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Detuned Power Steering on a c900
Posted by DamienS [Email] (more from DamienS) on Wed, 22 Oct 2014 09:39:29
Test car is 1987 900 Turbo Convertible 200k+ miles
In an effort to improve steering feel because I never liked the high level power assist on my cars, I figured that had the following options:
1) Install an off-the-shelf pressure bleed-off device between the high and low pressure lines, which would be effective and offer an adjustment feature, but be leak-prone and clunky and expensive.
2) Modify the steering rack internally to give less assist. I'm sure the amount of assist is set by internal geometry of fluid passages and piston sizes and whatnot, but I really don't know how a power steering rack works. Since I just installed a fresh rebuilt rack that I don't want to wreck, this option was out.
3) Move the mounting points of the tie rod ends forward/closer to the wheel hubs, which would give the steering rack less mechanical advantage to do it's job, in other words increasing effort to turn the wheels. But this has strong secondary effects a) it would quicken the steering ratio which would be _awesome_, but b) unknown effect of wheels turning too much in the wheel wells - it would probably need hard stops fabricated, c) unknown effect of having tie rods angling forward as viewed from above, probably wreck the ackermann or give some non-linear funky steering response. The rack would have to also move forward to keep things in line. Too complicated for my current ambition level.
4) I did this: I was replacing the power steering pump anyway, and I realized that it has an internal pressure relief valve just like the engine oil pump. In both cases the fluid pressure can build until a certain level, at which point a spring is overcome, a ball or piston moves which exposes a passageway, and the excess pressure escapes. The output pressure is determined by the strength of the spring. So I totally guessed and clipped off 2 coils from this spring, ground the end flat, and reassembled.
So far it drives like I had hoped - which is like 20% of the power assist is gone, and the steering wheel has a new weight to it. Parallel parking is still no problem. I don't see any downsides to this, only upsides...like less wear and tear on the PS pump, belt, pressure hoses, and steering rack because of lower system pressure. A little less power robbed from the crankshaft too. Anyone else try this?
posted by 72.52.9...
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