Its been done before, but not by me: Advice wanted

[IanJThor]

Hello Endless-Sphere people,

I am working on a side project and hopefully you will be able to help me out. I'm sourcing parts and figuring out what I need for a project I hope to start in the next month or so. It is essentially a "dirtsurfer" type of inline board (http://en.wikipedia.org/wiki/Dirtsurfing) that I plan on designing and building myself. I'm a mechanical engineering student at SMU here in Dallas and I am working part time in the machine shop. I have plenty of time, but not a lot of experience with electric motors and batteries so hopefully you can help me out.

So far a 15ah at 36V set up is what has been recommended from Gaston over at Ypedal.com . My primary objective is to maximize ride time and range, climbing hills is a distant second. Since I wont be dealing with hills much the regenerative braking is not nearly as important as the ability to freewheel. I'm guesstimating that the weight would be approximately 300lbs (200lbs for the rider, 50ish lbs for the cromoly frame, then whatever the wheels, motor and batteries weigh). I want to put a geared hub motor into a rear 20X2in bicycle tire. I'm hoping to have a respectable top speed (at least 20 but 30ish would be awesome) while still having enough torque to start from a stand still and travel at least 20 miles on flat ground unassisted before having to recharge/swap batteries. I know this thing isn't a bicycle exactly but it has two bike tires, I'll have to balance, and it will weigh roughly the same so I figured the application of the motor/batteries would be similar.

The area this will be used is basically a flat dusty desert. Its going to be subject to a hot sun, lots of dirt/dust and possibly some rainfall. I'm building this thing for Burning Man in August of 2012, its in North Western Nevada in a dried lake bed. I've never been and I wanted to have a unique method of transportation around the week long event, which covers many square miles.

What kind of set up would you recommend for this type of application? Batteries, controller, hub-motor etc. Thanks for any feedback you have.

Cheers

Ian

 

[Drunkskunk]

Well if Gaston says a 36V 15Ah battery will do, I'd believe it.
But usually I'd recommend 1ah per mile you wanted to travel at 20mph. Especially so on dirt.

geared motor sounds easy enough. for flat ground running at 20mph, you probably don't need bigger than a 350W motor, but a dirt surfer isn't going to be kind to the gears. Shock loading (I.E. landing after a jump or tricks, or kicking back with your legs against the board while balancing) can kill the gears, so a 500W minimum just for the larger gears. and even then, you'll have to be careful to be power off when stressing the board.

Something to consider. A direct drive is near to totaly dead silent, and won't suffer from shock loads. The cogging effect people mention is not a serious issue in most cases.

 

[IanJThor]

Wow, did not expect such a quick reply, thank you! I have a couple questions:

a dirt surfer isn't going to be kind to the gears. Shock loading (I.E. landing after a jump or tricks, or kicking back with your legs against the board while balancing) can kill the gears, so a 500W minimum just for the larger gears. and even then, you'll have to be careful to be power off when stressing the board.

I don't plan on jumping this thing but I'm sure I will run over bumps, small rocks ets. BUT, It will be used mostly on flat ground and I am planning to incorporate a frame over the rear tire to hold saddle bags for the batteries and misc items i want to carry (maybe books when I use it at school ). Since I am building the frame myself I have the option of incorporating a suspension system. I was toying with the idea of a suspension similar to the motocross board (http://expensiva.com/gadgets-new/motocross-board-prototype/) but I decided against it after seeing the success and relative smooth ride that the dirt surfer seemed to have with a rigid frame and no suspension. Although, after thinking about it, a suspension wouldn't help the motor out, just the rider. If a bumpy ride is going to damage the gearing then I might have to consider other options.

Do you think its a better option to swap to direct drive if its only going to be bouncing over small bumps not ramps/jumps/etc ?

Something to consider. A direct drive is near to totaly dead silent, and won't suffer from shock loads. The cogging effect people mention is not a serious issue in most cases.

How bad will the drag/cogging effect of a direct drive hub motor have on the ride over flat ground? Bad enough to make coasting in the wind unpractical? The free wheel was a big perk because it would allow me to use a type of sail and take advantage of the wind from time to time like these guys: http://www.wingsurfer.co.uk/ . Pretty cool.

so a 500W minimum just for the larger gears. and even then, you'll have to be careful to be power off when stressing the board.

500w min for the larger gears? I'm not sure I'm following, sorry I'm a newbie at this. Do you mean a 500 watt motor will take the stresses better?

Thanks for your insight, I hadn't expected such a quick reply!

 

[Drunkskunk]

At the upper speeds, the cogging might effect a Direct drive more. But i ride my bike often without even the battery, and while I can feel there is a small amount of drag, it doesn't stop me from wanting to keep riding. Its about the same amount of drag you would get if you ran knobby tires instead of slicks on the street.

But the faster you go, the more of a factor it becomes. So if those windsurfers are getting over 30mph, you might start to notice the drag.


the 500 watt geared motors are physically bigger than the 350watt, with bigger gears meant to take more power, so they aren't as fragile. if you really are going to just putter along smoothly then there isn't much danger, and normal bumps shouldn't matter much. a 350W motor should get you to 20 on smooth ground and will weigh less, But you said off road, and I've seen the way some people ride those things, it would be hell on the internal gears. Those gears are disengaged when the motor is freewheeling, so they are safe when the power is off.

Lithium batteries should fir under the deck between the rails easily. The only person I've seen commuting with one had his backpack between his feet. I imagine you would want the weight as low and centered as possible, but I'm not a rider

 

[IanJThor]

I like the idea of a totally silent drive but its not critical, the freewheel is a big factor on this project though. Also, based on what I've read from other people and their first project, after they start toying with these motors they have so much fun that they always want more power and more speed. I will probably still go with a geared drive, however I will definitively be looking at the 500W motors as a starting point.

I will probably need more juice to power a larger motor so I will get less range for the same size battery bank. I'm OK with having a large battery bank, I already anticipate the batteries making up the bulk of my budget. I'm hoping to use Lithium ion if I can afford them.

I had thought of putting the batteries under, or behind the deck but I think I want to protect them more. The saddle bags would be positioned at or below the center point of the wheel. Hopefully that will keep the batteries low enough that their weight won't effect the stability too much.

I just ordered the cromoly tubing today, 20ft of it. When it comes in I can start shaping the frame and figuring out if my homemade design is going to work out or not.