GB2428074A - An automatic torque biasing differential in a racing kart - Google Patents

Abstract

A racing kart 110 has an automatic torque biasing differential 126 mounted between bearings 128, 130 to the rear and left of a drivers seat 114 so that drive and brake torque is transmitted from the differential 126 to rear wheels 138, 140 via a pair of driveshafts 142, 144 fitted with constant velocity joints. A brake disc 132 is mounted on the differential 126, spaced from a sprocket 14, and is associated with a bake calliper 134 secured to a steel bracket 136 forming part of a structure of a main chassis 112. The chassis 112 is planar in nature and formed from welded or brazed tubular steel. A steering mechanism having a steering rack and track-rods acting on steering arms integral with a stub axle assemble is also provided. The driven and steerable wheels have a negative chamber angle of between 0-2 degrees.

Description

Improvements in or relating to Racing Karts

FIELD OF THE INVENTION

The present inventions relates to racing karts.

BACKGROUND ART

Karting is a form of motorsport characterised by smaller, lower cost, simpler vehicles. Their design is constrained by Federation Internationale De L'Automobile (FIA) rules that prohibit much of the complexity present in modern-day cars. The intention is to preserve the kart as a low cost and hence accessible form of motorsport.

All previous and current kart designs use a solid rear axle to connect the : .. rear wheels and to transmit the drive and braking torque to the wheels. This is * mainly an historic consideration, as the rules drawn up in the early days of the sport specified this arrangement for simplicity and cost reasons. The use of a differential is specifically forbidden in the regulations.

In conventional road and race vehicles, when turning a corner the driven : * road wheels rotate at divergent speeds as the wheel on the inside of the curve * travels a shorter distance than the wheel on the outside of the curve. For this reason, in order to avoid unpleasant driving characteristics, the drive shaft from the vehicle engine cannot be coupled directly to a continuous axle that carries the driven road wheels. Instead, the drive shaft must be coupled to a differential gear mechanism that drives two separate half shafts. Each half shaft carries a respective wheel. En this manner the wheels can be driven at different speeds of rotation.

In a vehicle having a conventional differential gear system, the vehicle drive shaft, which forms a connection from the gearbox or transmission case, has a bevel pinion that engages a larger bevel gear wheel called a crown wheel.

The crown wheel is secured to a differential casing in which differential pinions are mounted. The differential pinions are rotatably mounted in the casing.

When the casing rotates due to rotation of the crown wheel to which it is secured, the differential pinions revolve around the wheels' axis as they are carried by the rotating casing.

When the vehicle is travelling straight ahead, the casing rotates, but the differential pinions do not spin about their individual longitudinal axes. These pinions are engaged with bevel gears that are rigidly connected to the inner ends of the half shafts so that the latter rotate at the same speed. When the vehicle goes around a corner, one half shaft rotates at a slower speed than the other, causing the differential pinions to spin about their respective axes in the casing.

The action of the differential pinions retards rotation of the bevel wheel of one half shaft and at the same time accelerates the bevel wheel of the other half shaft.

SUMMARY OF THE INVENTION * ** * * I **..

** The solid rear axle constrains the rear wheels to rotate at the same speed. **S.

In a cornering vehicle, however, the inner wheel will travel at a lesser forward * speed than the outer wheel since the lower radius arc along which it is travelling will be shorter in its circumferential length than the arc travelled by the outer * * wheel. Thus, the use of a solid axle requires at least one of the tyres to scrub S.., during cornering. **aS

The current solution to this problem is to design a kart with large amounts of caster and offset in the steering geometry. This causes the kart to jack up upon application of steering lock. With lock applied, the kart is thus forced to pivot about an axis defined by the inner front and outer rear wheel, with the result that the kart tips over onto the outside front wheel due to cornering forces present, raising the inside rear wheel off the ground, and thus allowing the inner wheel to rotate at the same speed as the outer wheel.

The resulting dynamic behaviour of the kart requires significant amounts of lock to be applied at the initial stage of cornering to lift the rear wheel, with subsequent reduction in steering lock to maintain the required line. This requires a driving technique colloquially referred to as the "karter's flick" whereby the driver "flicks" the steering wheel to create the initial jacking effect.

Such techniques mean that the racing style of a kart driver is distinctly different to that of a car driver. In the wet, in particular, the need to "flick" is more pronounced and requires an exaggerated and overly aggressive driving style that is at odds with the driving style required to race cars.

The fact that the chassis design is used in order to provide acceptable cornering characteristics for the kart means that those characteristics are finely dependent on that chassis shape. Subsequent changes to that shape, for example through crash damage, will alter that shape and will lead to a general degradation of those characteristics.

To the extent that karting is intended to be an accessible form of motorsport from which talented drivers can progress, this is problematic in that : such differences mean that some talented kart drivers find the transition difficult. * * **I*

The present invention therefore provides a kart with a pair of driven * ** wheels disposed one either side thereof, and a respective pair of axle sections transmitting torque to the respective driven wheel, wherein a differential is * provided, receiving torque from an engine and transmitting torque to each of the **** axle sections. * . **S.

In this way, the kart can be driven in a manner much more analogous to the style appropriate to a normal car. Drivers with significant experience of conventional cars will find karting less difficult, and aspirant drivers using karting as a stepping-stone will find the transition to car racing easier.

The differential is preferably an automatic torque biasing differential.

These typically include a member that rotates when the rotational speeds of the two axle elements is different, and a brake adapted to slow that rotation. That brake thus restrains slippage of a wheel with little traction and, through the differential casing, transfers torque to the wheel with traction.

A braking system will generally be required. A convenient manner of doing so is by way of applying a frictional force to a disc mounted to the differential.

The use of a differential allows other aspects of the kart, such as its steering, to be optimised to other considerations rather than having to enable the driver to cope with the absence of a differential. Thus, the driven wheels can now include a variation in the static negative camber of up to 2 and the toe angle, compared to the current situation where both are constrained to be zero by definition. The steered wheels can have less dramatic caster angles of between 3 and 5 in order to reduce the amount of induced positive camber and weight transfer effects during cornering, while still providing sufficient self- centring of the steering, and more optimum negative camber angles of between 0 and 2 . The kingpin offset can also be minimised to reduce the required steering effort, to a level of (for example) below 70mm as compared with : significantly over 100mm in existing designs. ***

An additional benefit is the reduction in "scrubbing" of both front and rear tyres, leading to longer tyre life and a greater racing time before the tyres "go off" and begin to lose grip. *I.

I

BRIEF DESCRIPTION OF THE DRAWINGS S. *

S S...

S... An embodiment of the present invention will now be described by way of *Se example, with reference to the accompanying figures, in which; Figure 1 shows a schematic view from above of a kart according to the present invention; and Figures 2 and 3 show sections through an automatic torque biasing differential, figure 2 being a section on Il-Il of figure 3 and figure 3 being a section on Ill-Ill of figure 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to figure 1, a kart 110 has a welded tubular steel frame chassis 112 on which is mounted a seat 114 for a driver. The main part of this chassis is planar in nature, as opposed to previous designs in which more complex designs were necessitated. This simplicity will allow more economic manufacture. A pair of front wheels 116, 118 are mounted on the chassis and are steerable through the use of a conventional steering mechanism (not shown). Although convention in construction, the specific settings for the steering may be different as will be described.

An engine 120 is mounted on the chassis 112 to the left of the driver's seat 114. Drive from the engine 120 is transmitted by a chain 122 to a sprocket 124 mounted on a differential 126. The differential 126 is mounted between bearings 128, 130 to the rear and left of the driver's seat 114. A brake disc 132 is also mounted on the differential 126, spaced from the sprocket 124, with an associated calliper 134 secured to a steel bracket 136 forming part of the structure of the main chassis 112. * **

Drive and brake torque is transmitted from the differential 126 to rear wheels 138, 140 via a respective pair of driveshafts 142, 144 fitted with *.*.* constant-velocity (CV) joints (not shown), and machined hubs. The rear wheels * S. 138, 140 are mounted on wheel carriers such that their axial position on the hub can be adjusted in order to vary the rear track. The rear attachment geometry *s.

: is such that the static negative camber is between 0-2 . *** * S *SSs

The differential 126 is an Automatic Torque Biasing (ATB) differential such as is available from R.T. Quaife Engineering Limited of Sevenoaks, Kent, England and shown in US-A-6634979. This can be contrasted to a conventional "open" differential, which splits the torque equally between the two wheels at all times.

In such differentials, the maximum torque delivered is constrained to be that of the lightest loaded wheel and during hard cornering the inside wheel can begin to spin due to the reduction in vertical load upon it. This reduces the available torque at the other wheel limiting the amount of propulsion.

An ATB differential uses a system of gears to automatically bias the torque to the wheel with the most grip, and has the advantage of not requiring regular adjustment and maintenance when compared to a traditional plate (or Salisbury) type Limited Slip Differential.

A disadvantage of conventional differential mechanisms is that they permit one of the driven wheels to spin excessively if that wheel has less tractive ability than the other wheel, such as occurs under conditions of ice or mud. The total tractive ability of the vehicle is then limited to only one wheel. As the slipping wheel overspins, the differential pinions reactively spin and consequently reduce the torque supplied to the non-slipping wheel.

Several differential mechanisms have been proposed to overcome this disadvantage. One particularly advantageous design is the ATB differential. This differential gear system includes a casing 12 in which two collinear sun gears 28, are journaled, the casing 22 configured to rotate about the axis of rotation of the vehicle's wheels. Each sun gear 28, 30 has a splined connection to one of the half shafts 24 that are connected to the wheels, such that rotation of a wheel * causes likewise rotation of its respective sun gear 28, 30. Two sets of planetary differential pinions 46, 60 are journaled in cylindrical pockets 44 in the casing *.SS *.. 22, the pockets 44 being parallel to the axis of rotation of the wheels. Each set of pinions 46, 60 is engaged with one of the sun gears 28, 30, such that the pinions 46, 60 surround the sun gear 28, 30. Also, each pinion 46, 60 of one set is adjacent to and engaged with two pinions 60, 46 of the other set, such that as S. * : one set of pinions 46, 60 rotates in one direction, the other set of pinions 60, 46 *. rotates in the opposite direction.

Collectively, the two sets of engaged differential pinions 46, 60 surround the axis of wheel rotation. The sun gears 28, 30 and the differential pinions 46, have helical teeth 28a, 30a. The casing 22 includes end plates that enclose the pinions 46, 60 in the cylindrical packets of the casing 22. Each of the packets also encloses a thrust plate on the outer axial end of the enclosed differential pinion 46, 60. The thrust plates are non-rotatably secured with respect to the casing.

This differential mechanism operates as follows. When the vehicle is travelling straight ahead, the casing 22 rotates. This causes the differential pinions 46, 60 to revolve about the half shafts 24. However, the pinions 46, 60 do not spin about their respective longitudinal axes. Since the differential pinions 46, 60 remain stationary with respect to the casing 22, the sun gears 28, 30, which are engaged with the pinions 46, 60, likewise do not rotate with respect to the casing. Thus, the sun gears 28, 30, along with their respective half shafts 24 and wheels, rotate at the same speed as the casing 22. In this manner, rotation of the casing 22 causes rotation of the wheels.

When the vehicle travels along a curve, the differential pinions 46, 60 rotate to slightly accelerate the speed of the wheel on the outside of the curve and to slightly decelerate the speed of the wheel on the inside of the curve.

When one wheel slips (for example, due to ice or mud on the road surface), the acceleration of the slipping wheel causes its respective sun gear 28, 30 to accelerate. The helical teeth of the sun gear 28, 30 mesh with the complementary helical teeth of the engaged set of differential pinions 46, 60, causing the sun gear 28, 30 to be thrust axially inward and the differential ::. pinions 46, 60 to be thrust axially outward against their respective thrust plates.

**.. The thrust plates apply a frictional force against the pinions 46, 60 that retards **..

the spinning of the pinions 46, 60 and the slipping wheel. This frictional * ** *.* resistance to differential action prevents to some extent a reduction of torque to the non-slipping wheel. I. *

*..: US-A-6,634,979 describes a further refinement to the ATB differential in that the thrust plates 48 are adjustable by way of an externally accessible threaded member. Thus, the degree to which the differential intervenes can be adjusted.

The fitment of the differential 126 allows the specification of the steering geometry to be made on the basis of desired steering characteristics rather than to compensate for the solid rear axle. This provides a kart that is more like driving a car, easing the transition from kart to car racing. Thus, the front wheels 116, 118 are mounted on stub axles 146, 148 designed such that the caster and negative camber take values of between 3-5 and 0-2 respectively.

The steering is by a steering rack and track-rods acting on steering arms integral to the stub axle assembly.

Our design therefore radically changes the design of a racing kart with the inclusion of an ATB differential. As described above, the ATB differential transfers torque to the outside rear wheel when grip is lost on the inside rear. In the context of a kart race, there is a high probability that the inside rear wheel will lift slightly simply due to the leveraging effect of the driver on the outside rear wheel. It is therefore important in optimising the cornering ability of our racing kart that the differential does not transfer torque to the inside rear when grip is reduced, otherwise the inside rear could start losing grip all together because of the increase in torque. Therefore to maintain power throughout the corner the differential should transfer torque to the outside rear wheel, i.e. the wheel with the most weight acting upon it (due to the leveraging effect of the driver) and therefore the one which provides the most grip in a corner. At present, an ATB differential is the only design which achieves this within the constraints of reasonable cost, weight, reliability, maintenance requirements etc. : .. An open differential would not provide optimal cornering characteristics in a competitive setting, as the lifted wheel would receive the majority of the *S.

torque resulting in no drive being transmitted to the wheel still in contact with the track, therefore losing drive. It would however provide the desired handling characteristics to the kart and could therefore be employed in the early stages of * training at lower speeds, for example.

* A limited slip (or Salisbury) differential would be better than an open differential as drive is not entirely lost in such a situation. However, limited slip differentials need regular adjustment and maintenance as they have moving parts which are likely to wear quickly. Also, they are a compromise in that when adjusted to a high-slip setting, they will transfer a lot of the torque to a lifted drive wheel (like an open differential), and when adjusted to a a low-slip setting will mimic a solid axle to a greater degree resulting in understeer. Therefore, using an Automatic Torque Biasing differential is the preferred solution to this problem.

The kingpin offset can be reduced significantly compared to existing kart designs. The precise value adopted will be a compromise affected by several other factors, but the removal of the constraint imposed by on previous designs will allow a better compromise to be selected.

The stub axles 146, 148 are attached to the chassis 112 by means of a bolted joint, passing through the chassis_spacer-bearing-spacer-bearingspacer chassis, where the bearings are fitted to the stub axle.

It should be understood that the above-described kart is a racing kart, not an off-road kart. An off-road kart is shown, for example, in US2004/0129489.

The major difference in practice between a racing kart and an off road kart is that a racing kart is specifically designed to make use of the high levels of grip provided by a tarmacced surface, while an off road kart is capable of steering to an acceptable degree on both low and high grip surfaces. It is almost impossible for a racing kart to turn in to a corner without the high levels of grip provided by a tarmacced surface. This is why a racing kart can not be used on off road low * grip surfaces and indeed why a racing kart is exceptionally difficult to turn in on a wet tarmacced surface because the grip levels are much lower. It is also the S... * .

primary reason why racing kart drivers suffer from the "karting flick" where they aggressively turn the kart in to the corner rather than progressively and smoothly as one needs to do when driving a racing car.

* In terms of design, an off-road kart will be characterised by a soft *. *.

suspension system such as is clearly visible in US2004/0129489. A racing kart will typically have no suspension at all or very hard suspension in which the permitted vertical movement of the wheels is small (as for example compared to their diameter).

Generally, racing karts can be distinguished from off-road karts by a number of features, usually including one or more of: - Brazed/welded steel tubular frame - No suspension other than that provided by the flex of the chassis and the deformation of the tyres - Wheels less than approximately 6" outer diameter - Planar chassis - A maximum ground clearance of approximately 50mm, offering the low centre of gravity necessary to achieve high cornering speeds - Slick tyres to maximise grip when circuit is dry - Front, rear and side bumpers to protect kart from impacts during races and to reduce likelihood of wheel-to-wheel contact Chassis dimensions limited to the minimum needed to encompass driver, seat, engine, steering mechanisms, braking mechanisms and crash structures in order to keep weight and size to a minimum - Single seated - Internal combustion engine or electric motor providing the motive force : **, Four wheels S...

U..... - Braking system that applies to either the rear two, or all four of the * *. wheels * S * * *.

A racing kart is distinct from other types of vehicle commonly referred to as karts' and is usually designed for the sole purpose of being driven on a hard- surfaced circuit, usually constructed from tarmac or concrete. A circuit can be defined as a course That begins and ends at the same point.

The chassis of a racing kart is the structure to which all other components are attached, such as the seat, engine, wheels etc. For a racing kart this structure is predominantly planar, with the majority of tubes lying in a horizontal plane, with only minimum protrusion in ffie vertical plane for seat and steering column mounts. Current racing karts are constrained to use a chassis fabricated from steel tubing by the regulations, though a chassis could also be constructed using any other structural alloy, from plastic materials or fibre-reinforced composites.

A racing kart differs from many other types of vehicle in that if does not have "suspension", which can be defined as a mechanism or group of mechanisms or links, which connect the chassis to the wheels, but allow each to move independently of each other through a prescribed locus defined by the geometry of the links. These mechanisms typically have a spring incorporated, and a method of dissipating the energy due to vibrations. The absorption of energy from cornering loads and bumps in a racing kart is instead achieved through deflection of the tyres, and the chassis itself.

The wheels are typically less than 155mm in diameter and as a consequence the ground clearance, the minimum distance between the ground and a part of the kart other than a wheel, is typically 50mm or less. A racing kart uses pneumatic tyres, mounted without an inner tube on wheel rims, and are "slick" (tread-less) for dry use, and treaded for wet use, and has four wheels upon which it is suspended. * I.

No roll-over protection is typically provided by a racing kart, with passive safety features taking the form of plastic "bumpers" and "sidepods" mounted to the front, rear and sides of the kart to provide energy absorption during * ** * collisions. S..

* The size of a racing kart is limited by the regulations but is such that it is no longer, wider or taller than it needs to be to incorporate all the components required to make if move, and a seat for the single occupant.

Motive power can be provided via one or more internal combustion engines (utilising either the two or four stroke cycle) or one or more electric motors. Retardation is normally achieved via a disc brake system, operating on either the rear wheels only, or on all four wheels.

It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. * * S S **** S... * S *S.. * .. * S S * S.

S *5* *5 *

S **S. * . *S..

Claims (15)

1. A racing kart with a pair of driven wheels disposed one either side thereof, and a respective pair of axle sections transmitting torque to the respective driven wheel, wherein a differential is provided, receiving torque from an engine and transmitting torque to each of the axle sections.

2. A kart according to claim 1 in which the differential is an automatic torque biasing differential.

3. A kart according to claim 1 in which the differential includes a member that rotates when the rotational speeds of the two axle elements is different, and a brake adapted to slow that rotation.

4. A kart according to any one of the preceding claims including a braking system in which a frictional force is applied to a disc mounted to the differential.

5. A kart according to any one of the preceding claims having a welded or brazed tubular steel frame chassis

6. A kart according to any one of the preceding claims in which the attachment geometry of the driven wheels includes static negative camber of between 0-2 .

7. A kart according to any one of the preceding claims in which the : *. attachment geometry of the driven wheels includes substantially zero toeIn. * S S...

8. A kart according to any one of the preceding claims having a pair of * steerable wheels mounted on stub axles.

9. A kart according to claim 8 in which the steerable wheels have caster a. * : angles between 3 and 50 *..S

S

10. A kart according to claim 8 or claim 9 in which the steerable wheels have a negative camber angle of between 0 and 2 .

11. A kart according to any one of claims 8 to 10 in which the steerable wheels are controlled by way of a steering rack and track-rods acting on steering arms integral to the stub axle assembly.

12. A kart according to any one of claims 8 to ilin which the driven wheels and the steerable wheels are distinct.

13. A kart according to claim 12 in which the driven wheels are to the rear of the kart and the steerable wheels are to the front of the kart.

14. A kart according to any one of the preceding claims in which the axle sections and/or the stub axles are connected to the remainder of the kart in a substantially rigid manner.

15. A kart substantially as herein described with reference to and/or as illustrated in the accompanying figure 1 and/or figures 1, 2 and 3. * ., * *S.* I., * * ***. * ** * * S * S. *5*

S S. *

S S... *..S

S S...