I just left work and this guy's truck wouldn't start. I said, "Sorry, I wish I could help you!" But on my way home I was wondering what if I had a dynohub and peddled really really fast? Is there some way to attach jumper cables to the back hub? Maybe I should call mythbusters.

Car starters use about a kilowatt; bike generators a few watts -- off by three orders of magnitude. If the truck had a manual transmission you might have been able to push him fast enough.

Paul

Yea .. Pedal fast .. Very very fast

When I was in college a guy I knew put together a contraption to hook the rear hub of a bike to an alternator which in turn was wired to a light socket. If you pedalled really hard you could you could get the bulb to light up.

I'd say not. Cars normally use 12-volt electrical systems, and they draw one heck of a lot of current (tens of amps) when the starter motor turns. I think the Sturmey-Archer Dynohub is six volts, so to get the power required to start a car, you'd need to draw twice the amount of current compared to what the motor draws at 12 volts (power=voltage x current). As the heating effect of a current on a wire is proportional to the square of the current, you would burn the hub out practically instantly. That's assuming you had the power in your legs, which you most likely wouldn't. Myth busted, unfortunately.

Edit: That's assuming a rear dynohub, with the back wheel off the ground. A front would be even worse.

You can build your own 300 watt bike generator, but it will only put out about 14 amps; about 386 shy of what you need to start a car.

Well, if the problem is a dead battery, he technically doesn't need to put out 400 amps. He just needs to charge the battery. If the driver was really patient...

If the Skipper and Little Buddy can do it, you certainly can.

lol... I was about to edit... This device would certainly CHARGE the battery. I have no idea how long it would take to develop a cranking charge.
This has given me a new idea. Put this thing on a BOB trailer and go into the car starting business!

You'll have plenty of competition. They call them wreckers and often they call them cabs.

If the battery was drained (left their lights on, etc.) you could charge it. If the battery is bad (dead plate, etc.), you could pedal for days and never get it charged.

You could make a fake video of you charging a dead battery with your bike and starting the car, post it on youtube, then email it to Mythbusters. Make 'em ride like madmen.

Couple of things that baffle me. Is it not most dyno hub are mostly 6volt? Are most of them AC output unless they are either 1/2 or full wave rectified? If yes to both question, then you may stand a chance of ruining your dynohub.

Theoretically you could, but it would take a lot of time as you would need to charge the battery with enough energy to output a killowatt. Basically, you'll need to be on the bike for about 15-30 minutes to charge up the battery.

Edit: Sorry, I miscalculated. I forgot about the amount of time that the power would need to be supplied for.

I'd estimate the turnover time period to be roughly 2-3 seconds, so you'd have to be on the bike for 2-3 times what I said.

It would be easier to ride down to the auto parts store and buy him another battery.

Possible - yes, practical - no.

What you would have to do is use the dynohub to charge the car battery. A dynohub generates about 3W, so at 12V, this is ~ 0.25A. A typical car battery is about 45Ah capacity, so if the charging was 100% efficient, you would have to pedal for about 8 days (24 hrs per day) to charge the battery. Much easier to use the bike to ride to the auto shop, and either buy a new battery, or charge the dead battery.

Just gives you an idea of how much more efficient a bicycle is than a car!

Hm... is that right? I was going with 3W = 3 J/s. Pedaling with 3 J/s loads the battery with 3000 Joules in about 1000 seconds (assuming 100% efficiency). 3000 Joules is enough energy to provide 1 kW for three seconds.

Of course, I didn't account for the fact that the generator would provide AC current. So you'd have to double that time. I was already raising my eyebrow at the fact that my calculations implied I could charge a car battery with enough power to start a car within a hour, so please let me know where I messed up .

Woo -

It's probably somewhere between our two estimates. Standard car battery is rated at 45 Ampere Hours, which comes out to about 2MJ - You don't need all of the battery charge to actually start the car, and chances are, the battery is not completely dead. My estimate of 8 days is to bring a battery with no charge up to full capacity, which is plenty of juice for several starts.

I think that while starting the car, the battery has to supply 100 - 200A, for a time of a few seconds - if we say 200A for 5 seconds then the actual power used to start the car would be would be 12kJ - if the dynohub generates 3J/sec, then that is 4,000 seconds or about an hour to generate the power needed to start the car.

Dude just needs to drive a stick shift. Stick shifts all the way bro.

You guys're amazing. But I wouldn't have to charge the whole battery for him to start the car, right? A battery can be 1/4 charged to start a car, so maybe I'd just have to bike for two or three days... Or have several bikes all hooked up together!

Damn, I forgot that, I think they're usually AC, so you'd have no chance of even charging the battery off them.

Not true. You just have to use a rectifier as mentioned by colleen.

could make a profit at that, charging (sic) for a fully charged battery.

I think you'd be better off having a tandem and offering him a ride.

True, but if you just rode past the guy and saw he needed a jump start, you probably wouldn't have the rectifier, unless some of the new dynohubs have one built in.