On second thought ...

After thinking about how to handle the torque transfer from the drive axle to the sprocket cassette, I decided to go the more difficult but more elegant route. I had intended to use a kart hub to handle the torque load(previous post) and afix the cassette housing to it -- also sleeving the shaft a bit since the cassette's ID was slightly bigger than our axle.

After sleeping on it I decided that I would build an indexing jig and spline the shaft itself. By custom milling both the keyway and the keystock, I could size the keys to handle both the radial loads and the torque.

I started with a round piece of plyood and using my previously contructed hub holder created a indexed wheel.

I set up a little marker system off to the side of the mill table and used this to locate each spline location on the axle. Milling ensued.

Custom milling the keystock.

I left enough room in the keyways to move the sprocket group back and forth for perfect alignment. The shaft collars are only used to keep the sprocket from sliding back and forth on the axle.

Next ...

Our first tests with the vehicle will be on a dynamometer where we collect data on the prop at static thrust (wind speed). We will also use the vehicle as a 'dynamic prop stand' by running it (push or pull?) up and down the runway in still air at different speeds, collecting data from our instrumented hub.

Before we build our two spools for the spool transmission, we want to make sure we get our gear ratios optimized. I ordered up a large recumbent bike sprocket for the prop shaft (65t) and purchased a 9 speed cassette for the drive axle. This will give us a selection of ratios to work with. Additionally, the chain drive will be our less efficient (we believe) backup transmission if our spool drive adventures don't play out.

The next challenge will be to mate the cassette on the left to the hub on the right. Not exactly sure yet how to get that done ... thinking cap on.

One way out

We don't have a differential on the drive axle -- too heavy for our tastes and besides, the vehicle is designed to pretty much stick to a straight line. We still need a way however to ensure we aren't binding as we drift around off the straight and narrow. We also need a one way device on the prop shaft to keep the prop from backdriving the transmission (especially the spool transmission) if we get a sudden wind shift.

The solution was three ratchet hubs that will mate up to our kart axles. Of course as has been the case with most of the parts on this vehicle, no one seems to stock DDWFTTW ratchet hubs anymore. After about 16 hours (12 in the first one and 2 hrs each in the others) I finally finished them up this evening

Construction details in previous post.

When you have more time than money

(Or to put it another way ... what to do when DDWFTTW Vehicles R Us is out of stock on ratchet hubs)

To make the ratchet hubs, we again started with standard kart hub blanks.


Previously, I searched around and found a ratchet box wrench that I felt would handle our torque needs and was also reversable. We'll use the reversing feature to allow us to tow the cart around free from the transmission.

After putting the hub on a diet, I milled out a shape that matched the shape of the wrench head.

I grabbed a square swatch of aluminum for a lid -- drilled, tapped and cap screwed it on and then milled it round to match the hub.

I ground off the handle of the box wrench and installed three set screws to use for centering the wrench in the hub.

Once I fit, centered and locked in the wrench head with the set screws, I mixed up a small batch of epoxy and heated up the assembly (so the epoxy would flow). I let the epoxy fill up the space between the hub and the wrench head to make a nice, tight, torque resistant fit.

Here's a shot of the three in a progressive state of assembly.

Second chances

At the last group design meeting we all decided that the SJSU students would build a second propeller. They'll come up with a different design optimized for slightly different conditions and then we'll have another choice come test days.

The students spent the weekend working on test templates and practicing foam cutting. They are sharpening their hands-on skills on this cheap foam in preparation for the more expensive Spyder foam.

Their first task after this practice will be to make a test prop section about 2ft long with twist, taper and a spar. They will glass and sand this section as if it were part of the final prop. This test section will give them experience with every aspect of the construction and then they can go after the final product.

Here's a few shots of the guys cutting, shaping and sanding the test templates.

Here they are hot-wiring out some foam.

Three's a crowd

Finished the three brake disc hubs and mounted the discs to them.

They won't all be side by side as shown here -- I just slid them on the axle to shoot the pic.

Can we stop this?

The first three hubs I'm modifying are for the braking system. We're using three mountain bike disk brakes mounted to these hubs. There will be two on the drive axle and the third will mount to the propeller shaft.

Following is a sequence of shots I took last night from the process -- reduce hub size to something more appropriate, drill and tap the hole circle to match the disks and then install said disk.

Please hold me

As we're using kart axles to spin all our bits, we have quite a number of kart hubs that we need to modify to fit our various components. We're fitting bicycle disk brakes rather than kart brakes (too heavy), three freewheel ratchets (one per wheel and one for prop) and of course our actual drive components (sprockets, spools, etc. depending on the phase of the project.

Using the milling maching, I carved out a working fixture from delrin that bolts to the indexing table and accepts the kart hubs and allows them to be held for machining.

Where the rubber meets the road

I'm not pleased with how many hours we have in them, but I'm very pleased with the results.



We should have the remaining parts for the Prop hub in today or tomorrow and that's where we'll go next.

Never too late to learn

How many wheels have I trued in my life? This one and one more will make two.

Speed

The Faster than the Wind Team would like to take a moment to thank our primary sponsor Joby Energy. Not only have the folks at Joby generously given of funds and advice, but they have also allowed us to accept associate sponsorship from a company whose very business is based on speed.

Welcome aboard Google. Thank you for supporting our search for DDWFTTW.

Is it true?

The first wheel is all laced up, but our custom hub isn't going to fit any truing stand that's readily available -- so as with most of the project, we'll make our own.

I'll probably build the other wheel before I start the truing. It's a rather time consuming process so perhaps I'll set up where I can tweak on them while watching the NFC/AFC Championships tomorrow -- two birds, one stone.

Don't take me half the way.

50% of the spokes in, 50% to go.

It's too little to lace

Loading the hub up with the first round of spokes.


Installing the 'key" spoke.

Apparently the online spoke calculators aren't all that accurate when it comes to unusually large hubs -- once the leading spokes were in place, the trailing spokes were too short to lace in. Off to the bike shop to see if we can scare up a couple of extra millimeters.

Rim Shot

Two Velocity Dyad 27" rim blanks -- the toughest tandem bike rims on the planet we are told.

Get in the groove

Short spokes and a wide hub make for a nasty bend as the outer spokes leave for the wheel. A bit of a groove rolling towards the center creates a smooth path for the spoke. (yes, I had to lace the wheel up before figuring this out. Sometimes you must go backwards to go forwards)

My what big hubs you have!

With the work on the initial prop winding down, we're out of the garage and back into the shop. The milling machine will be the center of activity for the next few weeks as we churn out the custom drivetrain bits. We'll start with the drive wheels.

We decided to use 29" bike wheels as there is a good selection of heavy duty (tandem bike rated) components available. Unfortunately, we didn't feel like a standard width hub would give us the side loading strength we needed. Bikes lean for turning and thus other than asymmetric pedal forces, they don't get a lot of lateral loading. Narrow hubs work fine in that application, but our vehicle doesn't lean when it turns. Additionally, we need to transmit force to our axle whereas a bike hub rotates in its axle. The solution was to pick up racing kart wheels and modify them to create a hub that suits all of our needs.

We started by facing off the front and back of the kart wheels so there would be a flat area for the spoke holes.

With the wheel clamped to the index fixture, spoke holes were drilled every 20 degrees around the rim.

Here's a comparison with the original.