Calculate Steering Ratio

[LegendaryPuma]

Can I calculate the steering ratio with this information? If so, how?

Pinion travel = 3/4 of a turn (270 degrees)
Rack travel = 2 1/8"

Thanks!

[BillCobb]

No.

[LegendaryPuma]

Then what are the ways to calculate steering ratio?

[Will M]

(deep breath)
How do you think it can be done?

-William

[LegendaryPuma]

Well the rack goes 2 1/8". I figured that if I take a pencil and place it straight (90 degrees) then move an end (without moving the other end, rotate) 2 1/8" i get an angle. I then use a protractor to measure the angle. Then divide the 360 degrees of a full rotation on the steering wheel with the angle given by the pencil (which simulates the tire) and I get the steering ratio.

I am not sure if this is legit. That is why I am asking...

[Shebert]

If you answer that question, I will help you:

What is the definition of steering ratio and why do you want to know it(please don't say it is because they ask for it in the spec sheet, even if it is partly a good answer)?

[LegendaryPuma]

Steering ratio is the ratio between the degrees a steering wheel turns in relation with the degrees the wheels turn. I want to know how to calculate steering ratio so I can determine which is the best rack and pinion for our car. I want to find the best steering angle without much effort. I know that we don't need a lot of steering angle but enough to make the car turn, not understeer and that it wont wear down the drivers quickly (and at all if possible). And I am 300% sure it is going to be asked either on the specs sheets or the judges.

[Shebert]

So steering ratio is the relation between steering rotation and wheel rotation.

Steering ==> Rack ==> Wheel

From the data you have you should be able to find relation between steering and rack (if the steering is directly acting on the pinion). Then what do you need to know to find the relation between rack displacement and wheel rotation?

[LegendaryPuma]

wheel radius and rack travel I think

The pinion only goes 3/4 of a turn so its 270 degrees. Wheel radius is 6.5".

I figured that if I make a triangle. One side measures 6.5" @ 90 degrees. the bottom side is the rack travel 2 1/8". Now I connect both sides with the hypotenuse. No i get the angles 71.5 and 18.5 degrees.

Now for the steering ratio, is it 270 degrees/18.5 degrees = 14.59:1?

[onemaniac]

Originally posted by LegendaryPuma:
Steering ratio is the ratio between the degrees a steering wheel turns in relation with the degrees the wheels turn. I want to know how to calculate steering ratio so I can determine which is the best rack and pinion for our car. I want to find the best steering angle without much effort. I know that we don't need a lot of steering angle but enough to make the car turn, not understeer and that it wont wear down the drivers quickly (and at all if possible). And I am 300% sure it is going to be asked either on the specs sheets or the judges.

Users of this forum get very confused when questions like this are asked because:
-the question is very subjective that doesn't have one solid correct answer; or
-the question already contains the hint for the answer except the poster doesn't realize it.

Seems like you know the definition of what you're looking for. (regardless of its correctness) Since you know the definition so well, you just need to figure out the kinematics. Why post here?
Also, when you say the 'best' steering angle... what is the best steering angle? I honestly don't know what it's supposed to mean.
What makes the car turn? Some might say steering, but what really makes the car turn??? Answer this first.

[BillCobb]

If you connect the rack to the tire, there will be a problem soon after traveling about 2 mm. If you connect the rack to the wheel, the tire bead will start smoking soon afterwards, depending on the air pressure in it.

Think of a way to connect the rack to something else. You might TRY LOOKING UNDERNEATH A REAL CAR TO SEE HOW IN HELL THEY DO IT.

Just a thought: Please consider using the Forum SEARCH feature... .

[LegendaryPuma]

onemaniac you are totally right. By best I meant optimum and that well, I have to discuss it with the rest of the team.

BillCobb, yeah I know. And yes already gave a search.

This whole discussion is because I cant find the answer. Please refer to my previous post where I give what I think is a way to find the steering angle by using a triangle. All I want to know if its correct or not. That is all.

[Shebert]

http://fsae.com/eve/forums/a/t...5607348/m/1956095883 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/1956095883)

You should look for some literature, starts with ''tune to win'' its should answer your questions for while.

[acedeuce802]

That is not correct.

Why does your wheel radius matter at all? Is your wheel touching the ground?

Think about how the rack attaches to the corner assembly. Think about what would change in the kinematics if that mounting point moved. The linear movement of the rack has to be translated to rotational movement about the steering axis.

[siw]

Yeah drop your radius you don't need it. But before you do, I'd just re evaluate that one, it appears you don't require tyres..

Next, YOU need to define your steering ratio..start with your lock angles, and for them you need to look at the track layout. Then work yourself through the system to the steering wheel. Gather your variables and make informed choices..

The 'best' ratio for me might be the worst for you.. But why? What changes with each design? What do you consider the best steering ratio? Which events are you racing at? Why does this matter? Who's driving? What are there preferences? Have they had their Weetabix?

Just try and understand what you need to know and why and you will have better luck with getting answers which are helpfull http://fsae.com/groupee_common/emoticons/icon_wink.gif

And just some further advice. Knowing your steering ratio will not help when the judge asks why you've chosen it. The value is almost insignificant, they just want to know how and why you chose it. Not that you read 6:1 is great for the tighter courses, on an internet forum.

And it also sounds like your planning on using a steering system that you don't even know the ratio of. I'd get that in check!

[BillCobb]

Sorry, but the ratio chosen and any non-linear characteristics of it are very important to the driver because:

The gain (steering sensitivity: g/100 deg swa) and the rimforce or torque gradient (N or Nm per g) of the car must simultaneously make sense to the operator. Using a steering work function may help you out but since you are going relatively slow, you probably have a small rim diameter, low air pressure and an unpower assisted gear, things need to be synthesized early on in the chassis design process. If not, your car will suck.

Given the traditional rack and pinion type gear, the steer ratio can be reasonably computed as the ratio of the steer arm length to the pinion radius. If there are tierod angles of any sort, things will be a little different. If there are cyclic or other nonlinearities in the system, you will need the derivative of SWA by RWA to get by, especially if you ever intend to measure or compute the understeer or oversteer in the car.

FWIW, I once computed the understeer of my bass boat out on Woodland lake. A 3, 4 or 5 bladed propeller makes a pretty big difference. The 5 blade was the Mercury Hi-Five deal with air bleed adjustment screws. Anyone wanna take a guess on how I did it ?