650b Conversion Clearance

Some of you may be familiar with converting old road bikes to 650b, smaller rim, fatter tire, possibly more fun.  I am working on an older Fuji  for a friend at the moment and had to share this.  It has and will have the original 27×1-1/4 tires for a bit due to budget constraints.  But look at those perfect dimples, it could not get any better if you built it that way on purpose. 

Frosting on the cake is the bottom bracket drop of 60m or so.  If I had the money I would convert it and put Berthoud fenders on it just because it seems like the right thing to do.  I took it for a test ride, and even with no front load, trail in the low 40s and the big hoops it handles great.  Still, it begs for fatter rubber.


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6 Responses to 650b Conversion Clearance

  1. Michael_S says:

    Rob, I am looking at picking up a 1981 Fuji Royale which uses the same frame as the S12-S for a 650b conversion. Did you see on this bike if the 750 centerpulls would have worked?


    • Rob says:

      Mike, The answer is a big maybe. I think there is enough variation from frame to frame and front to rear on the production bikes that you need to almost try it to work it out. I have measured a couple too many times now to have it not work in practice when it is really close. I would throw in a couple of 700c wheels you have on hand and determine where the reach falls. If you are in the 55-56 range a 750 brake will very likely reach, and a tektro bigmouth might just make it. If you are slammed at the bottom of a 57mm reach brake to make it work with a 700c wheel, it is going to be really close. The math, 57mm + 18mm to get to 650b puts you right at 75mm. Every mm will make a difference when you are playing close to the margins and going from 27″ to 650b on a production bike. You will also want to think about braking power a bit when you are pushing the reach this far. At the bottom of a 750 brake with everything tightened up and Kool Stop salmons the stopping power will be passable, but far from epic.

      On the plus side, you could probably run 700x35mm tires on the fuji with fenders. The limiting point on the two I have recently had was the fork crown width, but there is plenty of vertical space to move the fender up and achieve the needed clearance. You could even run the 610 center pull that has a bit better performance. While these Fuji’s are not as elegant as a purpose built newer bike, they sure are a heck of a value when you put a little love into them. I still have a 54 in the garage, I can pull some measurements, but it may not be as good as working with what you have.

      • Michael_S says:

        I got this bike for my ERUB (early retirement Urban bike) with upright bars and a front basket. With 700×35′ s it’s still has fairly low trail. I actually have a decent 700C wheelset in the garage that I’ll start with. If 650B won’t work It will be just fine. Thinking about a set of those new Resist Nomads in 700x35mm for it anyway.

        • Rob says:

          Honestly out of all the treks, miyatas and other UJ bikes I have had come through the garage this year, the Fuji s12s were some of the nicest riding. Even with the 27″x1.25 tires and no luggage they were just spot on, ride with no hands etc. and still calc out to low trail. As the tires and wheels go smaller in OD it just keeps getting better. IMO the low trail bikes body steer instead of steer from the bars. This kind of makes sense when I am putting surfers on them, who have less bike experience, but hop on and fly around with a giant grin and hands over their head etc. The only downside to the fuji line was the really high bottom bracket, but it is no higher than a cross check or my old Rawland, just not rivendell low. You are going to have a ton of fun with it.

          Let me know what you think of the nomads. They are dirt cheep at universal cycles. I have a set of the 42mm with my contractor to test fit into the frame, but they went straight to him and I have not had a chance to play with them yet. Also, if you have anything in your stable that will fit a 700×42 keep your eyes over at Bruce Gordon’s sight once and a while. I heard that his old tire might be making a comeback later this spring. I know that 70×42 is too big for the fuji, I tried with my marathon XR, it went in but was very close.

          • Michael_S says:

            got the Fuji today. Paint is pretty good, brakes adjust to fit 700c although there is barely room for JB’s in the back. I’ll try my 650B wheels with Hetres tomorrow. Rake measures about 63mm… this is going to be a good bike for around town.

          • Rob says:

            Glad it will be a decent bike. The barely room for JB tires is a wow sort of thing that confirms my earlier statement that you need to have the bike in hand to make these kind of conversion calls. Are the stays dimpled? The round stays on the S12-S have super deep dimples. I measured at 330 from the axle, bulge on a JB, and there is 48mm between the stays on the S12 in my garage. If you want more pictures of the dimple I could throw some up. You could probably get away with some decent dimples now and spread the rear at the same time. This is easier than you may thing with a bench vise and a couple of basic alignment tools. I have most of what we would need in my garage if you ever wanted to try it out? Maybe bring it with you if we do Sulphur MTN in a couple months.

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