Library / Terramechanics and off-road mobility / TER-01
Terramechanics and off-road mobility
Drawbar pull
The net pulling force a vehicle has left for towing or accelerating on level ground: the gross tractive thrust minus all resistances from the terrain and the vehicle running losses.
- Ref
- TER-01
- Rev
- v1
- Field
- Terramechanics and off-road mobility
- Result
- DBP [kN]
H is the gross tractive thrust: the total forward force the wheels or tracks develop against the ground. Rmotion is everything that resists motion on level ground: soil compaction, bulldozing, rolling resistance, internal friction and air drag. DBP is what is left for towing or accelerating. Grade and the towed load are separate terms in the full balance, H − Rmotion − m g sin θ − Ftow = m U̇. Count each loss once: if soil compaction is already inside H (a net thrust), do not subtract it again in Rmotion.
The net pulling force a vehicle has left for towing or accelerating on level ground: the gross tractive thrust minus all resistances from the terrain and the vehicle running losses.
Calculator
- Ratio of net to gross thrust, DBP / H0.625
- Share of the thrust lost to motion resistance37.5 %
- Motion resistance as a multiple of the drawbar pull, Rmotion / DBP0.6
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- With 40 kN of thrust and 15 kN of resistance, 62.5 % of the thrust is left as drawbar pull (25 kN). If the ground softens and the resistance doubles to 30 kN, the pull falls to 10 kN, a loss of 60 %.
- This is why vehicles get stuck in soft ground without losing any engine power: the resistance takes more and more of the thrust until nothing is left for the pull.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (kN) | Local step | Change in result |
|---|---|---|---|---|---|---|
| H Gross tractive thrust | 10 to 150 kN | −5 | 135 | 140 | +10% | +16 % |
| Rmotion Total motion resistance | 2 to 100 kN | −60 | 38 | 98 | +10% | −6 % |
Sweep of Total motion resistance
| Total motion resistance (kN) | Drawbar pull (kN) |
|---|---|
| 2 | 38 |
| 14.25 | 25.75 |
| 26.5 | 13.5 |
| 38.75 | 1.25 |
| 51 | −11 |
| 63.25 | −23.25 |
| 75.5 | −35.5 |
| 87.75 | −47.75 |
| 100 | −60 |
Sweep one variable
Variables and how each one contributes
Drawbar pull
What the equation gives you. Everything else feeds it.
Gross tractive thrust
The gross tractive thrust developed by the running gear against the ground, in kN. Adds directly. Because the resistance is subtracted, the drawbar pull changes by a larger percentage than the thrust itself. Raising it by 10 % (to 44) moves the result by +16 %, from 25 to 29 kN.
Total motion resistance
The total motion resistance on level ground, in kN, from soil compaction, bulldozing, rolling and internal friction. Subtracts directly. On soft ground this term grows quickly, which is what limits the pull. Raising it by 10 % (to 16.5) moves the result by −6 %, from 25 to 23.5 kN.
Worked example with the values above
- Gross thrust: H = 40 kN. Motion resistance: Rmotion = 15 kN.
- Subtract: DBP = H − Rmotion = 40 − 15.
- DBP = 25 kN, which is 62.5 % of the gross thrust.
Assumptions and limits
- Steady level-ground travel, with the thrust and the resistance measured over the same conditions of slip and sinkage.
- The resistances are counted once and in the same place as the thrust definition (gross or net) used.
Common mistakes
- Subtracting the soil compaction twice, once in the resistance and once in a net thrust.
- Using the engine force at the tyre as H on soft ground. The soil, not the engine, limits the thrust.
Statistics and uncertainty
DBP is a difference of two uncertain numbers, so its relative error is larger than either. With H = 40 ± 4 kN and Rmotion = 15 ± 4.5 kN (soil resistance is rarely known better than ±30 %) the pull is 25 ± 6 kN, an error of about 24 %. Measured drawbar pull from towing tests is far more reliable than a calculated value on soft ground.
Where it is used
- Towing, recovery and winch assessments for off-road vehicles.
- Comparing tyres and tracks by the pull they deliver on a given soil.
Maths you need
- Forces in equilibrium: net force = thrust − resistance
- Units: kN and N