Never put a load into the middle of an unsupported strut

[minimutly]

When I,bought my trike I had some misgivings about it's rear suspension -IRS, double wishbone, coil sprung, with coilover spax adjustables. The point being the sloping shock is tied between the frame at the top and the mid-point of one of the elements of the lower wishbone.
Now, I learnt long ago not to do this, and have already spent some time trying to figure out a way to strengthen the arrangement, which was allready showing signs of bending. Well this afternoon after a spin out to the coast, I had a feeling the bend had worsened on one side. Bloody hell - if this had failed it could have been serious!
Well so much for profesionally converted trikes!
This brings me to a question, do I have a recourse to the converters? I have decided on how to fix it - replace 2 of the 3 wishbone elements with bigger tube, add a link plate under them to join them, and move the shock nearer the middle, to better share the load. I may look at moving the shock further outboard, but that will mean fabricating a new top mount, and putting it where I want to put some new silencers. I don't expect the makers to pay for the fix, but something towards it might be a good gesture?

 

[Paul.]

I would suggest that the weld has created a stress raiser there, it should have had a spreader plate fitted to the tube and particular care made of the welding to make sure there is no undercut or notching of the welds, you may have the same problem with the anti roll bar bracket, that welding is pretty poor for a pro job.

 

[Mr Roo]

I would suggest that the weld has created a stress riser there...

that welding is pretty poor for a pro job.

Yep. And yep.
the weld has def created a stress riser, which shows a real disregard for actual design vs. “feck it, it’ll do” mechanics.

as for the quality of the welds, also yes. i’d be expecting a quality TIG weld on a professionally-built trike, not a “pff, just mig it” job.

 

[8ob]

Is the anti roll bar causing the issue? The spring pushes down on the bracket evenly but one side of the wishbone has the resistance/twisting effect of the antiroll bar on every bump. Just thoughts.

Bob

 

[minimutly]

Agree about the stress riser next to the weld, but really it's the bending load that's a no-no. That said the fix will be similar in that the bending load will still exist, but divided by 2 and a bit, more if I bite the bullet and move the shock outboard.

 

[Wallace]

Hardly surprising the tube has cracked there, the wishbone is being twisted over bumps as the weight is being taken on that tube.

 

[minimutly]

Is the anti roll bar causing the issue? The spring pushes down on the bracket evenly but one side of the wishbone has the resistance/twisting effect of the antiroll bar on every bump. Just thoughts.

Bob

No, the antirole bar is neither here nor there - it's too soft for that, the issue is the weight of the trike is being taken by the spring/shock, and put into the side of the lower tube, and not even half way out. So, if the rear end weighs 280kg laden, half that 140, times the ratio of outboard to inner, times the ratio of the amount out of vertical, I suspect over 300kg pressure on that tube. That's a lot of bending load, plus shock loading of course. Damn, I'm frightening myself now....

 

[brightspark]

it looks rather long and a lot of overhang from the bottom of the unit not the best of design

 

[roofman]

Given the thin size of that tube then that shock should of been spread loaded between the 2 bottom tubes equally and so should of the roll bar too imho

 

[Ton-up]

Looks like that shock is mounted to the side of only one tube? Thats a terrible design in itself, probably twisting that tube on a weld and with the anti roll bar on the other tube as well.
A simple tube across (like on the top wishbone) would improve matters greatly.

 

[Joe 90]

I dont think its the welding. That's were it had to go. Its a very bad design. It should be on both tubes and further out like this kit car wishbone.

 

[minimutly]

Looks like that shock is mounted to the side of only one tube? Thats a terrible design in itself, probably twisting that tube on a weld and with the anti roll bar on the other tube as well.
A simple tube across (like on the top wishbone) would improve matters greatly.

You are 100% correct, not only is it bending the tube, it's twisting it too.

 

[pedrobedro]

It needs remaking like the Locost design with a plate to spread the weight and a proper bracket to support both ends of the bolt.

 

[Wallace]

What are the other two tubes connected to that are partially hidden by the tyre and driveshaft in the second photo?

 

[minimutly]

What are the other two tubes connected to that are partially hidden by the tyre and driveshaft in the second photo?

They are the rest of the "wishbone" - it's actually two triangles, or a parallelogram with a diagonal, there are two pivot points inboard and outboard so the tracking is set and can't be adjustable. Hence when I said I could link three tubes - but obviously unless the shock is in the middle the loads won't be equal.
There is a limit how far out the shock can go due to the narrowness of the frame and the width of track, so there will allways be some ratio to it. Anything that reduces the load on one element will be better, creating another failure point won't help.

 

[Steveblade]

Given the thin size of that tube then that shock should of been spread loaded between the 2 bottom tubes equally and so should of the roll bar too imho

I fully agree .

I dont think the anti roll bar will put much forces into the suspension components compared to the spring and shock but best over engineer it anyway.

 

[minimutly]

That antiroll bar is too puny to case any issues, but will prevent me moving the shock out. Maybe need to think long term and add moving this out over the winter. Another job to add to the list then....

 

[Wallace]

Plenty of scope there for improvement. Have they sold many?

 

[northwest]

That is quite shocking really, who thought that was going to work long term, your first instincts were right. The "designer" of that has no mechanical sympathy whatsoever. I would guess they were thinking the anti-roll bar mounted equally as badly would even things out. Just goes to prove two wrongs don't make a right!

 

[angellonewolf]

how long has it been built ?

if a year or 2 yes goback to the builder id be doing a bit more research on how the shock is mounted on most trikes and if most are mounted on the bottem lower bars

getting them to change it is going to be the hard bit most people dont like getting told they done it wrong

but a chat might get you some where