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Old 24th September 2009, 21:15   #1
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Question Steering rotation to Wheel rotation ratio

Hi,

Just out of curiosity, this question popped up in my mind while driving back to home tonight. What is the ratio of steering rotation to wheel rotation? I mean, I am sure that when we rotate the steering by 90 degrees, the wheels don't turn by 90 degrees .

So here goes my list of questions:
  1. When we turn the steering by say 45 degrees, the wheels turn by how many degrees?
  2. Is the answer to 1st question different for slow speed driving and high speed driving? (I think yes).
  3. Is the answer to 1st question different for different cars? (I think yes).
  4. If answer to 3rd question is yes, which car has the best suitable ratio? (I mean, wheels shouldn't turn too much with a small rotation of steering and vice versa).
  5. How does EPS and HPS affect this scenario or come into picture?
Thanks in advance.

P.S. : My knowledge is very limited (read negligible) with respect to technicalities of Cars.
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Old 24th September 2009, 21:27   #2
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Quote:
Originally Posted by Gandhi View Post
Hi,

Just out of curiosity, this question popped up in my mind while driving back to home tonight. What is the ratio of steering rotation to wheel rotation? I mean, I am sure that when we rotate the steering by 90 degrees, the wheels don't turn by 90 degrees .

So here goes my list of questions:
  1. When we turn the steering by say 45 degrees, the wheels turn by how many degrees?
  2. Is the answer to 1st question different for slow speed driving and high speed driving? (I think yes).
  3. Is the answer to 1st question different for different cars? (I think yes).
  4. If answer to 3rd question is yes, which car has the best suitable ratio? (I mean, wheels shouldn't turn too much with a small rotation of steering and vice versa).
  5. How does EPS and HPS affect this scenario or come into picture?
Thanks in advance.

P.S. : My knowledge is very limited (read negligible) with respect to technicalities of Cars.
1. The wheel will turn based on the steering box gear ratio.
2. No, it is not different for slow speed or high speed driving
3. Yes, the steering box gear ratio is different for different cars - Also, the left and the right wheels do not turn the same degree for a turn.

4. It is a personal choice - I prefer the FIAT UNO's Power steering to be one of the best in feedback as well as feel.

5. EPS and HPS affect the scenario : At higher speeds, the steering naturally becomes lighter due to physics and hence the power assist required either by EPS or HPS is minimal. EPS has a sensor based input to reduce the assist while HPS has a adjustable vane that reduces the pressure in the pump which automatically reduces the assist provided.

Hope this is useful!
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Old 24th September 2009, 21:54   #3
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5. EPS and HPS are power assists, which means they "add" power to the steering wheel.
How do they do this?
The assists work at zero and low speeds as this is when you need more strength to move the steering linkages that move the front wheels.
When you turn the steering wheel by a tiny amount (and the linkage is yet to follow the turn) an "error" is created as the linkage is not yet exactly with the steering wheel. This error is fed to a feedback circuit that moves the linkage in the appropriate direction to reduce the "error"; that is get the linkage in agreement with the steering direction.
To understand this better, visualize a tandem bicycle. The two riders, both pedal to push the same wheel. Similarly the power steering motor moves the linkage in tandem with your hand muscles. The power assistance has to be carefully calibrated so that the assistance is not overdone. At high speeds the power assist is nulled; thank God!
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Old 24th September 2009, 22:20   #4
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Quote:
Originally Posted by headers View Post
1. The wheel will turn based on the steering box gear ratio.
Do you mean that wheels turn differently in different gears? Please elaborate.

Quote:
Originally Posted by headers View Post
2. No, it is not different for slow speed or high speed driving
So the amount of wheel turn is same for slow speed driving and high speed driving, but the force required to turn the wheels is different for different speeds.

Quote:
Originally Posted by headers View Post
3. Yes, the steering box gear ratio is different for different cars - Also, the left and the right wheels do not turn the same degree for a turn.
Couldn't digest the wheels turning at different degrees. Please elaborate.

Quote:
Originally Posted by headers View Post
5. EPS and HPS affect the scenario : At higher speeds, the steering naturally becomes lighter due to physics and hence the power assist required either by EPS or HPS is minimal. EPS has a sensor based input to reduce the assist while HPS has a adjustable vane that reduces the pressure in the pump which automatically reduces the assist provided.

Hope this is useful!
Yes it is. Thanks.

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Originally Posted by gostel View Post
5. EPS and HPS are power assists, which means they "add" power to the steering wheel.
Very well explained. Thanks.
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Old 24th September 2009, 22:36   #5
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Originally Posted by headers View Post
3. Yes, the steering box gear ratio is different for different cars - Also, the left and the right wheels do not turn the same degree for a turn.
Can't stomach this. Street cars mostly will have a toe in, but when you turn the steering wheel, both will have to turn BY the same angle, though the angle the tires make to the car's body-line will differ because of the toe-in (or toe-out) Correct me if I am wrong.
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Old 25th September 2009, 01:32   #6
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Quote:
Originally Posted by Gandhi View Post
Do you mean that wheels turn differently in different gears? Please elaborate.

So the amount of wheel turn is same for slow speed driving and high speed driving, but the force required to turn the wheels is different for different speeds.
In order to understand the 1st reply : consider a truck's steering wheel to that of a open wheeler racing car (say F2 or F1)... The truck driver has to turn-and-turn-and-turn in order to take a U-turn. Whereas the F1 racer needs any where between 180deg to 270deg (of steering wheel turns) to achieve a turn (without oversteering, ofcourse). The difference lies in the steering-pinion to rack gear ratio called a Steering ratio.

quoted from wiki :
//Steering ratio refers to the ratio between the turn of the steering wheel (in degrees) or handlebars and the turn of the wheels (in degrees). In motorcycles and bicycles, the steering ratio is always 1:1, while in most passenger cars, it is between 12 and 20:1. Example: If one complete turn of the steering wheel (360 degrees) causes the wheels to turn 24 degrees, then the ratio is 15:1 (360/24=15)//

Amount of wheel turn is same across speed ranges. However, the effort varies. Generally, in most of Power assisted steering cars, The PS pump is throttled off at speeds beyond 60kph. This is a safety measure as well as a efficient step : amount of steering effort required decreases as speed increases. [compare the manual effort required to turn the steering wheel when the vehicle is stationary (engine off) to that when driving at say 30kph] Safety measure: because you wouldn't want your vehicle to sway around @ 90kph just by a slight touch. Steering feedback is very important (for the driver) as speed increases

Last edited by Manas : 25th September 2009 at 01:50. Reason: definition added
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Old 25th September 2009, 01:47   #7
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Originally Posted by clevermax View Post
Can't stomach this. Street cars mostly will have a toe in, but when you turn the steering wheel, both will have to turn BY the same angle, though the angle the tires make to the car's body-line will differ because of the toe-in (or toe-out) Correct me if I am wrong.
In order to understand the reason, picture this:
you are taking a left turn and your vehicle's front wheels are set at zero toe-in condition. When the turn is initiated, the inside wheel (left, in this case) shall need to take a steeper turn than the outer wheel in order to negotiate the turn with minimum scrubbing. this is due to the geometrical limitations of the steering system: refer pic for Ackerman's steering mechanism.The pic is sourced from here and is self explanatory. (pl. note that Ackerman is an age old system and has been improved upon over the years)
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Old 25th September 2009, 06:10   #8
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Originally Posted by gostel View Post
5. EPS and HPS are power assists, which means they "add" power to the steering wheel.
How do they do this?
At high speeds the power assist is nulled; thank God!
This is true only for Power Speed sensitive Power assisted steering system. In the normal Power Steerings the power assistance to the steerings remains same for all speeds.

Normally the speed responsive Power Steerings are found in the high ended vehicles.

Pls find some Info on the Power steerings below.

Power steering

From Wikipedia, the free encyclopedia



History
The earliest known patent related to power steering was that by Frederick William Lanchester in the UK, in February 1902. His invention was to "cause the steering mechanism to be actuated by hydraulic power" (as recorded in 'The Lanchester Legacy' book). The next design was filed (as recorded by the US Patent Office) on August 30, 1932, by Klara Gailis, from Belmont, MA. There is another inventor credited with the invention of power steering by the name of Charles F. Hammond (an American, born in Detroit), who filed similar patents, the first of which was filed (as recorded by the Canadian Intellectual Property Office) on February 16, 1954.
introduced the first commercially available power steering system on the 1951 under the name Hydraguide. Most new vehicles now have power steering, owing to the trends toward , greater vehicle mass and wider , which all increase the required steering effort. Modern vehicles would be extremely difficult to maneuver at low speeds (e.g., when parking) without assistance.
Hydraulic systems

Main article:
Most power steering systems work by using a to turn the vehicle's wheels. The hydraulic pressure is usually provided by a or rotary vane pump driven by the vehicle's engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn applies a torque to the steering axis of the roadwheels. The flow to the cylinder is controlled by operated by the steering wheel; the more torque the driver applies to the steering wheel and the shaft it is attached to, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels in the appropriate direction.
One design for measuring the torque applied to the steering wheel is to fix a torsion bar to the end of the steering shaft. As the steering wheel rotates, so does the attached steering shaft, and so does the top end of the attached torsion bar. Since the torsion bar is relatively thin and flexible and the bottom end is not completely free to rotate, the bar will soak up some of the torque; the bottom end will not rotate as far as the top end. The difference in rotation between the top and bottom ends of the torsion bar can be used to control the valve that allows fluid to flow to the cylinder which provides steering assistance; the greater the "twist" of the torsion bar, the more steering assistance will be provided.
Since the pumps employed are of the positive displacement type, the flow rate they deliver is directly proportional to the speed of the engine. This means that at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow control valve are used to direct some of the pump's output back to the hydraulic reservoir at high engine speeds. A pressure relief valve is also used to prevent a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of the cylinder.
Some modern implementations also include an electronic pressure relief valve which can reduce the hydraulic pressure in the power steering lines as the vehicle's speed increases (this is known as variable assist power steering).
DIRAVI

Main article: DIRAVI
In the DIRAVI system invented by CitroŽn, the force turning the wheels comes from the car's high pressure hydraulic system and is always the same no matter what the road speed is. As the steering wheel is turned, the wheels are turned simultaneously to a corresponding angle by a hydraulic piston. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to centre position. The amount of pressure applied is proportional to road speed, so that at low speeds the steering is very light, and at high speeds it is very difficult to move more than a small amount from the centre position.
As long as there is pressure in the car's hydraulic system, there is no mechanical connection between the steering wheel and the roadwheels. This system was first introduced in the CitroŽn SM in 1970, and was known as 'VariPower' in the UK and 'SpeedFeel' in the U.S.
While DIRAVI is not the mechanical template for all modern power steering arrangements, it did innovate the now common benefit of speed adjustable steering. The force of the centering device increases as the car's road speed increases.
Electro-hydraulic systems

Electro-hydraulic power steering systems, sometimes abbreviated EHPS, and also sometimes called "hybrid" systems, use the same hydraulic assist technology as standard systems, but the hydraulic pressure is provided by a pump driven by an electric motor instead of being belt-driven by the engine. These systems can be found in some cars by Ford, Volkswagen, Audi, Peugeot, Citroen, SEAT, Skoda, Opel, MINI, Toyota, Mazda[/COLOR].
Servotronic

Servotronic offers true speed-dependent power steering, in which the amount of servo assist depends on road speed, and thus provides even more comfort for the driver. The amount of power assist is greatest at low speeds, for example when parking the car. The greater assist makes it easier to maneuver the car. At higher speeds, an electronic sensing system gradually reduces the level of power assist. In this way, the driver can control the car even more precisely than with conventional power steering. Servotronic is used by a number of automakers, including Audi, BMW, Volkswagen, Volvo and Porsche. Servotronic is a trademark of AM General Corp.[3]
Electric systems

Electric power steering (EPS or EPAS) is designed to use an electric motor to reduce effort by providing steering assist to the driver of a vehicle. Sensors detect the motion and torque of the steering column, and a computer module applies assistive torque via an electric motor coupled directly to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. On Fiat group cars the amount of assistance can be regulated using a button named "CITY" that switches between two different assist curves, while most other EPS systems have variable assist, which allows for more assistance as the speed of a vehicle decreases and less assistance from the system during high-speed situations. In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems.
Electric systems have a slight advantage in fuel efficiency because there is no belt-driven hydraulic pump constantly running, whether assistance is required or not, and this is a major reason for their introduction. Another major advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control electric power steering systems can instantly vary torque assist levels to aid the driver in evasive manoeuvres, Toyota has been doing so since 2002.
The peak power output of the electrical system of a vehicle limits the capability of electric steering assist. A 12 volt electrical system, for example, is limited to about 80 amps of current which, in turn, limits the size of the motor to less than 1 kilowatt. (12.5 volts times 80 amps equals 1000 watts.) This amount of power would be adequate for smaller vehicles. It would probably be considered insufficient for larger vehicles such as trucks and SUVs. There are other types of electrical systems such as the 24-volt electrical system and other variants used for hybrid and electric vehicles. These have greater capacity that enable use of multi kilowatt motors needed for large and mid-size vehicles. Toyota's latest LS 460 (4775 lbs) premium luxury car, Highlander and RX 350[/COLOR] midsize SUV's use electric power steering.
The first electric power steering system appeared on the Honda NSX in 1990 and then the Honda S2000 in 1999. Today a number of manufactures use electric power steering, Toyota has been working to replace hydraulic systems with electric types on their redesigned cars and car-based platforms.
Reviews in the automotive press often comment that certain steering systems with electric assist do not have a satisfactory amount of "road feel". Road feel refers to the relationship between the force needed to steer the vehicle and the force that the driver exerts on the steering wheel. Road feel gives the driver the subjective perception that they are engaged in steering the vehicle. The amount of road feel is controlled by the computer module that operates the electric power steering system. In theory, the software should be able to adjust the amount of road feel to satisfy drivers. In practice, it has been difficult to reconcile various design constraints while producing a more pronounced road feel[citation needed]. It should be noted that the same argument has been applied to hydraulic power steering as well.
Variable gear ratio steering

In 2000, Honda launched the S2000 Type V equipped with the world's first power variable gear ratio steering (VGS) system.[4][5] In 2002, Toyota developed then their own "Variable Gear Ratio Steering (VGRS)" system introduced on the Lexus LX 470 and Landcruiser Cygnus, and also incorporated the electronic stability control system to alter steering gear ratios and steering assist levels. In 2003, BMW introduced their "5-series.[6]
This system should not be confused with variable assist power steering which varies steering assist torque not steering ratios, nor with systems where the gear ratio is only varied as a function of steering angle.

Last edited by Dippy : 25th September 2009 at 11:21.
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Old 25th September 2009, 07:17   #9
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The steering ratio is measured as lock to lock turns. Which is how many turns of steering wheel do you need to turn the wheels from complete left to complete right.

There is no best possible Steering ratio. Prior to power steering days the steering ration were dependent on the weight of the vehicle. The heavier the vehicle the more turns you need to complete the turn. even the 800 used to have 3.75 turns. Today there are cars that can do lock-to-lock with 2.75 turns. Once we have drive by wire steering it will do down to less than 1 turn possibly 0.75 turns.

Cars with power steering also have variable ratio on a single turn. From the middle the first half of the turn will be slower than the last half of the turn. This is done so that the steering appears quite normal for highway driving but in City where you tend to turn the steering more you need a taller geering(quicker steering). This is exacly what ZipDrive bought to Santro. But it was there in almost all the cars when they bought power steering.

Higher end cars also have Variable Geometry steering, where the number of turns lock-to-lock depends on the speed of the car. In all other cars its not dependent on speed.

About EPS its just a power steering system. It has been adopted as it will reduce the energy losses of a traditional Power steering system. The earlier versions of the EPS were designed not to give any feedback to the driver. But this has been addressed in the newer versions. Its all in the control module that drives the Power steering motor.
That said EPS will only work for light cars as it directly works on the steering rack rather than on the pinion. The traditional power steering system will move the pinion itself and so reducing the load on the steering system.
But EPS systems have virtually no maintainance compared to the non EPS systems.
EPS also has speed sensitive assistance. Where the assistance will reduce once the speeds climb. This will make the drivers feel more in direct control of the car.

Last edited by arunmur : 25th September 2009 at 07:19.
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Old 25th September 2009, 13:06   #10
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Quote:
Originally Posted by Manas View Post
In order to understand the reason, picture this:
you are taking a left turn and your vehicle's front wheels are set at zero toe-in condition. When the turn is initiated, the inside wheel (left, in this case) shall need to take a steeper turn than the outer wheel in order to negotiate the turn with minimum scrubbing.
Still can't understand this. Why is the angle measured from a point outside the cars. The angle I was talking about is from the center of the vehicle.

In the first picture, let say both the front wheels are at 0 degrees. The angle for both the wheels in second picture is similar with respect to rear wheels (to my naked eyes at least).
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Old 25th September 2009, 14:03   #11
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Quote:
Originally Posted by Gandhi View Post
angle for both the wheels in second picture is similar with respect to rear wheels (to my naked eyes at least).

had that been the case than lines would not merge. Lines would be parallel to each other.

Lines are perpendicular to wheels and if lines converge than angle has to be different.


Rehaan posted an excellent video about this on some other thread.
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Old 26th September 2009, 09:45   #12
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Quote:
Originally Posted by Manas View Post
In order to understand the reason, picture this:
you are taking a left turn and your vehicle's front wheels are set at zero toe-in condition. When the turn is in......

................... Ackerman is an age old system and has been improved upon over the years)

That is well understood. Thanks Manas for the explanation and the picture.
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Old 26th September 2009, 10:14   #13
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Good info Manas. That post and the pics cleared a miscconception I had with steering.
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Old 26th September 2009, 12:41   #14
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Originally Posted by NetfreakBombay View Post
had that been the case than lines would not merge. Lines would be parallel to each other.
Ahh, Yes. I get it now. This is a revelation to me the way steering and wheels work.
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Old 29th September 2009, 08:38   #15
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@sudharma, I just did a quick scan of your post and thank you for the very informative details, which I will soak in in the next few sittings.
Nonetheless, Gandhi had asked a simple question and I tried to give a simple answer without compromising the correctness of the answer. According to the Maruti sales-person, my humble Alto cuts off the EPS at higher speeds. Cutting off is simple, but varying the power assistance in accordance with speed of car runs into complexity, which one may expect only in high end cars. Moreover low-end cars are also lighter and do not really need steering assistance once you are out of the parking lot. So I completely agree with you
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