Shock Therapy: RV Shock Absorber Test

For motorhome owners seeking a better ride and handling, the most popular modification to
make to a coach is to replace its shock absorbers. Even brand-new motorhomes are open to
improvements due to the higher quality (and cost) of the aftermarket suspension components
available today. In order to determine the benefits of these aftermarket shocks, we
gathered several models and put them through our own test.

Currently there are three major
suppliers of aftermarket shocks to the motorhome market: Bilstein, Koni and RoadKing. Many
new rigs come equipped with Sachs shock absorbers, as was the case on our test coach – a
2008 Tiffin Phaeton 36QSH 36-foot diesel-pusher on a Freightliner XC raised-rail chassis
with a 228-inch wheelbase. In order to evaluate the performance of these aftermarket
shocks, we ordered shocks specifically for this chassis. The coach has a gross vehicle
weight rating (gvwr) of 32,000 pounds with 20,000-pound rear and 12,000-pound front axle
ratings.

During the test we installed and removed the shocks several times, driving the
coach on the same 1,000-mile route repeatedly during a three-month test program. During
each shock test, my copilot and I took detailed notes on performance, and at the conclusion
of round one we retested a few shocks to confirm our initial impressions.

Sachs
The first trip was made with the factory Sachs shocks, and initially
we thought they performed fairly well – with one main issue. Due to the lack of low speed
compression or rebound damping, the front of the coach raised or lowered 2-3 inches with
the release or press of the brake pedal. We found this annoying at times; in some cases it
caused the rear of the coach to drag on steep angles if I didn’t pay particular attention
to the brake pedal. We also thought the Sachs shocks were too soft on the road when it came
to cornering and overall body roll control.

Koni
Next, the Koni FSD shocks
were tested. Koni shocks are made in Holland and are well known in the racing world. The
first thing we noted about the Koni shocks was the complete lack of front-end bounce at low
speed while braking. On smooth roads the Konis were great, and they exhibited much less
body roll than the Sachs. The coach handled better and driving was less stressful due to
the lack of sway. However, when the road got really rough with potholes, highway expansion
strips, or any sharp bump, we noticed the Konis were considerably harsher than the original
Sachs shocks. Unfortunately, the Konis did not perform as well under poor road conditions.
The Konis for this motorhome carry an msrp of $181 each.

Bilstein
We had
the German-made Bilstein Comfitrac shocks installed at our local Camping World and set out
on our test trip. Bilstein is also a well-known worldwide producer of shocks for motorhome
chassis, racing and other high-performance applications. The Bilsteins exhibited a similar
level of low-speed control while going on and off the brakes as compared to the Konis. On
the smooth parts of the road the Bilsteins handled the ride well. They were comfortable but
still maintained nearly the same level of roll control as the Konis. When we hit the
interstate the real test began. Within a few miles it was clear that these were
significantly less harsh compared to the Konis, though they still possessed similar
handling and roll control. At an msrp of $125 each these are the least expensive of the
group.

RoadKing
The RoadKings were the last set of shocks in our test.
They are a new entry into the motorhome market, though RoadKing has been producing shocks
for 18-wheelers for years. Two of the main claims of RoadKing are the ultralong life of the
product and the fact that its shock is rebuildable. The company claims the typical life is
up to 350,000 miles. Once we got on the road, the RoadKings also showed an immediate
advantage over the stock Sachs shocks. In fact, every shock we tried was an improvement
over the OEM shock in one aspect or another and, as with the Bilsteins and Konis, the
brake-induced dive/rise was completely gone. The RoadKings displayed perhaps the best
handling of the group during slow and highway speed handling on smooth roads. Body lean was
minimal and the low-frequency bumps were handled with ease. They had a bit more firmness on
smooth roads compared to the Bilsteins. The next leg of the trip was the interstate and,
just as with the Konis, the RoadKings were quite harsh. Both damping and compression were
just too stiff for a coach of this weight. At an msrp of $400 each they are easily the most
expensive product on the market. Though we didn’t have the ability to measure the life of
the shocks tested, these would have a lot of cost to overcome when compared to the other
products tested.

The Results
When comparing the shocks and the resulting
ride, we thought the Sachs were most like a big, heavy sedan – such as an older-model Buick
or Cadillac – soft and mushy with a lot of body roll on turns and a lot of bounce over
bumps. The Bilsteins felt more like a BMW – smooth and comfortable yet in control. The
Konis and RoadKings have great handling and body roll control but, like a track-ready
sports car, they were just too firm for our liking. After driving thousands of miles
comparing all these shocks, it was clear which ones performed best in each category. We
rated them all on a scale of 1 to 5 (with 5 being the best), then added a value score based
on the cost of each product. Because the Bilstein shocks showed the best performance and
the lowest cost they received the highest score. Of course, everyone has a different
opinion of what the most important aspect of an ideal shock is; with this in mind, we have
given individual ratings to help you select your ideal product in the event you have a
different list of needs for your shocks.

How Shock Absorbers Work
Technically, shock absorbers are damping devices, because their real function is to slow
and dampen the compression and rebound of your suspension. This is done by controlling the
rate of movement of the piston inside the shock housing. Each manufacturer has its own
design philosophy on the best way to achieve this function, but in the end they all serve
to dampen the up-and-down motion of a motorhome while in motion. If you took high school
physics, you were taught about the conservation of energy. As applied to a suspension
system, this means that the energy in the suspension cannot be destroyed. This kinetic
energy is simply transformed into thermal energy (heat) and is dissipated into the
atmosphere as the coach moves down the road. The shocks turn the bounce into heat; the best
ones do a great job of it, while others do not, based on our tests.

There were two types of
shocks used in our test: twin-tube and gas-pressurized monotube shocks. The RoadKing and
the Bilstein Comfitrac shocks are both gas-pressurized monotubes, while the Koni FSDs and
the factory-installed Sachs shocks are both of the twin-tube variety. The twin-tube design
works using two separate cylinders, or tubes, inside the body of the shock. One is an inner
tube and the outer tube (or reservoir) is simply the body of the shock.

In order to dampen
the linear motion of the shock, a piston inside the shock is made with tiny holes to
restrict the flow of the oil used inside the shock. The sizes of the metering holes as well
as the viscosity of the shock oil are two of the factors that determine the level of
damping. The main drawback of the twin-tube design is that during rapid motion this
constant pumping of the piston causes the oil to overheat and foam. Once the shock oil
foams it greatly reduces its ability to be precisely metered and the shock loses its
ability to dampen. The gas-filled monotube shocks, such as the RoadKing and Bilstein,
overcome this foaming issue by putting the fluid under a high pressure condition. Due to
the high pressure within the shock, the entire assembly has to be precisely machined, which
can result in a more expensive shock to produce. Without this level of precision, shocks
under this type of pressure would simply leak and ultimately fail.

In this monotube design
a high-pressure chamber sits below the oil chamber of the shock and with the oil under
extreme pressure the piston can move as rapidly as needed without the foaming problem that
is common in the twin-tube shock. These shocks can be distinguished from twin tubes because
the high pressure causes the shock to always fully extend itself due to the pressure inside
the shock, whereas the twin tubes we tested can be fully compressed and they will stay
there indefinitely.

Within each type of shock there are differences and the Koni, RoadKing
and Bilstein shocks we tested each feature unique properties. The Koni FSD twin-tube shock
uses its own proprietary system called Frequency Selective Dampers (FSD). According to
Koni, the internal FSD system allows the shock to remain firm on smooth (low frequency)
bumps, but when the coach is subjected to rough (high frequency) bumps an internal valve is
employed to help reduce the typical harshness that would result if the shock used
single-stage valving. Koni offers a lifetime shock warranty for the original owner.

The
Bilstein Comfitrac gas-charged shock has a special 1.81-inch diameter piston that allows
the shock to react normally over smooth roads, but when the road gets rough the shock uses
a bypass valve to divert the flow of shock oil, and the result was noticeable as shown by
our ratings. Bilstein backs its product with a lifetime aftermarket warranty. RoadKing
modifies the gas pressurized system by greatly increasing the size of the internal piston.
This shock uses a 25/16-inch piston bore, which results in a large increase in total piston
area inside the shock. This greater diameter allows the shock to spread out the damping
load over a greater area and reduce the overall harshness over rough roads while still
maintaining control on smooth roads and corners according to RoadKing.

As you can see in
our ratings, each shock has its strengths and weaknesses. Because we are not suspension
engineers, we can only comment on the results obtained in our tests as opposed to deciding
a winner based on engineering or design. Now that you are armed with more information,
hopefully you will be able to select the shock that most suits your needs.

2 COMMENTS

  1. I don’t know if it’s just me or if everybody else experiencing issues with your website.

    It appears as if some of the written text in your posts are running off the screen. Can someone else please
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    them too? This might be a issue with my internet browser because I’ve had this happen before.
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  2. I have used Monroe, Gabriel, KYB, Tokiko, Koni & Beilstein shocks and struts for Ford Ranger, 2500 & 3500 GMC & Dodge trucks and a lot of small to full size cars & SUV…… By far Beilstein, Tokiko & Koni performance & service life is worth the extra expense.

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