I mentioned taking delivery of a pair of new front shock absorbers for our coach. The current set of shocks was installed just two years ago, but the trip up I-5 made it obvious that the front shocks weren’t holding up. A vehicle weighing more than 30,000 pounds has to have high spring rates and serious dampers to control the suspension.
Shock absorbers exist to damp the action of the springs. Undamped, the springs would compress over a bump, storing energy in the spring. The spring would then extend, releasing this energy as it elongates past its resting length, only to return to a compressed state. This cycle would repeat until the energy is dissipated through friction and heat. The vehicle would bob up and down like a boat going over swells in the ocean.
Getting the damping rate right isn’t a simple matter. Hydraulic shock absorbers have a piston inside a cylinder filled with fluid. The piston is connected to a shaft that extends from the cylinder. Typical installation has a fastener at the closed end of the cylinder opposite where the shaft exits and another fastener on the end of the shaft. Suspension movement strokes the piston in the cylinder, moving it through the fluid.
Slow movement of the shaft, such as compression of the front suspension when braking, or cornering, requires a high damping rate to limit the movement of the suspension. You don’t want the front suspension to compress or dive excessively while braking or roll from side to side when cornering. This is accomplished by forcing the fluid through a small port in the piston. The size of this orifice sets the low piston speed resistance – the damping rate.
Conversely, when the piston moves at high velocity – such as when hitting a square edge bump like an expansion joint where the road meets a bridge or a pot hole – we want the spring to compress and take the hit without transmitting it to the chassis. The orifice used for low-speed damping is insufficient to move the fluid quickly enough through the piston and the shock would hydraulically lock. What the shock designers typically do is add a number of high-speed ports – a ring of orifices through the piston.
This ring of orifices allows much more fluid movement – but how to keep fluid from moving through these holes when we want stiff low-speed damping? The most common way is to cover these high-speed ports with a thin metal shim. A stack of shims can be made to gain the proper stiffness of the cover over the hole. When the piston moves through the fluid at a high enough speed, the shim flexes as the fluid is forced against it – as it flexes away from the holes, it uncovers them and the fluid can pass.
Our current front shocks are Koni FSD series 8805. These shocks are designed pretty much as I described above. The piston in these shocks have a 36mm diameter. Hitting sharp irregularities in the road wasn’t an issue when these shocks were new. Over time, they gradually lost the ability to transition from high-speed damping to low-speed damping after hitting a sharp bump. It’s like the high-speed ports stay open too long, allowing the coach to bounce on the springs three or four times after hitting the bump.
About five months after I bought these shocks, Koni came out with a new replacement for use on the Alpine Coach Peak Chassis and Monaco Roadmaster RR4 chassis. This new shock, designated the Evo 99 series, has a 50mm piston and revised valving. The 40% larger piston allows the shock to operate at much lower fluid pressures and should enhance durability.
My task on Thursday was to remove the old front shocks and install the new series 99 shocks. It’s only four bolts – one on each end of the two shocks. How hard could that be, right? Well, I remembered when I had the shocks installed, the guys used cordless electric impact wrenches to remove and install the shock bolts. The mounting bolts were stubborn and it took two mechanics about an hour to replace four shocks.
The first thing I had to do was buy a 28mm socket and a large breaker bar to break the bolts loose. I bought a 24″ breaker bar and I couldn’t get the bolt to budge. I needed an impact driver. I borrowed my daughter Alana’s car and drove to Harbor Freight in Everett. I used to have a few impact wrenches in different power levels, but they’re long gone thanks to those Dirty, Rotten Thieves. I figured I could buy an air operated impact driver at Harbor Freight for about $40.
When I got there, it occurred to me that I only have a small, portable air compressor with only a few gallons capacity. In my sticks-and-bricks garage, I had a 60-gallon compressor that powered air tools easily. Although my little compressor can reach 150 psi, it doesn’t have a fast enough flow rate to power a big impact driver. I needed an electrically operated impact driver. I found a cordless impact driver with a 330 ft-lbs torque rating. It was branded Chicago Electric – Harbor Freight’s Chinese sourced house brand. I paid over $100 for it.
By the time I got home with it, it was after 1pm. I needed to charge the 18-volt battery before I could use it. It has an 18-volt nickel-cadmium (NiCad) battery. NiCad is old technology, but it works. The first charge on a NiCad is important as it forms the cell. I charged it for 90 minutes – the day was getting away from me. It takes three or four full charges before a NiCad reaches it its full capacity. After charging the battery, I had to allow another 15 minutes for it to cool before using it.
I started banging away on the first bolt with the cordless impact driver. I would hit it for several seconds, then stop to keep from overheating the motor and battery pack. After about 10 minutes of this, I needed to charge the battery pack again. Arrgh! This entails a cooling period for the battery before charging, about an hour or so of charge time, then another cooling period before attacking the bolt again. I had the first bolt off by 5pm and called it a day!
Friday morning I was back at it with a fully charged battery and started on the second bolt. After two charge cycles, I had it off. Removing the shock and installing the new one was fairly easy – it only took about 10 minutes. Getting the new shock in place entailed holding it at arm’s length with one hand while I was bent over the front tire with my head in the wheel well, then inserting the mounting bolt with my other hand. It was a workout!
Old 8805 FSD shock with top bolt removed
Comparison of 8805 and 99 series. Note – no dust cover on the 99 series
New shock installed
Now I had to start the charging – working – charging – working cycle on the other side. By the end of the day, I had one bolt removed from the other shock before I gave up. It was beer-thirty and I was ready for a cold one.
IPA from Seattle’s Fremont Brewing
Today I’ll start over and remove the last bolt. A lot of work for four bolts!
On Thursday evening, we received tragic news. Our friend Sini Schmitt texted Donna to inform her of the passing of her husband, Bob. Bob died unexpectedly while they were vacationing in the British Virgin Islands. We met Bob and Sini in San Diego a few months after we first hit the road. Since then, we’ve met up with them in Arizona and again in California and always had fun times together. I’m deeply saddened by Bob’s untimely passing. He was a great guy and will be missed by many.
