This is a series of photos to help explain what I'm trying to do with adding a spacer to the top of the shock and trading it for shock travel.
To explain the diagram and what your looking at.. The rectangle is the tyre, and the 173mm from the top of the tyre to the dotted line, is the arch gap. Where the 9.50mm is the shock, to simplify it just is attached to the lower arm and chassis via the dotted line, you can see the piston inside the shock. The angle is the driveshaft angle.
So this first image is "as is" now, when the weight of the vehicle, as you can see with the weight of the vehicle fully loaded the suspension moves down 9.5mm of the travel of the shaft, total is 100mm. As you'll see in the next photo, when fully drooped (all weight off and the shock at 0mm travel), which is 20mm down travel at the wheel, this shows there is a motion ratio of around 0.5.
Hopefully you can see the wheel has moved down 20.365mm, and the shock is fully topped out.
Suspension fully compressed, at 100mm shock travel. (note this isn't taking into spring bind, or lower arm hitting bump stop.. I'm fairly sure the bump stop will come first but need to measure this).
So in summary, in the stock position the following is true.
Wheel
Droop = 20.4mm
Compression = 189.8mm
Total Travel = 210.2mm
Shock
Stroke at ride height - 9.5mm out of 100mm (9.5% of shock travel).
The main point here is that your only 9.5mm down the shock travel, which is 20mm at the wheel.. as you drive down the road the wheel dropping away from vehicle more than 20mm will top out the shock, now imagine your going round a corner and the vehicle is leaning over, that 20 mm could be more like 10-0mm causing harsh ride and bad handling.
Also worth mentioning (and I don't know it to be a fact with the BP-51) but normally shocks close to the topping out have a high resistance zone to dampen the shock from crashing out, so running so close could actually cause the shock to give a horrible ride. Normally we aim to be sitting 1/3 of the shock travel at ride height for comfort.
Okey, so this is now the same 3 photos (well in order as above) but with a 10mm spacer added between the top of the shock and chassis, spacing the whole assembly down. SO it will add 10mm at the top, and will be 20mm at the wheel. (So will lift the vehicle).
So should go without saying that adding the 10mm spacer at the top, will not change the travel down the shock as the same preload is holding the vehicle up.
So at fully droop.. the same amount however as the starting position is 20mm down to start with, it's now 40mm. (This is where shaft angle is asking for trouble).
At 100mm shock position, you can see the wheel doesn't go higher than the arch now, its 3.7mm below (Again might not even get this high due to the stop).
So this doesn't help the shock position, as I'm still at 9.5mm down the travel. As expected its just moved the whole travel down by 10mm at the shock and 20mm at the wheel.
Wheel
Droop = 41.8mmm
Compression = 169.3mm
Total Travel = 211.1mm
Shock
Stroke at ride height - 9.5mm out of 100mm (9.5% of shock travel).
Okey, so guess to get to the point lol.. This is with 10mm spacer as previous but now reducing the preload by 10mm. The shock is currently at +10mm preload setting, so when I say reducing by 10mm, i.e setting it to zero or fully wound off.
So you'll see the shock is 19.50mm down the shaft now, which is getting closer to where I want to be, ideally 33mm down (33%, or 1/3), but much better then where I'm currently sitting. You'll also see that adding the spacer and removing 10mm preload has cancelled out, so the ride height is same first set of 3 photos, however so no lift, but I've traded the lift for shaft travel.
The difference you'll notice is, the shock now has more droop.. and the shock doesn't have the travel it had before.. but again if the bump stops or coil bind happens first.. I'm bring some shock travel into play that would normally be out of reach.
Fully Droop (Same as 10mm spacer alone with no preload) this is the most dangerous consequence of the mod.
Full compression, notice same 3.7mm to fender.
Wheel
Droop = 41.8mmm
Compression = 169.3mm
Total Travel = 211.1mm
Shock
Stroke at ride height - 19.5mm out of 100mm (19.5% of shock travel).
Summary
Adding a 10mm spacer, and removing 10mm preload, perfectly canceled out and doesn't change ride height.. however what it does is moves the position of the shaft and allows more downward travel, which is traded for compression.. however as I'm sure would never get full travel before coil bind or lower arm hitting the bump stop, this is part of the shock that is normally not used.
I know this is very long winded.. but I thought best to be detailed and not leave it open to misunderstanding and better to explain it fully.
My under standing that fitting 35mm spacers to gain lift is where the issues lies and why people think they are bad and to be avoided. but that is a 65-70mm lift so that is the issue and causing ball joints to bind.
Hopefully you'll understand why I need to give the shock more than 9.5mm of travel at ride height. IF I want to lift the truck further than this I would add more spacers maybe upto 15mm tops.. but I'm going to see what angles of the shafts cause an issue.. again getting a proper diff drop is the key to this without causing major issues.
I also want to install limiting straps on the front to protect the shocks topping out.
Let me know your thoughts
IF you make it to the end of all this, you've my kind of person