On / off power steering?

[JohnLear]

I've recently removed the power steer belt on my daily driver Honda Accord, and very much like the change in steering feel this creates, yet I do miss the PS in parking lots (and my wife would simply not be able to park the car if she had to drive it, and might have some difficulty steering it in tighter corners). It occurred to me that what I want is a PS system that could be easily switched on / off at driver discretion.

Has anyone ever tried, or heard of anyone, fitting an AC style clutch to the PS pump pulley? Such a clutch might be actuated by the driver simply pressing a button (on steering wheel maybe), so assist only exists when it's really needed, and not when it isn't (which is most of the time in my case).

Of course PS robs power, and removing the PS belt has resulted in a noticable 'butt dyno' measured improvement to the cars performance in both full and light throttle driving (more than placebo I think), and better engine response when 'blipping' for downshifts.

While I haven't yet carefully measured the economy, it does appear that the fuel guage is dropping significantly more slowly during my routine driving, i.e. the guage is dropping to / around X by Thursaday rather than by Wedensday (hardly an exact measurement I know...).

It would seem a win / win / win to me if the PS could be switched on / off at will, but are there any reasons why:
a) It seems to never (or rarely) to have been done?
b) Why it would be a bad idea?

I would think that such a system might appeal to quite a few people, especially more 'sports' oriented drivers. I know that quite a few people do disable their PS...

Regards,
John

 

[GregLocock]

The torsion bar in the valve has a maximum twist of several degrees, at which point (or just before) it is fully open, hydraulically. This is limited by a mechanical stop.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I guess when I hit that positive stop was when the steering got quite heavy before the belt came off.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[NormPeterson]

Is it possible to fabricate a T-bar of variable torsional stiffness and place the variability under direct user control?


Norm

 

[cibachrome]

Yes it is. The Delphi MagnaSteer is a t-bar plus variable stiffness augmentation provided by electromagnetic control. Thus a base t-bar at 1.0 Nm/deg can be stiffend or softened with current. Its not under user control in production, but controllers are available for tuning purposes to change the speed vs. current tables in the computer/control module. Typically the system is designed to use no current at the most probable speed to reduce power consumption. I believe ZF also has such a system (Servo-Tronic) that changes t-bar stiffness using mechanical action. These systems add considerable cost, mass, and packaging volume to the design space. What's always interesting to me is how so many companies with K&C equipment fail to properly set the speed sensor(s) values in the car controllers when performing tests. Since the default speed is ZERO, the steering compliance values can be very much in error. Of course, even now knowing whether or not a vehicle has automatic speed variable steering adjustments is a puzzle. (not on your own products of course, but on competitive vehicles)!

 

[cibachrome]

Back to the previous discussion about understeer...

Understeering effects include more than just tire slip angle effects. The chassis is steering the front and rear spindles as well as yourself. The is because of load dependent properties of the chassis (linear and nonlinear), geometry (steer by roll and steer by camber, tractive force steer, ect) and of course difference in tire sideslip due to size, load, rim width and pressure settings. Plotting understeer vs. lateral g produces a graph that usually looks like a scripted W. (showing + and - g levels for left and right turns). The values of the middle of the W (+- .3g) are higher than the .5 g because the steering system is dominant. Since there's no equivalent nonlinear compliance in the unsteered axle, the understeer curve gets to be bathtub shaped. After about .5 g, the tires are dominant as you point out, so the curve heads upwards to a limiting asymmtote. This should tell you that dynamic roll couple adjustments are only effective when the tires listen (or care to react) the the vertical loads (inside and out included). This can be more difficult to design with tires that have low load sensitivity (they could care less what loads are on them). Pressure, load reserve, rim width, contruction type and brand have strong influence on dynamic load transfer performance. At low lateral g (legal maneuvering) the load transfer stuff is pretty small IMHO).

So, with your Accord, we test the chassis on a K&C rig, test a front and rear tire on a tire tester, estimate some inertias (unnecessary for steady state analysis), put it all together and:

Understeer total is 2.0 to 2.5 deg/g at about 0.25 g. A pie chart of this scalar number shows:

.5 from tire effects,
1.0 from front steering effects
.5 from roll geometry (steer and camber)
a smidge more from the aligning moment rigid body reaction.(there's 4 tires with twisting moments on them, remember?)

This number crawls down to maybe 1.5 to 1.75 deg/g at .5 g and goes North to 5 --> 12 deg /g at .75 g.

As to your other interests, a sporty model has tire changes as well as steering changes. Higher cornering stiffness usually results in lower tire aligning moments so the steering loads go down. Less load = less understeer from the tie road load gradient. Less understeer means higher gain (g/100 degSWA). Sporty tires usually have "faster response", too. To me that means shorter relaxation distance due to stiffer sidewall, lower section height, wider rims and lower loads (higher load reserve).

Finally, yes you can screw it up. If your pinion radius is pretty small and the gear cuts are cheap, the rack pushaway forces act to increase rack compliance and add understeer. You then get lower gain (g/100degSWA) in spite of the faster ratio in this case. Yes you can crank up the rack slipper bearing tension to force the engagement, but the resulting friction increase will drive you bonkers (I hope I said that in the correct context). Low tire pressures (for ride purposes) will jack up the tierod loads and further the problem.

Hows zat?

 

[patprimmer]

If the torsion bar segment had a splined surface (yuck) you could change its effective length and therefore stiffness in torque (is that what its called).

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[JohnLear]

"The torsion bar in the valve has a maximum twist of several degrees, at which point (or just before) it is fully open, hydraulically. This is limited by a mechanical stop.
Cheers
Greg Locock"

Thanks Greg, I thought that must be the case.

 

[JohnLear]

"I guess when I hit that positive stop was when the steering got quite heavy before the belt came off.
Regards Pat"

Wouldn't the valve be fully open at that point, so assist should be at max? My bet is some restriction causing limited flow when the steering wheel is moved quickly (or attempted to be moved quickly).

It's many years since I've driven a PAS that wasn't a rack and pinion, but I can recall some PAS 'boxes' being very unhappy with fast inputs...

 

[patprimmer]

You simply get ahead of the power offered by the boost when you have very fast inputs. If you have full power available from the power boost and you are still inputting a lot, the torsion bar will stay wound up and you will be inputing against the stop. The load increases when the belt comes of, but as you already have a heavy load, so are somewhat prepared and the %age increase in load is somewhat reduced.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[GregLocock]

That's called steering catch. Typically the system is sized to cope with steering wheel velocities of 360-720 degrees per second.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[farna]

Hot rodders do things like this, but as mentioned, I don't think an OEM would touch it. The Honda Accord uses a power rack and pinion, I believe. It works a bit differently than a GM style power steering box. I don't think I'd try this with a power steering box, but definitely would with a rack and pinion. I've run racks with the hoses looped to circulate fluid and prevent a hydraulic lock with no problems. The steering box type units don't respond so well. More play in the steering and more difficult steering than expected without power assist.