All About the Hummer Shocks

The newest Hummer H1 is now 6 years old. I remember when some of my friends got 2000's which in my mind is considered new. Now those trucks are 12 years old. My 98 is 14 years old. Since most of the Hummers have changed hands, most owners don't have a feel how the trucks are supposed to handle. The shocks in most of the trucks have never been replaced. How many of you have a car that's 10 to 14 years old? There a real good chance that the shocks and struts have probably been replaced at least once or twice. This is probably the most overlooked maintenance item on the truck.

Lower A arm

What do Shocks do?

SpringsThe Hummer has a 4 wheel independent suspension with a coil spring and shock located in the center of the spring at each wheel. The shock absorbers job is to control the springs. When you hit a bump it's the spring that absorbs the lions share of the energy. It will compress and then 'spring back'. In fact without a shock the spring will continue to oscillate back and forth (twang) until the energy in the spring is used up. This, of course causes instability in vehicle handling. Imagine hitting a big off road dip. You want your truck to come back up and stop, not to continue bouncing up and down. Shocks are often called dampers because that's what they actually are. Their main job is to dampen the spring oscillation. Worn shocks can cause premature tire, steering and suspension component wear. They also cause dangerous instability through excessive sway.

A spring is made of steel and like any steel it weakens every time it flexes. If your shocks are bad or not properly set up for the kind of driving you do the spring will not be damped and continue to cycle. This causes the steel to fatigue which is why springs prematurely wear out.

How Would I know if I Need Shocks for my Hummer?

Old ShockThis is hard to notice because the shocks deteriorate over time and the problem slowly worsens. It just doesn't happen all at once unless you out and out break one. Years ago my vacuum pump went bad. As you know the vacuum runs the turbo wastegate. Over time I didn't even notice I wasn't getting boost. Now I have a turbo boost gauge but things went bad so slowly I didn't notice what I was missing till I got it all back at once.

Here's the typical reaction right after installing a set of Cepek shocks. “Thank you very much for your advice on the shocks. I had a chance to drive it alot today. They are a night to day comparison to the old ones. I had no idea that the shocks did that much. It corrected a number of issues including the bad shakes, crashing sounds and vibrations on undeveloped roads. Handling now is unbelievable for what this Hummer is. I took several tight exit ramps at double the speed limit with no slip at all on the 4/3 setting.”

Because our trucks are so heavy and the springs are so heavy duty you usually can't just bounce a corner of the truck and see if the truck continues to bounce up and down like you can with a car. One way to test them is to drive into a dip at about 3 to 5 mph. When you come up and out of the dip the truck will shoot up, come down and compress a bit and should rest level. If your shocks are weak the truck will shoot way up high, come down fast then rise up a second time and depending on how bad the shocks are it could just bounce. The spring is what takes the energy from the hit. The shock is meant to damp the springs (keep the spring from 'twanging').

The other way to know if your shocks are worn is if your truck sways in the corners. This is another one of those 'I don't remember how it's suppose to be' things. In fact I'll bet that most of the owners never experienced how well their truck handled when it was new because they bought their trucks used. If you're going around a corner and the truck feels 'sloppy' and sways some it's a shock thing. All I can say is that if you ever felt like you could lose it getting off on a curved highway exit ramp then these shocks will go a long way to alleviate that fear.

Tom Cepek told me that Rod Hall once said that if you have 2,500 miles on your factory shocks you need new ones. The way the story goes is that Rod picked up a new Hummer at a dealer out of state and had it driven back to his shop. When it arrived they had to replace the shocks. My take on the situation is that Rod Hall is much more demanding of their suspension system due to the extreme punishment they put a truck through during a race. The factory shocks are generic oem's made by Monroe. My guess is if you have over 20,000 miles on your truck you probably need new shocks.

The Factory OEM Shocks are Weak

Note the hump on the bottom of the canister and the angle of the eye ring which is bent on the OEM shock. This is fairly common with hard use on a factory shock. The can just collapsed at the bottom and the eyering is bent sideways on the factory shock.

GT Shock Factory Shocks

Options for Hummer Shocks?

Once I decided I needed new shocks I quickly found out that there are not many choices available. The only ones I found are the factory shocks, Rod Hall Racing shocks and the Cepek's. Note that the front and rear factory shocks are different part numbers because they are valved differently. The Rod Hall shocks are designed for punishing off road racing where the truck is traversing rough ground at high speeds. The suspension is being worked at a rate that far exceeds anything I'm going to put my truck through; this is why you need the remote reservoir (for cooling).

The Cepek's are adjustable, physically larger and have a lifetime warranty. Because they are adjustable you use the same shock on all 4 corners of the truck. They also have a remote adjustment option which will let me tailor the shock stiffness to the kind of driving I'm doing while sitting in the truck. This will let me experiment with different road conditions.


There's no contest when it comes to the quality of components on the GT Shock

One of the major things that creates a problem is that the factory shock does no use an O-Ring on the piston, it just runs inside the steel tubing, and that's like running your engine with no piston rings on the pistons.

GT Cepek Shock OEM Factory Shock

What People are Saying about the Hummer Shocks

I've had the shocks installed for awhile now and I'm getting spoiled. When I first received them I thought I would install the shocks without the remote adjustable feature. I asked myself "Why do I need to play around with the shock damping?" Now that I have it I can't live without it. If I'm on the highway I set them up stiffer just to see how fast I can come off the exit ramp. When I'm cruising I set them soft for a nice smooth highway ride. Around town I want enough firmness for good handling but I don't want to feel every bump in the street. I even find myself softening up the shocks when I'm coming to a railroad crossing or a potholed street in the city.

I tested the shocks out at the Hummer Club's Durango Moab event in September 2003. Almost all of the trails were slow extreme rock crawling. When the truck goes through what I call a 'whoop te do' (2 to 3 mph) where the right and the left go through a smooth deep dip at different times causing the truck to heave slowly around in all directions the soft setting, 0 would let the truck rise up pretty high. It wouldn't oscillate, but it did heave. Turning the shock damping up to 9 seemed to really tame it down and give me more overall control.

Many of the moves consisted of riding the brakes down a steep stair step while trying to go slow enough so you don't bash your bottom and trailer hitch on the rocks. I started by setting the shocks on the softest setting, 0. I found that the truck was bottoming out on many of the deep dips. I experimented with higher settings (more damping) and ended up running with the front and rear set on 7. This seemed to give me much better control and kept the truck from banging the bottom. I did use 9 on some of the extreme stuff. Raising the stiffness didn't affect the bumpiness of the ride because of the slow speed and 18 to 25 psi tire pressure.


I'm running a 98 wagon with gsa's at 37psi cold. When I first installed them I didn't have the electronic Remote control (ERC) hooked up yet because I couldn't decide where I wanted to put the pump and display. I drove around with the shocks for about 2 weeks at their softest settings. The ride was soft and real smooth over bumps but a bit too mushy for my taste. The truck did sway in the curves, but not as much as my worn out AMG shocks. When I finally installed the erc and started playing around I found myself driving with the fronts at 4 or 5 and the rears one step lower. I was getting spoiled because whenever I would end up on a bumpy pot holed road I would lower the settings to 0 on the bumps and stiffen up the shocks when I got back to normal pavement.

Off the road is very different. I was running 15psi in my tires. I thought that the softest shock settings would be the best. What I found is when I was moving slowly through rough areas where the truck was constantly going through dips, over rocks and down into washes my front was heaving right, left and up and down. As I began to firm up the shocks I noticed that the truck started to settle down and wasn't being thrown around as much anymore. I was running as high as 8-7 on some of the stuff in Moab.


I thought I'd give the list my opinion of the Cepek adjustable shocks we installed last week on my 2000 wagon. First I'd like to thank Chuck for selling the shocks at a great price and maintaining a website that made the install much easier ( I read pictures better than words) and Tom Cepek for all his trouble shooting help, answering the phone well after normal business hours and immediately caking care of a small problem that we had. It is very refreshing to deal with gentlemen who really care about what they make and/or sell. Both want to make certain that you have the knowledge of the system and how to get all the benefits from it. I had just over 40,000 miles on the set of stock shocks that we replaced, the immediate difference without playing too much with the 10 different settings that the control panel can be set for is amazing. With the shocks set at 5 in the front and 4 in the rear the wagon feels very tight and I no longer experience the body roll that I had when cornering with the old shocks. I've also noticed that most of the rocking is gone when braking on the steeper hills of San Francisco. My wife actually made mention that the ride is much smoother when we went out to dinner last night, I had the shocks aired down to 3 in the front and 2 in the back. I can't wait to go off road next weekend when I head up to the country to see what these shocks can do on the old logging roads in the Six Rivers Nat'l Forest and the gravel bars of the Klamath River. Gary Collings 2000 wagon 3/21/2004


A couple weeks ago I purchased a set of the Cepek Remote control shocks from Chuck. The installation took a couple of days, mostly cause I was the only one tackling the task the first day. Anyway once installed, as with all newly installed toys needed to do a road test. Conferred with my wife, packed a carry-on bag and hit the road. Presently we started from Tennessee heading to see my son and family in Little Rock, Arkansas. As Chuck had indicated the first day had a bit of a leak, but by the afternoon the air was holding fine. During the first day had fun playing with adjusting the pressure from a soft setting of 1 to very hard 9. I run a '95 XLC2, an extended cab 2-door with a camper top. I found that in Tennessee a setting of 5 in front and 3 in back was good on the interstate until I got to Arkansas where I had to back down to 4 and 2 - the roads were a lot rougher. In between times the various settings provided different results from loping (1) to teeth-jarring (8). Visited my kids - impressed their neighbors with the Hummer, a real H1- not H2, which always earns points with the son.


I did the install on my new shocks yesterday. I went to a buddy's shop and we were able to install all four in about an hour and a half. Thanks to Chuck and his website for detailed instructions. Most of the bolts came out very easy, however we did have to heat a few of them up. I am very happy with the new shocks and have noticed a significant improvement in the handling and and ride of the truck.


Mark Weber May 8, 2003 I canít believe the difference they make, and they are so simple to use compared to other systems I have worked with that adjusting them every time the pavement changes seems like it will be a habit.


Pat Mahan November 12, 2003 I just installed the shocks on my friend's 12,500 heavy Hummer. Now granted I do not drive the truck very often but when I drove it before it would scare me since it wondered around a lot. If you had to corner very hard the body roll was actually scary. When I test drove after doing the shocks, I pumped them all the way up and the truck felt like mine does when the shocks are set around 3 or 4. I really like the way they increased the performance on my truck and I think they made an even bigger improvement in the heavy truck.


There has been some commentary written about our beast's rough riding and adjustable shocks. I mounted a set of these shocks on my '95 XLC2 (Hummer extended 2 dr.) last October. It took a good day and then some to do the job myself with the help of a cousin and a couple six-packs of beer. Yeah a bit pricey, but I've spent money on many a' foolish doodads with a lot less results.

First of all, it is hard to believe how much road feedback shock absorbers pass back to the driver and passenger. With these shocks turned up to a high setting, every small expansion joint even on the interstate is felt - let's not even mention what it is like on a dirt road. On the soft setting of 1 or 2 you seem to float down the interstate and on the rough stuff much of the jarring disappears. Besides what fun to raise and lower the shock setting on the run, add to that continual tinkering with the CTIS - and you almost get an E ticket ride. I might mention that if you have a co-pilot along expect a reasonable amount of lecturing on keeping your eyes on the road.

After 4,000 miles of driving around 6 states and Canada, was it worth it? Maybe not from a pure benefit/cost ratio, but definitely big points when bellied up to the bar and the BS is waist-deep on who has what on their rig. You decide. Meanwhile, I can choose 9 programmed settings that automatically control front and rear shock settings, so there.
Myk '95 XLC2


I rode them quite hard over some trails, with what could be described as Whoop-de-dos, and the only thing I can say is that I slowed down due to common sense before the suspension started to bottom out. We were almost airborne in some cases and the shocks did great. I was able to maintain a much higher speed in these conditions with the Cepek's than the stock shocks. I have run the same area for years, so the results are quite real.

On rough roads the truck has more control and stability, as well as the ability to run at much higher speeds comfortably. Stock shocks would get to a point of such incredible vibration on washboard roads that I had to slow down for fear of the truck coming apart. This was greatly reduced with the Cepek's.

On regular roads I was able to stiffen the ride so much that I felt I could corner like a sports car with almost zero body roll or instability. I continually left them on a setting of 7 as I liked the stiffer ride, but down to a 2 or 3 off road.

Of course everyone will have different experiences, but I can't say enough good things about the cost-to-benefit of the Cepek system. It's truly one of the best things you can do to your truck for the ride.


How the Cepek/ GT H1 Hummer Shocks Work.

Each shock has a spring loaded piston valve at the bottom that regulates the shock's stiffness. With the valve all the way up and no pressure the shock is at it's softest setting. As the valve is pushed down the shock gets stiffer. The shock valve is adjusted manually by using the dial which is nothing more then a screw pushing the piston.

The remote adjustable unit works by regulating the air pressure pushing down on the shock piston valve. A small computer controls a miniature compressor that maintains the pressure at a user defined set point independently for the front and rear pair. By monitoring the air pressure the computer knows when a problem such as a leak occurs.

This is what the manual says about the shocks.

The G T Inc. (Cepek) Digital Remotely Adjustable Shock Control Unit is one that is very user friendly and easy to use. Once you understand the basics of the system, what it will do, and what you can expect from the programmed functions.

Basically, the system allows you to set the damping of the front and rear shocks independently, with each pair of shocks having an allowable setting parameter of 0-9, or 10 total positions of damping, 0 being the softest, and 9 being the stiffest, as well as a memory channel that again, allows settings of 0-9 or 10 programmable settings.

The user can input shock damping settings into each of the 10 memory channels and store those settings for future and continued use. The user then, only has to cycle through the memory channels by cycling through those numbers with the center push button and then stop on the preset shock damping setting they would like use.

As described above, the system:

  1. Allows the user to set shock damping independently front and rear,
  2. Has user programmable memory channels
  3. Provides aural and visual warnings
  4. Maintains pre-selected shock damping by constantly measuring line pressures and adjusting them as necessary,
  5. Gives a warning if the compressor adjusts line pressure more than 6 times in one hour if on the same setting
  6. Shuts down the compressor should a line break, be cut or pulled loose to prevent compressor damage
  7. Allows manual changes to pre-selected shocks settings without changing the pre-programmed settings.

The unit draws between 10 and 15 amps, typically 12 amps when the compressor is running. The control unit draw is negligible.

It is suggested that you may want to group your settings in order to keep like terrain, street, off-road, severe handling and other settings in close proximity in order to make it easier to remember the location of each setting. For example 0-2 could be your street handling settings, 3-6 could be your off-road settings, and 7-9 could be your mountain driving and/or towing settings.

Common Questions Answered:

The Cepek shocks are basically the same length as the factory shock, with the exception that there are no built in hydraulic stops in the adjustable shocks. The factory shock during fairly fast internal piston speeds limit the stock shock travel to approximately 2 inches of travel, but in the adjustable you get full travel at any internal piston speed, which would provide twice the amount of usable shock travel, or a bit over 4".

Shock is Compressed.= 12.388" Extended. = 16.452" Travel = 4.064"

These measurements are pretty much cast in stone because of the suspension geometry of the Hummer. The A arm can only travel a given distance before the half shaft CV joint will start to bind. The shock actually limits the A arm travel to prevent this. Unless you were to modify the suspension you have to use a shock with the stock compressed and extended lengths.

That being said my shock provides more usable travel, since it's not restricted by hydraulic stops. The factory shock is setup to highly damp the piston at the beginning and end of it's travel to prevent the shock from slamming into it's end stops and destroying itself. Our shock uses an external bumper for compression which keeps the shock from bottoming out, and allows for a smooth transition for the final 1" of compression. Extended length is limited by the shock topping out, which rarely happens because of the damping properties of the shock. Sometimes if you are rock crawling at slow speeds you can get to the full extended travel of the shock. Our massive striker head prevents topping the shock causing no negative effect on the shock whatsoever.

We don't provide valving specs, since it will make little if any sense to the average user. If you are designing your own shocks where you have control over the internal construction, piston diameter, fluid viscosity etc valving specs would make sense. Because our shocks are designed for the Hummer and there are no options, valving specs in this case are a moot point. Added to this is the fact that our shocks are adjustable so the valving actually has 10 steps.

The Rod Hall shocks are excellent, but may not be what you need unless you are racing your truck, and as far as reservoirs are concerned, not necessary for the average user, since the shock never gets hot enough to require the extra fluid and cooling the reservoir provides. If you are going to drive your truck at 60 mph off road all day then I'd be looking at Hall shocks. Our shocks have a lifetime guarantee.

There is a huge difference between my shocks and the other types in that, the amount of damping of my shock can be changed on the fly, by the use of the ERC, (Electronic Remote Control) unit, from the cab, which allows you to have anywhere from a soft stock type ride to something that will ride very stiff, and reduce body roll to just about Zero. Nobody else has this for the Hummer.

Buy Cepek Hummer Shocks