pic: Coaxial Swerve Drive Module with 2-speed Ball Drive and Nitrile Tread

[Kevin Ainsworth]

Originally Posted by Ether

Thanks for the clarification.

Couple of follow-up questions:

1) You are using the same 82% factor for both hi and lo speeds. I would have expected them to be somewhat different. Does your data show any correlation between gear ratio and actual-to-free ratio?

2) Do you have any actual data on swerve drives, or is your 82% number based on all non-swerve drives?

1) We haven't tested low speed since 2012, we really only cared about top speed and time to travel a distance up until now so that we can get an approximate speed. I therefore can't give an accurate real world relationship between gear ratio and free speed to theoretical speed. From a standing start to 20 feet our 2012 was 69% efficient in low and 50% efficient in high. These numbers are misleading because they aren't max speed they include acceleration time. We haven't been very scientific about our testing and I can only give an average or a specific value for a given drive train on an unknown date in an unknown condition. You have got me thinking and tire wear, gear wear, bearing wear, etc can all effect the efficiency. I really don't even want to relate my data to efficiency for that reason, I think there are too many variables that can effect this calculation based on how you got to your theoretical value.

2) Our 2014 swerve bot started with 5" OD tires and they are 4.6" OD now, that is a 88% to 81% efficiency variation. I am not sure what diameter they were when we tested the speed so I can only say the 2014 swerve bot was somewhere between 81% and 88%. I also can't prove we were at a constant speed the entire 20 feet.

Does a smaller wheel allow for a higher speed due to less torque required to drive the wheel (torque required at a given radius) so do they offset each other?

Does gear wear help at first to reduce friction and then hurt later when the gears start to wear out?

What effect does the gear ratio have like you asked?

What is the real world loss when an extra stage of gearing is added?
Should be around 4% per theory.

Our 2013 8WD had 40mm OD/28mm ID bearings has the least rolling resistance I can imagine (unless we attempt air bearings someday). But I can't test efficiency accurately because the aluminum Vex gears are half gone. It was only 70% efficient after testing it against the 2014 swerve bot but I know that it was much, much faster when it was brand new. We also calculate speed with nitrile tread at a compression factor. A nitrile tread that measures 4.25" at the OD is probably more like 4" at the base of the tread so
who knows who takes that into account in their calculations.

I guess in the end I can't trust a stated efficiency, there are too many variables that someone can make a mistake on when calculating their percentage. I care about real world performance. I just want to be able to predict the top speeds and time to travel a distance accurately.

I know that there are spreadsheets that have been developed for this and I think they are good tools to use.
I trust real world times over theoretical calculations.

Originally Posted by JesseK

Average top speed after "full" acceleration? Or after how many seconds from a dead stop?

Sorry for the nitpick, there's a reason for such a specific question, and either a forthcoming thread or a paper for a 'better rule of thumb'.

I hope you can gather this because I would be very interested in the data.
Maybe a standing start to 20 and 40 feet.
This would show acceleration and usable times to cover almost the entire field.

For our 2014 swerve bot: (free speed calculated = 13.3FPS)
2 seconds to accelerate from a standstill to 20 feet=10 FPS
1.7 seconds to cover 20 feet already at full speed=11.76 FPS
So it only added around .3 seconds to cover 20 feet from a standstill.

For our 2013 8WD dual CIM: (free speed calculated = 17.69FPS)
2.2 seconds to accellerate from a standstill to 20 feet=9.09 FPS
1.6 seconds to cover 20 feet (not sure if it hit full speed) =12.5 FPS
This bot is worn out, has bent chassis rails (scrubbing due to angles wheels).
Acceleration suffers but time to cover the distance is reduced.
I wish I had numbers for this one when it was new.

These tests were an average of three runs with stopwatches.
I would like to use the encoders and data logging to get more accurate results.

Originally Posted by Bryce2471

Challenge excepted! lol

In all seriousness, I've considered CADing a light weight ball shifting swerve module for quite a while now, so I'll try my hand at it, and see what I come up with.

I still curious about one more thing:
Have you done a BOM for this design? If so, what was the cost per module?

Bench racing is fun! And there is always a different way to do something.
Cost per module is probably pretty high, no data is available yet.
Looks like only half the parts have costs associated to them.