Calculating Rear Roll Center for Leaf Spring Cars

[68Formula]

So, I understand how to calculate the roll center for the front with control arms. How do you apply this to the rear on a leaf spring car?

Then, what are some good ways to adjust the roll center of the rear?

[dennis68]

There is an imaginary line that runs through both of the spring eyes, the height at which the wheel vertical center line intersects is the RRCH. Changing the RRCH on a hotchkis type suspension is very limited and requires either new parts or a friend that can de-arch springs.

[Steve Chryssos]

So how do extreme lowering blocks (say 2" plus) affect the geometry? Where the wheel centerline may be moved above the plane created by the front and rear spring eyes. Also, I assume a shorter tire has an impact since it moves all of the points closer to the ground.

[dennis68]

My bad, I should prefaced that statement with "to properly" before making any definitive statements. I would think anyone serious enough to be researching moving RRC would not be running 2" blocks on the axle, thats just me.

Yes, shorter tires would lower the RRCH but thats not a reasonable change as what you gain in the shorter sidewall makes more of impact than the 1" or so of lowered RCH. I think most people already have their wheel /tire combo set up and wouldn't be likely to change it to make a small adjustment.

[Steve Chryssos]

Oh. I thought it would be bad. Do you want the three points to interesect? Front eye, axle centerline, rear eye? Or is it better if the axle centerline is higher than the other two points.

I've often considered raising the front spring eye location (a lot of work) such that the line created by the front and rear eyes is at least horizontal. Then I could return to springs with a little more arch.

[dennis68]

Oh. I thought it would be bad. Do you want the three points to intersect? Front eye, axle centerline, rear eye? Or is it better if the axle centerline is higher than the other two points.
It really doesn't matter where along the spring pivot axis the vertical CL intersects except to determine RCH. With a 285/35R18 the RRCH would be ~12.9" if you positioned the spring so that the horizontal wheel centerline was in direct line with both spring eyes.

Repositioning the spring mounts could be advantageous however, to modifying existing anti's. I don't know if moving the spring pivot locations would have a measurable impact on roll steer characteristics....anyobody have any ideas?

[jeffandre]

I read on a few leaf spring sites that for advantageous cornering the front spring eyes should be about 9" below the rear spring eyes, as well as moved inward 1-3" each. We had a discussion about this a while back on the old board.

I currently have about 9" lower front spring eyes (rear sliders), and 3" inward mounting of each front spring eye. It also looks like the line between the center of the spring eyes goes through the center of the axle, maybe a tad higher than the center.

[David Pozzi]

Here is some leaf spring tech from AFCO.
Their website link to their tech page is dead but I found last years tech page on the "Wayback Machine" here: http://web.archive.org/web/20041126011147/http://www.rpmnet.com/techart/leaf.shtml

[Steve Chryssos]

Awesome. I see it now. Doesn't matter how much above or below the imaginary line the axle centerline is with respect to RCH The intersection of the vertical line created by the axle centerline and the imaginary line.
https://static1.pt-content.com/images/pt/2005/05/LEAF_5-1.gif

I'd say that file needs to be added to our little fountain of knowledge.

[jeffandre]

Awesome. I see it now. Doesn't matter how much above or below the imaginary line the axle centerline is with respect to RCH The intersection of the vertical line created by the axle centerline and the imaginary line.
https://static1.pt-content.com/images/pt/2005/05/LEAF_5-1.gif

I'd say that file needs to be added to our little fountain of knowledge.

That's exactly what my setup looks like. I still am a ways off from testing it on the road though...

[ks71z28]

On the 2nd gen Camaros it is popular to relocate the front leaf spring eye 1" higher to raise the RRC, and lower the car about 1/2". By that graph it appears that it would accomplish both by just lowering the car, not by the geometry change. I'm thinking of doing this to reduce my 1" lowering blocks to 1/2". It wheel hops terribly when hooked.

[dennis68]

On the 2nd gen Camaros it is popular to relocate the front leaf spring eye 1" higher to raise the RRC, and lower the car about 1/2". By that graph it appears that it would accomplish both by just lowering the car, not by the geometry change. I'm thinking of doing this to reduce my 1" lowering blocks to 1/2". It wheel hops terribly when hooked.
I'm curious what theory describes raising the RRCH? On almost any leaf sprung car RRCH is going to be at least 12-14". Ideally RRCH should be shorter than that in almost any situation.

[Norm Peterson]

What somebody needs to do is post is Figure 17.44 from RCVD (page 662).

There's a kinematic construction line that can generally be taken as 0.375L. This is essentially a virtual link between the front eye and wherever the 0.375L intersects the center of the leaf as seen in side view. The leaf spring suspension's own roll axis is parallel to this construction and intersects at the rear roll center point in the pic that's already been posted. An 0.375L virtual swing arm is also parallel to this construction, and this extends forward from the axle center. This virtual swing arm is what defines the fore/aft motion of the axle at the leaf attachment lateral coordinates. And the slope of all three is the rollsteer percent.

Note that this is kind of theoretical, as bushing compliance, lateral deflection of the shackle assembly, and torsional deformation in the leaves themselves is likely to throw the constructions "off" a bit. Even the slight flattening or exaggeration of the curvature as the suspension moves has a slight effect.

Norm

[StaticAGE]

On the 2nd gen Camaros it is popular to relocate the front leaf spring eye 1" higher to raise the RRC, and lower the car about 1/2". By that graph it appears that it would accomplish both by just lowering the car, not by the geometry change. I'm thinking of doing this to reduce my 1" lowering blocks to 1/2". It wheel hops terribly when hooked.

A couple of months late, I know. Moving the spring eye up isn't done to raise the RRC, but to improve Anti Squat as well as lower the car a bit. I believe Herb Adams used that trick when he was racing Trans Ams.

[Boyd]

....Moving the spring eye up isn't done to raise the RRC, but to improve Anti Squat as well as lower the car a bit. I believe Herb Adams used that trick when he was racing Trans Ams.

How exactly is this accomplished?

[David Pozzi]

How exactly is this accomplished?
On a second gen you have the bolt hole lower than on a first gen (similar bracket, but it's got a lower hole), you can cut a "window" in the top of the mount, then the front spring eye has a little room to go higher. Herb Adams says 3/4" higher is max available.
There isn't room on a first gen to do this mod.

Your rear ride height will drop almost 3/4".
Anti-Squat will increase.

[Chevy350]

Here you go: (Though a bit late!)
https://static1.pt-content.com/images/pt/2006/03/RCVD1-1.jpg
https://static1.pt-content.com/images/pt/2006/03/RCVD2-1.jpg

So (ignoring the fact that the spring bends and 'moves'), to get roll understeer, I want the point 'R' to be higher than point 'Q', giving a slope lower in the front on the roll axis? And the front portion of the spring (between the front spring eye and the axle) to be as straight/flat as possible?

From other books I have read, I have just thought getting the spring eyes almost at the same level (a little up in the rear), to minimize the built in roll understeer. Is the rear rollcenter height given from the line drawn through the front spring eye and the frame side of the shackle, as shown on the picture above, or through the spring eyes (ignoring shackle length)?

[Boyd]

Thanks DP.

[68Formula]

On a second gen you have the bolt hole lower than on a first gen (similar bracket, but it's got a lower hole), you can cut a "window" in the top of the mount, then the front spring eye has a little room to go higher. Herb Adams says 3/4" higher is max available.
There isn't room on a first gen to do this mod.

Your rear ride height will drop almost 3/4".
Anti-Squat will increase.

So that's it for the first gen? No room at all?

[MrQuick]

...ahh you can but it involves cutting up the floor pan. It can get ugly.

[68Formula]

...ahh you can but it involves cutting up the floor pan. It can get ugly.

Nevermind then. Can't stomach cutting into prestine virgin metal.