E02JRX - Singlespeed 29er (Prototype)

After the first test ride.

After the first test ride.

Singlespeed 29er

The first MTB I bought was a rigid singlespeed (Specialized Crave SL). I basically got it because the price was right - and it looked like an overgrown BMX. Being from a BMX background, that really appealed to me. That bike served me well for a few years and I love the simplicity and feel of a singlespeed mountainbike.

As much as I loved the Crave, there were a few things I thought I'd change when starting this frame build. I wanted the bike to be even more like a BMX - playful and responsive - but still be a capable cross country machine. One thing that had always bothered me a bit about the Crave was that it felt jittery at higher speeds and it was never very good in the air (I know, I know... big 29ers are not for jumping). The Crave also came specced with a rather short (60mm) stem, so the weight distribution when riding was a bit far back.

Starting the design I knew I wanted to maintain a BMX aesthetic, so I opted for a straight downtube and a quite low slung top tube. Geometry wise, I based it around the Crave geo, but changed a few key numbers. I slackened out the HT angle to give me a bit more stability at speed, shortened the TT ever so slightly to compensate for the longer stem that was going on the final build. I also reduced the BB drop a bit, mainly because I like a high BB on a BMX, so I wanted to see how that translated over to a 29er.

I had originally planned to just use straight gauge 4130 cromoly for the main triangle and single bend round tubing for the rear triangle. But being that I currently have no means of bending tubes, I ended up acquiring a full Columbus Zona tube set, including a tapered headtube for the frame. With limited tooling it sure makes it easier to build with chainstays and seatstays that are already bent.

The frame build was a huge learning experience (check the build process here: 1 2 3 4) and it was very satisfying when I was able to finally start assembling the bike with the components I had acquired over the past few months.

Update 20.5.2018

After about a year and 3000km of riding this thing quite hard, the bike is still going strong. The geometry changes I made from my previous hardtail were exactly what I was after. The bike is stable at speed (I've done up towards 70km/h down steep fire roads) and it's still nimble on tight and technical singletrack. It's an XC machine, so obviously it's not meant for jumping etc, but those times I do get airborne I feel a lot more stable in the air and I feel bunnyhops are also easier on this rig compared to the old Crave SL.

I've been through a set of tyres, a couple of chains and brakepads. I've also changed the dropout inserts to a better version with a slightly different position of the ISO tab mounts as the original ones I used restricted fore/aft movement in the sliding dropout. The cageless bottle mounts turned out to be a more aesthetic choice rather than functional, so they've been substituted for a pair of good carbon bottle cages.

During the first couple of months of riding I did notice that I had some slight crank arm rub (actually, it was only the crank arm boot that was rubbing) on the driveside when under high torque. This was partly due to a miscalculation on my part, not giving enough clearance for the intended cranks. However, this provided an opportunity to experiment. Originally, I left the frame without a chainstay bridge - so in an attempt to reduce the rear end flex and thus the crank arm rub, I brazed in a bridge.
First ride after this modification I took the bike up a rather steep climb where I would have been guaranteed crank arm rub and it turned out my modification was successful - no more rubbing! Excellent result.