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PostPosted: August 11, 2017, 2:24 pm 
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I'm stuck on this one....

Background: I designed a new suspension system for a kit car company, who apparently was sold but none the less the prototype car was built which my buddy now owns.

Here is the design
Attachment:
Old suspension design.JPG



He got a chance to drive it the other day and heard a weird sound. The LCA on the rear wheel was hitting the wheel! The wheels didn't match the cad model since they bought them after the design was done so that was an issue. He thought he had fixed the problem with a spacer since when he moved the wheel up and down way past full travel it didn't hit (coil over off) but once we put everything back together with the coilover... bam! it hit again. We spent hours looking for the issue then my calibrated eyes seemed to notice that the LCA was twisting. With some measurements we verified that it is twisting or something is allowing the shock side to drop significantly further then the otherside.

I did a simulation and it looks like it should only be deflecting .031" at 1000lbs. I fixed the the ball joint cup and chassis pivots (I cannot simulated the ball joint movement or the bushings so there is some of the issue).

Where I am stuck is trying to figure out what is moving. So either my simulation is way off (maybe more complex than i can do with my system) or I am completely missing something. Thoughts? I am going to try to set it up in my way more advanced sim software but that will take a while.


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PostPosted: August 11, 2017, 3:39 pm 
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I wonder if the lower control arm ball joint cup needs to welded in at an angle........?

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PostPosted: August 11, 2017, 4:13 pm 
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If it's really like the model it seems very precarious. The car hits a bump, the shock attempts to resist the upward movement of the arm - putting a twisting force into the arm with a moment arm between the lower ball joint and the shock's lower attachment point. Depending on both spring rate and shock valving, the force could vary between a little and enough to quickly fatigue-fail the arm at the outer end - the fact that the outer end is hitting the wheel is proof that it's happening. I'm curious how you came up with the force to use in the simulation because it isn't trivial figuring out what it'll be. The problem is that there is no calibrated potholes... After it's all said and done however, you already know it's failing and that it has to be strengthen. I'd extend that "tube to nowhere" the rest of the way out to the outer end, and also redo the whole thing with larger-OD tubing. I'd also add a cross tube along the inward edge of the flat plate.

For "fun", measure how much it's deflecting now to reach the wheel, then back- calculate the force needed to achieve it. Note that the deflection could be far worse, except that the wheel stops it from deflecting further.

Edit: rereading the original post, apparently it's happening at rest. While I'd still redesign the arm, in your specific case about it's likely that it's not modelled right.

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PostPosted: August 11, 2017, 5:07 pm 
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Quote:
I wonder if the lower control arm ball joint cup needs to welded in at an angle........?


Not until you really figure out what is going on!

There may be a lot going on here, please excuse me if I mention some obvious things, but some people reading this might benefit from that. I did some FEA for a member here and we found the lower rear arm was flexing a great deal. I don't remember the number but it was well over an inch. At that he said that this explained why the jam nuts on his rod ends refused to stay tight. The rod ends couldn't come loose because of how they were bolted on, but never the less, the jam nuts always backed out. When you don't use trailing arms there can be a lot of torque on the arms from braking and the offset of the coilover mount. So I advise the trailing arms, they make for a simpler set up that is easier to understand. I know that doesn't help you now though...

Another thing is that the spindle, axle pin and rim also flex, none of which is helping you.

For the FEA, I would set the balljoint fixed vertically but allow rotation and movement horizontally. The inner pivots should be fixed vertically and horizontally but allow rotation. The FEA is showing you the stress in that picture but not the strain, I assume. So the flat plate may not be stressed but allowing a lot of bending at the same time. Welding the tubes to the ball joint collar will help with that.

On my car's lower arm I found significant stiffness with a tube crossing about where the back of the plate is. I'll try to find my pictures of this.

Your spindle may also be rotating under braking, that would make it look worse on one side compared to the other side of the car.

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PostPosted: August 11, 2017, 5:33 pm 
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I definitely wouldn't say that design gives me any warm n' fuzzies. If nothing else I'd at least cut a section of tube in half lengthwise and weld it in, running between the legs and directly under the coil over bracket, as well as extend the legs much closer to the ball joint cup. With the existing design, the latter might look something like essentially fabricating a conical section that tapers down as it approaches the ball joint cup.

If the constraints in your analysis have 0 degrees of freedom at all of the joints, then your results cannot be trusted. Over-constraining the system can make a big difference, and not in a good way. As always: junk in = junk out.

Also, unless you're running rigid bushings, there would be another source of deflection to investigate.

How much clearance is there to the wheel without the coilover installed?

He should be thankful that he didn't go with any larger of wheels, which would have masked the problem, and could have ended in an ill timed catastrophic failure at speed.

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PostPosted: August 11, 2017, 8:32 pm 
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What kind of ball joint is it? Is it possible to get one with a shorter stud?


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PostPosted: August 11, 2017, 11:12 pm 
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Yeah I already redesigned a new control arm a few days ago the way I think it should be with the tubes intersecting the joint cup and reinforcement under the plate. I also moved the shock mount in as much as possible.

I just didn't want to fab up a new one without making sure it's the arm that is flexing. I think this will work but I won't know until it's on the car.


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PostPosted: August 12, 2017, 11:09 am 
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Here's pictures of a couple of iterations on my rear lower arm. After I did the FEA I landed up using a larger diameter rear leg on the arm. I also made the arm less wide which shortened the car's wheelbase by 3 inches. I really, really wanted the coilover mounted on the perpendicular leg of the control arm, but just wasn't able to work it out.

The coilover attaches to the arm where the short green rod goes thru the bracket.

The pictures don't really help you because it's a different shape arm.

Above I mentioned that if the problem is caused by braking, it would show up on just one side of the car - but that is wrong. The arms are left/right, so on the other side you have the same problem.

Consider applying a 1000 lb. load at the contact patch, pointed rearwards to simulate braking. This torque will be resisted by the control arms and you need to figure the leverage on them, they will see maybe more like 2000 lbs. if the they are half way the distance from the road to the axle spindle.

Get the FEA to agree with what is happening in the real world before you settle on a new design.


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PostPosted: August 12, 2017, 12:19 pm 
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Related to what horizonjob said, try to move the lower shock mount as far outboard as you can, which will minimize the twisting torque. Perfection would be attaching the shock directly to the upright.

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PostPosted: August 12, 2017, 1:00 pm 
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That's as close to the upright and inward as I can get it.

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PostPosted: August 12, 2017, 5:19 pm 
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Markus, I'm not picturing how your coilover is attaching. I assume the two inner pivots (At the top of the pic) are for the chassis mounts and the outer pivot is for the upright correct?

I also don't see twisting of the lca on my personal car but the push rod is really close to center and to the upright. My legs are also much further apart. I originally did a sim on my a long time ago but I don't have access to that version of solidworks anymore. I do have Autodesk mechanical simulation though but I am still learning it. I think in mechanical I can add the ball joint and upright in and get way better results. It keeps crashing on the dims though

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PostPosted: August 14, 2017, 12:35 pm 
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It wasn't really helpful to post those pictures. My car uses a reverse wishbone with trailing arms. It has a single inboard mounting point and the two outboard mounts connect to the bottom of a Ford IRS upright. The trailing arms run along the outside of the cockpit.

The problem we run into with control arms like the one you're working with is that they have a short vertical height so they have a hard time resisting twisting forces. If you look at the OEM control arms that go with the FORD IRS I am using they are pretty massive and are more than twice the vertical height of the tubing we tend to use. This makes them a lot more stiff.

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PostPosted: August 14, 2017, 1:03 pm 
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Yeah the only other thing I think i can do is to add anther support to the rear or maybe one diagonally and/or increase the tube size.

The more complicated FEA is a no go. it keeps failing.


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PostPosted: August 14, 2017, 3:52 pm 
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Maybe consider a cross tube that goes to the middle of the curve on curved tube? Then the shape is a little more like two large triangles and I think it will resist fore and aft stress better.

Maybe you can change the FEA to use different but simpler shapes. For instance remove the plate or the ball joint collar. I understand you aren't simulating the real world, but it might still be educational in terms of how some generic thing like this reacts to forces. The FEA program I use, Grape, would handle this without the plate and using a mitered joint instead of a curved tube.

Larger tube sizes would make sense if since the forces don't seem to be simple tension / compression.

Failing software solutions, there is certainly nothing to be embarrassed about doing this in the real world, bolt the wishbone down to a workbench and use a 2x4 to apply a stress to it. They do that with commercial airliners still and they don't seem embarrassed. Impressive to see a wing with stacks of sandbags on it!

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PostPosted: August 14, 2017, 4:36 pm 
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So more like this...


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