Bob's Hardtail Project

Jan 3, 2000:
I'm attaching some shots of how I developed the hardtail conversion for the XS frame. What I'm building is not really what I'd call a "chopper" in that it won't have any radical changes in frame geometry, but is more aptly termed a "bobber". The style preceeded the raked and extended fork style that became popular in the 60's. "Bobbers",or "bob jobs", have been popular since the beginning of motorcycling, when riders shortened fenders, went to smaller head and tail lights, and smaller or no mufflers in an attempt to improve performance by drastically cutting weight.

The style probably originated with American V- Twins and was soon adapted to the post World War II British twins that showed up in the U.S. That's where the influence for this bike comes from -- the mid 50's Triumph bobber. With that in mind, I compared the bare XS frame to a '53 Triumph rigid frame that I have and I made a sketch to scale from a drawing in the shop manual. This allowed me to see what changes would have to be made in order to get the proportions similar to those of the Triumph frame. The most obvious change would be in the backbone and seat post; the Yamaha engine cradle being much shorter than the Triumph's due to the XS's compact engine. As you can see from the sketch, I moved the seat post back 2" at the bottom, leaned it rearward 4" at the top, and extended the backbone back to meet it. This puts the sprung solo seat mount in the correct place and should also give me enough room to run a single carb, something I've been wanting to do that can't be easily done on the stock frame. Moving the seat post back requires that new engine mounts be fabricated but that was no problem. I also moved the rear wheel forward 1.5" to give a 55" wheelbase, the same as the Triumph's.

As for the actual fabrication, I determined the ride height I wanted on the XS frame and secured the swing arm in that position. I built a jig on an H-beam and worked off of a centerline established on the beam. I built a centering rod to project the centerline of the steering neck onto the centerline of the jig and located the rear axle on the jig centerline at the proper height. The front section of the frame was braced securely and the cuts were made and the rear section removed. I then attached mock up axle plates made of plywood to the rear axle in the jig and finalized their shape. Finally, the tube shapes and lengths were mocked up with PVC pipe.

The axle plates were then made from steel and the steel tube frame members bent using templates made from the PVC patterns.The chassis is now mostly complete and is a "roller" at this point, but I've still got a lot of little details to work out. I'll follow with more pics as the project moves ahead.

Best regards,
Bob Scogin
Slidell,La. USA

Jan 7, 2000:
If you look closely at the the frame shots, you can see the butt joint on the backbone where the original round tube transitions into the square tube that I used for the modifications. Compare this to the stock frame and you can see just how much the backbone under the tank was lengthened. At the bottom of the seat post, between the bottom frame tubes, is the new crossmember that was installed to accommodate the new tube location. It's located 2" rearward of the original location. Also note that the engine mounts are extended forward of the seat post to reach the original mounting bosses on the engine. The fact that the top rear mounting
bolts originally went "through" the seat post on the stock frame is a good reference as to just how far back the new seatpost is. You'll note also that, unlike the stock frame, the bottom rear mount bolts on to the seat post, making engine installation much easier. The other two shots are of the rolling chassis using an 18" on the rear and stock front end. The tank is a stock tank and the fender is a left over 6" wide flat fender left over from another project and supported by a home made stainless steel fender bracket.

Jan 31, 2000:
Since I've made a little more progress, I thought I'd send you another picture of the "bob job". As you can see, I've got a dummy engine installed for the purpose of locating and fabricating all the neccessary brackets and parts. The footpegs, brake pivot, fender and seat mounts are done. The stock gas tank is moved back one inch and the rear mount moved to the underside to clear the mount under the front of the seat The exhaust set-up is trumpet tip megs, typical of the 50's, but is subject to change. I still have to design and fabricate a mounting arrangement for the battery and electrical components. You'll note that the single carb and manifold are installed. The manifold is subject to revision as I'd like to get the carb a little closer to the engine. It's a tight squeeze even with the modified frame. The carb that's on it now is a flat slide 34mm Mikuni. I've still got a long way to go but progress is steady. More to come.

June 7, 2000
Thought you guys might be interested in a coupla' pics of the bobber with gaitered XS 850 standard fork and handlebars, and a little paint squirted on the tank. She's gettin' close to completion!