Yet another Rear control arm design

[tube80z]

Plus, I don't think the force that will get to the strut will be linear at all with the toe adjuster link. I mean, you hit a bump and there is going to be a spike in side force which will tend to make the strut want to bind right at the point where it should be moving. That's my uneducated view of it anyway. Am I missing something?

 

So how does the front not bind solid when you brake and hit a bump? Lower triangles and toe links are used on a number of WRC cars. I figure if it works for the front there's no reason it can't work in back. But I know we've agreed to disagree on this one.

 

Cary

[74_5.0L_Z]

I have had great luck with the control arms that I built. They are light and strong. My control arms, the stock control arms, and the ones sold by AZC are all examples of an H-arm strut (Milliken pg 641). The other option would be an A-arm and toe link strut. Each has their advantage and neither is meant to resist the twisting moment applied to the rear suspension when the brakes are applied (That function is performed by the strut).

 

 

The H-arm strut has a limitation that is described by Milliken, "...the inner bushing pivots must be perpendicular to the axis of motion of the strut at all times or bending of the strut will occur." What this means is that as the control arm rotates about an axis, and the end that attaches to the strut follows the arc of a circle. This is true for all rigid parts of the control arm. With our control arms (H-arm), the entire control arm is rigid and the bottom of the strut is rigidly captured. The strut tube is (supposedly) perpendicular to axis of the spindle pin. So, if the axis of the spindle pin is parallel to the axis of the inner pivot, then the strut will always be perpendicular to the axis of rotation and no binding will occur.

 

Now lets try a mental experiment. Let us adjust the heim in the front to get some toe-in. We have now turned the axis of the spindle pin so that it is no longer parallel to the axis of the inner pivots. This rotates the strut housing by a small amount and because the strut tube is angled away from the center of the hub, the top of the strut will try to rotate toward the back of the car. Well guess what: The top of the strut is captured by the bearing in your camber plates. You just put your strut in a bind. If you have rubber isolators and bushings you will probably get away with this. The rubber will compress before the strut bends. If you have spherical metal bearings everywhere, you will see evidence of the strut binding (springs hitting threaded collars). Interestingly, those cheesy aluminum/delrin eccentric bushings don't cause the same problem because the axis of the spindle pin and axis of rotation stay parallel.

 

Now for the other option: What are the advantages of the A-arm toe link strut? The A-arm, like the H-arm is constrained to rotate about an axis fixed to the chassis. The A-arm however only has one point forced to follow an arc. That rigid point connects to the strut and forces the attach point to follow the same arc. The link between the strut and A-arm is spherical and as such has two degrees of freedom. The connection is free to rotate about the axis of the spindle pin and to rotate about the axis if the strut housing. We need an extra constraint to control rotation about the axis of the strut housing. We need a toe link to do this. This is the important part: The toe link should only control the rotation of the strut about its axis. The toe link MUST only add one constraint. To accomplish this both ends of the tie link must be free to rotate in all planes, both ends must be heim joints. Allowing the toe link to rotate freely allows the strut to rotate about the rigid end of the control arm and prevents binding. If either end is constrained to stay in plane (clevis connection), then we are back to a H-arm and its limitations. Every control arm that I have seen made for a Z of this type was made wrong.

 

So lets try the same mental exercise with the A-arm toe link strut. We adjust to toe link to add some toe-in. We'll use the toe link in the front and the rigid link in the back. The spindle pin is again rotated so that it is not parallel to the axis of the inner bushings, and the top strut tube tries to moved slight back on an arc, but it is constrained at the top by the camber bearing. What happens? The whole strut rotates around the pivot at the rigid point on the control arm. As the strut compresses, the strut houing tilts further and further forward to keep the axis of the strut housing aligned with the bearings at the rigid end of the control arm and the camber plate. Because the toe link is not constrained, the strut housing is not in a bind and the control arm sees no torque.

 

 

So, after all of this I have come to a couple of conclusions:

1. If you have an H-arm set-up, keep the axis of the spindle pin parallel to axis of the inner pivot bushings.

2. I will only adjust the toe of an H-arm set-up using the inner bushings.

3. I am going to build a set of A-arm toelink control arms so that I and have more adjustment possiblility without binding.

 

Damn it, I used to be satisfied with my control arms. Oh well, back to the drawing board.

[tube80z]

Thanks for the detailed reply. I'll file this away for later use when I make my toe-control arms. And this time I'll use double heims.

 

Cary

[heavy85]

Jon - here's something else to think about and an experiment that I think would help validate your thought. Disconnect the top of the strut from the car. Now push the top of the strut forward and backwards with a reasonable amount of force. How much does the top of the strut move? If it's more than very very little then your control arms are not doing anything to help with rotational loads into the strut. Reason is that the strut/housing would have to deflect that amount before the control arm would even start to take load. If the top of the strut moves very very little then the LCA may take some load. One could argue that a stiff control arm would put more load into the strut. In reality the unibody will move around relative to the LCA. By making the LCA stiffer or not using a toe-link type of set-up you are taking away that degree of freedom (flexible LCA/soft bushings from the stock set-up) and as the chassis flexes and gets out of perfect alignment then it could bind up the strut - similar to what is said above. Not saying in the real world that's going to happen nor do I think there is anything wrong with your design I'm just giving some things to think about and a little experiment in your free time.

 

Cameron

[Flexicoker]

74_5.0, I think you just said more clearly what I was trying to say. My point about the strut 'seeing more stress' was that if you simplified the forces the bottom of the strut was seeing to either a linear force (like an A-arm would provide) or a moment (imagine the control arm being replaced with a torsion spring that was free to move for and aft) that the moment acting on the strut to restrain its movement would cause more stress within the strut. I wasn't trying to say that a stiffer control arm would make the problem worse, I was just trying to show one extreme vs. the other. I also was not trying to imply that an H style arm was wrong, just that I beleive it is overkill, and a better strength to weight ratio could be acheived with an A-arm and toe link setup.

 

I might be slightly biased since I welded all of these, but I beleive this is pretty close to an ideal setup:

 

http://img.photobucket.com/albums/v237/Flexicoker/F06/FSAEWest6-14-2006Alex038.jpg

 

All of the bearings are captured speherical bearings, no rod-ends in bending, and the alignment is done with shims. If there was no toe-link (like the bottom arm on our racecar), the inner bearings could be replaced with rod-ends, and they would see no bending loads, however, since the axial load from the toe-link is not in the same direction as the the rod-end is, it would be wise to keep it a spherical bearing. The outer bearing in an A-arm should never be a rod-end, as it will see bending loads, and I think that all 4 points in an H style control arm will see bending loads.

[74_5.0L_Z]

Flexicoker,

 

What you have for an upper rear control arm is precisely what we need for our lower control arms. In our case, however I think the rigid point of the control arm needs to be in the rear below the axis of the strut tube, and the toe link in the front. By doing it that way, we can decouple the toe and camber adjustments.

 

You also brought up a good point about not using rod ends at the wheel end of the control arms. What I would really like is a ball joint below the bottom of the strut tube and the toe link in the front.

[JMortensen]

Here is something interesting from Milliken p 642:

"One negative factor of this design (the H arm) is that any flex in the arm or distortion of the bushings due to braking loads or bump loads causes side loading on the strut."

 

That sounds familiar! Of course I've read the book, so maybe I can't take credit for that. Certainly I think it proves my point about the A arm toe link modification to the Z strut.

 

It goes on:

"With strut side load comes friction or resistance to axial motion. The suspension cannot isolate and perform all of its other functions properly when friction is present. This suspension has been thought of as a lower cost version of the A arm and toe link. It almost is, but not quite because of this potential for causing friction."

 

And after reading this I had the eureka moment. What would be necessary for the A arm and toe link to work without putting side loads on the strut would be for the A arm to terminate directly below the strut shaft. If you had that, then you could pivot all you want with a toe link and it makes no difference. That also answers your question Cary about why the WRC cars can do it.

 

Cameron, since my entire suspension is on heims joints there is no slop in there, and I can assure you that there will be very little movement up top. Terry Oxandale and I hemmed and hawed over what the result would be of the lack of flex in the system.

[74_5.0L_Z]

Jon,

 

If you read my post immediately above, then you will see that we had the same eureka moment.

 

Dude, get out of my head:mrgreen:

[JMortensen]

Now I'm wondering about cutting a section out of the tube that the spindle pin goes through and making an A arm that fits right up underneath the strut, then using a toe link (presumably in front because the strut tube is offset to the rear).

[JMortensen]

Jon,

 

If you my post immediately above, then you will see that we had the same eureka moment.

 

Dude, get out of my head:mrgreen:

I was writing mine when you posted yours...

[heavy85]

http://img.photobucket.com/albums/v237/Flexicoker/F06/FSAEWest6-14-2006Alex038.jpg

 

Not to divert too far but those were fun times ... back in the day ... late '90's for me. Back then if memory serves UTA (assuming from your avatar) was using some exotic Jap spec engine. 250 cc vs everyone else running 600 cc which given the ~17k RPM (from memory) could still choke the restrictor. Sounded cool as hell and shifted constantly but the judges didn't take kindly to the $$$. Also remember an aircraft type skin for bodywork but maybe that was another team. Back then mountain bike shocks with push/pull rods were just started to emerge. I was over in the Georgia Tech camp.

 

Cameron

[Flexicoker]

yeah, that was the 2006 incarnation of that powerplant. CBR250RR, 72 hp, 20k redline. It would have been a really killer car if it weren't cursed with terminal design related engine and brake problems. The motor isn't really that exotic, they're a dime a dozen in Japan, but we had to get them shipped here. I believe we choked the restrictor around 12k, the engine couldn't N/A. Fun times is right, that thing was a blast to drive.

 

[JMortensen]

Now I'm wondering about cutting a section out of the tube that the spindle pin goes through and making an A arm that fits right up underneath the strut, then using a toe link (presumably in front because the strut tube is offset to the rear).

Actually now that I think about it the strut is really totally behind the spindle pin area. So that makes it even easier. It would be necessary to somehow weld a tab or bracket to the back of the strut housing so that the rear rod end was in double shear. It seems like it would be pretty easy to then use a longer spindle pin and just hook the toe link up at the front of the strut housing as usual.

 

Arm in red, toe link in blue (duh):

 

So who is going to make the first one? My money is on Terry. He always seems to be the first out of the gate, and I know how concerned he was about binding the strut shaft...

[74_5.0L_Z]

Jon,

 

That is the exact design that I had in mind.

[RTz]

Jon, I'm thinking the side loading is caused by the difference between the wheel CL and strut CL. The location of the pick-up point is going to have relativly little effect, I think

[JMortensen]

I'm not sure what a CL is. I'm pretty sure you're not going for coefficient of lift.

 

Here is another example Ron. Take a shock with eyelets top and bottom. What comresses the shock without side loading: Pushing directly on the bottom of the shock, or putting a lever through the bottom eyelet and pushing up on the lever? Obviously if you tried to compress the shock at the end of the lever you would lift the shock until the eyelet bushings or the rod ends bottomed, and then the force on the shock would come at an angle. Same deal on our cars.

 

If the pickup is directly below the strut tube, you should get no side loads from the weight of the car and the suspension in bump. You'll still get some side forces from braking, but that's going to be a lot less of a hindrance than what you'd see from the weight of the car or the suspension in jounce.

[JMortensen]

I think I figured it out, you must be talking about the angle of the strut to the wheel angle. Yes, that absolutely will side load the strut as well, and that is as I understand it the major limitation of the strut suspension. That's why the strut is leaned inwards as I recall, so that under cornering loads the suspension can still move somewhat freely. I think the design is a compromise between straight bump side loads and cornering bump side loads. At least that's the way I had it figured in my mind.

 

The point that Dan and I are trying to make is that you don't need to exacerbate that problem by adding a lot MORE side force to the system.

[tube80z]

Here is another example Ron. Take a shock with eyelets top and bottom. What comresses the shock without side loading: Pushing directly on the bottom of the shock, or putting a lever through the bottom eyelet and pushing up on the lever? Obviously if you tried to compress the shock at the end of the lever you would lift the shock until the eyelet bushings or the rod ends bottomed, and then the force on the shock would come at an angle. Same deal on our cars.

 

CL is center line. Your example is not the same as what we have in the back of our cars. The spindle pin doesn't move up and down more in the front or the back because it is all rigidly connected to the strut. The control arms can be connected anywhere along the spindle pin and it will still move up and down. In order to make the installation stiff you want to have the max distance between these points.

 

In your example arm the toe link is going to be in compression under power. Most race cars I've seen lately put the toe link in tension. And for our cars it would be a simpler design with more distance between the points (on the outer end).

 

Cary

[74_5.0L_Z]

Having the toe rod in compression won't be a problem as long as it is not too slender.

 

I believe that the correct location of the hard point is below the strut tube. Having it there decouples the camber and toe adjustment.

 

One thing that I want to explore is the length of the toe rod and its affect on roll steer.

 

I have the fixture fom the construction of my old control arms. It will not be hard to modify it to make some of these. I expect to have some made within a month.

[JMortensen]

I don't think you are right that since everything attaches to the spindle pin that there won't be any side forces. There will be side forces on the strut regardless of the pin length. Like the Millikens said, any flex in the arm will create side forces. Why? Because flexion lowers or raises one end of the arm, attempting to rotate the spindle pin. The top of the strut stops that rotation as you say, and the rotational force is applied along the side of the strut shaft, so it gets concentrated at the bushing inside the strut, hence stiction. Put the pressure directly beneath the strut shaft and there is no longer any attempted rotation of the spindle pin.

 

About the toe link in compression, I'm not sure how you could design around that or whether it would be necessary if the strength of the link were sufficient.