Hey all, so I’m trying to draft up my dream MTB (for a casual), and I’m looking at some unusual preferences:
very low (sub 160) Q factor for riding comfort. This leads to a possible need of a 68mm BB shell - which, on its own may not be that problematic but will be exarcerbated if I want to make the shell 38mm wide and tacked on a super tight 3D printed yoke and potentially focus the stress as well.
if I were to follow up on the above and get that very niche yoke (85mm inner width, 123mm outer width), my goals would be: 49mm 1x chainline, 132mm between cranks as in White R30. This is very tight but then my 2nd choice might even be worse: Square tapered BB and crank for a low Q
the last but most flexible is tubing choice. I should be fine with a DZB 34.9 downtube 1.1-9-6-9, but would 28.6 8-5-8 top tube be too fragile? My top tube is ~54, DT ~67cm long.
I’m 130lbs, dont ride too heavy but will do some light-moderate drops with the help of a 100mm suspension fork.
Sot to recap, I’m asking for opinions on 3 questionable ideas for a hardtail
1/ 68mm , 38mm wide BB
2/ Squared taper BB + cranks
3/ Lighter tubing
But I’m happy to hear and learn what are the other no-gos from everyone as well. Thanks!
By “38mm wide” you mean the diameter of the BB shell? These are usually 38.1mm, or sometimes 40mm. Either way I think there’s more than enough wall thickness there if you have a BSA bottom bracket (it will be 1.55mm in the 38.1mm case).
I think that TT sounds fine for non-professional use. I used .8/.5/.8 TT and DT on a hard-tail I built (Columbus Zona, which were available at the time in MTB-appropriate diameters and lengths, but with moderate wall thickness). The owner has been crashing it over rocky paths in Scotland and the north of England for the last few years without any issues.
I also think square taper is perfectly fine. We never had any issues with that. Larger diameter axle standards like BB30 were originally invented by Cannondale because they were obsessed with using aluminium for everything, and now it’s all about carbon fibre.
Yes! Sorry my head blanked out trying to describe it…
//
And one more questions: for the typically available chainstays and seatstays where the small ends that meet the rear dropout are around 12mm, is that something to be concerned about?
I have read somewhere that (from @Daniel_Y maybe) that where the stays ended are the most common point of failure.
At your weight and riding style etc. I would easily drop the DT to a 35 9/6/9 tube and be perfectly comfortable with that. 32 8/5/8 TT…..but that would be my choice.
the 12mm dia of the end of seat and chain stays is fine. It’s how you fabricate and design the connection to the front end of drop outs. Don’t design sharp corners or big abrupt changes in material thickness.
I have read somewhere that (from @Daniel_Y maybe) that where the stays ended are the most common point of failure.
That’s news to me! But @Daniel_Y will have more experience of such things. Usually that’s all dictated for me by where I want any bend (on a pre-bent chainstay) to go, and what pre-made dropouts I’m using. IIRC you usually end up with a bit more diameter than 12mm on typical MTB style stays, especially if using dropouts with a built-in disk caliper mount as those are quite long.
Thank you! I’m debating to stick with 28.6 8/5/8 vs beef up its diameter or thickness…
Maybe I can find similar bikes to try out before deciding
Yeah I’m not sure where I would end up cause the stays I’m looking at does not indicate how are the segments distribute…By the way, is there pros/cons of having a bigger flat mount dropout vs extending the CS and tackle the brake mount on instead? I guess the former saves some weight?
I think the main reason to put the flat mount in the dropout is just convenience. Then you have the mount in exactly the right place.
I’ve used dropouts with the mount in them for a few post-mount frames (and in one case I converted a post-mount dropout into a flat-mount one because it was what I had
I think the next time I make a flat-mount frame I will still get the dropouts with the mounts in them from Ceeway. I don’t like drilling holes in tubes, especially not thin-walled chainstays at the tapered end.
As for the fork, that’s even harder with flat-mount. I handled that by cutting a strip of 5 or 6mm plate and brazing that on. It was thick enough to tap mounting screws into it without needing them to go all the way through into the fork leg.
