Getting to the bottom on the Thump

The Thumping diagnosed

After a great 2 hour session of trying to debug why Jane would thump loudly, lose power and stop.  It seemed like the controller was cutting power to the motor for some unexplained reason.  We ran it for 4 full minutes on jack stands while applying the brakes to simulate a reasonable load.  The motor was working as hard as it does on the local hills The brakes could apply enough to stall the motor but the thumping never occurred on the stands.  The mystery: what was different on the street versus on the stands.

Well today, first thing, I swapped out the 18V power supply that powers the Kelly KHB72701 controller.  Switched it back to a an old AC adapter with an 400W inverter, very Rube Goldberg.  First 6 inches up the drive and the thumping returned, as bad as ever.

So I opened the hood, had Carly gently apply some gas while I watched the drive shafts.  Sure enough, the driver side inner CV joint was slipping.  I could see the inner shaft rotate from the differential while the outer shaft slipped out about an 1/8" and then jumped back.  This was accompanied by the thumping sound.  Aha.

Here is a picture of what the CV joint looks like. 

When the car is elevated and the wheels drop, a few of the balls are pushed into the hub.  However, on level ground, the balls move out to the edge (like the ones on the left above).  When all six are near the edge, the slightest of force will cause all of them to slip out of their groove, only to jump back into the next slot, ready to thump again.  If the joint is at an angle, 1 or more of the balls will be deep in its groove and unable to slip.

As with anything, once you know what is it, it is easy to find information.  The following video shows another Mini having the exact same problem.

Jane was not thumping quite this bad, was was still violent.

Root Cause

Looking underneath, it became obvious the motor had slipped a bit forward.  Due to the electric, the motor only has one mount, the horizontal stabilizer and a strap holding it to the frame.  The strap allowed the  passenger side of the motor to slide forward about 1 inch, which caused the CV joint to align almost straight from the differential to the wheel hub when off the stands.  Well, it seems like having a constant angle on the CV joint actually prevents all of the balls from being on the edge of the grooves, where the slipping (groove jumping) can occur.  When the joint is perfectly straight, it is quite easy to see how the slipping could occur.  The horizontal angle was straight due to the engine misalignment and  when the car was lowered, the vertical angle would also line up near zero.  While on the stands, there was always a good angle at the CV joint, so slipping was impossible, explaining why the above test revealed nothing.  It is not logical that the controller would know whether the wheels were up and it seems like logic prevails, again.

Solution

So it is time to figure out a better mount for the motor which will prevent the movement.  With the motor sitting in the right position, the drive shafts look symmetric which is a great sign including a permanent angle on the CV joints.  Now, just to engineer a good mount.  With the motor out of position, one shaft was fairly straight (limited angle on the CV) and the other had quite a bit of angle.  Symmetry is a good thing.

If the new mount works, battery testing will resume.  A 600A ammeter is on its way.  This will allow for real current monitoring.  The Kelly Controller provides a 0%-100% current scale, but even under minimal load, it jumps to 20% and seem fairly rough at measuring phase current (Motor current).  The new ammeter will report battery current which when multiplied by the voltage will yield instantaneous power, the real indicator of how things are working.  I am intrigued to be working with such large currents, when most of my career has been spent in the milliamp range of the scale.  A 600A fuse is impressive.