The commissioned mech-tank hybrid, pictured above, is a core component of a story I'm writing and I'm still not sure of some of it's specification. Things like weight and mobility, and I'm stuck trying to make it somewhat realistic for my readers.
There are a few hexapedal gaits that have been used in robotics. Wikipedia lists Alternating tripod, Quadruped, and Crawl (move just one leg at a time). Insects use the alternating tripod, which is stable because tripods are stable, and is good for covering rough ground.
The fastest (not most stable) is described here. Direct link to the video here; the summary is that it's quicker to bounce on two legs at a time, which yes brings the body up off the ground quite a bit.
Generally speaking, the lower the center of mass of a moving object, the lower the risk of it flipping when changing direction.
Translated to a vehicle, if you want to make a sharp turn, the lower the center of mass, the quicker you can turn, all the rest being the same.
This is why race cars are as low as possible and trucks sometimes flip over when turning too fast.
You can try a little experiment by yourself: put an empty plastic bottle on a tray and walk fast with the tray, then turn sharply. You will see that it is easier for the bottle to fall when it's laying on its short face than when it's laying on the long face.
In your case, the lower you can keep it, the faster it will be able to change direction. Since I assume you are not planning for having your mech going always and only straight, keep it low.
As L.Dutch stated, for turning purposes a lower profile is needed.
However for realism, I'd say speed is by far not the only thing to be concerned about! Also consider the following points:
- Keeping a small (dense) profile to present less surface area to be hit by enemy fire.
- Keeping low so the vehicle can take advantage of terrain variations to lower its visibility to enemies.
- A wider stance is more stable and precise when firing heavy ordinance.
- Spread out legs will be more stable in less ideal terrain conditions.
- A higher profile could be faster and possibly easier on fuel.
This will all have been considered by the engineers, so the lower stance would most likely be default with different stances available for higher profile situations.
Its faster high up, slightly bend legs. For turns you can always go into a lower stance if necessary, but considering the legs are already supports farther away from the main body it seems kind of a mute point.
With the legs bent you reduce the maximum length of a single stride. So unless you stretch the legs almost horizontally, which I don't recommend, the stride length will always be smaller if you use a lower stance. Additionally it is more likely to be slowed down by terrain if you use more horizontal legs.
So stand tall! Until your speed is less important than your visibility that is.
Also since people may bring it up: dont shoot the legs! Unlike tracks or wheels you can put armor on them entirely (even if its light armor) and the legs are the most erratic moving parts of the vehicle. If anyone tries to shoot them, have them shoot where the legs meet the chassis.
For an alternative idea, check out the Tachikoma from Ghost in the Shell.
The core idea is they have multiple legs (only 4 in the canon but no reason to not go to six for larger models) but each leg has its own wheel, presumably driven by independent motors (just like some modern electric cars have a motor for each wheel). This means they can walk when needed, but also just drive at high speeds.
If the legs are completly stretched out, no. If the legs are closer to the body and the body is closer to the ground, it would walk more efficiently and with greater force, but slower. However, you could use the back legs to move forward like a jump, like bird-like creatures do or grasshoppers.
In short, the human body (and other animals aswell) is composed of "biological levers" that work with a certain amount of mechanical disadvantage most of the time.
(I couldn't find any examples with insects, unfortunately)
The same is applied to wheels and for mech legs.
If you put the legs of your mech far away from the mech's body and walking very low, the extended legs would work like giant levers, the same could be said about fully extending these vertically. However, just like Marky stated in the comments: humans walk efficiently by falling and catching up over and over again. But I don't know if such walking cycle would work for a 6 legged creature.
So, if you know about mechanical levers, the tip of the legs will move faster, but with much less force, since the speed of movement is related to the torque and RPM of a car or, in this case, a mech, then it wouldn't go very fast, but it would lower its profile on the horizon.
However, the back legs would be able to work better if these were used to step as close to the body as possible and then pushing it forward. It would work just like a grasshopper or a bird jump/walking, and the other legs could be used "just" to keep it from falling instead of actually making a lot of effort to move.
(sorry for the big illustration, I couldn't find it in another resolution)
Unlike what people think, the bird-like leg is not actually a leg, it is the bird's ankle, the actual knee is closer to the body than to the ground. It is like they are perpetually on the tip of their toes, not particularly comfortable, but certaintly easier to run very fast.
So, the distance from the thighs to the ground could be seen as more or less a straight lever to the ground, if the ankle is centered to the axis of the thighs. It would be like crouching before jumping or running, you could put way more energy in the movement instead of your normal walk.
You could use such mechanism to walk forward, but eventually the back legs of your tank would still need to fully contract, it could be with the kness moving laterally or vertically, as long as the inital and middle movement part are closer to the body, such as the bird example.
Here is a video of a flea jumping in slow motion, its legs are similar to bird legs.
Of course, your tank wouldn't fly as high as a flea, but it could move forward more efficiently and faster.
And the same can be applied to grasshoppers.
(The back legs of your hexapod tank are already pretty "grasshoppy" for this mechanical movement)
The effect of long legs is the same as riding a bicycle: it requires generally more effort to move them if uphill but about the same or less on flat terrain, and if the legs move with the same angular velocity at the body, the speed increases with making them longer. It may require less energy to move with longer legs because this means less steps, and the step likely to include additional activity not relevant to pushing forward (lift the leg up for the step, reverse the direction, and the like). Hence a really fast robot I think should have long legs, as much as practical. There is such a term as "bicycle step", how far do you drive on one revolution of the pedals that is roughly equivalent to one step of walking. It is many meters, hence cyclists are much faster than walkers.
However, the longer the legs, the more difficult it will be uphill. Also, the creature gets wider and may not easily pass everywhere.