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Tuesday, May 12, 2009

Wheelchair Trike Layout Ideas

After some pondering, I've decided to build the front frame "from scratch", rather than modifying a complete existing bike frame. It should be lighter this way, and being purpose-built it will allow me to have all the bits I need in all the right places without quite so much hacking and working around, and probably end up being completed faster as a result.

It should also be shorter by perhaps a foot, since not so much clearance between the front wheel and the pedals is actually needed as is provided by the frame itself, with the pedalling position I am in recumbent-style.

One possible layout for it, with the cargopod atop the frame:

It's all just laid out on the ground flat, on it's side, to get an idea of possible placements of things.

The frame would not be quite as simple as it looks here, as it would have a triangular cross-section from the front as well. The downtube will split Y-style somewhere around the front edge of the seat, so that it can be then bent upwards and be bolted inside of the chair frame tubing where it used to go to footrests. It won't actually go as far back as it looks in the pic.

The top tube will end just at the center of the crosstube at the top of the chair frame, and be bolted/clamped to it.

Those three connections should be enough to make a stable and sturdy frame out of the chair and tubing. If they're not, well, I'll find out soon enough after it's built. :-)

The actual length is not yet determined, I have to measure out where the seat will go, then the angles I'll be sitting at, how long a leg-extension I'll need and thus where the pedals will go, and how much clearance I need from pedals to tire and thus where the headtube will go. I'm guessing it'll be just under 8 feet long, about like the CrazyBike2. Even though it's front tire is smaller and the cranks are closer to the tire, all that extra space goes behind the seat so I can have more cargo in *front* of the wheel's axle than I can with the CrazyBike2. I might make it 9 feet long *just* to get more cargo space in front of the wheels but still behind the seat.

There are several elements not shown at all, such as the steering tie-rod from the front steering tube back to the handlebars, which are also not shown. I will probably use underseat steering, because I should have clearance to make it work on this bike, as long as I give up the idea of under-seat cargo pods.

Currently it has no planned suspension, but it is possible I will at least make a front suspension fork similar to a number of freakbikes I've seen on the FBN forum, etc. I have a number of partial concepts for a rear suspension, but all of them require modifying the chair frame in significant ways, so until I can implement one without doing that, they'll just remain concepts.

The "simplest" would be to bolt the wheel support plate not to the frame itself, but to a pair of tubes clamped around the vertical frame risers. Springs would be installed just above those tubes, mounted around the frame risers, so the tubes holding the wheel would press against the springs and give some amount of vertical damping (probably not much, with such short springs, given that I have no springs I could use there yet, either).

The next best would be to bolt the wheel support plate to a car leaf spring's center, then support that leaf spring at both ends from the frame. This would not be very easy to do and still provide enough space for cargo *in front* of the wheel, as it would move the wheel forward by almost half the frame's length.

Another variation on that would put the wheel support plate at the far end of the leaf spring, with the spring's middle and center mounted to the frame itself. This would actually allow the wheel to be extended out behind the frame, assuming the leaf spring can bear the bike's weight (estimated at 100-120 pounds due to the 3rd battery and complex drivetrain) when suspended at it's end that way, which is an unknown to me.

Any shock system I come up with for the rear needs to be independent for each wheel, partly so that I can turn easier using the tilt of the trike to turn with, just as with a bike.

I have another idea for a linkage from the steering that during a turn would proportionately force the appropriate rear wheel upward along it's suspension, tilting the trike toward that direction (the direction being steered toward). It may be completely impractical, and might not even be possible for me to make using parts I have available, but I may try it.

The major thing I needed to establish was how the drivetrain might work. Assuming I will also motor-assist this one, then the drivetrain will be a little complex, marginally more so than it currently is on the Crazybike2.

The primary reason for that is there is no provision on the wheelchair hub for power input from anything other than hands on the tires or handrail. The probable complexity of a rugged solution including a freewheel and cassette necessitates a separate stage in the drivetrain for that part, and simply bolting a chainring (the orange one) to the hub, or more likely to the spokes themselves, the way it was done on Eric Peltzer's ebike.

Working forward from the wheel, the chain will go to a hub (represented here by the small white chainring/hub part just forward of the wheel, below the chair frame). That hub will be mounted on a dropout (including derailer hanger) that will hang from the chair frame, bolted/clamped into place. The left side of that hub will have a chainring the same size as the one on the wheel itself bolted to it. The right side of the hub will remain a normal freewheel/cassette unit, which will have it's own chain going up to the next part of the drivetrain. There will be a derailer on this cassette to shift gears just as if it were still on a rear wheel in a normal bike drivetrain.

This next part will be the most complex piece, as it will take the power from both pedals and motor and transfer it to the wheel's drivetrain. On it's right side, as normal, will be a triple chainring with a derailer, so that it can be shifted as normal. On the left side will be a device I've wished for previously, but will redescribe in detail below. Basically all it does is allow two independent freewheels to be installed, without fear of them unthreading themselves from the crankshaft, on the left side of a square-taper shaft.

It's a 3" long cylinder the same diameter as the threaded portion of a rear hub, and threaded like that, so freewheels can be screwed onto it. There is a thick lip left unmachined on the outer left edge, so that anything threaded onto it (from the right side, before attaching it to the crankshaft) can't unscrew itself.

To set it up, just thread the freewheels on, put the device on the crankshaft, line up your chains by threading the freewheels back and forth until at the desired point, then measure their positions, take it and them off, and insert spacer rings of the desired thickness of whatever material is available (large steel washers would be good, but old rear cassette spacer rings should work, too), then thread the freewheels back on, and reinstall the device on the crankshaft.

Hook the chains back up, and voila! there is now a freewheel for keeping both of the power sources isolated from each other, and still able to input power into the rear drivetrain (which now does not need it's own freewheel, either).

Now we come to the power input to the drivetrain. On one of the freewheels of the device above, a chain goes up to the pedals, which have their chainring on the left side rather than the right, for this reason. The pedals would only need one chainring, but if great hill-climbing ability turns out to be needed, two or more could be used, with the derailer as they would normally have, or stopping and manually moving the chain from one to another can be done. A chain tensioner plus throttle will be installed on the top of the chainline as described a couple of posts back.

The other freewheel of the above device would have it's chain running back to the motor's output chainring. The motor is represented by the rusty-chrome sprocket within the chair frame, and is currently planned to be the treadmill motor, which has not yet been used on a bike yet (discounting stand-tests on the previous one). It's chainring would be small on the motor, and as large as possible on the freewheel. With the 36V setup this bike will have, most likely a double-reduction would be required, and so the device I made for that using the belt from the motor with 3:1 reduction, then small chain with about 10:1 reduction would take care of that.

The motor chain tension will be adjusted by sliding the motor (and it's intermediate reducer, if needed) back and forth in slots within it's mounting plate, which will be bolted to the chair frame.

The motor will be just in front of the batteries, which will be in a box directly between the wheels along the line of their axles. The box should be able to contain the controller, charger, and batteries. Since it will be aluminum, I can simply bolt the controller and charger to it's walls, and the box itself should help dissipate the heat. The controller's MOSFETs will if possible be mounted so they are on the inside of the box at a point where a heatsink can be bolted to the outside surface exposed to maximum airflow, but at as little risk of impact or tampering as possible.

The battery box won't be accessible without unbolting the cargo pod from the chair frame, partly for security of the contents.

In the space leftover around the motor and it's drivetrain components, I'll have a toolbox bolted in place with whatever I might need to fix the bike on the road at least enough to get me home. Again, it wont' be accessible without removing the cargo pod, but hopefully I won't need it very often. Because of this, the tools to unbolt the pod itself need to be something small I can carry with me, so I'll probably use allen-head bolts.

The front fork I've decided on for now is the one that came with the Magna frame that's now on the CrazyBike2. I chose it primarily because it's lighter than any of the others I have in that size, yet still strong and apparently well-made. It also has brake-pivot studs already on it, so I can use that type of brake instead of caliper brakes in front if I choose (and if I have enough working parts to do it with).

The seat is just the plywood version of my other seat, and will not be used on this bike--I'll probably build a v2.0 of the tubing/mesh seat I now use on the CrazyBike2, just for this bike.

That's the basic overview of how this one is currently being planned.

This is just a pic without the cargo pod, but otherwise the same.

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