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To verify your spring rate, add a ziptie to the the fork tube.  Make sure it is snug so it stays in the place it was last pushed too.  Once the zip tie is on the fork tube, you push it up to rest it against the upper fork. 

Bike sag:  lift the front fork off the ground and measure the gap (ziptie to fork gap) created by the front weight of the bike.

Rider sag:  Now push ziptie back up the fork, then sit on the bike and raise your feet so all of your weight is carried through the suspension.  Then carefully dismount and put kickstand down.  Measure the gap (ziptie to fork) created by your weight.  Add this gap measurement to the bike sag measurement, this is the total sag.  This measurement should be around 25% of the working range of the front suspension.   Now that is a rule of thumb, it can be higher or lower than 25%.  It depends on what is happening when you are loading up the suspension (upper rend). 

Checking the upper end by using the zip tie.  After riding, check the location of the ziptie, the gap from the fork to the ziptie.  Lets say the measurement is 5".  Then you would know the total max travel used was 5" + bike sag measurement.  That total measurement should never equal the total suspension travel amount (6", I think based on your posting).  If it equal, then you are bottoming out.  You would need to increase the preload and see if this keeps you from bottoming out.  If you use max preload and are till bottoming out, You will need stronger springs.  If you find you are only using 75% to 80% of the max suspension travel, you can reduce the preload spacer length some or you can  live with this.  If you are only using ~50% or less of the suspension travel, your springs are probably too stiff.  You should swap springs to a lower rate spring.

Hopefully, some helpful guides.

Jerry

If they can get close to an hour, the market can expand when recharge stations come on line; say the I-95 corridor.  If you can get a 200 mile ride in, stop for lunch and charge, then go, you can cover ~ 400 miles (or more if you stop again). 

Jerry

I do not have a Zero yet (perhaps in a year or so) but am studying the bike and like what I see.  Zero is doing an awesome job with design and manufacturing.  They are also putting in some really quality hardware, especially suspension and braking to go along with what they are doing in the power department. 

I like this new thread because someone out there is also looking at the geometry and asking a good question.  I don't know if I can answer the question, but thought I'd post some graphics from data I kept from another bike project (CB700sc) involving common bike metrics.

Pic 1.  Rake vs Trail.  A somewhat tight "V" design relationship.  Zero S/SR is the black dot with cyan ring.  I consider the Zero a naked bike (black dots).  The sport bikes are the red dots and the cruisers are the magenta dots way out there.  I would say Zero is in the V, but the low trail number makes it stand out.



Pic 2.  Wheelbase vs. Weight.  The tightest "V" design relationship I found; Zero picked a safe spot for the S/SR but low down the V.



Pic 3.  Trail vs. Weight.  A loose V design relationship and Zero S/SR is a bit of an outlyer because of the low trail but it still is in the V.



Pic 4.  Rake vs. Weight.  Another loose V and Zero S/SR is low in the V but in the middle.



Pic 5.  Trail vs. Wheelbase.  A "//" design boundary relationship(?).  Here the trail number doesn't stand out but looks like it is in the middle.  That said, the other manufactures seem to like the left side boundary.



Pic 6. Rake vs. Wheelbase.  Like pic 5 above, Zero S/SR is safely in the middle. 



So yes, the trail numbers are low, but not out of the norm.  The Zero numbers show they designed the bikes to be agile performers and they have given them quality components and a drivetrain that can really make the bike a great riding machine. 

You guys indicate the Zero has a low CG, but my visual inspection suggest the heavy batteries are high up in the bike, so I really wonder how "low the CG is on the bike.  What I would suggest on the bike is that it has has a tighter CG, that is it has a lower polar moment of inertia design, so the rake and trail numbers could reduced to take advantage if this design aspect to produce a nimble machine. 

As a side note, the 2011 S had a rake and trail of 22.7 degrees and 2.8 inches - MUCH smaller than the 2015 Zero S/SR.  They got away with these tiny numbers because it had less battery mass (even tighter CG / low polar moment of inertia).

If the tighter mass is an advantage to the electric bike, I would guess as power improves, it will overcome the gas powered bike in [short] races because it can out handle the gas bike.

Just a thought,
Jerry

Its a stock fork.  I'm not sure what the oil level is.  Is there any diagrams and specs on the fork.
I would think you would have to get a factory manual to help guide you, or you would google how to set the oil level for a USD fork (in general).  I am guessing you have worked on suspensions before and know how to do these kinds of things?  It isn't too difficult and the info on the internet is very good.

I did not think that the fork was pressurized or had any air spring.
It is not pressurized at rest.  But, when the fork compresses, the air [volume] above the oil gets compressed.  Here is a graphic of the work when I was setting up the front suspension on my HD XL1200T:


The dotted line represented the front fork (single fork leg) spring performance.  You can see the max capacity before bottoming out was 214 lbs.  When you add the fork oil, you can see the affect of the oil fill height; the more oil you have, the less air you have, so it gets compressed to higher pressures as the fork compresses.  The oem level alone raises the max capacity of a single fork from 214 lbs to 275 lbs - a 61 lb or 21% increase.

You can see if you add too much oil and fill to about 74% oil level (just a little over spec) the fork will not fully compress.

Zero customer service has still not gotten back to me, but I only emailed twice.  I might have to talk to a dealer.  Yuck.
I don't think a dealer is going to be much help [sadly] as they lack setting up suspension knowledge.  You have to talk to someone who knows what they are doing.  I was trying to help you see there are options without having to spend money.  You just be willing to play with the suspension a bit.  If you were close to N. Va, I could help you.
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