A-Arm Design pro's / con's?

[Hermit]

Hey all, so im considering my A-arm design, not exact measurements but just the general shape and location of adjustment.

First I would like to discuss the difference between the classis "A" shape arm and what seems to be on more modern suspension with the rear leg running more parallel to the body like so

front-lowers s.jpg

Im guessing the benefit of this style is you have a more direct adjustment of Camber/Caster individually with one not effecting the other as much. Also with the rear arm running more parallel to the body it would have better resistance against braking forces? I believe you only really see this style in the lower arm? is this a superior design or does it have its own drawbacks?


Next is in relation to the upper arm and more so on the location of the camber adjustments. I see this style quite often with the Camber adjustment directly on the ball joint. I believe this pic is even of a locost (found through google image search)

8e434c358b94e11e5f34eb3604bdb3ce.jpg

Benefit of this design is one location to adjust to +/- camber but the big negative I see is it takes up alot of real estate right where you want your coil over to be if your trying to get it as close to the lower ball joint as possible?

Im leaning more towards this style with the ball joint area as small as possible and only adjustment on the body side. this is a billet example but first image I found

RaptorBillet_UCA-555x396.png

The benefits I can see is more room for the spring and also less weight at the ball joint end and more on the body. The only drawback I see is if you only want to adjust camber you have 2 locations you need to adjust equally?

[TooBusy]

The top design looks to me at least to have more freedom of movement out at the ball joint end. The center triangle is fairly small and has long legs hanging off which leads to bending and twisting.

The traditional A Arm is more of a triangle and triangles are strong.

[rx7locost]

I assume wee are talking about upper front control arms? They carry much lower loads than the lower A-arm. BTW, there is a reason they call them A-arms.

The first pic has no space for the coilover. It is best suited for inboard shocks IMO

The second pic is essentially a book control arm. It gives a lot of adjustment capability.

The third pic, while allowing space for the coilover, has adjustability only at the chassis end and is not easily adaptable to various suspension geometries being cast.

They all can work. IMO there is no "BEST". Every design is a compromise. A lot of us build within our skills and tools.

[rx7locost]

I assume we are talking about upper front control arms? They carry much lower loads than the lower A-arm. BTW, there is a reason they call them A-arms.

The first pic has no space for the coilover. It is best suited for inboard shocks IMO

The second pic is essentially a book control arm. It gives a lot of adjustment capability.

The third pic, while allowing space for the coilover, has adjustability only at the chassis end and is not easily adaptable to various suspension geometries being cast.

They all can work. IMO there is no "BEST". Every design is a compromise. A lot of us build within our skills and tools.

[Driven5]

I think the main reason you see lower control arms like your first pic is packaging, to not get in the way of the steering. The secondary benefit in production cars is that it largely separates the lateral and longitudinal reactions, which allows finer tuning of the 'feel' through the bushings used. This is not as important when rod ends or typical poly bushings are used. I can't think of any production cars I have seen in which the camber or caster adjustments are built into this arm design.

The design in your second pic provides easy adjustment, but is my weaker by the way the threads are loaded in bending. It has worked on plenty of these little lightweight cars, although I wouldn't use anything less than 5/8" or 16mm fine threads. Many 'book' type builds can get away with it due to laying the coilover at a relatively steep angle, which also often leads to a 'falling rate' suspension.

As you note, the design in your third pic is more difficult to adjust, but more robust and allows for better coilover placement.

Even more so than that I tend to gravitate towards this general design, either with or without the crossbar:

[Hermit]

Thanks for your thoughts Justin, do you like the design you pictured because the threads are abit less under bending forces and more so tension/compression?

I do like that you can have left and right hand threads at either end and provides very easy on car adjustment

I think we might be on a winner (in my eyes anyway)

Is there any reason you couldnt have the connections on the ball joint end both pivoting like it is on the right, instead of the left solid and right pivoting.

I know its not needed but purely to appease my OCD and love of symmetry...

[benny_toe]

That's the design I should have used. Burien Larry had those on his Locost and I am copying his design. Need to get some caster!

[KB58]

The problem with buying someone else's A-arms is that you're stuck with their design limitations. I don't mean that they'll break; I mean that their lengths bracket in your suspension design, and it'll be random chance whether you can make them work. Well, they'll "work", but it all depends if the resulting RC position and movement will satisfy your requirements.

[Lonnie-S]

For my 2 cents worth, I'd add the design Justin shows is for the upper A-arm only, and you can see an interesting application of it to a Locost (with some added adjustability) by watching the 3-part McSorley video on YouTube. You can find part 1 by mousing on this link.

Also, for the lower A-arm, it has been shown that for the #2 example type. running a tube across the base of the "A" near the chassis bushings will add a lot of strength and help resist twisting under braking.

Cheers,

[Hermit]

The problem with buying someone else's A-arms is that you're stuck with their design limitations.

Yes, im not planning on buying an off the shelf arm, was just looking at design considerations / styles for when I build my own.

Thanks for linking the video Lonnie, I have thought about incorporating some body side mount adjustment BUT im hoping I can get everything bang on in Vsusp to suit all my components and find the ideal location for me which will make it simpler and less weight.

But in saying that if you ever change wheel size or want to tune roll centers in future it might be a worthwhile endeavor.

[TooBusy]

I think the main reason you see lower control arms like your first pic is packaging, to not get in the way of the steering. The secondary benefit in production cars is that it largely separates the lateral and longitudinal reactions, which allows finer tuning of the 'feel' through the bushings used. This is not as important when rod ends or typical poly bushings are used. I can't think of any production cars I have seen in which the camber or caster adjustments are built into this arm design.

The design in your second pic provides easy adjustment, but is my weaker by the way the threads are loaded in bending. It has worked on plenty of these little lightweight cars, although I wouldn't use anything less than 5/8" or 16mm fine threads. Many 'book' type builds can get away with it due to laying the coilover at a relatively steep angle, which also often leads to a 'falling rate' suspension.

As you note, the design in your third pic is more difficult to adjust, but more robust and allows for better coilover placement.

Even more so than that I tend to gravitate towards this general design, either with or without the crossbar:

These are basic circle track off the shelf items. You can buy dang near any length swaged tube or you can use tubes and welded bungs.
These are exactly what I had on the R1 powered car.

[benny_toe]

Coilover interference is why I have to re-do the upper control arms. To get the required caster I need another 1/2" of clearance. If I move the upright any further back the coilover hits the arm hard.

Right now I'm looking for a ball joint that fits the Miata upright and screws into the holder.

Cheers!

[Hermit]

Benny/Larry I believe these fit and are what I plan to use. You can get the ball joint stud different lengths also.

https://www.speedwaymotors.com/QA1-1210-105-K772-Upper-Ball-Joint-1968-89-Chrysler-Steel-Cap,57575.html

https://www.speedwaymotors.com/Steel-Upper-Ball-Joint-Sleeve-K772-Style,2126.html

[Bent Wrench]

These arms are not adjustable for ball joint position other than some camber because of the welded brace.

[rx7locost]

These arms are not adjustable for ball joint position other than some camber because of the welded brace.

OK, school me on that one.

IMO. the brace being welded, doesn't have any affect on locating the BJ. It all comes down to the 3 points, the BJ, the front Heim and the rear Heim. How they are connected has nothing to do with how those 3 points move, or don't move. That is of course, so long as there are no other adjustment provisions made in the A-arm.

As I see it, with that design, the camber is "mostly" adjustable with the one Heim while caster is "mostly adjusted by the other. There is of course some interaction between the two.