Thursday, January 22, 2015

Converting my Gunnar Roadie to low-trail 650B, part 1

My 2002 Gunnar Roadie is the best bike I've ever ridden. It's fast (it's painted orange; so what did you expect...), looks good, is reliable. Since I first built it up in 2010, I've made some changes, but they were mostly minor -- Grand Bois handlebars replaced the modern bars, as did a 1" steel stem the 1 1/8" aluminum stem, and I upgraded to a generator hub. But now it's time for something more drastic: Converting to 584mm/650B wheels and a fork that will make the bike low trail.
Gunnar with 25-622 wheels/tires

Why change an already great bike?

There are two main reasons for this project. The first one is that there is one shortcoming of the frame. It was built to accommodate tires up to 25mm wide, without fenders.  I can actually make 28mm tires work, but the clearances at the chain stay then get a little sketchy. Relatively narrow tires have served me pretty well so far. A good tire in combination with the compliant carbon fork make for a comfortable ride, and I've even ridden pretty rough gravel roads on Gunnar. The lack of fenders, on the other hand, is limiting, turning Gunnar basically into a fair-weather bike. I have little patience for getting road spray all over myself, and when the forecast predicts rain, I therefore switch to my fully-fendered Cross-Check. The reason for going to low-trail at the same as changing wheel size is curiosity. Bicycle Quarterly, my favorite bike magazine, has long praised the advantages in handling that low-trail geometry and front loading can bring. Now I don't nearly perceive high trail and rear loads as pernicious as some people make it to be. I regularly ride my Cross-Check with two Ortlieb Back-Rollers full of heavy groceries and yet I'm still alive. But then of course I lack a point of comparison.

Converting Gunnar is thus the answer to these two points, unlike converting my Cross-Check with ample clearance for wide tires and fenders. I'm on a tight budget and so I spent several months accumulating the parts required for the conversion: A 1" threadless Rawland Stag fork, Shimano Ultegra 6600 rear hub, Shutter Precision PV-8 generator hub for the front, Pacenti SL-23 rims, Sapim double-butted spokes.


Currently I'm running Dura-Ace caliper brakes with a reach of 44mm. Switching to 584mm rims necessitates another 19mm of reach for a total of 63mm. At the local community bike shop I found a pair of Mafac Racer centerpull brakes that at least should work for the rear. The Stag fork is intended to be used with cantilever brakes; so I might do centerpull in the rear and canti in the front.
Nice pair of Mafac Racer centerpull brakes


There are two, interconnected challenges with the conversion, tire clearance and ground clearance. As I said, currently the largest 622mm tires that fit are 28mm wide. A 584mm wheel moves the tire further towards the hub and therefore increases clearance at the chain stays—but not by much. The way to figure this out in advance is to measure the distance between the chain stays at 320mm distance from the rear axle. I got about 40mm, but the measurement is a little finicky. So in the best case, I could fit a 38-584 tire without fenders—which of course somewhat defeats one of the main reasons for this conversion—or a 32mm tire with fenders. With the latter option then comes the question whether my pedal-to-ground clearance is still going to be sufficient. When using a typical wide 584mm/650B tire like the Hetre with its 42mm diameter, the outer diameter of a wheel is actually about the same as that of a 622/700C wheel with 28mm tires. Using a narrower tire will lower the bottom bracket and consequently lead to potential pedal strike in corners. The Roadie probably has a bottom bracket of 70mm (though I have to confirm this by measurement), which is on the high end for a 650B conversion, especially when running 175mm cranks.


Low-trail bikes can have problems with shimmy more frequently than high-trail bikes. One possible remedy is the use of a rollerbearing headset to slightly increase friction. For now I will stick with the headset I already have and see how it goes. 

What's next?

I'm still waiting for some wheel parts to arrive before I can build them up. I also haven't decided what to do about tires yet. I'm hesitant to spend a lot of money on tires that then potentially are to wide to fit the bike. I could also potentially go for narrow-ish tires in the rear and 42mm tires in the front—but that doesn't make that much sense given the load distribution of a bike. I will report back once I'm getting closer to completion of this project.

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