Suspension geometry analysis

[Rick_L]

I was doing some reading over at pro-touring.com regarding SAI. Before we go too far I will say that I am just mentioning this and don't endorse it (or the opposite). Anyway, the article was discussing what happens as the wheels are steered that's influenced by SAI and caster. It suggests that the 18.5 degree SAI of the late C4 front suspension is way too much. Part of this is evidenced by the fact that the C5 spindle has much less SAI (also C6), i.e., they fixed it for the later versions.

Chevynut, have you read that article? Any comment? I know you posted a different article about this same subject, but I don't think it expressed an ideal range or what the limit might be.

Anyway, and this is kind of lame - just calling attention to it without having my own opinion yet. Hopefully I will. We're getting into 3D geometry here and that takes more tools to analyze.

[Rick_L]

This is the thread I'm referring to:

http://www.pro-touring.com/threads/1...ck-Performance

[55 Rescue Dog]

This is the thread I'm referring to:

http://www.pro-touring.com/threads/1...ck-Performance

Ron Sutton definitely has a grip on DYNAMIC handling, which is totally different than what it looks like on paper, which isn't pavement!

[oceangoer]

Well, thanks to all of you for the massive amount of work and effort.

Laz, Hit the Nail exactly on the Head !! He stated it's about "Driving our Tri5's", plain and simple.

I read many of these posts and others of various sus topics, but usually I don't sit down with Pen and Paper so I can understand each and every detail. I simply don't have the drive and time. But I do manage to learn a great deal and in the case of suspension details in post like this one and I do appreciate the info, I do often dig for an old dirty sus book for better understanding at times.

My '56 Chevy, I didn't install the C4 IFS & IRS at the time when We installed the LS1, T56 and Dutchman's 9 inch. I had all the parts, garage, tools,, at the time. But I did go with some bolt on's that,,, in the end is so much better than the stock Tri5 suspension. I still kinda regret not installing the C4 sus front and rear. But my little '56 is a very comfortable and responsive driver, even a few longer trips out of state. She's certainly not C4 quality, but close enough for this Car. The basic bolt on's are, Heidt's dropped spindles and polished S/S UCA & LCA, R&P rear steer, Helwig S bar, Wilwood 4 piston caliper kits,,,,

I live rural out in the forest area 25 miles out, and my sparsely populate hill is a twisty three mile hill uphill grade. I changed the front sus parts and braking so I can manage these Oregon Roads and the Freeways too. And, to safely navigate those unknown moments that pop up at us. Those are items like idiot or drunk drivers, Deer, Cougars, gravel in a corner, or a panic stop in a wet corner - This is what matters to me.

But, I'm just starting a full frame build with C4 sus front and rear and I'm helping my neighbor install C4 on both ends of his '57, now bare and naked frame. So I'm paying close attention. But I'm not going to mod major components !!

Yes, Like Laz I'm an Engineer (except retired), as was my Dad, Uncle and Grand Dad,, all which were Street Rod builder and drivers. So I grew up with this great past time and I keep it enjoyable. Being an Engineer, we all need details and no loose or unknowns... It drives me to learn the important details.

Just saying, thanks for all the effort every provides. I'll not need most of it, but I know where to look.....

Michael...

[chevynut]

Michael, I assume you know that I sell kits for installing an IRS and clips for the front. It could save you a lot of work.

Here's another thread with a lot of C4 information:

http://www.trifivechevys.com/showthr...nt-suspensions

[oceangoer]

Laz,

Oh yes, I've reviewed all that I could find.

I have a front C4 suspension cross member that Don @ Flat Out Engineering made custom for my '37 Chevy build. I don't need a front clip for either project. I've fabricated a few frames, and many Mod's to C3 and C4 suspension over the years. But this is my first complete frame build utilizing C4 front and rear suspension. I've modified my C4 Convert sus a few times, and right now I regret installing the OEM rubber control arm bushings in 2000, there nearly frozen in place, prob due to not enough driving.

My life long friend Steve (ME) designed the frame rails on Solid Works. The frame rails will be Fabricated by AME, they have CNC square tubing benders that will bend the frame rails to spec, And We can drive up to their shop and pick them up - Hoping for a tour.. I also have Dutchman's 9 inch IRS Diffy and the dual tube cross member ready. I'm purchasing the frame rails from AME because I just don't feel like fab'g another pair this time...

We already have the C4 rear bat wing cross member with mounts in the '57 Desoto frame and the front late C4 engine cradle install is well under way. This frame is very similar to a Chevy Tri5 except for two major differences, at the rear kick up the 57 Desoto frame is approx. 1 1/2 inches narrower and the front (did) utilizes torsion bars so there's no large spring pockets.

The C4 with a Dutchman's 9 inch IRS diffy might be of interest to some Tri5 Chevy owners. I'll be happy to post progress with photo's. But I think that will prob get started in about 4 to 6 weeks. That's because of AME's lead time and getting the Frame Table set up over at my place after the C4 engine cradle brackets are finished, plus some added fudge time.

We'll set the LCA to "0" degrees and allow for 1/2 in tire compression on the light weight '37 and 1 inch on the '57. Seem correct too you ??

Michael

[chevynut]

OG, I've been to AME's plant and Art gave me a personal tour of the facility. They're great guys and I got to see the tube benders and frame jigs. That's been close to 10 years ago now so I'm sure things have changed. It was before they moved to make room for a highway.

I think I've seen another company that makes a bolt-in differential for a C4 IRS. But they're like $4500 if I remember correctly. A Dana 44 is a lot cheaper.

I think 1" is too much radius change for a loaded tire. For example, a 245/40-17 tire is 25.7" unloaded diameter and turns 810 revs/mile. That's a loaded 24.9" diameter. So the radius changes by .44". I think all of them will be on the order of 1/2" and imo if you're trying to get closer than that you're kidding yourself. If you feel the need to get closer, find a car with the tires you want to run, inflate the tires properly and measure it.

[oceangoer]

Laz,

Plant tour yes, I can't wait. I love plant tours of factory with automation !! I hope they let me snap a few photos...


Ride Height // Loaded Tire Height


I've always been fascinated with ride height and all of the complex geometry that results when People change it up or down. Even with the stupid ill informed decisions re ride height I'm amazed in that Cars that are raised or lowered 2 or 3 inches still can be driven reasonably well, 50 MPH or less. I wouldn't want to ride in one at high speeds. Both of these builds we are going to set the front an rear sus to as close as we can get to factory spec's


RE the compressed tire height, initially I had thought that a half inch would be appropriate for the '57 HT. But I measured the 2004 FRC ZO6 Le Mans Com, and the fronts are compressed a full inch, and I re-check just now.. The front tires are the originals, G/Y 265x40x17 run flats (detest this tire). And this model is supposed to be the lightest in the C5 ZO6 lineup..

I'll have to re-check the C6 wheels and tires for the '57. But I think the front are 255x40x18. We'll definitely install them and get a good loaded measurement. But, the motor sitting in the corner of the shop os a 392 Blown Hemi, and that's at least 350 pounds heavier than an all aluminum LS6.

I'm thinking stick with an inch for tire compression for the '57 HT.. Your Thoughts ???

I agree, getting the compress tire height within a "Half Inch" will be the goal. Plus, tire diameter and their compression numbers vary some.


So, the plan is to set the lower control height to a level LCA and Plumb, fore and aft, then finish the metal fabrication to bolt in the C4 engine cradle.

Don'y faint, but We are going to bolt the engine cradle in place initially. Once that done we will flip the frame over and finish anything on the bottom of the frame. Then the plan is to re-install the front and rear sus and tires. We'll set the ride height via the shock mounts. Then haul the roller frame down for a optical alignment check, before we make the final welds.

Since we are working on two frames at the same time, I'm considering to fab a Fixture to secure the front hubs parallel and at the selected height. Prob something simple like 3/4 plywood base and uprights with some bracing. something to hold the hubs in place while we finish up some welds and do the various measurements to square to hubs etc to the frame. You know the importance of the front end sus placement.

We've measured and re-measured the '57 frame and it is amazingly "Square" for a old '57, esp since it was a Police Car. She still has the original black sides and white top paint job. Mostly gone and faded, but very cool. Anyways the frame measures within a 1/16 Cross triangulated in every place we have checked. Yes, We did install two vertical pins at the peak of the rear kick up for the fixed reference point.

As far as the Rear, the Bat Wing mounts / cross member is in place, square and plumb. Once the front hubs are aligned we'll align the rear hubs and screw the rear control arm brackets in place. We'e not going to weld them in place until after the optical check.


IRS Diffy

I did consider hunting down a Dana 44 Diffy core. But this '37 Chevy build is so different in most regards I decided to mate the C4 IRS to the Dutchman's 9 inch IRS Diffy. The Diffy case is wider, so each half shaft will require ~1.030 shortening. I chose 35 spline alloy axles and the the larger 1350 Yoke. The lower Camber control arms will require a lower Bird Cage, that's time consuming but not the difficult. We have all the drawings, so Steve should be able to design the cage pretty easily, We will be adhering to the late lower control arm length. Another reason for the 9 inch IRS Diffy is the ease of installing a a different 3rd member assembly. Plus 9 inch cases are much simpler to assemble and wider parts selection.

The Toe control assembly in the C4 IRS has always be a Thorn in my side. The Pivot point on the rear of the Dana 44 has baffled me for over a decade. We'll use the spindle mount for the inter pivot. The outer will be an interesting challenge. I'm very interested to see what happens when the inner pivot point is moved closed to the inner u-joint C/L. Anyone ???

I know Todd (@ Dutchman's) pretty well and He's always been been fair and honest in all regards. So I trust his knowledge and technical advice, re his 9 inch IRS Diffy. But, I'm still uncertain as to which "Traction Control" to utilize with the Corvette C4 IRS. Currently I'm leaning towards the Truetrac. The Wavetrak is another nice device, esp with the life time warranty. I'm deliberately don't desire a clutched device nor another locker for such a light this weight car. Do you have any experience with the Truetrak or Wavetrak ???

So, there's the plan,, frames first...

Michael

[chevynut]

I'm thinking stick with an inch for tire compression for the '57 HT.. Your Thoughts ???

I already told you my thoughts ....I think an inch is too much. You could get anal about it and calculate the size of the contact patch using the corner weight and pressure of the tire. Then all you have to do is figure out what radius gives you that contact patch area.

Don'y faint, but We are going to bolt the engine cradle in place initially. Once that done we will flip the frame over and finish anything on the bottom of the frame. Then the plan is to re-install the front and rear sus and tires. We'll set the ride height via the shock mounts. Then haul the roller frame down for a optical alignment check, before we make the final welds.

And I thought I was anal. Are you going to re-check after welding? things WILL want to move around a little....at least a few thousandths

We've measured and re-measured the '57 frame and it is amazingly "Square" for a old '57, esp since it was a Police Car. She still has the original black sides and white top paint job. Mostly gone and faded, but very cool. Anyways the frame measures within a 1/16 Cross triangulated in every place we have checked.

You're lucky. I've seen these frames off quite a bit in all the years I've been doing it. Our jig is square so once we cut the front end off the frame we can correct just about anything. The squareness of the original frame really doesn't matter to us.

We will be adhering to the late lower control arm length. Another reason for the 9 inch IRS Diffy is the ease of installing a a different 3rd member assembly. Plus 9 inch cases are much simpler to assemble and wider parts selection.

All C4 lower camber arms are the same length. I'm curious that since you're going through all the trouble of narrowing halfshafts for the differential, why wouldn't you narrow the whole rearend? I made mine 60" wide at the wheel mounting surfaces. When you shorten the halfshafts and not the strut rods you're going to affect the geometry, but I haven't thought about what it does. First impression is that it should increase camber gain. I heard that they lowered the inboard strut rod mount on the late C4 to decrease camber gain.

Yes I hear the 9" rears are easy to work on compared to GM. How much is the whole differential with case, carrier, and gears? I always wondered what a "birdcage" was.

The Toe control assembly in the C4 IRS has always be a Thorn in my side. The Pivot point on the rear of the Dana 44 has baffled me for over a decade. We'll use the spindle mount for the inter pivot. The outer will be an interesting challenge. I'm very interested to see what happens when the inner pivot point is moved closed to the inner u-joint C/L. Anyone ???

I guess I'm not understanding why the toe rod is such a problem. It's about the same length as the halfshaft stock, so it doesn't affect toe much as it moves. What the "spindle mount" that you're referring to on the inboard pivot? Once you cut the halfshaft down, you will affect it if the toe rod is left the stock length. Seems like you need to shorten it and move the inner pivot outboard. Why is the outer connection a challenge? Aren't you just using the knuckle arm?

Do you have any experience with the Truetrak or Wavetrak ???

I don't know anything about them.

I'd be interested in seeing your progress once you get the parts.

[oceangoer]

Laz,

Tires, well We'll mount a few and measure the results.

Frame, oh yes will check after welding. But My TIG isn't up to the Pro level. So at that juncture is off to a friends home shop. He's a Profession
fabricator now with his shop a home. And has several frame tables. Yes, I'm a bit finicky re the frame, back bone on the Car


Diffy, lower control arms same length. The 9 inch IRS Diffy case is wider, hence the narrowing of the half shafts to maintain the original width