Quick Truck Arm Question

[Jiggs]

I have been lurking here for some time, and have decided to post. I have been reading as much as possible and have searched, but I can't find an answer to this question. I am trying to find the virtual point of intersection height of a truck arm setup. The intersection point looking from the top down is a no brainer, but the side view-height is confusing. On paper, the line would go through the center of the arm's front bushing, but what about the rear point. Where is it located? I have 7" of vertical mass (diff tube, mounting plate, and arm itself) to choose from. Thanks for the help.

[Jiggs]

Maybe I should rephrase my question. How do you find side view instant center on a truck arm rear suspension?

[wendell]

Hell if I know. Call Hot Rods to Hell. Then build a three link.

[Beige]

The rear point would be based on the panhard bar/watts link, like anything else that uses one.

[Jiggs]

That would be the roll axis, would it not?

[wendell]

No way the instant center is defined by the Pbar. I'm a little surprised that the HTH crew hasn't chimed in on this one.

[Mean 69]

SVSA is the projected length of the truck arms themselves, in side view (so a bit shorter than the actual length of the arms due to the skew in plan view). The SVSA is NOT what is used to find the roll axis, by the way, but IS used to determine Anti-squat, Anti-lift.

The roll axis is defined by the rear roll center height, (which in most cases is defined by the good ole' boy "J-Bar," or Panhard bar) and the virtual intersection of the convergence point of the truck arms in plan view, projected to side view to establish the height and distance (that probably didn't make sense, but that's how it's found). In other words, draw a line in side view of the truck arm axis, forget about the bushing, just continue to project it forward at the same angle. Figure out the length of the intersection in plan (top) view, and where this length resides in side view on the projected line is the forward lateral restraint point of the system, used to determine the roll axis. Connect this point, to the RRCH, and you have your axis.

I'll bite too though, if you are building a scratch suspension system, why use the Truck Arm?

Mark

[Norm Peterson]

It looks more related to a torque tube arrangement than anything, though the Kirban traction arm is similar.

I'm not so sure that the SVIC isn't right at the center of the front bushings. Other than the effects of the bushing compliance and arm torsional deflection that is necessary to allow suspension motion, that's what the motion of the arm and axle as a single rigid body rotates about in side view.

Norm

[Jiggs]

Mark; everything you said made sense. I am not scratch building this suspension. It's in a '68 chevy truck. The reason I asked the question, is I have lowered it about 7" and was wondering what I did to the geometry. Is this a pro-touring vehicle? Not really, but I want to learn and know the reasons things do what they do. If moving the chassis points to improve the IC, and improve traction, is what is needed, then I will. Fabrication is my forte, knowledge is needed. I appriciate the responses.

[Beige]

That would be the roll axis, would it not?

I misread your question and thought that's what you meant. Sorry.

[Mean 69]

Did anyone catch the NASCAR Sears Point race over the weekend? There was some great camera work, and commentary. One thing I noted in particular was pretty nasty brake hop going into the hairpin turn, they even had a little tech segment showing a cutaway rear end, Jeff Hammond was explaining why the brake hop was such a big deal, in that it could/would really hurt the ring and pinion setup due to the violence. Of course, I have my own take on that, my primary fear would be losing control of the car and ending up in hitting something immovable!

Mark