Why are bulldozers so slow?

Question

Tracked-type tractors and bulldozers are very slow engineering vehicles. Their top speed is less than 15 km/h and when pushing it is even lower (< 5 km/h). For example: the Caterpillar D9's top speed in forward gear is 11.0 km/h while in reverse 13.5 km/h, while in 1st gear it is 3.5 km/h.

From elementary physics,

$$ P = \frac{dW}{dt} = \frac{d}{dt} \int_{\Delta t} \mathbf{F} \cdot \mathbf {v} \, dt = \mathbf{F} \cdot \mathbf {v}, $$

which means that for a given output $F = P/v$, that is: to increase the force one pays with decreasing speed.

I know that a vehicle drag force or pushing force is expressed in terms of torque (in units such as newton$\times$meters), but I don't know how to relate that to the previous formula other than putting the torque $$\tau = |\mathbf{r} \times \mathbf{F}| = r \cdot F = r \cdot P/v$$ which says that in order to increase torque one needs to decrease speed.

However, why can't we engineer an engine with a planetary shift that has both low gears for the maximal force at low speed while having also high gears for a reasonable speed (> 30 km/h) for better mobility?

Answer 1

I wonder why can't we engineer..

We CAN engineer a vehicle which is both fast, and has a lot of torque. (E.g. a tank, as you mentioned yourself)

The reason why we don't do this for a bulldozer (I.e. "Why bulldozers are so slow?") is because doing so is unnecessary - a bulldozer does not need to be fast - and it would be more expensive.

Answer 2

From memory, but pretty much realistic: a tank will use e.g. 1000 liters of diesel fuel just to move 100 km and will probably require a track maintenance afterwards. A truck with the same tank on its back will do the same with 50 liters of diesel and will be good for another 20000 km with the same set of tires.

Tracks are impressively expensive to operate both in terms of energy efficiency and wear/maintenance. This is even worse at speed.

Tracks are also neither too much reliable (especially at speed) nor capable of graceful failure. Losing a track is a common failure mode for a tank and in a battle this amounts to losing the tank.

All of the above combined forces tank (or bulldozer, see below) operators to transport them on trucks and trains as much as possible. It is simply cheaper to operate a whole additional heavy truck compared to running long distance on tracks.

This is also why even the expenses-tolerant military technology feels a great deal of pressure to migrate as much as possible equipment on wheels (just watch the news).

Bulldozers (and the other track-based equipment like excavators) are not much different, except that they can be made way cheaper by limiting their maximum speed to 15 km/h instead of 80-100 km/h for tanks.

Answer 3

Because the tracks or tyres and suspension needed for sufficient ground contact when pushing, dozing or grading does not match the suspension etc needed for higher speed situations.

Even the suspension and tyres used in F1 are not suitable for ordinary road use.

As for engines with low and high gears then there are several vehicles with 2, 3 and even 4 ranges of gears. Some have low and high with a splitter to give 4 ranges effectively. Others end up with 18 or 24 speeds etc...

Answer 4

They need to be heavy for traction, which makes them too heavy for speed.

To have enough friction between ground and tracks, the machine needs to be very heavy. The Caterpillar D9 you mention weighs 50 tons. With a typical coefficient of friction of about 0.5 between the tracks and ground, that puts a limit of 250 kN on its pushing force. The manufacturer specifications are in the 150 - 200 kN range depending on attachments.

With so much weight on the tracks, they wear out fast. They also wear down the ground surface, you wouldn't want to ride on public roads. The faster the speed, the higher the impulse forces.

Answer 5

You’d just need a lot of engine power and all the suspension, drivetrain (including tracks), brakes, safety equipment (seatbelts, airbags) etc. would have to be designed for the high speed. Just changing a few gear ratios won’t be enough.

Tanks need huge engines to achieve their speeds. Especially in early-ish tanks engine power was a big limitation for tank speed and lots of tanks didn’t manage to reach the speeds which were initially planned. Tanks also need a lot of maintenance and are still quite unreliable.

Answer 6

The objective of a bulldozer is to apply very large forces to very heavy loads of rock and soil. This requires a large engine combined with very low gearing mounted in a massive structure to generate the required shear force against the ground surface to push the load forward. This means the bulldozer moves a huge load at the speed of a slow walk, in normal operation. The power required to move a fully-loaded bulldozer blade at, say, 30 MPH would far exceed the available output of any engine of practical size and cost and so no one builds them that way; the application simply does not require those speeds in any case.

Answer 7

Where would you be driving it to? Would you be taking a paved highway? Have you ever noticed the grousers on the track pads? Would you want to drive your car on such a highway after dozers have made a few thousand trips over it?