Instant center

[Oborny71]

how do i go about changing the instant center on my 67' for better traction?

[SCOTI]

I would think if you moved the position of the trailing arm mounts up/down on the crossmember that would change instant center. I'm not 100% sure, but height adjustments could possibly alter it as well.

[Shev]

Doesn't instant center have to do with a traditional four link? I was under the impression it was the point the two bars come to a point in space...not sure how that would apply to a two link...teach us o great ones!!!

[Captkaos]

Every car has an instant center, but in talking about trailing arms, it is where the end of the trailing arm bolt center bolts to the frame. You have to move that location to change it.

[Oborny71]

would that be up or down for improved traction?

[68LSS1]

Well, this will be long and won't answer your question but I can't find the answers either. So here goes. Normally the intersecting points of the upper and lower control arms where it meets a imaginary line drawn from the middle of the front wheel and at the height of CG to the rear wheel centerline is your instant center. If this intersecting point is above the front axle center line than you have more than 100% anti-squat. If it is below this than you have less than 100% anti-squat. Figuring CG (center of gravity) would take some scales and geometry to figure out and as mine is in parts and is far from stock so it would be specific to mine anyway. Moving the instant center (or effective center which would be more correct because as the suspension moves the instant center changes) changes anti-squat or the "built in" ability of the suspension to handle loading. Anti-squat is determined by weight of the vehicle, CG height and length of wheelbase. That was the easy part. Now for the hard part, applying it to trailing arm suspensions and this is what I haven't been able to accomplish. I am installing a Hot Rods to Hell truck arm improvement kit as I plan on having a 800rwhp and almost as much torque power plant in my truck. I talked with Steve @HTH many times before buying his kit. Read a lot of feedback from the Camaro, Nova, Chevelle, etc people who installed his truck arm kits in their cars. Bought and read "Chassis Engineering" by Herb Adams (which I highly recommend) among other books. Hung out on Pro-touring.com (next project will be one). Anyway with his kit he has you move remove the brackets on the cross member where the forward part of the trailing arm connects and move them down one set of "holes". Meaning the original holes for the middle two rivets now become the top. I did call Steve and question moving the mounts down and although I can say I am very satisfied with the product and customer support, his answer didn't have the "math" behind it I was looking for. Normally moving the intersecting point up would change the effective center and increase anti-squat which is what we want to do but do the characteristics of the trailing arm suspensions follow the norm? I don't think they do. We really have only one pivot point on trailing arm suspensions. But I do plan on designing a cross member that will I will be able to move the brackets up and down (or unless I can figure out a definitive answer soon). Unfortunately I'm aways from having this thing on the road so if someone can determine how the anti-squat principles apply to a trailing arm rear suspension you can either go with the norm of raising the mounts to increase anti-squat or go with what the kit from HTH has you do and lower them. The HTH kit was designed for handling as well as trips to the drag strip so there could be a compromise being made their also as making the trailing arms parallel with the ground minimizes roll steer but because of the length of the trailing arms you still maintain good anti-squat characteristics.

Cliff notes: Didn't help you I'm sure but I think I helped myself a little. And if I'm right I would raise the mounts. If I'm wrong then lower them.

[SanitysBane]

so are the trailing arms mounted to the frame or a cross memeber?

[SCOTI]

Originally Posted by SanitysBane

so are the trailing arms mounted to the frame or a cross memeber?

Crossmember. The crossmember is riveted to the frame.

[68LSS1]

Also, whatever you do, ensure that the panhard is parallel with the ground with the truck at race (ride) height. Or as close as possible. This will help to ensure straighter launches. If its not parallel it will have a tendency to push/pull the rearend side to side which results in you not being able to stay in the groove. Yes, other things can account for this and contribute to it but this is real cheap and easy to correct.