Leaf spring tuning and setup - excerpt from the 72 GMC build thread

I wanted to repost this to the main forum to see how the other folks thought...

Here are the pictures of the rear suspension. I think I can move the upper damper mount along the frame rail pretty easily. In addition, I think I may just entirely remove the center support bearing crossmember altogether, because I'll be adding a fuel tank under there which may take up the area next to the driveshaft, Thoughts?

I don't have much hope in lowering this thing. Has anyone done a de-arching? Anyone know where or the cost? Has anyone ever added leaves to a leaf spring pack? How is that done?

adding leaves is not hard.

you buy a new center pin and remove the rear axle or do one side at a time.

have no pressure on springs and remove center pin. add leaf in the pack where it belongs. use big c-clamps to pull it all together and install new center pin. this will lift the truck most likely. i've built beefy spring packs from multiple stock packs. i actually always keep a few extra spring packs around to build from.

your best bet for dropping while keeping your capacity would be to move the hangers and shackle mounts up, or buy ones that move the spring eye up.

i wish i had your frame and you had mine, my c20 is a coil spring truck and i was wanting leaves for ease of rear end swapping so i could take out my old dana rear axle and put in a 14 bolt.

Well there are options. You could cut the frame and make room above the leafs for the axle. There, you just lowered your truck by the thickness of the axle tube. You could always find leaf springs made lifting the truck then go to a 'spring under' style mount, because then you probably wouldn't need to cut the frame up. You might only lose a little bit. I would contact one of the companies that make custom leaf springs for a quote. My dad got some made for his '90 suburban recently.

Go to junkyard and grab some leaf springs the same width. Unbolt your square u bolts.... Get longer ones Add a few leafs. No big deal. My C10 has 12 leaf springs.... Yes, 12. It was a tow truck in its past life.

Lowering shackles are cheap and readily available.

Have you considered air bags or cheap air shocks? Doesn't raise truck but when you need to tow it gives you more versatility.

The shackles look like they would only drop the rear of the leaf. As I understand that, the drop will understeer more, so I was hoping to be able to raise the front of the spring as well, but that bracket looks pretty permanent. Can I flip it upside down and drill holes to mount it to the frame with grade 8 bolts?

This bracket:

A flip job is a big move - like 6", right? That's too much. Maybe I could do that and get a shorter shackle to make up the difference instead.

I'll have to find a 67-72 for the leaves - because they are super narrow, but I can do that. Which leaves do you add? Where do you get the center pins? How do the ends attach? Actually...

Maybe a flip kit and increasing the spring rate would lower it just the right amount?

These guys are talking a lot about lowering these trucks. http://67-72chevytrucks.com/vboard/showthread.php?t=473599

My truck just has a progressively shorter spring added. I'm not sure what he means by center bolt.

new center pins/bolts

http://www.summitracing.com/parts/tfy-92038/overview/

you dont need the same model springs as long as you find the same width.

here is a handy chart i found that i keep bookmarked for leaf dimensions.

http://littlekeylime.com/MrN/mrnimages/leaf_springs.png

the ends of leaf springs do not attach. the main leaf has the eyes that bolt to the truck. the rest of them just set there, or like in your pics one leaf toward the bottom has a bracket riveted on that lines most of them up.

patgizz wrote: new center pins/bolts http://www.summitracing.com/parts/tfy-92038/overview/ you dont need the same model springs as long as you find the same width. here is a handy chart i found that i keep bookmarked for leaf dimensions. http://littlekeylime.com/MrN/mrnimages/leaf_springs.png the ends of leaf springs do not attach. the main leaf has the eyes that bolt to the truck. the rest of them just set there, or like in your pics one leaf toward the bottom has a bracket riveted on that lines most of them up.

Mine are 2 1/4" wide, nothing on your chart matches that - I am fairly certain it's a rather unique and silly width. Still, it sounds like I can add a leaf or two easily and then flip the axle to the top of the stack. if I do this and then relocate that one damper, I ought to have a fairly decent rear suspension. Is there any easy way to replace the bushings (they wear, I presume) in the eye of the main leaf?

I had to drill out the rubber... Then cut out the bushing steel with a hack saw. A hammer and a large bolt. I then replaced them with aluminum bushings from Calvert racing. It was not easy and I don't think there is an easy way to do it without total leaf removal and a press. I now have a shop press and would take that route next time.

springs can be de-arched by a spring shop/truck related.

Extreme lowering. Mount the springs under the axle.

Fred Puhn in; "How to make your car handle" has an excellent section on arching/dearching springs to achieve the desired ride height and maintain or improve the handling.

Key point that I remember is to keep the front spring eye height at or slightly below the hub height. (Minimizes roll steer)

Rog

You can take the middle spring off and flip it over. This will lower the rear of the truck and maintain the springrate.

Leaf springs are awesome for tunability! You just get a spare set and add a spring here, flip a spring there, flip another spring.... bingo bango you have the ride height and spring rate you want.

Just make sure the end of the flipped spring doesn't dig into the other springs.

wvumtnbkr wrote: You can take the middle spring off and flip it over. This will lower the rear of the truck and maintain the springrate. Leaf springs are awesome for tunability! You just get a spare set and add a spring here, flip a spring there, flip another spring.... bingo bango you have the ride height and spring rate you want. Just make sure the end of the flipped spring doesn't dig into the other springs.

I think I want more spring rate. If I just add leaves, I will add height and rate. If I do this and flip the front bracket, I will likely be even with what I have currently as far as height and have the higher rate. I am not sure how to get the 3-4" drop, though, unless I add enough leaves to get me an extra 2-3" of ride height with the flipped bracket and then flip the axle.

emodspitfire wrote: Fred Puhn in; "How to make your car handle" has an excellent section on arching/dearching springs to achieve the desired ride height and maintain or improve the handling. Key point that I remember is to keep the front spring eye height at or slightly below the hub height. (Minimizes roll steer) Rog

You can dearch your own?

tuna55 wrote: The shackles look like they would only drop the rear of the leaf. As I understand that, the drop will understeer more, so I was hoping to be able to raise the front of the spring as well, but that bracket looks pretty permanent. Can I flip it upside down and drill holes to mount it to the frame with grade 8 bolts?

Yes, you can grind off the rivets and flip it. I have done that. The only problem is that some times the bracket will limit(bind) at full suspension droop. I was able to make it better by application of a BFH but it still hit some. It will require the use of longer shackles at the rear to even things out.

Axle flip. It will drop the rear about 4 inches.

tuna55 wrote: You can dearch your own?

I did. Like this:

It had its own realm of related quirks, and if I were to do it again, I would have 4-linked it instead.

http://www.gwellwood.com/project-cars/the-hideous-hardbody/hideously-de-arced/

Hal wrote:

tuna55 wrote: The shackles look like they would only drop the rear of the leaf. As I understand that, the drop will understeer more, so I was hoping to be able to raise the front of the spring as well, but that bracket looks pretty permanent. Can I flip it upside down and drill holes to mount it to the frame with grade 8 bolts?

Yes, you can grind off the rivets and flip it. I have done that. The only problem is that some times the bracket will limit(bind) at full suspension droop. I was able to make it better by application of a BFH but it still hit some. It will require the use of longer shackles at the rear to even things out.

Did you also swap them L for R? It looks like that may be the hot ticket for not having interference.

So, I haven't measured. I am bench building still. If I:

Swap brackets upside down and L/R
Add leaves to increase ride height by 2-3"
Flip axle with existing hardware (no parts to buy)

I can literally drop this thing about 2-3 inches net, increase the drivability and increase the load carrying capacity, and do it for the cost of the extra leaves and that's it.

Does anyone have any thoughts regarding an "optimal" angle for the dampers? It seems to me as if you'd want 1:1 motion ratio, so, straight up and down as ideal. Now, I can't get that because my dampers would be crazy-short. If I move both upper brackets and buy 7" drop dampers (shortest I can find), I can decrease the angle of inclination from 35 degrees to 18ish. I think that's worth it, what say ye?

tuna55 wrote:

Hal wrote:

tuna55 wrote: The shackles look like they would only drop the rear of the leaf. As I understand that, the drop will understeer more, so I was hoping to be able to raise the front of the spring as well, but that bracket looks pretty permanent. Can I flip it upside down and drill holes to mount it to the frame with grade 8 bolts?

Yes, you can grind off the rivets and flip it. I have done that. The only problem is that some times the bracket will limit(bind) at full suspension droop. I was able to make it better by application of a BFH but it still hit some. It will require the use of longer shackles at the rear to even things out.

Did you also swap them L for R? It looks like that may be the hot ticket for not having interference.

I didn't but that was on an old Ford 20 years ago. I can't tell from the photos but if a side to side swap works better for clearance I can't see why it wouldn't work. You just need to make sure the eyebolt position is lined up vertically with the old position.

When I dropped my 97 F150 I used a kit from Beltech. The front hangers on the F150 were riveted to the frame and held the eyebolt ~3 inches below the bottom of the frame. Beltech's new hangers put the eyebolt up in the center of the frame.

Just grind off the old rivets and bolt the new hangers up using the same holes.

tuna55 wrote:

wvumtnbkr wrote: You can take the middle spring off and flip it over. This will lower the rear of the truck and maintain the springrate. Leaf springs are awesome for tunability! You just get a spare set and add a spring here, flip a spring there, flip another spring.... bingo bango you have the ride height and spring rate you want. Just make sure the end of the flipped spring doesn't dig into the other springs.

I think I want more spring rate. If I just add leaves, I will add height and rate. If I do this and flip the front bracket, I will likely be even with what I have currently as far as height and have the higher rate. I am not sure how to get the 3-4" drop, though, unless I add enough leaves to get me an extra 2-3" of ride height with the flipped bracket and then flip the axle.

That would be why you would add springs, but add them upside down. This will give you more spring rate AND lower the rear.

This way is as adjustable as you want. Nothing permanent. You can change it in a couple of hours to whatever else you want if you are not happy with it.

In reply to wvumtnbkr:

Let me be super clear to see if I understand this.

If I have a ride height with a spring pack, and add a leaf to it in the middle, upside down, the ride height will decrease and the spring rate will increase?

Really? is that how it works?

If you added a leaf upside down AND flipped one of the existing leafs upside down. Yes, it would be stiffer AND lower.

If you just flipped one of the existing leafs upside down, it would lower the vehicle.

If you added a leaf rightside up AND flipped a leaf in the existing stack (assuming the leaf(s) have the same curvature and length) the ride height should stay the same and the springrate will increase.

Alternately, if you add a spring upside down and do nothing to the existing pack, it will have a pretty similar ride height to stock and have an increased ride height.

I am currently working with some friends on a LeMons car that has rear leaf springs. We have been experimenting with all of these options. It is really neat!

If you look hard enough around the internet, there are a few people that have calculations to help determine spring rate.

wvumtnbkr wrote: Alternately, if you add a spring upside down and do nothing to the existing pack, it will have a pretty similar ride height to stock and have an increased ride height.

whoa whoa - one of these should say "spring rate", I am getting the latter use of "ride height"

I think I may understand. If I add one upside down and flip one of the current ones along with the flipped front bracket, I may get the mild drop I want without flipping the axle and get better handling and increased spring rate to boot. Now to find a spring pack to use...