I'm looking to be able to achieve a speed of a least 30mph on level ground without peddling and have enough to torque to climb small hills also without peddling. It would be nice if it could carry a 400lbs load counting the rider, deep cycle battery and some groceries with a 15 mile range.
Feel free to tell me I'm out of my mind.
Posted Apr 6, 2012, 4:24 AM UTC · edited
You cannot directly related 40 pounds (rather, pounds-force) to horsepower. HP is a measure of power (surprise?), where the 40 pounds your motor is rated for is a force (if that's not immediately clear, think about this - no one sells trolling motors that say "this will make you boat go 10mph").
So, have a look at this relationship: 40 pounds-force * 10 mph in horsepower. If we assume that your 40lbf motor can get you to 10mph, we can relate it to HP. However, I'm not sure the best way to incorporate the necessary velocity in your situation.
I suspect you'd have to figure out the terminal velocity of your system, which requires knowing a couple specific things which may be difficult to figure out, like the drag coefficient and projected area. You mention a suspected requirement of HP, but that's really a backward way of thinking about your design. Calculating your terminal velocity is probably exactly what you're really interested in.
Then, the usefulness of the contraption is figuring out the current draw of the motor at that load and the capacity of the battery.
Plus it really doesn't take that much power to over come air drag. Some quick googling and some cars doing 50 mph only really need 10 hp. (now that's proportional to the square of velocity) but it's more about getting there.
So if I have motor power and peddle power on the bike, the peddles are used to overcome the initial air drag which would allow me to use a motor with less torque?
This is like a foreign language to me, so please excuse my lack fluency. Thanks.
Thanks, loved the WolframAlpha link. Im sure I will using it for a multiplicity of things in the future.
Since I've put my ignorance on display already, I'll continue. Is 40 pounds of "marine torque" equivalent to torque in general? Based on other advice here, I am looking at alternate motor options, so this just an academic question.
My head is spinning, so I'll stop here for now as it is obvious I don't know enough at this time.
Thanks again.
I haven't the foggiest notion. Maybe call a marina and ask? Google research will generally turn something up.
I have much better idea on what I am googling to find than before. I did a lot of googling, I wasn't asking the right questions.
You will probably hear from me again, but it may a few weeks as you kind folks have given me much to digest.
400lbs?!!
A 200 amp hour battery weighs over 100 itself. With groceries, a rider, a battery, and some sort of apparatus to hold everything this is not an entirely unreasonable figure.
I dunno, man. I ride a motorcycle. 4-cylinder sport touring bike with hard cases, a shaft drive, fairing, two headlamps, 5 gallons of gas, etc...
... that entire bike weighs 530 lbs. if you're toting 400 lbs something is seriously wrong.
Yeah, but his 400 pounds includes 250 pounds of human... yours doesn't. Batteries are heavy, motors are heavy. It seems pretty reasonable to me.
sweet jesus, you're right.
he needs a diet or a harley.
1 hp = approx 7.4 kw electric motor.
see how many watts the electric trolling motor provides to get that number in horse power.
for then it's really an early motorcycle equivalence.
early motor cycles were bikes with motor stuck on. often a 5 hp motor was enough for level surface. the 400 pounds is mostly irrelevant if on a level surface. weight only really factors in if you are going up hills or your tires are very low pressure and then weight increases rolling resistance.
the drag of a person on a bicycle is google
Motorcyvcle and rider 1.8 frontal area Bike Upright Commuter 1.1 5.5 Bike racing 0.88 3.9
early google motorcycles
" The factory offered four singles with either 26- or 28-inch wheels (wheels were measured from the outer edge of the tire back then), and either battery or magneto ignition. These were referred to as the Model 7. Single cylinder, single-belt drive, battery ignition, 4.34 HP, 35 c.i., 28-(or 26) inch wheels." http://www.virginiawind.com/byways/history_03.asp
by the way gearing will help in both being able to start rolling and maximum speed and battery range becuase it will help your electric engine efficiency .
btw you can make an electric bike with a bicycle and an electric drill
see the video halfway down at www.kickinggas.org
I hope Mr. Ropers demise in the bike race is not an omen of my death by bike.
The electric drill powered bike looks intriguing. It would probably require multiple battery packs to achieve a 15 mile range I would think and the 36 volt drill to approach 30mph, though 25mph would be satisfactory. I really like the idea of the drill powered bike, though Id like to use a standard sized bike.
Based on all of the great advice provided I am rethinking my approach before I start throwing money at it.
Thanks much.
electric drill motors are junk, don't bother. They're just extremely cheap can motors - not very efficient.
Do you feel the same way about DeWalt products?
It's not the brand, it's the construction.
There is no reason to put a nice motor in a drill - it would jump the price up by a huge margin and would only have performance benefits for a minority of users. I have never seen a battery powered drill which had a decent motor in it. For intermittent high-torque applications cheap motors are perfectly suitable.
DeWalt makes very good tools, I use their drills all the time, but the motors just aren't suitable for an electric bike.
The pounds of thrust is what the motor develops with a prop attached and pushing against water. You need to know the torque developed by the motor, and based on that you can decided what kind of gear ratio you need that will give you reasonable acceleration and still allow you to reach your goal top speed without overspeeding the motor.
A lot of people who bolt these cheap chinese (gasoline) motor kits onto bicycles use a separate chain drive on the other side of the bike. You can look at how they attach the extra sprocket and get some ideas for how you could set up the drive train for your motor.
You should also look at using lithium-ion or lithium-polymer batteries to keep the weight down. Lead-acid batteries do not have a very high energy storage density. This becomes more practical from a cost standpoint if you can find some used but still working packs.
All that said... A trolling motor is not going to be the best motor for this type of thing. You should look for a fairly large brushless DC motor. These require drive electronics but they are much smaller and lighter than conventional electric motors and the torque they develop is mind-boggling.
Thank you for the reply. It appears that I need to do some serious rethinking all around. I have $100 bet that I can get this built within a year. It isn't the money, I just hate losing.
set up the drive train for your motor.
I've looked at the gearing used for some the kits and it appears to be pretty straight forward using standard bike sprockets. Am I being ignorant to assume that I can use the same ratios used to power bikes that use smaller motors.
lithium-ion or lithium-polymer batteries...
No doubt lighter will better. Most trolling motors recommended deep cycle lead/acid so I was just going with the manufacturer's standard as a guide.
You should look for a fairly large brushless DC motor.
What do you consider "fairly large"? And what are brushless DC motors used for. I've read that some individuals have used a starter from a semi-truck ito power a bike, but they didn't give many details and the top claimed speed was 18mph (the legal motorized bike speed in most states) after getting things started by peddling. I'm ok with peddling to provide the initial torque, but desire greater speed.
Gearing: You need to match the gear ratio to the motor's output. Depending on how much torque your electric motor develops you might be able to use the same sprocket kits that the chinese gasoline motor kits do.
Batteries: The reason "deep cycle" marine batteries are recommended is because conventional lead acid batteries do not like to be drained more than about half their total electrical capacity: it damages the battery and shortens their lifespan. Deep cycle lead acid batteries are specifically designed to be drained further before being recharged without being damaged. In this case, the recommendation the manufacturer of the trolling motor is saying more like "don't use normal lead acid batteries with this, use deep cycle batteries instead". In your case, you want to avoid lead acid batteries altogether so you can pretty much ignore this recommendation.
derphurr suggested that I look at the gearing used by Chinese gasoline bike motors. I'm pretty sure that will do the trick.
Since Im most likely going abandon the idea of using a trolling motor, I will abandon the deep cycle battery too. I really appreciate everybody explaining things at an elementary level. I'm a bit slow until things gel in mind, once they gel I tend go like a bat out hell. However I suspect Im going stay in hell for awhile.
Have you thought about a cheap junkyard car starter motor? You would need a pretty cheap DC motor controller for speed control, potentially gearing like with a derailer.
There is a YT video of a guy with a large model airplane prop bolted to the back of a two seater bicycle that goes 60mph, but that seems loud and dangerous.
I've looked at many YT videos that are pretty outrageous, but missed this one. I model airplane prop is definitely winner in terms of over the top.
I did some reading and I'm confident you are correct in suggesting that I replicate the gearing used by Chinese gas powered motors.
The video link, provided by another poster in this thread, for a drill powered bike looks like a reasonable approach for someone with a weak engineering skill set such as myself. I also got some useful google hits for brushless dc motors designed for lawn mowers, carts, wheelchairs, trikes and bikes. Based on scanning the specs on these, it appears Im looking for a 1000 to 1500 watt motor of this type. These also have matching controllers which is a big plus. I'll need to see what I can find on craigslist as these are a bit pricey new.
What bike? What's your budget? Do you have a motor? 400 pounds!?
I have none of the parts at this time. A guesstimate budget of $500 to $700 for new, but would like to get the cost down with a used motor perhaps.
Bike weight 30lbs with narrow racing tires, I weight 250lbs, the battery deep cycle batter weighs 50 to 60lbs and 50lbs for carrying stuff from store to home. A crude estimate based on browsing the hardware specs.
The wheel hub motors they sell for bikes don't provide enough power. A friend suggested an electric trolling motor might do the trick and it sounded interesting because they come with a speed control included with the motor.
I envision reversing the sprockets so that the small sprocket is rotated by the motor which chain drives the larger sprocket on the back wheel.
I'm not stuck on a trolling motor, so I'm open to other off the shelf electric motors that anyone might suggest. I've looked at other options, but the cost seems to go way up because the electric motor and control are sold separately.
What I probably need to know is how to convert pounds of marine thrust into horse power. I'm pretty good a putting things together, however I have a low level confidence in making a judgement on how powerful a trolling motor I need. My guess is that I'll need 2 to 2.5 horsepower, however I don't have clue how many pounds of thrust are equivalent to those horsepower numbers.
My goal is to build a functional grocery getter for suburban use.
IIRC the trolling motors usually use the surrounding water for cooling, and generally advise not running them when they're not submerged.
For a good reference point: a 50cc 4stroke scooter makes about 4HP. They'll do about 35mph flat out and unrestricted. I think 2.5HP would be a pretty reasonable minimum if you can keep your drivetrain losses down (scooters use belts and wide tires which waste a lot of power).
I would keep the ability to shift, though. You'll find that 0-30mph is probably too large a range for a motor to operate efficiently. 3 speeds would probably do you a lot of good - hillclimb gear, general cruising gear, and like a downhill/flat high speed gear.
EDIT: a little searching turns up the tidbit that trolling motors pretty reliably pull about 1 amp per pound of thrust at full throttle.. That means a 55lb thrust motor would pull just shy of 1HP (55A *12V = 660 watts, ~750 watts per HP).
This guy probably knows more than I on the topic. Listen to him.
My advice would be more find someone else who has conquered a similar task.
This guy seems to know his hardware. I would either try to get in contact with him and pick his brain with very specific questions. I know you aren't looking for a deep answer on electronics, but he could cross off ideas that you might think of trying.
This Guy made a scooter that supports and moves his weight. Ignore the programming and electronics end, and just ask him about his components selection. An old motorized wheel chair with a proper gear box or shifter, could solve your issue for cheap.
Sorry for the delayed response. I don't log in often. Best of luck, and please post an update when you actually begin the project. We love photos.