Is This Motor a Good Choice?

[mikegrundvig]

Hi all; I'm competing in the Power Wheels Racing Series next year for the heck of it and a chance to get better at engineering and machining. With that said, I know of no place with more experts on electric vehicles and motors than this forum so I figured I'd ask for some advice here. This is the competition:
http://powerracingseries.org/

The pertinent rules are max 36v (ugh) and max $500 raw parts/materials cost minus safety equipment (breaks are safety equipment). It has to have a Power Wheels body but that's about it. All of the better competitors had basically built frames and dropped a body on top with some careful hack saw work. The vehicle has to compete in three races for points at two different Maker Faire events: drag race, short 2 heat race, and a 75 minute endurance race. Pit stops are allowed in the endurance race to switch batteries and the like.

Now with all that said, I have no interest in just tossing a body on a simple box of steel and being done with it. I really want to make something crazy out of this project, basically to take the time and effort to make something really one of a kind. To that end, I'm designing it to have four wheel independent suspension Formula 1 style like this: http://www.serpent.com/image.php?ImageID=1234568279 with a full homemade spur-gear differential. I'd love for it also to be 4-wheel drive as well but that's going to be determined on how well my initial homemade U-joints work and the weight/cost of making the differential.

The existing karts max out at a top speed of about 20mph and their acceleration is pretty poor. It's clear having a good bit of punch coming out of the corners combined with good handling will ensure a much better chance at winning. High top speed is of limited use because the courses are reasonably short and a bit twisty (though not TIGHT, just lots of sweeping turns) but the straightaways are long enough that it's important to not be slower than the 20mph or so.

So with all that said and keeping budget in mind, this is the motor I was thinking about:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5142

And this is the controller:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8920

I'm assuming I'll have to water cool the motor for the endurance race but that should be doable. Many of the racers are using scooter-style motors like this one (though generally lower wattage):
http://www.tncscooters.com/product.php?sku=106160

I have that motor as well for another project, would it be a better choice? I like the RC motor as it's both smaller and lighter while being much stronger. I'd love any input or advice anyone has on this. Thanks!

-Mike

 

[REdiculous]

If you've only got $500 to work with I wouldn't go RC. I think the cost is the main factor; $212 vs $112 (not counting shipping)...the RC stuff is almost twice as expensive. If you've already got the 1000w tncscooter motor then it makes even more sense since you don't have to wait for anything except maybe the controller, and they probably have those in stock....there's a small waiting list for the RC motor (stock was at -13 when I checked).

 

[mikegrundvig]

I have the controller + motor already but it still counts against me. The price is something like 150 vs. 200. I have 9 months to build the kart so out-of-stock isn't an issue at all.

 

[REdiculous]

Do the batteries count against you too?...just curious.

The RC gear comes out to $212 and the cheap stuff comes out to $112 (not counting shipping, throttle and brake switch). I see a $100 difference between them, which is 1/5 the budget.

At that rate I'd consider getting 2 of the cheap motors and 2 controllers ($224 is only $12 more than RC motor+controller would cost). It might make 4wd easier, more oomph off the line, redundancy is good, and you could potentially switch between them so that each motor only has to deal with half a lap (or whatever) to make cooling a non-issue.

That's my line of thinking, anyway.

 

[theRealFury]

Well, if $500 has to cover motor, controller, batteries and all other construction materials i dont think id want to go for it. Batteries are going to be the killer in that budget :S If i was looking at the two motors just for performance purposes then id go with the RC turnigy motor, The Turnigy motor gives more rpm/v so more scope for gearing down to provide better accelleration and reducind stess on the motor. Also the 7lb's of weight saved with the RC motor has got to be a +, you can get 7lb's of lipo's and still have a few gramms left for an RC ESC. I wouldnt run the RC motor at close to 6500w though unless it was actively and forcefully cooled though as i doubt it would last long especially if under cover.

As for 4 wheel drive, something tells me you will really struggle to do that especially with the tight budget, if it were me i would stick to RWD.

Also i dont see the benefit of independant suspension on such a small form factor gocart style EV, if your going to be racing on a cart track would suspension even be needed? most of the carts(admittedly not done so for a few years now ) i have raced didnt have any suspension other than the tyre rubber and the seat of my pants .

What ever you do choose to go with, i wish you luck and hope to see some pictures & footage of the event(s).

 

[mikegrundvig]

Thanks for all the comments. The rules are getting modified for next year, the big ones are that no one can race with lithium chemistry batteries - lipos are specifically called out, people are still arguing lifepo stuff. SLAs, nicad, etc. are all fully supported. There were a fair number of fires with poorly constructed vehicles (generally overheating or melting down) and so people are nervous about high-energy batteries like lipos going up due to shoddy construction. Batteries also only cost 50% against the budget. So $150 in batteries counts $75 against the budget. Confusing but useful. Another change, though not to the rules, is that they are going to ensure the courses are short enough that top speed is limited. They feel 20mph is about the max safe speed in one of these races so while they won't limit the technology, they will design short straightaways. This means acceleration is even more important than before.

Whatever motor I go with will be actively water cooled with the water running through a transmission radiator. Too many other vehicles burned up their motors in the endurance race. Less then half were even capable of finishing.

As for the suspension, this is purely an engineering and welding/machining exercise for me. I know it won't be practical or even useful but I want to see how exotic I can go. I do want to win, but I'm more interesting in pushing the limit of what I can make first and foremost. I've actually come quite far on the design and with surplus purchases it's going to be relatively inexpensive. Thanks!

-Mike