[Wageslave]

I just bought a car from a member of the forum with what looked like just an issue of worn out leaf spring shackle bushings. After a lot of reading on leaf spring retrofits I found out about the 1 1/2 inch difference in the drivers side frame rail. The builders of the car evidently didn't know this or ignored how the rear of the leaf springs were lining up. What I have is the front of the leaf springs mounted equal distance from the centerline of the car and the rear of the springs forced to line up with frame rails. Evidently this is why the bushings failed. The rear end out of a 1990 TA and is centered in the body. I can't get it in my head how the rear end is centered and the rear end of the leaf springs is so far off. I am at a loss at what to do next. Some of you may recognize the car, the previous owner posted on the forum for help about tire rub not recognizing the root cause of the rub.

[Sirrotica]

Since I have done a IRS to leaf spring conversion about 50 years ago, if memory serves the rear frame rails are offset to one side. I know I used the rear crossmember from the donor car to attach the shackles to because of the frame rail offset. I had to notch the crossmember to fit over the Tempest frame rails so the leaf springs ran straight back from the front mounting points.

Donor car was a 1962 corvette, and the original fuel tank wouldn't fit under the car after the conversion, I used one from a 62 chevy, and had to move it into the trunk area.


BTW did you mean a 1980 F body instead of a 1990?

[Peter Serio]

That car is a uni-body car but spot welded underneath the floor pan are basically your frame rails. That said the rails on that year car are not perfectly aligned (left to right) to the outside lower edges of the rocker panels and the quarter panel.

One side is closer and the other is further away.. Not sure why GM made the body that way, they must have had some reason.

[Wageslave]

It is a 1980 TA rear end. The builder did a very nice job on the front eye mounts, they are set in the frame rails but both mounts are centered on the rails not allowing for the offset of the rear frame rail on the D.S.

[Sirrotica]

Originally Posted by Peter Serio

That car is a uni-body car but spot welded underneath the floor pan are basically your frame rails. That said the rails on that year car are not perfectly aligned (left to right) to the outside lower edges of the rocker panels and the quarter panel.

One side is closer and the other is further away.. Not sure why GM made the body that way, they must have had some reason.

If I remember correctly the spare tire well was the reason the frame rails were offset, but it's been a long time and a lot of water has passed under the bridge.

[Wageslave]

Thanks the diagrams Peter.
I found out about the offset a bit to late, I already bought the car, now I have to fix it if it is fixable. I was hoping to not have to scrap the existing suspension. Would an offset shackle work on one side of the car. Detroit Speed makes an
1 3/4 offset shackle. Would the suspension operate normaly?

Originally Posted by Peter Serio

That car is a uni-body car but spot welded underneath the floor pan are basically your frame rails. That said the rails on that year car are not perfectly aligned (left to right) to the outside lower edges of the rocker panels and the quarter panel.

One side is closer and the other is further away.. Not sure why GM made the body that way, they must have had some reason.

[Wageslave]

What I read in a post here on the Forum is that it is a carry over from the gas filler being located on the drivers side of the 1962's, doesn't really matter why now. Racking my brain for a workable solution when there may not be one.

Originally Posted by Sirrotica

If I remember correctly the spare tire well was the reason the frame rails were offset, but it's been a long time and a lot of water has passed under the bridge.

[Sirrotica]

There is, if the front is mounted correctly, there have been countless Tempests converted to leaf springs. Each person does it differently, as I did using the corvette piece to mount my rear shackles to. An offset shackle would work as long as it was tailored to what you need for your application. Close enough, isn't going to be close enough. It's going to have to be exact. You can see what close enough gets you. You may not be able to buy shackles to fit your needs and they will have to be fabricated to fit.

I was 16 at the time I did my conversion, I had to learn to weld so I could do the conversion in my fathers garage laying on my back, in the winter with no heat....NOT FUN.

[Wageslave]

I'm a metal fabricator/welder for 40 years. I'm just not sure if having an offset shackle just on one side of the car will create new problems.

[Sirrotica]

As long as it doesn't bind during spring deflection the minor offset should be fine. You should have plenty of skills to make up the shackles you need.

I can't recall what vehicle I've seen them on, but I do remember seeing factory offset shackles on some piece of Detroit iron over the years.

From your pictures I believe you're going to have to make shackles for both sides, neither side seems to be in alignment with the frame.

[Wageslave]

A lot of what you see on the P.S. is due to the bushings being destroyed and the load pushing the rear end toward the P.S.

Originally Posted by Sirrotica

As long as it doesn't bind during spring deflection the minor offset should be fine. You should have plenty of skills to make up the shackles you need.

I can't recall what vehicle I've seen them on, but I do remember seeing factory offset shackles on some piece of Detroit iron over the years.

From your pictures I believe you're going to have to make shackles for both sides, neither side seems to be in alignment with the frame.

[Sirrotica]

The tape measure will be the end judge when you get to that point.

[Wageslave]

Yep, time to break out the plumb bob and my good tape measure

Originally Posted by Sirrotica

The tape measure will be the end judge when you get to that point.

[Doug]

I saw the car on Ebay a few times and wondered about how the rear end was set up. The car is very nice!

Attached for reference are copies of a couple of pages out of the car's factory BODY SHOP MANUAL.

Paragraph "j" on page 11-4 of the car's BODY SHOP MANUAL has this note:
".....NOTE: Center of the compartment pan left side assembly (gas tank filler neck side) is located 1 5/16 inches further from body centerline than center of compartment pan right side rail assembly....."

The above note refers to the “gas tank filler neck side“. In previous years the same floor pan was used in both cars and station wagons and gas was filled from the side on both the cars and wagons, necessitating the frame rail offset design. In 63, the same floor pan was used for both cars and station wagons but only the wagons were filled from the side with the cars having the gas filler in the center of the rear of the car.

The original rear tread/track width of the car is 58 inches. The rear tread/track width of the TA rear end is 60 inches so you already are “too wide”.

You indicate the front mounts of the leafs are mounted “centered” on the frame rails, equal distance from each other. The centerline of car can be determined by measuring equal distance from corresponding points on the frame rails. Note that the driveshaft floor pan tunnel for these cars is "offset" about an inch to the left (driver side) and if one uses the centerline of that tunnel, it is not "centerline of car".

With reference to the chassis dimension information drawing on page 11-2 of the car's BODY SHOP MANUAL, the overall distance (dimension "X") between the frame rail centerlines where your leaf spring shackles mount is 42-1/32 inches as shown in the chart on page 11-3. Allowing for the driver side offset of 1-5/16 inches, that means the centerline of your passenger side spring (frame rail) is 20-23/64 inches (effectively 20-3/8 inches) from "centerline of car".

Again, with reference to the chassis dimension information drawing on page 11-2 of the car's BODY SHOP MANUAL, at about the point where your leaf spring front mounts are mounted, the overall distance (dimension "P") between the frame rail centerlines shackles mount is 40-5/32 inches. Or, in other words, the front mounts are 20-5/64 inches off centerline of car.

Accordingly, that means the passenger side front mount centerline is 9/32 inches (20-23/64 inches minus 20-5/64 inches equals 18/64 inches (9/32 inches)) closer to centerline car than the centerline of the passenger side rear shackle mount is to centerline of car.

From your pictures, the passenger side spring "appears" to be straightly parallel with centerline of car. However, if it is 9/32 inches out of alignment, it would be difficult to see.

I would suggest you do a bunch of measuring of the distances between your spring mounts on both ends, confirming that the dimensions are relative to the centerline of car and confirming, at least, whether or not the passenger side spring is parallel to the centerline of car.

Your pictures indicate that you have tires that are much wider than stock tires and you have rims of wider backspacing. I don’t know what the factory distance is between the spring perches on the TA rear end. To achieve the space needed for the wide/backspaced tires may have required relocation of the spring perches closer together than the factory spacing.

The frame rail where your shackles mount is only 2.25 inches wide. I don’t know what the width is of your leaf spring, I assume it is 2.5 inches but may be wider. In any case, your shackle straps are going to be wider apart top-to-bottom (unless some spacer is used at the top to compensate).

Depending on what your measuring finds out for the passenger side, it appears that you can substantially correct the alignment by simply cutting your shackle straps about an inch or so (whatever the bolt hole spacing is at the bottom of the strap) above the bottom bolt, lapping the bottom pieces on the INSIDE (toward centerline of car side) of the top pieces and welding them back together.

For the driver side, it appears that you can substantially correct the alignment by simply cutting your shackle straps and lapping the bottom pieces on the OUTSIDE (away from centerline of car side) of the top pieces and welding them back together.

Your photo of the driver side shackle mount appears to show the bolt not going through the rail squarely. If that is the case, it should be corrected when you work on the car. Similarly, the frame rail metal is “thin” and you may want to plate each side where the bolts go through.

Keep us posted, work safe!

Good luck!

P.S. I hope my math is close to correct!

[Wageslave]

The previous owner tried several times to sell on Ebay with no results. He is a very nice guy just didn't know all he needed to about fixing what was wrong with the car so decided to pass it on. I'm glad he did we really like the car even with it's issues, like no heat or AC, no E-brake, worn out shifter and a host of other small issues.

Just got off the floor of the garage with my plumb bob and tape. I measured a difference side to side on the frame rails of 1 1/4 so pretty close.

My leaf spring CL is 42 1/2, just the case of the way they were installed by the builder. The springs run parallel to each other front to back + or - 1/8.

The spring perches on the rear axle are off about 1/8 from one side to the other, as reinstalled by the builder.

My intention is to build an offset shackle for the DS using 3/8 x 2 flat bar in an offset H pattern.

Unfortunately I discovered that the leafs are "S"ed from the 500 ftlbs of torque and I'm going to replace the springs while I have it apart.

My big question now is the proper spring rate, any suggestions would be greatly appreciated.

AS to the rear track width the builder didn't get all the offset they should have on the rear wheels. I put smaller tires (from 275's to 255"s) to fix the issue for now. All a matter of money $400 for tires or $1200 for new wheels, the tires won.

Thanks for all your help Doug and the help from all the other generous folks here on the Forum.

And your math was good

Mike

Originally Posted by Doug

I saw the car on Ebay a few times and wondered about how the rear end was set up. The car is very nice!

Attached for reference are copies of a couple of pages out of the car's factory BODY SHOP MANUAL.

Paragraph "j" on page 11-4 of the car's BODY SHOP MANUAL has this note:
".....NOTE: Center of the compartment pan left side assembly (gas tank filler neck side) is located 1 5/16 inches further from body centerline than center of compartment pan right side rail assembly....."

The above note refers to the “gas tank filler neck side“. In previous years the same floor pan was used in both cars and station wagons and gas was filled from the side on both the cars and wagons, necessitating the frame rail offset design. In 63, the same floor pan was used for both cars and station wagons but only the wagons were filled from the side with the cars having the gas filler in the center of the rear of the car.

The original rear tread/track width of the car is 58 inches. The rear tread/track width of the TA rear end is 60 inches so you already are “too wide”.

You indicate the front mounts of the leafs are mounted “centered” on the frame rails, equal distance from each other. The centerline of car can be determined by measuring equal distance from corresponding points on the frame rails. Note that the driveshaft floor pan tunnel for these cars is "offset" about an inch to the left (driver side) and if one uses the centerline of that tunnel, it is not "centerline of car".

With reference to the chassis dimension information drawing on page 11-2 of the car's BODY SHOP MANUAL, the overall distance (dimension "X") between the frame rail centerlines where your leaf spring shackles mount is 42-1/32 inches as shown in the chart on page 11-3. Allowing for the driver side offset of 1-5/16 inches, that means the centerline of your passenger side spring (frame rail) is 20-23/64 inches (effectively 20-3/8 inches) from "centerline of car".

Again, with reference to the chassis dimension information drawing on page 11-2 of the car's BODY SHOP MANUAL, at about the point where your leaf spring front mounts are mounted, the overall distance (dimension "P") between the frame rail centerlines shackles mount is 40-5/32 inches. Or, in other words, the front mounts are 20-5/64 inches off centerline of car.

Accordingly, that means the passenger side front mount centerline is 9/32 inches (20-23/64 inches minus 20-5/64 inches equals 18/64 inches (9/32 inches)) closer to centerline car than the centerline of the passenger side rear shackle mount is to centerline of car.

From your pictures, the passenger side spring "appears" to be straightly parallel with centerline of car. However, if it is 9/32 inches out of alignment, it would be difficult to see.

I would suggest you do a bunch of measuring of the distances between your spring mounts on both ends, confirming that the dimensions are relative to the centerline of car and confirming, at least, whether or not the passenger side spring is parallel to the centerline of car.

Your pictures indicate that you have tires that are much wider than stock tires and you have rims of wider backspacing. I don’t know what the factory distance is between the spring perches on the TA rear end. To achieve the space needed for the wide/backspaced tires may have required relocation of the spring perches closer together than the factory spacing.

The frame rail where your shackles mount is only 2.25 inches wide. I don’t know what the width is of your leaf spring, I assume it is 2.5 inches but may be wider. In any case, your shackle straps are going to be wider apart top-to-bottom (unless some spacer is used at the top to compensate).

Depending on what your measuring finds out for the passenger side, it appears that you can substantially correct the alignment by simply cutting your shackle straps about an inch or so (whatever the bolt hole spacing is at the bottom of the strap) above the bottom bolt, lapping the bottom pieces on the INSIDE (toward centerline of car side) of the top pieces and welding them back together.

For the driver side, it appears that you can substantially correct the alignment by simply cutting your shackle straps and lapping the bottom pieces on the OUTSIDE (away from centerline of car side) of the top pieces and welding them back together.

Your photo of the driver side shackle mount appears to show the bolt not going through the rail squarely. If that is the case, it should be corrected when you work on the car. Similarly, the frame rail metal is “thin” and you may want to plate each side where the bolts go through.

Keep us posted, work safe!

Good luck!

P.S. I hope my math is close to correct!

[tom s]

I did my first change over in 1964 and have done a few since.I have really never seen a leaf spring setup in thease cars I liked.I have always used coil springs.The last 3 are coil overs with ladder bars.Since your a welder might be the time to do it right.Tom