Low BB, Short Crank MTB Build Report

I just finished my latest frame that I designed for smoothish higher speed trails. I decided to take the leap and design around short cranks and low BB. I ride single speed, so my plan is to have two bikes, one setup for slower technical trails with an easier gear (current bike), and one for faster smoother trails with a faster gear (this new bike).

The frame stats (unsagged):
HA 66.25°
SA 75°
RC 405-445
FC 815
ETT 670
BB drop 74
BB height 297
120mm SID, 44mm OS

I’m 6’-1" with pretty long legs. I’ve ridden on 180s for the past 20 years and swear by the extra leverage for riding single speed, but with all the chatter about short cranks I decided to try it. I dropped my BB 22mm lower than my current frame from 52mm to 74mm. 165mm cranks make up 15mm difference, then dropped it another 7mm just to embrace the concept of a low BB.

I also reduced my fork travel from 130mm on my current bike to 120mm, and shortened the head tube from 115mm to 110mm. With cranks horizontal my feet are 22mm lower and hands are 15mm lower than my current bike.

I rode today on the type of trails I designed it for and it felt really good, especially balanced and planted in the corners. The short cranks really wind-up fast, which is nice coming out of corners. I definitely feel the reduced leverage on steep climbs, but didn’t have much climbing today. I’m looking forward to riding on the terrain I didn’t design it for to see how it stacks up. I have a feeling I’ll be redesigning my slow/technical bike after some time on this bike.

Other notes:

  • I used Aircraft Spruce .035 x 5/8" for seat and chain stays. I bent them using a Ridgid 358 bender that I bought on eBay for $150. It worked quite well.
  • I used @ben.land101’s yoke design. This is my fourth frame using a version of that yoke, it’s great in all respects.
  • I designed dropouts that give me 40mm of adjustment so I can play with CS length.
  • I still suck and welding, hopefully it won’t break.

Lot’s of valuable forum knowledge went into this frame, which I’m very thankful for!

-Garth

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Looks great! I’m about the same, 6’1", been riding 180s forever. A lower center of gravity is always better. I can get of pedal strikes when I ride aggressively on my current hardtail, maybe it’s time for shorter cranks.

Most dimensions on a bicycle are directly proportional to rider dimensions, except cranks. Tall riders with a 34" inseam have traditionally used 180 cranks, a shorter rider with a 30" inseam will use 165. That’s 100 mm difference in inseam, but only 15 mm in crank length. I think what has been available from manufacturers and conventional wisdom about BB height/drop has limited experimentation with crank length.

Looking forward to seeing the long term evolution of shorter cranks!

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I have a 30" inseam and went from 170mm to 160mm cranks. I live in a very rocky / at times slow tech area and the difference has been amazing. I absolutely love them for punchy climbs. They spin up so fast and I find myself able to accelerate easily.

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Awesome build! Thanks for sharing.

I am digging the 5/8 chainstays. We use 3/4, but honestly, I have no idea if that is too much or too little! The rear triangle is braced with the rear hub really well.

I think there are two parameters being tuned here: BB height and BB drop

  • BB height: lowers your CG. Which is good for stability and cornering, but bad for pedal strikes
  • BB drop: affects how easily your front wheel can lift. Too lilttle BB drop, and the bike lifts off on climbs. Too much and the bike is hard to pump and wheel lift.

BB drop vs BB height is my theory of why people are trending towards mullets on their big downhill and enduro sleds. The smaller back wheel allows you to have a low BB for cornering without making the bike too planted and static which comes with a large BB drop.

Anyways, I think it’s a great bike. The geometry is very well thought out and “makes sense” to me! Curious to see how you feel about the low BB over time after you have adapted to it.

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@wzrd mentioned they use 5/8” on XC bikes, and I was driven by only having the tooling to bend 5/8”.

It seems like the absolute strength and stiffness of the CS would be driven by the ovalized/dimpled/etc pinch point at the chainring. Having a stiff CS section attached to a thin section doesn’t seem to gain much overall stiffness or strength? If it’s going to bend/flex/break it should be at that weakest point. Can printed yokes overcome that weak point with internal structures? My basic understanding of structure is that the overall shape and size is really what drives the flexural strength.

Interesting point about the bike being too planted with a low BB, I’ll pay attention to that while riding.

-Garth

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Update on the low BB, short crank build…

I’ve ridden every type of terrain I can get to:

  • Fast, smoothish, berm corners, well graded climbs, lean turning
  • Slower very technical, punchy hard climbs, slow steer turning
  • Enduro type downhills, chunky with good speed, just trying to hold-on and not crash

The short cranks feel weird right at first if I’ve been on long cranks, but I adapt within minutes. The only time I notice them feeling short is hard slow climbs when my legs are feeling tired. I think it’s more a matter of psychological expectations- my body is still used to the feel of the 180s, and expects that feel. With more riding on the 165s I bet my expectations will adapt, and I won’t notice.

I really like the way the 165s wind-up fast, especially coming out of turns. They also feel like they roll through the dead spot faster. This doesn’t make sense since they’re turning at the same speed as 180s, and technically slower foot speed. The reduced bending in my knees and hips must make it feel easier to push/pull through that dead zone.

All in all, I don’t see any real losses, and possibly some improvement going from 180s to 165s. Of course this is all “feel”, no power measurements, etc. I guess my crank length beliefs have been wrong for the past 20 years?

The real winner is the lower BB allowed by the 165s. This bike has an amazing sweet-spot cornering. It feels so stable and predictable, much more so than my other bike that has the same front and rear centers and a 22mm higher BB. I’ve started two wheel drifting a few times in corners, and the thing maintains an amazing poise.

I haven’t noticed any issue lifting the front end, or any other downsides to the lower BB.

I would say if you want to play with geo, drop that BB and see how it feels!

My goal with this new frame was to have two bikes, the new one tuned for fast riding, and the other for technical riding. Turns out the new one is better for both, which means only one thing: time to make another frame!

-Garth

5 Likes

Thanks for the follow-up report!

I have noticed this too (lower BB gives more cornering confidence), but still haven’t really found a connection with the physics involved. Maybe lower CG = less centripetal force = less force causing you to slide out? At the moment, I chalk it up to cornering magic.

I did a bit of poking, and the deepest “production” bb drop was 65mm: https://sour.bike/en/candy-shop/bikes/crumble/

Your 74mm bbd is quite a bit lower!

Not a bad thing :rofl:. What’s next in the queue then?

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I think it’s pretty much all magic, and we just apply numbers to pretend we know what we’re talking about and make it replicable.

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