Saturday, March 2, 2013

Frame Modifications

Surly posted a comment the other day on my Superbike post about roadracing frame geometry being much different than the drag racers I used to fiddle with. Short answer, yes. Long answer, yes, but let's discuss it.

Part of the Frame Building Library Collection
I'm not sure why, but ever since I learned how to weld I've been a student of the art of frame building. I always thought it would be cool to be the go to guy for a custom racing frame. There are guys who have made a living doing that. The first name that comes to my mind is Rickman. They made dirt bike frames and road going frames. Beautiful craftsmanship, fillet brazed and nickle plated.

Rickman CR 900
If you're talking drag racing, Kosman; flat tracking, Champion and C&J; roadracing & custom, Harris, Seeley, Bakker, Bimota, Spondon, North, and many others. Lots of chopper builders out there. I've never been a big fan of choppers but always liked the look of the bikes created by Arlen Ness. And there are more than a few dabblers out there who have made one or two.

I've made a couple of complete frames over the years. I made a duplicate frame for a Triumph Trident that a guy was roadracing. I made the thing out of chrome-moly to make it lighter and stronger but he could still pass through inspection as a stocker once it was painted. Mostly, however, I did drag racing stuff. I raked frames, gusseted them up and made extended swingarms. I would have liked to have done more with  roadracing but the opportunity never really arose.

Drag racing is pretty simple in theory and in chassis design, at least compared to road racing. Frame alterations usually are designed to get the bike lower, get it to hook up and keep it pointed straight. Other than overcoming the tendency of the chain to pull the wheel sideways when launching, most of the force applied is in a straight line forward once the tendency to wheelie is overcome.

Road racing is a whole bunch different. You have to contend with both acceleration and braking forces and get it to go around corners. Here we're looking at both art and science. The Cycle article points out many of the changes made to get the Vetter bike to corner. The steering head was replaced and gusseted. The stock Kaw steering head is thin and is fitted with ball bearings. If you bang it down hard after a wheelie, the bearing races become dented and the neck distorts. The Vetter racer was fitted with tapered rollers in the new neck and was braced to prevent flex from cornering forces. The frame work was done by none other than Rob North. I assume this is the same Rob North famous for the three cylinder Triumph and BSA frames.

Rob North BSA Frame - Braze Welded. Typical British Construction Method
My bike frame came from a guy who ran it into a parked car. Not only did he bend up the neck, he hit it hard enough to bend the front down tubes. I put it on my frame jig and put on a new neck with tapered rollers, braced up the front of the frame and changed things around a little on the downtubes to compensate for the non-factory bends.

The stock Kawasaki engine mounts allow the engine to move around a little in the frame. The plates are about 3/16" thick and the bolts are all a loose fit in everything they pass through. Like the Vetter frame, I made up some new plates out of heavier material and ream fit larger bolts. Now the engine is a stressed member of the frame rather than jumping around in the frame. The Vetter bike also changed the swingarm pivot bolt to take all the slop out of it and they strengthed the swingarm. I didn't see the need to go with a larger pivot bolt but I made a rectangular section swingarm and I'm going to bridge the bottom.

The Vetter bike ran a WM-7 rear wheel and a slick. My tire now measures 4-1/2" wide. The slick was probably 6". That by itself really changes the loading on the frame. When I was dragracing I had a big rim on the back and ran a Goodyear roadracing slick low on air so it would have a nice footprint. When I put it on the street bike with the sidecar I mounted up a 140 width Metzler and it looked huge. Baby size tire now but that's what I'm shooting for on the rear again. A 140/70-18 should fit under the bike with no further modification other than a wider rim. No offset sprocket required and it will look like it should have back in 1978.

The Vetter bike has the top shock mounts moved forward over four inches. This used to be common practice on endurance racers. Shock absorber technology wasn't what it is now and this was before mono-shocks and linkages. This allows a more compliant ride as well as more wheel travel. This in turn gives you more freedom in setting the ride height and helping with weight transfer under braking. I'm thinking I don't need to get that elaborate with my bike. If I was building a replica, yes. I'm planning on a little more hooligan/streetfighter than pure Superbike.

If you look back a couple of posts and see the picture of Pridmore putting the bike over on to the points cover, the article shows how they shortened that up to allow more clearance for just such manuevers and the removal of the alternator to allow the same type of cornering shenanigans on the other side. Professional rider on a closed course - don't try this at home. Pridmore also raced sidecars during his career, as seen in the photo below, and as I mentioned to Surly when I responded to his comment, I once had the pleasure of meeting Mr. Pridmore and talked a little sidecar racing. Nice guy.

Pridmore Pilots Rig On The Isle of Man

Summing up, yep. Roadracing frame building considerations are  much different from drag racing. Other than bridging the swingarm, I've already made most of the same alterations as the Vetter frame (this project has been around almost since the time that article came out). I need to build the pipe, bridge the swingarm, widen the rear wheel and then most of the fabrication work will be done. Just mechanical and paint after that. Lots simpler than building a world beater Superbike.

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