March 2000 - On The Street 9

Turning Ability–Exploding the MythIn discussion with various departments I have found that there

is not a clear understanding of the definitions and or meaningsof many of the technical terms used in explaining themaneuverability of a vehicle. A clear understanding of thesteering system and its performance is required to properlyevaluate your bid proposals. In this issue I hope to explode themyth of steering systems.

The definitions:

Cramp Angle – As shown in Diagram A the cramp angle isdefined as the angular measurement of the tire centerline fromthe straight ahead condition to the farthest point in the turn. Thecramp angle is always measured on the inside tire of the turn. Inother words the left tire is measured in a left turn and the righttire in a right turn. This is because of Ackermann Steering effectsthat will be defined later.

Turning Radius – The distance from the turning center to thecenter of tire contact with the road of the wheel describing thelargest circle while the vehicle is executing its sharpest practicableturn. Diagram B shows the measurement for the turning radius.

Turning Diameter – Twice the turning radius.

Turning Diameter – Wall to Wall (Bumper Swing) – Thediameter of the smallest circle which will enclose the outermostpoints of projection of the vehicle while executing its sharpestpracticable turn. This is equal to the minimum turning diameterplus twice the radial overhang beyond the turning radius. DiagramC shows the measurement for the bumper swing.

Turning Diameter – Curb to Curb– The diameter of thesmallest circle within the vehicle will clear a curb 150 mm (5.9inch) high, while the vehicle is executing its sharpest practicableturn. This is equal to the turning diameter plus twice the horizontaldistance from the center of tire contact with the road to the arcsubtended by a chord drawn between the points intersection ofthe outermost projection of the tire shoulder on a horizontal plane150 mm above the surface on which the tire rests. Diagram Dillustrates the measurement for Curb Clearance.

Continued on next page

Diagram A

Diagram B

Diagram C

Diagram D

March 2000 - On The Street 10

Ackerman Steering - The outer ends of the steering arms arebent slightly inward so that when the truck is making a turn, theinside wheel will turn more sharply than the outer wheel. This isdone to compensate for the greater distance the outside wheelmust travel.

At the maximum turning angle, there is normally Ackermangeometry error between the front wheels. This error will result intire scrub of both front tires. For turning calculations, equalslippage of both front tires is assumed. In operation greaterpressure on the outer wheel caused by centrifugal force and otherinfluences will cause tire wear in vehicles maneuvering throughurban streets.

Ideal Ackerman Steering occurs when a line is drawn throughthe axle kingpin intersecting the tie rod connection and continuedto the chassis centerline (The line from T1 through T2 in DiagramE). This line should intersect the chassis centerline at the centerlineof the rear ale for a perfect Ackerman Steering set-up.

In the real world axle manufacturers cannot tool up for asteering tie rod arm for each and every length of wheelbase thata truck manufacturer can build. There is a tolerance ofapproximately 40 inches of wheelbase change that a tie rodassembly can be used without a severe scrub on the tires.

In the case of large tires used in aerial applications for 21,500lbs. there is another unique circumstance. Attempting to use thecorrect tie rod arm is an attempt to break the law of physics. Nomatter the effort two objects cannot occupy the same place atthe same time. In this case the tie rod arm that can be used and fitwithin the rim profile without colliding with the tire and or rim isoutside the optimum for correct Ackerman Steering. The resultis extreme tire wear, and compounded with a poor Ackerman onan apparatus that is constantly turning those expensive 425 tireswill wear out in 5,000 to 6,000 miles. Some steering shops attemptto bend the tie rod arms to reduce the wear on the tires due toturning scrub. Don’t do it! If an OEM component is not availablereplacement or alteration is not recomended.

In the chart below is an example of tie rod arm use with theresulting steering error based upon the steer angle of the axle.

The Ackerman Chart shows that the steering angle errorincreases with the turning cramp angle. Therefore when a 425tire on an aerial is cramped to a 45° angle the error resulting intire scrub increases, causing the tire to have increased wear and ashortened life.

To calculate the turning diameter (TD) for the vehicle inDiagram F the following formula is be used:

TD = 2[(L / sin (90-A))+OS]Dimension OS is shown below:

Diagram EDiagram F

Turning continued on page 15

March 2000 - On The Street 15

Type W h e e l Turning

Rear

Live

35º

45º

35’ 9”

33’ 2”

Cramp

210”

240”

Now some real world case studies of configurations of trucksand the effects of cramp angle on their maneuverability.

First a single rear axle apparatus is examined.

The picture above illustrates the difference made when a singleaxle chassis is improved by using a 45°-cramp angle.

Wheelbase CrampAngle

TurningRadius

180

180

35º

45º

31’ 6”

26’ 5”

Wheelbase CrampAngle

TurningRadius

240

240

35º

45º

40’ 1”

33’ 2”

As the chart shows change in axle cramp angle reduces theturning radius requirement of the example by just over 5 feet!

Now the tandem rear axle apparatus.

To determine the turning ability of a three-axled vehicle it iscustomary to measure the wheelbase from the front axle centerlineto a point midway between the two rear axles. At this pointconsider a transverse line drawn as the equivalent of the center

Turning continued from page 10 of the rear axle of a two-axled vehicle.This is the result of comparing a tandem axle apparatus under

the same 35° / 45° comparison:

The change in axle cramp angle reduces the turning radiusrequirement of the longer wheelbase aerial by over 7 feet!

To explore the newer ideas of multi-axle steering is the followingdiagram:

In the case of the chassis shown on the left, the Drive / SteerTag combination, the turning center of the rear axle is at thecenterline of the driver or lead axle in the rear axle set.

On the right is a live driving tandem where the centerline of thetandem axle set is used to determine the maneuverability of thechassis.

Armed with the information in this article you can now askinformed questions and understand when reviewing bidinformation. Oh yes, one last thing. Make sure that the advertisedcramp angle of a manufacturer is with the tires installed. <sigh>

Drive/Steer Tag Live Tandem

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