Zr2 front suspension

[Epsilon Plus]

It's all 1.5". The diff drop corrects the CV axle angle. Keep it mind 1.5" is barely anything. People ride on janky lifts with janky angles all the time for thousands of miles. Some of those monster lift trucks out there have CVs that look like a capital A underneath. They must never go into 4WD with load resistance on them or something, I dunno.

 

There's been some people I've seen speculate that one could potentially just do the UCAs and correct the ball joint angle for a torsion crank and get most of the benefits of this whole thing.

 

I chose to do the whole thing because like GM, I ain't janky (maybe I am, but also OCD so I wanted all the factory correct stuff).

[kylant]

On 3/17/2025 at 11:02 AM, GMCnewbie said:

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

 

On 3/17/2025 at 12:34 PM, Chuck FB said:

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

 

just for reference, my truck 2025 2500 AT4, 11350 GVWR has 5600 torsion bars. 
I bet it would ride pretty nice with the 4800 bars. I am also willing to bet the cognito bars are 4400s

Edited March 29, 2025 by kylant

[Chuck FB]

48 minutes ago, kylant said:

 

just for reference, my truck 2025 2500 AT4, 11350 GVWR has 5600 torsion bars. 
I bet it would ride pretty nice with the 4800 bars. I am also willing to bet the cognito bars are 4400s

Yes, so your Duramax crew cab short box has the standard 5600 torsion bars for the diesel and my gas with crew cab short box with the standard 4800 are in theory matched for their weights on the front. Taking your truck and installing 4800's should in theory soften that ride up enough to make a noticeable difference. I wonder if it would require new reindexed keys to give enough adjustment to reset to factory ride height, that would be the question mark. As you might have seen where I posted the 2024 model year charts for the 3/4 and 1 ton truck spring rates, the 4400 torsion bars are only offered in the two wheel drive trucks of a standard cab or double cab. Up here in Alberta I would not be surprised if a dealer has NOT brought in a two wheel drive HD truck for years, the truck that was the norm years ago is a unicorn now so I know it would be impossible for me to find one to crawl under and measure its torsion bar diameter to see what a 4400 compares to the Cognito.

 

I am curious if you have ever weighed your trucks axles when you were fairly empty in the box and rear seat and just you in the truck. I haven't a clue what the Duramax version to mine would weigh on the front as it would be an interesting comparison. I did not get the sunroof and that would add a little bit of weight, however I have the optional second battery and the two alternators so that adds up some over a standard gas version. The 1 ton aspect weight of mine would be over the rear axles anyway with the extra leaf pack so not affect the front weight.

 

Speaking of ride and my truck also has the 275/65R20 tires with the somewhat skimpy sidewall height for the diameter of the tire, based on the industry tire inflation chart and my axle weights, I am experimenting with running 40 pounds front and 35 rear. I am almost on the line with 40 front for its weight and the 35 rear is based on the minimum 35 pounds that an LT tire type should be run at highway speed as the bedmat, bakflip and what I have in the bed at the moment is well under what 35 psi can carry. It sure takes the harshness out of pavement cracks and mellows it out to a more sedate level vs the 60 front and 80 rear the truck came off the assembly line with. The diesel would never be able to safely get away with 40 on the front with the factory size tires but would be interesting to know what if any it can be dropped from 60.

[kylant]

any updates?

[kylant]

are the cv axles different on the zr2/atx? i read a package description and it mentioned new axle shafts.

[newdude]

42 minutes ago, kylant said:

are the cv axles different on the zr2/atx? i read a package description and it mentioned new axle shafts.

 

 

Only on the 1500s.  HDs use the same axles regular 4x4 and ZR2/AT4X.  They can do this because they drop the diff on the ZR2/AT4X hence the different part #s for the diff mounting brackets.  

[kylant]

On 4/4/2025 at 6:48 AM, newdude said:

 

 

Only on the 1500s.  HDs use the same axles regular 4x4 and ZR2/AT4X.  They can do this because they drop the diff on the ZR2/AT4X hence the different part #s for the diff mounting brackets.  

that's what i thought. thanks

 

is there any new hardware required for the diff drops? or anything else except the LCA bolts?

[GMCnewbie]

On 4/5/2025 at 8:51 AM, kylant said:

is there any new hardware required for the diff drops? or anything else except the LCA bolts?

I'm assuming you're referencing bolts and nuts.  For that, the only bolts/nuts that I'm aware of that need to be replaced are the LCA bolts and the axle nut.  Both are expensive.  I strongly suggest ordering online to save money.

[GMCnewbie]

On 4/2/2025 at 6:41 AM, kylant said:

any updates?

I finally wrapped up the front end and took the truck out for a ride.  For reference, I replaced UCAs, LCAs, knuckles, differential carrier brackets, torsion bars, and shocks.  I'm still waiting on a part for the rear-end. 

 

That said, the ride feels good.  It's a much more controlled ride over speed bumps, dips, and humps in the road.  The ride feels much nicer.  Is it worth the price paid?  Time will tell.

 

Still waiting for my new wheels and tires as the wheels are backordered.  Hoping they'll arrive in the next few weeks.

 

I still need to dial-in the lift in the front, but i think I'm gonna wait until the wheels and tires get here before I try to make any final adjustments.  On that note, I initially installed Cognito torsion keys with the new bars (per the Cognito rep), but the front height was about 4" higher than stock.  So I swapped them out for the stock keys and that was about 1" lift over stock.  I'll play around more with the height when the wheels/tires get here.

Edited April 6, 2025 by GMCnewbie

[kylant]

1 hour ago, GMCnewbie said:

I finally wrapped up the front end and took the truck out for a ride.  For reference, I replaced UCAs, LCAs, knuckles, differential carrier brackets, torsion bars, and shocks.  I'm still waiting on a part for the rear-end. 

 

That said, the ride feels good.  It's a much more controlled ride over speed bumps, dips, and humps in the road.  The ride feels much nicer.  Is it worth the price paid?  Time will tell.

 

Still waiting for my new wheels and tires as the wheels are backordered.  Hoping they'll arrive in the next few weeks.

 

I still need to dial-in the lift in the front, but i think I'm gonna wait until the wheels and tires get here before I try to make any final adjustments.  On that note, I initially installed Cognito torsion keys with the new bars (per the Cognito rep), but the front height was about 4" higher than stock.  So I swapped them out for the stock keys and that was about 1" lift over stock.  I'll play around more with the height when the wheels/tires get here.

nice.
with stock keys, did you have to crank them more than the stock adjustment?

[Chuck FB]

1 hour ago, GMCnewbie said:

I finally wrapped up the front end and took the truck out for a ride.  For reference, I replaced UCAs, LCAs, knuckles, differential carrier brackets, torsion bars, and shocks.  I'm still waiting on a part for the rear-end. 

 

That said, the ride feels good.  It's a much more controlled ride over speed bumps, dips, and humps in the road.  The ride feels much nicer.  Is it worth the price paid?  Time will tell.

 

Still waiting for my new wheels and tires as the wheels are backordered.  Hoping they'll arrive in the next few weeks.

 

I still need to dial-in the lift in the front, but i think I'm gonna wait until the wheels and tires get here before I try to make any final adjustments.  On that note, I initially installed Cognito torsion keys with the new bars (per the Cognito rep), but the front height was about 4" higher than stock.  So I swapped them out for the stock keys and that was about 1" lift over stock.  I'll play around more with the height when the wheels/tires get here.

I can guess that the amount the torsion bars would have to be turned up to obtain a certain ride height would vary a fair bit depending on if the truck has the gas engine vs the diesel. Cognito does not designate anything on the website so it would be more what they have found when experimenting with them on the different truck platforms that they have not put into print.

[GMCnewbie]

1 hour ago, kylant said:

with stock keys, did you have to crank them more than the stock adjustment?

I set them to the same point as stock.  That gave me roughly 1" lift with stock wheels and tires.  I haven't gone back to adjust because I'm too lazy to lift the truck again right now.  I think the Cognito keys would've settled lower than the 4" had i tried driving around with it to get the suspension to settle.  I may try that down the road depending on how things shake out with the new wheels/tires.

[Chuck FB]

10 minutes ago, GMCnewbie said:

I set them to the same point as stock.  That gave me roughly 1" lift with stock wheels and tires.  I haven't gone back to adjust because I'm too lazy to lift the truck again right now.  I think the Cognito keys would've settled lower than the 4" had i tried driving around with it to get the suspension to settle.  I may try that down the road depending on how things shake out with the new wheels/tires.

Its probably been said early in the thread but what is the actual "official" height increase with the new suspension parts, I've seen the comment that its 1 1/2 inches higher but is that the suspension or the tires and suspension as a whole.

[GMCnewbie]

I've heard 1.5" as well.  I honestly never asked the question as to suspension vs tire size.  What I can say is the AT4X/ZR2 components combined with Cognito torsion bars, stock keys, and stock AT4 wheels/tires gave me a 1" lift in the front.  

[Chuck FB]

11 minutes ago, GMCnewbie said:

I've heard 1.5" as well.  I honestly never asked the question as to suspension vs tire size.  What I can say is the AT4X/ZR2 components combined with Cognito torsion bars, stock keys, and stock AT4 wheels/tires gave me a 1" lift in the front.  

And as you measured the stock bars to the cognito and stands to reason they are somewhat softer and would loose some height due to them, how much is hard to say but it would be some. One thing you could do if there are any AT4X around assuming their fender trim is the same as all the other GM trucks so your measuring apples to apples, is to measure the best you can from the underside of the fender trim straight down to the center of the hub. That takes away all tire height and tire inflation variables. Do the same right now with your truck before you do anything more ( make sure the truck was driven to the flat area you park it on and not jacked up and then set back down ) and measure it the same way so you have that fender to center of hub referrence as a guide. Then when you swap wheels etc, its always that same hub to fender spec when parked on a flat surface to do your fine adjustments to get the truck side to side height like you want as well as total suspension lift.

 

On my truck I bought recently I discovered that the left side was a 1/2 inch higher than the right side and yes the fuel tank was full and not really anything else in the truck. That really showed up on the key bolts as the right hand bolt was sticking out quite a bit more, GM seems to do a rather poor adjustment at the factory for the ride height I have found, the trucks are not consistent off the assembly line.