Can you ID this frame?

[WideAWAKE]

After looking at Tom`s Champion framed 750 I took another closer look at my old 750 and it has the same "Wishbone" style of frame brace in front of the swingarm as the Shell frame. No carb clearance problem there. Mine had a Shell tail section and a Champion tank. Those look like 32mm Ceriani`s. That might be a Shell decal on the tail section?? The bike had uncontrollable speed wobble.
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Did either of you race these?? Or just for street use?

Lack of a front brake seems like maybe it hit the track??

 

[halfmile]

I never raced the 750. I raced a 250 Bultaco Pursang. The 750 was unsafe at speed and it probably needed a fork brace as well as bigger diameter forks?

 

[atom4488]

"... it probably needed a fork brace as well as bigger diameter forks". Not to derail this thread, but...

Can someone confirm my rudimentary and largely intuitive understanding that the causes of speed wobble or other high-speed instability would be overall fork stiffness (tube diameter, length, bracing, headstock/headstock bearings) and perhaps more importantly front end geometry? This is of interest to me because of an upcoming build (Project XS).

 

[Kevin Werner]

"... it probably needed a fork brace as well as bigger diameter forks". Not to derail this thread, but...

Can someone confirm my rudimentary and largely intuitive understanding that the causes of speed wobble or other high-speed instability would be overall fork stiffness (tube diameter, length, bracing, headstock/headstock bearings) and perhaps more importantly front end geometry? This is of interest to me because of an upcoming build (Project XS).

I think it has more to due with harmonics. GP bikes have very steep headstock and don't wobble. Nortons and Triumph and BSA had skinny forks and didn't (reportedly) wobble. 77 and later XS650 have 35mm forks and wobble a bit less with the same frame geometry. My 78E became a wobbler overnite when I spooned on a new front tire. I don't see Speedway bikes wobble. I illuminate some extremes here. More food for thought.

 

[RC4MAN]

Kevin is correct in that there are many variables that can cause wobble. On a particular bike it can often be traced to a single thing, yet that same thing on another machine with other differences may not cause a wobble.
Most of the cases that I've personally experienced on my own machines have all been directly attributable to steering head bearings, either too tight, rusted or damaged (dimpled)

 

[atom4488]

I would have thought that a lot of the stability is a function of offset in the triples, with more trail (less offset)= more stability, less trail (more offset) = quicker steering less stability (quote taken from Mule's Insta page). If that is true, headstock angle would have less to do with stability, hence the steep headstock angle of speedway/GP/flat-track bikes is not as much of a factor?

 

[arcticXS]

I would have thought that a lot of the stability is a function of offset in the triples, with more trail (less offset)= more stability, less trail (more offset) = quicker steering less stability (quote taken from Mule's Insta page). If that is true, headstock angle would have less to do with stability, hence the steep headstock angle of speedway/GP/flat-track bikes is not as much of a factor?

In his book "Motorcycle chassis design" Tony Foale describes and shows pictures of various experimental front ends he had built, to establish what is most important for stability, trail or rake. And he states that it is trail. The rake can be almost vertical without loss of stability, as long as trail is within the normal range (roughly 3.5-5 inches)
I think that the rake is more of a compromise to reduce bending forces, and hence friction under braking in telescopic forks. And I guess also to provide clearance between front wheel and engine without the need for a very long frame neck,

 

[atom4488]

@arcticXS, like your answer, makes sense. I was searching my failing memory banks for the name you cited; Tony Foale. Will see if I can get myself a copy of his book.

I have a number of sets of triples from different bikes (XS 650, XT350, Kawa ZX11, CRF450, other smaller machines) which I will measure to see the differences in offset. My XT (which I have posted pictures of often enough) has a USD fork conversion including of course the triples. I suspect that it changed the front-end characteristic's quite a bit from the original. It doesn't wobble, but it sure is sensitive when compared to the XS.

 

[MrBultaco]

I used mine as a street tracker, sparingly, however. The guy I sold it to raced it up Pike's Peak when it was mostly dirt 1982. I entered it in the Tri-State Auto Show...it won it's division. Yeah, I never should have sold it if you want to know.

 

[MrBultaco]

Your frame might be a Series 200 Champion frame...used for smaller engines...halfmile's and mine are Series 500