Weird Speed Limit - Unity and Metr

Im running 12s4p 30q pack on an off road board with dual 6880 190kv and 5:1 gear drive with 165mm (6.5") wheels.

The board has plenty of power however i cant get it to go over 26 mph. It feels like the speed is being artificially limited by something but i cant work out why.

Once i realized this i did a short speed run so that i could log some data and try and work out whats happening.

In this log you can see that i go full throttle at around 06:57:47.
I have reached the full motor current of 120A by 06:57:49.
Motors stay at 120A untill 06:57:51 where battery current reaches battery limit of 80A and motor current starts dropping.
As soon as the speed starts to reach 25mph at 06:57:53 both the motor current and battery current drop off.
After a few seconds im stuck at 25mph only using 40A motor current and 35A battery current, but it wont go any faster. Top 60% of my throttle feels like a deadband that makes no difference to the speed.

Try again and you can see the exact same series of events happen at 06:59:01

Im using modes in Metr but the maximum speed is set to 33mph which should be 59k ERPM on my setup.

Any ideas what is limiting the speed?

Have you ceheck the WATT & ERPM limit in the config?

i know the Watt is not being limited because during acceleration im pulling 80A at ~45V so around 4kW from the battery, but once at 26mph its only pulling 30A so around 1.5kW.

With regards to ERPM my maths says that 26mph should be around 47k ERPM. i have the ERPM limit set to 60K which i think should be around 33mph. Additionally i have this set in each metr mode and can verify that correct values are being sent because slower speed limit settings work correctly in some of my other modes.

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According to the calculator your max speed should be just around 27mph with those numbers at 85% efficiency.

Interesting. If that what is causing this then i didn’t expect it to be such a solid wall.

Previously i had a similar battery and motor setup on street wheels and i was able to go over 30mph. I chose this motor / gears / tire combo specifically to match my previous top speed.

I guess its possible that the air filled wheels are much less efficient and that is causing me to only achieve the lower estimate for top speed.

Maybe its time for me to order some larger wheels :slight_smile:

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Or make your Gear ratio more like 4.5:1 if possible…

If it is being caused by maximum speed of the motor then a larger wheel would be easier than changing the gear drive ratio, however ideally i did not want to change either of these.

Im a big guy and i like the torque that i get from the higher gear ratio. Currently i don’t have to slow down for hills at all. If I swap to larger wheels or change the ratio then i fear that my hill climb ability will start to suffer :frowning:

Urethane wheels will have around 90% speed efficiency but pneumatics will have around 75%…and the diameter can be slightly smaller depending on your weight and tire pressure



Guess this is probably the issue then. dont think i would of accounted for this when choosing parts.

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I think I had 71% efficiency on 8" pneumatics, but the battery was also ass so the efficiency was probably a little better

4 posts were merged into an existing topic: Off topic, debate , i don’t agree


On pneumatics?

@BillGordon this part of my post was on topic (and true)

^regardless of your tire size and kv, if you look at the no load ground speed values on this chart, and set your gear ratio accordingly (@ 42v), to a very close approximation you’ll get the same performance. so, for example, if you set your gear ratio such that your no load ground speed is 62.2mph, your top speed on flat ground with 60a battery current limit per motor will be approximately 57mph.

Sorry then. Was following flag protocol.

@BillGordon I’m slightly worried my other posts were mis-flagged as off topic as they all dealt with issues limiting speed. :stuck_out_tongue_winking_eye:

I understand, but others felt differently and flagged them.

Well this is on topic and useful information.

Did you make this spread sheet?
Is it available for others to play with anywhere?

You can see in the log you are hitting 95% duty cycle when your speed maxes out so that is the limit you are running up against/feeling. Run out of voltage headroom to drive more current through the motors as the back emf (kvĂ—rpm) added to the voltage drop caused by winding resistanceĂ—motor current approaches battery voltage.