Steeringangle/ Slipangel/ Side slipangle

[Ben A]

Hello,

i have a newbee question about steering geometrie, Slipangle and so on.

Basic Design steeringangle in Skinpad.

How can I easy calculate the side slip angle of the car?
I need this to calculate the optimum steerangle in Skinpad. To get the best use of maximum latral force on the tire.

How can I easy do that?

What I have done:

I did a excelsheet to calculate the Wheel loads with an 1,5 g latral acc.
With that wheel load I look at the tire data. For each wheel I can look where the slipangle is for Maximum lateral force.

Is that ok?

But what came after that? Is there equation to calculate the steerangles?

Thank you guys and Sorry for that noob question it is not easy to learn and read the stuff but it is very interesting and you get a motivation to make it all perfect and very good.

[Old Greg]

Sorry, no equation and not particularly easy.

First find the slip angles at each contact patch for a given cg slip angle and steering angle. There's an equation on p.147 of RCVD but it needs to be modified to account for track width. combine that with your weight transfer spreadsheet.

Use those slip angles and normal forces with your tire data, then repeat with different values for cg slip angle and steering angle until you have your desired lateral acceleration and zero angular acceleration (only for steady state).


There was a thread a few months ago entitled "Cornering Force Distribution" that you might find to be good reading. See also the TTC forum.

[Ben A]

Hi,

thank you for your comment.

Mhmm... is it possibel to do it in the Cad? I´ve seen one team did it like this:

They draw 2 D skech of the car in Skidpad.
They calculate the yaw velocity and and the circumferential velocity (They say 1,5 g in skinpad). With a sideslipangle they choose, they project the velocity vectors from CG to each wheel. with the Slip Angle from the Tiredata/Max Fy an the adding of the Yaw vektor and the global velocity vector they have therer Steerangle? I think this way is easy but is this correct or only a rapprochement?

I´ve look at the RCVD Book, Is that right that i have first calculate the velocitys for eachs wheel? How can i add the track width to this calculation on page 147?

How i have to combine that to my Weightdistr. sheet? do you mean i can calculate the FY for each wheel with the same sheet?

Sorry for the noob questions i´m very new in Suspension desgin and Tiredata and so on.

[oz_olly]

I certainly learnt alot from doing a simple 2d cad drawing of the car cornering. You can see what happens to the rear slip angles with toe in and toe out. You also see how the turn centre moves forward relative to the vehicle with increasing lateral acceleration (ie the rear slip angle must increase to produce more lateral acceleration and the only way for this to occur is for the turn centre to move forward). You can also see that if the steering angle is held constant and the rear slip angle is increased the the front slip angle will also increase until you get closer to the limit where you will either need lots more or lots less steering angle to balance the yaw moments about the cg until you eventually reach a lateral acceleration where you can no longer reach a condition where the yaw moments about the cg are equal to zero.

You will probably also learn something if you were to put a line in at each wheel representing the lateral force vector and then measuring the perpendicular distance to the cg for each wheel. This will help give you some indication as to which wheel is or isn't contributing to yaw moment.

[Vittorio]

Hi Ben.

The method you described is accurate only if the sum of all moments around the CG is zero and if the sum of all forces is exactly lateral (no longitudinal component), otherwise the sideslip angle will change according to the situation (in particular according to the moment around the CG), so you have to be careful with this method. I'm not a suspension designer, but if I was you I would definitely look for a better way to design your suspension. Start reading the post suggested "cornering force distribution", you have to create an iterative model to calculate accurately your suspension behaviour.
Hope this helped.
Good luck!

[Charlie]

Originally posted by Ben A:
With a sideslipangle they choose,

And that's exactly it. You can't easily measure the car's side-slip angle (there are ways, just expensive). If you assume one, getting the tire slip angles is a simple exercise, but you have to assume you guessed it correctly.

[Ben A]

Thanks for your Replys. It helps me a lot.

is it right that i need the cornering stiffnes for front rear? to calculate the Moments and total lateral tire forces? I looked on page 148/149 on RCVD. But the corneringstiffness isn´t linear on that point or?

How can i match the formula on these pages to a 4 Wheel car? Because of differnt Slipangles and steerangles on one axle.

thx
Ben

[Old Greg]

Originally posted by Ben A:
How can i match the formula on these pages to a 4 Wheel car? Because of differnt Slipangles and steerangles on one axle.
Ben

Do you see the +ar and -br components? They're accounting for the lateral velocity due to rotational velocity and the longitudinal distance from the cg. What you need to do is account for the longitudinal velocity due to rotational velocity and lateral distance from the cg.

The results from doing so are very close to the numbers you'll get from a cad drawing (in my case, about 0.01 degree difference between the two). You should do both as well, if for no other reason than to check your calculations.


Originally posted by Ben A:
How i have to combine that to my Weightdistr. sheet? do you mean i can calculate the FY for each wheel with the same sheet?


Yeah, just to streamline things a little. You'd end up with one spreadsheet that will tell you the FZ and alpha at each corner (for your specified conditions). There will still be a lot of manual labor involved, but it should make things a bit easier. Like Vittorio implied, the next step is to get your spreadsheet (or matlab) to look at your tire data for you and calculate what your beta and delta will be through iteration. You're making a good start, and if you choose to pursue this thing all the way to the end your current work will be directly applicable.


Originally posted by Ben A:
is it right that i need the cornering stiffnes for front rear? to calculate the Moments and total lateral tire forces? I looked on page 148/149 on RCVD. But the corneringstiffness isn´t linear on that point or?


You're right, cornering force is non-linear. Those equations are based on the assumption that it is. Understand those equations and why they're written the way that they are, but don't use them directly.