D. P. R. Chibikart Garage Hoonage

Over the weekend, I took Chibikart (and a few tagalongs) to the Ol’ Silley Vehicule Proving Grounds and took a few metered runs up:

It was actually slower than Chibikart1 by a fair margin, hitting only a 72 second best time, compared to Chibikart 1’s best of 62 seconds. On the whole, though, it was more efficient, consuming only 11Wh of battery during that run. The best product score was 784.8 Wh*s.

Neither result – that it’s slower but more efficient overall – is surprising. First, we already know that hub motors are less efficient than indirect drive systems – they have to pull more current, generally, to perform the same amount of work and being would for high torque also necessarily increases the motor resistance (for the same form factor).

But DPRC is slower because the Turnigy 5065 motors have a much lower torque produced per amp even after accounting for the 2.5:1 geardown between them and the wheels. From my adventure building the new motors, I know their torque constant Kt to be roughly 0.12 Nm/A. For the Turnigy motor, at 236 RPM/V, that translates to a Kt of 0.04 [1] – multiply by the 2.5:1 torque increase of the chain drive that that comes right out to 0.1 Nm/A.

This different alone isn’t enough – Chibikart 1 has four motors, for a grand total (lumped parameter) of 0.48 Nm/A, whereas DPRC only has 2 motors for a total of 0.2 Nm/A. Given that the 350W Jasontroller is safely limited to 25A output in all cases, DPRC can only produce half of the acceleration of Chibikart 1. But most of the garage race is spent flooring it, or at near-constant velocity, so only significant speed changes will contribute to the time. Hence why the discrepancy isn’t, say, 50% slower or something.

I have a feeling that Chibikart 1 on 2 motors will get a much worse result than DPRC – it’s only a ~16% time gain (7/6ths) for half of the available torque!