Land vehicles – Wheeled – Occupant propelled type
Patent
1984-03-07
1986-12-16
Mitchell, David M.
Land vehicles
Wheeled
Occupant propelled type
280286, B60K 2102
Patent
active
046292055
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a motorcycle front fork assembly.
Improvements in motorcycle engine power and tire "grip" in recent years have drawn attention to deficiencies inherent in the design of conventional front forks, which are the means commonly applied to support the front wheel of the vehicle and to steer, control and balance it.
In particular the increase in performance arising from these improvements has highlighted those factors that tend to cause steering instability of the kind that is termed "flutter" or "weave". This form of instability is characterised by a side-to-side oscillation of the front forks and wheel which in extreme circumstances can lead to structural damage or loss of control. The most significant factors that can induce this oscillation are believed to be (a) changes in the steering geometry of the vehicle arising from suspension travel and (b) torsional flexibility in the fork assembly.
The overwhelming majority of motorcycles currently produced use the "telescopic" form of front fork. This comprises two parallel legs each formed as a pair of concentric tubular members having resilient suspension means acting between the tubular members. A wheel spindle is rigidly connected to, and extends between, the lower ends of the two legs. For there to be sufficient clearance between the tubular members of each leg to allow relative telescopic movement, there must also be some degree of relative lateral movement, and therefore lateral stiffness is impaired. Further, because the tubular members of each telescopic leg are free to rotate relative to each other, it is not possible to exploit the torsional properties of the individual components of the fork. Thus, the torsional rigidity of the assembly as a whole is poor. A further defect is the friction developed by telescopic forks that often prevents smooth functioning over road irregularities; this condition is known as "stiction" and becomes more apparent as the forks are raked away from the vertical.
However, a far more serious defect occurs with variations in steering geometry produced, e.g. by road irregularities.
The variations in steering geometry are best illustrated with reference to the change in angle of the steering head axis. The angle of the steering head axis is specified at the normal weight of the vehicle when statically loaded, which would generally be at one third of suspension travel at both wheels. However, it will be apparent that under practical road conditions the effective steering head angle can vary given that each wheel can move independently from full extension to full compression.
Thus under heavy braking, weight transfer will compress the front suspension fully whilst allowing the rear suspension to become fully extended, thus giving rise to a steeper head angle. Similarly, hard acceleration will cause the front suspension to become fully extended and the rear suspension to be fully compressed, thus giving rise to a shallower head angle. Road irregularities can produce a similar situation. Thus when the vehicle passes over a large bump in the road surface, first the front suspension and then the rear suspension will be compressed, thus causing changes in the effective head angle.
This change in head angle can in itself cause disturbances in the steering characteristics of the vehicle, but the major consequence is to the castor effect of the front wheel which is normally termed "trail". Trail is best described as the amount the front wheel "trails" behind the steering head axis. In practice, trail is generally specified as the distance between the tire contact patch and the steering head axis at ground level. As trail is of major importance in determining the steering characteristics of the vehicle, and in particular the self aligning or stabilising forces that contribute to the vehicle stability, any change in this value can have a serious effect on stability. Moreover it is believed that dynamic effects are generated as these changes in geometry inter-act with other forces produced by the machine's passage--
REFERENCES:
patent: 4179135 (1979-12-01), Slater
patent: 4184695 (1980-01-01), Roe et al.
patent: 4186936 (1980-02-01), Offenstadt et al.
patent: 4367882 (1983-01-01), Alexander et al.
patent: 4401316 (1983-08-01), Miyakoshi
patent: 4410196 (1983-10-01), Ribi
Dukes Mark C.
Mitchell David M.
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