Front Control Arm Design

[Mike Mileski]

I have been putting together a new front control arm design. The outer portion of the main arm will have a rod end with high misalignment bushing in lieu of an outer ball joint. I used a double adjuster welded inside of piece of 1 1//2" OD tubing for the outer portion. The little rod end on top is for the sway bar link. The big difference is the TC rod. It uses a steering rack inner tie rod on one end and a pivoting rod end where it attaches to the main arm. That's the part I'm wondering about, will the pivoting rod end really make any difference compared to the normal solid connection? Any thoughts or comments on this?

 

Mike Mileski

Tucson, AZ

[RebekahsZ]

It might make the control arm flop around since there is nothing to stop rotation. What inner tie-rod did you use?

[helghast7]

I feel like the biggest problem is durability compared to gains.

 

And also you need to take into consideration what that extra movement is doing to the suspension geometry and what kind of negative effects it could have. It could possibly make the car feel unstable during rapid weight movements like going through a chicane or even something as simple as changing lanes.

[Mike Mileski]

Thanks for the comment. That's why I'm putting this out there. I can envision the control arm rotating a little bit, one way or the other, as it moves up and down but I can't envision any flopping around, in a static position, since the TC rod is a fixed length. The inner tie rod is a Moog EV162.

 

Mike Mileski

Tucson, AZ

[rejracer]

I've been wanting to do something like this, but use the stock TC rod and cut it down, thread it and use a inner tie rod in the rear like you are doing. 

 

For a design using all heim joints to work you will need to get the TC rod front pivot point to be centered on the line between the inner and outer control arm pivot points. In other words, I don't think there is a way to safely build it using a heim joint for the front TC rod mount. 

 

I can envision the control arm rotating a little bit, one way or the other, as it moves up and down

 

The rotating that Rebekahsz is referring to is during braking.  As the front tires are pushing back it will want to naturally twist the part that that the TC arm connects to.  Since the Heim joints  on the control arm will run out of normal motion in this direction in about 15 to 20*, you will run the risk of the following:

1. loosening the outer and inner control arm jam nuts. 

2. Changing caster during braking.

3. Changing tow in during braking.

4. Pulling the anti sway bar backwards or pushing it forward depending on if the front TC rod heim joint twists above or below the control arm. 

 

 I can't envision any flopping around, in a static position, since the TC rod is a fixed length.

 

Mike Mileski

Tucson, AZ

 

The TC rod is fixed, but as braking force increases the control arm will be pushed backwards.  This backward force will cause the front TC rod mount piece to twist because the TC rod front mount center point is behind the inner and outer control arm heim joint center points. 

 

The twisting motion will have the same affect on alignment as a TC being shortened and causing the problems listed above. 

 

To test this, build a jig, (or install on a mock up car).  Grab the outer heim joint and push backwards.  You will notice it twisting.  This is the force that will get you in trouble when braking hard. 

 

If you were to change the design so that the front TC rod heim joint is eliminated and replaced by a sturdy U shape bracket, this twisting could be completely eliminated.  In my opinion once the suspension geometry is set, you do not want any motion between the control arm and TC rod.  There is another design on the market today that allow for some flexing, and I think the design has the same weakness.  This basic design is implemented by Arizona Z car and Datsun parts LLC, with the former being a much stouter implementation. 

 

There is a reason the OEM TC rod is 16mm in diameter up front and bolted to the control arm using 2 10mm bolts.  That reason is to prevent the front control arm from twisting. 

 

By the way, nice execution, it looks well built aside from the TC rod heim joint. 

Edited October 30, 2013 by rejracer

[RebekahsZ]

That's what I meant by flop but I really meant what he said. Bolt it up and see what happens, the construction looks good, like all your stuff always does. You are one heck of a craftsman. I don't think anything will come apart, I just think you will have an unstable alignment over bumps and under transition from braking thru acceleration.

[Mike Mileski]

I think that, based on what you guys are saying, and thinking more about it, that I'm going to nix this idea.

 

Mike Mileski

Tucson, AZ

[johnc]

An A-Arm with one leg pivoting at the point of the "A" is a bad idea.

[heavy85]

Does anyone have experience using that Moog steering tie rod as TC pivot? I remember there was someone who sold TC rods that used them but dont know how they hold up. Seems like a good idea.

[RebekahsZ]

heavy85- Johnc turned me onto some TC rods two years ago that use a tie rod end. Not a lot of miles, but almost all of it racing-some tracks with rough pavement. So far so good-car goes straight and doesn't wander on the highway, drag strip or under threshold braking. Hey, have you gotten my PMs asking for a review on your driver's seat?

[tube80z]

Is there a reason you are making them so strong?  You really want them to rip off or fold up if they are hit and not damage the frame rail.

 

My personal preference is to put all the heavy adjusters on the inboard end where they move less.  UB Maching and others offer adjsuters that allow you to use a 3/4 bung and run a 5/8 rod end in a left/right combo so you can adjust in place.  For the tire end rather than putting a rod end in bending I'd use a spherical in a cup.  You can weld that to the end of the control arm tube.  For the TC to control arm weld some thin wall tube laterally across the TC tube and then hook this to ears welded onto the lower control arm.  This is very light and strong and pretty easy to do.  Very similar to this http://store.motorsportsconcepts.com/dtm-front-suspension-e30-e36.html

 

Hope that helps,

Cary

[NCchris]

A simple design I came up with. The existing strut rods can be shortened and threaded 5/8 UNF (fine thread). The adjuster tubes are thinwall chromoly with LH/RH ends already welded. These are available from Speedway Motors in 1 inch increments. A 5/8" L/H rod end with a 1/2" hole will attach to the machined aluminum adapters that will attach in the stock location.

 

Suggestions?

chris

[RebekahsZ]

Freaking beautiful! Please post up a full thread on building these when you have the job done completely. Everything from speedway is pretty cheap too, and shipped and on your door in a couple days. Please do a complete write up with part numbers if you can.

[tube80z]

A simple design I came up with. The existing strut rods can be shortened and threaded 5/8 UNF (fine thread). The adjuster tubes are thinwall chromoly with LH/RH ends already welded. These are available from Speedway Motors in 1 inch increments. A 5/8" L/H rod end with a 1/2" hole will attach to the machined aluminum adapters that will attach in the stock location.

front strut rods2.JPG

 

Suggestions?

chris

 

This works better if you have more than factory caster

 

http://forums.hybridz.org/topic/41611-tc-rod-pivot-relocation-bad-dog-subframe-connectors-slotted-crossmember/page-6?do=findComment&comment=567622

 

Cary

[rejracer]

In all of what I say below, keep in mind I am approaching this from the viewpoint of a typical daily driver.  The part gets bolted on, and it's not looked at for 20k+ miles.  If you are a racer and doing regular inspections, most of the points I highlight would be identified before it becomes a problem, assuming the design and build is sufficient.  My fear is that with these parts being more and more available, that some kid is going to bolt these things onto his car and forget about them for too many miles.  Then one fine day he gets the money for some sticky tires and decides to go canyon carving, failure occurs and aforementioned kid kills himself and/or someone else. Regardless of that I think what I am saying could have some benefit to the racing community in designing lighter, longer lasting and safer parts. 

 

Is there a reason you are making them so strong?  You really want them to rip off or fold up if they are hit and not damage the frame rail.

 

This is very light and strong and pretty easy to do.  Very similar to this http://store.motorsportsconcepts.com/dtm-front-suspension-e30-e36.html

 

Most of the stock TC rods get bent because of using poly bushings on both sides and fail while braking hard in a corner. The rear rod mounts need to flex. If they are not flexible enough then with normal wheel travel the TC rod will flex.  Add braking forces, and you have recipe for disaster.  Will it work?  Sure, for a while.  Will it fail.  It might.  Engineer accordingly. 

If the TC rod is supported correctly in the rear (either stock bushings not over torqued or some type of ball pivot) then the rod will almost always damage the frame rail in a collision.  I've proven that point on my car, I nearly tore a rear TC rod mount cup off the right frame rail in 1997.  The rod showed no damage at all.  I'm using the same rod to this day.  My guess is the reason companies are overbuilding these things is sometimes they will be used in a racing application, and not everyone knows that a poly bushing should only be used on the front side of a TC rod.  So what ends up happening is they put a spherical joint on the rear of the TC rod, which eliminates play while allowing full travel with no binding.  Then they overbuild the rest partly due to the size of rear Heim joint needed, and partly due to the size of the materials that are readily available to fabricate that design.  Are they overcoming a basic design flaw they already fixed by putting a spherical rod end in the rear?  In my opinion, yes.  Is it a problem?  I don't see a downside, aside from weight.  

Would I build them to act as a breakaway part so not to damage a frame rail?  No way!.  I've not seen many TC rod failures where the resulting damage is less than a damaged frame rail.  I've never seen or heard of any crashes from a frame rail failures on a car that was roadworthy in the first place.  Not saying it has not happened, It's just when the S30 chassis has a major suspension failure, I've only heard of the TC rods bending.  Perhaps someone with more experience will enlighten me.  

The BMW part in the quoted link has the same basic design flaw:  The TC rod front pivot point is behind the center line of the control arm.  This BMW part is controlling the twisting forces of the control arm by using a clevis type link.  It's the same as the AZC design, just weaker due to the smaller dimensions of the rearward tab.  Move the clevis pin to the center of the control arm and there is no tendency of the control arm to twist.   

 

 

A simple design I came up with. The existing strut rods can be shortened and threaded 5/8 UNF (fine thread). The adjuster tubes are thinwall chromoly with LH/RH ends already welded. These are available from Speedway Motors in 1 inch increments. A 5/8" L/H rod end with a 1/2" hole will attach to the machined aluminum adapters that will attach in the stock location.

front strut rods2.JPG

 

Suggestions?

chris

Brilliant!  
This design controls control arm twist by being firmly attached to the TC rod.  The TC rod is girthy enough to handle the loads.  The rear of the TC rod has no built in binding issue.  In my opinion a well designed racing part.  Only downside, is no rear bushing, so more road noise transferred to the chassis, and Heim joints are inherently high wear parts.

My suggestion:
1. If intended as a racing part, and occasional replacement of Heim joint is acceptable, leave as is.
2. If intended as a racing part and occasional replacement of joint is not desireable, change rear TC mount to a inner tie rod end type.  
3. If intended as a street part, change rear to a bushing setup, I think the part is strong enough  to handle the loads.  

 

My design (which I have not built yet) is what you have done plus a inner ball joint at the rear of the TC rod, plus the control arm is adjustable length. 

 

 

This works better if you have more than factory caster

 

http://forums.hybridz.org/topic/41611-tc-rod-pivot-relocation-bad-dog-subframe-connectors-slotted-crossmember/page-6?do=findComment&comment=567622

 

Cary

 

Why would this design work better if there is more than factory caster? 

 

While this one appears to be built out of much stronger materials, the TC rod front pivot point is still behind the center line of the control arm.  In my opinion it's a basic design flaw that has to be overcome by adding a lot of width to the control arm (this is what AZC does) or much stronger materials which is what the above link appears to be doing.  Regardless it's a solution to a problem which should never have existed in the first place.  The rearward pivot points will always be a high stress, high wear part of the design.  

I'm amazed that no one has thought of moving the pivot point forward. This way there is no tendency for the control arm to twist under braking, and the only short coming then is it's a high wear point, which is fine for racing applications that are inspected on a regular basis.  

 

[JMortensen]

The reason the clevis works better for larger amounts of caster is that you run out of room on the TC rod mounting holes. If you disconnected the TC rod from the control arm and then adjusted the caster to 7 degrees, you would no longer be able to get the bolts back in, because the forward angle of the control arm would mean that the TC rod holes would be in the wrong position. There is no simple way to fix this without a joint between the arm and the TC rod. You could oval out one of the mounting holes, but that's a crappy fix. Bending the rod is also less than ideal. If you decide to make a major change to caster later, you have the same problem again.

 

That little clevis piece that Dan made (and I copied) fixes that problem. It bolts in the stock location and it also allows the pivot bolt to be loosened when adjusting caster so that you're not springing the arm by making the adjustment. 

 

As to the relative weakness of the tab with the TC rod bolted to it, that's something that I have pointed out as well. As of yet, haven't seen one break though.

Edited November 5, 2013 by JMortensen

[clarkspeed]

I've been thinking about some front control arm ideas for a new race car that I am building (future build thread).  I wanted to use as many off the shelf circle track parts as possible.  This combo seems to work.

 

1. Allstar 56292 Screw in Ball joint Holder

http://www.allstarperformance.com/product.htm?prod=221&part=ALL56292&add=no

 

2. Moog K772 style ball joint, many options including adjustable and low friction.

 

3. Combine with swaged tubing, clevis,  and rodends of course for the CA and TC rod

 

The steering knuckle needs to be modified with 1.5"/ft (7deg) ball joint reamer and my aluminum bump steer spacers would need a little work to get the nut to clear.  I haven't checked if this combo interferes with rotor or wheel yet.

 

Any thoughts?

[JMortensen]

I tried to get the TC rod in line with the ball joint. Had issue with the bolts interfering with the steer knuckle. Since your is simplified you may avoid that situation. Another issue is that you'll lose some of your clearance between the tire and TC rod, so you may have to put spacers in the rack to allow less lock. The reason to get that rod inline with the ball joint is to keep the loads on the TC rod inline, so stressing the rod less. In practical terms I don't think it matters that much, so long as the bracket and rod are strong enough to handle the stress out of line, which pretty much every aftermarket solution does without problem.