Sidecar Design Formula - IMPORTANT!

Nobody else would ever get away with that

I could do that.

Thanks for the post Claude. I have the pic already and it’s given me some ideas.

Claude, what bike is this subframe for?

That's Claudes design for 1150gs

I like that.

Holy mackerel, what did I is the past months. Here is the subframe for my 1150 pulling a 130kg rolling chassis. Why the need for such a heavy subframe ?

That's easy there are plenty of folk who feel the rear subframe on the 1150 gs is too light to just clamp too, tying both sides of the tug together can't hurt and it appears you used rose or heim joints in the two lower points Claude uses mounts that bolt up solidly.
Underbuilt or overbuilt.

Thanks. I'm also one for over engineering. Mine just came out like that because I knew nothing and copied PowderMonkey's build. I did use rose joints. Here is how I engineered the rear top of the stabilizer mounting point spreading the load off the subframe.

Here is my whole to-bike mounting set-up. Criticism is welcome. In the meantime I have strengthened mounting of the the brace to the front of the subframe to the crashbar mount under the tank and re-mounted the stabilisers perpendicular to the car.

Just remember when u fly the chair u have a 4ft lever trying to bend the crap out of everything.
Like a tool designer once told me when I was working for him “We don’t engineer anything we just make it as strong as we can and hope it doesn’t break”.
But I do agree with your thoughts, when is heavier just extra weight without the need? I dunno. When I was designing my upper rear mount I calculated the flex on an unsupported cantilevered 1 1/4”!tube, anchored on the lower subframe but not supported at the top. The calculated deflection was scary. I didn’t think the bike rear subframe was designed to transfer the load back to the engine mounts either so I went all in and built an exoskeleton frame to do the job. Overbuilt? I hope so. I don’t have the engineering tools to do a proper analysis so I default to: “We don’t engineer anything we just make it as strong as we can and hope it doesn’t break”.

Mine. It weighs 47lbs.

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I trust the Germans followed that same engineering philosophy. But does the subframe not in fact bear on that rear engine mount ? I'm busy with a second sidecar to 1150 so still doing some R&D.

The rear sub frame on a stock 1150gs, holds up the seat and muffler,bags if you run them. It's not overbuilt.

The frame failure I’ve seen ( google search) look to be cracks in the relatively thin tubing. I think the frame is designed to support vertical forces but not so much horizontal. I picked up the same engine mount, transmission mount (at the foot peg bracket) but add in the lower subframe. On both sides of the bike. But the biggest deal is its 1” diameter steel tubing with an 1/8” sidewall, If the welds are good I don’t think it will break. But who really knows.

Part of the trouble is when designing something to sell the bean counters get the final say not the engineers, things are built to a price point.

That's why so many modern bikes make the engine do double duty as the main frame- It's not better engineering, it's just cheaper that way!

And Phil Vincent just turned in his grave... View attachment 2370441

As did Dr T from Ducati.