Zr2 front suspension

[kylant]

1 hour ago, GMCnewbie said:

Finally gonna do the suspension swap on my '24 AT4.  Parts have been sitting in my garage since mid-Nov.  Weather should be nice next weekend.  New wheels/tires are on order and should be here in the next week or so.  I also went ahead and bought the Cognito Comfort Ride torsion bars and leaf springs.  Not sure what to expect from all of this, but I'm excited to dig into this project.  I decided to do the Cognito parts simply because I don't plan to haul super heavy loads anytime soon.  A 22% loss in capacity shouldn't affect my usage much at all.  That and the fact that I got a really good deal on my truck last year from the dealer that even after ordering all these parts and wheels, my out-the-door price on the truck and parts included is still less than MSRP.  That's how I'm gonna justify it in my head at least.  lol

 

I'll post pics next week when it's all wrapped up.  

 

On a side note, I apparently ordered a pair of the rear suspension spacers that for the Zr2 only according to the GM parts diagrams.  I clearly wasn't paying attention when I placed my order.  Let me know if anyone needs/wants them.  I think I paid around $100 for the pair.  Not sure what GM is charging for them these days.

be sure to post how it goes. 

i am really interested in the full

leaf replacement. i was planning on doing the mini leafs instead of the blocks. but have also been thinking about the full pack. 

i've done full leaf replacements on my Ford's but they didn't make that much of a difference. curious if they will on the GM...

Edited March 16, 2025 by kylant

[GMCnewbie]

1 hour ago, Epsilon Plus said:

Are you doing the full front/rear swap? Technically, you'll have a ZR2 after and will need them. What spacers are these that you refer to?

They're the rear jounce spacers (85565250).  Based on the parts diagrams, the AT4X doesn't used them which is confusing since all other parts are the same with the ZR2.

 

Forgot to answer your initial question.  Yes, full swap is in order here.  Replacing the UCA, LCA, knuckles, shocks, leafs, leaf spacers, and torsion bars.

Edited March 16, 2025 by GMCnewbie

[KvetL5P]

Has anyone completed the full swap on a 3500? I'm curious about what they did for the leaf pack. I assume the zr2 block and u bolts won't work becuase they are too short and would need an aftermarket block? 

[Chuck FB]

9 hours ago, GMCnewbie said:

Finally gonna do the suspension swap on my '24 AT4.  Parts have been sitting in my garage since mid-Nov.  Weather should be nice next weekend.  New wheels/tires are on order and should be here in the next week or so.  I also went ahead and bought the Cognito Comfort Ride torsion bars and leaf springs.  Not sure what to expect from all of this, but I'm excited to dig into this project.  I decided to do the Cognito parts simply because I don't plan to haul super heavy loads anytime soon.  A 22% loss in capacity shouldn't affect my usage much at all.  That and the fact that I got a really good deal on my truck last year from the dealer that even after ordering all these parts and wheels, my out-the-door price on the truck and parts included is still less than MSRP.  That's how I'm gonna justify it in my head at least.  lol

 

I'll post pics next week when it's all wrapped up.  

 

On a side note, I apparently ordered a pair of the rear suspension spacers that for the Zr2 only according to the GM parts diagrams.  I clearly wasn't paying attention when I placed my order.  Let me know if anyone needs/wants them.  I think I paid around $100 for the pair.  Not sure what GM is charging for them these days.

I noticed you have the Cognito torsion bars on order and have been curious as to what they really are rated at ( relative to the various GM rated bars depending on the factory build options and their physical diameter ). I have never called them to quiz them but had sent them a note months ago and that must have fallen by the wayside as I had no reply. If you wouldn't mind once you have them in your hands and have the time, if you have access to an accurate slide caliper or actual micrometer, to measure the diameter right down to the thousands of an inch and at the same time measure and record the diameter of your own stock bars.

 

Your AT4, what is the rating of your front axle on the tag on the door pillar ?

[GMCnewbie]

58 minutes ago, Chuck FB said:

I noticed you have the Cognito torsion bars on order and have been curious as to what they really are rated at ( relative to the various GM rated bars depending on the factory build options and their physical diameter ). I have never called them to quiz them but had sent them a note months ago and that must have fallen by the wayside as I had no reply. If you wouldn't mind once you have them in your hands and have the time, if you have access to an accurate slide caliper or actual micrometer, to measure the diameter right down to the thousands of an inch and at the same time measure and record the diameter of your own stock bars.

 

Your AT4, what is the rating of your front axle on the tag on the door pillar ?

I have them in the garage now, but I'll have to check the measurements tomorrow.  That said, my AT4 has the 4,600lb torsion bars.  I want to say the Cognito bars are rated at 3,700lb.  I could be wrong on that, but I seem to recall reading that somewhere. 

[Chuck FB]

On 9/2/2024 at 9:09 AM, Epsilon Plus said:

A bit softer. Like going from a covered wagon to a Model T. Spring rate is just too high on the torsion bars and rear leafs for the shocks to even be involved in everyday driving unless you are loaded down. The only way I think you can really soften this truck up is with Cognito's comfort ride torsion and leaf set up. -25% rate, but also -25% payload/capability. I'm not willing to un-HD my HD.

 

I'm on 37s with 45psi front, 40 rear and the wife still has to be mindful of slight cracks in the road when taking a drink of her water cup or she'll be wearing it.

 

I would do it again. Price was right. No aftermarket jank. Factory driving characteristics/capability. Perfect for me stance. Fit 37s. Cons are that it is a ton of work for a baby lift. Being that you have a 24 though you might be able to run the DSSVs. Still a mystery on whether all 24s got the hard brake line change to allow for the massive shock bodies.

I was reading through this thread a while back with interest due to the theme of not just the lift aspect but of seeking out a better ride if possible on an HD truck. You mentioned in this comment about your ride improving due to the tire size but for your truck use had not gone down the road of swapping to lighter springs because you still want it capable of doing "HD" things. I just joined up on this forum recently as the truck I had ordered is finally in my hands and contemplating any changes I may decide to make to it but have been reading through this forum and others for some time before deciding on the brand and model of truck I did get. So about the cognito torsion bars which to me still seem to be a rather fuzzy mystery as to what they are actually rated at, I was wondering what your stock torsion bars are rated at. Assuming you haven't swapped them from what came with your truck, what is the rating of your front axle on your pillar sticker as that is the easiest way of figuring out what your truck came with. I purposely did order mine with the standard 4800 rated front end as at this point don't have a plan of hanging a bunch of weight on the front of the truck.

 

Also I got thinking about your tire size and the weights they are capable of carrying at lower pressures ( according to official tire inflation charts ) and wondered if you have weighed your truck axle by axle when weighted like you may typically drive it ( of course if you are piling weight into the box at times or towing a heavy hitch trailer, that changes that picture ). If you have never looked at the tire charts and lets just say you do often drive the truck fairly much unloaded, it might surprise you as to how much lower you can actually run your tires than what you currently are in a scenario like that.

 

Hopefully I am  not derailing this thread too much with my questions and comments, at least that's not my intent !

[Chuck FB]

16 minutes ago, GMCnewbie said:

I have them in the garage now, but I'll have to check the measurements tomorrow.  That said, my AT4 has the 4,600lb torsion bars.  I want to say the Cognito bars are rated at 3,700lb.  I could be wrong on that, but I seem to recall reading that somewhere. 

No rush to measure them, mostly just need the precise tool if you don't happen to own one as a measuring tape doesn't cut it !. As to your stock bars, I can't say I have heard of a stock GM bar rating of 4600, just recheck the sticker on your truck as I suspect it will say 4800 ?. In fact the factory part number tags may still be on your bars before they get scraped off when removing the bars and on that theme, look for any tags on the congnito bars and write down those numbers.

 

Like I said so far I haven't heard anything on the actual rating of the congnito bars, other than what you had come across ( and its easy to get rumours flowing on an item and fiction becomes fact sometimes ). And here I go now making a comment that I had seen a youtuber who had done a cognito swap some time ago "speculate" as he was looking at his congnito bars, he seemed to have a sense that they were possibly actual GM OEM bars but of a smaller size. He never confirmed that but it did get me wondering, who actually makes them and what if they did turn out to be an off the shelf lighter capacity GM bar. But like I said, I don't want to get some wild rumour going because some guy had some comment in his video about his cognito bars he installed.

[GMCnewbie]

I haven't been outside to look at my truck.  My guess was 4,600lb on the stock bars, but you're probably correct with 4,800lb.  My torsion bars are the lightest available from GM whatever that is.  I have a digital caliper and will measure the Cognito bars tomorrow afternoon for you. 

[Chuck FB]

9 minutes ago, GMCnewbie said:

I haven't been outside to look at my truck.  My guess was 4,600lb on the stock bars, but you're probably correct with 4,800lb.  My torsion bars are the lightest available from GM whatever that is.  I have a digital caliper and will measure the Cognito bars tomorrow afternoon for you. 

That sounds good, probably the most important item is not to destroy any information if there is any on the congnito bars before your able to jot that down and/or take photos of the tags.

[kylant]

pretty positive the bars are actually gm bars. unsure on the load rating. i am speculating they are from the old 1/2 ton trucks 

[kylant]

here's a question for you guys...

 

will the diff drop mounts work with an aftermarket leveling kit instead of doing the whole GM swap?

instead of doing the knuckles and lower control arms, just leave those stock. 
I don't see why that wouldn't work??

 

thoughts, insight?

[GMCnewbie]

13 hours ago, Chuck FB said:

That sounds good, probably the most important item is not to destroy any information if there is any on the congnito bars before your able to jot that down and/or take photos of the tags.

Confirmed that my truck has the 4,800lb bars installed.  I took photos of both the Cognito and stock bars.  The Cognito bars only had a single sticker that was wrapped around them.  I had to take 3 pics to get the whole thing.  The Cognito bar is 1.02mm smaller than the stock bar.

 

 

 

[GMCnewbie]

Here's the other images of the Cognito sticker.  Nothing really helpful to identify the spring rate.

[Chuck FB]

1 hour ago, GMCnewbie said:

Confirmed that my truck has the 4,800lb bars installed.  I took photos of both the Cognito and stock bars.  The Cognito bars only had a single sticker that was wrapped around them.  I had to take 3 pics to get the whole thing.  The Cognito bar is 1.02mm smaller than the stock bar.

 

 

 

Excellent, this will be very helpful to others trying to dissect what bars they have relative to the stock 4800 lb rated ones in a gas short box crew cab. I don't know the diameters of the various GM OEM sizes but in another thread in this forum I had put a couple of PDF charts in which lists the ratings of the 2500 trucks and  another for the 3500 trucks for the model year 2024. In both cases the lightest rated bars tended to be on the two wheel drive standard cab gas powered trucks and they are 4400 lb bars, that's the lightest rating I have seen but that is not to say GM didn't have a lighter rating on a prior year. I've also never looked into if trucks prior to 2020 used the same bar or prior to 2017 models. Of the ratings for 2024 that I mention in that PDF, they go from steps of 4400/4800/5200/5600/6000

 

It would be interesting if someone found a truck with a 4400 pound rated front end and was able to measure those torsion bars to compare with the diameter of the cognito bars.

 

My bars also being 4800 and granted it was my micrometer so there's that difference but they were around 1.523 thou but they were not exactly the same, manufacturing tolerance variables for sure. I may as well put the conversion in this post as yes my eyes crossed a bit at the mm even though I am in Canada.

 

4800 lb rated GM OEM torsion bar diameter 38.76 mm = 1.526 inches

 

Cognito unknown rated torsion bar diameter 37.74 mm = 1.486 inches

 

[Epsilon Plus]

On 3/16/2025 at 11:19 AM, GMCnewbie said:

They're the rear jounce spacers (85565250).  Based on the parts diagrams, the AT4X doesn't used them which is confusing since all other parts are the same with the ZR2.

 

They are just plastic clip-on things that pop right on.

 

On 3/16/2025 at 11:45 AM, KvetL5P said:

Has anyone completed the full swap on a 3500? I'm curious about what they did for the leaf pack. I assume the zr2 block and u bolts won't work becuase they are too short and would need an aftermarket block? 

 

The u-bolts wouldn't but the block just sits under the lowest leaf between the leaf and the axle perch. Would prob just need to source longer u-bolts from somewhere. I can measure my block if you need to know exact lift. If the 3500s are already 1.5" taller in the rear from the different spring packs, then it's a wash anyway.

 

Rear ZR2 lift is only needed for those who want the stock rake or just to say they have the full lift. Those just looking for the factory "level" can leave the rear stuff out.