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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?

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    $\begingroup$ Am I correct in thinking that tanks have suspension on each wheel and bulldozers generally don't or have fixed wheel arrangement? $\endgroup$
    – Transistor
    Mar 27, 2023 at 17:29
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    $\begingroup$ Bulldozers are meant to provide fine control of the blade when cutting grade, ripping or pushing other equipment. They often encounter rocks in this work, which lead to high forces transmitted to the undercarriage, frame and blade. The momentum of contacting rocks at high speed with certainly damage the operator or the machine. $\endgroup$
    – Takala
    Mar 27, 2023 at 22:50
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    $\begingroup$ @Transistor For tanks the answer is generally yes (barring some very early designs), though the suspension for each wheel is not always fully independent of the other wheels (see for example the vertical volute spring suspension or Horstmann suspension systems used on many WWII-era allied tracked vehicles). Bulldozers typically instead have fixed rollers and idlers because this results in a much simpler (and easier to maintain) design. $\endgroup$ Mar 28, 2023 at 0:28
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    $\begingroup$ @ToddTakala: That's the reason you wouldn't want to use a high gear (if one existed) while the blade was down, but the OP wasn't suggesting that. They were suggesting having a higher gear for just driving between places where bulldozing needs to be done. ("for mobility"). The "why not" answers to that question are different (but do exist). $\endgroup$ Mar 28, 2023 at 2:42
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    $\begingroup$ Dirt does not mind waiting. $\endgroup$ Mar 29, 2023 at 5:58

7 Answers 7

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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.

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    $\begingroup$ In addition to this, depending on the application of the bulldozer, such as pushing material down a steep slope, it would be too dangerous for a bulldozer to be capable of high speeds. $\endgroup$
    – Fred
    Mar 28, 2023 at 0:12
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    $\begingroup$ @Fred hypothetically you could interlock an overdrive gear (you've got the power so it would be just gearing) to the blade, so you can only go fast with the blade up $\endgroup$
    – Chris H
    Mar 28, 2023 at 12:22
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    $\begingroup$ Also never underestimate the human error. If you have the option that the bulldozer can go fast, there will be at least one operator who tries to go fast when slow is required, regardless of training, instructions or common sense. $\endgroup$
    – Zibelas
    Mar 29, 2023 at 11:27
  • $\begingroup$ @Zibelas given the number of car accidents caused by pressing the accelerator instead of the brake, your point is a good one - that's why I mentioned interlocking, but of course it doesn't cover lots of other risky situations $\endgroup$
    – Chris H
    Mar 29, 2023 at 15:45
  • $\begingroup$ @Fred Not only that, but also for the general safety of other workers. Speed limits on construction sites or for construction equipment are sometimes mandated by law, or otherwise heavily recommended. $\endgroup$ Mar 30, 2023 at 13:19
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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.

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  • $\begingroup$ I totally agree that transporting heavy-tracked vehicles on a semi-trailer truck is much more economic than letting them do it all the way on their tracks. However, in some combat scenarios, you can't deploy the truck all way to the frontline and need that the tanks or bulldozers will travel some way on their own. In civilian applications, a bulldozer can be transported and unpacked by a truck all the way to its work zone. $\endgroup$ Mar 28, 2023 at 17:32
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    $\begingroup$ @Triceratops this is why tanks are made to run at 100 km/h. Bulldozers sometimes happen to crawl few hours from the paved road to the work zone and this is still cheaper than making them move faster. $\endgroup$
    – fraxinus
    Mar 28, 2023 at 20:21
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    $\begingroup$ @Triceratops What top speed do you think vehicles need in a work zone (i.e. shared pedestrian/vehicle area)? In similar areas e.g. parking spots the max speeds are usually limited to 5-10 km/h to reduce probability and severity of accidents. $\endgroup$ Mar 29, 2023 at 13:23
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    $\begingroup$ @user3819867 one could always benefit from the higher maximum speed when not in a shared area. and still drive carefully when other obstacles are around. But this "always" is in practice quite rare. This could amount for e.g. 10 minutes per day saved for 2x the maximum speed and these 10 minutes will cost e.g. 2x more expensive bulldozer (different engine, transmission, suspension, tracks) and 30-50% more operating expenses. Not a deal, really. $\endgroup$
    – fraxinus
    Mar 29, 2023 at 14:11
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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...

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    $\begingroup$ Because they have different budget constraints. Will you pay for the servicing? Fancy running one of those tanks you mention as your daily driver to work? any idea of the fuel and maintenance required?" $\endgroup$
    – Solar Mike
    Mar 27, 2023 at 12:30
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    $\begingroup$ @Triceratops It isn't really obvious why a military bulldozer needs to be as fast as a tank. If a bulldozer needs to move long distances you put it on a truck. Otherwise when actually doing work it only needs to move slowly. $\endgroup$
    – Eric S
    Mar 27, 2023 at 18:04
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    $\begingroup$ @Triceratops "So why tracked combat tanks can develop a speed of > 50 km/h". When it's your money, and you don't need for a bulldozer to go 50+ km/hr, are you going to pay 10x more for a 50+ km/hr bulldozer? $\endgroup$
    – RonJohn
    Mar 27, 2023 at 18:23
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    $\begingroup$ @MichaelRichardson Combat engineering vehicles exist. They're tanks whose modifications include dozer blades on the front to clear mines and destroy earthen fortifications. M60 based en.wikipedia.org/wiki/M728_Combat_Engineer_Vehicle M1 Abrams based: en.wikipedia.org/wiki/M1150_Assault_Breacher_Vehicle Leopard 2 based: en.wikipedia.org/wiki/AEV_3_Kodiak T72 based: en.wikipedia.org/wiki/IMR-2 $\endgroup$ Mar 27, 2023 at 21:16
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    $\begingroup$ I would think another constraint would be the environment. Normally, I see bulldozers being used with other people around, and the visibility isn't that great. I don't want one moving at 30mph. Tanks on the other hand... That squishing-screaming noise was probably an enemy combatant, so that's fine. $\endgroup$
    – user34314
    Mar 27, 2023 at 21:25
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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.

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    $\begingroup$ Tanks are heavy too. They just use huge engines (or even turbines) to compensate. $\endgroup$
    – Michael
    Mar 28, 2023 at 12:44
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    $\begingroup$ @Michael True. I guess it would be more reasonable to say "it's harder to make a heavy vehicle drive fast", especially with tracks. $\endgroup$
    – jpa
    Mar 28, 2023 at 14:25
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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.

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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.

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  • $\begingroup$ So why don't they also have high gears? $\endgroup$
    – user253751
    Mar 30, 2023 at 12:08
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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?

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