Laying out A arms

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bgkast
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Laying out A arms

Post by bgkast » 19 Mar 2014, 16:47

I'm trying to lay out my A arms to make my fixtures and am having a difficult time figuring how to go from the center to center drawing (easy enough to draw once you use the law of sines to get the other angles) to the layout of the tubes. Because the tubes do not lay directly on top of the lines between the points it's difficult to figure out where to start. Without cutting out the tubes and tabs, laying them all out on the table and trying to trace them what is the best way to approach this? My goal is to get a line drawing of the arm drawn so that I can lay the tubes and tabs on it to build the arm. The front lower arm is the one that I started with and is particularly vexing.

Thanks

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Re: Laying out A arms

Post by Midlana1 » 20 Mar 2014, 07:29

Here's what I did:

Using a sheet of plywood, mark the three end points, then drill and install appropriately sized bolts in each hole. Over each, place a rod end. Screw the threated weld bungs onto the rod ends so that approximately half the rod end thread is visible. At this point, the arm pretty much designs itself. All you have to do is "connect the dots", adding tubing to pick up the bungs.

For the forward lower arm, I'd build the outboard box sub assembly first, bolt the Miata ball joint to it, then measure relative to it, again, on a sheet of plywood.

A method that Langan used was to create a template of each A-arm using thin metal (think of it as a 2D projection). Then, the actual parts are layed on it, taped down, then welded.

Regardless how it's done, frequently check the fabricated arm against the suspension brackets on the chassis. It's of course critical that everything lines up. Once the arm is tacked together, check it again. It's easy at that point to bend it slighty for perfect alignment - not so easily once it's welded!

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Re: Laying out A arms

Post by bgkast » 20 Mar 2014, 08:34

Thanks. I guessed that that was probably the easiest way to do it. The idea behind drawing it out is to 1. Make a drawing to build the arm over similar to what Langan did and 2: to project the two arms on top of each other to get an idea of where to run my outboard shock and spring.

I did overlap the two point to point drawings (with the upper arm correctly spaced outboard of the lower arm) and noticed that the outboard (ball joint) end of the lower arm is almost 2" farther out and about 1" forward of the outboard point on the upper A arm (numbers rounded to protect top secret suspension geometry ;) ). I understand that there is some caster and king pin inclination in there, but comparing them to the Miata upright the two points are farther apart than I expected. Can anybody confirm that this is correct?

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Re: Laying out A arms

Post by Midlana1 » 20 Mar 2014, 09:30

That sounds about right. On top of whatever the Miata upright gives, I added about 1" of castor (top pivot 1" aft of the lower). Do you have the Miata uprights yet?

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Re: Laying out A arms

Post by bgkast » 20 Mar 2014, 11:37

I do. They seemed to have about 1" of king pin inclination from my rough measurement which is why the lower arm sitting out 2" from the upper arm stuck out to me.

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Re: Laying out A arms

Post by Midlana1 » 20 Mar 2014, 11:55

Don't forget that the combination of threaded tube ends and how much the rod end is screwed in makes a huge difference in overall length. And, the OEM lower balljoint sticks of the arm assembly a bit.

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Re: Laying out A arms

Post by bgkast » 20 Mar 2014, 12:12

True, but I was using the center to center diagrams which locate the pivot points of each arm, right? Just want do double check before I make them out of steel! :mrgreen:

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Re: Laying out A arms

Post by bgkast » 20 Mar 2014, 16:08

Well I was able to find the angle between the lower A arm tubes by using the angles in figure 24.21 and the assumption that the plate that attaches to the end is flat (180 degrees). Are the tube the lengths given in that figure the center length of the tube or the length to the longest end of the angled cut? Is it also safe to assume that 2.8" should be added to each tube length to get to the pivot point?

This is quite the math problem! :mrgreen: Thanks

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Re: Laying out A arms

Post by bgkast » 20 Mar 2014, 17:55

This is maddening!!! I tried drawing it up using the above calculated angle between the tube and it's close, but the center of my tab B ends up being .4" farther back that it should be relative to the outer point on the center to center drawing dimensions. Are the angles shown in figure 24.21 exact (calculated or measured from CAD), or are they approximate? I'd like to see how Freakynami drafts this! :shock:

If Kurt can confirm the center to center points are correct I may just disregard everything but the points and redesign the whole thing. :?

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Re: Laying out A arms

Post by freakynami » 20 Mar 2014, 22:08

I haven't jumped in here yet, I only just got to the first suspension arm yesterday. I've started with the (easy?) front upper, and plotted the points out based on the three distances, and the angle in the diagram 24.7 matches up. Then I've put in the rod ends and spherical, followed by the threaded bungs and finally the tube. That is as far as I've got for now, but next is to trim / shape the tube ends and make up the clevis.
AArms01_1.png
AArms01_2.png
AArms01_3.png
AArms01_4.png
I'll move onto the lower front next to see if I can find anything out...
*edit, and yes, I just realised I have the rod-ends clocked out by 90° - I'll fix that...
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Last edited by freakynami on 08 Jul 2017, 06:26, edited 1 time in total.

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