I want to be able to believe the numbers I get when I do drive train calculations. But I think I’m doing something wrong.

Let’s say I’m using 6” wheels and I want to optimize a robot’s gear ratio to travel 15 ft distances as quickly as possible.

**What I try:**

Plug the numbers into a spreadsheet (JesseK, JVN, etc) and figure out what ratio minimizes the travel time. I also plug them into a spreadsheet that I made that basically traces JVN’s first drive train spreadsheet (my sheet attached below).

Acceleration Spreadsheet.xlsx (110 KB)

**What I get:**

I find that a ratio of about 7:1, providing a theoretical top speed of about 20 ft/s, gets me there in 1.26 seconds from a stop. This is in a range of gear ratios that get me the approximate shortest sprint time. I plug those numbers into JesseK’s spreadsheet and get a similar time. According to the spreadsheet I made (mostly copying JVN’s original spreadsheet), the robot gets up to around 17 ft/s by the time it crosses the 15 foot line. And according to the numbers in my spreadsheet, it’s traction limited, because the gearbox can provide more force than my maximum tractive force in the first few instants of acceleration.

Those results seem wrong. What gives? I assume that a 20 ft/s drive with a CoF=1.1 is not traction limited, so what did I do wrong? Based on what I just calculated, my design decision would be to completely throw out those numbers and design something that’s perhaps 12-13 ft/s, because based on experience that seems like a nice speed that’s not too fast or too slow. I’d like to figure out what’s going on with my numbers so I don’t have to ignore the math.

**Turning**

I understand that in the end, the need to turn will constrain me into choosing a slower gear ratio than what I’d choose if I’m only considering sprint time over a certain distance. That means I can forget these problems to some extent. But I still want to understand what’s going on. If I’m designing for a strafe wheel (we may consider this depending on the game), I don’t care about the same sort of turning math anymore. Then I’ll be back to a case when I just want to know what ratio is the fastest over a certain distance. I want to know how the slower acceleration balances out with a faster top speed over a certain distance, and how hard we can push the motors without frying them or tripping breakers.

Speaking of tripping breakers,

**Current Constraints?**

I am basically ignoring the 40 amp limit, because the motor is going to spike initially anyway and the breakers can handle temporary overages. How should I decide exactly how much I dare to push those current limits? According to my spreadsheet, I’m pulling more than 40 amps for the first 0.70 seconds if I floor it from a stop, including a span of 0.29 seconds over 80 amps. That’s fine, right? The snap action breakers can handle (if I’m looking at the right spec sheet) a 200% overload for approx. 1.5-3.9 seconds, or 300% overload for 0.5-1.1 seconds. How much do I push that? Are there any good rules of thumb for figuring out how much you can push an RS-550 or FP motor?

**Inefficiencies **

Right now I have a 0.85 gearbox efficiency and a 0.93 carpet to wheel efficiency built into the spreadsheet. What other terms eat up power and slow down the robot? I lifted that 0.93 directly from JVN’s spreadsheet, and I have no idea if it’s anywhere close to a realistic number.

Thanks in advance for any guidance you may decide to offer.

Acceleration Spreadsheet.xlsx (110 KB)

Acceleration Spreadsheet.xlsx (110 KB)