Wednesday, June 24, 2015

Sprint Report

I received a comment from Frankie Flood at Handverker on my post "345". Seems he's got a 350 Sprint that he'd like to make into a racer and wanted a little more info about the  one I built.

Here's the frame I used. Doesn't look like much, especially with about twenty years of dust on it. It's a 250 frame but I ran a 350 motor in it. The 350 frame was a big heavy double loop thing, while the 250 is pretty much the bare minimum necessary to carry the engine and the two wheels required to actually be a motorcycle. The front end is off a 650 Yamaha if I remember correctly. I chopped the steering head off the Sprint and welded the one off the Yamaha on. I could have just as easily ran the 250 front end or the 350, for that matter, but I had my reasons at the time. In fact you can see both the 250 & 350 front ends hanging in the left of the photo. I think the rear swingarm is totally scratch built from 4130 tube. The main tubes are a little smaller in diameter but the bridging underneath gives it all kinds of support. There is a crossover tube to move the shifter from the left side to the right - the opposite of what most guys do. My rider was also racing some little Ducati's and they all shifted on the right side as did the early Sprints. The 350 was already switched over to left side shifting from the factory but my guy wanted it switched back. The aluminum can with the yellow sticker is the catch can.

The above photo shows the bolt in piece to make the 250 frame a double loop frame like the 350. The top of the triangle bolts into a hole in the tab below the steering head, the bottom bolts directly into two holes in the engine. It's made from 3/4" 4130, also. Stiffens up the frame a bit and has Dzus mounts for the fairing. The fairing lowers are hanging there, as you can see. Fabbed from aluminum sheet as was the rest of the fairing. The windscreen on the fairing was a stock EMGO item. The top of the fairing had a steel armature inside it that bolted up through the same holes as the windscreen to keep things from rattling apart and supported the mounting peg that slipped into a piece of tube on the steering head to hold it in place.

The rear wheel was a stock 250 hub laced to an aluminum rim. The stock set-up had a thick sprocket with rubber bushings inside for a cush drive. Not much of a selection of sprockets available that way. I machined up a magnesium spacer and then machined up blank plate sprockets from the farm store to fit. I had access to a CNC mill at that time so I made a fixture, wrote a program and away I went. Worked really well.

The front wheel was one that I made myself. It used a Triumph backing plate and brakes but the hub was an aluminum fabrication bolted to a steel pipe. Lots of welding and machine work but the guy said it stopped really well. 

The rear of the frame is a little tweaked as a result of the crash at Daytona. It wouldn't take much much to straighten it out however. Likewise, the top of the fairing took a hit but it too could be straightened out without a lot of work. However, if I was going to go racing again, I'd think about making a whole new fairing. The craftsmanship on mine was fine - in fact I was rather proud of it - but the aesthetics weren't the best. I'd try to knock out one like the photo below. For one, it would have the classic Aermacchi racer look, and secondly, that's the kind of stuff I enjoy doing and would like to get better at.

The gas tank should still be use-able but the seat is probably junk. They are both made from the same aluminum sheet as the fairing. I didn't think about it when I took the frame photos or I would have shot a couple of them also. I'm sure they're still around, I never throw anything away.

I imagine I've got a couple of pictures of this thing around somewhere. I posted one without the fairing a while back from the first season I ran it. It was painted Guard's red, rather than the black cherry. I'll see if I can find a photo of it all put together and then put it up here. I've got a small book that I had printed up that's a collection of articles on the Sprint racers, by the way. The majority of the information in it was originally put together by Syd Lawton, I believe.  There's some good stuff in it. Let me know if anyone's interested in a copy. 

No comments: