CB750 Suspension Issue

Hey guys,

I have a 74 CB750 I was working on. I cut the sub frame and started working on a monoshock conversion but was cut short. I left the rear shocks as is for now (I ride like once every two weeks for 15 miles at best, it’s just a hobby). I know my tail end isn’t supported from the upward force the shock gets but I wanted opinions. Any suspension recommendations that will allow me to have a mono or dual shock with my current under seat oil reservoir? Any help is appreciated. I am attaching a photo of my bike and another bike I saw posted with a dual style suspension set up I had never seen. Can it be done to a cb? Thanks folks


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It can certainly be done, but do you have the time and energy to do it? Your sub frame and shock attachment is going to fail if not soon then definitely later. It's just not triangulated well enough.

So if you are going to scrap the current design and go for that on the BMW you will need to start looking into motion ratios, start calculating the forces that the frame will see, and start fabricating. The BWM might produce a nice rising ratio, but it looks like the forces will be very high, probably not ideals on an oldschool tube frame. It can be done, we know that. Or you could just triangulate your subframe and go riding.
Sorry to say, you likely will be well served to restore the original frame components to support both the rear suspension and yourself. Certainly, with enough time, planning and money you can build anything you like, including the novel suspension on the BMW. Very often you will see very nice looking motorcycles that have had plenty of time and money lavished upon them with altogether too little planning involved. By planning I mean proper engineering. The classic and formerly nearly exclusive twin shock design your bike had originally was made that way for massively good reasons. It is cheap in material, cheap in manufacture, very strong, very safe, and places minimal loads on the rest of the chassis. You have to be very clever indeed to improve on this scheme. And you have to have very compelling reasons to justify the huge increase in complexity required to do pretty much anything else. There are two principle issues involved in most single shock schemes. Ignoring the very important considerations for the suspension bits themselves and whether or not they can be fitted within the available spaces on an existing bike, the parts you are starting with were not intended to endure the new, very much larger loads required. 1) The swing arm itself will require so much additional bracing to support a single shock (for most popular current schemes), you will likely be better off going with an entirely new arm or starting from scratch. This is because the original design places a great deal of the loads - which start at the rear axle where they are introduced into the arm - directly into the shocks which are attached very close to the axle. Very much of the loading never has to travel along the arms and into the pivot, so the arm as an assembly can be pretty feeble and still do a good job. Also observe that there are two shocks instead of one, so the load per shock and all the mountings is half of what one shock would be, and that is if a single shock were located way out back near the axle. Add to this doubling by moving the attaching points farther from the axle and closer to the pivot and you can see the leverage makes the loads truly skyrocket. Which is fine, as long as you increase the strength of all the components accordingly, as well as the spring and damping capabilities of the shock - which now must do the same job as two much longer travel units. 2) all of these now vastly increased loads have to be dumped into the frame. And in your existing frame, not only is there no existing place to put them, but should you create strong enough mounts for everything, the chassis itself was never intended to take those sorts of loads in the first place, and if you thought your frame was on the wimpy side with the factory twin shocks, you won't believe how bendy it will be without all that extra triangulation and the much higher and more concentrated loads all fed into pretty much the worst possible locations. Likely you could increase the strength and stiffness of your existing frame to at least make a safe motorcycle, but you will either be starting from scratch to design a frame around the loads to be placed upon it or end up with a very, very much heavier frame.

Certainly, people do these sorts of conversions all the time. Many are perfectly fine and safe as well. Many are not. Exceedingly few are equal to, never mind an actual improvement over the original design, though that is not to say it can't be done. If you are actually wishing to improve the performance of your bike, you have a very hard road indeed ahead of you if you wish to undertake such extensive engineering challenges. If, on the other hand, you are looking for a more interesting, good looking, more novel or simply more custom bike, than this may be worth pursuing.

If all of the above sounds perfectly UN-helpful, I'll offer this: If you are determined to go single shock, rob the ENTIRE system from a bike that has something you like. Swing arm, shock, all linkages and mounts if possible. Make whatever measurements needed to install it on your motorcycle EXACTLY as it was installed on the original bike. That way, at least that portion will be ok as a system, and you will only have to alter your bike to fit the new parts and be strong enough to withstand the new loads.
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