Leaf shackle difference

[geometryjeff]

Can someone help me understand if the leaf spring shackles should be at the same angle on both sides. The '68 Camaro frame diagram shows original dimensions from 5/8 diameter locator points which I used for reference. The car is up in the air with engine mounted and no interior. The car is level with front springs replaced with turnbuckles for shock length and rear end axle is supported with no shocks, and I used plumb bobs at a datum plane from locator points. The 'side to side' and 'cross' dimensions are within 3/32". I indicated the rear wheels for parallel axies and measured from front to back parallel lines to the front edge of the rockers; both sides are the same within 1/8" so the axle is centered and square. I also measured the static distance between the rocker end to the rear tires with less than 1/16" difference. I believe the car is square to its self. The problem is that the angle of the shackles differ from side to side noticeably - maybe 5 degrees or more. Should I consider relocating the shackle mounting hole (that only changes appearance since the center to center relationship remains the same)? Can a difference in the torque on the shock plate U-bolts affect the rear of the springs appreciably? Or is it possible that the Hotchkiss spring rates are slightly different? Since the rear of the spring is along for the ride and if there is no bind with the rear panel, does it matter? Any thoughts are appreciated. Thanks.


Jeff

[Ron Sutton]

Hey Jeff,

A few quickie answers to help out.


Can someone help me understand if the leaf spring shackles should be at the same angle on both sides.
The angle of the shackles themselves have no bearing. but they are an indicator something is different side to side ... be it weight, spring rate, ride height, etc.


The '68 Camaro frame diagram shows original dimensions from 5/8 diameter locator points which I used for reference. The car is up in the air with engine mounted and no interior. The car is level with front springs replaced with turnbuckles for shock length and rear end axle is supported with no shocks, and I used plumb bobs at a datum plane from locator points. The 'side to side' and 'cross' dimensions are within 3/32".
It needs to be a smidge closer if you can get it. Don't use fenders for reference, as they vary too much.

I indicated the rear wheels for parallel axies and measured from front to back parallel lines to the front edge of the rockers; both sides are the same within 1/8" so the axle is centered and square. I also measured the static distance between the rocker end to the rear tires with less than 1/16" difference.
Don't trust the rockers. Production tolerances vary too much.

I believe the car is square to its self. The problem is that the angle of the shackles differ from side to side noticeably - maybe 5 degrees or more. Should I consider relocating the shackle mounting hole (that only changes appearance since the center to center relationship remains the same)?
Shackle angle itself will have no bearing on performance. But something is different side to side that could affect performance.

Can a difference in the torque on the shock plate U-bolts affect the rear of the springs appreciably?
I didn't understand this question.

Or is it possible that the Hotchkiss spring rates are slightly different?
All spring vary. All springs. Ultra high quality pro racing coil springs from Renton & Draco that come with a spring rate dyno graph will vary a few pounds. Lower grade springs vary more. A quick way to tell is to take them off & compare them to each other. First, simply put them side by side upside down on the floor. Are they the same height at the pad, same shape & same length eye-to eye? If they're close, consider yourself fortunate. Second use some some crude (but repeatable) test to your leaf springs rate in your garage.

Example:
On a long workbench with a vise ... run a bolt through on eye of Spring #1 & clamp the bolt in the vise so it can't move. Rest the eye on the other end on a simple grease plate (2 pieces of clean sheet metal with a thin layer of grease in between). measure the height from the pad surface to the workbench ... then apply a large weight. Ideally, it would be something that weighs similar to your car's rear corner weight (say approximately 800#). But this isn't easy, nor practical. So figure out the best way you have to put a lot of weight on the spring pad ... and measure its height now. Record the compression amount & repeat this on leaf spring #2.

Again ... if they're close ... consider yourself fortunate. If they vary a lot ... you may want to replace one of them. I doubt any company will let you return them, because you've bolted them up. The fact the spring rate varies isn't the company's fault. It is the nature of how springs are made.



Since the rear of the spring is along for the ride and if there is no bind with the rear panel, does it matter?
If the spring rate varies significantly, the car's handling will be affected. If the car sits lower on one side significantly, the car's handling will be affected. But if you have the front spring eyes at the same location as each other ... same spring rate ... and same ride height ... the rear shackle angle won't matter.


Any thoughts are appreciated. Thanks.

One last thing, a lot of winning road racers that run on leaf springs (mandated by their race series rules) adjust ride height ... to match the car side to side ... with shims under the pad & with different length shackles. Just make sure you have all the car's weight in place, including driver weight, tire PSI, gas in the tank, etc.


Best wishes.

[geometryjeff]

Thanks for input. I know that the car had a repair done to the right back corner a long time ago and had the frame pulled; I assumed it is correct because the car tracked straight for years before this rebuild. I will pull springs, as you suggest, to quick check rate difference and double check centerline of frame eyes to rear locator hole.
Your query on mounting plate - Because the perch is flat and the mounting plate is flat, wouldn't a higher amount of torque on the U-bolts try to flatten the arch in the spring in that area? Would that be enough to cause a minor change to the rear of the spring. Also, I was assuming that to maximize performance, the drivetrain should be on a parallel line to the body (mine is offset 1/2" to right) and rear axle should be square to body, leaf springs centerline and drivetrain with front suspension centered and square (disregarding caster, camber, toe, bump steer and all the other important geometry for now). I'm using laser lines, string lines and plumb bobs for centering; I think I can get things to +/- 1/32 with a little more effort. Again, Thanks for input. FYI - I found that using a tapered cork with a hole through the center to hold a plumb bob string works well to find locator hole centers.

[Ron Sutton]

Thanks for input. I know that the car had a repair done to the right back corner a long time ago and had the frame pulled; I assumed it is correct because the car tracked straight for years before this rebuild. I will pull springs, as you suggest, to quick check rate difference and double check centerline of frame eyes to rear locator hole.
Your query on mounting plate - Because the perch is flat and the mounting plate is flat, wouldn't a higher amount of torque on the U-bolts try to flatten the arch in the spring in that area? Would that be enough to cause a minor change to the rear of the spring.
It sounds like your spring is arched in the short pad area, so I guess it could. If that's the case, just tighten the U-bolts to the same torque.

Also, I was assuming that to maximize performance, the drivetrain should be on a parallel line to the body (mine is offset 1/2" to right)
Most production cars have the entire powertrain offset 1/2" to the passenger side. Engine, trans, driveshaft & pinion in the rear end are often offset together. Still parallel to the car ... just offset to the passenger side for better weight balance.

and rear axle should be square to body, leaf springs centerline and drivetrain with front suspension centered and square (disregarding caster, camber, toe, bump steer and all the other important geometry for now). I'm using laser lines, string lines and plumb bobs for centering; I think I can get things to +/- 1/32 with a little more effort. Again, Thanks for input. FYI - I found that using a tapered cork with a hole through the center to hold a plumb bob string works well to find locator hole centers.

Cool. Best wishes.

[geometryjeff]

Thanks again! Springs are slightly different static height and torquing incrementally gave slight improvement to shackle angle. Next question - I think I saw in a post here (couldn't find it again) that the best shock location is vertical in both 'X' (side/side) & 'Y' (front/back) planes with a max tilt of 10 degrees toward outside at lowest 'Z' ('X') plane behind and closest to axle. Since engineering faces packaging issues, most shocks are moved inboard to allow for rim/tire clearance and for ideal shock length. What are thoughts on making box cuts through the frame rails for 2x4 3/16" rectangular tubing to go up through the floor for longer shock lengths and location above the leaf spring and behind the axle vertically? That would also allow for a brace from the shock towers to the roll cage to triangulate the boxes to the frame rails. Thanks in advance for any and all thoughts.

Jeff