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Old 03-24-2017, 02:16 PM   #76 (permalink)
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Old 07-25-2017, 01:32 AM   #77 (permalink)
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Thanks to all who provided info in this thread. I wanted to bump it to see if anyone has some more info on the 05+ and the 11+ Super Duty boxes. from what I can tell the SD boxes are prime candidates for swaps because of the flat front facing pitman arm, beefcake internals, plenty of power from the large piston and they are also readily available in junk yards for about $250. I'm looking to snag one if its worth it for my 05 chevy SAS project since the factory box isnt going to cut it and its in a horrible spot.
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Old 08-06-2017, 05:53 PM   #78 (permalink)
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Quote:
Originally Posted by AgitatedPancake View Post
Thanks, trying to keep it objective as possible to help develop my own understanding further, and so we can all gain from it in the end. I love tech discussions like this, so thank you for keeping it along as well

So on this one, let me explain how I believe the servo/control valve works. First the general stuff, then the specifics Honestly these things are pretty cool-

The first thing in the line of things connected to the steering wheel in a power steering system is the control valve. This valve regulates the hydraulic fluid to provide assistance when necessary. It is NOT RPM sensitive, but it IS torque sensitive due to the torsion bar. If whatever is downstream (steering box, rack) provides no resistance, like when the tires are off the ground, the valve freely spins but never opens the ports to provide hydraulic assistance. Now if the tires hit resistance so the output of this control valve doesn't want to spin, but you twist hard on the steering wheel attempting to make things move - the torsion bar inside the valve actually twists however many degrees to open a hydraulic channel for assistance. These torsion bars are a replaceable and tuneable option that range from fingers on the steering wheel assistance at one end, to having to crank on the wheel and hardly even noticing you have power steering at all.

So as long as the control valve is the first thing connected to the steering wheel and gets that resistance, all it knows is "hey, you're twisting that steering wheel with 20ft-lbs or torque, let me send fluid to help". You can change the ratios of the box downstream, or add quickeners as you please without changing the force required to get that fluid flowing. If you put a quickener before the control valve though, you have to account for the reduction in torque (corresponding to ratio) the torsion bar will actually see for a given input.

No matter what ratio box is chosen, the actual volume and surface area the hydraulic assist has to work with doesn't change, which means the responsiveness of the system shouldn't change. In the case of a steering box, let's throw some arbitrary numbers around and say the piston has 3" travel, and rotates the sector shaft 90* in the process of doing so. No matter what ratio box you choose, that piston is still going to travel the same distance per rotation of the sector shaft. So you would get the same amount of hydraulic assist out of a 6:1 box, as you would a 13:1 box (fixed ratio for simplicity). The only difference would be that it would be easier to demand more out of the 6:1 box due to the fact that you could keep the torsion bar in the control valve twisted and at max assist easier. You might run out of pump capacity when dealing with the ability to go lock to lock in 1 second versus 2 seconds before (random numbers for example). I'd bet restrictions in the lines, fittings, and box itself would become apparent at some point though I'm pretty sure some tinkering can be done there
Talking about this, are all the servos the same, other than torsion bar size? Or does the servo change with piston size?

I'm trying to find an outside frame reverse ratio box. Ford 78-9 seems to be the biggest?

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Old 09-22-2017, 11:58 PM   #79 (permalink)
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Bump

Trying to figure out if I can swap servos (corkscrew piece) without changing the piston since the pistons are different sizes. Anyone done it with different ratios? I have a 3.5 turn AGR box and want to swap to a 2.75 turn servo from an older Saginaw box.
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Old 09-24-2017, 07:21 PM   #80 (permalink)
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Have anyone ever tried or looked into installing EPAS to existing power steering? my google search show that EPAS is popular in old cars that have manual rack & pinion. I couldn't find anything on putting an EPAS to power assisted steering box for more steering power.
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Old 09-24-2017, 07:37 PM   #81 (permalink)
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Have anyone ever tried or looked into installing EPAS to existing power steering? my google search show that EPAS is popular in old cars that have manual rack & pinion. I couldn't find anything on putting an EPAS to power assisted steering box for more steering power.
For off road EPAS wouldn't add anything except making the steering feel lighter when you're not taxing it at all. If a hydraulic steering box can't move the tires how you want then EPAS is either gonna stall or break something.

When combined with a (big) check valve that lets fluid flow from the reservoir to the pressure side of the pump EPAS might help ram-assist steering in on road applications where you're trying to go from lock to lock quickly but the pump is having a hard time keeping up with the ram.
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Old 09-24-2017, 07:49 PM   #82 (permalink)
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What I really meant that will EPAS work well to use instead of hydro assist? To maintain fast steering ability but with more steering power in rocks. Even there are stuff out there to make EPAS power level tunable with a knob
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Old 09-24-2017, 08:10 PM   #83 (permalink)
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What I really meant that will EPAS work well to use instead of hydro assist? To maintain fast steering ability but with more steering power in rocks. Even there are stuff out there to make EPAS power level tunable with a knob
Maybe. I don't know of anything close to off the shelf that's powerful enough without some serious downstream gear reduction which would kind of defeat the point of being fast.

The OEMs are light years ahead of aftermarket when it comes to making good use of electronically controlled systems. Look at some of the adapting chassis control tech (sway bars, shocks, etc) and none of them use EPAS in anything "heavy duty"

I mean the technology has its place but I don't think there's enough pros over current hydraulic systems to be worth it in the kind of big tire and rough terrain applications common here.
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Old 05-27-2018, 04:15 PM   #84 (permalink)
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Old 02-13-2019, 09:21 AM   #85 (permalink)
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Any more info on the SD boxes. I am building an IFS and wondering if the SD box has enough power on it's own to not run hydro assist. I could add it after if needed. Someone mentioned the dodge box as well. I am looking for a forward pitman arm like the SD uses.
From number that I found. WJ box + 1.75 dia hyro assist=8.344 in sq
ford 3.18 box= 7.942 in sq
97 ram box=9.457 in sq (front facing pitman?)
SD box=??
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Old 02-13-2019, 09:39 AM   #86 (permalink)
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It's not just about piston area. You need to consider motion ratio as well. The ratio of motion between the end of the pitman arm and internal piston is not 1:1.
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Old 02-13-2019, 09:50 AM   #87 (permalink)
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Could you explain? Is the box ratio and pitman arm length stuff or something else that I am missing.
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Old 02-13-2019, 10:59 AM   #88 (permalink)
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Could you explain? Is the box ratio and pitman arm length stuff or something else that I am missing.
If the piston has 1,000lb pushing on it and has 5" of travel and the end of the pitman arm has 10" of travel then pitman arm has 500lb pushing on it. Obviously those numbers are made up but the point is that there's a ratio of motion between the pitman arm and the piston so 1lb of force on the piston is not necessarily 1lb of force at the end of the pitman arm.

You can't just compare steering box piston area to the area of a hydro assist cylinder. You need to account for the difference in ratio.
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