What is haul road rolling resistance, and why it drives cycle time
Every haul truck spec sheet gives you a rimpull curve: the pull available at the drawbar against speed, at a given gross weight. Compare that curve to the resistance the truck is actually fighting on your road and you get the speed it can hold, which sets your cycle time. Two forces make up that resistance. One is grade, which is fixed by the ramp design. The other is rolling resistance, and that one moves around depending on how the road is kept.
Rolling resistance is the force a tire has to overcome just to keep rolling across the surface in front of it, separate from any slope. A hard, well-bladed, well-watered haul road might sit around 2% rolling resistance. Let that same road go soft, rutted, or loose on top and it can climb past 6-8% without anyone changing the grade at all. For a loaded 240-tonne-class truck, that difference is the equivalent of adding several percent of grade resistance on top of whatever the ramp already costs it. The engine doesn't know the difference between fighting a slope and fighting a soft road. It just sees total resistance, and the rimpull curve responds the same way either time: less speed available at the same gear and throttle position.
Where rolling resistance actually comes from
It's not one thing. A few conditions do most of the damage on a working haul road:
- Loose, uncompacted material on the running surface, especially after a grader pass that hasn't been rolled
- Potholes and washboarding, which force the suspension to work and bleed speed on every cycle
- Standing water or a road that's too wet, which adds drag without adding traction
- A road that's too dry and dusty, where the surface breaks down under tire load instead of supporting it
- Poor crown and drainage, so water sits instead of running off and the base stays soft underneath
Any one of these raises rolling resistance on its own stretch of road. A haul route usually has several of them at once, in different sections, which is why two trucks running the same route on the same shift can post noticeably different cycle times depending on exactly when they ran it relative to the last grader pass or the last rain.
Why road condition matters more than people think for cycle time
Cycle time is built from load, haul, dump, and return, and the haul and return legs are where road condition bites. A truck that should hold 35 km/h loaded on a given ramp at design rolling resistance might only manage 28 km/h if that ramp hasn't been bladed in two shifts. Multiply that gap across a full shift of cycles and a full fleet, and you've got a fuel burn increase and a productivity shortfall that never shows up as a single alarming number. It shows up as a slow bleed in tonnes-per-hour that's easy to blame on operators, weather, or dispatch before anyone checks the road.
This is also why rolling resistance is the harder half of the resistance equation to manage from outside the fence. Grade is on the design drawings. Rolling resistance is a day-to-day, shift-to-shift condition that depends on maintenance crew timing, weather, and traffic since the last grader pass. An operator benchmarking their own fleet utilisation needs a read on road condition that updates as often as the road does, not a static assumption baked into a cycle-time model. An analyst trying to estimate a competitor's output from outside has even less to go on: no telematics, no payload data, just whatever can be read from the haul network itself.
That's the gap a daily, image-based read on truck counts and road condition is built to close. Mine Fleet Activity gives you a count of trucks moving on the haul network alongside a read on road condition, refreshed daily from VHR imagery, so a change in cycle time has a visible cause attached to it instead of just a number that moved.
If you're trying to separate a genuine productivity problem from a road that's just due for a grader pass, that's worth a look.