steering ratio

[mayuri]

Hi !

We will be using the rack and pinion steering of a car having overall steering ratio 39.6mm/revolution and steering turn lock to lock as 3.3.

please guide me if this is a correct consideration..as the race cars should have steering ratios between 5:1 to 10:1

Thanks

[ibanezplayer]

... not sure on the mm/rev but our steering rack has 0.61 turns lock-to-lock (220 degrees) and is a bit to much IMO.

I don't think this rack will be suitable, but i am not a chassis/suspension guy.

Goodluck

[Drew Price]

Mayuri,

Your ultimate steering rate lock-to-lock will be dependent on the steering arm length you choose. Distance from the 'kingpin' axis to the outer steering pickup.

This distance also has a bit to do with upright stiffness depending on your layout, a very short steer arm length can give unwanted non-linearity to the steer input also.

Just something to consider.

For a first car especially if you're tight on money and have easy access to this it wouldn't be the absolute worst thing of your drivers had to shuffle their hands if the steer ratio was too slow. It might even be better than having it so fast that the driver's arms are tired out after only 10 minutes at race speed, like our first car was.

What school are you with? You might want to put it into your personal profile to identify yourself, it's good practice.

Best,
Drew

[mayuri]

Thanks ibanezplayer and Drew Price for those worthy advices.

Drew Price ,Wat i have interpreted from your post is that the ultimate steering rate lock-to-lock will change if we'll change our steering arm length and distance from the 'kingpin' axis to the outer steering pickup.That means the value i have suggested about the steering system of a car that we would be using(steering turn lock to lock as 3.3) would be irrelevent when we will put it in our car because we can change it to our desired value.

I would appreciate if you could suggest about overall steering ratio 39.6mm/revolution .This was mentioned in the car's(of which we are using steering)details.I wonder how to convert it in simple ratio form.....

[Drew Price]

Originally posted by mayuri:
That means the value i have suggested about the steering system of a car that we would be using would be irrelevent when we will put it in our car because we can change it to our desired value.



Exactly.


About figuring out the ratio, I believe the general ratio is degrees of steering wheel rotation to degrees of steer at the wheel. A 5:1 steer ratio would mean that 5 degrees angular rotation at the steering wheel gives 1 degree of wheel steer. A max steer angle at the wheel of 20 degrees would then be 20*5 or 100 degrees of wheel rotation, a little past a quarter of a turn.

It's just top view geometry (trig) to see what rack travel gives you how much steer angle.

What ratio you actually decide on depends on so many things that the easiest thing I can suggest is to make your steering pickups bolt-on pieces, so that you can quickly change the ratio when you are testing if you have no baseline to go off of. If I could do it all again, that's the methodology I would use.

Best,
Drew

[mayuri]

I am in the steering department of my team(1st timers).I have read Race Car Vehicle Dynamics( steering chapter), it explains the concepts nicely but I am unable to find the strategy as to how and from where I should start with steering .

I know the steering ratio needs to be in between 5:1 to 8:1 for a better feel but how to finalize one particular value. Is it like..Ok I think 4:1 is good keeping in mind its benefits and faults..Or is their any strategy for coming to a particular value..? The same question is valid for deciding the max. Steering angle of the wheel.

I would highly appreciate if any one of you guys could provide me with any strategy for designing the STEERING SYSTEM of my car...

Thanks

[Drew Price]

What is it that gives a particular ratio it's 'feel?'

It's the force you have to put into the steering wheel to turn the car.

You have a system of combined mechanical advantages to turn the road wheels. The dia. of the steering wheel, the gearing of the rack, the length of the steering arms all determine the steering ratio.

The force comes from the tire being scrubbed against the ground, and for non-zero camber, caster, and kingpin inclination the weight of the car.

You have another moment arm to consider, it's the distance from the point where the steer axis (kingping axis) intersects the ground, and the centroid of pressure of the tire contact patch.

Both points move as the car does different things.

The tire also has it's own self-aligning torque about the contact patch centroid.

Read RCVD section 2.2, on aligning torque and pneumatic trail. Read up on mechanical trail and scrub radius as well.

They all effect what you want the steering ratio to be.

As for how to design the steering system as a whole, one good approach is to decide how much force it takes to hold the car steered at speed you want the driver to have to exert. There are published reports of this, but somewhere between 4-8kg in each hand should at least be a starting point. 8kg is kind of a lot, you may find your drivers get tired easily, less than 3-4kg and you may find the steering has no feedback anymore, and the wheel needs too much input (hands shuffle too much).

You can guess a rough figure for what all the info you don't know is that seems reasonable (coeff. of friction of the tire and ground = 1.0 for example), or spend the money for the tire data (if you think you will be able to make use of it - sounds to me like you should be spending your time elsewhere than staring at the computer monitor for 3 weeks straight making sense of the tire data, like I did).

Best,
Drew

[mayuri]

Hey thanks Drew for all those valuable inputs..
Am going through tune to win..i wish to reach the position of designing my steering as soon as possible http://fsae.com/groupee_common/emoticons/icon_smile.gif..

thanks!