Cobra Z gets new rear billet control arms today...pics

[arizonazcar]

The bushings are machined specifically to hold the tubes tightly in crush mode, If under impact you're able to rotate one, then the floor of the car will limit the travel upward only allowing some positive camber, conversley any downward

travel is limited by the half-shaft bottoming out. This may limit chassis damage and the camber can easily be reset in seconds That's assuming you didn't install the set screws.

 

Using a .250-inch diameter grade 8 fastener gives you the following shear capability:

 

A = Cross-sectional area of the fastener size (since bolt bodies/shanks have circular cross-sections, use area of a circle) = Pi x r2 where R (radius) = .250/2 = .125, therefore A = Pi x (.125)2 = .0491 square inches (in2)

Capability in shear = 91,000 lbs / in2 x .0491 in2 = 4468 lbs

 

After 25 years full time of making performance & racing parts for the Z (sold in 24 countries ) you guess I don't take that kind of thing into account before I hang a part on someones car?

 

Design was intended to allow:

 

Raising inboard pickup points to correct geometry on extremely lowered race cars

 

Provide track width adjustment for tire clearance etc.

 

Toe adjustment

 

Fore & aft adjustment for centering of the strut into upper mount or camber-plate

 

Reduce unsprung weight

 

Look so damn good!

 

Was not intended for you:

 

If you're going to be bouncing off concrete walls at a 150 MPH

or

If you have no need for the adjustments

or

you don't trust aluminum suspenion components ( even though every Corvette made since 1984 has all aluminum suspension arms )

or

you don't like the way they look

or

the design doesn't meet your particular application..........

or

You live in Chicago, you drive your car all winter long on pot-holed streets covered with salt.

 

Dave

[JMortensen]

After 25 years full time of making performance & racing parts for the Z (sold in 24 countries ) you guess I don't take that kind of thing into account before I hang a part on someones car?

Your qualifications don't exclude you or your parts from criticism. Here are some of the problems I see with your arms and your statement above.

 

You're figuring your shear strength wrong. It appears you've neglected to reduce the diameter by the depth of the threads x 2. You're loading a screw with built in stress risers (threads). I don't know exactly what that does to the numbers since I'm not an engineer, but it makes me want at the very least a larger margin of safety, and I'd want a relatively huge margin to account for metal fatigue since this thing is going to be constantly under stress.

 

The nylon bushings are "tight". I've seen plenty of pressed on metal parts spin on their shafts, so "tight" nylon bushings seems like a bushing that will have some stiction, not a secure method to keep a part from rotating.

 

You're telling people to rotate the brackets for the control arm up. The more perpendicular to the ground the bracket is, the more the leverage the control arm has on the bushings and the set screw. Raising the pivots isn't in itself a bad idea. The way that the brackets clamp in makes raising them in your design a bad idea.

 

You've deleted your toe adjuster, so now toe adjustments have to be made one turn of the rod end at a time, which is a gross adjustment, not a fine tune. Terry Oxandale figured out that with a 3/4-16 rod end 1/2 turn of the rod end equated to 1/8" toe change minimum. I think you're using 5/8"-18 rod ends, so it might be a little finer adjustment, but still not exactly fine tuning. And unless the taps were started in exactly the same position on the front and back holes on both sides, it seems unlikely that the rod ends would be at the same depth for a given number of turns. That said I think it's unlikely that you could even get both wheels to point straight ahead.

 

You've shortened the control arms which exacerbates the camber curve.

[arizonazcar]

With pickup points rasied , the mounts are up against the bottom of chassis....

can't go anywhere no matter what force is applied, no matter how loose.

5/8-18 thread is plenty fine adjustment for toe.

Steeper camber curve is disired due to limited travel with coil-overs etc.

Arms aren't necessarily shorter, depends on adjustment....can swap jam nuts with full thickness nuts so rod ends arent hanging out in the breeze.

Whole 35 year old chassis has metal fatigue: reccommend you make mold of chassis and reproduce in carbon fiber injected with structual foam, make suspension components from billet titainium, replace engine with Garrett TPE-331 (has 0% latency cycle compared to wastefull 75% latency of piston engine) makes 1100 HP from 150 LBS at 100% duty cycle

Have personally driven vehicle so equipped at 375 MPH (Mitsubishi MU2)

verfied on GPS.

As a side note I still hold Commercial pilot license single engine, commercial pilot multi-engine

Instrument rating single engine & mullti engine

in High performance & complex aircraft

Just for kicks

[JMortensen]

With pickup points rasied , the mounts are up against the bottom of chassis....

can't go anywhere no matter what force is applied, no matter how loose.

So the sheet metal floor pan will prevent ANY force from moving your part no matter how loose? That's what would hit so far as I can see from the pics. Let's assume for a minute that the floorpan is strong enough to prevent movement. What prevents the inside tire in the corner from moving in the opposite direction in this example?

5/8-18 thread is plenty fine adjustment for toe.

Not in my opinion.

Steeper camber curve is disired due to limited travel with coil-overs etc.

Riiiiiiiiight. That's why racers are always trying to make their control arms shorter.

Arms aren't necessarily shorter, depends on adjustment....can swap jam nuts with full thickness nuts so rod ends arent hanging out in the breeze.

I thought the rule of thumb here was that you want 1.5x the thread engaged in the arm itself, not that you just want it engaged in any thread. If there isn't enough thread engaged in the tube the arms would not be able to support the load, even if the rod end was fine. It wasn't the rod end that Kipperman broke on his arms: http://forums.hybridz.org/showthread.php?t=123263

Whole 35 year old chassis has metal fatigue: reccommend you make mold of chassis and reproduce in carbon fiber injected with structual foam, make suspension components from billet titainium, replace engine with Garrett TPE-331 (has 0% latency cycle compared to wastefull 75% latency of piston engine) makes 1100 HP from 150 LBS at 100% duty cycle

Have personally driven vehicle so equipped at 375 MPH (Mitsubishi MU2)

verfied on GPS.

As a side note I still hold Commercial pilot license single engine, commercial pilot multi-engine

Instrument rating single engine & mullti engine

in High performance & complex aircraft

Just for kicks

There is nothing useful in this part of your post.

[JustinOlson]

What is nylons creep characteristics over time? Most plastics have trouble with creep when loaded. I would be worried a crush fit could turn into a loose fit over time.

 

Justin

[arizonazcar]

So you took a big enough impact to shear off 2 5/16" diam bolts, overcome

the clamp load on the tubes, and punch the mounts through the bottom of the chassis......what do you think is gonna be left of the car?

By the way the rod ends have 1.625" of thread so there's plenty of room for

adjustment while maintaining adequate thread engagement.

Gaining neg. camber under load is the whole idea behind raising the inboard

pickup points.

Cross section of fastener shear data was for 1/4", but using 5/16"

I called 'em set screws but you're actually bolting through the bottom of the cap on the doglegs and the front diff X-member through the tubes, (once you have it ll where you want it) you'd have to shear them both off before you can rotate anything.....the bottom of the chassis is your backup stop.

Dave

[JMortensen]

So you took a big enough impact to shear off 2 5/16" diam bolts, overcome

the clamp load on the tubes, and punch the mounts through the bottom of the chassis......what do you think is gonna be left of the car?

Probably the rest of the car looking fairly normal assuming it didn't dig a rim and flip.

By the way the rod ends have 1.625" of thread so there's plenty of room for

adjustment while maintaining adequate thread engagement.

By whose definition? You saying "it's adequate thread engagement" or "the toe adjustment is fine enough" isn't enough proof for me. Aurora bearing says 1.5x the diameter of the fastener, so that's 15/16" engagement. Last I checked the threaded section of a rod end is 1.25", which leaves 5/16" adjustment.

Gaining neg. camber under load is the whole idea behind raising the inboard

pickup points.

Here I was thinking that it was to raise the roll center. It's good to know that you did not have roll centers in mind, and it was actually the camber curve that you were thinking of. Especially helpful when you consider the negative effect that these will have on the camber curve.

Cross section of fastener shear data was for 1/4", but using 5/16"

I called 'em set screws but you're actually bolting through the bottom of the cap on the doglegs and the front diff X-member through the tubes, (once you have it ll where you want it) you'd have to shear them both off before you can rotate anything.....the bottom of the chassis is your backup stop.

Better, but I still wouldn't trust it. There are many different ways to secure the brackets you've designed. Installing them in a nylon bushing with a set screw or a 5/16" bolt is about the least effective way I can think of.

[evildky]

wow those are beautiful and I'll let you too argue the finer points of engineering but if you wanted to effectively lengthen the arms longer rod ends and rotate the inner tube so that the pickup points are centered beneath it would get the length back although I'm sure there is some engineering problem crated by placing the arms at an angle? and to prevent the tubes from rotating all you have to do it drill a hole in each rear cap and put a bolt through the cap and into the rod it will turn no longer

 

and I gotta say with all that aluminum and individual re-engineered pieces why not a simple cradle that serves ad diff mount and moved the lca pickuppoints inboard for better geometry and a matching set of longer LCA's? it just seems to me taht for someone wanting to go for the whole nine yards you could make a better unit that would locate the diff, fit the finned cover, allow for longer LCA's etc

[JMortensen]

and I gotta say with all that aluminum and individual re-engineered pieces why not a simple cradle that serves ad diff mount and moved the lca pickuppoints inboard for better geometry and a matching set of longer LCA's? it just seems to me taht for someone wanting to go for the whole nine yards you could make a better unit that would locate the diff, fit the finned cover, allow for longer LCA's etc

There's ONE far superior way to solve the problem.

[arizonazcar]

I'm not using Aurora rod ends, correct length of thread is 1.625" as stated.

1.625" - .375 for the jam nut leaves 1.25" of thread, keep .93 of engagement

and you have just under 3/8 of adjustment left. Since you can do both inboard and outboard rod ends there's about 3/4" of total to play with.

 

The inboard rod ends are 12" apart, turn one in half a turn and you have made 1/36 of an inch adjutsment, multiply by 2 for a 24" tire and your toe has changed 1/18 of an inch at the tread (pretty fine) , I may have mod for finer adjustment if needed later.

 

As the car is lowered and the rear arms (outboard side) go up past lateral the camber goes positive under load, raising the inboard pickup point corrects this and allows an increase in negative camber under load.

 

I'll post a pic of the end cap with screw later.

Dave

[JMortensen]

I'm not using Aurora rod ends, correct length of thread is 1.625" as stated.

1.625" - .375 for the jam nut leaves 1.25" of thread, keep .93 of engagement

and you have just under 3/8 of adjustment left. Since you can do both inboard and outboard rod ends there's about 3/4" of total to play with.

We're agreeing here, I just remembered the length wrong. So you end up with 5/16" adjustment on each rod end, or 5/8" total.

 

The inboard rod ends are 12" apart, turn one in half a turn and you have made 1/36 of an inch adjutsment, multiply by 2 for a 24" tire and your toe has changed 1/18 of an inch at the tread (pretty fine) , I may have mod for finer adjustment if needed later.

That is not how the toe adjustment works. It's not a linear 1:2 ratio of the threads to the toe change. There is some trigonometry involved in figuring it out, and I happen to suck at math so I won't try to explain it, but Terry Oxandale previously showed that it is not a direct relationship between the length of the thread and the toe change. The angle of the spindle pin change is what is affected and the diameter of the tire also affect the outcome. The bigger the tire the more coarse the adjustment becomes, I can tell you that much.

 

As the car is lowered and the rear arms (outboard side) go up past lateral the camber goes positive under load, raising the inboard pickup point corrects this and allows an increase in negative camber under load.

This is not correct. On the stock vehicle it doesn't matter where the control arm is in relation to the horizontal. The Z gains negative camber until the control arm is perpendicular to the STRUT HOUSING, not the ground. It is correct to say that it gains negative camber a lot more slowly once the arm passes horizontal, but it is not correct to say that the camber starts to go positive past horizontal. I don't know if it is possible to lower the car so much or travel enough for that to happen. I want to say that it's nowhere near possible, but I'd have to check to be sure.

 

Here is a diagram that shows what is going on also courtesy of Terry Oxandale:

http://www.fototime.com/ftweb/bin/ft.dll/detailfs?userid={7DC317B0-8EDB-4B2E-A837-F708D07C9769}&ndx=19&slideshow=0&AlbumId={E19B86F3-5BFB-4733-AF72-09BD805A6F1A}&GroupId={832D28D1-26F8-4E35-B3D1-6BB2152C089B}&screenheight=768

[JMortensen]

Also you really wouldn't want to adjust toe at the inner rod ends because that would swing the outer end of the control arm at an angle. The farther the strut traveled from it's intended path (perpendicular to the inner control arm joints) the more bind you would put the strut shaft and the control arm in. This is a BIIIIIIIIIG no-no with an H arm strut setup.

 

EDIT--Actually I think I'm wrong here. I was imagining the effect of moving one pivot up and the other down

[blueovalz]

The inboard rod ends are 12" apart, turn one in half a turn and you have made 1/36 of an inch adjustment, multiply by 2 for a 24" tire and your toe has changed 1/18 of an inch at the tread (pretty fine) , I may have mod for finer adjustment if needed later.

 

This is correct. As Jon point out earlier, the 3/4" coarser pitch being used on the OUTER rod ends to adjust toe is really too coarse for fine tuning. The advantage of using the inner bearing adjustment with these 5/8" rod ends (slightly finer threads, and spread out at about double the length of the outer rod end spacing) does allow a finer adjustment.

 

I over engineer everything, but I assume the 5/8" has sufficient shear and tension failure figures. The 5/8” bearing would have made my conversion a dream-job, but I couldn’t see using anything less the ¾”, but again, I’m sure you’ve looked this over well. These arms look nice, and appear to function well. With that said...

 

I welcome a long term test of the arms:

1) oriented vertically, because that is where side forces are going to try to rotate the bearing brackets inboard. BTW, have you considered a split bushing instead, or a split metal bushing?

 

2) The effect of a stiff large sway bar pulling the inner bearing brackets up or pushing down as well (if the brackets are instead oriented horizontally as shown in the photos). I guess I would envision a splined bushing design that rigidly held the bushing in place rather than a poly (or what ever the material is) bushing.

 

3) The effects of heat in that area are also a consideration as well. What effect will heat have on the "clamping" ability of the bushings if one were to run their exhaust close to, or under the bushings? Aluminum has a different expansion coeficient than the steel tube inside or the poly bushing. What effect will heat cycling, under compression, have over time?

 

4) I can see where the pin (bolt) can be used to prevent twisting, but then that kinda disposes the "track tuning" advantage in that now you're locked into a specific setting unless you want a drastic change that allows another hole to be drilled in the tube further around the circumference.

 

Lastly, again an issue I brought up about the exact spacing required for mounting sperical bearings in the orientation "we" all like to use in our custom arms. The formula car orientation in the earlier post is 90º rotated from our use (your's and mine), which helps a lot. It at least places the spacing error issue on the stronger part of the bearing race. In our designs, if that spacing is off even a few thousanths (which is a common difference from one strut to another), you're going to bind that bearing in an axis it is not designed for, which is why I had to use very thin shims to get mine exactly right (which was then built into the design on the next set). In tube-type arms, the tube may deflect a little which reduces the stress on the bearing itself, but the billet arm looks like it will not have this advantage.

 

Please do not take this as any criticism, but instead as an opportunity to educate us on your design.

[G.I.jonas]

Wow!! this is some serious tech info here!!-good stuff!

 

Perhaps i'll just keep reading until i have no desire whatsoever to design my own arms.

[Jeff]

Wrong thread, oops.

[PresaMax]

Awesome thread. What's great about this site is all the expertise and experience people put into what they write. After reading all the above posts, who's product (rear and front adjustable control arms) works the best? Modern Motorsports? AZC? I'm just a curious consumer looking to invest in these aftermarket products. Thanks.

 

Julius

[Titus_RacerX]

I wonder what Jon thinks about the Modern Motorsports parts http://www.modern-motorsports.com/catalog/product_info.php?products_id=68

 

Jon, your ideas?

[JMortensen]

I wonder what Jon thinks about the Modern Motorsports parts http://www.modern-motorsports.com/catalog/product_info.php?products_id=68

 

Jon, your ideas?

Start another thread and I'll give you my opinion. I don't think its appropriate to do it here.

[arizonazcar]

The front to rear adjustment may only need to be done once during initial setup, the pickup points can be raised as desired for a given ride height. I don't expect to see many at the track changes to these settings such as you would with camber.

 

Exhaust heat should be kept away from all sensitive components such as

brakes, suspension, fuel system, electrical etc. This is standard racing practice.....Header wrap is a good idea where required.

 

The lower arms should glide into place onto the bottom of the strut housing

(no sledge hammers on my beautiful parts please)

If it's too tight a fit we sand/grind/file as required, too loose and we add shim

washers untill a proper fit is acheived, I may include some shims with the kit.

 

Dave

[rudypoochris]

Coyote Gary any update on how these are working out for you? Any rotation of the shaft w/o the set screw (or did you use the screw)? Thanks!