C20 Rear Suspension Questions (Setting the suspension up for towing)

[handloader90]

I've recently picked up a 1969 C20. It will be driven a few times a week and used as a parts hauler on the weekends. I do have intentions of using this truck to tow a '14 Jeep Wrangler halfway across the country in about two years or so. My main focus is setting up a suspension that is best for towing.

The truck is getting a 6.0 gas engine that came from an '04 Yukon Denali and a 2WD NV4500.

I have a 14 Bolt in the garage with 4.10 gears, disc brakes and a Detroit.

I have everything needed to swap the front end of the truck over to disc brakes and I plan on using an airbag kit in the rear of the truck with new shocks, Bilsteins if they have some that will work properly.

Truck Details:

Currently has what is believed to be a 305 mated to a Saginaw 3 speed that came out of a car.

It has leaf springs instead of the control arm and coil spring setup. I've read that the 67-72 trucks could be special ordered with leaf springs but I believe at some point one of the owners of this truck swapped leaf springs and the associated hardware onto this frame.

The current springs are 52" long measured from spring eye to spring eye following the curve of the spring. I haven't measured from spring eye to center pin yet to determine if the spring is split evenly or if it has an offset.

The spring is not as wide as a 73-87 spring, the spring (not including the bushing and sleeve) comes out to 2.25" wide... the opening on the front/rear spring hanger is smaller than 3", somewhere between 2.5"-2.75", shackle is obviously the same too.

What I am wanting to do is use a 73-87 spring with poly bushings. I will have to use the spring, shackle hangers and shackles for a 73 or newer truck. I'm gonna look around but something (spring and shackle hangers) off of an OBS truck may be a better fit.

Questions:

1.) I will be buying the springs new and am curious if I should stick with 52's or go with 56's.

2.) How many leaves for max towing capacity? As many as possibly? I would like to have a smooth ride when there is no load hooked up to the truck.

3.) Stay spring over or go ahead and do a spring under?

4.) What is the best way to mount and orientate the rear shocks to use their full potential and maintain a stable ride? Same as how we do it for offroad rigs, inboarded with a crossmember or?

There's a few pics of the truck, hopefully that will hold a few of you guys' interest.

 

[sweetk30]

if the current system works fine leave it alone .

then best bang for the buck as you already said is air bag kit to help when loaded and still smooth ride empty . only other thing I would look at is timbren bump stops and no bags if you don't want to plan on air up/down .

my years of doing bolt on stuff like this I have never had anyone complain about the ride with air bags or timbren bump stops . add-a-leafs or HD thicker spring packs ride like crap when empty and got the most complaints .

 

[diesel4me]

I'm no expert on spring swaps,but I can tell you I had a '71 GMC K1500 ,that someone put rear leafs in it that had 10 leafs!..and yet it still rode decently empty,though a bit choppy..I put Monroe Magnum 60 shocks on it and they helped a lot to reduce the rebound effect..

Truck also had stock 2 leaf front springs,and it was the front that rode harsh and it bottomed out easily--only had maybe 1" of space between the bump stops..with the plow on,you were on the bump stops 100% of the time,but the weight make it feel smoother riding..

As for the over or under,I'd stick with "over" the axle--putting the leafs under the axle puts 100% of the stress on the u-bolts..

Though GM at least used 9/16" or 5/8" U-bolts with some beef (unlike many Fords,that have puny 7/16" U-bolts with the top arched area flattened even thinner on many trucks,and I'm amazed they dont break!)--I like the fact the U-bolts only clamp the spring to the axle tubes and keep it from parting ways when you get airborne ,they do not support the entire load of the trucks weight and what your carrying or towing..

You can get rear helper springs that only "kick in" when extra weight is put on the truck,that helps keep the un-loaded ride smoother..

I liked using the clamp on coil type overload springs because they rode well even unloaded and could be easily removed if desired--the leaf type helpers require frame brackets they bear up against when you load the truck down..

You can see in this photo that despite it having 10 leaves in the rear springs,it didn't have that "jacked up" look really..small stock tires do make it look like there was a lot of "lift",but it actually sat fairly level still..

 

[handloader90]

if the current system works fine leave it alone .

then best bang for the buck as you already said is air bag kit to help when loaded and still smooth ride empty . only other thing I would look at is timbren bump stops and no bags if you don't want to plan on air up/down .

my years of doing bolt on stuff like this I have never had anyone complain about the ride with air bags or timbren bump stops . add-a-leafs or HD thicker spring packs ride like crap when empty and got the most complaints .

I had an Airlift kit thay was installed on a '79 K20 and that thing was tits! Told myself I would never have a trucks without bags again after that.

 

[handloader90]

I'm no expert on spring swaps,but I can tell you I had a '71 GMC K1500 ,that someone put rear leafs in it that had 10 leafs!..and yet it still rode decently empty,though a bit choppy..I put Monroe Magnum 60 shocks on it and they helped a lot to reduce the rebound effect..

Truck also had stock 2 leaf front springs,and it was the front that rode harsh and it bottomed out easily--only had maybe 1" of space between the bump stops..with the plow on,you were on the bump stops 100% of the time,but the weight make it feel smoother riding..

As for the over or under,I'd stick with "over" the axle--putting the leafs under the axle puts 100% of the stress on the u-bolts..

Though GM at least used 9/16" or 5/8" U-bolts with some beef (unlike many Fords,that have puny 7/16" U-bolts with the top arched area flattened even thinner on many trucks,and I'm amazed they dont break!)--I like the fact the U-bolts only clamp the spring to the axle tubes and keep it from parting ways when you get airborne ,they do not support the entire load of the trucks weight and what your carrying or towing..

You can get rear helper springs that only "kick in" when extra weight is put on the truck,that helps keep the un-loaded ride smoother..

I liked using the clamp on coil type overload springs because they rode well even unloaded and could be easily removed if desired--the leaf type helpers require frame brackets they bear up against when you load the truck down..

You can see in this photo that despite it having 10 leaves in the rear springs,it didn't have that "jacked up" look really..small stock tires do make it look like there was a lot of "lift",but it actually sat fairly level still..View attachment 269616

Thanks for all of the info!

We've got the same color scheme going on too!

 

[diesel4me]

Sadly I sold that truck after I owned it maybe 5-6 years--bought it in the mid 80's and sold it around 1992 I think...

I did a lot to that truck--had a dying 305 when I got it,from a Chevy Monza,I did a valve job on only the drivers side head and after I replaced the very burnt exhaust valve one cylinder had (the dead one!)--it smoked like a tire fire!..so bad I got pulled over nearly every time I drove it..

A friend had a mint '75 El-Camino and his wife slid on ice in it one snowy day and crashed it into a tree broadside,totaled it--I bought the 350 from it with 68K on it from him for $150..paid $500 for the truck--I plowed the parking lot at work with it for $20 a pop and it soon paid for itself..had a TH350/NP 205..

I will agree with Sweet K30,my older brother tried a pair of those Timbren "jelly bump stop" load handlers on his home built beaveratail ramp truck he made out of a '79 E-350 cube van..after adding those to the stock leafs,it rode very smooth,loaded or unloaded,and no sagging..
I've been tempted to try a pair of the front ones they now list for my '82 K2500,but the price is close to $300,and I'm not really into spending much on it--its gotten rusty and I dont need to stress crack the frame on it..but I bet it would ride nice with those,and not have the springs sag with the plow on it..

 

[handloader90]

Sadly I sold that truck after I owned it maybe 5-6 years--bought it in the mid 80's and sold it around 1992 I think...

I did a lot to that truck--had a dying 305 when I got it,from a Chevy Monza,I did a valve job on only the drivers side head and after I replaced the very burnt exhaust valve one cylinder had (the dead one!)--it smoked like a tire fire!..so bad I got pulled over nearly every time I drove it..

A friend had a mint '75 El-Camino and his wife slid on ice in it one snowy day and crashed it into a tree broadside,totaled it--I bought the 350 from it with 68K on it from him for $150..paid $500 for the truck--I plowed the parking lot at work with it for $20 a pop and it soon paid for itself..had a TH350/NP 205..

I will agree with Sweet K30,my older brother tried a pair of those Timbren "jelly bump stop" load handlers on his home built beaveratail ramp truck he made out of a '79 E-350 cube van..after adding those to the stock leafs,it rode very smooth,loaded or unloaded,and no sagging..
I've been tempted to try a pair of the front ones they now list for my '82 K2500,but the price is close to $300,and I'm not really into spending much on it--its gotten rusty and I dont need to stress crack the frame on it..but I bet it would ride nice with those,and not have the springs sag with the plow on it..

Yeah, those Timbrens cost about the same as a good airbag kit.

 

[diesel4me]

Brand new front springs ,even 2-3" lift ones,are cheaper too..

 

[yellowK20]

Swap in the arm suspension, and use air bags instead of coils it will ride like a dream loaded our unloaded.