hello guys, and merry christmas
i hope that you dont mind me dredging this old thread up, but i think that it is possible to create a pedal-powered tracked vehicle which can be steered in a relatively conventional way, not involving turning a crank to turn the vehicle
as in, rather than using 2 seperate sources of motion and power for steering and forwards motion, it is possible to divert the motion from a single source, i.e. the foot pedals, to cause the vehicle to move and turn
it depends on the use of a pair of CVT's meant for bicycles, with the ability to gear from 1:1 to almost nil output... CVT's are becoming more and more popular nowadays , so it should be easier to get a pair of them... and you also need some method of actuating the CVT's from the rider's position
the idea is , by having a pair of concentric gears, one inside another, rotating in opposite directions, and with planetary gears connecting them... if both of the gears are rotating such that the linear velocities of their teeth are equal, the planetary gears will be at a standstill... however if one of the concentric gears is slowed down or sped up, there will be movement of the planetary gears
it is this movement of the planetary gears which will drive the driven wheels
therefore, if one of the concentric gears is geared directly from the input , while the other is geared via the CVT's, and the CVT-driven gear can rotate twice as fast as the directly driven gear, the driven wheels can rotate both ways at equal rates depending on the ratio of the CVT's
here is a diagram i have slapped together, hopefully it will not hinder what im trying to say... this is one side of the transmission, the other side will be a mirror image
the numbers next to the gears mean their relative number of teeth, and the colours filling them represent their different rates of rotation
1. the cyan gears are the input, driven directly from the chain from the foot pedals
2. the magenta gear is merely turning in the opposite direction of the cyan gear, this is the input of the CVT
3.the yellow gear is the output of the CVT, as denoted by the variable x representing its ratio
with those points out of the way, lets ignore the movement of the large dark blue gear for now, associated with the ratio
1/2c ... imagine, instead that the gear is fixed, it does not rotate, as in
c = 0 ...
the dark brown gear is a differential gear, it's rate of rotation will be the average of the two gears connecting to it... since one side has 0 rotation, it will rotate at half the rate of the gear turning it
therefore, it rotates at a rate which is a quarter of the CVT output, but because it has twice the radius, its linear speed is
1/2 that of the CVT output
the orange gear, which is the
outer concentric gear, will also be rotating with a linear speed that is
1/2 of the CVT output...
and to mantain the ratios needed for the wheels to be driven in both directions equally, the portion of the purple gear inside the orange gear, that is, the
inner concetric gear, will need to spin at a linear velocity
1/4 of the cyan gears
therefore, nominally,
d = 1/2 .... the relative linear speeds of the axels of the plantary gears will then correspond to
(1/4 - 0)/2 = 1/8 in the forwards direction,
(1/4 - 1/2)/2 = -1/8 in reverse , and obviously
(1/4 - 1/4)/2 = 0 , which is geared neutral ... notice that i use positive numbers to mean forward motion, and negative numbers to mean reverse
i dont know how many of you would have read this far... but you will notice that using the transmission involves constantly turning 2 different sets of small gears, the spider gears in the differential, and the planetary gears driving the output... this causes friction, and it is inefficient...
this is why there is the gear combination in the bracket with a combined ratio of
2/c , it creates a 'forward bias' in the concentric gears , in the sense that when the CVT outputs are slow enough, it will cause both the concentric gears to rotate in the same direction, and minimize the friction caused by the planetary gears ...
this applies to the brown differential gear too, since the slower the CVT outputs, the slower the spider gears in the differential turn... however, the higher the value of
c , the more it offsets this effect since it rotates the other side of the differential in the opposite direction... hence
c will likely be a low value
the forward bias would throw the ratios off, and to mantain equal forward and reverse speeds,
with c > 0 , d would no longer be 1/2 , etc etc... i am leaving
c and
d , as well as the teeth ratios of the un-numbered gears, open to interpetation , since gears only come in integer amounts of teeth, gear ratios can only take certain values, and different people might prefer different amounts of forward bias or have different space requirements etc... i chose 2 as an arbitary ratio for some of the gears to demonstrate the concept
also, the other purpose of
c is to counter the minimum ratio that the CVT's can give, since they do not actually give 0 output ... the values of
c and
d will have to be adjusted to accomodate for this
so the vehicle will travel faster forwards than it does in reverse, unless the CVT's are also able to overdrive, which in that case the vehicle is still more efficient going forwards due to the lower amounts of internal movement
here is the same diagram in terms of linear speeds, rather than rates of rotation, i hope it makes it easier to understand
now i dont know if any of you would have even reached this far, but for those of you who are still with me, you will notice that with the presence of the very large gear in the transmission, it will be turning at the rate of
c , which as i have said is likely to be very low... if the gear extends off the right of the diagram i.e. across the center of the transmission, you could use the middle as a sort of pinion gear to drive much larger gears, resulting in very high torque... this can be geared to different applications which are torque-based, such as a winch...
so imagine, a pedal powered tractor with full manerverability and enhanced forward mobility, with a pedal powered operating winch... i can see it being used in festivals as a sort of mantainence vehicle