My team is using the KOP chassis and tried running the numbers through a calculator to predict the speed and we ran into numbers which seemed unrealistic. Can anyone help clear stuff up?
We’re using the 6 in KOP wheels with the Toughbox mini (16 to 48 and 14 to 50 gearings) attached to two CIM motors per gearbox. We got upwards of 10 ft/sec which is way faster than expected.
Can anyone help clear stuff up?
that sounds just right. You really want to go about 15 fps, of course 
What gear ratios would get you up to 15 fps?
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If you use the CIM motor speeds from VEX, then use the available gear ratios from AndyMark, and a bit of math with the wheel diameter, you can find linear velocity!
The gear ratio to linear velocity conversions for 6" wheels are also already listed in your manual on building the KOP chassis, but it’s more fun to learn the math behind it since it is applicable to all gearbox sizing.
Swap out the 16:48 to 19:45 or 24:40. Assuming you stick with TB-minis, you can easily buy the extra pinion and output gears to swap out. Scroll to “Optional Gear Ratios” down here. As stated earlier 10 ft/s seems reasonable but you want to hit 14-16.
Approximately 7.5:1 overall reduction for a 2CIM-per-side drivetrain. Just want to show you the math that would get you there. Warning: this should be used just to estimate. There’s a lot of factors that affect the speed of your drivetrain.
15 fps = 180 in/sec
6 inch wheel diameter * pi = 18.85 inch per rotation
(180 in/sec)/(18.85 inch/rotation) = 9.55 rotations/sec of the wheel.
From the CIM page on AndyMark, we find that the CIMs spin at about 5310 rotations per minute = 88.5 rotations per second. Assuming an 81% speed loss when the drivetrain is loaded[1] (pretty typical percentages), we can convert this to 88.5*0.81 = 71.69 rotations per second of the CIM.
Now we know our input speed and our output speed: 71.69 rotations per second and 9.55 rotations per second. Simply divide in/out and you get a reduction of 7.51:1. If you check this with JVN design calculator you should get the same answer a bit quicker :). To achieve this with a Toughbox Mini, I would recommend keeping the first stage gearing at 50:40, but then changing the second stage from 50:14 to 44:20. This will get you to an overall 7.86:1, which will be a bit slower at 14.39 feet per second adjusted, but that should be fine.
[1] When the drivetrain is running free, it will spin at a certain speed. In general, once you put the robot on the ground, it will spin about 81% slower due to the additional load from the weight of the robot, friction, etc.
Please let me know if I’ve made any mistakes here! I am sure I have :rolleyes:
I just want to add that the default ratio for the Toughbox Minis is set where it is for a reason. Your theoretical free speed will be somewhere around 13 ft/s, but your actual speed should be somewhere 11-12 ft/s. That is a perfectly good speed for this game, but I encourage you to find out what ratio works best for you. Faster does NOT always mean better.
Also, what speeds were you expecting?
We were wondering a similar question. With the KOP come this gear ratio table:http://pix4chiefdelphi.weebly.com/uploads/3/7/5/4/3754356/toughbox-mini-gear-combos_orig.png these values are at 75% free speed according to the published table
We would like to know from other’s experience what the trade-off is to go from the included 10.71:1 which is rated at 10. ft/sec, down to the 5.95:1 rated at 18.0 ft/sec.
How much slower will the 5.95 accelerate off the blocks?
Is there a greater battery amp consumption?
Do the motor controllers get hotter?
Can you “brown out” the bot in a shoving match sooner?
…and any other considerations we haven’t yet thought of?
Thank you for your assistance.
We are asking, because we have neither the time nor money to order the interchangeable gears AND try them out sufficiently to be confident that the above and other issues not mentioned might show up.
The answer to these is “yes”.
5.95 is really pushing what is reasonable with 6" wheels. Teams that run that kind of high speed usually do so with 6 motor or shifting gearboxes, and even then you need to analyze if the field will ever be open enough to take advantage of it.
We have increased the wheel size or gear ratio to go one level faster the past 3 seasons without issues. You can do 8.45:1 with the kit wheels and 4 CIMs without much concern. You can do it with traction wheels even if your drivers can remember not to get in pushing matches since you aren’t geared for them. I’m considering that for this season since we like to be able to not be pushed much from a side hit even though we have no intent of trying to push others. It works well for positional defense where you try to stay in front of and perpendicular to your opponent.
Was 44:20 just theoretical? Andymark doesn’t seem to sell 44 or 20 tooth gears for the Toughboxes on the All Ratios Gear Kit for Toughboxes page.
Yes, those were theoretical. I’m not too familiar with the Toughboxes unfortunately. I guess I assumed that any 1/2" hex bore gears could be used in the Toughbox Mini, which was potentially a bad assumption. Can someone confirm whether or not that is possible? If they can be, then the Vex 1/2" gears also share the 20DP and 14.5 degree pressure angle specs. If not then follow others’ recommendations.
Either way, thanks for catching that.
Thanks for the suggestion to go down just one level to 8.45:1. That helps our planning!
The second stage gears for the 8.45:1 configuration are 19t and 45t, so you’re only one tooth off.
