Swap K3500 Front on K1500 Suburban?

GoreMaker said:

In retrospect, this is probably the wrong category for this thread, since I'm neither planning to lift the vehicle nor drive it offroad, but I couldn't find an appropriate category in the Street section

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Honestly you're probably in the right spot, I think the offroad guys are more commonly looking to swap to bigger axles/front ends since we use bigger tires that break 1/2 ton stuff, especially since your truck is 4x4. It sounds like a cool build though, especially with the NP149. It's easy to find the 6 lug SF with 3.73s and a G80 though, and just as easy to find a matching 8.25 in the pick-and-pull type yards. It's a direct swap too, all you need is a conversion U joint. I know the SUV's used shorter overall length springs but I think the mounts are the same as the pickups and should bolt right up with anything but a dually axle.

The big chain yards that use flat-rate pricing (here the Pick-N-Pulls just call it a "truck axle" doesn't matter if it's a 10 bolt or a D60) are the best place to go since you can save a ton of money vs someone who knows what they have. That way you won't be out TOO much money vs using the one you already have. I think it's a worthwhile swap just for the big brakes alone, especially in a Suburban full of kids.

There are differences involved here.

GoreMaker said:

That's not a concern, leaf spring pads can be cut and re-welded easily.

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It's the front springs mounts that are different to accommodate the shorter leafs, rear end goes right in. I read through the other posts as well. On an upper control arm .75 inches is a HUGE amount. You might never achieve acceptable alignment numbers. The's more to it than a camber number. Not sure what thread Schurkey posted or what it entails but from memory, guys that have tried this before have also needed cv axle spaces and a few other things. Track width differences, spindle height differences, the differences between how far apart the uca and lca mounts are, etc all need to be investigated. A few other have started threads and disappeared into the night half-way through never to never to be heard from again.
A 2500 frame has been suggested which makes it bolt on and go. If your properly equipped its less than a 2 day job.

3/4" is hardly a huge number for control arm length. I just checked both my vehicles, and they're each running the cam bolts at about the middle. In each case, the camber is set at stock spec (+0.5 degree). That means I've got at least 1/2" of outward adjustment in there, plus I wouldn't want stock spec anyways. I want -0.5 degrees, and there's about one degree of adjustment for every 1/4" of movement at both mounting points. With 3/4" shorter uppers, I probably wouldn't even need to max out the cam bolts to achieve that. Even if I did, there's offset bushings available to gain even more adjustment.

This all assumes the K3500 setup uses the same geometry as the K1500 with 3/4" longer control arms on both the uppers and lowers, and that the steering knuckles have the same relative mounting point locations for the ball joints. I haven't measured those yet to know for sure.

As for the torsion bar lengths, that's a whole nother thing I hadn't considered.

GoreMaker said:

As for the torsion bar lengths, that's a whole nother thing I hadn't considered.

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That's where it gets easy. All the pickups, as well as the 2 door SUV, and Suburban are the same length. The hex is the same too, 1500 to 3500. Means 1 ton bars fit in half ton trucks...

The only different length torsion bar is found in the 4 door Tahoe/Yukon. They're about 1.5" shorter that all the rest. 52.5 vs 54.

Ish...

Front upper arms inboard mounts are farther apart, mount farther inboard and higher on the frame, and the frame is taller. Front diff isn't terrible to swap in, but IFS 8 lug on a 6 lug frame ain't gonna happen the way GM delivered it.

Supercharged111 said:

Front upper arms inboard mounts are farther apart, mount farther inboard and higher on the frame, and the frame is taller. Front diff isn't terrible to swap in, but IFS 8 lug on a 6 lug frame ain't gonna happen the way GM delivered it.

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Hmm... does that mean the distance between the upper and lower ball joints on the steering knuckles is greater on 8-lug than 6-lug?

GoreMaker said:

Hmm... does that mean the distance between the upper and lower ball joints on the steering knuckles is greater on 8-lug than 6-lug?

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Yes, the knuckles are taller too. And they wear a 4 bolt hub as opposed to the 1500 having a 3 bolt hub.

GoreMaker said:

As for the torsion bar lengths, that's a whole nother thing I hadn't considered.

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T-bars are direct swap.

GoreMaker said:

3/4" is hardly a huge number for control arm length.

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It can be, Control arms swing in arc so the distance of the UBJ and LBJ to the vehicle centerline changes. It's not a vertically linear movement as the suspension compresses and rebounds. You're talking about a Short-arm/Long/arm suspension system and keeping the too short uca. SLA suspension systems experience some camber changes through the rebound and compression cycle moreso than equal length or double wishbone systems. Even with the arm moved outboard to properly locate the UBJ, the tighter arc it swings in will still accentuate these camber change.

Aligmnents specs are static readings and are not wholly indicative of whats going to happen dynamically going down the road. You can have caster, camber, and toe correct and still have damaged components on the vehicle.

GoreMaker said:

Hmm... does that mean the distance between the upper and lower ball joints on the steering knuckles is greater on 8-lug than 6-lug?

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That's one of the things I mentioned you need to dig deeper into in a previous post, there's more to it then bolting it together. Everything suspension moves in arcs. Where the control arms mounts are as it corresponds to knuckle heights, steering linkage angles, all can affect how it performs. Ride height changes alone can change things.

Hipster said:

It can be, Control arms swing in arc so the distance of the UBJ and LBJ to the vehicle centerline changes. It's not a vertically linear movement as the suspension compresses and rebounds. You're talking about a Short-arm/Long/arm suspension system and keeping the too short uca. SLA suspension systems experience some camber changes through the rebound and compression cycle moreso than equal length or double wishbone systems. Even with the arm moved outboard to properly locate the UBJ, the tighter arc it swings in will still accentuate these camber change.

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Oh I'm well aware of this, hence my comment about camber gain. The GMT400 layout has barely any camber gain from the factory, which is odd in a vehicle with so much body roll. Reducing the upper control arm length by 3/4" would be a great way to add that. On F-bodies, I've experimented with adjustable upper control arms that provide 1.25" of length adjustment. I tried the control arms at stock length with almost no shims to get the camber into spec, and at their shortest length with huge shim stacks. I also tried achieving desired caster using just the arms, just shim stacks, and a combination of both. The setup that ended up working best was having the upper control arms at their shortest, because the camber gain really kept the tire planted in the turns.

Having said that, if the steering knuckle is too tall and the ball joints make that worse, the geometry might end up being too extreme. I'll have to spend some time taking measurement and entering them into my suspension geometry calculator

boy&hisdogs said:

Honestly you're probably in the right spot, I think the offroad guys are more commonly looking to swap to bigger axles/front ends since we use bigger tires that break 1/2 ton stuff, especially since your truck is 4x4. It sounds like a cool build though, especially with the NP149. It's easy to find the 6 lug SF with 3.73s and a G80 though, and just as easy to find a matching 8.25 in the pick-and-pull type yards. It's a direct swap too, all you need is a conversion U joint. I know the SUV's used shorter overall length springs but I think the mounts are the same as the pickups and should bolt right up with anything but a dually axle.

The big chain yards that use flat-rate pricing (here the Pick-N-Pulls just call it a "truck axle" doesn't matter if it's a 10 bolt or a D60) are the best place to go since you can save a ton of money vs someone who knows what they have. That way you won't be out TOO much money vs using the one you already have. I think it's a worthwhile swap just for the big brakes alone, especially in a Suburban full of kids.

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I missed this reply earlier (I don't get the email notifications I expect from this forum despite having my settings changed to instant notification). Thank you, that was very useful. And you're right, part of the reason for acquiring a Suburban is to be able to haul many kids safely in winter. Over the Spring/Summer/Fall months, I use a 96 Roadmaster wagon to carry 3 to 6 kids aged 3 to 8 to miscellaneous destinations. But that's not an appropriate vehicle for Vermont winters and I'm hoping the Suburban with really good dedicated winter tires and some upgraded brakes will serve the same purpose.