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This has been an on-going research project of mine. I discovered when I inquired about having the Trail Boss (TB) factory lift kit (GM part# 84629787) added to my 2019 LT Z71, that I could not have the kit installed due to the secondary composite leaf springs on my truck. Upon further investigation, I found that ONLY the LT with 5.3L (and LT with 3.0 diesel and short box) had the composite leaf springs preventing the truck from having a TB lift added. On the GMC side, I found out the Sierra Denali also uses the same leaf springs with composite secondary as the LT 5.3L. My first impression was, “why the heck would GM do this?” Why wouldn’t the LTZ or High Country (HC) have the the composite leaf? Why would only the mid-level LT have it? The Sierra Denali has it and it’s a $60K+ truck. My LT averages around $45K. I might be a little upset as a LTZ or HC owner to find out my rear suspension is the same as on any Custom trim truck, or even, the base W/T model. I’ve doing a lot of reading on composite leaf springs and there are more advantages to them than disadvantages. Though they are dimensionally bigger (thicker) than the steel counterpart, they weight a lot less, last longer, and offer better damping properties than steel. GM has had a good track record using composite leaf springs. The same technology has been used on the corvette since 1997 with the introduction of the C5. Disadvantages are minimal, such as, they are more expensive to produce, can burn up in a fire, and in this case, can’t be further bent or altered out of original specification. This must be the reason GM won’t allow the lift to be added to vehicles with a composite leaf. I’m sure the leaf could take the additional stress, GM engineers just want to play it safer than sorry. So with this, the Silverado LT 5.3L (and LT 3.0 diesel with short bed) and the Sierra Denali are the only trucks to use the composite leaf. Though I’m disappointed I won’t be able to get the TB lift added, I’m now pretty stoked to find out my LT has more mechanical innovation put into the suspension and I didn’t have to spend $15K+ more for a Sierra Denali to get it. However with this, it seems that if someone is looking for a top-end truck, the Denali would be the better choice over the HC which lacks the particular composite leaf spring feature, if that mattered to the buyer. For reference, attached here are the GM compatibility charts for the Silverado and Sierra to see if you can add the TB lift kit. Also here is a link to the previous discussion about the TB lift kit where you can find more detail about installation and issues surrounding the composite leaf springs.
