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 resistanceAnd a full 40.7 ton load (total vehicle weight 150,200 lbs)?

We use theRimpull curveTo determinethis

Start by Finding Your Vehicle Weight

Most rimpull charts show theWeight of an empty truckAnd a truck loaded to fullWeight Capacity.

Otherwise start with theWeight down here.

Determine the Total Resistance

12.5% positive grade resistance working against the truck + 2.5% positive rollingResistance 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 isThe rimpull curve

Now Read Down the Maximum Speed the Machine Power Can

Sustain

We can go 5mph with a full loadUp 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% rollingResistance opposing the truck movement (rolling resistance is always against you)Give a total effective grade of -10% - minus means a favorable grade and the needTo 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

• One can then add up the time for 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.

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MPH

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