sytech steering design

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S y t r i c s A u t o m o t i v e

D e s i g n C o n s u l t a n c y

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Steering Design Tip

How to really design Steering Geometry for your vehicle is

being demonstrated in this paper.Also taking you through

steering geometry basics to help you make better steering

design for future.

Sytrics-Steering

Design


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Sytrics-Steering Design 2010

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When youre designing steering kinematics, the goal is to orient the tire to the road in the optimal

orientation. But, how do you know the optimal orientation? Tire data, of course! One of the basic

decisions when designing a steering system is how much Ackermann you want. The answer to this is

determined directly by the tire characteristics, and you can answer this question by using Sytrics Design.

The Ackermann Steering Geometry

Ackermann steering geometry was patented by Rudolph Ackermann in 1817. The wheels in this

system pivot on a rotating member. The pivot point of the rotating member is attached to the

end of the axle while the end of the arm is attached to a translating linkage directly, or through

another linkage. When the vehicle is moving in a straight line, the attachment points of the

rotating member to the other points is parallel to the direction of travel. As the translating

member moves toward one side, the wheels each pivot about the axle point, causing the car to

turn. However the original geometry had failed to solve the problem that whe n turning the

wheels turned at the same angle causing scrubbing of the wheels over the ground. In the

modified Ackermann geometry, when the vehicle is moving in a straight line, the lines formed

by the pivot points of the rotating member converge to a point at or between the front and

rear axles. The result is that when turning, the wheel on the inside of the turn rotates at a

greater angle than the outside wheel. In these systems the location of the translating member

could be in front of or behind the front axle. The illustration below shows the improved

Ackermann geometry with the convergent point at the rear axle and its translating member

behind the axle.The steering geometry was analysed from the plan view. The rotating T, tie-rods andsteering arms were assumed to be rigid, rotating bodies.


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Bump Steer

Our aim should be designing steering system to an extreme condition; therefore it is essential that it

may track uneven terrain. The vehicle steering system should therefore not be subject to bump steer.

Bump steer is when the wheels essentially steer themselves whilst traversing difficult terrain. Bump

steer may be prevented by using the proper length and angle of tie-rod. The tie-rod should extend to

where the Stub axle carriers.

Steering in Muddy Terrain

Ackermann steering principle reduces the minimum turning circle and the lateral forces on the tyres

when rounding corners. However, Ackermann steering is not as beneficial on muddy terrain because

tyre friction and hence lateral forces are reduced. Therefore, the Ackermann steering principle should

not be implemented to the detriment of suspension and handling.

Handle Bar Steering System

The design of the system is to have the handle bars attached to the steering column via a stem. The

handle bars are the source of user input to the system. The outer steering column is connected to thechassis via a plate. The inner column rotates and is attached to the T-piece. The T-piece is in turn

connected to the tie-rods via bearings. The tie-rods act as linkages to the steering arms. The steering

arm in this vehicle design is apart of the stub axle carrier assembly.


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Modelling

The steering geometry was analysed using a combination of AutoCAD and Microsoft Excel.

Excel Steering Mode

The steering geometry was analysed using Microsoft Excel. The excel document uses the Ackermann

Equations and Trigonometry. The appropriate equations were inputted into a excel spreadsheet

capable of calculating the design angles of the steering system.

For Ackermann Steering: and

Where L is the wheelbase and R is the turning radius from centre of rotation to inside wheel & B is the

track width.

User Input: -

Offset = La (m)

T bar Length = L1 (m)

T bar Transverse Length = L2 (m)

Steering Arm Length = L3 (m)

Steering Arm Angle = (degrees)

Handlebar Rotation = (degrees)

Starting Coordinates: -

a = 0, La

b1 = -L2, (La + L1)

c1 = L2, (La + L1)


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d1 = ,

e1 = ,

Therefore: -

ax = 0

ay = La

b1x = -L2

b1y = La + L1

c1x = L2

c1y = La + L1

d1x =

d1y =

e1x =

e1y =

Length of Tie Rod: -

= a tan (L2/L1)

For a Rotation : -

b2x =

b2y =

c2x =


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c2y =

Left hand Tie Rod and Steering Arm

Length L5L =

1 =

=

1,2 =

1 = (= 0 = outer wheel steer angle)

All equations and inputs can be put into the excel sheet in an intuitive manner so it could easily be used

by designers. Due to the continuous nature of any project, Sytrics strive to make any documents as

accessible and easy to understand as possible so that future groups may benefit.

sytech steering design

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