Motor Brush Advance

[bennelson]

Hey Tom, I have been thinking a bit about brush timing lately.

Any new info about your advancement, thoughts now that you have been driving it a bit?

[TomEV]

After a couple of weeks I put it back to where it was. Turns out (at least with this motor / voltage / controller / vehicle weight configuration) that advancing it made it somewhat slower (7.5 seconds 0-25 compared to 7.0 standard) and less efficient (about 300 wh/mile compared to ~250 standard).

There may be a happy medium somewhere, but without drilling a few more holes (or constructing a variable ring) where it was seems to be the best position. Eventually, I may drill holes at 5 degrees advance, and 5 degrees retard to see how those positions may change power/efficiency.

On a side note, it appears that this motor may be maxed out for this 1250 lb car.

Today I put 13” tires on the Citicar to see how it would affect speed and acceleration. For baseline, my Citicar has a top speed of 32 / 36 (normal / field weakened) and accelerates from 0-25 in an average of 7.0 seconds with 145R12 tires. (R in this configuration implies an 82 aspect ratio.)

I have a pair of tires from a 1981 Comutacar with 13” rims. They have 135R13 tires with the same bolt pattern as my 1976 Citicar. For the test, I put the 135R13 tires on the back of the Citicar.

Using a GPS, I determined that the speedometer was spot on with the 12” tires. With 13” tires, the speedometer registered just under 24 MPH when the Citicar was actually moving 25 MPH.

Acceleration from 0 – 25 (actual) remained the same with the 13” tires as with 12” tires – 7.0 seconds average (5 tests).

More interestingly, the actual (GPS measured) top speed was exactly the same (32/36) with 12” and with 13” tires, although motor RPM was lower with 13” tires. This leads me to believe the main limitation with my Citicar is not in gear ratio, maximum RPM of the motor, voltage, or amps available, but in the ultimate power of the motor. Last year, I put an extra battery in series (for 54 volts) and the top speed was also 32/36.

For reference, at 3000 RPM / 6.8:1 the Citicar is going 27.74 MPH, and with 13’ tires it is going 29.05 MPH.

Looks like I'm running out of ideas to improve speed and acceleration of this motor. The best / most successful experiment so far was field weakening... (other experiments were with increased voltage, increased amperage - direct contactor - increased tire size and advanced timing. All others experiments did not shown much/any gain. But again, this is with this particular motor... If your motor is not severely overloaded as this one appears to be, you may have better success...

If you just want a higher top speed, my money would be on field weakening, and a second step would be to adjust timing a few degrees positive or negative to see where your particular setup stands.

Tom

[bennelson]

Thanks for all the numbers.

I have always thought it would be fun to put a bigger motor on a Citicar, although a Warp13 may be overkill....

[MetroMPG]

Tom: boat tail! I suspect that even at 30 mph, more than half your power is being consumed by aerodynamic drag.

It would be interesting to see a Citi/Comuta car owner do a complete "rebody" job with aero in mind.

[bennelson]

Ok, so it's off topic of brush timing, but YES, I would LOVE to see somebody do a full new body of a C-Car!

Many of them have very beat up bodies, and it would be fairly easy to make a new one from scratch!

I am planning on going to the C-Car World Get Together next month. I will make sure to take lots of photos, and I'm sure I will see a body mod or two out there.

[rmay635703]

Originally Posted by TomEV

After a couple of weeks I put it back to where it was. Turns out (at least with this motor / voltage / controller / vehicle weight configuration) that advancing it made it somewhat slower (7.5 seconds 0-25 compared to 7.0 standard) and less efficient (about 300 wh/mile compared to ~250 standard).

On a side note, it appears that this motor may be maxed out for this 1250 lb car.

Today I put 13” tires on the Citicar to see how it would affect speed and acceleration. For baseline, my Citicar has a top speed of 32 / 36 (normal / field weakened) and accelerates from 0-25 in an average of 7.0 seconds with 145R12 tires. (R in this configuration implies an 82 aspect ratio.)

If you just want a higher top speed, my money would be on field weakening, and a second step would be to adjust timing a few degrees positive or negative to see where your particular setup stands.

Tom

If field weakening has an effect, a larger tire or different gearing or different voltage should have an effect.

Trouble is your WEIGHT and rolling resistance must also stay the same another battery must have added weight.

Was your amp draw the same with bigger tires? Its possible they had more rolling resistance.

My recommendation to you is to locate some geo metro 105x13" tires for $5 from a salvage yard (be very carefull mounting them), fill to 60psi (sidewall rating) and test.

What PSI do you run your tires at now? Remember their "rating" is at speeds in excess of normal highway speeds, the slower and lighter the vehicle the more air you can safely put in, if you aren't already test at a much higher air pressure, sure ride sucks but hey it does anyway? I would say 50psi-60psi, this is how I got my miles zx40 up to 30mph from 24mph.

My c-cars top speed is effected 5mph if I change from 60psi space savers to normal 40psi radials and my amp draw increases, I am sure you would see a different if you increase pressure.

Good Luck

[TomEV]

I thought that larger tires would increase the speed as well, but with field weakening both on and off, the speed was the same with it on/off w/145r12 tires. Probably the 3.5 motor is just at the end of its capability, and wind drag, etc. combine to defeat it going any faster than 36.

The tires are run at 40 psi, mostly because I can't stand the road noise and bumpiness - even at 42 psi it gets bad. I may try 60 psi for a test to see how much of a difference it makes. If it'll do 45 mph at 45 psi, I may just learn to tolerate a bit more noise. 8-/

I did a cd calculation, and the Citicar ended up being around .68 if memory serves. Not quite a brick, but close... If I can figure out a way to mount a temporary support structure to the body without making holes in the plastic, I may try a boat tail and see what change happens.

I should be at the c-car meeting in mid-May as well - see you there...

[bennelson]

I always thought an interesting aerodynamics test would be to bolt two citicars together - back end to back end.

Put the back one in reverse and the front one in forward and go. It would be a slightly more aerodynamic four-wheel drive vehicle!

It would also look suspiciously like a STAR TREK shuttle craft....

[TomEV]

... or the Star Tours shuttle craft at Disneyland. Maybe that's the mod needed - plasma drive - just as long as the plasma doesn't come out of the 3.5

[TomEV]

I took the motor out and drilled two more holes at 5 degrees advance. Hard to say if there has been an improvement, but there are a couple of interesting data points -

Over the past 56 miles I have been testing accel times and in general driving faster than normal. Even with the 'non optimal' driving, wh/mile is at 243. This is about the wh/mile I get by trying to be efficient.

During a longer than normal drive today (16 miles) the top non field-weakened speed crept up to 34+ MPH. The normal max is 32 on that stretch of road. And that was after about 11 of the total 16 miles, so the batteries were slightly tired.

Acceleration time 0-25 now ranges from 6.0 to 6.6 seconds. Normally, it would be 7.0.

Other than the weather being somewhat warmer these days the only change I am aware of is the 5 degree timing advance. I'll keep it this way for a while to see how it works out. Perhaps 10 degrees was too much, and zero is OK for the stock motor at its rated 36v, with five being a better fit for 48v...