# How fast can we go?

Seeing as securing the stacks of balls will be important to strategy, how fast will this season’s drivetrains be? I would expect faster than 1:1.6 for the better teams maybe with transmissions for pushing especially for the robots that are light to accommodate elevating. Where might the limit be?

I am doing an X-Drive Motor-Wheel with a speed internal gearing on my motors… Havent actually calculated it yet.

If anyone wants to use it, I made a table with the different direct drive ratios. I haven’t yet uploaded it elsewhere, but it is still here:

That’s a very useful table, thank you.

Not saying your idea is bad but many teams plan on making a ramp and an X drive might not be the best way to get up it.

We have multiple teams at our school some are working on ramps and it’s working right now

It all depends on how many motors you put on your drive. If you want to be able to drive defensively, than either 4 motor 1.3, 6 motor 1.6, or 8 motor 1.95. If you’re all about speed, then do 4 motor 1.6, 6 motor 1.95, or 8 motor 2.4. These are all direct drive ratios with 4" or 3.25" wheels.

Transmissions are always an option, they’re fun to build and they might even end up being useful in this game. Historically, though, the simplicity and efficiency of direct drive drivetrains have come out on top.

Actually if you look at the chart provided above a Holonomic X Drive with Speed internal gearing is a 1:2.26

Sorry, I’m talking tank drives

Some of these values seem off. Like how can a 4" wheeled X drive ever outrun a 5" tank drive robot with the same gear ratio…

Ditto to a few other quick comparisons made. How are you calculating the x drive speed?

The reason why the x-drive is faster is because the wheels are at an angle and move in both dimensions compared to the robot at once, making the robot go ~1.4 (root2) times faster

This should give a better explanation:
http://www.aura.org.nz/archives/1137

I predict the fastest for the high/middle school division would be a 6 motor 2.4:1 speed on 4" wheels since that gives the same torque output as the reliable, internal 1.6 to 1 speed ratios. The challenge there is how to make an intake which can keep up with the speed of the drive and a shooter that can keep up with the intake

I remember this being a strong discussion a while back. Did anyone ever end up proving this empirically?
When trying to take this article and apply it to a real life robot, the assumption that the x drive will travel vertically (in their first picture) at the same speed (free speed of the motors) as the tank drive, considering the x drive will only have two motors to the tank’s four in the same direction, is a bit faulty in my opinion. I think an x drive isn’t just a gear train, but also a power sink. You will gain the strafing mobility at a cost of a portion of your motor power. Less motor power means less top speed under the same loading conditions.
Everything in the article is correctly done, I just don’t think it actually applies to real world conditions.

Has anyone ever tried doing a side by side comparison of this? Have 2 nearly identical robots, the only difference being the drive, and see which one is faster.

Although an 8 motor drive can easily go up to 1:3 4 inch wheel internal high torque, I would say that anything above 1:2 is not so easy to drive and master. Plus, you need that slow speed and low momentum for precision in autonomous.

As for stacks of balls, I would approach that similarly to stacks of sacks: top roller, drive forward, suck in the base ball, wait correct amount of time, and then drive forward to suck all of them in. Special mechanism for picking up stacks amused me for a while, but with the no expansion rule that might be a little hassle to design. Nonetheless I am excited to see skill bots designed to pick up and shoot 4 at a time.

Not really a side by side comparison, but AURA also posted http://www.aura.org.nz/archives/1141 a while back. It’s a pneumatic shifting x-drive. While driving around you can see the difference in speed between tank drive mode and x-drive mode when it shifts.

That’s not quite what the original question was regarding I don’t think. The point was that an X drive while moving in a diagonal (if viewing from the X view) is only using power from 2 of the motors. In this sense, it would probably move slower than a classic tank drive simply because of the number of motors providing power, not gear ratios. In all other movement, when all 4 motors of an X drive are being used, the X drive should be faster because of the addition of both vectors of movement.

That is sort of what I am saying.
When you are designing a drive train, it will have the highest top speed if you gear it such that the motors operate at max power at that point (50rpm/6.75in*lbs for a 393 torque configuration).
If you have a tank drive geared to operate at maximum power at top speed and put it next to an x drive geared to operate at maximum power at top speed (note that the gear systems in each of these robots is different; both motors are running at 50rpm) the tank drive will have a higher top speed. This assumes that the “repulsive forces” in the direction of travel (bearing friction, back emf, gear friction, etc.) for each robot is the same which should be fairly valid in VEX.

I am not saying an x drive is slower than an identical geared (external gears) tank drive.
I am saying that a optimally geared tank drive is faster than a optimally geared x drive.
If you are shifting between the two (like in the above video) then everything I said goes out the window because both drive trains are not optimized.

I think this is a valid statement. The VEX motors (geared the same) output the same torque no matter what drive train you have. Since the x-drive has an inherent amount of friction due to the nature of how it works, I think it’s fair to say that an “optimally” geared system for each would result in the tank being faster (and more efficient).

The X-drive isn’t popular because of its speed/efficiency, I think it is used because often its maneuverability is worth the sacrifices.

Actually, the x-drive is more efficient than most people give credit for. Straight omni-drives require a lot of motor power to turn, while x-drives turn more slowly with less strain on the motors. Often, the limit on the drive speed (for straight drives) is in its turn rather than in its forward movement.

Looking back at the toss up season, the fastest 4-motor drive was an x drive (by team 7682)!