"center" for power steering?

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Protege5 2003
I've read that power steering provides little or no assistance at the center and more as the wheel is turned farther from the center. Something more or less like this, where X is the steering position, Xo is the center steering position, and f is the function mapping the steering displacement to steering assist:

Assist = f (X - Xo)

Assuming that's more or less correct, how does the power assist "know" where the center (Xo) is? Does it a follow from the steering/suspension geometry, or is there a specific adjustment for Xo, for instance, on a rack and pinion steering, marking a particular point on the rack?
 
toucci said:

Thanks.

According to that the center is set by the steering/suspension geometry. If you drive along not holding the wheel there will be no torque on the torque bar and so no power assist. If the alignment is right this should also be straight down the road.

Do torsion bars ever fail? From that article it sounds like if one broke it would disconnect the steering wheel from the steering gears. The article also describes it as a thin rod of metal that twists when torque is applied to it. Twisting a thin rod of metal back and forth seems like a good way to break it eventually.
 
pasadena_commut said:
Thanks.

According to that the center is set by the steering/suspension geometry. If you drive along not holding the wheel there will be no torque on the torque bar and so no power assist. If the alignment is right this should also be straight down the road.

Do torsion bars ever fail? From that article it sounds like if one broke it would disconnect the steering wheel from the steering gears. The article also describes it as a thin rod of metal that twists when torque is applied to it. Twisting a thin rod of metal back and forth seems like a good way to break it eventually.

The degree of caster in your suspension alignment is what causes the steering to return to center after turning. Nothing in the rack affects this unless something is binding up internally.

With the wheels straight ahead, the fluid coming from the pump bypasses the valves in the rack altogether and gets returned right back to the pump. In other words, there is no power assist, because it is not needed.

When you make a turn, a torsion bar is attached to a valve inside the pinion, and the valve opens up a porthole in the pinion and allows the fluid to go into the rack to pressurize one side of the rack or the other (which is what gives the power assist). The thickness of the torsion bar determines how "heavy" the steering is, ie how much effort is required to turn the wheel and open the valve in the rack.

r_valves_both_600.jpg


If a torsion bar broke or failed (which is very rare), you would lose power assist, but the car would still steer. It'd be just like a manual steering rack. The rack and pinion gears are what actually move the tires. The torsion bar and valving are only related to the power assist.
 
LTD-Scott said:
If a torsion bar broke or failed (which is very rare), you would lose power assist, but the car would still steer.

Once the torsion bar is gone what is left to connect the steering wheel to the pinion? The picture in the article cited above doesn't show any other connection. Is it maybe that when the inner part of the valve turns far enough it locks up with the outer part and so transmits torque from the input shaft to the pinion that way?
 
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