Topic: Custom frames

[MJL]

I saved these pics from a thread here awhile back and am looking into recreating one.  Has anyone done a custom frame besides a chopper or bobber?

[sparty]

Neat idea, but I really don't believe that a CB750 engine case would be stout enough to be used as a stressed member of the frame.  It wasn't engineered for those added stresses and may slowly crack.

Sparty

[mkramer1121]

Neat idea, but I really don't believe that a CB750 engine case would be stout enough to be used as a stressed member of the frame.  It wasn't engineered for those added stresses and may slowly crack.

Sparty

Sparty,

check out satanicmechanic.org  There was a ton of manufacturers that made aftermarket frames for CB750's (Egli, Martin, which that pic is) that used the engine as a stressed member.  Someone must have done the research to figure out it could be used that way.

[Ecosse]

That's Moto Martin I believe; those guys know what they're doing. Upon close  examination you see that the above engine is a partially stressed member and not like many formula (car) racers where the rear suspension links to the gear box and the rest of the unit bolts to the front of the engine making the engine a complete stress member.

Having said that Sparty's thought crosses my mind when I look at similar situations.

Does anyone have any thoughts on the spine-type of frames such as how they resist torsional loads and flexing?

[voxonda]

Been in business over 30 years and seen a lot of 'aftermarket' frames, and never ever seen a cracked case in either a Martin, Egli or whatever frame. Think I can say that the engine can handle more than we think. Up to a certain limit.

Cheers, Rob

[sparty]

Upon close  examination you see that the above engine is a partially stressed member and not like many formula (car) racers where the rear suspension links to the gear box and the rest of the unit bolts to the front of the engine making the engine a complete stress member.

I didn't notice that. Okay, that makes a huge difference in the amount of stress that case is subjected to under load. 

[KingCustomCycles.com]

Most engine mounted frames would benefit from a boxed back bone to absorb torsional as well as flexing loads.  But then, the engine is basically being held in a sling of tubes under the load carrying backbone.  Man, you would hit the pavement fast if anything went wrong.

[goon 1492]

Yeah it looks like the motor is hanging more like a sling, and it does look like it could ge top heavy

[Geeto67]

An easier way to do it woul dbe to adapt a semi modern sportbike frame to take the cb750 engine. An aluminum wishbone frame is basically the same design as the martin except boxed alloy instead of steel tube trellis. Compared to some of the more modern sport bike inline 4 engines, the cb is physically  an anvil.

Some possible canidates would be:

Early fzr600
cbr600F2
first gen Ex500 (although these and the zx6's from the late 1980s use a bolt in engine cradle that runs underneath the engine as well).

I wonder if the bike would actually be lighter not being water cooled?

[MJL]

I'm talking the idea over with a friend who does tube bending for Jeep roll cages and such.  I think 1-1/4 .090 wall DOM would be good.


Here is another Martin framed CB750.

[tinyrobot]

Isn't eh CBX a partial frame as well?  That motor is bigger, heavier, and made of the same materials. 

[malcolmgb]

I think I posted this link before but check it out, not a 750 but it may give you some help.

http://www.denoonsp.com/serv011.htm

[Ecosse]

I think I posted this link before but check it out, not a 750 but it may give you some help.

http://www.denoonsp.com/serv011.htm

Yeah that's the spine frame I referred to. I'd love to build something like that one of these days. I wonder how it compares structurally and by weight compared to trellis frames and others.

[MJL]

I think I posted this link before but check it out, not a 750 but it may give you some help.

http://www.denoonsp.com/serv011.htm

Thanks for that, it should help some.

[void909]

thats a cool frame

[MJL]

After some more searching I find that Martin is back in the motorcycle biz, but he lives in France.  Going to be expensive to have a frame shipped over.