A frame and wheel spacing

[Bill Musarra]

I have never seen this info posted so I thought I would put a feeler out for it. On the lower A-frames, where there are rod ends that attach to the uprights(DRBs and others), how far out should the rod end be placed in order to get proper alignment. I realize the short answer is none, but for proper alignment and safety, how much can the rod end be let out. This of course puts the weight on the threads which can't be good, even when using bolts on both sides of the mounting. Especially if you change tires to a different size(width). I understand this is influenced by the "proper look" of the tire to wheel arch. I guess I am also asking what is the inner measurement for tire to tire on the rear. What influence are there for the proper placement of the uprights. Being a non engineer I have no clue about these things, and a good explanation with pros, cons and considerations would serve all on the forum. How does placement affect handling and such?? As a non engineer I'm not sure exactly what to ask. How does wheel size(width) to tire width work into this? I understand a little about offsets/centerlines and the like. I realize this is a lot to talk about, but I can't remember any discussions on it. Very soon I want to take my car to one of the race shops to get all this setup and sorted before getting it turned loose on the road. I have the articles on alignment in the home shop and they have helped me get things aligned enough to get the car to roll around without the tires squealing on the garage floor. Any and all info would be appreciated.

Bill

 

[Terry Oxandale]

Bill,

Are you doing this solely for alignment purposes, or are you also looking to relocate the upright assembly outboard for more pleasing aesthetics (tire to wheel opening)? The reason I ask is it would seem that a great deal of toe can be found with only a single turn of either joint, perhaps two or three turns at the most, which should be safe. Slotting the inner pivot bracket mounting holes so that it can slide fore or aft would provide some toe as well.

If not...

Have you instead considerd shimming the inner pivot bracket that locates the polyurethane bushings (based on photos I've seen of your build) outboard to move the lower arm outboard and thus maintain the shortest length possible on the rose joints? This obviously pushes the trailing arms outboard a bit, requires extending the upper link (which can handle the extended joints much better (or install a longer link)). it would also change the angle of the shock, which would change the spring rate a small amount, but the forces on the inner pivot would not change as a result of this (from what I can see. In short it would take a little more work, but nothing bad, and IMHO, would be mechanically the best option short of welding and coating a new set of lower arms.

Another option (again, I realize your arms appear to already be powder coated), is weld threaded tube-ends into the angle bracket that holds the rose joints. This alone would move the bottom of the upright outboard another 3/8" minimum depending on the design of the tube-ends chosen.

In regards to the tire's relationship to the upright, I've often wondered, as I look at some race-car uprights, what kind of forces are involved with the hugely offset tire centerline/rose joint centerline delta. This was one reason why I chose to make the upright as "upright" as possible so that the tire centerline (or more accurately, the contact patch) was as close to the centerline of the outer rose joints as possible.

 

[Bill Musarra]

Terry,
The only alteration I have made to the A arms is at the frame mount. Pete(Economan) had some info from the factory that the angle of the mount should be altered 4* to assist in the castor of the setup. Don't suppose you could comment on that? I don't quite understand the action of all the pieces in the rear setup and how to get maximum effect or setup. I understand the concept of castor in the tilting of the upright around on its axis. But how that affects the trailing arms, I don't. On the front end i understand its affect with anti dive characteristics(correct??). This probably shows how much I don't understand this stuff.
As you know I have added extended flares to my rear clip. I noticed when I finally took the car off the stand that the tires extended about an inch past the wheel well, and the arch hit the tires. That was with the rod ends at their shortest setting. But I am still curious about my questions.
What you have said about moving the items outward makes sense. Are there any bad things that, that action can bring about? Like how far out can the rods go before something is compromised? Some of this information isn't for me, but may be beneficial to others. What will be the actions of the rear with the numbers(castor , camber, toe in/out) set incorrectly. I don't plan to take my car immediaely to the track, but given the actions of some on the road when they see these cars, I would like to know what can happen when I try to avoid them. I will see the track soon enough.

Bill

 

[Jim Cowden]

BILL QUOTE
I understand the concept of castor in the tilting of the upright around on its axis. But how that affects the trailing arms, I don't. On the front end i understand its affect with anti dive characteristics(correct??). This probably shows how much I don't understand this stuff


We are talking rear upright???.If the rear end pick up points are in the optimum location you should not need caster.
Rotating the upright will raise or lower the trailing link pick up points on the upright ,as you go into pos caster the pick up points will rise, as you go neg caster they will fall.

If you have an inverted lower A arm in the rear they generally get bump steer if the forward pick up points on the chassis are to high or low.
Pulling caster on the rear upright will bring the hight of the rear trailing pick up points into a similar hight to the front points reducing the bump steer.
It is reduced as you are optimising the arch the trailing arms are traveling in.

It will also give squat or anti squat due to the thrust angles of the trailing links.

As far as pushing out the rear upright are you governed by the trailing links passing through chassis areas.

I dont think I would want any more than 1/2 the rose joint hanging out Bill or 1.5 the thread dia supported at least.

Wheel off sets on the rear I am with terry on that a central wheel offset for bearing load is good.
If you dont you just have to do extra maintenance.

Sometimes pictures or motioning with your hands make it so much easier when talking suspension..

Jim

 

[Terry Oxandale]

I hope I'm not contributing to your confusion when I take a shot at some of this. In the most simplest of terms, this is what I use to get started; if nothing else is changed, moving the lower lateral arm frame pivot fore or aft should only change your static toe setting. After that, only the lower trailing arm's geometry influences the dynamic toe as the wheel moves up or down. The upper trailing arm influences rear caster; the upper link (and/or lower arm) influences camber changes. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com

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These simple relationships become more complicated when you introduce geometry that provides anti-squat, or move the axis between the lower trailing arm/lower lateral arm pickup points away from being parallel with the centerline of the car (which almost all of our cars are that way), or make the trailing arms non-parallel to each other in the horizontal planes, or where your static geometry is to begin with. <o></o>
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[Bill Musarra]

I want to thank each of you for the information you shared here. I am sure the mechanical crowd or the engineering mind set picked up on all that you put in writing. I guess I need someone to move their hands while I am reading these post so I can picture in my mind what it is you guys have said. If You were talking about the innervation of the inguinal canal(where you guys get hernias from lifting those engines), I would have said, Oh yeah, now I see how it works. Maybe this ad about the Turbo Encabulator is for you what the vision is for me.

Turbo Encabulator - Rockwell International

Bill

 

[Terry Oxandale]

Boy, that's what I feel like at work sometimes when a vendor comes to us with delta design notes on new software for a major change in our applications. GAD!