Saturday, June 7, 2008

Knee Walker - not done yet...

Since I last posted about the knee walker, a fair bit has happened. For one, we've finished building the thing.

Here's an end-on view of the disc brake mounted up to the rear axle. I just like the symmetry of it.

The rear axle mounted up to the underside of the frame. The red and black thingamajigger is the disc brake caliper. As things have progressed, using this stop bike part was a good call. It was a little tricky making sure it got mounted and aligned just right, but it will provide great stopping power, and saved a lot of time and effort in designing some other braking mechanism.

We took the whole thing over to the bike shop last Friday when we picked up a brake cable and housing. They got a kick out of seeing the finished product. When I ordered the brake assembly from them, I could only give them a hand-wavy description of the project, because I was still in the middle of designing it!

The wheels we are using have what are called "split rims," - symmetric left and right halves that are bolted together with the tire in the middle. This is unlike the wheels on your car or bicycle, which are one-piece and have the tire pried over the rim. The tires also have an inner tube. What it all means is that, if one tries to unbolt the wheels before deflating the tires, you'll get about halfway before the 50 psi in the inner tube blows the two halves apart. I should have known this beforehand, but instead learned it the hard way with quite a surprise. Hence the warning label we added to the frame.

This is how the finished device looked when we brought it over to adjust it to the patient and have him try it out. Keep in mind that this guy is big - over seven feet and nearly 600 lbs, so it looks much smaller next to him. The time spent having him try it out was well worth it - I was able to pick out some things that we'll need to change and adjust before he uses it full-time. So, after that brief time over at the hospital, it has been back in our Shop for revision.

One of the major changes will be swapping out the front wheels, which are fixed, for swivel casters. It seems obvious that you would want to be able to steer something like this, but we were initially told that the front wheels should be fixed. The PTs and docs we were working with were concerned that, like the commercially available knee walkers that have handlebar-steered front wheels, the device would be too tippy. Well, without some amount of steering, the device is too difficult to maneuver, making it a good bet that he simply won't use it at all, which is a bigger concern than it being too tippy.

Next, the arms that the front wheels attach to need to be shortened up, because the hallways in his house aren't as wide as we thought.

The uprights for the seat need to be lengthened, because even with the seat all the way up, it still needs to be an inch or two higher.

The handlebar stem needs to be a bit longer, too.

The attachment for the handlebar stem needs to be beefed up, because we was able to twist it around much too easily by leaning into it.

The frame, although strong enough for the load, needs to be stiffened. As it is, with a full load on the seat, the frame twists a bit more than I would like to see.

So, that's what has been keeping me busy 12-14 hours/day for the latter half of the week - designing these changes, putting together drawings of new parts for the Shop to make, and assembling the new parts together. With luck, that should be complete by the middle of next week, and we can give it another go with the patient.

We've been told by his doctor and family that, once this guy's leg heals up, we should think on how to build him a bicycle!

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