Front Ball Joint Discussion Thread and Potential Safety Concern

[Turn In Concepts]

Has anyone other than Bob and I given thought to the way the outer lower factory ball joint that is located in the bottom of the knuckle has the potential to be popped out of the socket since the force path has been changed with the relocated spring/damper?

In the factory Subaru configuration, under a heavy upward movement of the suspension (hitting a pot hole) the force is applied upward from the wheel through the hub and knuckle and into the spring/damper and then the chassis. The lower ball joint simply follows this motion and is under little stress. On the 818, the spring/damper is relocated from above the knuckle to the lower control arm. Now when a bump is hit the lower control arm is applying the force directly to the damper and into the chassis from the wheel and knuckle and relies on the connection of the ball joint to the knuckle to do so. There is now a separating force on that ball joint that is taking place and pulling in a direction that could separate the ball from the socket. In all other double wishbone designs the lower outer ball joint is reversed from the 818 configuration such that when the force is applied there is a closed end to the joint on which the ball can bottom out so as not to get pulled from the socket.

I worry that in a very hard bump the front suspension could pull apart.

This is all just theory as I have done no actual FEA or force path analysis to determine if it would hold together, but figured I would put it out there.

Tony


This picture is the 818. You can see the socket end of the ball joint in the knuckle.



Here's a NA/NB Miata which has a double wishbone setup. You can see the lower ball joint is oppositely arranged. The ball joint is behind the tie rod end, but in the same configuration.

[Rasmus]

Has anyone other than Bob and I given thought to the way the outer lower factory ball joint that is located in the bottom of the knuckle has the potential to be popped out of the socket since the force path has been changed with the relocated spring/damper?

Yes. There are several old threads discussing this very topic.

[Frank818]

I worry that in a very hard bump the front suspension could pull apart.

I live in the place with probably the worst road conditions in a non-3rd World country, so I'll let you know how the suspension handles next summer after I start driving the car.

[Boog]

After the spring relocation isn't it the same separation force regardless of the orientation of the ball joint?

Is the concern that the ball joint shaft will leave its housing? or that the ball joint as a whole will detach from the knuckle?

[xxguitarist]

Boog, No, not in this case. Previously, the spring/damper acted on the top of the knuckle, so there was a direct force line from the hub, through the knuckle, into the shock. The ball joint just kept things aligned. The lower control arm was unsprung, just a geometry controlling feature.

Now, a force on the hub will go through the ball joint, pulling upwards on the lower arm, which then compresses the shock. The ball joint will see significant tension loading.

Note, I'm not making any claims either way for how much of a concern this is, or isn't.

Here's a crummy sketch of the mechanics in place.
Hub/knuckle is red, control arms green. Ball joint black.

[billjr212]

i agree that the concern is valid and the design change is significant. however, the design of the 818 front suspension is similar to what has been used on the roadsters and I'm not aware of any failures of the nature that we are discussing here on that model.

[xxguitarist]

Bill,
Do you know what ball joint is used on the roadsters?

[billjr212]

Not sure. My roadster used the donor lower control arms from the Fox Body mustang. I *think* I swapped out the ball joints with OEM equivalent ones from NAPA, but since it was ~7 years ago that I built it, I can't say for certain.

[C.Plavan]

Remember this is a very light car also. Those ball joints are beefy

[Jaime]

That ball joint isn't just clamped in there, the bolt sits in a ridge in the ball joint body. It would take one heck of a lot of force to separate it.

Also, this issue is more important than being discussed here - the front end of the car literally hangs from that joint, even when you are not hitting a bump.

[Buzz Skyline]

Yes. There are several old threads discussing this very topic.

I've been hunting around for a while and haven't found one of the threads that covers it yet. Can you point us to one?

It shouldn't be that hard to determine how much force it would take to rip out the ball joint, I would think. Has anyone done a calculation?

[Rasmus]

Assuming two things:

1. The 818 weighs 818kg
2. The weight distribution is 40/60 (front/rear)

We can calculate that ~164kg of static weight rides on the ball joints.

My WRX Wagon, in SCCA STX trim, ran ~426 kg of static weight per front corner. Granted the weight wasn't suspended through the lower ball joints. But the braking forces and lateral turning forces still ran though those ball joints.

[Buzz Skyline]

Here's a NA/NB Miata which has a double wishbone setup. You can see the lower ball joint is oppositely arranged. The ball joint is behind the tie rod end, but in the same configuration.

I can't really make out what's going on with the Miata ball joint. Do you think there's a way to modify the 818 design to take the load off the ball joint? Now that you've brought this up, it really concerns me.

[Buzz Skyline]

Here's a picture of the Miata knuckle design. It seems much better, with the ball joint in compression.

miata knuckle.jpg

http://rev9autosport.com/catalog/pro...e/1915/id/324/

[Buzz Skyline]

Here's a picture of a separated ball joint in a Beetle, which has the same effective problem as the 818

Ball joint separation.jpg

I got it from this page, which discusses mods to correct the design flaw for Beetles:
http://www.thesamba.com/vw/forum/viewtopic.php?t=491123

[wleehendrick]

Right, but that failure was experienced by overloading the Beetle suspension in a Dune buggy; certainly pushing the original design beyond it's intended use.

Yes, having the joint reversed would be ideal, but since the 818 is so much lighter on the front end than the WRX (and I'm not planning on jumping or rallying mine!) I'm not terribly concerned given the apparent lack of failures in the similar application in Roadsters.

[Buzz Skyline]

I found a paper that includes pull-out tests for six light truck ball joints.

http://www.hindawi.com/journals/tswj/2014/971679/

The failure force ranged between 43 and 45 kilonewtons, which is a range of 9666 pounds to 10116 pounds. With 350 pound/inch springs that means you'd have to compress the springs by 27 inches to generate enough force to pull the ball out of the housing.

That's a lot more travel than the 818 suspension has, so as long as you don't bottom out, the joint probably won't fail.

On the other hand, it would be nice to have a margin of safety. If you go with a factor of three, then compressing the springs by 9 inches reaches a conservative upper limit. That would still bottom out the the 818. But considering that bottoming out would put a lot more force on the joint than the spring does, I'd be a little worried that it wouldn't take too many potholes to bust a ball joint.

I assume the strength of the joints probably decreases with age and wear. So at the very least, I think I'll order a new set of quality joints before I hit the road.

I don't know the stiffness of the 818R springs. I'm sure they are a lot stiffer, which would put a lot more stress on the joints for a given compression, but hopefully they will spend most of their time on smooth tracks where they won't likely bottom out.

[Buzz Skyline]

Yes, having the joint reversed would be ideal, but since the 818 is so much lighter on the front end than the WRX (and I'm not planning on jumping or rallying mine!) I'm not terribly concerned given the apparent lack of failures in the similar application in Roadsters.

On the WRX, however, the joint is in compression, not tension. If it fails on a WRX, it just stops working as a bearing (i.e. it gets noisy and loose), which will hurt your handling. On the 818, failure means your frame will hit the ground and you might rip off a wheel.

[EODTech87]

I don't know the stiffness of the 818R springs. I'm sure they are a lot stiffer, which would put a lot more stress on the joints for a given compression, but hopefully they will spend most of their time on smooth tracks where they won't likely bottom out.

The Front Springs on the 818R are actually softer at 300lbs.

[Buzz Skyline]

OK, I feel better now. According to the Moog page, the most common configuration for load-carrying suspension ball joints is in tension, like on the 818, and the joints are designed to handle it.

http://www.moogchassistraining.com/sus_balljoints_t.php

I think I'll buy some new ones from Moog.

[wleehendrick]

On the 818, failure means your frame will hit the ground and you might rip off a wheel.

I realize that, I was referring to the fact that the FFR Roadster repurposes Mustang hardware similarly and doesn't have reported failures. The 818 takes even more weight off the front.

I think I'll buy some new ones from Moog.

Good to know; I already put new Moogs on!

[fact5racer]

I live in the place with probably the worst road conditions in a non-3rd World country, so I'll let you know how the suspension handles next summer after I start driving the car.

Sorry, but many and myself now consider the US as a third world country.

[Boog]

Boog, No, not in this case. Previously, the spring/damper acted on the top of the knuckle, so there was a direct force line from the hub, through the knuckle, into the shock. The ball joint just kept things aligned. The lower control arm was unsprung, just a geometry controlling feature.

Now, a force on the hub will go through the ball joint, pulling upwards on the lower arm, which then compresses the shock. The ball joint will see significant tension loading.

Note, I'm not making any claims either way for how much of a concern this is, or isn't.

Here's a crummy sketch of the mechanics in place.
Hub/knuckle is red, control arms green. Ball joint black.

I think there is a misunderstanding here, probably because I needed an extra comma. The original complaint/concern was not that the ball joint was under tension but rather the orientation of the ball joint now that it is under tension. I think all double wishbone cars have a lower ball joint that operates under tension.


I am curious if the concern, due to the inverted ball joint orientation when compared to most double wishbone setups, is about a separation of the ball joint shaft from its housing or about a separation of the housing from the knuckle.

[DMC7492]

Load-Carrying Type
Load carrying ball joints can be divided into two types. In the tension-loaded type, forces applied to the joint housing are pulling the stud against the housing. In the compression-loaded type, the same forces are pushing the stud into the housing. The tension-loaded is the more popular design.
The catalog will list vehicles that have four ball joints. If you are replacing the lower ball joints, it’s a good idea to check the uppers. NOTE: Never order a ball joint based on its size alone. Ball joints of the same size may have very different, application-specific engineering. Installing the wrong part could cause safety issues for which you could be liable.

These are from the MOOG pages,
The 818 is Load carrying in the tension loaded type, and so what if it is the more popular design, the main concern in the 818 is... Was this Subaru ball joint which is a following type initially from Subaru, engineered to be used in a tension load? I think NOT.

Can the engineers at FFR shed a light on this design decision? Thanks Don

[DMC7492]

http://thefactoryfiveforum.com/attac...5&d=1403731074

Here you can see how the stud will be pulling out of the ball joint housing when impacting the wheel!

[Mechie3]

The concern isn't the ball joint pulling out of the knuckle, but the ball joint itself ripping in two. It's because we're using "non-loaded" type ball joints in a "loaded type" application.

http://www.aa1car.com/library/ball_joints.htm

My gut feel is that with the same G loading we're going to see much less force. Would I do the same for a stock Subaru? No. For an 818? It's probably ok. Not that I want to find out the hard way.

[JAubin]

Aside from looking at the design critically, if anyone had a spare knuckle and ball joint they could pretty easily set up a test with some weights to get a rough idea of the failure point. Though I'd certainly be careful trying to do that as it's likely to involve a lot of exploding parts when it fails. If only I still worked somewhere that had an Instron, it would be pretty easy. Alternatively you could probably set up a welded in piece to limit the travel of the upper control arm so that it tops out as you're compressing the bump stop. If you limit the travel there it should prevent the displacement necessary to generate enough force to separate it would be my guess, but that's pure speculation...

[xxguitarist]

The major source of force that is being ignored here isn't the springs, or even the static weight of the car. It's a matter of shock loading, with firm dampers.
The dampers will respond to high speed step changes with many times the force from the springs.

An instron+ a couple simple fixtures would be the perfect solution to determining the moog ball joint separation force. I don't have access to one currently, though.

[Mechie3]

I only have a 250lb Chatillion here. Not exactly ball busting (lol) power.

[Buzz Skyline]

The concern isn't the ball joint pulling out of the knuckle, but the ball joint itself ripping in two. It's because we're using "non-loaded" type ball joints in a "loaded type" application.

The problem is when you try to source replacement ball joints, you'll always get the wrong type (compression) if you look for an OEM replacement for the WRX. What we need is a way to identify tension-type ball joints that will fit the Subaru control arms.

Anyone have an idea what those might be? I imagine someone at Moog might be able to help.

[tirod]

I think the concern it valid. And as with a lot of other hot rod parts, a kit to invert the joint would likely sell.

So, mocking up the parts to fit, can it be done? The adapters add another "interface" that has to carry the loads, and introduce the challenge of maintaining the same installed height.

The optimal solution has already been F5's path for other compromises - they now offer a complete spindle to correct the SAI and lack of a ball joint "interface" the Mustang strut spindle was missing. Expect the same solution, with the inverted joint to "correct" the load handling, and the addition of a top ball joint stud hole.

Donor parts are compromises for economical kit building, provide enough demand and you get the optimal part on the market to replace them. A true racing spindle for the 818 could be fabbed from aluminum plate with stud receptacles and would be lighter.

Here's a thread on some mouth watering fabricated spindles with some billet designs thrown in: http://www.locostusa.com/forums/view...hp?f=5&t=12384

40 year old F1 tech now applied to street, track, and Pro Touring cars. "Detroit" just does them in cast iron because it's cheap mass production - note they do make forged aluminum control arms for trucks, SUV's and cars now . . .

[longislandwrx]

buy the highest quality, properly sized pinch bolts you can find.

[matteo92065]

I only have a 250lb Chatillion here. Not exactly ball busting (lol) power.

Our Tensile testing machine only goes up to 50N.... or 11lbs.
I'm no help testing this.
But I would like to redesign and build new front hub/spindle assembly that is specifically for the 818, aluminum, one piece, and addresses the ball joint design. Think really light, simple and strong. Who's with me?

[Buzz Skyline]

I just called Moog's technical support line. The tech acknowledged that non-load bearing ball joints should not be used in load bearing situations. Unfortunately, he had no way of identifying a replacement for me with the info he had available. He said he doesn't have the size specifications available to him. He suggested measuring the WRX ball joint and going through a variety of load bearing joints to find one that fits.

I have no idea how to do that short of getting a job at an autoparts store. Anyone have a buddy at a Napa warehouse who could spend a few lunch hours finding a bolt-retained, load bearing (in tension) ball joint for us?

Here's the load bearing joint for a VW Beetle (1974), but it's a press in type, and I have no idea what size it is.

http://www.fme-cat.com/overlays/part...%20BEETLE&vin=

Here's the non-load bearing joint used on the same car.

http://www.fme-cat.com/overlays/part...%20BEETLE&vin=

[Buzz Skyline]

But I would like to redesign and build new front hub/spindle assembly that is specifically for the 818, aluminum, one piece, and addresses the ball joint design. Think really light, simple and strong. Who's with me?

I'd support that. If it's too costly or time consuming, I still at least want to find a better ball joint.

[Frank818]

if anyone had a spare knuckle and ball joint they could pretty easily set up a test with some weights to get a rough idea of the failure point.

I have 4 spare knuckles but 0 balls. I mean 0 ball joints!

A true racing spindle for the 818 could be fabbed from aluminum plate with stud receptacles and would be lighter.

Craig are you in?

But I would like to redesign and build new front hub/spindle assembly that is specifically for the 818, aluminum, one piece, and addresses the ball joint design. Think really light, simple and strong. Who's with me?

Maybe Craig, he can do anything cuz he's the best bracket guy on the market. That's a big bracket but hey, we must always have challenges, right.

[Turn In Concepts]

I found a paper that includes pull-out tests for six light truck ball joints.

http://www.hindawi.com/journals/tswj/2014/971679/

The failure force ranged between 43 and 45 kilonewtons, which is a range of 9666 pounds to 10116 pounds. With 350 pound/inch springs that means you'd have to compress the springs by 27 inches to generate enough force to pull the ball out of the housing.

That's a lot more travel than the 818 suspension has, so as long as you don't bottom out, the joint probably won't fail.

On the other hand, it would be nice to have a margin of safety. If you go with a factor of three, then compressing the springs by 9 inches reaches a conservative upper limit. That would still bottom out the the 818. But considering that bottoming out would put a lot more force on the joint than the spring does, I'd be a little worried that it wouldn't take too many potholes to bust a ball joint.

I assume the strength of the joints probably decreases with age and wear. So at the very least, I think I'll order a new set of quality joints before I hit the road.

I don't know the stiffness of the 818R springs. I'm sure they are a lot stiffer, which would put a lot more stress on the joints for a given compression, but hopefully they will spend most of their time on smooth tracks where they won't likely bottom out.

That is certainly a lot of force, but keep in mind that as soon as the damper contacts the bump stop and compresses it fully the suspension goes solid and all forces is transmitted to the suspension components and frame.

Let me also be clear that I was bringing this up as a matter of discussion. I am personally considering building an 818 and would in no way would this topic persuade me to not build one.

There are higher strength ball joints that are on the market than the factory Subaru units. Moog is one manufacturer that comes to mind.

Here's something else to consider. This is a company, Mooresport, in Canada who we work with frequently. They make high end rally cars and a lot of very nice parts. They fabricate aluminum knuckles in house that can be used with a stud coming out of the lower part of the knuckle with a spherical bearing for the outer ball joint connection. These have been proven in rally to hold up without issue. Only downside is cost as a pair of these is about $5000.



Tony

[Buzz Skyline]

I couldn't swing $5000, but I'm definitely interested in a solution.

It seems to me that it wouldn't be that hard to make a bracket that takes the load off the ball joint and puts it back on the knuckle. Here's where I could see it attaching.

bracket.png

[Turn In Concepts]

The problem with that bracket is it would have to hinge between the shock mount and the ball joint pinch bolt.

[Buzz Skyline]

I would ditch the attachment to the LCA and put a perch high enough that it wouldn't contact the control arm. Hopefully, we could do it without modifying the suspension geometry much.

It could be of the same gauge steel as the FFR knuckle bracket. It could attach to the ball joint bolt to make a truss instead of a cantilever.