alistairolsen
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| posted on 25/11/13 at 01:45 AM |
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Elderly PAS equipped cars pulling?
When I got my first PAS equipped car I was annoyed to discover it pulled in the direction of camber (normally left), requiring constant light pressure
on the wheel. After years of non-PAS cars, this was annoying and at first I thought, just a symtom of that car.
Since then I've driven a lot more cars, old and new and PAS and non PAS and the tendency seems to be in older cars with PAS racks. Now all the
ones which I've owned have had good balljoints, TREs etc, so I know its not that, I've been through tyres and pressures and it's not
that either.
What is it that controls the assistance level in a PAS rack? I've seen a spool valve in bits, but I muct confess didnt take the time to
thoroughly understand how it actually worked in detail!
Any thoughts?
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Slimy38
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| posted on 25/11/13 at 06:22 AM |
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I've found they tend to pull when the alignment has been done badly, and the steering wheel no longer sits horizontal when the car is going
forward. It looks fine when it's on the ramp and the two front wheels are aligned with each other, but on the road there are other forces in
play that mean the steering wheel has to be turned slightly to keep the car going forward, and obviously it wants to move back.
The last time I had it was on my tintop after a Halfords alignment... after taking it to a decent garage they dialled it out no problem.
But having said all that, I didn't think PAS made any difference to self centering, I thought it was still the suspension geometry that did
that? PAS just applies a constant force to make steering easier. The newer cars seem to be a bit cleverer, and only apply assistance when appropriate,
but older stuff is just all the time.
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nick205
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| posted on 25/11/13 at 01:10 PM |
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As far as I'm aware, PAS shouldn't be the issue if the car's pulling. It's down to the geometry, alignment, tyres, plus any
wear and tear. The road surface including camber and wear pattern (HGV tramlines etc.) will also play a big part.
Thinking of several cars I've had from new (Seat, VW, Volvo) and had the alignment checked on, all have had a tendency to pull towards the verge
on even a mildly cambered road. As you describe, all have required a slight steering input to counter this. Older vehicles I drive with PAS also
behave the same.
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britishtrident
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| posted on 25/11/13 at 03:19 PM |
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With hydraulic power steering it usually means the steering rack needs centred up by adjusting the track rods (shorten one and lengthen the
other by the same amount) after that the steering wheel needs centering.
Electric power steering can can be the above or the pot sensor is out of calibration.
As to how hydraulic pas operates the valve it depends on a tiny amount of lost motion. Old style (ie 1970s era as used on BMC 1800 and BL
Princess) ) used sideways movement of the rack pinion to operate an external spool valve that sat on top of the rack this type was adjustable.
Modern (for example TRW racks) use lost rotary motion on rotary valve built into the the rack pinion housing.
[Edited on 25/11/13 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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alistairolsen
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| posted on 25/11/13 at 07:26 PM |
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But this is my point guys, this is on cars that ive spent a lot of time and money ensuring are "right", wheels checked for roundness and
true with new tyres with pressures checked regularly, new bushes, balljoints, dampers and springs in may cases and still a gentle pull to the left due
to road camber, but definitely more marked than their non PAS equivalent.
BT ,what do you mean by lost motion?
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Dingz
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| posted on 25/11/13 at 07:37 PM |
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The valve is torque operated so there is a slight amount of 'springiness' in the unit as you apply steering input.
Phoned the local ramblers club today, but the bloke who answered just
went on and on.
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coyoteboy
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| posted on 25/11/13 at 09:21 PM |
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Is it not possible that the geometry is different for the PAS version (otherwise the PAS version would be ludicrously easier than the nonPAS version)?
Leading to (or allowing) having more caster or scrub radius, leading to more sensitivity to road surfaces which normally wouldn't bepossible but
isn't a problem as the PAS removes that heavy feedback?
FWIW my 1991 PAS car doesn't pull unless I run very low profile tyres or larger scrub, when it can get as bad as being classed as snatching.
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