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 Post subject: A-arm minimum lengths
PostPosted: September 17, 2019, 1:38 pm 
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Hey all, I’m building a non traditional, so my chassis dimensions are completely different than everything else. I’m trying not to have my wheels stick out too much, but I’m worried that my wishbones will be too short, due to my frame width.

Is there a length that would be considered too short and not to go shorter? Obviously the upper will be shorter as well, but mainly concerned with the minimum lower length, and still have good geometry

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PostPosted: September 17, 2019, 6:06 pm 
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Shortening the arms is after you've moved the left and right suspension together as close as possible.
Then fit a rack that puts the inner tie rod pivots the required width apart and height for no bumpsteer.

You can also run more backspace on the rims and design the arms to clear for adequate but reduced turning circle.

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PostPosted: September 17, 2019, 7:15 pm 
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I’ll be fabricating wishbones from scratch, so not shortening anything on this one. Just wondering if for example, 12” long lower wishbones is too short

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PostPosted: September 17, 2019, 11:18 pm 
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I happened to have a Miata LCA in my van, so in response to this query, I scampered out in the rain (rain in September, in western Oregon?) and measured it. 13" from pivot line to ball joint, so I'd opine that nope, 12" is kinda shortish, and certainly shorter than a Locost, but not too terribly short. If that's what your chassis demands, then GFI.

Glossary:
LCA = Lower Control Arm
GFI = Go For It

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PostPosted: September 18, 2019, 2:09 am 
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JackMcCornack wrote:
I happened to have a Miata LCA in my van, so in response to this query, I scampered out in the rain (rain in September, in western Oregon?) and measured it. 13" from pivot line to ball joint, so I'd opine that nope, 12" is kinda shortish, and certainly shorter than a Locost, but not too terribly short. If that's what your chassis demands, then GFI.

Glossary:
LCA = Lower Control Arm
GFI = Go For It



Thanks! As it stands now, I might be in that neighborhood. From outside of chassis to body is just over 17” now and with brackets and upright/hub/brake/rim I fear it might even be shorter.

I’ve been playing around with numbers on a suspension simulator but can’t seem to get detrimental effects from going that short.

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PostPosted: September 29, 2019, 8:07 am 
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Velix wrote:
JackMcCornack wrote:
I happened to have a Miata LCA in my van, so in response to this query, I scampered out in the rain (rain in September, in western Oregon?) and measured it. 13" from pivot line to ball joint, so I'd opine that nope, 12" is kinda shortish, and certainly shorter than a Locost, but not too terribly short. If that's what your chassis demands, then GFI.

Glossary:
LCA = Lower Control Arm
GFI = Go For It



Thanks! As it stands now, I might be in that neighborhood. From outside of chassis to body is just over 17” now and with brackets and upright/hub/brake/rim I fear it might even be shorter.

I’ve been playing around with numbers on a suspension simulator but can’t seem to get detrimental effects from going that short.



Shorter arms lead to more Track width change during suspension travel...
Or more Camber Change....
depending on the geometry.
A good effect is that you can use smaller dampers and springs if going conventional.
If going pushrod, then there´s no difference except you are adding ligthness...


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PostPosted: September 29, 2019, 7:11 pm 
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Light cars have less suspension travel than heavier ones, so the negative effects of shorter arms are reduced.
Push rods add weight but improve motion ratio, if its too low, or can create a variable (ie rising) ratio. Typically, the further the shock/spring is from vertical, the more potential benefit there is from pushrods. They can be done wrong too, of course ;(
The springs don't care about motion ratios but the shocks don't like low ones (ie a lot more suspension travel than shock travel)


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PostPosted: September 30, 2019, 1:16 pm 
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I measured the distance from my lower A-arm pivots to the outer edge of my 195-section tires at 22". The wheels I'm running have less offset than they should, so with the proper wheels and maybe skinnier tires I could get this down to 21". That said, my lower A-arms are 14" long. In order to fit the tires under the body you're planning to use, my A-arms would have to be only 10" long, the uppers maybe only 6 or 7". That would bother me. Maybe something like a VW Beetle front end would work. Or fender flares.


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PostPosted: September 30, 2019, 5:25 pm 
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Thanks for the input everyone!

What is the recommended wall thickness these days for control arms? Seems people are going thicker now, due to the book ones not lasting. I was hoping to use 1” chromoly just haven’t decided how thick is needed. Overall car weight should be around the 1200# range.

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PostPosted: October 1, 2019, 8:07 am 
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I believe must of the control arm failures have been when builders use .75" Dia. tube and .06" walls.
I'm using 1" with .072" wall for the lower, and .87" tube with .082" wall for the upper. The lower has a fish mouth gusset design, and both arms have been rigorously tested i.e. [Michigan roads :ack:]
I actually wanted a thinner wall for the upper arm but could find DOM thinner wall tubing locally.
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PostPosted: October 1, 2019, 8:50 am 
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I cheated and bought front LCAs for the Slotus. They were 1-inch X .120 wall tubing. When I built the control arms for the rear, using the engineering principle of "Monkey See, Monkey Do" I used (wait for it...) 1-inch X .120 wall tubing. To date, I have had no failures of any kind on any of the control arms. YMMV, just my .02, etc, etc. Oh, and "I ain't no Enga-Neer!"

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PostPosted: October 1, 2019, 1:19 pm 
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I used 3/4" tubing with a .095" wall and haven't had any problems for more than 50K miles. IMO the failures of lower A-arms were due to stress risers caused by poorly-designed shock mounting plates. The failures I've seen were all at the point of maximum stress, right where the tube meets the inboard edge of the shock plate.


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PostPosted: October 1, 2019, 9:32 pm 
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.095 is *way* easier to MIG weld than .062.


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PostPosted: October 2, 2019, 12:06 am 
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TRX wrote:
.095 is *way* easier to MIG weld than .062.


Not an issue for me. I only have a tig. Sometimes I wish I bought a mig instead though!

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PostPosted: October 2, 2019, 12:08 am 
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nick47 wrote:
I used 3/4" tubing with a .095" wall and haven't had any problems for more than 50K miles. IMO the failures of lower A-arms were due to stress risers caused by poorly-designed shock mounting plates. The failures I've seen were all at the point of maximum stress, right where the tube meets the inboard edge of the shock plate.


Any pics of yours to show how you improved the design?

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