Rear Lower Control Arm Design (LCAs): H-arm VS. A-arm

[tube80z]

I agree with jt1. Although I think the A-arm is the optimum shape, that single rod-end in bending is carrying the vast majority of the force due to acceleration and braking. spherical bearings should be loaded radially, and rod-ends in bending are bad news.

 

I'm curious how you came up with this. I don't understand how there's a single rod end in bending outside of attached ARB loads.

 

Cary

[johnc]

Rod ends are used in bending all the time in race cars. Yes, as an Internet Engineering Masturbation Exercise its not a good idea, but no cars ever get built in the Internet. They are built in the real world where compromises have to be made for a whole assortment of reasons.

[TheNick]

I'm still waiting for someone with more patience than myself to come up with a way to adjust anti squat. In my head I see the inner CA pivots cut off and a nut welded in place (vertical/face of the nut pointing down towards the ground) and then an enormous rod end used to adjust the Y-axis/vertical axis height of the front and rear CA pivots.

 

That also a pretty decent way to control the rear roll center too.

[Flexicoker]

I'm curious how you came up with this. I don't understand how there's a single rod end in bending outside of attached ARB loads.

 

Cary

 

Look at the rodend that connects the A-portion of the control arm to the strut. If your car is accelerating, that force is going to have to go from the strut to the control arm. Since the toe link is about perpendicular to the direction of this force, its not going to carry any of the load. The direction of that force is not along the threads, and is going to put a bending moment on the threaded portion of the rod-end. Any force other than a lateral force is going to be bending that rod-end.

[rsicard]

Still don't understand why the "A" arms are better than the TTT H arms.

[Flexicoker]

Still don't understand why the "A" arms are better than the TTT H arms.

 

Imagine a lamp hanging from the ceiling, but its on a really long chain so its at about shoulder level. Grab that lamp with both arms, one on each side, and swing the lamp from the left to the right. As the lamp goes to the left, your left arm is going to go up, and your right arm will go down (relative to the left) The lamp and chain are the strut, and your arms are an H-style control arm. Your arms changing angle relative to one another is the twist thats going to be happening with an H-arm if you change the toe, or move the wheel front or back. The design of an A-arm with a separate toe-link accommodates that movement without flexing the arm or binding the strut. In most cases the geometry change from stock is going to be so minimal that nothing bad will happen, but there are probably situations with alot of toe, or trying to center the wheel in the wheel well were it could become a problem.

[rsicard]

Imagine a lamp hanging from the ceiling, but its on a really long chain so its at about shoulder level. Grab that lamp with both arms, one on each side, and swing the lamp from the left to the right. As the lamp goes to the left, your left arm is going to go up, and your right arm will go down (relative to the left) The lamp and chain are the strut, and your arms are an H-style control arm. Your arms changing angle relative to one another is the twist thats going to be happening with an H-arm if you change the toe, or move the wheel front or back. The design of an A-arm with a separate toe-link accommodates that movement without flexing the arm or binding the strut. In most cases the geometry change from stock is going to be so minimal that nothing bad will happen, but there are probably situations with alot of toe, or trying to center the wheel in the wheel well were it could become a problem.

Flexicoker: Thanks very much for your explanation. As for the design of the A-arm I agree with you. There is something about it that appears to be weak to me. Perhaps not. As for the rear A-arm, when the Datsun is level the aft A-arm should be a zero degrees caster. Caster should make not difference in the rear. If the A-arm is at near zero caster and a H design A-arm is use with a little toe-in if there is any twisting of the strut when transitioning from full down to full up, the heim joints can rotate the small amount is there is any twisting at all. The arcs that both heim joints swing through should remain constant from what I can imagine. If they both remain a constant arc then I don't see any twisting motion of the strut. Perhaps I am not visualizing it proper but I just cannot imagine same.

[260DET]

I'm still waiting for someone with more patience than myself to come up with a way to adjust anti squat. In my head I see the inner CA pivots cut off and a nut welded in place (vertical/face of the nut pointing down towards the ground) and then an enormous rod end used to adjust the Y-axis/vertical axis height of the front and rear CA pivots.

 

That also a pretty decent way to control the rear roll center too.

 

Its getting a bit off topic isn't it but without thinking about it much adjusting squat and raising the roll center would require two vertically adjustable pivots, both of which would have to be adjustable higher than the stock locations. Which obviously would require some severe 'reengineering' back there.

 

The mention of raising the roll center is deliberate, I can't imagine it would need to be lowered on any road course/circuit type S30.

[EMWHYR0HEN]

Look at the rodend that connects the A-portion of the control arm to the strut. If your car is accelerating, that force is going to have to go from the strut to the control arm. Since the toe link is about perpendicular to the direction of this force, its not going to carry any of the load. The direction of that force is not along the threads, and is going to put a bending moment on the threaded portion of the rod-end. Any force other than a lateral force is going to be bending that rod-end.

 

Realistically is this even possible to avoid? This always seems to be the argument but no real solution has come up.

[JMortensen]

Realistically is this even possible to avoid? This always seems to be the argument but no real solution has come up.

The stock control arm has no threads in bending.

[EMWHYR0HEN]

Were talking about the after market/modified arms here right?

[JMortensen]

Were talking about the after market/modified arms here right?

Look at my inner monoball mounts. They're welded in. That's one option. It could be done the same way on the outside end of the control arm, it would just require welding the control arm together with a captured monoball sleeve and a separate toe link. You wouldn't need to worry about threads in bending on the toe link, since it only gets acted upon radially. Building the arm this way would eliminate the ability to change the track or camber with the arm itself.

 

Personally, I don't think the threads in bending is a big deal. The only failure I've seen at that part of the control arm was with the AZC chromoly arms and that was probably due to a bad weld more than anything. If someone were inclined to build an arm without threads in bending I don't think it's that hard to figure out how to do it.

 

AZC arm failure:

http://forums.hybridz.org/showthread.php?t=123263

[TheNick]

Its getting a bit off topic isn't it but without thinking about it much adjusting squat and raising the roll center would require two vertically adjustable pivots, both of which would have to be adjustable higher than the stock locations. Which obviously would require some severe 'reengineering' back there.

 

The mention of raising the roll center is deliberate, I can't imagine it would need to be lowered on any road course/circuit type S30.

 

 

 

Yeah sorry if my explanation wasn't clear - you'd also need to rethink the rear mount for the control arms too.

 

I think its possible to do with some 2" square tube. Basically make a completely new rear subframe. Probably not too difficult if the car is on a rotisserie.

 

Of course I don't know what the antisquat % and roll center locations are like with the stock locations, so maybe there isn't any need to consider it anyway.

[Flexicoker]

Okay, after all my griping about rod-ends in bending without presenting a solution, heres a possibility. Probably not the best solution out there, but I think it will work. Little steel clevises that bolt on to the sides of the strut with the spindle pin. Now you can use an A-arm with a proper staked or c-clipped spherical bearing. The only disadvantages I see to this setup are that you can't adjust track/camber with it, but toe will be on-the-car adjustable. Also, sway bar loads are going to load the spherical bearing axially and try to pop the bearing out of its housing on the A-arm.

 

comments?

 

[tube80z]

Look at the rodend that connects the A-portion of the control arm to the strut. If your car is accelerating, that force is going to have to go from the strut to the control arm. Since the toe link is about perpendicular to the direction of this force, its not going to carry any of the load. The direction of that force is not along the threads, and is going to put a bending moment on the threaded portion of the rod-end. Any force other than a lateral force is going to be bending that rod-end.

 

Thanks, I may have misread you're previous post but what I got from your analysis was that all the force in accelerating or braking was going through the front heim. That's what I was having trouble understanding. With the top of the strut being fixed it's also going to pick up a good deal of the load. In playing with stock arms bolted to my bench I was amazed at how little resistance they offered when twisted. With the wheel being offset from the strut axis there's also a force trying to turn the wheel into the car (when accelerating) and this puts the toe-link into tension. And vice versa for braking.

 

I've had a number of cars with rod ends in bending. I'm not saying it's ideal but I know when the end is starting to get overloaded the jam nut has a habit of loosening up. And the next phase is usually some small bending you can feel. So far these parts have been beat on very hard and this hasn't been an issue. Not exactly an engineering analysis but I'm comfortable enough with them. If they can survive wheel hop that broke my back (and why I'm not racing this year) I think they should be okay. That said they are inspected before each race.

 

Cary

[tube80z]

Okay, after all my griping about rod-ends in bending without presenting a solution, heres a possibility. Probably not the best solution out there, but I think it will work. Little steel clevises that bolt on to the sides of the strut with the spindle pin. Now you can use an A-arm with a proper staked or c-clipped spherical bearing. The only disadvantages I see to this setup are that you can't adjust track/camber with it, but toe will be on-the-car adjustable. Also, sway bar loads are going to load the spherical bearing axially and try to pop the bearing out of its housing on the A-arm.

 

comments?

 

[ATTACH]13486[/ATTACH]

 

That's very similar to an idea a few of us have been talking about. The difference being the clevice was a single piece mounted under the strut and could be spaced down with spacers to control the rear RC.

 

Now, who'll be the first to build one?

 

CAry

[tube80z]

I think its possible to do with some 2" square tube. Basically make a completely new rear subframe. Probably not too difficult if the car is on a rotisserie.

 

Like the attached?

[thehelix112]

I was toying with the idea of using a bolt on attachment to the strut stub to move the pivots down. Look forward to seeing someone actually make one.

 

Dave

[JMortensen]

Okay, after all my griping about rod-ends in bending without presenting a solution, heres a possibility. Probably not the best solution out there, but I think it will work. Little steel clevises that bolt on to the sides of the strut with the spindle pin. Now you can use an A-arm with a proper staked or c-clipped spherical bearing. The only disadvantages I see to this setup are that you can't adjust track/camber with it, but toe will be on-the-car adjustable. Also, sway bar loads are going to load the spherical bearing axially and try to pop the bearing out of its housing on the A-arm.

 

comments?

 

[ATTACH]13486[/ATTACH]

The problem with the heim joint in this horizontal position is that it can easily bottom. Basically it will restrict the amount of travel available. I like the idea and especially the double shear mount of the rod end (could weld the clevis to the strut housing too, for that matter) but the implementation is a little tricky because of the range of motion with the rod end.

 

I was toying with the idea of using a bolt on attachment to the strut stub to move the pivots down. Look forward to seeing someone actually make one.

A guy on classiczcars.com made one like that and posted pics a couple years ago. I wasn't impressed, not because his machine work was subpar or anything, it just ended up looking like a clumsy solution and extremely expensive to machine all the bits.

 

I think making a cradle for the inboard ends is a good idea, because the rear control arm mounts are pretty horrible from the factory. The uprights with no cross bracing or anything just seems like a really bad design. The one thing is that if you're going to really raise the pivots in front you're probably going to have to cut into the floor for access, there just isn't much room to go up under the stock floor. There is a big plate reinforcing the diff there too. Doesn't make it impossible by any stretch, but it would be a lot harder to do than raising front LCA pivots for example.

[JMortensen]

Here's a couple links to that piece under the spindle pin from classiczcars.com.

http://www.classiczcars.com/photopost/showphoto.php?photo=13696

http://www.classiczcars.com/photopost/showphoto.php?photo=13694

 

If I were going to do this on a stock strut housing, I'd cut the bottom off the bottom of the housing and weld them longer like Terry did. Much simpler and less expensive.