Motorized bicycle project

My 6 year old son wants us to motorize his bicycle (he upgraded to a slightly larger one recently, and wants to add a motor to the smaller
one). The bikes cost almost nothing (garage sale), so I am not afraid of ruining that bicycle.
The plan is to go with battery power rather than fuel, for the obvious reasons.
I had him ride in front of me yesterday, at a pace that he likes, and found out that his pedals are doing roughly 80 RPM. To that end, I want to buy a 12 VDC angled gearmotor that makes about that many RPM, and connect it to the pedal hub. I would obviously remove the pedals.
The angled gearmotor would be on one side, and the battery would be on another side.
For motion control, I want to wire a momentary double pole double throw on-OFF-on switch. In one ON position the bike would go forward, and in another position the motor would reverse and actuate the brake on the rear wheel hub.
For battery, I want to buy a UPS battery of modest size.
I am eyeing a DC motor rated for 24VDC and about 187 RPM. The motor is 1/4 horsepower. I hope that at twice lower voltage, it would make about twice fewer RPM and a fraction of power. (I would not want the motor to produce much more power than a child can produce by himself, for safety reasons).
Would you say that my expectation of what the 24VDC motor would do at 12V, are justified. Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus26157 wrote:

I assume you're in the US, so this may be of limited use, but I expect the US has something similar: www.oatleyelectronics.com.au has a whole list of kit for building small electric scooters & suchlike. Including speed controls (You didn't mention that, but if it was me, I would want a speed control :)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I would encourage him to ride the non-motorized bicycle. Better yet, they make a bike that attaches to the rear of an adult bike so you and him can both ride and pedal together.
The paybacks in health will stay with him all his life.
I wish I had a link for what I am describing, perhaps someone else can.
Wes
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I think you'll only have about 1/4 of the power at 12 volts (assumes the resistance of the motor is more or less constant once its running). That's about 1/16 hp to drive the bike. With so little power, you won't get into any trouble, but may well be dissapointed. If you did have enough power to drive the bike at the speed you suggest, you would need some sort of speed control.
Interesting idea about reversing to apply the brake, but again, this system would apply FULL brake, all at once!!!!
Pete Stanaitis --------------------------
Ignoramus26157 wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Remember, it is a kid's bicycle. He weighs about 40 lbs. I did some quick power calculations. The most power I can put out is go up a stairs, skipping every other step, briskly. That amounts to 150-200 watts from a 190 lbs man and is a quite extreme effort.
A 40 lbs kid riding a bicycle, on a 40 lbs bike, uses a lot less power than that. I would say, safely 2/5 of that based on weight, further divided by perhaps 3 due to efficiency of bicycle. So, we get approximately 20 watt of a "boy on bicycle" output. This 1/4 HP motor, turned into a 1/16 HP motor, would deliver 46 watt, which is plenty or maybe even too much (acceleration too sharp).
I would rather err on a safe, low power side here.

I'll see. 12v motor drives are available. I think that I can start without one.

Good point. Reversing can be done through a series resistor.
His bike also has a handle actuated brake (not working at the moment).
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
He needs a speed control Otherwise he can't approach anything except full tilt. And full tilt will be much faster going downhill, slower going up. I doubt that the motor will be able to produce a panic stop, or even a quick stop--and if it can, then he will have too much braking most of the time.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus26157 wrote:

I remember recently seeing a guy with an electric assist on his bike and I stopped him to ask about it, the battery pack was fitted to the seat stay and was about 14" high x 2" wide x 3" deep and the power unit was in the front hub like a drum brake hub. very neat installation and he said it had a range of about 30 miles. I can't remember the maker unfortunately. Regarding your output estimates, I recall some years back with the man powered flight and the gossamer albatross figures being given and an extremely fit person, such as the top class amatuer cyclist involved, might be able to achieve 1/2hp for the duration required, whereas most normal fit people would do maybe 1/4hp and then only for limited periods. For good output the tour de France and I expect other races these days, many racers are fitted with telemetry and the average and instantaneous outputs are sometimes given, very impressive, but in a different league to the rest of us.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I think a motorized bicycle is a somewhat vicious machine for a six year old to handle. Since you like to make things, why don't you take it a step farther and build him an electric car. All you need is four small bicycle wheels, a simple welded frame and a "horseless carriage" body that resembles a 1903 Olds or the original Ford.
Jerry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

It would be vicious if it would go fast, which I want to avoid. I would want the max speed to be approximately his brisk, leisurely speed.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Jun 20, 6:10?pm, Ignoramus25967 <ignoramus25...@NOSPAM. 25967.invalid> wrote:

I suggest a twist grip control like a motorcycle that would return to the off position when released for safety in emergencys. I would also use a caliper type brake. if he has to reverse the motor to stop that will also stop the motor and cause it to overheat. Engineman
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Jun 21, 11:10 am, Ignoramus25967 <ignoramus25...@NOSPAM. 25967.invalid> wrote:

You need this Iggy:
http://cgi.ebay.com.au/Electric-bicycle-bike-motor-kit-26-NEW_W0QQitemZ170123600330QQihZ007QQcategoryZ58099QQrdZ1QQcmdZViewItem
Graham.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

http://cgi.ebay.com.au/Electric-bicycle-bike-motor-kit-26-NEW_W0QQitemZ170123600330QQihZ007QQcategoryZ58099QQrdZ1QQcmdZViewItem
Graham, it is a nice kit, but it is for adults (26" wheels).
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 20 Jun 2007 22:33:31 -0500, Ignoramus25967

Then make him a "chopper" bike, and run it on 12 or 24 volts to reduce the speed.
Brian Lawson
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 20 Jun 2007 22:33:31 -0500, with neither quill nor qualm,

It's also for sale only to AU, and even if it weren't, can you imagine the shipping costs to get it here? They want $75AUD to ship it locally in AU.
- Metaphors Be With You -
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
You don't want to eliminate the pedal system, since a failure of the motor drive would render the bike unrideable without them. Think small tire drive on the rear wheel rim with over riding clutch instead. JR Dweller in the cellar
Ignoramus26157 wrote:

--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 20 Jun 2007 20:29:13 -0700, JR North

Better yet, a multispeed rear sprocket set, and chain drive to the rear wheel with freewheel on the motor sprocket. Mount the motor assembly on a rear "carrier" type frame. That way you can pedal to assist, pedal with no power, or run under power.
Or install 2 freewheels on the rear wheel. Then motor and pedals are totally independent.A bottom bracket race can be pressed into duty to couple 2 freewheels as it has the same thread as the rear wheel hub where the freewheel fits, Space the first freewheel out with a bottom bracket lock nut so you have about half thread left for the bottom bracket race- and thread the second freewheel on. Not good for high power or a big man's bike, but plenty strong for a 40 pounder.
--
Posted via a free Usenet account from http://www.teranews.com


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
see:
http://www.kreuzotter.de/english/espeed.htm
for some interesting data on power use by bike.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 20 Jun 2007 20:29:13 -0700, JR North

And here is the drive for it: http://www.surpluscenter.com/item.asp?UID 07062117375898&catname=&qty=1&item=1-2317
--Andy Asberry-- ------Texas-----
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.