Inferno wrote:Thanks for the replies. It's 15 miles each way, including a rather large two mile hill.
Shame it'd need a controller, I want to keep the bike looking as bike like as possible - motor in a similar position to a water bottle, batteries in a back pack and a subtle drive line with minimal reduction gearing. I was hoping I could get up to around 15mph under my own steam and then just flick the go button.
I imagine a lot comes down to cost, but what sort of size am I looking at for a motor controller? The smaller the better, but I want/need reliability!
I've looked at hub motors, but I'm more in favour of direct drive.
One thing you need to remember is that the power-to-speed relationship goes approximately as the cube of speed ( P = X*(speed)^3 ) because you aren't just pushing yourself along, you're also battling wind resistance, so it'll take you 8 times as much power to go 30 mph as it will to go 15 mph, especially uphill.
Combine that with the heat you lose to electrical resistance is proportional to the current, not the voltage (which is why high power lines are running megavolts), you are going to heat things up much more at low V/high A than the other way around, leading to failures, particularly in the controller department.
That's why most high-speed bikes are running at very high voltages, and you'd be much better off with a 48(+) V setup than 24 V. My hub motor (which around here is called a direct-drive, there are no internal gears) goes 25 mph easily, I could possibly push it to 30, but around here things are so curvy that I haven't. It's as simple electronically and physically as you can get, no gears at all (rear hubs are commonly made now to take a freewheel, so you can still use the gears for your pedaling, but I put my motor on a 1-speed bike). A hub motor actually can look as bike-like as any, you've just got a big rear hub, which can be hidden at least on one side with a disk brake. Good battery packs (LiPo gives you the highest power density, at the moment) are not too hard to camouflage, especially if you have a rear rack, or put them where the water bottle would go. The controller often fits on the seatpost/downpost or under the rack, and is all but invisible unless you are looking for it.
The other thing you need to consider is: Is your bike built solid enough to go at 30 mph continuously? Most probably aren't, and the high-dollar bikes are optimized for the driver pedaling, not motor drive, so they often aren't as good for an ebike as a mid-cost, lots-of-steel-and-good-welds bike (go through dogman's posts, he's had a lot to say on this topic). Don't get rid of your rear brake. Braking power is a Good Thing (TM).
Where do you live that they'll let you drive 30 without hassles? Most places don't bother you for 25, but 30 is pushing it for daily on-road use (and would be a terror on bike trails, so unless you want trouble don't even think it).
If you don't know electronics, just get a good kit from a reputable dealer and you can be sure that everything will work together just fine. Then, once you get more comfortable with things, you can start tweaking.