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PostPosted: September 3, 2017, 5:04 pm 
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Joined: June 28, 2016, 9:21 pm
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Hello guys,

I am wondering if I can use heim joints as upper and lower ball joints. I don't see a lot of people doing that. The purpose of the vehicle is local cruising in summer and occasional track use. Are they strong enough? Will they hold up over time? What brand of heim joints has better quality for this job?


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PostPosted: September 3, 2017, 5:24 pm 
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That is completely fin but you have to buy the good ones. Check out Jack McKormicks site http://www.kineticvehicles.com/rodends.html he goes over the types in detail. You probably want to go 5/8" or bigger.

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PostPosted: September 3, 2017, 6:23 pm 
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And look over his control arm test. http://www.kineticvehicles.com/WArmTests.html

I used Jack's LX series 5/8" rod ends for my uppers. Upper control arms see a lot less stress than the lowers. I didn't have any problems with them there and after ~6k miles were still in very good shape. I really wouldn't use them on the lower arm myself. If you do decide to use them on the lower, spring loaded control arm, be careful how you orient them. They aren't rated as high when side loaded.

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PostPosted: September 4, 2017, 1:33 pm 
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I'm going to give a dissenting opinion, and here's why:

A spherical joint is much stronger when loaded radially than axially. A rod end (spherical joint with a threaded shank) loaded in bending of the shank but still also radially, as in Jack's experiment, is less than ideal compared to loading it purely in tension/compression. However, if oversized enough has proven to work. Jack's test proves it's rather strong in Yield, but for steel you also want to keep your loads less than half of yield load to maximize fatigue life. In addition to that, I personally wouldn't use a factor of safety less than 2. So now that means sticking to a load of no more than 1/4 of yield for general garage engineered steel products.

But even if your rod ends meed this criteria in radial bending, I would NOT recommend using a it as a lower ball joint in the front suspension, as it sound like is what you're specifically asking about. In order to maintain any reasonable semblance of steering range of motion, bump loads will feed into it axially. I believe the general guideline is that a spherical joint has an axial load capacity of 10% it's rated radial load capacity. So that 17,000 pound alloy steel rod end is now a 1,700 pound rod end. Yes you may only have 350lb each front wheel, but what is the impact force from a pothole? Is it 2G, or 4G, or 10G? By the time you start talking about a big enough rod end for me to be comfortable with it, you might as well use an automotive style ball joint.

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PostPosted: September 4, 2017, 5:04 pm 
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Driven5 wrote:
I'm going to give a dissenting opinion, and here's why:

A spherical joint is much stronger when loaded radially than axially. A rod end (spherical joint with a threaded shank) loaded in bending of the shank but still also radially, as in Jack's experiment, is less than ideal compared to loading it purely in tension/compression.


Gotta remember though, the inner end of the a-arms are heim joints. The upper arm in particular, is basically just keeping the wheel at the correct camber. Since it's pin jointed at both ends, it can never be in bending up and down. And the only way it would see stress fore-aft is under braking, where it would take a component of the braking force. In this case a heim upper balljoint is still not being loaded in the 'worst' direction, so for those reasons I'm going to side with the 'it's ok' crowd.


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PostPosted: September 4, 2017, 5:24 pm 
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The question was asked if both upper and lower ball joints. Simply saying "it's ok" means you think it is for the lower as well.

The uppers have been demonstrated to achieve acceptable results with an appropriately sized rod end/tie rod end, or better yet, spherical joint/mono ball.

However, the lower is inherently (as opposed to "never) placed in "bending up and down" by the shock/spring. It sees far greater forces than the inboard joints due to the leverage ratio of the outboard joint being much closer to the shock/spring than the inboard joints, in addition to it having only one joint outboard vs two inboard. Again, this is all occurring with the inboard lower joints also typically placed in radial bending relative to the spring/shock loading, as opposed to the outboard lower being in axial (worst case scenario) bending.

There is a reason that this configuration is almost never seen in the lower outboard location. At the very least, if you do use rod ends for lower ball joints, use rod end safety washers (not just regular washers) to catch it in case the ball gets forced out of the body.

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Last edited by Driven5 on September 4, 2017, 6:28 pm, edited 1 time in total.

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PostPosted: September 4, 2017, 6:24 pm 
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Driven5 wrote:
Simply saying "it's ok" means you think it is for the lower as well.


Oh. No, I only meant the upper.


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PostPosted: September 4, 2017, 11:43 pm 
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+1 for Driven5, with a few exceptions.
Driven5 wrote:
Simply saying "it's ok" means you think it is for the lower as well.
Nah,
it means he said it wrong.
Driven5 wrote:
It sees far greater forces than the inboard joints due to the leverage ratio of the outboard joint being much closer to the shock/spring than the inboard joints...
Right you are.
Driven5 wrote:
...in addition to it having only one joint outboard vs two inboard.
Right you are PROVIDED the distance between the two inboard joints is greater than the distance from the inboard joints to the outboard joint. Back in Ye Olde Days, when F1 cars had real front suspension and long skinny control arms, the inboard rod ends of the LCA could be the heaviest loaded joints in the car.
Driven5 wrote:
At the very least, if you do use rod ends for lower ball joints, use rod end safety washers (not just regular washers) to catch it in case the ball gets forced out of the body.
My two cent's worth on this subject: don't use a rod end as a LCA ball joint. It'll end up carrying the weight of the car plus whatever loads come from bumps, and the rated axial load for a rod end is typically 10%--15% of its radial load. Yes you can make it work with a big enough rod end, but a big enough automotive ball joint is smaller and lighter than a big enough rod end. And while it's unlikely that a ball will leave a rod end in an UCA, unlikely ain't never, and you may not get past the tech inspectors unless you have rod end safety washers on your UCA (as Driven5 described for LCAs).

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PostPosted: September 5, 2017, 2:50 pm 
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Just another data point: many of the long travel type dune buggies use rod ends for the outer lower control arm. They are actually loaded radially. The rod end, when combined with tapered spacers, gives enough travel for steering. These long travel buggies are approaching 2000 pounds; many have 500+ HP; and they are routinely jumped. Also, keep in mind they live in an environment made purely of sand, so they are able to live (for some length of time) in a dirty application.

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PostPosted: September 5, 2017, 9:29 pm 
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Well dang, Ken's right too! I didn't mention it on the assumption you were going to use them with donor-car spindles, but there's nothing really to indicate that, is there?

So if you're making your own spindles, and you can mount the outboard rod end with its bolt horizontal (which will probably call for high misalignment spacers, or a very limited steering lock and a very big turning circle), then you'll have radial loads, and the failure point will be the bending load on the rod end shank.
Attachment:
File comment: These can be had with 5/8" through bolt and 3/4" shank.
TEMP.jpg
TEMP.jpg [ 39.09 KiB | Viewed 1599 times ]

However, having oriented your rod ends for radial loads in up-and-down suspension operations, now your brake loads are axial loads, and they'll be roughly double what the tire sees (since the LBJ is the fulcrum between the tire contact patch and the UBJ) and that probably doesn't matter much on a sand dune, but could be a big deal with slicks on asphalt.

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PostPosted: September 6, 2017, 12:29 am 
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I have an additional take on this. Don't use a rod end because the entire load is appearing on the threads of the shaft in bending. Both vertically and horizontally. That's just not good practice, using a piece of steel with big notches in it ( threads ).

If you would like to do this use a spherical bearing in a weld on cup. This removes the issue of threads and these bearings are available with up to a %20 axial load rating, if I remember. This is a typical formula car setup. You would expect to use quite a large unit for this, but it could save space. They sell these for this purpose and others in the catalogs.

A spherical bearing is like a rod end bearing but has no male or female shaft, it is just the bearing part. In oval track catalogs they are sometimes referred to as "uniball" bearings.

Do not skimp on the size for a control arm that has a coilover connected to it.

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