A friend of mine is a mechanic, and long ago he told me if my motorcycle’s battery fails and I can’t get another vehicle to help me with jumper cables, what I have to do is have the ignition ready, get it into 4th gear, and after giving it some momentum to suddenly release the clutch.
How does this work, and why 4th gear specifically?
In: Engineering
It sounds like your friend is describing a push start, or at least that’s what I’ve heard it called when talking about cars, bikers might have some different lingo.
But what’s happening is that by getting the bike, or vehicle, rolling with the clutch popped, you build up some momentum, when you engage the clutch part of that momentum is suddenly going to be resisted by the shaft and the engine, rotating and moving the parts.
Since the engine is already starting to spin this lowers the amount of energy necessary to get it to turn over and start, meaning you don’t need the battery to work perfectly to do it.
Though be warned if the battery is truly completely dead, this won’t work, it stills needs to be able to spark the engine to life.
As for why is has to be in 4th gear, I’m not sure since in cars you typically do it in second gear, but my guess is that putting it in higher gears means there’s more resistance to spinning, so you don’t get the engine and parts spinning too fast to start. Though that might be different with a bike.
The main thing that starts an internal combustion engine is the starter motor. This is why cars/motorcycles don’t need dangerous hand cranks. Some motorcycles and dirt bikes have kick starters.
The idea, is to put it into a gear that will spin the engine easily, get it moving so that gas will get sucked into the chambers then assuming the battery has enough charge to spark the spark plugs, it’ll start working. Once the engine is running, it’ll recharge the battery. (it’s easier for the battery to trigger the spark plugs vs spinning the starter motor from a stop)
So I’m not sure about bikes, but cars have alternators. Alterators, when you first start your car, your using power straight from the battery, but as soon as the car starts running, there’s a bunch of loops of copper and a magnet in the middle, inside your alternator. So, your engines on, the alternator is spinning the magnet which creates enough power to recharge the little bit of juice taken from the battery and run your lights, radio, etc. I can’t imagine why motorcycles wouldn’t have alternators. So what he’s saying is, put it in 4th gear, so that the alternator spins when you push it, let it spin fast enough and long enough and it will charge the battery just enough to start the bike. Let it sit a minute and run and the battery will charge up.
Hope this helps. My first big explanation on this page.
Tl:dr Car tires spin, magnets spins, electricity is generated, car go brrrrr
The magneto (generator) for the bike is comparatively stronger than in a car. So all you need is a circuit path (ie, key turned in ignition), and turn the engine over quick enough. The magneto turns when the engine does, so if the engine is turning fast enough ***for any reason*** you’ll have power getting to the spark plugs. So turn the engine with the key on, the engine just fires right up. Done it many, many times with my bikes.
Use 4th gear so that the least movement of the rear wheel results in the most revolutions of the engine. If the tire skids instead of turning when in top gear, lower the gear setting. The tire will more strongly resist turning in higher gears, so lowering it will make the wheel turn easier. You may have to push the bike further, but a lower gear will usually work, too.
(biker long ago, tech may have changed)
To start, the engine just needs to be spun fast enough that it can ‘catch’. Releasing the clutch connects the wheels to the engine, causing the bike’s momentum to spin the engine if the bike is moving.
Each gear will spin the engine a different speed. Lower gears will spin it faster but with less efficient use of the bike’s momentum. It sounds like your mechanic friend has decided that fourth gear is the right balance of speed and momentum-efficiency.
Interestingly, this can also be done with manual transmission cars.
The old bump start.
Essentially, the hardest work your battery has to do is start your engine. It’s really hard and requires a lot of energy. But if you can manually get the bike/car rolling by pushing it and get going to say 4-5 mph and then sudden engage the engine… thats enough to do 90% of the work. If the battery has any life left in it at all, it’ll ignite the spark plugs and then your generator will recharge your battery while powering the engine.
It’s a good trick to know if you ever find yourself in a pickle. Starter switches on motorcycles are notoriously prone to failure.
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