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Tuesday, August 30, 2011

EHB updates

I shrunk the innerside bearing in the CAD. I decided that a 1x2x.5 inch bearing is a little lot ridiculous, so I downsized it to a 25x37x7mm bearing (which happens to have about the same load carrying capacity as the 12x28x8 stock bearings). This shrunk the size of the width of the motor by about 4mm, freed up some space around the coils inside, and shaved a few ounces off the estimated weight of the motors, which is hovering right around 1kg each...less than half the weight of ELB's motors (5lbs each!).

Another advantage of shrinking the motor by this much is that it allows me to use MBS Matrix trucks (in addition to the Bionic trucks I currently plan on using). They have the same size axles, except the 1/2" section on the Matrix trucks is 3mm shorter than on the Bionics. If I end up breaking the Bionics, and Ground Industries really does go out of business, then I'll have an alternative. I could also just buy Matrix Wides, which have an axle even longer than the Bionics, but smaller motors are more covert :) . There's actually another difference between the MBS and GI trucks... the GI axles are held to much tighter tolerances than the MBS ones. In fact, much of GI's machining seems to be of higher quality than MBS's. Too bad they seem to be going out of business.

Unlike ELB, I'm trying very hard with EHB to design for flexibility. ELB's motors were designed only to work with the MBS skate-style trucks, only to have polyurethane strip tires (I got lucky and found an alternative), designed never to be opened once they're closed, etc... I'm addressing all of these and other issues with EHB's motors.

Despite the cost, and previous conclusions, I'm leaning towards having the rotors waterjet cut by Big Blue Saw. It'll be about $260 for twelve 1/4" thick rotors, which seems absolutely insane to me. I re-ran the numbers, though, and if I was getting paid for the time I'd spend cutting (along with material and operating costs), it actually about evens out. In other words, waterjetting is expensive unless you have free access to one.

The other large expense will be the motor controllers. I really wish I had time to learn how to design brushless PM motor controllers, but there's no way in heck it'd be cost efficient for me to do so.  I'm thinking 4 Kelly KBS controllers, which is ~ $320 with the water proofing option. OUCH.

As you can probably tell, I did a preliminary cost estimate. The total cost for EHB is looking to be about $1600 (not counting labor of course). That's a shit ton of money. I already have $345 invested in it (stators and board).

I had a realization: 2 motors will be a lot cheaper than 4. Probably not half, because the 2 motors will have to be larger, but at least 30% cheaper...I'll keep that in mind for V3. Of course, with two motors, I lose 4WD, which is necessary for the wintery conditions I hoped to drive ELB in and hope to drive EHB in.

Classes start in a week.


  1. ok, take that 1600 dollars and then double it.
    I went into my build with 3500 bucks and am down to 400 bucks now. yikes! You're right, the 2wd would be a lot cheaper, considering you would also only need 2 Kelly controllers. BTW, why bother with the water-proofing? If you squeeze them into a battery-box then I don't think you will need it. And if they do, that's what Automotive GOOP is for :)
    Good luck dude, my boards getting there. (I also had a LOT of parts water-jet, so I feel your pain in the wallet)