GB2484675A

GB2484675A - Motorcycle front fork with asymmetric stiffness

Info

Publication number: GB2484675A
Application number: GB1017590.9A
Authority: GB
Inventors: Tejpal Singh Bharij
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-10-19
Filing date: 2010-10-19
Publication date: 2012-04-25
Also published as: GB201017590D0

Abstract

A motorcycle front suspension comprises a telescopic fork in which the tubes and the sliders are rectangular in cross section, one on each side of the wheel. The rectangular tube is clamped in the yokes such that the wider dimension of the rectangular tubes is parallel to the plane of rotation of the front wheel, providing greater longitudinal rigidity (L) and lower sidewise rigidity (S). Alternatively tubes and matching sliders of any non-circular shape and cross-section may be substituted for the rectangular tubes. In another embodiment, two or more tubes/sliders on may be used either side of the wheel, arranged one behind the other, parallel to the direction of travel, or if the steering wheel is not centred, parallel to the plane of rotation of the front wheel. Four tubes 9 are clamped in yokes 4 and 6, while a pair of slider assemblies 8 carry the front wheel 2, free to rotate around axle 5.

Description

Motorcycle Front Suspension This invention relates to motorcycle front suspension intended primarily for high performance racing motorcycles as used in MotoGP and World Superbikes. It may also be used in all other types of motorcycles including high performance machines for normal road use.

Similarity to present-day road-racing motorcycle front suspension has been retained only in the following aspect: Telescopic forks, whether upside-down or normal, have been retained. The reason for this is to maintain the vitally important feedback a rider receives, i.e."feel" which is absolutely necessary for a rider to confidently execute his/her duties. For the purpose of this invention the words "telescopic unit" is to be understood to mean the complete assembly of a fork tube and it's slider, with all the internal shock-absorber mechanisms in place and functional, shock-absorbing medium or hydraulic oil present to correct amount, the spring duly fitted inside and the unit pressurised if so designed. Two adjectives are used for forces, deflections and br vibrations in this article also for the purpose of this invention and they are: "longitudinal" which should be understood to mean "in the direction of travel of motorcycle or the front wheel if the steering is not exactly centred and a direction exactly 180 degrees to this direction", in simple words "back and forth" , and lateral" which should be understood to mean "the two opposing directions which are perpendicular to longitudinal" in simple words "sideways". See figure 1,where I stands for longitudinal forces or deflections, and S stands for lateral forces or deflections.

The drawings are of an inferior standard as they are all drawn by hand in order to avoid involving a third party for security reasons. The drawings may not show a complete item, the purpose being to clarify with minimum sketching.

In the past five or six decades there have been tremendous achievements and technological advances in the design and development of motorcycle frames, brakes, wheels, rear suspension and engines. In all these separate systems the power, capacity, and performance have improved considerably. The only exception is the motorcycle front suspension. The telescopic forks have basically remained the same in the last 60 years. Slight improvements were made by fitting them upside-down, better internal damping mechanisms and pressurising them. Some manufacturers opted for wider diameter telescopic units in order to achieve improved power/capacity and to achieve more rigidity. The result was disappointing because at maximum angles of lean the front wheel lost grip far easier than with the original narrower-diameter tubes/sliders.

As the motorcycle reaches its cornering limit it is leaned over at around 50 to 60 degrees and the suspension is usually at the end or very near the end of its travel (fully compressed) . At this stage the only form of shock-absorption available is the tyre itself or the flexibility of the telescopic units.

The importance of the "sideways", lateral flexibility of the front suspension must not be underestimated. The bumps and irregularities in the road surface will always be in an upright ( vertical) direction whereas at maximum angles of lean the suspension travel tends to be more horizontally effective and less effective vertically hence very little use in accommodating vertical variations in the road surface. If Q is the angle of lean from the vertical, (riding in a straight line the motorcycle is upright, i.e.vertical, so Q = 0), and h is the total suspension travel, the effective vertical travel of the suspension is given by hcosQ. Clearly as the angle of lean increases the value of hcosQ diminishes. Hence at greater angles of lean there is urgent necessity of providing an alternative form of shock-absorption other than the normal suspension travel. This invention addresses this problem primarily, at the same time addressing some of the other problems

mentioned further down in the description.

The rigidity/flexibility of the present-day motorcycle front suspension has to be analysed in order to fully understand and appreciate this invention. All the forces acting on and the resultant deflections and/or vibrations caused by the front wheel moving over irregular surfaces are first translated to the front-wheel axle and the lower ends of each fork leg. Hence these will be considered. The ideal front suspension would be very rigid in the direction of travel of the front wheel yet be flexible laterally or "sideways" to offer shock-absorption at greater angles of motorcycle-lean. This is where this invention considers directional forces and deflections rather than the omni-directional rigidity/flexibility taken for granted by the present-day telescopic unit (each fork leg is equally rigid in all directions around its longitudinal axis, be it in the direction of travel of front wheel or laterally i.e.at right angles or "sideways") The situation is made worse by the fact that the lower fork-ends being rigidly secured to the wheel-axle provides a box-section (formed by the axle-bolt,the two fork legs and the steering head clamp)which is far more rigid laterally(sideways) than in the direction of front wheel travel. See figures 2 and 3. Figure 2 is a diagrammatic representation of the forces acting on the front suspension, for example when braking, or when encountering a substantial road-irregularity, and the shaded area to the right represents the effective area of cross-section being forced to bend. The forces are longitudinal, i.e. in the plane of direction of travel. Similarly figure 3 is a diagrammatic representation of the forces acting on the front suspension laterally, i.e. "sideways"or at right angles to the direction of travel. To the right of this is a shaded area which represents the effective area being forced to bend. It is clear that the shaded area in figure 3 is much larger than that in figure 2, which means that a conventional front suspension is far more rigid laterally ( sideways) than it is longitudinally. Ideally what is required is the opposite of this.

Preferably the front suspension should be very rigid in the direction of front wheel travel but have some amount of flexibility laterally. See figure 4 which shows two solid metal beams where the upper beam is omni-directionally rigid whereas the lower beam is rigid when forced against its narrower surface but quite flexible when forced against its wider surface. In figure 3 "C" stands for concrete base, "R" for Rigid, and "F" for Flexible.

The invention proposes a front suspension which while maintaining the telescopic fork principle has each fork leg with a cross-sectional shape that is narrow when viewed from the front of the front wheel yet is wide when viewed from the side of the front wheel. The cross-sectional form may take any shape such as a rectangle, ellipse, oblong, symmetrical aerofoil or any other shape such that the wider dimension of the cross-section is parallel to the direction of travel of the front wheel. This invention greatly increases the longitudinal (fore-and-aft) rigidity of the assembly while at the same time imparts greater flexibility laterally (sideways).

Considering that the fabrication, and manufacture of a rectangular-shaped or an elliptical-shaped telescopic unit is more complicated and financially prohibitive, each side of the fork leg or side may also consist of a multitude of cylindrical-shape telescopic units, the minimum being two units per side arranged one behind the other forming a row(see fIgureS) which is parallel to the direction of travel of the front wheel, the intention being that a multitude of cylindrical units assembled in a row have the same effective rigidity/flexibility as a single rectangular-shaped or ellipse-shaped telescopic unit See figure 6, which represents the side view of the front suspension and wheel. The two telescopic unfts shown on one side of the wheel have a cross-section marked "C" and the effect of this cross-section is as shown by "D".

In order to retain the original power/capacity the tubes/sliders are much nan'ower. It is assumed that the total damping power of the suspension is proportional to the total volume of the damping medium(hydraulic fluid). Since suspension-travel is the same as original so the volume is proportional to the cross-sectional area of the base. This area of cross-section originally shared by two circles ( one on each side of the wheel) is now shared by four circles (two on each side of the wheel). Hence each tube/slider (telescopic unit) will have half the area of cross-section as the original. However, the diameter of the telescopic units is not halved, as will be shown by following example: Consider modern racing forks of diameter of 48 mm. Area = nr2 = 1809 sq. mm. Halving this gives 904.5 sq. mm., radius: 16.96 mm, hence diameter = 33.94 mm. Consider also the surface the area in both cases, if nominal length of fork is 760mm, then the 48 mm fork circumference is 150.8 mm., and surface area = 760 mmX 150.8 mm = 114608 sq. mm.

The 33.94 mm fork tubes give circumference 106.6 mmX 760 mm = 81035 sq. mm., but there are two, 81035 X 2, hence surface area = 162070 sq. mm.

It can be seen that this invention gives an increased surface area for a given volume of damping fluid. This results in increased efficiency of dissipating the heat produced by the damping process.

Increased surface area also means that more material is needed for manufacture, which in turn means that the material for manufacture can be much thinner gauge while still maintaining the structural integrity of the assembly. Thinner gauge material further helps in heat-dissipation through the fact that the damping fluid is nearer to the outside cooling air.

The following formula has been derived for calculating the radii r of a multitude of tubes replacing a single original tube (telescopic unit) of radius R, where the original volume is now shared by n number of tubes: r = . Also if A1 is the surface area of the original single tube and A2 is the total surface area of n tubes replacing it, then it can be derived that A2: A1 is equal to n:1/31 Additional advantages of multiple telescopic assemblies per side of front wheel over the present-day conventional front racing-forks: 1. The frontal area for ram air (necessary for engine power) is increased. See figure 7.

2. More precise steering: The gyroscopic inertial forces working against turning left or right are overcome more easily by the four-tube assembly than the conventional two tubes. See figure 8.

3. Less handle-bar vibration ( around the steering axis caused by transmission of road irregularities), hence less possibility of" arm-pump" or "compartment syndrome": Please refer to figure 16 A and 16 B. Figure 16 A which represents conventional front suspension as viewed from the front of the motorcycle being deflected by a road irregularity (the dotted lines show an exaggerated deflection of the forks and wheel axle 5.) In 16 A it is clear that the front wheel axis S is being tilted (angular displacement, with the angular axis being in the middle of the steering head-stock, perpendicular to the steering axis) This to-and-fro tilting of the wheel-axis is translated,due to the gyroscopic precession of the rotating wheel, as angular vibrations of the steering handle-bars. Compare this with figure 16 8 which represents a multi-tube front suspension assembly which through design being flexible laterally(sideways) will flex rather in the form of two adjacent "S"es. The reason for this form of flexing is the fact that, a, the wheel axle is fixed rigidly at right angles at each end to the fork assemblies at the lower end, and b. the gyroscopic inertia of the rotating wheel will allow linear translation of the wheel-axis rather than allow tilting of the wheel-axis.

4. Lower possibility of front-wheel chatter: Vibrations in the acoustic range generated by the front disc brakes, especially the ones with carbon discs, have a substantial negative effect on the handling of the motorcycle. By having multiple telescopic units on either side of the front wheel the natural acoustic frequency of the front fork assembly is at least halved, so the possibility of resonance Is appreciably reduced. Vibrations caused by the front brakes are further countered by the very rigid multiple-telescopic unit assembly. Please note that vibrations due to front brake action are directional i.e. acting in planes parallel to plane of wheel rotation, where this invention is more rigid than conventional front racing forks One of the reasons why motorcycle manufacturers opted for the "upside-down" configuration was to gain more rigidity by clamping the wider-diameter sliders in the clamps/yokes rather than the narrower-diameter tubes. This is no more a problem with this system because the two tubes on each side of the wheel form a box-section which is very rigid in the direction of motorcycle-travel but also allows a small amount of flexibility sideways, necessary to absorb the bumps when motorcycle is leaned over at maximum angle. For this reason there is no more a necessity to have "upside-down" configuration of the front suspension, in fact clamping the tubes in the steering-head clamps rather than the sliders will increase the lateral flexibility without jeopardising the longitudinal rigidity.

Un-sprung weight of the front suspension is an important consideration. Because two or more sliders on either side of the wheel, braced together are inherently stronger structurally than a single slider they can be designed smaller and lighter. Moreover since sliders are preferably made from a material (aluminium alloy or magnesium alloy) less dense than the steel tubes the possibility of reducing the un-sprung weight is enhanced. See figure 9 which shows one variation of the lower slider assembly (the lugs for attaching the front brake callipers are omitted for simplicity of the drawing).ln figure 6 the letter "E" stands for cast braces holding the two sliders together, and "F" is the hole for the front axle bolt or rod.

Consider the evolution of the rear swing-arm in order to see the improvements achieved. See figure 10. On the left is a typical 1960's swing-arm which has equal rigidity/flexibility both up-and-down and sideways. On the right is a typical present-day racing swing-arm with the tremendous improvement of having good rigidity up-and-down, yet is flexible enough laterally( sideways)

S

Power/Capacity: This invention allows designing the front suspension with a varied choice of power/capacity. If four tubes/sliders of the original diameter are used the power/capacity would be doubled. This is hardly likely to be used. Or one can have the same power/capacity as the original conventional suspension by reducing the diameters of the four tubes as per calculations shown earlier in the article. So the choice of increasing power/capacity goes from 0 % to 100%! There is definitely a need to increase the power/capacity of the present-day front suspension, perhaps of the order of 20-30%. This is because engine power, and most importantly the front brake power has increased tremendously since the advent of the carbon brakes. Manufacturers have found it difficult to increase the power/capacity of their front suspensions. Some manufacturers increased the diameter of their tubes/sliders to solve this problem. The result was devastating with numerous crashes through losing front wheel grip at maximum lean angles. These manufacturers promptly reverted back to narrower-diameter tubes. This invention solves this problem The invention proposes a motorcycle front suspension where the tubes are uppermost, with two tubes on either side of the wheel, clamped in top yoke and bottom yoke to accept two rather than the conventional one tube per side ( each yoke will have four holes, rather than the normal two holes per yoke for the tubes),see figures. The slider assembly, see figure 9, will be at the lower end of the suspension attached to the wheel axle on either side. The internal damping mechanisms and springs may be of any design, and the suspension units may be pressurised. The "upside-down" configuration may be used, however, maximum benefit from the Invention is obtained as suggested.

The invention will now be described solely by way of example and with reference to accompanying drawing, using figure lLThe main motorcycle body 1 is attached rigidly to the steering head-stock 7.

The steering stem which Is part of the lower yoke 6 is connected via bearings to the upper yoke 4 which is secured tight with a nut to the steering stem. The yokes 4 and 6 have four holes each rather than the conventional two holes each. The clamping mechanism for each hole ( slit and bolt) have been omitted solely for simplicity of drawing, similarly handlebar 3 which is firmly attached to the upper yoke 4 does not show any attachment bracket details. Four fork tubes 9 are fitted firmly in yokes 4 and 6. Two slider-assemblies 8, one on each side of the front wheel 2 are fitted as shown. It is to be understood that each tube/slider unit has all the internal damping mechanisms, springs and damping fluid in place. The front wheel 2 ( where the wheel rim, spokes, hub and the complete brake assembly have been omitted solely for simplicity of drawing)is free to rotate around wheel axle 5 which is firmly attached to slider assemblies 8 using pinch-bolts 10. Normal left-to-right and vice versa steering is achieved by turning handlebar 3 which in turn turns the wheel 2. Normal suspension travel is provided to move the wheel up and down.

Claims

Claims 1. A motorcycle front suspension comprising mainly of a front wheel, telescopic forks, top yoke, lower yoke, steering stem and handlebars providing three degrees of freedom for the front wheel namely wheelspin and linear translation of the said front wheel relative to the steering head and main body of the said motorcycle(suspension travel) and angular movement of the front suspension assembly about the steering axis(steering travel,left and right) whereby each leg of the fork,one on each side of the wheel, comprises a tube of rectangular cross-section sliding into a slider of matching rectangular cross-section, arranged and clamped firm by the upper and lower yokes in a manner that the wider side of the rectangular tubes is parallel to the plane of rotation of the front wheel.
2. A motorcycle front suspension according to claim 1 wherein the said sliders are attached at their lower ends to the axle of the front wheel, and the said suspension is complete with internal mechanisms, fluids,gas pressure if required, brake assembly and any other equipment necessary for the proper functioning and operation of the motorcycle.
3. A motorcycle front suspension according to claims 1 and2, wherein the said tubes and sliders are of any cross-sectional shape other than circular.
4. A motorcycle front suspension according to claims 1 and 2, wherein each leg of the said fork comprises a multitude of circular tubes and the tubes' sliders, a minimum of two on each side of the front wheel, arranged in a pair of rows which are parallel to the plane of rotation of the front wheel.
5. A motorcycle front suspension according to claims 1 and 2, 3,and 4 wherein the position of the tubes and sliders is reversed ("upside-down" arrangement).