Speed-Sensitive Power Steerings

Preamble
Week before last week we had a look at as to how Power Steerings appeared in India and then sampled a somewhat an ‘extortionist’ kind of a problem faced by one in real life.

Well, I thought it’d end with that but it didn’t, as one well informed car owner bowled another googly at me. This is how it went…

Question
A small doubt SKG: You have said that ‘power assist’ in such steering systems (hydraulic or electric) is cancelled out at road speeds higher than 40Kph. Is it true for all cars, especially those with hydraulic power-assist? I’m asking this coz in my Indigo which has hydraulic power steering, I noticed that if I give even a small twist to the steering wheel at expressway speeds, the car suddenly swerves, which could unnerve you, if you are not prepared for it! In fact, the corrective action you may take on reflex could cause an over-shoot in the opposite direction, making the car go zig-zag at high speeds and thus create a dangerous condition, especially if there are other vehicles near-by or trying to overtake. VG/Mumbai.

Answer
Thanks for the input/feedback VG. Let me try and rise to the occasion:

Although the EPS/HPS fellows positively claim ’speed sensitive’ power-assist, it’s relatively easier to achieve in EPS’s, as it’s a simple matter of programming the EPS/EPROM that takes a ‘road speed’ signal from the main ECU of the vehicle.

However, in HPS’s, as can be imagined, the things are a little more complex. As far as I know, there’s a ‘Pressure Regulating Valve’ of sorts that’s interposed between the steering wheel shaft and the actual steering gear/linkage.

This PRV has 2-slotted discs or cylinders wiping against each other and in their ‘reset’ i.e. ‘straight-ahead’ position, the slots are more or less offset, thus allowing minimum fluid flow thro’ them. Since one side of the PVR is tied down to the front road wheel side of the steering system and the other thro’ a ‘torsion link’ to the steering column, it can be visualised that the ‘through-put’ of Hydraulic (fluid) Pressure thro’ the PVR can be controlled by the difference of torques that it’s subjected to on its 2-sides.

In other words, if the effort on the steering wheel side is significantly higher than the road wheel side of the PVR, it’ll sense it as an attempt to turn the vehicle at low road speed and thus allow higher HP thro’ it, which in turn will result in higher ‘degree of assist’. The opposite is true when the effort-differential is low, meaning a higher road speed and therefore, lower the degree of assist.

So as can be seen, the design AND ‘calibration’ of the PVR is crucial in controlling not only the overall degree of assist but also in imparting a ’speed-sensitivity’ to the system. That’s how one keeps reading in professional road test reports of ‘over or under assisted’ systems on a particular vehicle, thereby losing that ‘feel of the road’ or vice-versa. In some HPS’s, the setting of the PVR is ‘service-adjustable’ but in most they’re not.

Coming now to your real question, since I haven’t lived with an Indigo, I really can’t tell, except that there seems to be something amiss with its PVR, either by way of design/calibration or later malfunction. What you can do is to carefully check as to whether the effort required at the steering wheel is nearly the same at crawling as well as cruising speeds. If so, then it means that the PVR is working, as it should. On the other hand, if it gets lighter as the vehicle road speed increases, then the PVR is not ok i.e. it’s not as speed sensitive as it should be - assuming that it’s there in the first place in a cost-cut Vehicle!

For more on HPS/Variable Assist systems, you may like to browse thro’:
http://auto.howstuffworks.com/steering4.htm

As it provides detailed graphics, which is not possible in the present format. This will make understanding of the whole concept a lot easier.