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PostPosted: July 31, 2019, 10:33 am 
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Hi people, I'm a newbie designing my own car... When it comes to using rod ends for suspension control arms, I noticed there are people mounting rod ends horizontally and some do it vertically.

Or both, like this one I saw in another post:
(sorry for stealing the image, just using as an example)
Image

I'm not a mechanical engineer so my knowledge is limited.. What is the better orientation to install them?

Mounting rod ends to a horizontal clevis (like the upper control arm on the pic above) would give it much larger degree of free articulation.
I also checked out some FSAE designs and about 90% of them mount rod ends to a vertical clevis (like the lower control arm on the pic), but the travel angle is a lot more limited. Is there other benefits to doing this? Maybe for better lubrication? :roll: :roll: Thanks!


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PostPosted: July 31, 2019, 12:29 pm 
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In general, you pick the orientation that favors the more necessary degree of freedom. Other factors are access, maintainability, and cost, in the case of having to go with a high-misalignment part. In your picture, the orientation of the rod end used for steering is correct, since the orientation favors the need to steer +/-45 degrees, while vertical angling changes will be a lot less. Same for the upper A-arms, where there's a lot of vertical motion, and very little angular change.

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PostPosted: July 31, 2019, 1:11 pm 
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I'd spin that LCA mount 90 degrees. Makes me wonder if there is some other constraint that isn't obvious to me from the picture.

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PostPosted: July 31, 2019, 2:30 pm 
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Agree about the lower A-arm inboard rod ends. Not sure why they wouldn't be oriented the same as the uppers. That said, "if" the car will have limited suspension travel such that it never exceeds the rod-end limits, then it's fine as-is. Of course, from a practicality standpoint, another reason to orient them the same as the upper A-arm is that when servicing the shocks, the A-arms can swing down fully, rather than resting on the shoulder of the ball against the bolts.

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PostPosted: July 31, 2019, 2:33 pm 
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TooBusy wrote:
I'd spin that LCA mount 90 degrees. Makes me wonder if there is some other constraint that isn't obvious to me from the picture.

Me too. A Heim joint's rated load is about 5-10 times higher in radial load than axial load. The lower control arm sees more load than the upper.

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PostPosted: July 31, 2019, 9:25 pm 
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rx7locost wrote:
TooBusy wrote:
I'd spin that LCA mount 90 degrees. Makes me wonder if there is some other constraint that isn't obvious to me from the picture.

Me too. A Heim joint's rated load is about 5-10 times higher in radial load than axial load. The lower control arm sees more load than the upper.


Good point. With the lower shock being between that and the balljoint, that heim is seeing a lot of axial load. No bueno.


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PostPosted: August 1, 2019, 10:12 am 
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The rod ends on the lower arm are oriented that way to resist braking loads. They do not have enough mis-alignment for that orientation on the upper arm, the lower arm carries the bulk of the brake loads and the rod end is stronger that direction than when you are trying to pull or push the ball out of the socket. Braking loads would be the highest loads the front suspension takes.

Those are beautiful fabricated uprights and I love the little fold he had to put in the sheet for the steering arm. This is a vey well and carefully thought out suspension.

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PostPosted: August 1, 2019, 10:31 am 
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horizenjob wrote:
The rod ends on the lower arm are oriented that way to resist braking loads. They do not have enough mis-alignment for that orientation on the upper arm, the lower arm carries the bulk of the brake loads and the rod end is stronger that direction than when you are trying to pull or push the ball out of the socket. Braking loads would be the highest loads the front suspension takes.

Those are beautiful fabricated uprights and I love the little fold he had to put in the sheet for the steering arm. This is a vey well and carefully thought out suspension.


Thanks Marcus. That's something I wasn't thinking about.

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PostPosted: August 1, 2019, 11:24 am 
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@horizenjob

Generally, I'm agreeing with everything you said, Marcus. However, I'm not seeing how the orientation of the joint helps much. If the forces at the inboard joints were pushing or pulling towards or away from the centerline of the car, yes I could see that.

What I'm visualizing is the wheel and rim rotating clockwise, and the brake caliper pinching down on the rotor with the forces that action generates going into the two bolts just behind the stub axle. Then, through the control arms, those forces get translated to the chassis. So, in gross terms, the 4 attachment points on the chassis must resist forces acting in a plane (pretty much, if not) parallel to a vertical plane through the car centerline, right?

I know from readings years ago that you're right about the lower control arm taking more of the braking forces, but I can't remember why. Do you recall?

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PostPosted: August 1, 2019, 12:07 pm 
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I disagree that the orientation of the rod ends matter on the LCA. On the other hand they do matter on the upper control arm as pictured.
The loads on the rod ends are point loads that are inline with the tubes of the LCA. In other words there are no effective radial loads at all. There would be radial loads if the shock was removed and the assembly was allowed to "hang" on the LCA. It is easier to assemble the LCA to the chassis in this configuration. The brackets on the chassis would have to be angled to be inline with the rod ends if the orientation was vertical and the top or bottom of the bracket be open to allow the rod end to be slipped in place.
The loads into the UCA rod ends are a combination of radial and axial loads. Any radial looseness in the upper rod ends will allow motion in the UCA but will be minimized in the LCA. The LCA is a better designed wishbone in my opinion.

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PostPosted: August 1, 2019, 12:22 pm 
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Lonnie-S wrote:
What I'm visualizing is the wheel and rim rotating clockwise, and the brake caliper pinching down on the rotor with the forces that action generates going into the two bolts just behind the stub axle. Then, through the control arms, those forces get translated to the chassis. So, in gross terms, the 4 attachment points on the chassis must resist forces acting in a plane (pretty much, if not) parallel to a vertical plane through the car centerline, right?
Lonnie, I may have this wrong, but think of the forces in the plane of the LCA under braking. The chassis mass and momentum apply a forward force at the lower inner joints while a resistive (backwards) braking force is applied at the lower ball joint end of this plane. I think Marcus' point is that in a horizontal orientation, the Heim joints are better suited to resist this loading.

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PostPosted: August 2, 2019, 8:06 am 
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The braking forces are actually rotational. Think you should look at the force as being applied at the tire/ground contact point. The lever arm wants to rotate about the axis. Davew


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PostPosted: August 2, 2019, 7:27 pm 
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davew wrote:
The braking forces are actually rotational. Think you should look at the force as being applied at the tire/ground contact point. The lever arm wants to rotate about the axis. Davew
I agree, Dave. The rotational braking force would be centered on the axle hub. The resultant forces would be applied to the upper and lower ball joints through the lever arms implicit in the upright. In the case of the LBJ, its force would be seen by the inner LCA joints through the plane of the control arm. Well, maybe... :?

In any event, Jack has a good write up on rod ends and loads that might help fourribs sort this out:
http://www.kineticvehicles.com/EdStraightArms.html

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PostPosted: August 3, 2019, 1:07 am 
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The loads on the rod ends are point loads that are inline with the tubes of the LCA. In other words there are no effective radial loads at all.


Kartracer is right and I don't know how I confused myself here. The arms would have no strength to apply a load to pull the balls out of the socket anyway... Sigh. I think it's nice to have the nut and exposed thread on the backside of the joint just to protect the threads a bit.

On the bottom arm it may be because he preferred to make the brackets that way.

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PostPosted: August 4, 2019, 12:10 pm 
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The lower arm is the correct design. I originally had the same orientation on the upper arm but discovered that it did not provide enough travel and I did not want to use high misalignment spacers or rod-ends.

Braking and cornering loads are both radial for the lower arm's rod-ends which is desirable since rod-ends are significantly stronger in this direction. The axial load is generated by the reaction force on the spring. This axial load is <10% of the load on the ball-joint so it is the optimum orientation for this force rather than braking or cornering loads.

Braking and accelerating requires forces in the fore/aft direction, cornering in the lateral direction, and bump in the vertical direction. These forces create a resultant vector. As Kartracer stated, this is in line with the tube's axis however the forces transmitted through the rod-ends that create this resultant force are not. There will almost always be off axis loading at the rod-end. Depending how the rod-end is oriented will determine whether it is axial or radial.

Mathematically it is easier to start at the tire contact patches and work in but if you are masochistic you could start at the CG and work out. Either way your numbers should be the same since you are dealing with Newton's Laws.

Two compromises were made in the upper arm -
1) The rod-ends were rotated to increase available angular movement as required by the shorter arm
2) The rod-ends were oriented perpendicular to the chassis centerline to accommodate the camber adjusters

It isn't the best engineering practice. If you are going to do it, the upper arm is the better location since its loads are lower due to the upright's geometry resulting in lower upper ball-joint loads. I'll let Rudy14 confirm since he now has the car but I believe these arms are doing just fine after 8 years.

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