School me on hydro steering

[HYDRODYNAMIC]

The problem with the math side is, it starts with a number of assumptions and guesses. I'm not saying don't do the math, in fact, quite the opposite. But understand the why behind.

Here's how it's "supposed to be done":

.

Here's how it's REALLY "supposed to be done": WELL REALLY JUST ANOTHER WAY

Step 1: Tire up against a rock needs to push the vehicle sideways and break traction of both front tires assuming tires are on flat dusty or dirty rock with a coefficient of friction of .5 .
Given: 40" tire on a 5000 lb rig with 3000 lb being on the front end. 3000lb load x .5CF = 1500 lb load x 20" ( half of tire) = 30,000 moment / 6" high steer arm = 5,000 lbs force.

Step 2: Calculate cylinder area
2.5" Bore - 1.5" Rod = 4.908-1.767= 3.141 Sq. In.

Step 3: Determine cylinder force
3.141 sq. in. x 1800 PSI (CBR pump with external relief) = 5653 lbs force, THIS IS GREATER THAN THE FORCE REQUIRED SO YOU HAVE ENOUGH CYLINDER AND PUMP FORCE

Step 4: Determine cylinder volume
8" cylinder travel x 3.141 sq. in. = 25.1 cubic inch lock to lock

Step 5: Select steering valve displacement
25.1 cubic inch cylinder / 9.51 cubic inch orbital = 2.64 turns lock to lock
156cc = 9.51 cubic inch /

Step 6: Calculate pump flow
The PSC CBR XR pump is 11.3cc/rev = .689ci/rev
The pulley size is close to the crank pulley guessing 1:1, if over or under driven, flow will change
at 4000 rpm 11.93 GPM
at 2000 rpm 5.97 GPM
at 1000 rpm 2.98 GPM

Step 7: Calculate orbital speed
8" cylinder travel x 3.141 sq. in. = 25.1 cubic inch lock to lock = 25.1 cui / 231 in/gal = .108 gal
5.97 gpm pump output / 60 sec/min= .0995 gal / sec.
.108 gal /.0995 gal sec = about 1 second lock to lock if driver can turn the wheel fast enough, 2.64 turns
Not that you need to turn lock to lock in 1 second, but you do need to turn 1/4 lock in .25 seconds to dodge a boulder when bombing straight ahead.

 

[ScottRS]

Back to the "is this pump enough" bit:
If you have a cylinder of X cubic inches, and want it to go lock to lock in Y seconds, you need a pump that can do just that. What valve you select, is just a means to tell it to do that. You can only move the cylinder as fast as pump flow allows. Too many end up with small valves and then blame the pump for a zillion turns lock to lock, because of starting from a too-small pump.

So for a real example, I have two CB pumps, good for 2.5 GPM each (using stock pump at-idle numbers here for bad-case-scenario), so a combined 5 GPM. This is slightly more than a healthy mildly hotrodded P pump, but not a lot. A gallon is 231 cubic inches, so my pumps are collectively going to move 1155 cubic inches per minute, or 19.25 cubic inches per second, of fluid. I have a 2.75" bore steering cylinder, 8" stroke, with a 1.5" rod, so lock to lock is 3.14 x 1.375^2 x 8 minus the rod volume of 3.14 x 0.75^2 x 8, total volume of 33.36 cubic inches. So at that, it will take my pumps 1.75 seconds to move my cylinder lock to lock. How fast can I spin the wheel? I have a 9.7 cubic inch valve, so I can (theoretically; if I can't physically do it, that's another topic) turn it at about 2 revolutions per second; it's about 3.4 turns lock to lock. Whether you consider that good, bad, whatever, is a matter of preference. I would encourage getting in something you like the steering of, with a friend, and have the friend time how fast you can go lock to lock.