the Surly website, these are my Chainring options;

94mm BCD x 30t, 31t, 32t, 33t, 34t, 35t and 36t (works with 5-8-speed chains)

104mm BCD x 32t, 33t, 34t, 35t and 36t (works with 5-8-speed chains)

110mm BCD x 34t, 35t, 36t, 38t (works with 5-8-speed chains)

110mm BCD x 38t*, 39t, 40t, 42t, 44t, 46t, 47t, 48t, 49t and 50t (works with 5-9-speed chains)

130mm BCD x 38t, 39t, 40t, 42t, 44t, 46t, 47t, 48t, 49t and 50t (works with 5-9-speed chains)


However the single-speed Crankset has the following Specs;


33t Surly stainless steel ring on 58/94mm BCD spider. Includes spindle for 68/73 shells. 175mm or 180mm. Black.


I understand that the smallest supported chainring on the Rholoff is 38t.

Am I correct in thinking that I cannot use replace the 33t chainring on the single-speed crankset with a 38t chainring (because the crankset spider is 94mm BCD, and no 38t is available for this spider)?

What the heck is BCD?

So, if I really want a Surly crankset, I need to follow the following advice from the Surly website;

Armsets are available in silver or black and come with all basic assembly hardware. If you buy an armset you’ll still need a Mr. Whirly spider, bearing set, Mr. Whirly spindle, chainring(s), and hardware to mount the ring(s) to the spider.

This being the case, I would elect to buy the following (the ** ** indicates the choice I would make when options are available);


Armsets: 165, 170, 175, 180, & 185mm. Silver or Black. (165mm in Silver only). Forged 7075-T6 aluminum.
Spiders: 58/94mm, 58/104mm, 58/110mm BCD. Silver or Black. Aluminum.
Spindles: Short to fit 68 and 73mm BB shells or long to fit 100mm BB shells. CroMoly.
Bearing Sets: Outboard-style cups and bearings. Includes three 2.5mm spacers. Black or Silver.
Hardware: All small bits are available in case you lose something. Hardened steel or aluminum, depending on application.


Do these look like good choices for a Big Dummy with a Rholoff Speedhub?

Thanks for your advise.

BCD is bolt circle diameter - which tells you where the bolts are located relative to the centre of the spindle.

So yeah if you want Surly products get a 110mm BCD crank set for a 68mm BB shell and the 110mm 38t chain ring. You'll be in business.

So far I've tried a 44T chainring with my Rohloff [stock 16T cog] and I'm now running a 40T chainring. I have the feeling I may not want to go any lower than a 40T. I'm going touring in the mtns with a heavy load this weekend so I'll have a better idea next week.

Good luck with your build...:thumb: