Crankarm Chronicles Pt.2

These days crank arm length is a hot topic. I almost avoided it - I've been happily turning over 175s for 35 years and didn't see a need to change. I even did some experiments 20 years ago. But the topic wouldn't disappear, and in the middle of 2023 of course I got curious. So this series of posts covers what I thought I knew before and what I've discovered since then.

People don't want short cranks, do they?

Part 1 of this series covers 35 years of happily riding 175s and even 180s. I tried 170s several times and they never worked out. I was convinced 175s were right for me.

But people kept talking about them. More and more articles praised them. My pal Peter Croce raved about them. I thought I gave them more than a fair shake, but did I miss something?

Framebuilder Matt Appleman was a convert, enough to take manufacturing nice cranks in short lengths into his own hands. Matt believed in them so much he even had research papers to back him up Appleman's comprehesive guide to cranks is a wonder and the place to start if you're curious. . I still tried to be skeptical - the Martin paper he cites shows 145 cranks generating more power than other length, but if you look closely (the asterisk on the 145 and 170 bar) you'll note the improvement over 170s wasn't statistically significant.

Then I saw this video from bike fitters Greg Choat and Chris Balser. One thing that caught my attention in the video is that Balser puts people who might benefit from short cranks on 155s because there's enough of a difference to get people to reset their nervous system. This had me wondering - maybe I needed to try something much smaller. I was also struck by these two fitters' focus on finding a fit that worked for each person's body instead of just adjusting some things to meet some rules. Finding these two led to other things too but that's another article.

So a plan was hatched to try short and shorter. I found a pair of 165mm Race Face cranks from Geartrade on the cheap, but trying something shorter on that didn't seem to have an affordable path, until I found these Goldix cranks on AliExpress. Soon I had 160 and 155 in the mix.

First try was 165s on the trail bike. The felt short at first, but after a few minutes... they sort of disappeared. Unlike the 170s, these didn't seem slower. On a steep jeep road section that is often tough to ride, I cleared it even though conditions were loose. Power wise things didn't seem that different, but it seemed like traction was in my favor. I'd best describe it as making more, less risky bets with each power stroke. Some people might call it "smoother", but it wasn't that I was spinning smooth circles. But there did seem to be something to be said for this size. My first descent with them felt slightly better on tighter turns and it was a little more natural riding switchfoot. But on their second ride with straighter trails I felt a little more unstable with my forward leg back further, a sort of tipping over the toes sensation.

After a couple rides, I dropped to the 160s. These always felt short and I didn't feel like they improved my climbing any further. Mostly the never disappeared and something about them always felt funny. The 155s were the same, but slightly worse. So it seemed like there was some advantages for me to go shorter, but I had already found the limit. I put the 165s back on to keep trying them out.

When all this crank shuffling was going on, my bikepacking bike's ancient 175s were starting to creak so I looked for a replacement for an upcoming trip, and all I had free were 180s, 160s, and 155s. I thought the 160s were too short, so at first I put on the 180s. But these now felt too long after riding the 165s, so my next best choice was 160s.

The bikepacking trip was the annual ride out to the Meteor Shower Campout in the Black Rock Desert. Usually we ride out 14 miles on Saturday morning to the campsite. The next morning there's usually a sunrise ride on the playa and then we ride back out. Less than 50 miles total and zero elevation gain. I figured the cranks would be good enough to get me through the weekend.

This year however I didn't want to hang out in the heat all day so I rode out Saturday after sundown. Pedaling in the dark on a flat dry lake bed there's not much stimulus so I settled in to pedaling. And settle I did. The bike disappeared. My knees just pistoned up and down and it felt strangely effortless. No tiredness in my quads. And to top it all off, I was riding in sandals and it all felt natural and efficient. And other than the first minute or two, the cranks didn't feel short even though the were 15mm shorter than my usual and had felt too short on my trailbike.

There was one other change - my new obsession with functional bike fit led me to discover pelvic rotation and I was riding an SMP saddle. But again the bike fit stuff is another story.

The bikepacking bike had never felt better either. This was because of a design choice that had been bugging me and I was sort of regretting. Since the bike was designed for comfort over distance, I built it with a slack seat tube angle. But right after I built it I started riding steeper and steeper seat tube angles to open up my hip angle, and I was really liking that direction. This left the bikepacker feeling too slack and I was thinking about retiring it for a geometry update. I'm glad I didn't!

However, when you shorten your cranks, it moves your pedal position at 3:00 backwards. This is equivalent to making your seat angle steeper. It was now clear why the cranks felt too short on my trail bike - it made the steep seat angle feel even steeper when pedaling. On the bikepacker the shorter crank put my lead foot in a natural position to generate power at 3:00. Since the cranks were shorter that meant my leg at the top of stroke was lower which gave me the more open hip angle I was looking for. I could even run my seat slightly lower without trouble getting over the top of the stroke. This meant less extension and ankling and the extra stability at my foot felt like I could deliver more power. This also worked great with sandals and a midfoot position. I didn't need a stiff shoe to get ankle extension without fatiguing my foot.

On the next trip, a three day route recon with longer mileage and two mountain passes, the bike continued to amaze me. I could pedal all day and my legs just didn't feel tired. From what I learned from all the bike fit videos, I was now pedaling mostly with my glutes, which are nice big durable muscles. I was so impressed I started eyeing the 155s. I knew they didn't work on the trail bike, but what about my gravel bike? It had a 74 degree seat angle so somewhere in between the bikepacker and the trail bike. This bike also had an SMP so I had plenty of saddle rail to make adjustments.

The first ride felt awful initially. I kept scooting the seat back and by the end of the ride I had a position where the short cranks started to click even though the reach to the bars was getting longer and longer. I swapped in a shorter stem before the second ride and things felt pretty good except for one thing - now the bike handled really weird with me sitting almost on top of the rear axle. Since the bike was built around a steeper seat tube I had picked shorter 420mm chainstays to balance things out. Now the weight distribution was out of whack. I liked the 155s but they weren't going to work out on this frame

I started brainstorming how to get the frame back in balance. I thought about removing the rear triangle and brazing a new one on with longer chainstays. But that also didn't account for the longer front end that also matched the steeper seat tube angle, or the now shorter stem.

What I really wanted was a way to keep my position and weight distribution the same, but how do you do that when you want to make the seat tube slacker to move back on the bike? Then I realized I could do that by flipping my frame of reference. Instead of looking at the bottom bracket as the reference point defining the frame, I started with my seat and bars as the reference. To make the seat tube angle slacker you could also rotate it "back" (counterclockwise from the drive side) by pivoting around the top of the seat post instead. That would mean your body stays in the same place and position, your weight distribution on the wheels stays about the same, and if the bottom bracket moves forwards the same amount you shorten your leg and pedal relationship stays more or less the same during power portion of your pedal stroke.

Now how to move a bottom bracket - I thought about a eccentric bottom brackets - maybe a oversized one with lots of throw? Or could you make a sliding bottom bracket on rails? Then I realized there was an even simpler solution - just add a second bottom bracket in front of the "normal" one.

I had an old retired gravel frame up in the attic, so I pulled it down, put a notch in the down tube in front of the existing bottom bracket and brazed a second one in. Bottom brackets are about 40mm in diameter so the delta was more than the difference in crank length. But on this bike that meant I had the existing seat angle of 74 degrees, and a second choice at 71 degrees. So with some saddle adjustment this gave me a good range between the original position and something like the bikepacker.

What I didn't grasp was that I was adding 40mm to the chainstays which were already long around 450mm. Chainstays getting close to 500mm long just seemed crazy. On paper, it didn't make sense. Cranks almost an inch shorter than I was used to. Chainstays longer than the longest touring bikes. A seat angle fit for a beach cruiser.

Once I got the fit tweaked, it felt fine. No, better than fine. Refined. Civilized. Sitting further back and lower I was balanced with little weight on my hands. I felt small in the wind, like I could curl up in a little ball. It felt like cheating.

One thing I didn't get around to doing when I set the bike up was adjusting the gear range. Physics says with shorter cranks you should switch to a lower gear to keep the force the same. With a derailleur you'll pick the gear that feels right anyway but I anticipated have to switch to a small chainring to keep my low gear feeling the same.

Matt Appleman suggests dropping only half the difference, which he got from Jim Martin. In a podcast interview The podcast "Bikefit Podcast" is defunct but some podcast clients still have it cached. Martin is in episode 9, John Cobb is in episode 11. I was hoping to have time stamps but can't find a player to link to., Martin explained that the joints themselves have variable leverage and with a shorter crank more of your power stroke can sit in the higher leverage zone so you don't need to compensate as much since you're removing weaker parts of the stroke. And to add to the confusing, on the same podcast in an interview with aero and short crank specialist John Cobb Cobb mentions the higher gearing also here if you can't find the podcast episode., he suggests not gearing down at all, even recommending short crank riders avoid the "compact road" 50 tooth big ring in favor of traditional 52 and 53 rings.

In my case, I was just lazy and swapped over my old 38 front ring and dug up an old 11-42 cassette. Dropping from 175 to 155 should have a 12% reduction in gearing, making my 38 ring feel like a 42! On top of that, this bike usually had a 11-46 cassette but I had stolen it for my fat bike over the winter. Taking it over one of my favorite local climbs, I had one of my best ascents, even though I was running about a 20% higher gear than usual. Again, with the position right, the short cranks continued to impress me.

I'm still riding that frame to fine tune the position further and then I'll probably build a fresh frame for my position. The only downside I've noticed is that it seems harder to just spin and go slow. I'm not sure why, is it because things feel so natural that I just want to push? Was my old technique actually inefficient in a way that going slow was a natural result? But it is getting easier so maybe I still need to get used to them more.

In some ways I'm thrilled that I've discovered so much new stuff about bikes in the last year. But I also feel like so many things about performance riding were almost lies. And bike design in the traditional sense feels like a cargo cult. I had always looked at from the bottom bracket outwards and divined meaning from angles based on what the bike world at large was saying. 75 degree seat angles "climb better". 450 chain stays are too long. I realized these numbers exist as constants only because they can be used to define and differentiate different model. We can only keep them constant by keeping the crank length within a short range. Once you let crank length vary by as much as chainstays vary, all hell breaks loose. Does chainstay length really have meaning? Did we really make progressive bikes longer or did we just swing the seat tube around the saddle to make the seat angle look steeper and the chainstays look shorter?

Taking the rider-centric fit methodology and applying it to frames led me to a new way to think about bikes I'm calling the Hourglass method, which will be the one of the next articles to come. After that, more Crankarm Chronicles with some investigation into the history of crank lengths. After that comes more about modern bike fit and where things may be going.