Factory Spring Part Numbers & Associated Lift

[Trail X]

Based on what I've found to be true thus far, the following appears to be mathematically sound.

PLEASE READ THE ENTIRE POST.

The table below works by looking up your stock front spring part number or tag color on the top line first. This can be found by looking at your stock spring tag, or by looking for the associated RPO code in the glove box (see below for RPO > Spring # conversion).

6## = Left front spring
7## = Right front spring

## = the letters below:

15125881 - FH, HM, HU, FZ, JC, CC
15125882 - HN, HW
15125883 - HP, HX, CB
15125884 - FL, HY, JL, HC
15125885 - FM, HD
15125886 - HT, JA, JR, JT, HF, JC
15125887 - RL, RX
15125888 - RM
15125889 - HK, HL

I think these particular springs were not used in all years. However, we do know that the I6 2wd received mainly lavendar and dark green springs. When you add in the 4x4, the SWB I6 trucks received mainly Grey and pink springs. The LWB 2WD vehicles also received Grey and Pink mainly. Finally, LWB 4x4s mainly received light blue and orange. Hopefully that lets you know approximately where your vehicle would fall (even if you don't have a spring in the provided part number range).

Once you know the spring on your current vehicle, look up or down the table to see which spring could give you the desired lift (listed on the left). KEEP IN MIND, THE NUMBER LISTED ON THE CHART IS SPRING LIFT, NOT WHEEL LIFT.

My lower control arm is 15.5" from the fulcrum to the ball joint centroid, and 10.5" from the fulcrum to the strut mount. So in theory, if you are supposed to get, say 20mm of lift based on the chart, divide that by 10.5, then multiply it by 15.5. You get 29.5mm of actual wheel lift. (Or to make it simple, just multiply the chart value by 1.47 to get estimated wheel lift.)

Going to spring # 15125889 will give the most lift possible. It is the stiffest possible spring for the TB. For someone with a stock Lavender spring, they can attain 1.39" of strut lift by going to the 89 spring. Using the conversion above, this gives 2.04" of wheel lift.



All the calculations were easier to do in metric, so I only converted the bottom row to inches for your reference. The conversion from mm to in is to divide by 25.4.

Hopefully that all makes sense. Feel free to ask if you have questions. I wanted to test this out myself by purchasing a new spring, but I don't forsee the funds for that immediately. So while this is mathematically sound, I have not yet confirmed this on my own vehicle.

Consider this your disclaimer. Use this information at your own risk. A higher spring number WILL produce a stiffer, rougher ride. If you lift your vehicle too far, you could break something. Don't blame me because you didn't do the math yourself to make sure you are within a safe CV angle range. This is based on information that has been handed down between a few sources and I did my best to put it all together and figure out the mystery.

[tasnat23]

Well James.... I searched for the -89 part number on gmpartsdirect.com, and they have them for $76 a piece. So I think it's going to be a go sometime soon, so I get rid of my damn chevy lean (leans a little to the driver side).

[Trail X]

Cool!

Just keep in mind that those springs will be very stiff... probably made more for someone with a heavy winch bumper.

What are your current springs? Did you calculate the lift you'd be getting? Remember to multiply by 1.47 to determine your actual wheel lift.

To give you an idea of the stiffness change, below are the spring rates for the springs:

Spring#.......Rate(N/mm)
15125881.....50
15125882.....53
15125883.....56
15125884.....60
15125885.....63
15125886.....67
15125887.....70
15125888.....74
15125889.....77

Conversion factor to english:

1 N / mm = 5.71 lbf / in

[Dizzlenator]

So, JD, in your humble opinion, my friend, what is optimal set up for our trucks?? Seeing as how they used to be brothers from other mothers (haha!) what do you think is best set up? I'll be going to the BDS lift hopefully before TECORE 10, Part I....figure I might as well change springs, etc if I need to. Eventually, I'll be doing a full bumper with winch in the front.

[Trail X]

Work backwards... if you lose 1" by adding a bumper and winch, then get a new spring to compensate for the 1"... (or maybe a little more, hehe).

I wouldn't go over an inch or so higher without adding weight because you may regret the stiff springs. Keep in mind this is opinion based on the spring rates alone. I don't have direct experience with the higher spring rates.

[The Roadie]

Work backwards... if you lose 1" by adding a bumper and winch,

My bumper and winch added about 200 pounds. Even though they cantilever in front of the CG, I don't think this adds more than 50% lever arm factor. Let's say 300 effective pounds. With the wussiest 440 lb-in springs (times two because there's two of 'em), that's 300/880 or about 1/3" sag. That agrees with the approx 1/2" I measured.

Ooops - I goofed, too. Lever arm effect on the strut mount means another 50% effective weight. So 400/880. Still around 1/2" for a SWAG, not 1".

Aw shucks, but I noticed you used the proper weasel word: "if". Carry on. Permission to get married and get her name changed over.

[Saxis]

Not sure if this is information you can use, but my Denali has RPO "6RT" and "7RT". I don't see that in your list at all... Could that possibly be one of the missing codes?

[Dizzlenator]

God, I feel dumb when I read half of what you guys post for technical questions.

I'll just and then pretend I know what the hell you are talking about. I used to think I knew what I was talking about. I guess I still got LOTS of learning to do....

[Saxis]

God, I feel dumb when I read half of what you guys post for technical questions.

I'll just and then pretend I know what the hell you are talking about. I used to think I knew what I was talking about. I guess I still got LOTS of learning to do....

[Trail X]

Work backwards... if you lose 1" by adding a bumper and winch,

My bumper and winch added about 200 pounds. Even though they cantilever in front of the CG, I don't think this adds more than 50% lever arm factor. Let's say 300 effective pounds. With the wussiest 440 lb-in springs (times two because there's two of 'em), that's 300/880 or about 1/3" sag. That agrees with the approx 1/2" I measured.

Ooops - I goofed, too. Lever arm effect on the strut mount means another 50% effective weight. So 400/880. Still around 1/2" for a SWAG, not 1".

Aw shucks, but I noticed you used the proper weasel word: "if". Carry on. Permission to get married and get her name changed over.

Wait a tic... doesn't the wussiest spring (50 N/mm) = 285 lbf / in?

So an effective 300 lbs on the lever arm produces 450 lbs on the front springs (2/3 factor due to lever arm), shared by two springs = 225 extra lbs per spring.

So that means 255 / 285 inches of droop... = 0.89".

The epic battle... EE vs. ME...

[The Roadie]

I'm gonna go in the corner and cry now.

The strongest spring is 440 and I don't quite have that even. That's why I wanted to get 550 or 650 on the coilovers.

Never mind.......

M.E. rulez. E.E. droolz. IT guys just have all the money during this round of play.

[Trail X]

The strongest spring is 440 and I don't quite have that even. That's why I wanted to get 550 or 650 on the coilovers.

Are you still wanting 550 or 650? (lbs/in?)

If it's the same units, 550 = 96 N/mm...

I don't think you'd be happy with it unless you're carrying a couple extra steel bumpers on your front end.

From what I've found, a stock 4x4 TB SWB has about 7500 N of force acting on each front spring (not wheel). That's 1686 lbs. This means your spring will only be compressed about 3" with the weight of the vehicle on the spring. A stock spring is compressed about 6 inches in comparison.

This means you won't have much up-articulation when you're going over big trail obstacles. Even with the entire front end weight on one spring, you may hit 3" of compression... that's my concern.

Stiff springs are good for fluctuating loads, as it means the vehicle won't sag much when extra weight is added... but for articulation, you're going to hurt yourself if you go too stiff. But maybe you're looking at it from a different point of view.

[tasnat23]

Cool!

Just keep in mind that those springs will be very stiff... probably made more for someone with a heavy winch bumper.

What are your current springs? Did you calculate the lift you'd be getting? Remember to multiply by 1.47 to determine your actual wheel lift.

well I'm pretty sure that they were light blue. Not sure now cuz the tags are no longer on there. Do you think going that far will be bad?

[Trail X]

I don't think it will be awful... but certainly a noticably harsher ride.

Do you have a lift already? If so the additional ~2" of lift probably won't be good and you'll likely be bouncing off your shock internal bump stop all the time.

[Philberto]

Not sure if this is information you can use, but my Denali has RPO "6RT" and "7RT". I don't see that in your list at all... Could that possibly be one of the missing codes?

If you check on compnine, you can see based on their diagram which part number that RPO matches up to... just fyi...

-edit: Looks like part number 15125884... middle of the chart... James, would you want to add those RPO numbers to the chart? Looks like you can get all the RPO codes by referencing compnine's parts lists with item numbers and RPO codes.

[tasnat23]

I don't think it will be awful... but certainly a noticably harsher ride.

Do you have a lift already? If so the additional ~2" of lift probably won't be good and you'll likely be bouncing off your shock internal bump stop all the time.

Well now I have the BDS susp lift.

[The Roadie]

But maybe you're looking at it from a different point of view.

Exactly. With more and more difficult trail experience, what kills me is whacking the fragile front suspension bits with too much compression. Or the radiator bottom. Articulation isn't the holy grail for my style of driving, since what thwarts me MOST OFTEN is lack of ground clearance, not traction or torque. And because of the locking effect of the TC clutches, torque doesn't leak out of a spinning front tire too often, so REAR articulation is more important, along with the G80.

If I'm alone, I can almost always unleash the winch. Or get a strap assist if I'm with somebody. Or turn around and give up. Traction can be obtained by external means. Not so for ground clearance.

Harsh, stiff springs are almost hidden by airing down. The tires soak up 4" or more of bumpiness, and add to the effective suspension travel!

The Icon coilovers will have a threaded collar for preload adjustment, and I expect to have to work on that a lot to find the best feel for me. And I might indeed ask for the springs to be softer. But I want desperately to keep the nose out of the rocks on downhill runs. I did a LOT of damage on this wet set of stairs in Sedona last Thanksgiving. Because of my sagged OEM springs, fixed later.

[Trail X]

James, would you want to add those RPO numbers to the chart? Looks like you can get all the RPO codes by referencing compnine's parts lists with item numbers and RPO codes.

You get me the data, I'll update it.

[Saxis]

James, would you want to add those RPO numbers to the chart? Looks like you can get all the RPO codes by referencing compnine's parts lists with item numbers and RPO codes.

You get me the data, I'll update it.

Looks like:
Spring 15125887 = RPO# RL, RX
Spring 15125888 = RPO# RM

[bgwolfpack]

But maybe you're looking at it from a different point of view.

Exactly. With more and more difficult trail experience, what kills me is whacking the fragile front suspension bits with too much compression. Or the radiator bottom. Articulation isn't the holy grail for my style of driving, since what thwarts me MOST OFTEN is lack of ground clearance, not traction or torque. And because of the locking effect of the TC clutches, torque doesn't leak out of a spinning front tire too often, so REAR articulation is more important, along with the G80.

If I'm alone, I can almost always unleash the winch. Or get a strap assist if I'm with somebody. Or turn around and give up. Traction can be obtained by external means. Not so for ground clearance.

Harsh, stiff springs are almost hidden by airing down. The tires soak up 4" or more of bumpiness, and add to the effective suspension travel!

The Icon coilovers will have a threaded collar for preload adjustment, and I expect to have to work on that a lot to find the best feel for me. And I might indeed ask for the springs to be softer. But I want desperately to keep the nose out of the rocks on downhill runs. I did a LOT of damage on this wet set of stairs in Sedona last Thanksgiving. Because of my sagged OEM springs, fixed later.

Deriving that a stiffer spring will alleviate “whacking the fragile front suspension” is a misnomer. First this platforms worst attribute is its inability to be raised high enough in the front suspension for the needed amount of travel in such a downward bouncy trail. To this, a stiffer spring will only kill the combinations of needed travel for the comfort you are looking for. That control being, the ability to drive downward and manage the vehicles attitude without an override of pucker factor or the uncontrolled banging in the bump stops or harsh unload at the bottom of suspension travel felt solely through the shock assembly. One may think that making the travel stiffer would reduce this, I believe this is wrong. By adding the weight of your homemade bumper and winch set-up, you’ve gained one ability, but changed the characteristics of how you need your suspension to work. Therefore, a stiffer spring at each corner would only be a step backward to what you’re concerned with unless you’re looking to bounce, bounce, bounce down a trail...
The answer you’re looking for does not reside in beefier springs but better shock control, multi-staged if possible. Limiting the front end components by not letting them reach their full articulation height because of too taunt of spring pressure will ruin the capability of the truck.
Since you’ve “fixed” the springs, stiffer I presume, have you returned to a temped this trail again? A different point of view.