GB2242400A - Vehicle suspension system - Google Patents

Abstract

The invention relates to wheel suspension systems for vehicles in which chassis mounted cantilevered arms 4, 5 are used to support at least one pair of wheelhubs in which the breadth to depth ratio at the section of maximum bending moment of the arm is greater than 4 to 1. Preferably the vehicle has two pairs of wheels and the cantilever arms are used to support both pairs of wheels. A pair of cantilever arms may be used to support each wheelhub; each pair of cantilever arms may comprise an upper and a lower arm attached to a kingpin which carries the wheelhub. The wheels may be fitted without pivots and the roll and pitch characteristics are selectable so that the system can provide both anti-squat and negative squat geometry. <IMAGE>

Description

VEHICLE SUSPENSION SYSTEM This invention relates to wheel suspension systems for vehicles in which the chassis also performs the suspension function by means of deflection.

Vehicles such as racing karts and certain low cost passenger and freight carrying designs have a rigid chassis or frame which carries the wheels directly. In particular racing karts have a tubular chassis with an extremely stiff 'suspension' which provides little control of either the roll or pitch axes; 'anti-dive' and 'anti-squat' features cannot be achieved with such suspension systems.

The present invention provides improved structural efficiency for direct chassis suspension systems by optimimising each chassis component for its particular function. The system provides improved dynamic traction and stability together with a stronger and safer driver compartment.

According to the present invention there is provided a wheel deflection support system for a vehicle of the type in which the chassis performs the suspension function, characterised in that chassis mounted cantilevered arms are used to support at least one pair of wheelhubs in which the breadth to depth ratio at the section of maximum bending moment of the arm is greater than 4 to 1.

The cantilevered hub support arms, having a large breadth to depth ratio, maximise wheel travel. This results in smaller camber angle changes and increased tyre footprint giving improved traction.

The relationship between the deflection of a cantilever chassis arm and stress is: y 2L2 f = 3ED in which y is the deflection in metres, f is the stress in Newtons per square metre, L is the length in metres, E is Young's Modulus of Elasticity for the material from which the arm is fabricated, D is the depth of the section in metres and B the breadth of the section in metres.

This relationship shows that the deflection of the cantilever arm which carries a wheel, and hence the wheel travel, is inversely proportional to the section depth of the arm. It will be seen that the thinner the cantilever arm the greater the distance the wheel will travel for a given amount of stress. In order to maintain the section modulus, BD2 /6 for a rectangular section, the breadth B must increase by the square of the depth reduction. Such an increase is easily accomodated and assists rigidity in the longitudinal plane. It is possible to use both upper and lower cantilevers in order to increase lateral stiffness by reducing the lateral bending moment. Each cantilever may be laminated to reduce further the section depth.

In a preferred embodiment a pair of cantilever arms is used to support each wheelhub. More preferably each pair of cantilever arms comprises an upper and a lower arm attached to a kingpin which carries the wheelhub.

Such an arrangement suits the front, steered, wheels of a kart. When the vehicle has two pairs of wheels cantilever arms may be used to support both pairs of wheels.

In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which: In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which: Figure 1 is a front elevational view of a deflection system according to the invention, Figure 2 is a rear elevational view of a deflection system according to the invention, and Figure 3 is a schematic side view of a kart.

A pair of front wheels 1 for a racing kart are carried by stub axles 2. The axles 2 are pivotably attached to a king pin allowing rotation about axes 3. Lower cantilever support arms 4 and upper cantilever support arms 5 locate the kingpin and are attached by bolts 6 to a chassis 7 which passes under the drivers legs, not shown, when the kart is driven. The lower support arms 4 deflect about points 8 and the upper support arms 5 about point 9 on the chassis 7.

A pair of rear wheels 10, see Figure 2, are attached to an axle 11 which is supported in bearing assemblies 12 attached to upper and lower wheelhub support arms 13 and 14.

The upper support arms 13 are attached to a chassis frame 15 by bolts 16 and the lower support arms 14 by bolts 17.

The upper arms 13 deflect about a point 18 and the lower arms 14 about a point 19. A drive sprocket 20 and a brake disc 21 are mounted on a sleeve 22 which is attached to the axle 11 by a key 23 at a point in line with the centre od gravity of the kart.

The relative angles of the support arms determine the origin of the virtual swing arm. A front wheel 30 and a rear wheel 31 of a kart, see Figure 3, have respective upper and lower support arms 32, 33, 34 and 35. The relative angles of the support arms 32 and 33 for the front wheel 30 meet at a point 36 ahead of the kart. The relative angles of the rear wheel 31 support arms 34 and 35 meet at the point 37 between the wheels. The point 36 represents the origin of the virtual swing arm. The position of the points 36 and 37 relative to the centre of gravity of the kart 39 determines the pitching of the chassis under acceleration and braking conditions. Class One karts use only a single rear brake, so it would be advantageous not only to minimise weight transfer off the rear wheels under braking conditions but to actually increase the weight transfer to the rear wheels.The intersection of support arms 34 and 35, point 37 is preferably the location of the crankshaft of the kart engine, not shown.

The tyre of the wheel 31 makes contact with the ground at a point 38. A line is shown drawn from the point 38 through the point 37. When this line is extended it cuts the line normal to the ground drawn through the centre of gravity 39 at a point 40. The point 40 is located above the centre of gravity 39 of the kart. This illustrates a negative squat geometry giving forward weight transfer under acceleration and rearward weight transfer under braking.

If the point 40 lies between the ground and the centre of gravity 39 its height would determine the percentage of anti-squat (reduced weight transfer to the rear wheels under acceleration). Conventional anti-dive geometries applied to braked front wheels have the origin of the virtual swing arm 36 behind the wheel. However for unbraked front wheels, as in Class One karts, some antidive effect can be achieved at the front wheels by using the inertia of the wheel assembly and an origin 36 of the virtual swing arm positioned ahead and above the wheel axle as shown.

Racing karts have a basic understeering characteristic due to the fact that driving torque is supplied direct to the driven wheels without the use of a differential gear. When turning, the inside wheel is rotating at the same speed as the outside wheel and produces forces which react against the turn and urging the vehicle to maintain a straight course. The conventional solution to this problem is to cause the rear inside wheel to lift when turning so as to reduce or eliminate any driving force from this wheel.

The lift on cornering is obtained by using large castor angles and wheel offsets (scrub radii) so that the front wheel jacks the rear inside wheel off the ground as the kart turns into a corner. This method of obtaining cornering lift has the disadvantage that with large castor angles the camber angle changes as the steering angle changes. The front inside wheel camber angle becomes more positive and the front outside wheel camber angle more negative as the steering angle increases. The width of the front tyre footprint is substantially reduced due to the resultant convex tyre tread profile, thus markedly reducing front end grip. However front end grip determines the total level of grip and its reduction is responsible for the low level of engine torque available when coming out of slow speed corners. In such a situation it is necessary for the rear tyres to spin initially in order that the engine can accelerate rapidly into a higher torque region of its speed range. The manoeuvre is not assisted by the large rearward weight transfer which occurs during acceleration which decks the rear inside wheel again.

The use of two cantilever arms per wheelhub requires a central chassis section having sufficient depth to locate the inner ends of the upper arms. The resultant chassis centre section has greater torsional stiffness and beam stiffness than the conventional single plane chassis and offers greater crash protection to the driver. By optimising each component for its particular function the need no longer arises for the centre section of the chassis to be flexible. The introduction of fundamentally correct solutions to the various design aspects of a kart avoids the need for conflicting compromises.

The use of upper and lower cantilevers in a kart according to the invetion with large breadth to depth ratios provides further advantages as well as improved ride with greater wheel travel. The kart will naturally lift its rear inside wheeel on turning as weight is transferred diagonally to the front outside wheel when going into a corner; jack steering is no longer necessary. The large castor angle can be eliminated and the camber angle much more closely controlled allowing the use of flat treaded, wide footprint, front tyres providing substantially increased front end grip.

The twin cantilevers can be considered as simulating the 'wishbone' suspension of sophisticated racing cars but without the use of pivots. The geometry of the cantilevers can be arranged to optimise camber angle changes in bump and roll, pitch and roll axes and anti-dive anti-squat characteristics. The application of anti-dive anti-squat principles eliminates the large weight transfer changes under braking and acceleration conditions and also increases braking efficiency when using rear axle braking systems in which the axle carries a single disk. The decking of the rear inside wheel under acceleration is delayed allowing earlier acceleration through the low engine speed range when coming out of corners.

Pairs of upper and lower cantilevers may also be used to support the rear axle carriers in order to introduce some rear wheel travel for improved traction over uneven or bumpy surfaces. the relative angles of the rear cantilevers in side elevation determine the anti-squat charactersitic.

The rear drive sprocket is conventionally located on the outer axle section between the rear wheel and the chassis frame of the kart. The sprocket becomes less vulnerable to kerb damage when positioned for minimum torque-steer effects on the central axis section inside the chassis frame. Such use also allows a narrower rear track to be used which assists driving over wet surfaces. The drive sprocket and brake disc can be mounted on a common hub.

The position at which the hub is attached to the axle will then determine the torque-steer effects from axle wind-up.

In an alternative construction the rear cantilever arms are turned through a right angle to operate as trailing arms.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims (12)

1. A wheel deflection support system for a vehicle of the type in which the chassis performs the suspension function, characterised in that a chassis mounted cantilevered arm is used to support each wheelhub of at least one pair in which the breadth to depth ratio at the section of maximum bending moment of the arm is greater than 4 to 1.

2. A wheel deflection support system according to claim 1, characterised in that the vehicle has two pairs of wheels and cantilever arms are used to support both pairs of wheels.

3. A wheel deflection support system according to claims 1 or 2, characterised in that a pair of cantilever arms is used to support each wheelhub.

4. A wheel deflection support system-according to claim 3, characterised in that each pair of cantilever- arms comprises an upper and a lower arm attached to a kingpin which carries the wheelhub.

5. A wheel deflection support system according to any of the preceding claims, characterised in that the chassis has a depth exceeding one quarter of the wheel diameter.

6. A wheel deflection support system according to any of the preceding claims, characterised'in that the rear wheels are carried by an axle driven from a centrally located drive sprocket.

7. A wheel deflection support system according to any of the preceding claims 1 to 5, characterised in that the rear wheels are carried by an axle fitted with a drive sprocket and a brake disc located in a position which provides minimum torque-steer effect.

8. A wheel deflection support system according to any of the preceding claims, characterised in that wheels are fitted without pivots and the roll and pitch characteristics are selectable.

9. A wheel deflection support system according to any of the preceding claims, characterised in that it can provide both anti-squat and negative squat geometry.

10. A wheel deflection support system according to claim 9, characterised in that the front wheel is unbraked and the geometry is anti-squat.

11. A wheel deflection support system as claimed in claim 1 and as herein described.

12. A wheel deflection support system as herein described and illustrated-in the accompanying drawings.