Lowering: When optimizing for wheel bump travel doesn't work

Context
Reverse trike on double-wishbone IFS with coilovers

Objective
Improve lateral grip
Decrease roll

Method
Reduce load transfer
-decrease CG
--lower ride height
Current static ground clearance: 6"
Target static ground clearance: 5"

I initially expected to upgrade to a threaded body shock of the same 16" extended length to preserve the smaller collapsed length to maximize available bump travel.  I believe I read Keith utilizes this methodology?

I would not expect that the next larger shock, sacrificing not only maximum wheel bump travel (due to longer compressed length), but also sacrificing the available shock extension travel due to packaging, would yield anything of interest -- but that's not what I found:

Current static ground clearance of 6" - target static ground clearance of 5" = 1" drop @ wheel * 0.84375 motion ratio = 0.8" @ shock
Current 16" extended shock rides at 15.5" - 0.8" = 14.7" new shock ride

Replacing with a threaded body shock of the same 16" extended length (11" collapsed / 15.87" extended) gives:
15.87" extended - 11" collapsed = 4.87" usable stroke
14.7" - 11" = 3.8" bump / 4.87" usable = 75% bump
15.87" - 14.7" ride target = 1.2" droop / 4.87" usable = 25% droop

Replacing with an 18" shock (12" collapsed / 17.87" extended), travel would be limited by packaging to 17 1/8". However,
17.125" packaging - 12" collapsed = 5.125" usable stroke
14.7" - 12" = 2.7" bump / 5.125" usable = 52% bump
17.125" - 14.7" ride target = 2.5" droop / 5.125" usable = 48% droop

Does that look right?

What would you do for a 100% street performance vehicle: 3.8" bump : 1.2" droop, or 2.7" bump : 2.5" droop?

Can you move the top of the shock up so you can get more bump with that longer shock?

Do reverse trikes have any particular weird dynamics that makes them want more of a particular type of travel?

I usually prioritize bump over droop, but you need some - and less than an inch is pretty minimal. 

In reply to Keith Tanner :

The packaging is already quite tight, and I'm not sure more bump would be usable -- see below.

I'm not sure of the dynamics, but I mentioned in the review thread that the stock ride is atrocious.  Finding that the shock droop lightly-laden is only 0.75" ( = 1.2" at the wheel), I wonder if that severely constrained droop travel is the cause of the poor ride quality.

I've revised my post to reflect that I am now targeting 5" static unladen ground clearance for serviceability, so actual laden ground clearance would be around 4-4.5", and thus even more biased towards bump than the numbers above.

With the longer shock fully compressed to 12" is 2.7" bump travel from ride height; that's 3.1" at the wheel.  If my static unladen ground clearance was 5", that puts the frame rail within 1.9" of flat ground.  That might be a good place to stop?

Because with the 11" shock, that's 3.7" bump at the shock and 4.3" at the wheel, within 0.7" of flat ground.

Once you're laying frame, you're out of bump travel :) I think you're on the right track. 
 

Lack of droop in ride quality feels choppy over crests. It's not so much that bumps are harsh but the car keeps checking itself when trying to "flow" upwards. 

In reply to Keith Tanner :

I thought I had this figured out, but I had the motion ratio wrong.

5" stroke gives 1.2" = 25% droop, 3.7" = 75% bump to within 0.7" of flat ground.

6" stroke gives 2.5" = 48% droop, 2.7" = 52% bump to within 1.9" of flat ground.

Now I'm not so sure.  This is unladen -- won't adding load decrease bump and increase droop?

Yes, with a vehicle where the occupants are a significant percentage of the overall weight you'd probably be best off looking at the loaded scenario. 
 

I have to think your limiting factor is ground clearance though. Less than an inch of clearance at full compression doesn't give much margin for rocks or sidewall squish. Are you taking bumpstops into account?

In reply to Keith Tanner :

Thanks Keith.  Comparing them side-by-side laden and with various spring rates, the shorter stroke and compressed length could settle right in the 60-70% bump travel sweet spot.

The shock dimensions account for the integrated bump stop cap on the shock.  I'll certainly be doing a lot of zip-tie data logging.

So it seems your method to optimize wheel bump travel first was indeed superior after all.