Adding air bumps, cant revalve; spring change?

Fifty

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TL;DR- Should I change spring rates after adding air bumps and unable to change shock valving.


I have a newish Chevy Colorado Zr2 with those newfangled Multimatic DSSV shocks that have extremely small shafts and am not sure will survive very many events. Based upon what the factory team, Hall racing, is doing, utilizing bumps front and rear to help control the truck at speed, I decided I would copy them. (There is more to it than that, but its the simplest excuse)

It seems the factory rubber bump is just a hair off the control arm and Im pretty sure the arm rides on it at static height when driver and gear are in it. It has been mentioned to me by people who are smarter than me, that the factory race truck may be using their "jounce shock" as an auxiliary shock on top of being a bump and recommended a short travel 2.5 inch bump.

As of yet, I dont have a way to revalve these multimatics should it be needed, and as soon as they pop, they will be replaced with something more user friendly. But in the mean time I have 3 spring rate choices: 14 inch 550#, 14 inch 687# and a 15.5 or 16 inch 500# spring (that once preloaded onto the shock assembly should act like a 600# spring but add a little extra height).

It is my understanding that the shock valving was based around the 687# spring. However that spring is for the diesel variant of the truck which is 400# heavier on the nose. The gas spring is the 550# spring and what came on my truck and felt like it packed a little...
I was advised by an engineering member involved with the Hall race team to try the heavier spring; and I have tested the 687# spring and it feels just a tad over sprung.

Considering I am going to adapt the threaded hydraulic bumps (front and rear), so I can adjust engagement positioning should it be needed, as well as nitrogen pressure, will this have an effect on spring rate?

Hydro/air bumps are new to me. So im not sure if its common to adjust the spring rate off the bat, especially if valving is not an option.

Use is/will be amateur "prerunning" events. Truck is almost tail heavy with the bed tire/gear rack, otherwise around 4900# and close to a 50/50 split.
 

Chris Tobin

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I don't have answers for you, but I hope to have a Diesel ZR2 Colorado one day so I am following along...
 

Fifty

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I don't have answers for you, but I hope to have a Diesel ZR2 Colorado one day so I am following along...
Lol. Yeah. The stock multimatics are a love hate relationship. But I’m hoping the bumps will help.
 

Chris Tobin

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Lol. Yeah. The stock multimatics are a love hate relationship. But I’m hoping the bumps will help.
Really... What do you love and hate about them??? I haven't driven one yet. But I was very impressed with the diesel Z71 Colorado so I ASSUMED that I would really like the diesel ZR2 as on paper it is a big improvement over the Z71...
 

Fifty

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Really small shaft. Great for a 30.5 tire, not so great if you are going to run 35’s.
Needs bumps for the front, almost like a bypass shock.
The multimatic doesn’t have any revalve support so your screwed. They have a unique lower shock mount so that sucks. Makes finding replacement shocks at the moment impossible until someone makes a bracket.

Wheel wells are tiny... 31 inch max and then have to do metal trimming tiny.
These are the biggest issues. At stock wheel size/weight, everything is pretty dang good.
The lockers are amazing.

I’m sure with a 35, some bumps and fiberglass (my route) it will change drastically for the better.
I’ll find out for sure on the 24th. Stock tires but bumps and a couple other small things.
 

Chris Tobin

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Thanks for the insight, hopefully someone more knowledgeable than me will chime in and help you with further suspension tuning...
 
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