Manual or Power Steering.

[scrimshaw]

I have a basic question regarding the installation of a manual or power steering rack on a car I am building. What are the basic parameters used to establish the installation of a power rack, obviously weight on the front wheels would be one but at what weight would a power rack be necessary. What about tyre size, wheel size, suspension design, etc.

Can it be simplified for someone like me without knowledge and resources available to professional car builders?

Or is it more of a personal decision based on the preferences of the user?

 

[evelrod]

Since the trend is toward "returning the driver to the womb"...All but the most aggressive sports car will have power everything. Not always a bad thing, but unnecessary for many applications. Even many of the current crop of racecars have power steering. I have raced a few cars with power assist and found them adequate and not uncomfortable.

Yes,for a street application, personal preference plays a large part in all this. For me, I find "one finger" steering uncomfortable. I prefer good "feedback" to "mindless ease". All said, I AM a dinosaur and feel more comfortable at speed in cars that I control and not the other way 'round!

Rod

 

[NormPeterson]

What sort of weight, and what sort of use is involved here?

A 3000 lb front engine V8/automatic transmission car is for the most part daily-driveable with non-assisted steering, even at ratios more consistent with power assisted steering. It will be somewhat physically difficult at low speeds.

It would almost certainly be a handicap at something as continuously frantic as autocross, assuming that things like suspension geometry, wheels and tires, etc., had been optimized for that activity. This (autocross) might be a good guideline with respect to predicting the ability to cope with emergency situations on the street, actually.


At 2200 lbs or so, autocrossing a front-engine car with unassisted steering gets a lot more reasonable.



In addition to your list, I think you have to at least consider acceptable force levels, rates of rack travel, steering arm length, and steering wheel diameter. Caster in particular affects steering effort. Racks have friction, though I don't think that specific information is readily available.


Norm

 

[GregLocock]

rack friction is around 30-40 lb, for a hydraulic rack. Manual will be less, never worked on one.

Cheers

Greg Locock

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

 

[scrimshaw]

Here are a few stats - you will probably be able to make a much more informed guess than me.

Weight around 2500 to 3000lbs. (Rough guess)
Weight split about 50/50 as near as I can calculate
140 inch wheelbase.
18x8 inch wheels with 155/75's.
Very mild street use.
Mustang II independent suspension with stock rack. I don't know ratios.
17 inch steering wheel.
Guy with big biceps driving.

Thanks for all replies.

 

[GregLocock]

140 inch wheelbase is interesting.

It really comes down to parking speed manouevres, and whether you want to twirl the wheel or not.



Cheers

Greg Locock

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

 

[scrimshaw]

Sorry - mistake on the tyre size.
That should be 255/70R18.

So if it comes down to parking does that mean in regards to steering the car if I go with power steering I will lose some feedback and the ratio will be different but otherwise no other bad/dangerous problems? ---- I was concerned that a car could be 'too light' for power steering and not feel right at highway speeds.

 

[patprimmer]

If you are designing from scratch rather than just building from a mismash of available components, you can make the steering ratio a lot faster with PS. You may or may not see that as a good thing, but it is the reason that PS is sometimes used on race cars.

Regards
Pat
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[NormPeterson]

I don't think that feedback is going to be of much concern during the stated "very mild street use" or parking.

Any decent power-assisted R&P will provide adequate feedback when you're driving a bit harder than that.


I think the Mustang II R&P is about 20:1 with 4.0 turns lock to lock. But at 140" WB you'll be turning the steering wheel a lot more than you would have in the 96-ish WB of its original MII fitment. Alternatively, you might consider installing a 1.5:1 steering quickener (at the expense of ~50% greater effort requirements).


140" wheelbase and under 3000 lbs?


Norm

 

[datstang]

I autocross my 5.0L powered Datsun 260Z without power steering, and I usually do not wish for the power assist. The car weighs 2590 lbs. full of fuel with me in it, and has 52 percent of the weight on the back tires. The car has Hoosier 23.5 x 9.50-15 radial slicks on all four corners, and uses the factory rack and pinion steering. I believe that the ratio is 16:1. Additionally, the car has six degrees of caster.

As long as the car is moving, the steering effort is reasonable. At a standstill, turning the wheels requires some effort. I am a reasonably strong and healthy 6 ft 185 pound male. To me the steering effort is acceptable, however someone with less strength may think otherwise.

 

[patprimmer]

I have seen actual data for lap times.

The car was late 80s Camero.

It was a group C touring car in Australia.

The tests were done to settle a debate between the two drivers who would drive the car in a 1000km race at Mount Panorama near Bathurst NSW Australia.

Both drivers were very well respected full time professional drivers. Both were very fit. One had a career mainly in rally and touring car, the other in open wheeler classes including F5000 and touring cars, but both had long and broad careers in motor sport. One was in his 30s and probably weighed about 170# to 180# and was probably about 5'9". The other was probably early 40s and probably about 190# and probably about 6'2"

Both lapped faster with power steering, including the guy arguing that manual steering would be faster. They raced with PS where a much greater advantage would have been gained after hours at the wheel vs a single lap test.

The consensus was that the quicker steering input that was possible with PS was responsible for the faster laps. The reduced fatigue was an added bonus that would help maintain the extra speed and allow a longer driving session if required for strategic reasons although fuel load was the major factor in driving sessions and rules also determined the maximum time for a session

Regards
Pat
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[LionelHutz]

For me, 3000lb would get power steering. I don't see the need to be fighting the steering.

No-one has mentioned it yet, but you could change the pressure to change the "feel". If you put in a manual rack then you get what you get.

Pat - I'd also wager the rally/touring driver was pro PS and the open wheel driver was against PS. Anyways, you're exactly right about the ratio and fatique.

 

[patprimmer]

Actually it was the other way round. The rally guy was also the younger smaller guy. Both had multiple Australian championships to their name in their respective fields and one had previous outright victories at Bathurst to his name.

The open wheeler guy was from an era where drivers did a lot of work on their cars so he was the better mechanic and engineer.

Regards
Pat
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[NormPeterson]

No-one has mentioned it yet, but you could change the pressure to change the "feel". If you put in a manual rack then you get what you get."

I don't think it's the pressure, per se, that you want to change.


Norm

 

[evelrod]

I guess all the NASCAR cars are now power assisted...One reason, I would think, that little 100 lb. Danica Patrick is talking about trying that series. Seems not to be a problem.

In my cars, the Lotus is so light in the steering dept. an assist would just waste power. In the Mini, I should be so lucky as to devise some form of assist that would not kill the power. That monster can make a strong man weak in a couple hours!!! I'm only good for about an hour on track without relief...Then again, I'm almost 70 and a bit totally out of shape.
I still have the fastest vintage Mini Cooper times at Willow Springs!!!!!

Rod

 

[GregLocock]

Rod, you could grab the EPAS from a new Fiesta, it is column mounted so there should be room -the motor sits under the steering wheel. Getting it to run is a whole nother game.

A basic PAS pump can pull 5 hp at 5000 rpm, so if you are on a long straight you will be losing power when you don't need assistance at all.

There are more sophisticated pumps which reduce this. Or you could go to EHPAS which drives the conventional PAS pump electrically.

For reasons I don't understand a hydraulic rack runs at full flow all the time, the assistance is generated by progressively blanking off the return port to the pump.

Cheers

Greg Locock

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

 

[evelrod]

"GREAT THHHUFFERIN' THHHUCCOTASH"---5hp? FIVE HORSEPOWER? Are you kidding? I would KILL for five hp at 5000 rpm !!!! I even have a switch that kills the drive loss to the alternator!

Rod-----With apologies to Mel Blanc and Sylvester.

 

[GregLocock]

Shocking isn't it? Of course the gearing of a conventional PAS is set to provide sufficient hydraulic power at idle, so for a racer you could slow it down a lot, although catch may become an issue.

Ah, that's the issue, you really need to drop the system pressure more than the delivery rate.

Cheers

Greg Locock

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

 

[FeX32]

Good luck parallel parking


Fe

 

[LionelHutz]

"I don't think it's the pressure, per se, that you want to change."

I didn't saw what changing the "feel" meant but people will often play with the valve in the pump outlet which basically adjusts the amount of assist. I believe that valve adjusts pressure but could also affect the volume.