Determining Truck Speeds using
Rimpull and Retarder Curves
Question
How fast can a Vovlo A40D go up a 12.5% grade with a 2.5% rolling resistance
And a full 40.7 ton load (total vehicle weight 150,200 lbs)?
We use the
Rimpull curve
To determine
this
Start by Finding Your Vehicle
Weight
Most rimpull charts show the
Weight of an empty truck
And a truck loaded to full
Weight Capacity.
Otherwise start with the
Weight down here.
Determine the Total Resistance
12.5% positive grade resistance working against the truck + 2.5% positive rolling
Resistance working against the truck = 15% total resistance.
On the grade scale find the total resistance
(15% in this case)
Read Up from the Weight Line to
the Grade Line
From This Point Read Over to the
Rimpull Curve
This blue line is
The rimpull curve
Now Read Down the Maximum
Speed the Machine Power Can
Sustain
We can go 5mph with a full load
Up a 15% effective grade.
Rimpull Curves Only Work for
Effective Grades Against a Vehicle
Retarder Curves are Used for Favorable Grades
Braking a Large Truck
• For large haul trucks drum or disk brakes
would heat up quickly
• To control speed on downgrades large
trucks use hydraulic resistance applied to
the drive train
– For very short distances one can go faster
and hold the truck back, but over longer hauls
the hydraulic resistance system would
overheat too.
How Fast can a Fully Loaded Volvo A40D go
down a 12.5% favorable grade against a
2.5% rolling resistance?
Figure the Resistance
12.5% negative or favorable grade trying to make the truck roll away + 2.5% rolling
Resistance opposing the truck movement (rolling resistance is always against you)
Give a total effective grade of -10% - minus means a favorable grade and the need
To use a retarder chart.
Take the Weight and Read Up to
the Favorable Grade Line
Now Read Over to the Retarder
Line
Yipes – Which Line?
That Depends on Your Gear and
Road
• If you are only going down a short
downgrade then you could be geared to
high or low range – read to either high or
low range.
• If it is a more sustained down grade or you
are regularly going downhill then don’t
overheat the retarder – use the continuous
curve.
Suppose it is a lot of downhill
You can go 18 mph.
Notes on Vehicle Travel Times
• The rimpull and retarder curves represent
maximum speed the vehicle can
mechanically achieve.
– You may have congested areas or sharp turns
where travel speed must be lower
• We normally brake haul roads into
segments when grades or conditions
change
– Each segment is assigned a speed based on
vehicle capability or another speed constraint
We Determine Vehicle Travel Time
for Each Road Segment
• Equation for Travel Time
Dis tan ce  in  feet
Travel  time  min utes 
MPH
)
• One can then add up the time for(88
all *
the
haulage
segment to get the travel time
• Another note – drivers seldom achieve maximum
allowed speed.
– One can divide by a driver efficiency to get actual time
– About a mile or more drive one will probably get about 90 to
95%
• Divide your time by (.9 or .95)
• Your travel time will rise to account for less than 100% perfect
drivers.