Minimum Steering Angle

Hi all,

What is consider the minimum required steering angle (at the wheels) for a street driven vehicle in order to have a reasonable turning radius?  This will be a function of the wheelbase, so if anyone has a handy formula available as well as any comment it would be greatly appreciated.  Thanks.

Joest

Here is the crude formula

radius=track/2+wheelbase/sin(average steer angle)

This does not define the wall to wall turning circle, for which you need to consider body overhangs.

Reasonable is a bit hard to define, it depends on the vehicle. For instance a London Taxi will have an 8m turning circle, whereas we'd say 11-12 m is reasonable.

For a sports car more than that would be acceptable.

You'll find that in practice your Ackerman has a strong effect on what is achievable, it may be worth exceeding 100% at full lock, as usually the back of the inside tyre hits the chassis rail before the front of the outside wheel.

Cheers

Greg Locock

i get some pretty wacky results using your formula, Greg.  also could you clarify "average steer angle"?  i'm assuming that means the average of the driver and passenger side angles, but you know what they say when you ASS-U-ME.

is there a typo in there somewhere?

Crashbox455,

I think the formula work as long as you enter everything is consistant units.  Also make sure your calculator is set for input in degrees and not radians if that is what your angles are in.

Joest

Sometimes it's the "obvious" - did you double the results from Greg's formula?

From a consumer standpoint - "reasonable" depends on the intended use of the car.  If it frequently involves tight turns, as in parallel parking or parking in lots with relatively narrow spaces and aisles, anything greater than about 10m will feel "clumsy".  Note that the above are common design features for increasing lot capacity, customer convenience be damned.

My '01 Nissan Maxima, which with 17" wheels requires ~10.5m/~41 ft, frequently demands a back-and-fill straightening maneuver in the above circumstances.  That's very annoying and impossible to avoid noticing, especially when another car that I have that features a wheelbase that's barely 5mm shorter than the Maxima almost never needs a second attempt to get square and centered in the space.

Norm

oh, i see.

Excel uses radians by default.  how clever.  using computer software to simplify things......

ok, so the formula works.  on my 61.75" track, 108" whlbase camaro, i'd need roughly 46 degrees of steering to pull a 10yd turning circle.  ouch....

anyone know off-hand what maximum steer angle is for a typical RWD production car?  i doubt it's that high.

not far off that, in our case. There's no magic

Let's see, typically (just less than) three turns lock to lock, through a 20:1 ratio that's 54 degrees.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

Volvo 240s and 740s turn pretty sharp.
maybe a little over 30 feet "between curbs"
240s have about a 108 inch wheel base, and at full lock the there is an im-modest amount of suspension exposed to the outside world.

Here's a data point for a vehicle that has pretty good maneuverability for a 4-door, 4/5 passenger sedan.

Wheelbase = 102.8"  (2611 mm)
Track = 59.1"  (1501 mm),  same for both front & rear tracks
Maximum steering angle, inner = 37* +/-2*
Maximum steering angle, outer = 32* +/-2*
Rack stroke = 5.39"  (137 mm)
Published ratio = 17.0:1
Steering wheel turns, lock to lock = 2.9 (sales brochure)
Steering wheel turns, lock to lock = 3.1 (service manual)
Turn circle diameter, curb to curb = 34.8 ft  (10.6 m)

Car is a 1995 Mazda 626.


I'd also like to correct my earlier statement to say that it is more like a 12 m turn circle where maneuvering in tight places has a clumsy feel.  (Math done in haste is also a waste.)  Apologies to Greg and anybody who assumed that the metric value was the primary data.

Norm