Sizing custom fabbed control arms

[Derek]

I am going to fabricate my upper & lower control arms and I am trying to figure out where I can go to determine what diameter and wall steel tubing I need. I am pretty set on using 1" diameter DOM for all of arms and pushrods, but I just don't know where to begin figuring wall thickness.

The pushrod bracket will be welded directly to the vertical tube that contains the tie rod on the lower control arm. The anti-roll bars will attach to the inboard springs, so there will not be bending forces on the a arms other than from the forces from the wheels.

These a arms will carry the C5 Corvette uprights.

I am not looking for featherweight arms that will not be reliable over an extended period of time, but I also don't want to carry unnecessary weight. Obviously these are CRITICAL components and a failure could be catastrophic so slightly overbuilding will be in order.

I am figuring the following variables will come into play:
Total vehicle weight
Max Cornering G, Braking G & Accel G
Length and Layout of A Arms ( Rears will be right triangles, fronts, isosceles triangles)
HP / TQ

If anyone could provide some input it would be great.

Derek

 

[Phil]

Derek:
I use 1" DOM .120 wall all the time or chromoly and it is plenty strong for wishbones, you might find that the chromoly is not that much more costly than DOM, and the dimensional tolerances are superb. Also if you are planning to add sleeves to the ends or front of the wishbones for bushings, spherical bearings or monobolts, chromoly is available in many more wall thicknesses and diameters to accomplish that.
Good Luck
Phil

 

[Derek]

I should also note that I am not committed to one thickness for all control arms. The lower control arms are more highly stressed than the uppers and I would think that the rears would need to be more stout than the front.

I was thinking front upper gauge X, front lower and rear upper gauge Y, and lower rear gauge Z.

eg. X = .083
Y = .095
Z = .120

I will not be using rod ends other than for the pushrod. All of the bushings will be Delrin AF with hardened steel tube inserts.

What I am trying to figure out is if I am overthinking this whole thing. The weight difference between the .120 and .083 is .3 lb/ft.

 

[Russ Noble]

so slightly overbuilding will be in order.


If anyone could provide some input it would be great.

Derek

IMHO what you propose will fulfill your criteria.

 

[Ian Clark]

I'd like to see a picture of the upright with proposed pickup location for the pushrod. Sounds like an interesting project, however you've really got to think several steps ahead of yourself to avoid expensive redoes.

 

[Howard Jones]

Go with the thick, .120 at both F & R on the lowers. The lowers take nearly all the load. I made my own uppers for my GTD and did in fact use the same .83 4130 CM.

Read up on CM welding. Not difficult but necessary steps. Preheat to about 400F, Then remove from oven and take to welding bench quickly. Complete welding and cover assembly with a box. I used a wooden crate. Let cool back to room temp in box. This isn't the whole process. Read up on CM welding.

 

[Derek]

I'd like to see a picture of the upright with proposed pickup location for the pushrod. Sounds like an interesting project, however you've really got to think several steps ahead of yourself to avoid expensive redoes.

Once my upright arrives, I will put together some final drawings. I am also planning on doing a mockup with 1" conduit to make sure things are going to go together how I plan them to.

 

[Jack]

Good idea for the conduit. Howard's method for preheat is excellent. No hydrogen embrittlement that way. Some also post heat after welding and allow to cool too (both steps).
I have exclusively (in the past) used .120 wall in both mild steel and CM and never had a failure. "Knock on wood"!

 

[Trevor Booth]

very broad statement to say lowers take all the load. distribution of load depends on vertical distance from axle centreline. In the event that upper and lower are the same distance from axle centreline the load is shared equally, load share alters in proportion to the dimensions from centreline if not equal, however one is in tension and one is in compression in all cases