LocostUSA.com

This is the second time this has occurred and it would be the third but I caught it before it happened. I don't understand why. What is the force on the car that is ripping this thing off? This last time I had welded and braced the mount so good that from the pic you can see it actually ripped the metal off this time. Its not like my weld broke and failed. The force is actually pulling the metal structurally apart. None of my other welds or mounts are failing. Just the lower rear mount on the front side. To describe it better it is the lower A-arm mount on the passenger side that is closest to the passenger.

Only thing I can think of is the camber isn't right? or caster? or what could be putting this CRAZY force on that mount?

J. R.

_________________
Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
My Car: Custom Build Lotus Super 7

I don't have any words of wisdom as to how it broke but I am very interested in what others have to say.

Where you going straight or turning when it completely broke in the past and what were you doing when you noticed it this time?

Is that a front lower arm forward mount? Any chance of getting a picture of the failed weld and the back side of the bracket?

I'd say the bracket seeing significant force and being welded to the middle of a thin walled tube is probably a contributing factor. Good thing you caught it.

Several builders have made mounts that are welded on the front and rear faces of the tube. The welds end up being in shear and the greater weld area helps to distribute the load. It also helps if the welds end on the edge of the tube since it feeds into the tube's sides rather than a perpendicular face. Plus you can size the bracket to increase the weld area.

Were there shims centering the rod-end on the bolt?

_________________
-Andrew
Build Log
Youtube

Couple of things.
First, noted with the red circle, where are your spacers? It "looks" like the rod end was able to slide back and forth on the pivot bolt, indicated by what looks like marks on the bolt itself. This could have certainly contributed to that pickup being literally 'beaten' off the chassis.
Second thing is that it would appear that the welds were not sufficient here as only the top area, green arrow, seemed to get the penetration. Most likely the last area to give way with the pickup being wiggled back and forth, it just fatigued the chassis and tore it away.
I might take a guess that the weld was too cold, being the chassis and pickup had very different wall thickness, it seems maybe you welded at the thin wall tolerance rather then the thick wall.

1. I had taken the shims off and not put them back this time, but in the past they have been there.

2. It has occurred every time when I am going slow pulling in or out of the driveway going full lock. This makes me think it is the caster as when I go full lock the caster causes the tire to tilt so that it will auto correct back to center. I used to run more caster but I have since backed it way down in what I thought would solve this problem and then this happened.

3. I agree the weld may have been cold. I usually use straight helium gas with my 110v welder since IT WILL cause the weld to run hotter and I am used to setting it up knowing that. I had borrowed a buddies C25 bottle which I usually only use with 240v welders. I will re-brace and weld the bracket this time with straight Helium and go for a little hotter welder and see what we get.

J. R.

_________________
Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
My Car: Custom Build Lotus Super 7

Does the jam nut come into contact with the plate below the mounting tabs when the suspension is in droop?

Let me add even more to the story. I have over braced this area now also because on the 2nd time this happened the bracket didn't fail, and non of my welds failed.....the actual metal between upper and lower parts of the frame that the bracket is welded too started to crack. NO WELD area the actual metal started failing. For some reason there is some CRAZY force being applied on that lower rear mount. What is strange is I have checked the driver side and it is fine. The driver side is still the original way, meaning it is not over braced but built like everyone else's cars are.

SO CONFUSED why so much force is being applied. Anyone got any ideas? I will try and experiment and report back

J. R.

_________________
Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
My Car: Custom Build Lotus Super 7

I am wondering about the jam nut hitting or if you have something in your geometry that causes the suspension to bind up.

_________________
Parts left out cost nothing and cause no problems!!

I don't see this as a weld issue. There's enough good weld area to hold the bracket in place under normal loads, and it's failed three times while none of the other welds has failed, and it's ripping the metal and not failing at the joint. I think JPS and seven13bt are on the right track. I think it's a bind issue.

First. ... Is the originally welded surface of the bracket laying horizontal in the provided picture while its "normal" position to have the rear (welded) face vertical?
It appears to me from the picture that this is the case which would make the a.moore's statement "Several builders have made mounts that are welded on the front and rear faces of the tube. The welds end up being in shear and the greater weld area helps to distribute the load. It also helps if the welds end on the edge of the tube since it feeds into the tube's sides rather than a perpendicular face. Plus you can size the bracket to increase the weld area." of greater import.
Putting the applied loads into a weld joint rather than the base metal (the chassis tube) focuses the applied forces on the welds themselves which are, by their nature, less resistant to fatigue than the parent metal.

2nd. .. The weld line directly underneath the green arrow looks suspiciously like it had rather poor penetration into the host material (the chassis tubes).
I suspect an incorrect penetration angle rather than a "cold joint" from what I see in the picture.
It may be something as simple as your physical access to the chassis making it difficult to hold yourself in the right position to get good access to the joint which would explain why it happens on one side but not the other.

3rd and probably the least telling. .. is this the front or rear tube of the control arm and is this leg of the control arm perpendicular to the chassis or at an angle?

Edit>>> you answered one half already, I just failed to note it. .. front LCA/rear mounting point.

Normally a suspension pickup should have no problems with force application you describe as "going in and out of the driveway", I rather suspect that this is actually when the loads occurs slowly enough that you can see the results.
In my opinion hitting a pothole at speed has a much greater potential for catastrophic failures by creating momentary point overloads well beyond the response rate of the suspension.
Typical loading of that weld is down and apart (in tension) at an angle shallower than the shock's included angle. .. imagine the lower shock mount as the pivot with the load pushing up on the lower ball joint.
The fact that the pivot moves during suspension travel doesn't change the length of the levers involved, it only lessens the applied load since some portion of load is absorbed by the spring.

Out of curiosity, what sort of potholes do you grow in your area? Out here they're the size of Yugo's and if, like most people, you tend to cross them with your offside the issue may be a simple matter of repeat overloads but I rather doubt it.

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Can you not turn the wheels full lock and get someone to hold tension on the wheel while you jack the arm up slowly.
You might be able to see or feel the bind?
Also you might have to remove or loosen the spring off so the car itself does not rise.

Al

_________________
Super Seven 3.4

An Excellent question. I don't know for sure and will check when I get home. GOOD THINKING!

J. R.

_________________
Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
My Car: Custom Build Lotus Super 7

I think SOME of the weld may have been cold, but I welded it on all four sides and you can see on the top weld that it ripped the metal right off.

I will double check for binding today and also look to make sure the jam nut is not hitting it in any way. Also to be clear it is mounted regular. In the photo the mount has fallen down to that position. That is NOT the position it is welded on the chassis.

I will go home and collect data and more photo's if necessary. I REALLY appreciate the feedback I am getting from you guys.

J. R.

_________________
Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
My Car: Custom Build Lotus Super 7

I suspect much the same, as it appears that a huge force is pulling the A-arm away from the chassis. Having something like the check-nut bind would create an enormous leverage where the arm could be "levered" away from the chassis.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Most rear lower control arm failures are caused by brake reaction forces. You typical see this type of failures under a high cycle stress input form the the twisting load applied. Check the front bracket connection, to make sure the front arm and bracket is sharing part of the load. Check that connection area also for blinding and clearance as well and the spacer(s) are tight. The 2nd problem is the repeated welding in the same area. This cause an heat effected zone which makes the surrounding metal brittle. Cut out that area or make a complete glove on three sides of the down tube also ties into the lower tube. The patch should be large enough that you are welding on virgin metal. Go with the suggested bracket design with the welds on the side of the down tube vs the front face. Dave W