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Breaking some laws....of physics


Hey Folks;

Engineering friends have made it clear to me that one of the most UN-stable structural forms is the cantilever....where something sticks out more or less unsupported but from the near side, leaving a substantial overhang.

Another always put it this way...."Overhang is the enemy of rigidity".

And looking at the sidecar in general, its pretty clear there is a good deal of cantilever and overhang going on.

So this points up one of the most difficult concepts I've had to deal with in trying to install my hack. There's so much overhang, and only so much strength in the bike frame and attachment system, that I feel as if I'm about to do something very dangerous. I can just see me and the bike going one way along I-40, while the sidecar, Mrs. Sahagan and the Shih-zhu head the other way....and it isn't a pleasant thought.

So my question is simple. How much strength must we build in? How important is rigidity in installation? Are there any lessons to be learned from horror stories? Are there any horror stories?

Thanks for your help.

The only time you have to face the cantilever condition is when flying the chair. The rest of the time the rig is pretty solidly based on three pretty substantial supports, the wheels. If properly triangulated, the strength of the supporting struts will only have to support the sidecar load and the added weight incurred by centripidal (?) force during turns.
From your description of the sub-frame you have fabricated, the dimensions of the materials used and assuming you possess welding skills approaching those of a (certified) welder, it sounds like you will have a pretty sturdy platform. (Assuming you get that fourth mount in of course).
Rigidity should be sufficient enough that there is no visible flexing of the mounting components or frames. Unless you are working with a Harley type setup where the flexing takes the place of a non-rigid suspension.
Unlike your friends, I'm no engineer, but I've been on a lot of freight trains. If you couldn't bend a few laws there wouldn't be any sidecarists.

If something is cantilevered it is not supported on the end but is supported part way towards that end and relies on the support point to hold up all the weight hanging out past it..yikes did that make any sense?
Anyhow..on a sidecar there is not cantilever as the sidecar tire is acting like a post to hold itself up. Until, like lonnie said the wheel is off the ground.
Yes there is a lot of stress that needs to be controlled by the sidecar frame and the mounts holding it to the bike. And Yes there is a lot of stress that is transferred to the bike frame..but if the mounts are sufficient and are working together to distribute these forces properly then there should be no problem.
The old addage or 'build it stout out of things you know about' goes a long way towards the peace of mind of the sidecarist.

The "old adage" as I've been quoted is: "When in doubt, build it stout, out of things you know about."
A sidecar axle attached on just one side is an example of cantelevering. The axle and wheel are cantelevered off the suspension.
In terms of attaching the sidecar to the motorcycle, the best alignment of struts is pointing generally toward the sidecar axle. And triangular shapes are much stronger than cantelevered shapes. When arranging the struts, try to have nice triangular shapes with the plane of the triangle pointing toward the sidecar axle.
Try to avoid having struts or connections that impart bending loads into a threaded fastener. If a threaded rod is bent, it will fracture at the thread roots (the bottom of any V groove) It's OK to use a threaded rod as one arm of a triangular structure, since the loads are primarily down the rod rather than across.
Remember that an eyebolt attached at right angles to the load is cantelevered, so keep it as short as possible, and make sure the smooth shank fits snugly in the bracket, with the threads only keeping it pulled into position. If you're using one of those "strap" clamps around a frame tube, try to position the clamp so the eyebolt is in line with the load, not at right angles.

>>If you're using one of those "strap" clamps around a frame tube, try to position the clamp so the eyebolt is in line with the load, not at right angles.<<
The quote from Dave's post is a very important one. Many times we see rigs where this has not been done. The results can be a clamp that tends to rotate around the frame tube which, of course, changes the initial setup. Is it the clamp's fault? Not is moreso the improper use of the clamp.Any strap type clamp that is installed at a right angle to the strut is being called upon to do more than it was designed to do.


Hey Guys;

All right!! In just three or four posts, you folks have given me more valuable information than I was able to get off of various sidecar websites, including their illustrations, over a several week course of research. And sometimes, the devil is in the details, just like that bit about threaded rods and bends versus straight applications. I was just about to make that mistake on my next and last connection....I'll change that!

I do have pretty nice triangulations with my struts, and they do trend decidedly toward the axle (which I put 15 percent of the wheelbase forward of my rear bike axle). Even without the fourth connection made, the set-up seems remarkably strong, so I guess I'm almost good to go.

I did wonder about cornering forces, for it seems the load would not only be pretty severe, but in going around two opposing curves, you'd end up reverses the forces as well, meaning you jump from push to pull constantly. I think I'll keep a very close watch on my bolts, struts and connections for the first few thousand miles, at least until I get a good read on how the set-up will take it.

And, if you add a 150 pound passenger, and fly the car (is this something you do often?), it seems the forces would be even greater, more so or less so, depending upon the distance between the frame of the bike and the sidecar itself.

I'm planning on continuing to use my right-side saddlebag, but only as a removal, and not as an 'open' on the bike; that will let me keep the sidecar pretty doggoned close to the bike. It seems to me this would help maintain more structural integrity in virtually all cases.

Again, thanks! I'm finally getting somewhere....


Don't feel you must mount the sidecar very close to the bike. Give yourself enough room between the bike and sidecar frame to do normal maintenance. You may not need that RH saddlebag, since you can now load up the sidecar trunk.
Another "rule of thumb" for mounting is to position the sidecar axle in line with the front of the rear wheel rim. So, for an 18" rear wheel, that means 9" of sidecar wheel lead. Of course, the rule of thumb goes out the window for machines with fat rear tires, auto tires, etc. etc.
I know that others will disagree, but I prefer mild steel clamps and high strength bolts. The clamps should bend and deform slightly, but the bolts should always be straight. If a bolt is bending, the clamp is wrong. And don't feel you must use the hardware that came with the sidecar. It's not that difficult to saw up some mild steel tubing, have a good welder stick the parts together, and make your own clamps to fit what you need.
In general, fasteners (bolts, nuts, etc.) should never be welded. Welding will anneal or crystalize the metal, which of course weakens the fastener. I learned this the hard way yeaaaaars ago. It was a real shock to look down and see the sidecar strut parting company with the bike.


Thanks for your reply....

In my current situation, I don't believe I'll need even a single clamp. I have seen them on bike/sidecar systems, and have felt that in some cases it could have been better and safer done with a sub-frame assembly of some sort. On the other hand, I've seen instances when clamps were the best answer on all accounts.

In fact, my bike has no front frame to speak of, and my sub-assembly may be a bit unusual. For in truth, I've mounted a heavy steel plate (on the right side of the engine) to three engine mount bolts (which of course do mount to the frame, but requiring a couple of 'set-offs' in order to get everything lined up and plumb) with upper and lower welded steel brackets for the struts....with another similar but smaller (no brackets needed on this one) steel plate on the left side of the engine. I then connected both steel plates (pretty nearly centered) across the front of the engine with a 5/8's inch all thread steel rod. To keep forces as equal as possible, I used nuts on the threaded rods both inside and outside the steel plate, adjusted them and tightened the fool out of them. That way the plates don't pull the engine apart to any degree, nor compress the sides of the engine too greatly either.

Of course, all of this required some careful consideration of just where to cut plastic, and how to re-install original baffling and engine heat it was a fun kind of undertaking. But it would be a mistake to say my bike is still 'stock'.

One worry was that by wrapping the engine so hard in steel, it might engender added vibration or buzziness....something the Concours is generally known for. In my bike though, that's been a complete non-issue, so long as I keep the carbs synced and the balance shaft adjusted properly. Of course, it occurred to me the wrapping could as readily lessen any vibration, and that's just what it did; although as I noted, such vibration as was present in my bike was so little as to be pretty much ignored anyway (and I came to the Concours from Gold Wings).

I'm kind of a shadetree mechanic as to many things, but I learned to weld on river barges when just out of high school. My welding LOOKS a bit flyblown, but has always been STRONG enough to pass muster. At this point, if my welded brackets are as strong as I suspect, it would take pulling the whole engine out of the bike in order to break my set-up.

On the other hand, I loaned my hack out to another fellow a few weeks ago and in his installation he cut my struts, pounded the ends into a slightly bigger steel tube (bent) and welded them on. In short, he pretty much destroyed my struts. So....and here's the kicker, I cut the threaded ends off, and welded splices in directly to the tubing, end to end. I'm wondering if my end to end welding of the tubing will prove as strong as the original. I've ground and sanded off all the over-run of welded material so the struts look as if they're one piece of tube. I think they'll hold just fine, but as I look at my system, I suspect they'd be the weakest point. (I might go back and weld 1 inch wide tube 'wraps' around each weld, if I can find a tubing the right inner diameter.)

I'm still trying to get my mind to accomodate the whole idea of the cantilever and the forces it has to represent on the frame of the know, there are many things we do that are actually very un-natural. Folks put contact lenses in their eyes, when every piece of their nature says you don't put things in your eyes. And not only so, but people can make all ten fingers go this way and that, at their own behest, while playing a piano, and that's certainly against human nature. So maybe I can get used to seeing all that weight and all that cantilever sticking out there, without it bothering me....

Again, thanks and we'll catch you later,


In butt welding tubing one should bevel the ends that are to be welded so that the first (or root) pass is at the bottom of the groove to insure complete penetration of the parent material. Subsequent weld passes should be left intact, including the final (or cover) pass, so the weldment is not below the dimensions of the parent metal. Grinding and smoothing the outside surface of tubing may make it look prettier, but without proper weld penetration any amount of material removed will weaken the weldment. Wrapping the welds as you mentioned is another option. You don't need tube (pipe) of the proper thickness to do this. You can split a section of tube and bend (hammer?) 2 pieces to weld wrap the joint for reinforcement. If I wish to have a smooth outer finish, I will drill holes near the butt ends, insert a short tube in the strut, weld the beveled butt ends to the insert, plug weld the predrilled holes and then sand it smooth. A little extra effort for a lot of extra strong.

>> If I wish to have a smooth outer finish, I will drill holes near the butt ends, insert a short tube in the strut, weld the beveled butt ends to the insert, plug weld the predrilled holes and then sand it smooth. A little extra effort for a lot of extra strong.<<

Amen to this is not hard and is the safeist and strongest.