2wd vs 4wd rear suspension question

[FLYNAVY30]

Anybody know why GM used a trailing arm set up on the 2wd trucks and a traditional leaf spring set up on the 4wd trucks in the rear?

I'm still batting around ideas for converting my C20 to 4wd and trying to decide what, if any, down sides there are to retaining the trailing arm set up in the rear in a 4wd application.

[notsolo]

Possibly a leaf spring truck has less movement in the drive shaft. Side to side etc.

[CG]

I cant seem to let this idea go either. I asked over in the 4x4 part of the board and there weren't a lot of answers but overall it was positive as long as you weren't building a rock crawler or crazy off road truck.

Need an adjustable track bar to keep the rear centered. May have to recenter the rear in the wheel well and of course make sure your pinion angle is correct.

They make kits for converting the front but they all seem to require the 52 inch spring which may or may not be a good thing (I had a thread on that over on the 4x4 section too). You're going to need the 4x4 front cross members and other assorted bits and pieces to make it all work, along with transfer case and shifter, correct trans, matching front and rear axels etc etc ... It really adds up.

[FLYNAVY30]

Oh yea, I know what I'm in for on the front side....some buddies and I have already started putting the list together.

We were just debating the pros and cons of converting the rear to leafs, or leaving the trailing arms.

I'm not going to be doing any rock crawling or, even serious off roading.....mostly camping trips that involve potentially muddy trails, sand on the beach, and snow when we go up north.

[Bigdav160]

My guess is lack of ground clearance with the trailing arms.

[gmikkels]

Hello,
Are you looking for stock or lifted truck, if lifted how much?
Thanks,
Gene

[FLYNAVY30]

Originally Posted by Bigdav160

My guess is lack of ground clearance with the trailing arms.

I'd have to do some measuring, but whether its leaf springs or trailing arms, I would think the low point in either set up is still the bottom of the differential.

[FLYNAVY30]

Originally Posted by gmikkels

Hello,
Are you looking for stock or lifted truck, if lifted how much?
Thanks,
Gene

Essentially stock K20 ride height.

[gmikkels]

OK, I do not know the difference in ride height between the stock c20 and k20.
So the first step is to raise the trunk to the height you want, the trailing arms travel thru a arc and as you raise the truck it will pull the tire forward in the wheel well. If this looks good to you great, if not you will need to drill out the rivets and pull the cross member back. The next issue issue is a k20 or the one I had used a single piece drive shaft. You may need to modify the bottom of the cross member for drive shaft clearance. You may want to use the (rear) leaf spring brackets off of a 2 wheel drive truck. The are unique in the fact that the frame is tapered in the location where the front hangers mount. You then would need to use blocks under the springs to get the desired ride height.

[FLYNAVY30]

As best I can tell, a stock C20 with the HD springs is about the same ride height as a stock K20. I think the way to go is to get a K20 donor truck, and weld the mounting brackets and cross member into my Suburban frame. Get the front all set up at stock ride height, then adjust the back to match. I imagine that will entail moving the rear cross member aft a few inches to keep the rear wheels centered in the wheel arches. The truck already has a 2 piece drive shaft, so I imagine I'd just need to measure and order a longer aft section of the drive shaft. Id also replace the stock panhard rod with an adjustable one to center everything up.

[Richard]

4wd more of a work truck? Leafs were an option on the 2wd. Leaf springs are more progressive in the spring rate while coils are linear. Depending on design, leaf springs tend to increase spring rate as they compress = more load carrying capacity. Coil spring rate stays the same through their travel = more compliant ride.

[LockDoc]

Originally Posted by Richard

Coil spring rate stays the same through their travel = more compliant ride.

That is not necessarily true. The "variable rate" or "progressive rate" coil springs change depending on the amount of weight loaded in the bed of the truck.... A lot of the '67 - '72 trucks had them.

LockDoc

[Richard]

Originally Posted by LockDoc

That is not necessarily true. The "variable rate" or "progressive rate" coil springs change depending on the amount of weight loaded in the bed of the truck.... A lot of the '67 - '72 trucks had them.

LockDoc

Yes, you are correct. There are variable or dual rate coils. Just as leaf packs can be made to be more linear. At any rate (get it?) my point was leaf springs tend to handle higher loads better than coils.
A while back my neighbor came by to check out progress on my truck. He was stating that he had converted his old 71 c20 with a big camper from coil to leaf and how much better it was after the change.

[A1971Blazer]

I've pondered, contemplated and researched the very same idea, except doing it with a short bed pickup....('67-68)
I've talked with DIY offroad and a couple of other companies about it. They all concur that the trailing arm rear set-up would be perfectly fine for a vehicle that is not intended for any hard core off road use....and would likely give a better quality ride.
Also, if you notice, most hard core rigs use some kind of link rear suspension as opposed to leaf springs.