vt951

I did not want to hijack some of the other threads about aftermarket control arms, so here's another new one. I think this might actually come to fruition, so hold your flames!

I am in the process of designing and analyzing some new control arms for my brother in law's shop to fabricate and possibly offer for sale to the community. The idea is to offer a lower cost, yet durable and easily serviceable, alternative to some of the other aftermarket alternatives that are popular with 944/951/968 racers.

Our design is a welded steel tube assembly that provides for direct bolt-on replacement of the oem cast aluminum arms. Not sure yet what all would be involved to replace the early steel arms. Early and late offset lengths are planned. They will use the early bolt-on ball joints because of low cost ($25 each) and easy replacement (3 bolts).

Anyway, I have modeled our design concept and am setting up a finite element analysis to size and optimize the material (basically, determine the necessary tube wall and plate thicknesses). Based on feedback in one of the other threads, I will be assuming simultaneous 5-g bump, 1.5-g cornering, and 1.5-g braking loads. So, I need to figure out the resultant forces on the control arm from these loads. I am missing a couple of pieces of information to do this:

1. dimensions of an aggressive racing sway bar. Basically, OD, ID, transverse length between mounting points, and longitudinal length between the sway bar brackets and mounting points (lever length). My assumptions for now are OD = 30 mm, ID = 25 mm, transverse length = 60 inches, lever length = 10 inches. If anyone has a Weltmeister or or other racing sway bar to take some measurements on, please let me know.

2. amount of front suspension travel on a moderately sprung track 944 (let's say 250# springs). Why moderately sprung rather than 1000# race springs? I believe the control arm loads are actually highest when using a big sway bar with loose springs, because the loose springs allow more suspension travel, which puts more load on the sway bars, resulting in the highest bending loads on the control arms.

Here's the concept model:

Chads996

Hey bud... I like the design, but I think the manufacture of the component would be more than the "less than $1000 per set"' rule. Please correct me if I am wrong, but here is what I am seeing:

1. Cost of materials
2. CNC Machining (Bearing flange)
3. CNC pipe bending
4. Time for material pre-fab
5. Welding (possibly CNC robot)


Furthermore, you should tie in the spherical bearing mount flange to the sway bar mount in on piece. Less fab, less overall welding and will likely result in a stronger part.

Overall, I like it. But I leave with a few questions...how much? ...and why can't I choose to put this on the track with stiffer springs? Isn't that one of the goals? (Example - my car has 500# and 600# springs.) For info on suspension travel, speak with Jay at Ground Control or Karl at Racer's Edge.

Thoughts?

C.

vt951

Chad,

Thanks for your comments. Our goal is to sell the set for well under $1000, including ball joints and any other hardware necessary to swap out the oem aluminum arms.

The plate parts will be cnc plasma cut. The tubes will be cut and bent the old-fashioned way, using a tube bending die and jigs. No cnc welding robot. With the low volume we're talking, it doesn't make sense IMHO. And my bro in law is a very skilled fabricator and welder who can do this kind of work very efficiently and high quality.

I like the idea of tying the sway bar mounting plate into the ball joint plate, but I couldn't think of a good way to do it. To help carry the bending load, the legs of the ball joint plate actually press into the tubes a few inches.

Regarding spring rates, what I was trying to say is that the worst case is soft springs with stiff sway bar. So, would actually be better off using stiffer springs as far as the control arm forces are concerned.

Chads996

Something else to consider as well. I think your sway bar mounting hole is in the wrong location. The hole is almost on the centerline of the spherical/inner mounting point. (Sorry for the lousy pic...This is all I have handy for reference...)

xsboost90

looks right to me- so alan am i going to get to be a test dummy for this first set?

vt951

It's exactly on the center line in my model. Must be an optical illusion in the picture.

vt951

Maybe? It's up to my bro in law really. I told him they were your control arms that he's borrowing... I'll let you know when he actually produces something and we'll see what we can talk him into.

billthe3

For sway bar measurements, my weltmeister is a solid OD 29mm, don't know the width.

vt951

Thanks for your help. Any chance you could measure the width and lever length?

Does anyone have any idea on the suspension travel?

vt951

bump... still need advice on suspension travel and sway bar dimensions. Thanks!

Lorax

What happens when a set fails on the track for whatever reason and someone comes after you for it?

vt951

They will be sold with a strict off-road use, no liability, etc, etc disclaimer. I don't think it will be a problem.

They are going to be very strong anyway. I'm thinking that the ball joint or other mounts will fail before these control arms.

adrial

For suspension travel ... you should calculate the estimated body roll at 1.5G cornering.
First calculate weight transfer ( (Weight * Lateral Accel)/Track), then apply that weight transfer to the resultant spring (spring + swaybar) and calculate the wheel travel and swaybar twist. Of course there will be more load imparted if someone hits some bumps with the outside wheel mid-corner, but that's what safety factor is for since it's very difficult to estimate that load. You can use this for fatigue (left turn/right turn/left/right..etc). The goal should be infinite life (IMO) including any knockdowns for weldments in the load path.

For static...since robustness is really the goal here... assume one wheel at full bump and the other wheel at full droop. If you can use this as a criteria and it doesn't make your design excessively bulky compared to existing designs, do it. Springs don't matter here, assume the stiffest swaybar you can find.

For suspension travel...you're probably gonna have to go measure a strut...and don't forget the motion ratio between the wheel, spring and swaybar.

Your swaybar mount looks weaker than most designs already on the market. If you put a plate on the opposing side and tie them together with a boss...you should be good to go. It's difficult to see what thickness plate you're using, but it looks to be fairly thick which may make this a design that works. Obviously the factory arm has pockets to hold the bushings...may be a good idea to include these if you can. It looks like your swaybar attachment point is above the spherical bearing (believe it is in plane on factory a-arms?), which will affect things a little. Better to put it in the exact same place as factory.

I'm sure you could also waterjet the flat stuff you have in there, not sure if that's any cheaper than plasma cut.

Finally I would chose to press in a spherical bearing similar to the Fabcar arms instead of using the early balljoints. Although this option gives you the possibility of making your own stronger, replaceable spherical bearing in the future .

Lorax

I was only mentioning this because I have heard that some manufacturers have had legal issues regarding this very topic.

Just make sure you have your bases covered

vt951

Good idea (full bump to full droop).

I agree. It's .200" thick in the design shown, and the prelim analysis I've done already confirms it needs to be stiffened one way or another. I am working on a redesign of the sway bar mount and ball joint mounts right now.

Well, those press in ball joints add a lot to the cost, both up front and every time they need to be replaced. And if someone really wants those, there are lots of other aftermarket options already.