Modified Ground Hog FWD -plus a new tricycle design

 I have managed to greatly simplify the “Ground Hog” universal joint transmission set up. The fundamental changes are shown in portions of my “project in work” drawings shown below.

There are two main differences. Firstly, the structure, which supports the universal joint, can now be fabricated much more simply from two sections of commercially available aluminum extrusions.

 The fixed (boom) portion consists of a 2.1” section of 4 x 4 x ¼” bolted to a piece of 1/8” plate which fits over the steering head and is brazed to it.

The pivoting (fork) portion is fabricated from a 2.1” section of 3 x 3 x ¼” which is clamped to the steerer by two nuts. Two small plates clamped to each side of the extrusion by two 1/8” bolts, which pass through the steerer, lock the extrusion to it and prevent rotation independent of the steerer. This arrangement also allows for some degree of vertical adjustment in assembly. Rigidity is provided by a bridge structure formed by the upper arms of the two sections of extrusion, which are joined by a bolt passing through them on the centreline of the steerer.

 The second main difference is that with this arrangement a standard BMX, cut down MTB fork or similar, splayed out to 130mm to accept a standard derailleur hub and cogset can be utilized.

 There are two features of such a UVJ system that should be stressed. Firstly, when travelling in a straight line, there are no transmission or power losses apart from the two chains and ball races. The universal joint is effectively a solid axle. It is only during turns where power is not a major factor that joint “knuckling” could cause some slight loss. The other factor is that the 30 degrees or so of turn normally available when pedaling can be increased by another 5 degrees or so when free-wheeling -if the joint is allowed to pivot to its maximum.

 I have applied this modified FWD system to a lean tricycle design, which I have called “TRIAD”. Its most significant feature is that it permits the separation of the steering and leaning functions. This would permit turns to be made with lean input being coordinated to suit any combination of speed or turning radius, rather like an aircraft.

 The main frame spine, which contains the front wheel, transmission and seat, is made from 1.75” dia tubing and is permitted to lean. The rear portion consists of a 1.875” (.058” thick) tube which fits over the main section and to which is attached the two rear wheels using a similar structure to the Brike. The difference between the internal diameter of the rear tube and the external diameter of the front tube is about 008”, which with a little lubricant applied should be the ideal clearance and will involve no machining.

The rear portion also includes the handlebar which, in addition to providing turn input, will allow the front portion to lean relative to it when it is pressed down (again similar to the Brike). The ability to “lean on demand” together with the 24” track should permit some quite aggressive cornering manoeuvres to be conducted without upset!

 The overall geometry is non-radical with a 15” seat height, 18.5” bottom bracket height, 24” track and 75-deg head angle. The frame has been arranged to simplify manufacture with a 45 deg mitre in front of the seat and two 60-deg mitres on either side of the steering head. The side view drawing segment shown illustrates the main characteristics.

 In conclusion, the objective has been to capitalize upon the inherent advantages in compactness and efficiency of front wheel transmission, whilst the ability to initiate lean totally independently of turn input overcome any stability disadvantages of the Delta configuration

Roy Bird: Feb.2001.