Once every 50 controllers or so we hear of someone having a problem with rough or jumpy starts on a Zilla. Since this just came up again, I figure I’ll write a FAQ about it.
First off, the Zilla is designed to be a very smooth controller. Rough starts are not normal. Your foot, the motor, drivetrain, potbox and controller make up a large dynamic system. The complexity of the system sometimes makes it hard to figure out what is causing the system to be rough. I will go over the problems starting with those that are most common and ending with those we rarely see.
1 ) Starting in first gear:
In most EV conversions first gear is never used. When the torque load on a motor is small due to the low gearing, and the difference in motor current between no load and normal load on the motor is only a few amps, then the inertia of the motor overpowers the flexibility in the drivetrain (motor mounts, axles, tires etc) and it is hard to get a smooth start. In this case the smooth motor current control of the Zilla can’t help you much since so little current is involved. Unless you have a unusual configuration you should not use first gear at all.
2 ) Lack of proper return springs on the throttle assembly:
All vehicles should have two return springs where each one is capable of returning the pedal by itself. This is not only a good idea for safety, but reasonable pressure on the accelerator pedal resists the tendency of your foot to be jostled during acceleration. A pedal that is too light will cause rough starts. And no, the return spring that comes on the Curtis pot box is not enough by itself.
3 ) Lack of an accelerator cable.
It is common to think that low drag is better, but this is not always true on the accelerator pedal. Running a pedal with a rod linkage, that is, without a cable, causes two problems. First of all the lack of stiction makes the above mentioned bouncing foot problem worse when starting off. Secondly the fact that your foot is always shaking a bit wears out the pot right in the part of travel that is used while cruising down the road. So just a bit of stiction in the system is a important trait to design in.
4 ) Bad/Worn/Defective Potbox.
The potboxes that are currently available are of poor quality. I have seen a number of brand new ones recently that do not smoothly transition near the beginning of travel. This can be checked with a meter set for reading ohms while you slowly move the pedal through it’s range. Although it is easier to see this with an analog meter, it is still common to miss a problem internal to the pot since it only takes a small jump to make the car surge. Sudden jumps in resistance when smoothly moving the pedal indicate a problem. Sometimes replacing the pot (maybe temporarily with a rotary pot that is not affected by foot feedback issues) is the best way to rule out other causes. The pot input can also be tested for smooth response using the DAQ 4 to read the value as described in the owners manual and other sections of the FAQ. The first value of DAQ 4 represents the potentiometer reading in a hexadecimal format. The pot input will be read by the Hairball even with the key off, so there is no need to turn on the high power contactor to do this evaluation. If the roughness started happeing after some time, and especially if it is in cruise mode then it may be a dirty pot. A quick temporary fix is to stomp on the accelerator five times (with the key off!) and then go out for a drive and see if it is resolved. If this fixes it then there is a good chance that your pot is on the way out.
5 ) Poor potbox mounting:
The potbox should be firmly mounted to a area of the car that does not change position with motor torque. I have seen situations where a potbox mounted on the motor assembly, with a cable that was not quite flexible enough, would cause the torque reaction of the motor to feed back into the pot position.
6 ) Loose motor mounting:
A motor with very sloppy motor mounts or one that is missing a torque bar (when the car was originally designed to use one) can cause oscillations in the drive system.
7 ) Corrupt Hairball settings.
There is a factory testing setting that disables the smooth current control on startup. If your Hairball settings were ever corrupted for any reason (this would be indicated by a 1113 Error unless the error count is over 5) then it is imperative that you reset the defaults using the special menu and then re-enter your desired settings. If there is any doubt, then I advise trying this fix first since it is such a easy fix.
8 ) Bad solder joint in inside the Hairball.
We make errors in production too, occasionally we don’t catch them before shipping. It is not common, but we did have one case years ago where the filter capacitor for the throttle input was not soldered down properly. If you suspect this problem, contact us to get a RMA number so you can send you Hairball back to the factory for inspection.
9 ) Poor pot box wiring or connections:
I put this last since the Hairball is very unlikely to react to potbox wiring noise (I tested the design by wrapping a potbox cable around the M+ motor lead) but it is possible that the wiring connections are loose, or that someone did not use the standard twisted pair wire that comes with a potbox and decided to make an antenna loop out of the wiring instead.
As a side note, roughness is much less common when using our HEPA pedal option since it eliminates quite a few of the common issues causing this issue.
I hope this helps,
-Otmar