Bigger motor, same gearing and kv, lower top speed?

DIY Builds
#1

Alright so my friend @Paul77 is having a weird issue and I’d like to help him get sorted:

He switched out his straight cut gears (64/23 - 2.78 ratio) and 6355 190kv TB motors to helical gears (75/27 - 2.77 ratio) and 6380 190kv TB motors and his top speed dropped from 36mph to 31mph. This seems like too much of a difference to be the 0.01 gear ratio change.

So what could it be?

  1. motor kv not being exactly identical?
  2. user error in programming values?
  3. helicals and straights transferring force differently?
  4. ghosts?

Just to ask more questions, @Paul77, you didn’t change wheel size or gain 50lbs during the change?

Your motor and battery settings are the same between the 2 setups?

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#2

on paper or real world? could just be an input error in the app?

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#3

See if you can check the motors true kv rating on the VESC tool

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#4

I appreciate all your help through this and, for real thanks for the thread start to figure it out.

Same wheel size same user weight, and the 6355’s I was running at 85A and the 6380’s I tried 85A, 90A and 100A to see if I could get more out of them but they are just 5-6 miles and hour top end slower. All other settings set the same.

I switched gears & motors around, and no matter what set up…slower

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#5

Definitely calculate kv for each set in vesc tool and see if there isna difference!

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#6

Where is that located to test KV, can you do it on the app?

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#7

My guesses, from most likely to least likely:

  1. Actual KV being different between the motors
  2. Duty cycle limit being set lower after upgrading firmware, possibly
  3. You counted the number of teeth on the gear wrong / was sent the wrong gears
  4. Battery no longer being full voltage
  5. ghosts
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#8

I can assure you 2,3,4 have all been ruled out by me multiple times

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#9

have you measured you top speed independently, aka GPS? same road conditions? Have you measured the speed of the old setup independently with GPS? Maybe you had the wrong settings before, who knows.

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#10

Did you take apart a skate that worked?

Sounds like motor Kv differences. Motor Kv is typically +/-10% from the labeling due to manufacturing variances.

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#11

This was my best guess. Do you know how to check motor kv via vesc tool or variESC tool? Seems like a good first step for the man to try out.

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#12

An example:

Typing “kv” in the vesc tool console while full throttling may or may not do the calculation for you

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#13

Sounds like that top speed drop roughly equates to the difference between a 190kv and 170kv motor. So, based on that, I would most likely go with what everyone else is saying. Definitely ghosts. :flushed:

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#14

Is there a way of calculating the KV with the numbers in the pic? This is unloading reading.

Screenshot_20210613-065814
Screenshot_20210613-0658141440×3200 274 KB

Screenshot_20210613-065848
Screenshot_20210613-0658481440×3200 332 KB

1 Like
#15

Calcs to ~163kv assuming 7 pole pairs and my quick math isnt trash and based on 49k erpm

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#16

Well that might be my mystery solved, they state 190kv

I will hook my 6355 up later and do the same thing to see where they hit.

How did you determine that from what #'s were given?

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#17

49000 erpm / 7 pole pairs = 7k rpm

7000rpm / 45v = ~156

156 / 0.95 to account for duty cycle cap gets you to ~163kv

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#18

Bookmarked and screen shot taken, thank you very much! That’s pretty far off 190, is that normal?

1 Like
#19

Definitely not, that would be fine for a motor sold as 170kv, not for 190kv.

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#20

I mean… It’s not that far from normal… Assuming the numbers we used / calced are correct. A ‘typical’ tolerance might be +/- 10% which puts a 190kv motor in the 170-210kv range… Given that i only used erpm to the nearest 1k and voltage as measured by vesc (which can be dodgy), our estimated kv could also be +/- call it 5%…which puts us pretty well within a normal tolerance

3 Likes