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[Gregski]

Do I need a 72 volt motor to get the highest MPH?  I'm just hoping for something respectable like 65 MPH.  I like the Mars ME-0709 motor, but couldn't I just use a 48 volt motor like the Mars ME-0308?  I noticed electric golf cart motors can usually only go 20 MPH why is that?

I guess what I need to ask is what will give me faster MPH volts or amps?  I bet it's a combination as in watts or horse power, HP.

I'm not looking for formulas or graphs just an explanation in layman's terms, please.

[NewAge]

Unfortunately, it's really all math and formulas, but in a nutshell,
Mars motors make 72 rpms per volt, so 48 volts equals 3456 rpms while 72 volts equals 5184 rpms.
This means that the 72 volt system has a broader power range.
The 48 volt system will easily do 65 mph, but it wouldn't be nearly as efficient as the 72 volt system,
so make sure that you balance your system to make it as efficient as possible.
You need to do the math. Say you have a 20" diameter rear wheel, now multiply that by 3.14 (pi), then multiply that by 5184 (motor rpms), now divide by 63360 (inches per mile) then multiply by 60 (minutes per hour), this gets you 308.29 mph if the motor ran at 1:1 with the rear wheel, too fast!! You need to reduce your speed through gearing, if you have a popular 60 tooth rear sprocket and you want a top speed of around 65 mph you would need a 13 tooth motor sprocket which would get you a top speed of 66.73 mph. (60/13=4.62) (308.29/4.62=66.73)

Remember, the higher the top speed, the slower the acceleration, so make sure that you balance your system. 

[Bogan]

newage is quite right, for dc motors like the mars range, the max current is constant for the motor, which gives a relatively constant acceleration throughout the rpm for a given gear ratio.
This means a high top speed gives slow acceleration and low top speed gives quick acceleration, there are two ways to get around this:
Use a gearbox, see http://electricmotorcycleforum.com/boards/index.php?topic=286.0 for discussion of this
Use a more advanced motor such as a brushless dc, or ac motor, these a more expensive and require more expensive controllers (i think) but may be worth it for large bikes

another option is to wait till the ttxgp runs and see what tech is the best! though id imagine thier tech is fairly expensive

these sort of posts are probly better put in the tech section

[Gregski]

OK, I get it I am in the wrong pigeon hole, I wish I could move this post to the Home Brew section, sorry guys.

[skadamo]

Moved to the Home Brew pigeon hole

[Gregski]

Time to move on to the What If portion of our show:

In order to save some money, (I believe most newbies and first time builders would probobly agree), and instead of buying a 72 volt 8 HP motor for $500 bucks, what if I picked up one of them used golf cart 36v 3 HP or 48v 4 HP motors on eBay for $100 bucks and just ran them at higher voltage.  I read somewhere (and this is where I get dangerous) that increasing the voltage increases the RPMs, and that you can run these motors at twice the volt ratings, but you're still limited on the amperage.  However those golf cart motors are rated for under 20 MPH, so could I get them up to 40 MPH.  I think what ever motorcycle (like a Ninja 900) I convert it will weigh less than a golf cart.

Assuming I don't care how long the motor lasts me, I'm not thinking years here I am thinking months.  Also I just need this for a 9 mile trip to work and back, so roughly 20 mile round trip.  That's maybe 15 minutes of run time each way, with stops at red lights etc.  Maybe occasionally run an errand, or pass a few gas stations and laugh. 

[Bogan]

I would expect heat build up would be the main enemy here, running a motor at a higher voltage with the same current increases the hp, and internal efficiency losses increase as well, so id imagine it would get quite hot, though some ducting and heatsink could probly take of that.
Another problem could be arcing on the brushes due to the higher voltage and rpm, but this would only decrease the brush life as you are already expecting anyway.

I wud imagine somebody would have done a project like this on the internet, a bit of googling shud get you someone who knows the ins and outs better than i

[NewAge]

Gregski, remember balance, the system that you're talking about putting it into a Ninja 900 would be kinda like putting a volkswagen engine into a cadillac. 48-72 volt systems work well in a 125 to 400 rolling chassis, a 900 chassis is pretty robust and is designed to handle around 100 horsepower, most 400's can handle around 50. Larger, heavier chassis equals less performance and range for your system. On the other hand a larger more robust chassis can carry larger, heavier batteries, the point is balance 

By the way, I run pretty much the same system in my minibike