Steering Angle

[hustler287]

I have looked through the forums and found numerous related topics but not a definitve answer. http://fsae.com/groupee_common/emoticons/icon_mad.gif

How are maximum and average steering angles calculated for all four wheels?

I appreciate they change for different values of Ackermann, w/b t/w radius of turn and slip angles.

Example (front wheel steer)
tw = 1500mm
wb = 1000mm
Ackermann = 100%
radius of turn = 5m
slip angles = ?


Reagards,

John (jack)

[hustler287]

I have looked through the forums and found numerous related topics but not a definitve answer. http://fsae.com/groupee_common/emoticons/icon_mad.gif

How are maximum and average steering angles calculated for all four wheels?

I appreciate they change for different values of Ackermann, w/b t/w radius of turn and slip angles.

Example (front wheel steer)
tw = 1500mm
wb = 1000mm
Ackermann = 100%
radius of turn = 5m
slip angles = ?


Reagards,

John (jack)

[exFSAE]

There's an "Ackermann angle" .. or steered angle dictated by geometry. Google it. It's a function of wheelbase and turn radius only.

While this may work at low speed it doesn't hold true at race speed. You've got understeer, you've got tire effects.. hell.. taking a 30' radius turn at 60mph may only take 3deg slip angle for one tire, but 5deg for another!

Figure out the Ackermann angle for minimum turn radius at comp, and then dial in a little more.

[Buckingham]

John (Jack),

If you know the complete geometry of your vehicle (track/ackermann/wb/etc), you can come up with Geometric Slip Angles for a given turn radius, vehicle sideslip angle, and driver steering wheel input. (GSA is my own jargon term, someone else may have come up with a better term at some point). The GSA would simply be the difference between the direction the car is headed (as defined by the turn radius) and the direction the tire is pointed (as defined by vehicle sideslip angle and driver steering angle).

A 2-D sketcher is the easiest method for determining GSA's. It can be done mathematically using trig (for importing into Matlab), but it will take you some time to come up with that.

I used the GSA method because I found it useful for evaluating how many different steady-state solutions were possible for a given turn radius. The more steady-state solutions possible that yield the same max lateral accel, the less sensitive the car is to driver skill. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Composites Guy]

exFSAE's advice sounds good to me.

I will tell you that I have been designing for a maximum of roughly 30 deg of wheel angle with respect to the center line of the car. This means having enough rack travel as well as making sure their aren't intereferences between suspension parts and wheel rims. This has proved to be an excess of wheel turn... i.e. when going around the skidpad at top speed you are a long way from the end of the rack. I think at max rack travel we were actually at 27 and 33 degrees respectively, which is somewhat less than ackerman.

I'm interested in hearing what some other teams choose for thier max wheel angles...

[flavorPacket]

the only place you'll use deltas above 30 deg is at formula student where we had to do a 180 around a single cone. We design for roughly 27 deg max.

[lporter]

We've been around 30 degrees for the last few years. It's typically a combination of the desired steering wheel angle at full lock (i.e. not having your arms cross) and the amount of clearance between the a-arms and the wheel rims.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> .. hell.. taking a 30' radius turn at 60mph may only take 3deg slip angle for one tire, but 5deg for another! </div></BLOCKQUOTE>

...actually it would probably take some magic ( ~ 8g) or at least Ricky Bobby behind the wheel.