A-arm question

I have always read that an A-arm with a straight leg is better. Does it matter if the angled leg is the front or back leg?

I would think that the front arm gets a bit less stress, at least on a RWD car.

The rear arm takes most of the braking force and bump force so I'd rather have the front leg angled if it were me.

This is for a front suspension and for packaging reasons I would prefer the back leg straight I just want a sanity check.

Curtis - but wouldn't the angled arm more directly oppose the braking force, and therefore be stronger?

Is there really a one-size-fits-all answer here?

I guess I'd tend to think most about what forces are involved. For the front of a RWD car, I'd think having the forward tube of the A-arm reaching forward would be preferable, as braking force would put it in tension (like ReverendDexter says). But the more angled it is, the less it can contribute to lateral loads, no? Though the more angled forward it is, the less it tries to compress the rear arm under braking as well, I think.

EDIT: Are we talking about upper or lower control arm? The forces above apply for the lower arm. If I'm not mistaken, the net combined force on the upper A-arm from the braking torque and braking force will actually be towards the front of the car.

I tend to try to visualize this stuff with vectors, while keeping in mind that fundamentally the tubes only want to resist tension and compression, and even compression causes more concern about buckling (though my impression is that a rule of thumb for that is that a reasonably well-triangulated and fabricated structure would probably be unacceptably flexible before it had too little absolute strength?)

And just to clarify, when you say "with a straight arm is better", are you referring to angle, as we've been discussing, or are you talking about the arm itself being curved in shape, or having an angle bent/formed into one of its arms?

Yeah I talking about the angle. And I am pretty sure angled arm forward is better in my case.

In the 944 / 924s the front a-arm mount is strait back towards the frame. The rear arm mount is back about 18 inches form the front mount so the rear arm of the a-arm is angled.

Think of it this way the forces on the front wheel are trying to push the wheel back. So what to do. This usually depends on how the a-arm is constructed. If the legs are stronger in compression then you want the angled arm in the rear. If they are stronger in tension then you would want them in the front. You also have to take into account what you are mounting them to.

Alot of manufacturers want the structure in front of the wheel center line to absorb energy and there for will usually not be nearly as strong (it is designed to crush and dissipate energy) there for many manufacturers will do as my Porsche does and design an arm that is strong in compression and put the angle leg of the a-arm back towards the firewall.

Ideally you would want the A-arm to be triangular and tubular. This way you can use the least amount of material and still have good strength in compression and tension as well as the triangulation will help reduce the possibility of a flexural failure of the a-arm material.

It is not a simple question you ask.

Being in the heart of stock car racing, I get to see all kinds of used a arms at the parts places. It does not matter one bit if the front leg is bent, curved, or straight. The big thing is to have the ball joint where it needs to be and use tubing material that's up to the task. I am also looking at some pictures of the Red Bull F1 car. The front leg appears to be almost perfectly straight in some pics, but others the arms are at equal angles. As long as the caster, camber, toe, and ackerman are correct, the A-arm angles are a non issue.

In reply to ncjay:

Hang on, the task that the tubing has to be up to is substantially defined by the angles involved and whether or not the tubing is in pure tension/compression or has bending loads. As soon as the tubing has a bend in it, there's no longer the possibility of pure tension/compression as any load between the ends becomes a bending load...

I'm sure that there have been plenty of fast cars with funny shaped arms, but if you've got a free hand to design this stuff, it seems like a good idea to put some thought into the weight/strength/stiffness concerns.

ReverendDexter wrote: Curtis - but wouldn't the angled arm more directly oppose the braking force, and therefore be stronger?

The arm shape has little or nothing to do with it. The forces imposed on the chassis have to do the angle between the ball joint and the bushing.

If the tube that goes from the ball joint to the rear bushing is straight, it can theoretically handle more stress than the bent tube for obvious reasons. I'm (of course) assuming that the OP is talking about A-arm design with stock pivot positions. If we're talking a clean slate design, I'd need charts and an easel

In reply to curtis73:

I believe the OP actually is talking about a relatively clean-sheet design. The word "straight" got used early on to describe an arm perpendicular to the vehicle's centerline, and all heck broke loo... well, sort of flopped out.

I believe the original question was, assuming that either the forward or rearward leg was going to be close to perpendicular to vehicle centerline and the other more angled rearward or forward, which arm should be angled and which straight?

I pretty much agree with your summary (though I'm inclined to worry about the difference in stiffness involved in bent tubes), but I think we're now discussing questions apart from what 93EXCivic asked... Fetch the easel!

I'm going to go back and reclarify my original statement. Most A-arms have two chassis mounting points and one outer ball joint. The majority have the ball joint centered between the two chassis mounting points, or at least fairly close to centered. I've also seen enough cars with the ball joint even with the front chassis mounting point and even the rear chassis mounting point. In some oddball cases, I've even seen it located past the chassis mounting point, but I think that's just someone being "artistic". The wall thickness on the tubing and weld quality is far more important than the A-arm angles. Look at enough pics of F1 cars and all types of open wheel cars and you'll see all kinds of variations on A-arm angles.