(HJWBITI PAPER
®

Performance-Based Brake Tester Suspension Effects
LIT NO: WP100412

DATE: October 2012

On April 1, 2008 CVSA approved a new method for
DOT inspectors to check commercial vehicle
brakes and enforce the minimum
brake
performance requirements in Federal Motor Carrier
Safety Regulation 393.52. The test uses a machine
called a Performance Based Brake Tester or PBBT.

These units are currently used by DOT inspectors
in seven states (WI, IN, TN, FL, GA, SC, & KS) plus
Alberta, Canada, but they are also being used by
fleets and Service Providers to proactively check
vehicles for compliance with the federal regulations,
as well as their own self-defined pass/fail criteria.

The intent of testing with this equipment is to verify
that a vehicle can provide a minimum level of brake
force as a percentage of its measured weight.
PBBT testers can come in flat plate or roller
dynamometer (roller dyno) form. This memo will
concentrate on the roller dyno styles. One style is
permanent and built into the ground with an access
pit (reference Figure 1).

Hendrickson has learned from our recent testing
that the roller dyno method of testing can produce
very different results depending on the style of
suspension tested. The critical detail in the testing
is when the weight on the suspension is recorded
for use in the calculation of the ratio of brake force
to vehicle weight (BFtot/GVW).
The test procedure calls for each axle to move onto
the machine rollers, the rollers spin 1-3 mph, and
the driver applies the brakes. The machine
captures the maximum brake force reached before
the tire slips. The brake forces for all axles are
added together and the number is divided by the
sum of the weights recorded for each axle, and
then this ratio is used for compliance and must be
above .435, according to FMCSA 393.52 and the
CVSA North America 0.0.S. criteria section 1.1.

The second style of PBBT is a portable unit that
can be carried behind a vehicle and used at a
weigh station or rest area, as shown in Figure 2.

There are two ways to record the weight in these
tests. The first is to weigh the axle when the rollers
start spinning, prior to the application of the brakes,
and use this weight in the calculation of BF
tot/GVW. This is called the static weight method.
The second method is to monitor the axle weight
continuously during the brake application and to
record the axle weight at the point the maximum
brake force takes place for use in the calculation.
This method is called the dynamic weight method.
The first method (static weight acquisition) has
been used for many years, and at the time of
implementation of PBBTs for enforcement of
FMCSR 393.52, it was also not known that the type
of suspension would significantly influence the
calculated results from the roller style PBBTs.

,

I . I

Page 2 of 4
What we have learned:
When a trailer with a trailing arm suspension is
tested on a roller dyno there is a substantial offloading (up to 30%) that takes place on the axle
being tested. The reduction in weight results in the
tires slipping on the drive rollers (known as "lockup") before the brake can reach full torque.
The brake torque reached when the tire slips on the
roller is the brake torque used in the calculation.
This off-loading takes place to a much lower degree
in non-trailing arm suspensions. This phenomenon
can only occur in this testing procedure on single
axle roller dynamometer tests. Some suspension
types are also known to gain weight during the
brake application.
Braking of a single axle's rotating tires, while the
remaining non rotating tire axles are held stationary
and locked in place by each brake's torque is not
possible in normal operation of a tractor-trailer. In
this test, the non tested axles on the tractor trailer
are not allowed to move with the trailing arm axle
being tested, which results in the non tested axles
wedging the trailer up as the tested brake creates
torque. This artificial "wedging" reduces the down
force on the tested axle and causes the tested tires
to slip before the brake can reach full torque
(reference Figure 3).

Apparent weight change during PBBT Test
9000
8000
7000
Ji 6000
; 5000

l

3:

.,.----~-~----··--·-····-·~-·-

it"'1~-----...;;;;;;;:;;;;;;;.-···
H·····-··-...::-·-·-·····~----r·-

4000 +--1f--------'
3000
2000

Suspension
Trailing Arm
Suspension

1000
0
0

1000 2000 3000 4000

sooo

6000

Brakt Force (lb)

Suspension Style vs. Brake Force (Axle Weight Off-loading)

Figure 3

During normal braking some axles may unload, but
others have to increase in load, as the total weight
of the vehicle does not change. Testing axles
individually, however, allows the effect of individual
unloading (or loading) to accumulate, which cannot
happen during an actual stop of the vehicle.
In addition to this effect, the rate that the brake is
applied can also affect the recorded force of that
axle. A slow steadily increasing pedal apply gives
the tires on that axle the best opportunity to get to
maximum grip. A rapid apply can cause a tire to slip
sooner before the maximum brake force can be
reached.
A second issue we have uncovered is that several
fleet customers are testing trailers independently.
The federal regulation specifies a minimum ratio of
total "braking force as a percentage of gross
vehicle [single unit] or combination weight",
commonly referred to as retardation, which is a
Total Combined Vehicle Measurement (Tractor
and Trailer). Testing trailers by themselves is not
equivalent to the federal requirement.
The problem:
If a PBBT test is performed using the static weight
method on a vehicle with trailing arm suspensions,
the total vehicle weight can be overrepresented in
the calculation.
Since the unloading of the axle during the test
results in the brake force locking the wheel (and
thus stopping the test) at a brake force value lower
than the maximum brake capacity, the test result
can indicate a lower ratio of total braking force to
total vehicle weight when compared to a vehicle
without trailing arm suspensions (Figure 4 ).
The method of acquiring the axle weight used in the
compliance calculation is not specified in the
current CVSA PBBT test procedure or FMCSA
Functional Specification.

Page 3of4

ll!lllllllPBllT"Stort"

The change to dynamic weighing will not impact the
non-trailing arm
testing of vehicles with
suspensions. The use of dynamic weight
acquisition on PBBTs is recognized and already
used in some countries outside the U.S. (Europe,
New Zealand and France) and is being considered
by others.

-Min, Req1rired

Another solution, though not as effective, is to
ensure that the brakes are applied slowly and at a
steadily increasing rate during the test. This will
ensure lock up does not occur prematurely, which
leads to an even lower brake force being delivered
to the PBBT rollers at the time the test is
terminated.

Fast.rah Slowllralre fa$t8rake ~low Drake
Apply, Stalk Appl\l, Static Apply,
Apply,
Wt
Wt.
OynwkWtO'{ltarllieWti

Figure4

Hendrickson has performed actual vehicle tests
comparing
the
fully
loaded
(80,000
lbs
GVWR)vehicle stopping performance at 20 mph
(per FMCSA 393.52) between different trailer
suspension types and found the following results,
as shown in Figure 5):
TraHer Suspension Style
''''''''~'''''

,,,,,,,,,,

,,,,,,,,,,

•Hendrickson TrallingArm !lli~ Non-Trailing Arm

g

-Legal Max

~

so

~

40

Ci
~

30

~

20

The most effective solution is a combination of both
dynamic weight acquisition and slow brake apply.
Additionally, PBBT machines are frequently
configured to automatically end a test if during the
test the brakes are not applied within some "time
out" duration period.
In the event that a PBBT machine is configured so
as to not allow a gradual brake application lasting
approximately ten seconds-the accepted duration
in the CVSA PBBT for enforcement procedureswithin the "time out" period, then it may be
necessary to increase the machine's "time out"
period.
What Hendrickson has done:

10

0 ""-----Stopping Distance from Link Engineering full vehiele test# HEN-66

Figure 5

Hendrickson met with CVSA in April 2011 and
alerted them of this issue. We have also informed
the PBBT manufacturers of this issue. All of the
PBBT manufacturers that attended CVSA annual
meeting expressed willingness to work with their
customers to accommodate changes, as needed,
to dynamic, rather than static weight measurement.

Possible solutions:
PBBT tests for enforcement use or confirming
compliance with the FMCSRs must use the
dynamic weight of the axle in the ratio calculation,
and test a full combination vehicle, not independent
trailers.

CVSA has agreed to share this information with
their PBBT user member jurisdictions. Fleets have
not been alerted to this issue.
What Hendrickson needs to do:
Hendrickson needs to pass this information to all of
our customers who have PBBTs so that they are

'H
lUJ

Page 4 of 4

aware of the potential issue. If needed, the
Hendrickson engineering group will go through the
detailed reports with fleets.
The fleets will need to contact the manufacturer of
their PBBT to have the equipment software
updated to use dynamic weight, if it is not used
already. Finally, once again, fleets need to be
reminded that this is a full combination vehicle test.
The Hendrickson Engineering Team will continue to
work with CVSA in improving the use and
understanding of PBBTs as they become used
more often across North America.

www.hendrlckson-intl.com

Hendrickson Canada
250 Chrysler Drive, Unit #3
Brampton, ON Canada L6S 686
905.789.1030
Fox 905.789.1033

'!roller Commerolal Vehicle Syslems
2070 Industrial Place SE
Canton, OH 44707-2641 USA

866.RIDEAIR (743.3247)
330.489.0045
Fox 800.696.4416

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Clrcuito El Marques Sur #29
Porque Industrial El Marques
Pob. El Colorado, Munlclpio El Marques,
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