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Re: Laying out A arms

Posted: 21 Mar 2014, 01:02
by freakynami
Okay, a wee bit more on the upper front:
AArms02_1.png
It looks like the tubes follow the virtual point to point lines nicely. I may ditch the clevis and weld directly, but need to run the numbers to see how much flex is possible, and if that is within reason. If so, I'll just put the clevis back in...

Re: Laying out A arms

Posted: 21 Mar 2014, 01:07
by freakynami
Now, onto the lower.

I've put the rod ends at the virtual points. I put the Miata lower at the outboard virtual point - using the centre of the ball.

Then I built the "box" around it (did I miss Tab A???). First cut, I've run the tubes along the virtual point to point lines - I can see that this won't work, as mentioned earlier.
AArms03_1.png
(bear in mind, I've built the "Left" hand first)
AArms03_2.png
AArms03_3.png
Next up, I'll try drifting the tubes across a bit and "aim" them at the Tab B... I'm pretty sure this is what Kurt meant in his earlier post?

Re: Laying out A arms

Posted: 21 Mar 2014, 01:53
by freakynami
Yep, that seems to have worked.

I put a couple of target points nicely spaced on Tab B and pointed the tube centrelines at those. Trimmed the ends and it looks good to me. You can see how the tubes don't follow the "virtual" point to point lines, both diverging forward, and slightly downward.

Hope this helps...
AArms04_1.png
AArms04_2.png
AArms04_3.png
AArms04_4.png

Re: Laying out A arms

Posted: 21 Mar 2014, 02:38
by freakynami
And the result...
AArms04_5.png

Re: Laying out A arms

Posted: 21 Mar 2014, 02:48
by freakynami
My apologies for the total thread-jack, but it is relevant, I promise :)

I didn't love the way the tubes stopped at Tab B, and was thinking about having a closer look at that later... Sure enough, at the end of the chapter was Kurt's comments along those lines. One option, spreading the tubes slightly and wrapping them around the side of Tab Cs a bit, is what I've done here.
AArms05_1.png
AArms05_2.png

Re: Laying out A arms

Posted: 21 Mar 2014, 06:21
by Midlana1
You two are actually... well... quite awesome. I'm very impressed to see significant thought going into things rather than blindly following, or just as bad.

Everything you both note is correct. What happens between the end points is very negotiable, and if the tubes are moved slightly it's not a problem at all. I fully recommend your modification of spreading the tubes slightly so they wrap around the corners - that goes far to give additional piece of mind of not having the butt welds in bending (from the side pressure from the springs).

Well done, both of you!

Re: Laying out A arms

Posted: 21 Mar 2014, 06:28
by Midlana1
BTW, freakynami, since you're doing accurate CAD drawings, check the physical size of the OEM Miata ball joint housing and confirm whether there's room for it within the outboard box assembly. My OEM part has what appeared to be a slight casting shift, such that the inboard mounting hole was offset in its casting. It caused the bolt to be so close to my A-arm that I had to grind off the edge of the ball joint housing slightly.

My point is that if it's a systemic issue, since you already have it in CAD, you can extend the A-arm end housing inboard (make it longer) to give clearance for ball joint variation. bgkast, it's not a show-stopper either way. If it doesn't fit as-is, you just grind down the corner of the OEM ball joint and it'll be fine.

Just remember, you're free to alter the A-arms "mid-assembly". As long as the end poings don't move around, you'll be fine.

Re: Laying out A arms

Posted: 21 Mar 2014, 07:29
by bgkast
Thanks! That does really help. I think that is really the only way to do it, make the point to point triangle, lay the pieces on it (virtual or otherwise) and connect the dots. The angle and tube lengths in fig 24.21 should be used as a check, but not necessarily to build from.

I spent some time with the lower ball joint and some calipers and am going to take another shot at drawing it. The big problem when I was drawing it before was I didn't have the physical parts with me because I was trying to draw it on the drafting table at work.

Re: Laying out A arms

Posted: 21 Mar 2014, 09:18
by bgkast
Sucess! My ball joint had the same extra casting that Kurt described so I bumped tab A back slightly.

Re: Laying out A arms

Posted: 21 Mar 2014, 09:56
by Langan
Now that you have the lower arm done, the real trick is to consider where the shock or push rod goes! Your CAD is great.