Can we replace a heavy steel chassis with a lighter aluminum spaceframe (or better, a monocoque), and how?

Not necessarily for drives, but for lifts that extra pound can absolutely make a difference. In In The Zone for example, I knew of teams that used nylon screws on their cone intake in order to reduce weight. The few grams that they save per screw wouldn’t make much of a difference on a drive base, but when that part of the robot is moving up to 5ft and down to the ground every few seconds, it all adds up.

This isn’t always the correct assumption. Many of the top teams will have enough time to optimize parts of their robots down to swapping steel screws for nylon screws as I mentioned above.

At a cost of losing nearly 25% of your pushing power. You were mentioning a cost-benefit analysis; there is nearly zero robot performance cost for optimizing weight, but there are significant trade offs to changing the gear ratio of a drive. To me, it would be a no brainer to use the lighter bearings, while I would have to debate with myself whether or not to change the drive gear ratio.


Again, I’m an empiricist and data guy at heart. When I see forum posts that say things like “Mecanum drives are slow/get pushed/can’t turn” and “steel chassis are terrible” without supporting evidence/documentation, it raises my suspicions. It could very well be true that the maths are right and X-Drive is ~1.4 times faster than tank.

Right, so I think what can happen is situational concerns can become over-generalized. Reducing a few grams of weight at the end of a 5 foot lever can have significant benefit. Reducing a few grams of weight in the hopes that the drive train will be faster seems misguided.

Maybe what I’m really driving at is: “Know the reason why you are doing things” and the corrollary: “‘Because the Internet said so’ is not a reason”. Data over Lore!

this verifies that a x drive is faster, because speed is inversely proportional to torque, so since this robot has greater torque when it’s in tank mode, it must have greater speed when it is in X drive mode.


With that established, let’s proceed empirically.

Generally, lighter robots are more reactive to driver input. In a straight line the difference may not be as large, but how often are robots driving in purely straight lines? Turns are significantly slower on heavier robots in my experience.

In a straight line? Not too much faster. Acceleration? Way faster. The drive base will be able to turn and change directions far faster than the full robot could ever hope to. This is verified through my 6 years of vex experience, having driven many drive bases with and without the rest of the robot on top of them.

The drive base in a tank configuration is visibly slower than in an x-drive configuration.

These are all issues that I have seen mecanum drives have in competition. I don’t know the math behind it (because I’ve never really looked at using them based on size grounds), but empirically this is the case.

In my experience, this is the case. Steel drive bases react slower to driver input than aluminum drive bases and cause motors to overheat much faster.

(switching away from full empiricism here)
How so? Math (and common sense) tells me that reducing weight will make the base acelerate faster, even if by a small amount. If a team has gotten to the point that reducing weight by swapping out steel screws is the most beneficial thing that they can do, then why should they not do it?

The teams that leave an extra few pounds on their robot because they didn’t feel like taking it off are generally not the teams that make it to freedom hall.


My son reminded me that we’d already run this experiment. We ran 2 chassis, both 200RPM Mecanum drive robots over 5 tiles (10 feet). One chassis weighed 5.4 pounds (just a chassis) and the other weighed 18.6 pounds (Tower Takeover competition bot).

Run chassis-only competition
1 2.94s 3.01s
2 2.95s 3.15s
3 3.08s 3.39s
avg 2.99s 3.18s

Times were gathered manually (e.g. Ready-Set-Go) on a stop-watch, rather than through automation, so some margin of error for human reaction time, both on the control and the stop-watch.

So a benefit of ~6% for a 13.2 pound weight difference. Assuming linearity of this benefit, that’s 0.5% speed increase for each 1 pound weight reduced.

Per the Vex site, a shoulder screw weighs 0.002 lbs; this seems a reasonable estimate for a screw+nut. So eliminating 500 screws would give a 0.5% drive speed increase.


like @sazrocks said, the turning and acceleration speeds are what makes the big difference, not long distance straight path speed. try timing how long it takes to spin 10 times around, probably would see a large difference.

First of all, this experiment is optimized for top speed, not acceleration as meng was talking about.

Secondly, 10 feet is unrealistic. Cut that back to 4.5 feet (rough distance that must be moved to get from the starting position to the autonomous line). Even with the numbers that you posted, 0.2 sec is plenty of time to make a difference when doing things like going for contested middle objects. Using some simple math, we can see that when the lighter robot crossed the finish line, the heavier robot was over 8 inches behind. This is again a large difference when trying to reach contested objects.

Additionally if you are willing to use a bit of math, this experiment is much easier. In code running on the robot, the moment the motors are started, start a timer. Stop this timer once it reaches the number of encoder ticks equal to the distance that you want to test. As long as the motor wheels do not spin, this should be very accurate and provide much more useful empirical data.


Idk, even if it makes hardly any difference I’d use aluminum (and always have) because it looks cleaner which is a very big part of being picked imo, if ur bot looks janky with a steel chassis but preforms just as well if not a bit better than a nice, clean, aluminum robot the clean bot is more likely to be picked in my experience


I think the takeaway here is that when it comes to vex, weight does not make a drastic difference, since the differences are minimal. Acceleration and speed differences only become noticeable when you push the motors to their limits. An 18 pound drive is no where close to the maximum weight that the motors can move. A more effective comparison would be between a 20 pound and 40 pound drivebase. Sort of how there isnt much difference in acceleration between throwing a tennis ball and baseball, but there is a massive difference in throwing a baseball and a bowling ball


And an aluminum chassis won’t deform, I pushed my wallbot into a wall with a 120 lb battlebot and it didn’t break :slight_smile: and it could even push the 120lb bot when it didn’t push back

1 Like

Fair. But assume a contest between a 18.6 pound robot and a 17.6 pound robot (and linearity in benefit) and we get less than a quarter inch difference in distance traveled, likely well within the difference between 2 drivers reaction times.

1 Like

This is the attitude that keeps teams from getting to the top IMHO. Accepting your premise, if your driver had a reaction time advantage you just gave it up. If your driver had equal reaction times to the opponents, you may lose out on position. If your drive had worse reaction times, this advantage is now 16 inches, which is basically a whole robot length and you definitely lost out on position. Matches at the highest level are extremely tight and are decided by these things. Maybe just one little thing at the very end of the match causes you to lose. Or maybe you fall behind as the sum of something like this happening a handful of times throughout the match. Regardless, you need to buy yourself every advantage possible. One might not make the difference, but the sum of the little things is what wins tough matches.


Perhaps most of what you say is true about getting to the top or making it to Freedom Hall, but this quote is the over-generalization that I think happens too frequently.

Where do you come up with a 16 inch difference? In all likelihood, you saw @sarocks comment that the 5.4 pound robot traveled 8 inches farther than the 18.6 pound robot in the same amount of time. This 13.6 pound difference in robot weight is context that is lost. I don’t know very many 5.4 pound vex robots that are competitive.

Obviously, everybody wants their robot to quickly accelerate and move around the field. But if you make the robot too light and gear your motors too high, you will be left with very little ability to push back against other robots.

If playing defense and counter-defense is a part of your strategy, then another important consideration is how much pushing power you can get out of your robot’s weight. This conversion is determined by the friction coefficient of the wheels that you are using.

For example, for 4" omnis it is about 1.3, which means that 15 lb robot on 4" omni wheels could generate 19.5 traction in respect to the ground, which is just slightly more than max 18 in*lbf force generated by 4 V5 motors with 200 rpm cartridges at 2" radius (more details here and here).


Good point; my son also measured the amount of pushing and pulling these chassis could do:

Push-Pull chassis-only competition
Push 47N 73N
Pull 47N 77N

Again chassis-only is 5.4lb and the competition is 18.6lb. So the heavier robot pushed with 55% more force but was ~6% slower. I’m not as confident that the pushing/pulling has a linear relationship to weight, but if so, it would be 4% more pushing power per pound gained while losing 0.5% speed per pound.

1 Like

Technik makes a good point here. If your friction is too low, then you will actually lose acceleration due to wheels slipping

1 Like

That’s simply not going to happen for your normal competition robot. In a pushing match, maybe. But I would avoid those.

Bottom line, there is no need for a steel chassis. The benefits of replacing some steel with aluminum may be marginal, but there should be no reason why aluminum doesn’t work

1 Like

I’m not sure how long you have been doing VEX, but I feel that you are misinterpreting how to actually be successful at a competition. Of course not everything will have the same percentage of effect per amount of time spent on it, thats a given.

But telling the students to focus on different parts of the robot because that hour of time will only increase performance by 3% for example is not the way to go about it imo.

A huge point that needs to be made here is that the best robot does not always win. You can perfectly optimize the robot using all of the “data” you have presented (which I would not trust given many differences even between two normal chassis’) and it still wont win you the tournament without the driver skill and reaction time that many hours of practice gives.

So what I am trying to get across is to optimize everything you possibly can given what resources you have, but don’t focus on the push and pull data from a flawed test you might have done. And as much as you can try to get the kids to do all of these tests and analysis in the end it is THEIR robot, so let them do what they believe is best, no matter the source.


This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.