Thread: Electric Power Steering

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There seemed to be several folks interested in the electric power steering I mentioned in my build thread so I'm starting this post to pass on what I have learned and learn in the future about the subject of EPS. These first posts will be mostly lots of reading with few pictures. Sorry.

Feel free to comment. I may be chasing rabbits down a blind hole and any insights would be appreciated. I'll be putting this up in several installments just to keep it from being a too much to read in one sitting.

I have researched electric power steering on the internet as much as I can, but reliable info is hard to find. I did have a chance to speak with a rep from a well known aftermarket steering gear manufacturer at a large show and will pass on what he told me. What he related was off the record, so I won't mention the name of the company I still have some questions about EPS, but think the only way I can answer them is to actually try it, so my car will become a guinea pig. Here is what I know so far.

Most EPS is either mounted somewhere in the steering upstream of the rack, usually in the steering column, or it can be in the rack itself. My Dodge Ram truck has it in the rack and I think late model Mustangs do to. The integral rack units seem to be in heavier applications. The units I'm going to be talking about are the ones mounted in the steering column. These tend to be small worm gear boxes with electric motors attached and come in smaller front drive cars.

EPS functions very similar to hydraulic steering. In the hydraulic units, a small torsion bar is connected to the steering wheel shaft and to the internal worm in the gearbox. A hydraulic valve is around the torsion bar and when the steering wheel is turned, the torsion bar twists and operates the valve. The more the bar is twisted, the more hydraulic fluid is allowed to pass to the steering units internal pistons.

EPS works the same, but instead of valves and pistons, it uses electronic sensors and a motor. The torsion bar is still there, but it now operates a pair of sensors that track their relation to one another. An electronic control unit monitors these sensors and sends varying amounts of power to the motor in an attempt to keep them aligned with each other.

For a hot rodder, EPS does appear to have some real advantages over traditional hydraulic steering.

It removes weight from the front of the car and eliminates the possibility of fluid leaks. What weight it does add is more toward the center of the car.

In theory it can be installed on virtually any car as long as you take the frontend weight into consideration. The Saturn Vue that my unit came off of tips the scales at around 3500 lbs and is front wheel drive. I don't think I would put it on my 4000 lb 46 coupe, but something like a 60's Mustang or 32 roadster would most likely be fine.

It can also give adjustable assist. In fact, this is how most cars that have it from the factory are set up. More boost for low speed and less for high speed. If you read that a car has speed sensitive steering it's most likely electric. This boost adjustment is controlled by the car's computer.

EPS doesn't use any real power until it's needed except for the small amount used by the control unit. It just requires a couple of heavy gauge wires from the battery for power.

Eliminating the hydraulic pump on some builds will greatly simplify fitting everything in a tight engine compartment and since the EPS unit is small and can be hidden behind the dash in many cars, it lends itself to being installed in cars that you might not normally even consider. Fenderless street rods come to mind.

Unfortunately, it also has some potential problems. This is the area where I had the most difficulty finding good info. Most of the threads I read were by people that were singing the praises of their installation and glossed over any issues they were having.

Some EPS units are reported to have a "dead feel" and don't give much road feedback to the driver. I've seen reviews of new cars that this disconnected feeling is reported to be present. This may be due to too much assist, but I never found any reliable info about this. I have driven older hydraulic steering cars that had this problem. I seem to remember it most on the older linkage type power steering setups rather than the more modern integral units. The older setups felt completely disconnected from the road, but you could literally steer a 2 ton sedan with your little finger. I remember one fellow describing it as feeling like you were driving on butter with bald tires!

The most serious issue I found reported is returnability. A good steering system will attempt to return to straight ahead (RTC - return to center) after a turn if you let go of the steering wheel. Some EPS systems are said to dampen the RTC. This would be a real deal killer for me as I think it's unsafe. I don't want a car that I have to "unsteer" after a turn, especially any performance car that might be driven, well, like a performance car.

The RTC issue is what I discussed with the steering rep. He told me that his company had looked into EPS, but had put it on hold because they experienced RTC issues with some, but not all, of their test cars.

I have found some info that suggests that this is a result of the steering control unit being out of adjustment since it reportedly keeps up with where center is located and is supposed to apply a small reverse assist to get back to center when a turn is completed. As far as I can tell, there doesn't appear to be a command from the vehicle computer that tells the steering to RTC, so the control unit out of adjustment thing may be true, but I doubt it. If that were the case, it seems to me that the units would have to be removed and reinstalled in exactly the same spot in the steering travel every time. That seems a little far fetched, but I guess it's possible.

My theory is that the RTC issue is purely mechanical. The cars I've researched that had do-it- yourself EPS added were originally non-power steering cars, mostly older muscle cars. Those old cars usually didn't have much caster to reduce steering effort and most older cars that came with manual steering had pretty high steering ratios for the same reason. Caster is one of the major players in getting normal steering to RTC.

Some of the comments I read said that with the power off there was no difference in effort required to steer the car, but I do know from handling them, that there is some internal mechanical drag from the EPS gearbox. The EPS units like mine are really just right angle worm drive gear boxes with a permanent magnet DC motor. Worm drives are known for their self holding ability even with low numbered gear ratios. Add in the drag from the permanent magnets in the motor and it makes sense that there would be some mechanical dampening. For what it's worth, the drag is high when you initially try to turn the unpowered EPS, but drops off after you get it turning.

Since a manual steering gear box has a high mechanical advantage and since the EPS unit is on the steering wheel side, any drag by the EPS would be multiplied by the steering box. Just to throw out some numbers, if the steering box had a 20 to 1 ratio, the drag of the EPS unit would, in theory at least, be 20 times greater at the spindle. Once I got the steering on my car hooked up with a manual MII rack and the EPS, I noticed that it takes quite a lot of effort move the steering by trying to move the spindle. Much more effort than I normally associate with moving manual steering the same way. This leads me even more to believe that the issue is mostly mechanical.

I did read one source that said that one of the big three (Ford I think) had to go to specially ground rack and pinion units to get one of their car models to RTC reliably. This also would seem to indicate that the problem is mechanical since if it could have been overcome by simply adjusting the steering control unit, I'm sure Ford would not have spent extra time and money on high precision rack parts. Of course, this info came courtesy of the good old internet, so you never really know if it's based in actual fact.

The solution could be as simple as increasing the caster in the front end to give a much stronger mechanical RTC signal. This and possibly using a faster(lower ratio) steering gear to reduce mechanical advantage the EPS drag has. Adding extra caster should be fairly simple on most cars, but changing a steering box could get expensive. Many applications may not even have a faster box available. We'll just have to see what we have when I get my car on the road and go from there.

At any rate, the Saturn EPS I'm adding is fairly inexpensive compared to the systems offered by the big name aftermarket companies. The system can be gutted if I don't like it and then it really won't add any drag to the steering. I can then switch to a hydraulic rack with no other changes. Everything is in place to go to hydraulic steering as plan "B". The gutted EPS system won't be seen behind the dash if I go this route.

That's enough for right now.
Stay tuned, more to come.

Interesting.
Thanks for taking the time and effort to post this.

Thanks a bunch Mike. Very good write up. Addressed two issues that and are a concern to me should I decide to go EPS on the 57. The first I was already considering......the weight of the vehicle (the 57 Plymouth with a HEMI sitting over the front tires) and whether the donor vehicle PS unit would be heavy duty enough.

The second is what the road feel would be like. So far in the reading I'd done nobody seems to have addressed this.

The comment you quoted "...... I remember one fellow describing it as feeling like you were driving on butter with bald tires!....." could also be used to to accurately describe a 57-61 Plymouth/Dodge with factory PS and is one of the reasons I'm looking at alternatives.

I really appreciate your effort in writing this up and look forward to seeing how it turns out.


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Thanks guys. I have a lot more to post, but I need to edit it first.

Denny, that EPAS system is one that I looked at early on. They push their application specific kits , but also have a universal kit (or they did when I talked to them) that's a little cheaper. On that one, all the building is up to you. I was looking for something cheaper because I wasn't sure I would like it and a grand was too much money to gamble with for me.

My Hemi Dodge truck has EPS, and it feels pretty good to me. It has a nice feel and returns to center just as it should, so it can be made to work. The main question is whether I can make it work flawlessly in a hot rod.