Re: Jeep bikes, with AWD!!!
Alan Braggins writes:http://www.jeepbikes.com/rubiconawd.html
>>> Besides having open bevel gears that do not work with dirt, the drive is transmitted by a 5-foot
>>> long shaft no more than 1/2" in diameter and apparently a flexible shaft, there being no
>>> universal joints at curves in the shaft line. If these guys are serious they ought to answer the
>>> question of torsional stiffness of the drive between rear and front wheel. That shaft is a long
>>> floppy torsion bar that will most likely exceed its torsional strength if the rear wheel slips
>>> on the level with the front wheel on dry pavement. Besides that, such a load will generate a
>>> large steering torque.
>> I note that it is claimed that there is an "Internal, rigid drive shaft system". I can't really
>> make it out, but it could run fairly short and straight through the down tube (with just one
>> joint as shown in the head tube), with drive taken from the bottom bracket axle.
> One of the smaller pictures seems to show a gear on the back axle (inside the brake disk). It's
> not clear, but it looks like maybe a flexible shaft behind the rear shock absorber, linking the
> back axle to the bevel gears shown in the cutaway view of the headset/ steerer area. (Though how
> it copes with changes in length because of that rear suspension I don't know - flexible shaft has
> splines and can slide into a shaft inside the top tube? Front suspension is even less clear.)
Rigid is a fuzzy word in this description since all practical materials have substantial elasticity
in the dimensions of this design. A 5-foot long 1/2 inch rod is a twisty spring that could absorb
as much as two revolutions twist... if it were steel. If it is a flexible (speedometer cable)
drive, then it is at least twice as torsionally soft and have insufficient strength to turn the
Also visible is the cumbersome and ineffective steering link from upper head bearing to the fork
crown, there bing no steer tube on this fork. This slightly curved plate has no torsional strength
and will act as a spring between the handle bar and the wheel. Beyond that, the fork is suspended
entirely by a pair of bearings under the head tube (aka a 2-inch long steer tube). Things like this
that make me doubtful of whether this bicycle has been ridden or is just a design exercise.
This is not a practical feature for the future or one for bicycles. Just its weight makes the
device ungainly and its execution makes it useless. By the way, slip joints in shafts do not absorb
length changes when under torque unless they are roller sleeves (heavy). I suppose, for special
events, like monster trucks, a two wheel drive might have an application, but it is not in the
future of MTB design.
Jobst Brandt firstname.lastname@example.org Palo Alto CA