When we conduct street interviews on Tanatos, the ultimate street MTB frame from our very own Libido Bike Co., we hear voices like these:
- “What’s street mountain biking anyway? I’m only interested in pump tracks, dirt jumps, slope style and skate parks, but not that one.”
- “They say the top and down tubes are triple butted and the whole frame is post-weld heat-treated to ensure optimal strength and toughness to be expected from chro-moly, right? Yet, the 2.5 kg weight of the frame is way sinful from my lightweight equipment fundamentalist’s viewpoint and thus to be persecuted.”
- “Yeah, that one. That stuff with all the nightmares when it comes down to parts to assemble on, huh? Especially, the BB and crankarm area are designed with BMX parts in mind, but I’m a MTBer so I’m clueless.”
Certainly. Let me explain.
- Street riding is literally where you jump off stairs, ride walls and so forth in the streetscape. Key points and features in equipment design are almost perfectly identical to those for pump, dirt, slope style and park. Spot on.
- Sorry, that was a design error. If only we designed it to weigh around 2 kg just like other aluminum frames intended for similar use, folks would have snapped the frames in several months to several years just as in other companies’ frames when ridden on the street, making us richer thanks to replacement demands. However, many who have had a chance to hop on ours have said “light and agile,” while none has said “heavy.”
- We’ve got great news for you today. The Shimano cranks fit.
Yes, you heard me right. Tanatos can actually accommodate MTB cranks made by Shimano. That also means there are rooms for you to try and fit other stuff like RaceFace as long as the spindle size is the same 24mm. Let’s take a look at some photos from our installation test.
Here are the parts required and points to note for the setup:
- Grab a 6805-series BB bearing set (= basically Shimano BB minus cups) for the bottom bracket
- 24mm ID spacers are needed for crank position adjustment
- Use a “Chain Line 3mm Outboard (2×11-speed)” crankset, a.k.a. Boost-compatible model, available in XT and SLX component series
- Use the original 24T or 26T front sprocket
- Choose any rear sprocket, typically 11 to 12T
Now let’s cover all the details.
#1, BB bearings.
Though you could hammer the bearings out of threaded Shimano BBs, there are tons of identical bearings w/ ID sleeves sold after market, so why not get them? If unsure what fits, whisper these magical words: “for Trek’s BB90.”
On the right you see the bearings with 24mm ID sleeves I got for this.
The plastic sleeves had a flange larger in diameter than the bearing, which need to be trimmed. To any size smaller than bearing OD. There seems to be a design intent to add extra sealing, but it’s just optional. The definitive sealability is ensured by the bearing itself.
Thus, there is no need to fine-trim it like this. I just wanted to, being hyped for the demo assembly this time. Then, press in the bearings and you’re done. Much easier than BMX-type BBs where you need to insert a metal sleeve between the bearings.
#2, spacers.
You will also need some 24mm ID spacers, which, again, you can just buy anywhere, whether being genuine Shimano parts or aftermarket. Your BB kit may also include some. You’ll need about 10mm thickness in total. If you’re a handy type, you could chop some 24 ID piping into spacers, turn stuff on the lathe, or even find 15/16″ washers that fit. By the way, 15/16″ may sound like but not really is such an odd number: you’ll find it in most BMX sprocket bores as well as brake master cylinders on some sports cars like 32 skyline, JFYI.
Sorry for the messy notes, as I was juggling between Boost-compatible and non-Boost cranksets. Anyhow, the final spacer setup was 7mm on the right and 2.5mm on the left. Though slightly dependent on individual frames, these are about the right numbers that ensure sprocket-to-frame clearance, correct chain line, equal Q-factors between right and left, and maximum engagement of the left crank arm onto the spindle. Which means, you can add a few millimeters worth of spacers if desired.
You get this much clearance with a 26T. As bike frames and parts deflect under stress, you’ll need at least about 1mm gap here. Currently it’s around 1.2mm.
Chain line measures at 49mm, matching that of 50mm at the hub. Roughly put, larger differences like 3mm or 5mm may (depending on the chain and sprocket) cause issues. Although, some people ride with like 10mm offset, and no instant failure is likely. That 1mm here is, from the bike engineering point of view, within the tolerance range of zero.
Then #3, the entree, the crankset.
B is your lucky letter of the day. It means it’s dedicated by the global giant Shimano to Li”B”ido Bikes.
This Boost specification is essential, as sprocket/frame interference will be an issue otherwise. If you avoid it by adding more spacers, you’ll run short of the engagement margin on the left crank. So just stick to the gift of “B.”
That’s it.
I kind of over-elaborated on it for the fear of misunderstandings, but it’s nothing more than a breeze for pro shop mechanics to assemble Shimano cranks on our frames.
*More unnecessary extra information from here on*
So for the black magic approach to just make it work, you’re all set. In case you had more whys and hows in your curious mind, I’m addressing further details below.
The keys here are BB standards and bearing compatibility.
Let’s start from basic industrial knowledge. Industrial bearings are standardized respectively for metric and imperial series with several selected ID and OD sizing. Application-specific standards like bike bearings may come with larger or smaller ID than normal, but still are based on some standard sizes in most cases. Among such standard OD sizes is 37mm, which applies to both “Spanish” BB bearings employed on Tanatos and Shimano’s Hollowtech 2 bearings, albeit the different IDs.
Now about BB standards on frames that accommodate them. I actually learned recently that the BB shell I designed for Tanatos is quite similar to the “BB90” standard from Trek.
Here’s my take on what BB90 is. It’s an integrated BB shell combining a threaded shell and outboard bearing cups. Historically, 68/73mm threaded shells were the golden standard for some time, which housed loose-ball bearings inside. However, through the transition from skinny, square tapered spindles to the fat and hollow ones for the sake of greater rigidity marketed by Shimano, the bearings were eventually chased out of the shell and found home outboard. The Hollowtech 2 era comes. In terms of bearing placement, it’s similar to traditional headsets, despite the difference of press-in and thread-in cups. Now, someone may have thought, “why not integrate the external bearing housing part into the frame as in the recent headtube innovations?” and voila, that renders about the exact dimensions of BB90 on the drawing board.
The BB90 and the “Spanish +” on Tanatos share the same 90mm shell width and 37mm bearing OD, with the only difference being the press-in depths. 7mm-wide 6805 bearings are used for BB90, while we use 9mm-wide 6904-based Spanish bearings. BB90 features flush mounting, while we press bearings 10mm in (1mm recess from shell edge), so there is just 3mm difference on either side if 6805 is pressed into our shell. Now, it’s so simple – just add spacers.
There also is an opposite hack called “Shimanish BB” where you press 6904-series bearings onto Shimano’s outboard BB cups to accommodate BMX cranksets. Either way, parts selection is key to success, in such manner as to ensure proper chain line, Q-factor, etc. For Shimanish, BB spindle length is probably the biggest requirement. For our case, front sprocket/frame interference was the highest hurdle, which just became solvable thanks to the release of the Boost-compatible cranks. I really appreciate all the requests, hints and information I have received from people that led to this fitting test.
The parts needed for this setup, like the BB bearings, are not our regular stock. However, there may be cases we can help you with if procurement is too difficult for you. Please don’t hesitate to ask.
Happy press fitting, everyone!