Oh no ... not again ... Street Fox fail

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May 15, 2011
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Hi Folks,

It's been while since I posted anything on here (although I have been a visitor in the background) and now I'm back asking questions. So my apologies for that.

But I just completed my weekly once over on the Fox and have discovered that the main body to boom joint has failed again 😕 Five years ago this happened and I took my Fox to the local agricultural fabricators and they re-welded my original joint. Since then I have been doing a weekly visual check of the Fox just to make sure I am not caught out or worst still CRASH !

I've really got into my trike again this year, having completely eliminated my shimmy issue altogether and doing over 500 miles since lockdown began here in the UK. Now this ....







I could go back to the fabricators and ask them to re-weld it again. But it is likely to fail again in the same place. So I need something a bit more permanent as a fix this time.

What do others do for this joint ?
I'm guessing that I'm not the only one to have this issue ?

Given the stresses on this joint, in all directions, this has got to be the most vulnerable joint on the Fox.

Some pics of your solutions would be really very helpful.

Thanks all, I appreciate your help and advice in advance.

Cheers
Si
 
Joined
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Looks like you need to add some gussets. You can make them from the same tube as the frame to fit fore and aft of the joint, and can also add a pair underneath, left and right. Given the angles of the joint your gussets may need to be made from several pieces.
 

SirJoey

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Precisely what I was thinking, but I didn't have any good pics displaying an example.
Good one, Popshot! (y)
***
 
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A simpler solution would be to make 4 plates shaped like this out of 2 or 3mm plate and weld them on fore and aft both sides. Make sure to weld both sides of the lower part and under the top tube for maximum strength. Either solution would require the grinder out around that crack and that welded up first. It'd be simple enough to notch the plates with a grinder at the point of that large weld so the plates sit flat. Either that or that weld needs flattening off. The former is the better option.

 
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Radical Brad

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The gussets will certainly do the trick.

It's actually a strange failure considering there is not much stress at this point of the frame. On a 4 wheeler, there would be, but on a trike, any side to side would not be met with resistance at all. I can only guess that there was some weld anomaly originally, and they just chased it when doing the repair.

Flap disc the paint away from the joint, weld in the gussets, and then reweld that joint, ahead of the crack.

Have to ask...

Are you a physical beast? By that I mean.... over 280 pounds and capably of dead lifting the front of a small car?

Brad
 
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Hi Folks,

I've really got into my trike again this year, having completely eliminated my shimmy issue altogether and doing over 500 miles since lockdown began here in the UK. Now this ....



Cheers Si
I think the clue lies in that phrase ?

I don't think you have eliminated shimmy ? I think you have added a steering damper on the left front wheel ?

I suspect that it is transferring all the energy of the shimmy into that left hand joint ?

Whether that is correct or not ? adding more weld will not fix this problem !

The weld just makes the joint thicker at that point and so the stress then concentrates on the nearest thin section , which is why the cracks appear alongside the weld/tubing junction.

The welds themselves have not failed the tubing has.
 
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I'd suggest that there is in fact a lot of stress there. Any potholes will jar a three wheeler quite easily and more so than a 4 wheeler. The three wheeler will always dip straight into any pothole as it finds the three lowest points of contact. A 4 wheeler will hold the wheel at the pothole up to some degree by balancing to some extent on the remaining three wheels. Any force applied to the wheel from pothole and rider has half of the track width to lever against that weld. In an ideal world the tubes would be the other way round with the main beam on top so all the weld was doing was holding the tubes fixed though I'm well aware the Streetfox is designed to be as simple to build as possible and a dropped axle would leave that ethos behind.
Paul's point on the damper is definitely valid. If there was a shimmy it was probably the cause of the original failure and after fitting the damper the force is still going into that weld but not at the same rate. Nothing will kill a weld faster than an oscillating force. The caster is probably the key to the shimmy as mentioned. Failing a castor issue I'd look to a wheel bearing or a bent axle.
 
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Not wanting to rain on your parade further ?

It looks to me like your front axles are 9mm ?

For your own safety they should be 14mm or greater.

Commercial trikes using Sturmey Archer hub brakes have 12mm axles , definitely the smallest diameter you should be using.
 
Joined
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Hi Folks,

Wow, thank you for such fast responses and great advice. I shall try to answer all the questions and queries.

I dug into my files for my Fox build to see if there were any clues there. Being my first time welding and doing a project like this I did try to follow the plans as closely as possible.

The axles are 14mm BMX, and from my notes it looks like I was shooting for a caster angle of 15 degrees, as per the plans.

Here, some pictures of the original fail.





As I say I took the frame to the fabricators in the village and they re-welded the joint.

BEFORE I did that I did mock up some gussets based on angled steel, which would be welded to the main tube, with a vertical plate which ties to the boom tube.



These would sandwich the main tube.



My idea here was that using angled steel as a base for the gusset, this would be one piece of steel, so no weld to fail at the right angle between the main and boom tubes. Then the vertical section is added. The appeal was also that apart from the three inner 90's the welds where are easy to get to.

Anyway the fabricators re-welded my joint made a much better job of it than I did and it looked OK.

Resent changes and upgrades to the Trike which could have contributed ...
  • I have added a steering damper. Before I had a shimmy, which I did post about years ago, with video on You Tube. I was using rubber washers on the tie rod to reduce this. The steering damper has stopped "the effects of the shimmy", and yes I agree with the comments it has not "cured it"
  • I have changed the front tyres having scored some brand new ones on eBay for less than half price. So I have gone to being able to run higher PSI up front than before with BMX tyres. So lost some tyre damping effect there in favour of less rolling resistance.
  • New rear wheel (doubt this, but a recent change nonetheless).
  • Other changes, well using it more than ever before. Covid charity challenge at work I wanted to complete on my home creation.
My journey to work is country roads, pot holes, some very rough surface and steep left camber, very steep climb, together with single speed bumps.

Before I spotted the fail after it had failed on both sides. This time I have spotted it early. As before the weld has not failed, but the steel next to the weld.

Finally to answer the size of the Fox engine question ..... 190lbs, 6ft1. Never tried to lift a car !

So I thought I would spend today with Sketchup and come up with some ideas, based on all your comments and advise, and tailored to my welding ability. Will post again with those ideas.

Whilst I am fixing the frame and everything is in bits, I will check and strip the front wheels and king pins.

I appreciate the helpful advise.

Cheers all,
Si
 
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Your strengthening plan is / was fine and should add plenty of strength. 14mm axles ought to hold up for a 190lb rider but Chinese steel is often less than high quality. One of the donors I've had recently had a bent axle which would launch the wheel from the frame according to the seller. The seller knew of the effect but not the cause. Such a defect may be spottable on simply spinning the wheels or may not. The one I had was only obvious on pulling the axle out.
 
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If you go down that route cut the ends of the plate against the main boom at 45' ?

So a 45' cut where the pulley axle is [ I assume that is what the bolt is ?] pointing forwards.

Cut a matching one at the front , then on the other side do the same but reverse them.

That way you won't create a sudden change in thickness of the main boom , that should help stop all the stress being concentrated in one place .



The thick bit is your weld and the thin bit the base material ?

So the lines of stress concentrate at the change in section , the 2 ends where the ' F's ' are , are stable the concentration point is where it will fail.

Your additional pieces attempt to move the stress and spread it over a bigger area hence reducing it's effect locally ?
 
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That is why I added a tria angle ad the front. I did it on the top of the main frame, but my arm is straight and had no angle in it. Better was to ad one in the rear also, but for now it is OK.

All forces are in the 2 welds that break. That is why I added the tri angle.

Next build will get the arms to the side, but I ad there also some support. Takes off the stress on the welds.
 
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The WD-2 trike made very effective use of "gussets" and I think that trike frame will be pretty bomb-proof.
 
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Hi Folks,

Thanks again for all your input. I have spent some time playing about in Sketchup.

Based on right angle steel (green) and flat plate (blue) I've come up with two gussets, which will sandwich the main boom. I am thinking this should be fairly straight forward to make. The welding on the other hand is another story. The welds on the inside of the two triangles underneath will be a challenge. ;) ha ha ..... I like a challenge !





I will cut the ends of the two green plates at 45 degrees as suggested (if I understand correctly). Just could not draw them in Sketchup.

This will also have to take into account chain routing .... another challenge !

Cheers
Si
 
Joined
Sep 16, 2018
Messages
333
Location
Netherlands
Hi Folks,

Thanks again for all your input. I have spent some time playing about in Sketchup.

Based on right angle steel (green) and flat plate (blue) I've come up with two gussets, which will sandwich the main boom. I am thinking this should be fairly straight forward to make. The welding on the other hand is another story. The welds on the inside of the two triangles underneath will be a challenge. ;) ha ha ..... I like a challenge !





I will cut the ends of the two green plates at 45 degrees as suggested (if I understand correctly). Just could not draw them in Sketchup.

This will also have to take into account chain routing .... another challenge !

Cheers
Si
You can also put them on top.
Than you have no problem with the chain.
 
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