the mechanics of electric assist power steering conversion questions

[bryant]

so ive seen in several threads reference to electric steering being bad for the steering gear. someone referenced that its like taking a large lever to the steering gear and another reference to the effect of it like taking a jack hammer to the steering gear.
now i just cant see how thats possible.
here is why.
when starting with a manual steering gear, the force applied to turn the wheels is a given amount. this amount is provided by the drivers force on the steering wheel. the same as applying a lever to the steering shaft. now if you add the electric steering assist to the steering column its going to apply the same force to the steering gear that the driver would have provided to make the wheels turn. the only time this could be a problem is when the steering gear reaches its full lock.

the oe manufactures are using theses systems on cars now for over 10 years. i could see them building their steering racks to take any extra abuse created by the electric assist but still i dont see any extra loads or forces being applied.



so the next factor in this question is in my personal car, im switching the manual steering ratio box over to the power steering ratio box. now my car is a maverick. the power steering cars had an assist ram that was after the steering gear, so the loads through the steering gear would be less than the loads through the manual steering gear. at this point i could see the possibility of the power steering gear box not being strong enough for the electric assist, but on my last maverick i did run the power box with out the assist. i have seen lots of fords running with the power assist removed and never have found any to have worn out steering boxs.

some one did reference that the twisting load on the steering column with the electric steering is substantial. i do recognize this and do plan on supporting the motor servo assembly to my dash bar in my roll cage. that should be plenty of support for the assembly

if im missing something as to why going to the electric steering is a bad idea, can some one please clarify as to why?

[Red67Mustang]

I believe your assessment is accurate. The simple truth is there are equal and opposite forces involved (Newton's third law). To turn the wheels you need only slightly more force than is required to overcome the opposing forces in the steering system / tire contact patch. In all cases the driver is able to input those required forces through the steering wheel (even with a PS system that has lost its assist). That ability is a fundamental safety / liability issue (arguably the only somewhat related notable failure would be the Chevy Cobalt recall). Significantly more force applied would simply 'overboost' the steering and result in a car that was just difficult to drive.

IMHO the whole 'jackhammer on the steering box' is an expression indicating either a lack of information or an contrary agenda.

[David Pozzi]

How many turns lock to lock is the Ford PS box?
It seems to me, your steering won't be all that quick.

[bryant]

the manual steering box is 20:1 4 5/8s turns and the power steering box is 16:1 3 3/4 turns.
i will be switching to the 16:1 but am interested is getting an even faster ratio than that. i plan on contacting lee power steering to see about getting a steering box with a faster ratio.
im sure ill want power steering with the faster ratio. i really dont want a power steering pump and lines in my engine compartment so im really interested in the electric steering. now that i think about ill see if tom lee at lee power steering has an opinion on the electric assist.

so does any body have any good reasons why the electric steer is not a good choice?

[Mateospeed]

so does any body have any good reasons why the electric steer is not a good choice?

Sure, I can hope to explain why some folks think there is excessive force on the steering gear. Think about the process of steering the car at rest, starting from the tires:

Take the un-assisted, manual steering box.

-The tires will require a certain force to BEGIN turning (to break static friction), of say, 500 lbf.
-The force input to the tire must occur along the (in your case) steering linkages, then translated to the steering box. Still 500 lbf here.
-The steering box is a gear-reduction device, meaning it trades angular displacement for torque. So, relatively high torque input, low displacement given to the steering linkage translates to relatively low torque input, high displacement on the steering-wheel side of the steering box. Meanwhile, it also converts the linear movement of the steering gear to a twist movement of the steering shaft. Say you have a 5:1 gearbox. that 500 lbf becomes 100 ft*lbf of torque required to turn the wheels.
-The driver, in the end, feels that torque translated up the steering shaft of 100 ft*lbf to make the tires start turning, with the vehicle at rest.

Now, consider an electric assist unit, installed in the steering column. Its job is to multiply the torque input to it, by an specified amount. Assume, for the purpose of this discussion, that this "torque multiplication" is constant, at 2:1. So, 100 ft*lbf input to the electric assist unit results in a 200 Ft*lbf output to the steering shaft.

Now, consider the same scenario, starting with the tires.

-Tires still need 500 lbf to start turning.
-steering linkage still sees 500 lbf - nothing changed here.
-Steering gear still reduces that to 100 ft*lbf. Nothing changed here.
-NOW, that torque is translated up the steering shaft to the assist unit. The assist unit does its job, and drop the force reacted to the driver by 2:1, as previously discussed.
-So, the driver now only exerts 50 ft*lbf to the steering wheel, to achieve the same force turning the wheels.

Now that we're on the same page, herein lies the rub: Essentially, the maximum force input to the steering box and linkage is *still* 500lbf, right? no change, right? Right. EXCEPT when you encounter the ability to raise that 500lbf requirement. Say your tire is sitting against a curb, or you've steered all the way to the steering stop. Now, whatever force you input to the steering wheel is multiplied by 2:1. Now you have the ability to exert much more torque into the steering box, translating to much more force into the steering linkage. If something can't handle the load, SNAP!

So, under normal driving circumstances, there's really no change - you won't be able to input significantly more load upon the system. Find something solid to push again, and you could pop something.

Now, to address your specific question - you want to change your steering box, too, to one with a lower ratio. Let's look at this new scenario:
-500lbf required at the wheels.
-500lbf at the linkage
-3:1 gearbox means 500 lbf /3 = 167 ft*lbf of torque.
-with manual steering, that would be too much to turn by yourself - so you add electric steering.
-2:1 assist: 167 ft*lbf / 2 = 83 ft*lbf at the steering wheel.

So, some components see the same load - namely, those between the gearbox and the tires (unless you find something solid - see above). BUT, any components between the gearbox and the assist unit see higher loads, by 67% (significant!). Things like your steering shaft, any u-joints in the steering shaft, the steering column and dash supports, will all see higher loads. The loads for that assist unit must be reacted by something solid for it to do anything - else it would just spin in place, right? So you have to support it to something solid - perhaps the dash support.

So, you could pop something. Steering is REALLY important - don't screw around with it! Calculate how much load increase your system will see and where, and beef everything up to handle that load! Then go have fun!

On the issue of jackhammering - that's a special case, caused by slop in the system (common with a steering box) and an improperly-programmed electric assist unit. You would need someone to provide the hardware, software, and programming who knew what they were doing to avoid this scenario.

Happy steering!
-Matt, your pocket engineer.