steering arm inclination

[mayuri]

Hi!!

If I find steering arm inclination (SAI)(steering arm is the arm that is attached with tie rods and if we'll extend it ,it'll meet at the center of rear axle ..for 100% ackermann) with the uprights, theoretically, it comes out to be 68 degrees (as front track width=1396mm and wheelbase = 1747mm) …using trig ..but this would come out to be equal on both sides...right?

Now if I’ll simulate the steering for different angles of front inner wheel ,my vehicle’s turning center(TC) will change…right…and if it becomes evident that the TC is always behind the rear axle…Does that mean I have to increase my SAI ..in order to get correct SAI , so that for some front inner wheel angles it is behind for others it is forward ?

OK! First of all tell me Is this the right criteria that I am using for analyzing?

I found, approx. Turning center radius = (track/2) + wheelbase/sin (average steer angle)
Please clarify as to “track” is front track width or rear?
“Average steer angle “is the one made by steering wheel or the angles made by front wheels when steered and you take their average…. Like if my steering wheel angle is 310 degrees, what would be average steer angle?



Thanks.

[mayuri]

Hi!!

If I find steering arm inclination (SAI)(steering arm is the arm that is attached with tie rods and if we'll extend it ,it'll meet at the center of rear axle ..for 100% ackermann) with the uprights, theoretically, it comes out to be 68 degrees (as front track width=1396mm and wheelbase = 1747mm) …using trig ..but this would come out to be equal on both sides...right?

Now if I’ll simulate the steering for different angles of front inner wheel ,my vehicle’s turning center(TC) will change…right…and if it becomes evident that the TC is always behind the rear axle…Does that mean I have to increase my SAI ..in order to get correct SAI , so that for some front inner wheel angles it is behind for others it is forward ?

OK! First of all tell me Is this the right criteria that I am using for analyzing?

I found, approx. Turning center radius = (track/2) + wheelbase/sin (average steer angle)
Please clarify as to “track” is front track width or rear?
“Average steer angle “is the one made by steering wheel or the angles made by front wheels when steered and you take their average…. Like if my steering wheel angle is 310 degrees, what would be average steer angle?



Thanks.

[ed_pratt]

Mayuri,

I think you are confusing SAI(steering axis inclination - otherwise known as kingpin inclination) with the angle of your steering arm in the plan (top down view).

As far as I am aware, the 100% Ackermann value for the centre of the rear axle only holds true if the steering rack and tie rods all lie in a straight line in the plan view.

Are you using front or rear steer geometry? i.e. is your steering attached to the upright ahead of or behind the front axle centreline?

Do you have any books or reference materials on suspension geometry?

A good place to start would be reading some of the older posts in this forum;

http://fsae.com/eve/forums/a/t...607348/m/19610317231 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/19610317231)

http://fsae.com/eve/forums/a/t...607348/m/50510335441 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/50510335441)

These are two pretty good posts on the subject.

As far as your turn centre goes, I can't remember from the top of my head but I thought the calculation assumes a turn centre in line with the rear axle?
..........perhaps someone else could clarify this?

If the equation is asking for track width it usually mean it has assumed a constant track width both front and rear. In this case I would use the larger of your tracks assuming the worse case scenario, again you should check this by reading some reference material on the subject.

Hope this helps

Ed

[exFSAE]

What are you talking about

[jrickert]

It sounds like you are sortof on the right path but there are a few things you should consider.

You do not have equal load/camber/tire temp/slip angle/etc on the inside and outside tires, so i don't really like averaging the inside and outside tire orientation when attempting to find your turn center. Averaging the front tire positions might be an okay approximation when talking about low g turns in a road car but is not good for a race car.

There is more to steering kinematics than just your steering arm angle. You also need to consider your rack placement and length.

In a two wheel steer car, your turn center will generally move rearward with increased steering angle.

[mayuri]

hey thanks jrickert for your advice.. but more than that , thanks for understanding what my question meant ... hehe

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
What are you talking about </div></BLOCKQUOTE>

[mayuri]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jrickert:
Averaging the front tire positions might be an okay approximation when talking about low g turns in a road car but is not good for a race car.

</div></BLOCKQUOTE>

Then what do we exactly mean by "average steer angle" in:
Turning center radius = (track/2) + wheelbase/sin (average steer angle)

I think there's no point in taking the average of steering angle made by the steering wheel ,because then there would be no point of averaging ...

what about track (front or rear of car)?

looking forward for your valuable inputs.

thanks

[Zac]

The first point I should make is that it generally is not a good engineering practice to just lift an equation from a textbook or website without a good understanding of the assumptions and limitations behind it. In this case, you are using pure geometry to define the turning radius but does that still apply when you have large amounts of slip angle? Are you sure you really want or need 100% Ackermann?

But to answer your question, it looks like the equation is wrong. You can either leave off the track term and use the average of the wheel angles, or you can add the track term and use the inside wheel angle(subtract if you want to use the outside wheel angle).

Also, Ed is correct, that equation assumes that the turn center is in line with the rear axle. You should be using the front track width.

[mayuri]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
In this case, you are using pure geometry to define the turning radius but does that still apply when you have large amounts of slip angle? Are you sure you really want or need 100% Ackermann?

But to answer your question, it looks like the equation is wrong. You can either leave off the track term and use the average of the wheel angles, or you can add the track term and use the inside wheel angle(subtract if you want to use the outside wheel angle).

Also, Ed is correct, that equation assumes that the turn center is in line with the rear axle. You should be using the front track width. </div></BLOCKQUOTE>

ya ! Zac
I think the formula i mentioned would be valid under low speed lateral acceleration, with almost no slip angles.

For the formula ,The turning circle of a car was defined as the diameter of the circle described by the outside wheels when turning on full lock..but then also turning radius should come out as wheelbase/sin(outside wheel angle) ,only.
yes ! for this formula the turning centre should fall on the line extending the rear axle
...which would not be the case at high lateral acceleraions....right?