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

[Guest 280ZForce]

I've been emailed, PM'd, IM'd, etc about what the differences are between everyone's rear lower control arms that are offered. As well as it has been posted several times members looking to make or buy the ideal rear lower control arm that suits all their needs and lessens their worries.

 

And people want to know what the differences between each makers LCAs are and here it is...

 

Which is if I'm allowed to do so on here, I don't see why not as it is only stating facts that are already known as far as H-arm designs and A-arm designs and I'm just pointing who makes which design and using a well known chassis building book as a reference.

 

I'm in no way stating anyone's product is inferior as ALL the DESIGNS have been TESTED, PROVEN & WORK... and of course everyone has their opinion and this is all open for discussion as it should be. We're all adults here and should act like it.

 

I have much respect to all the individuals, companies, etc out there building parts for our Zs, as this isn't an argument or a shot at who has the best or yada yada. They ALL make extremely nice and top quality parts (and I buy products from all of them as well), sometimes different designs are presented and the LCAs I'm having made are just different than theirs and it has peaked questions and interests for all parties.

 

This is merely just a thread about H-arm designs vs A-arm designs, so lets leave the discussion to just that.

 

H-arm: (Techno Toy Tuning, Modern Motorsports, Arizona Z Car & the stock design) vs. A-arm: (my "280zForce" run at LCAs & a few other guys who have DIY): A-arms provide a lil bit more adjustability and in doing so prevent binding better when adding toe.

 

Looks like I'm stacked against all odds here with the big classic Z car part vendors and being the only one to offer an A-arm design, but I'm not alone in thinking about it out loud and making it happen as other members have taken on their own A-arm designs including tube80z, 74_5.0L_Z, and jmortensen, among others.

 

Pics of each make for comparison so you can analyze and understand each design entail:

 

Click pics to enlarge:

.............AZC.............................MM...........................TTT.........................Z Force............

 

 

Both work designs work, BUT here's the differences laid out very nicely in an excerpt I've taken from one of the other LCA build threads.

 

This is just so it isn't buried among the posts anymore in other threads and has an actual thread itself...

 

I have had great luck with the control arms that I built. They are light and strong. My (74_5.0L_Z) control arms' date=' the stock control arms, and the ones sold by AZC are all examples of an H-arm strut[/u'] ***(Milliken pg 641)(book ref. below)***. 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).

 

74_5.0L_Z - H-arm Design... Click Pic to enlarge

 

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:

 

The A-arm: - What are the advantages of the A-arm toe link strut? , 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.

 

***For those who don't know, the men referenced above as Milliken (William F. Milliken and Douglas L. Milliken) are engineers who wrote this book called "Race Car Vehicle Dynamics" - Written for both the engineer and the automobile enthusiast, RCVD explores the engineering details governing the motions of automobiles in general and race cars in particular. Topics addressed include: Tire behavior, Aerodynamics, Steady-State and Transient Stability & Control, Wheel Load analysis, Steering Systems, Suspensions, Dampers, Force-Moment analysis, "g-g" Diagram analysis and much more. The historical chapter on vehicle dynamics development is a good read in itself for the non-mathematically oriented.

 

http://www.millikenresearch.com/rcvd.html

 

and also their complementary book with the help of Maurice Olley (one of the great automotive design, research and development engineers of the 20th century): "RCVD Chassis Design: Principles and Analysis"

 

Here's the back cover of it:

http://www.millikenresearch.com/olleybak.pdf***

[JMortensen]

The others are replicating what is on the car from the factory and improving on the H arm by making toe adjustable, etc. The A arm design fundamentally changes the way the strut gets loaded and the way twisting forces work on the strut and the arm. The A arm and toe link is a superior design.

[David K]

The others are replicating what is on the car from the factory and improving on the H arm by making toe adjustable, etc. The A arm design fundamentally changes the way the strut gets loaded and the way twisting forces work on the strut and the arm. The A arm and toe link is a superior design.

 

The "others" are bling pieces. A-arms are a true upgrade.

[AkumaNoZeta]

"H" versus "A" doesn't matter if modified for an upper link or control arm. It only matters if you're just keeping the strut type suspension, right? I'm working on either a double A-arm design or a multi-link, I was working on a few drawings but just ordered Chassis Engineering and Race and Rally Car Sourcebook to read so I can get a further understanding and make better designs.

 

What about the idea of using aluminum for the arms? I figure for a street car it would be a good idea for reduced weight but for a race car it wouldn't be able to handle the stress. Would aluminum tubing be alright for the manufacture, should solid rods be used instead, or does it have to be billet instead of welded together? The AZC ones are billet aluminum arent they?

[rsicard]

280ZForce: Thank you VERY much for your posting. This makes one think a lot more about the rear suspension geometry and its action/reaction using the two differing designs. It appears that on your design the front heim joint should be adjusted for camber and the rear will set the toe-in or toe-out. Is this correct? Please advise. Thanks.

[johnc]

The A arm and toe link is a superior design.

 

Yup.

[EMWHYR0HEN]

Not mentioned is the ease of adjustment with the A-arm. There's less bending loads on the A-arm design almost making it an entire "two force member" which can take huge amounts of load if designed properly.

 

If we were to make by example alone almost every purpose build race car has an A-arm design.. even with all different types of suspension setups.

[rsicard]

I've been emailed, PM'd, IM'd, etc about what the differences are between everyone's rear lower control arms that are offered. As well as it has been posted several times members looking to make or buy the ideal rear lower control arm that suits all their needs and lessens their worries.

 

And people want to know what the differences between each makers LCAs are and here it is...

 

Which is if I'm allowed to do so on here, I don't see why not as it is only stating facts that are already known as far as H-arm designs and A-arm designs and I'm just pointing who makes which design and using a well known chassis building book as a reference.

 

I'm in no way stating anyone's product is inferior as ALL the DESIGNS have been TESTED, PROVEN & WORK... and of course everyone has their opinion and this is all open for discussion as it should be. We're all adults here and should act like it.

 

I have much respect to all the individuals, companies, etc out there building parts for our Zs, as this isn't an argument or a shot at who has the best or yada yada. They ALL make extremely nice and top quality parts (and I buy products from all of them as well), sometimes different designs are presented and the LCAs I'm having made are just different than theirs and it has peaked questions and interests for all parties.

 

This is merely just a thread about H-arm designs vs A-arm designs, so lets leave the discussion to just that.

 

H-arm: (Techno Toy Tuning, Modern Motorsports, Arizona Z Car & the stock design) vs. A-arm: (my "280zForce" run at LCAs & a few other guys who have DIY): A-arms provide a lil bit more adjustability and in doing so prevent binding better when adding toe.

 

Looks like I'm stacked against all odds here with the big classic Z car part vendors and being the only one to offer an A-arm design, but I'm not alone in thinking about it out loud and making it happen as other members have taken on their own A-arm designs including tube80z, 74_5.0L_Z, and jmortensen, among others.

 

Pics of each make for comparison so you can analyze and understand each design entail:

 

Click pics to enlarge:

.............AZC.............................MM...........................TTT.........................Z Force............

 

 

Both work designs work, BUT here's the differences laid out very nicely in an excerpt I've taken from one of the other LCA build threads.

 

This is just so it isn't buried among the posts anymore in other threads and has an actual thread itself...

 

 

 

***For those who don't know, the men referenced above as Milliken (William F. Milliken and Douglas L. Milliken) are engineers who wrote this book called "Race Car Vehicle Dynamics" - Written for both the engineer and the automobile enthusiast, RCVD explores the engineering details governing the motions of automobiles in general and race cars in particular. Topics addressed include: Tire behavior, Aerodynamics, Steady-State and Transient Stability & Control, Wheel Load analysis, Steering Systems, Suspensions, Dampers, Force-Moment analysis, "g-g" Diagram analysis and much more. The historical chapter on vehicle dynamics development is a good read in itself for the non-mathematically oriented.

 

http://www.millikenresearch.com/rcvd.html

 

and also their complementary book with the help of Maurice Olley (one of the great automotive design, research and development engineers of the 20th century): "RCVD Chassis Design: Principles and Analysis"

 

Here's the back cover of it:

http://www.millikenresearch.com/olleybak.pdf***

280ZForce: With all due respect, I have read the description but cannot visualize this

 

"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."

 

I would expect that with camber plates there are spherical bearings which allow free movement in any of three axis. Just cannot visualize what was described here. If you try to explain it should be clear beyond any doubt to the person reading same. The person reading the explanation cannot visualize what is in the authors head without a very clear explanation. Not trying to be as ass about this but am trying to visualize EXACTLY the disadvantage of the "H" type arms.

[jt1]

The only reservation I have about the A-arm/toe link design is that the entire accel/deccel force generated by the rear tire is placed on the forward heim joint.

 

Has anyone done any analysis of the force on the forward joint?

 

jt

[johnc]

The only reservation I have about the A-arm/toe link design is that the entire accel/deccel force generated by the rear tire is placed on the forward heim joint.

 

Well... no. Torque is handled by the driveline, halfshafts, etc. If braking, acceleration, and cornering is at 1G then that's the load going through the control arm. There is some amount of load applied to the front bearing from the tires toeing under acceleration and braking but that is shared with the toe link.

[jt1]

What transfers the longitudinal force generated by the contact patch to the chassis?

 

It looks to me like the force path is thru the tire to the wheel, to the stub axle, to the bearings, to the spindle, and then all placed on the forward heim joint, in bending, on the threaded part of the heim joint.

 

jt

[johnc]

What transfers the longitudinal force generated by the contact patch to the chassis?

 

That's the 1G I mentioned in my post. The arm is seeing the acceleration loads from moving the entire vehicle mass. Sometimes people think the control arm also sees the torque loads from the driveline/wheel/tire/brake.

 

Its not that great of a load if the rod end is properly sized and the load is a radial load. On acceleration you're looking at 1,500 lbs. of load on each side of the car assuming a 3,000 lb. car and a 1G rate of acceleration - and that's assuming the tire can generate that level of grip.

 

A NHBB standard female 1/2" rod end has a radial load fatigue strength of 7,580 lbs. so you're never going to fatigue fail the bearing with the loads we're talking about.

Edited April 11, 2009 by johnc

[jt1]

It's not the radial load I'm concerned about, it's the bending moment and shear force on the threaded portion of the heim joint, and the pics above seem to me to show a male joint, not a female.

 

I'm not trying to be argumentative, or saying the design isn't adequate. I am saying the load on the forward joint is much higher with the toe link design, since it carries all the load, not a portion of it like with a h arm. The forward joint caries the weight of the car on that corner, the weight transferred to that corner by acceleration and cornering, and the force that is accelerating the car. All this combines as a bending and shear force on the threaded portion of the shank, if in fact it's a male joint. It's an important piece, and the specs of the joint should be given due consideration.

 

jt

[jt1]

The forward joint caries the weight of the car on that corner, the weight transferred to that corner by acceleration and cornering, and the force that is accelerating the car.

 

I worded that very poorly. I meant that the weight transfer can increase the force on one joint, for instance accelerating out of a corner.

[bodie]

ok i think you guys are splitting hairs, as you replace the rubber bushings with heim-ends you reduce the amount of toe in you would need to have to compensate for deflection in the suspension during load. ex you would normally have 15degrees total toe on a stock suspension you wouldnt need to run that much since your gonna have less deflection and you would bring it down closer to zero. reducing suspension bind.

[rztmartini]

i think that another added benefit to the A-arm design when compared to some of the other H-Arms (not all), is on-car adjustability. I have the earlier AZC rear control arms and while the toe is adjustable "on-car", the camber is not. getting the camber set is a pain, having to make a measurement, lift the car, take the "spindle bolt" out and adjust the heim joints, then lower the car and make another measurement.

[Flexicoker]

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.

 

Doing some super-simplified hand calculations, 1500 lb acceleration force, .5" from ball center to where it meets the jam-nut, and a .4405" diameter stud (minor diameter of 1/2-20 UNF, and not taking into account stress concentrations due to the threads) I get 89 ksi. Thats a whole lot considering that threads and fatigue weren't taken into account, and heat treated 4340 yields at ~200ksi

 

Also the axial proof load on a 1/2" Aurora male rod-end is only 2,040 lbs. And thats how the bearing is being loaded under accel.

 

Unfortunately I don't see a simple solution to the problem other than a bigger rod-end. Getting rid of the threads in bending is going to require alot different design, and probably not be on-car adjustable.

[JMortensen]

I'm using a 3/4" rod end. Most of these designs are using 5/8".

[JMortensen]

http://qa1.thomasnet.com/keyword/3-piece-precision-5/xm-rod-ends-2?&plpver=1001&pagesize=25&pagenum=1&filter=1&keyword=xm&key=product&keycateg=3001458&keyprod=3001488&SchType=2&keyType=P&Primary=1

 

From this page, the XM bearings 10/12 size (3/4 shank, 5/8 hole) have a radial load rating of 31,680. The -10 size (5/8) have a radial load rating of 17,955 lbs.

 

I had found previously on the QA1 site that the XM has a rather high spec for axial loading of 30%, I think most teflon lined 3 piece bearings have a rating of 10-20%, but couldn't find any spec on their site just now. Assuming 30% is right, that means my bearings can handle 9504 lbs, the smaller -10 bearing can handle 5386.5 lbs.

 

As far as threads in bending, ANY of the control arms in this thread has threads in bending. The worst offender appears to be the MM arms which seem (judging by the pictures) to have 2 or 3 inches of thread in bending.

[Flexicoker]

I can't run anymore calcs right now, but going from 1/2" to 5/8" makes a pretty big difference, its over double the moment of inertia (therefor half the stress). I think if you took stress concentrations due to the threads into account, and designed for infinite fatigue life with fully reversed bending, 5/8" would be right on the ragged edge.

 

The jist of what I'm saying is, with a big enough rod-end, periodic inspection, and minimal threads exposed, all of these designs will probably work fine. It should just be known that it does not have a very high safety factor, and is now a racing part, and racing parts need to be inspected frequently. That is all, I don't want anyone to think I'm bashing on their designs.