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"bump" starting - which gear is right?


elkroeger

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Okay guys. I never understood this. I proved it to myself this morning, but I still don't quite get it.

 

 

The battery on my 11RS was dead this morning. So I pushed it (30yds) over to the small hill, coasted down, and "bumped" it in 1st gear. All i got was a skid. Before I made it to the bottom, I selected 2nd and vroooooooom! Off I went.

 

I've heard it all my life: "don't push start in 1st gear". But it doesn't make sense to me. The lower the gear, the faster the engine has to turn, and thus, the easier it is to bump start. I know I'm wrong, but why? Also, if 2nd works, why wouldn't 5th be even better?

 

Can someone explain this to me? thanks!

 

 

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Dave in Doodah

Think of it this way - 1st gear gives the least amount of turns to the tire per turns of the crank shaft. That is so you can start a stopped tire with a relatively fast turning crankshaft.

 

When you bump start, you are doing the opposite - trying to turn the crank shaft with the rear tire. Because of the gearing, in 1st, you are trying to turn the crank VERY fast with very little rotation of the tire - way too much inertia/friction to overcome so the tire skids. Bump starting in a higher gear is using the slow turning tire to spin the crank at a more reasonable speed. You are only looking for enough engine rotation to turn the alternator for spark, and to move the pistons to drawn in fresh gas/air charge. The starter does not spin the engine fast - bump starting does not need to either.

 

I am sure there is a more factual, accurate explanation; but this is how I resolve it in my feeble brain.

 

Also, if you spin the rear tire to reach TDC for your valve adjustments (instead of turning the crank with a wrench), you realize that top gear is the easiest way to do it - for the same reasons.

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Mister Tee
Think of it this way - 1st gear gives the least amount of turns to the tire per turns of the crank shaft. That is so you can start a stopped tire with a relatively fast turning crankshaft.

 

When you bump start, you are doing the opposite - trying to turn the crank shaft with the rear tire. Because of the gearing, in 1st, you are trying to turn the crank VERY fast with very little rotation of the tire - way too much inertia/friction to overcome so the tire skids. Bump starting in a higher gear is using the slow turning tire to spin the crank at a more reasonable speed. You are only looking for enough engine rotation to turn the alternator for spark, and to move the pistons to drawn in fresh gas/air charge. The starter does not spin the engine fast - bump starting does not need to either.

 

I am sure there is a more factual, accurate explanation; but this is how I resolve it in my feeble brain.

 

Also, if you spin the rear tire to reach TDC for your valve adjustments (instead of turning the crank with a wrench), you realize that top gear is the easiest way to do it - for the same reasons.

 

Perfectly good explanation. Also note that on EFI and solid state ignition bikes, some amount of battery power is usually required to start the bike.

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Joe Frickin' Friday
Think of it this way - 1st gear gives the least amount of turns to the tire per turns of the crank shaft. That is so you can start a stopped tire with a relatively fast turning crankshaft.

 

When you bump start, you are doing the opposite - trying to turn the crank shaft with the rear tire. Because of the gearing, in 1st, you are trying to turn the crank VERY fast with very little rotation of the tire - way too much inertia/friction to overcome so the tire skids. Bump starting in a higher gear is using the slow turning tire to spin the crank at a more reasonable speed. You are only looking for enough engine rotation to turn the alternator for spark, and to move the pistons to drawn in fresh gas/air charge. The starter does not spin the engine fast - bump starting does not need to either.

 

I am sure there is a more factual, accurate explanation; but this is how I resolve it in my feeble brain.

 

Naw, that was pretty good. :thumbsup:

 

In first gear, the engine has a LOT of leverage against the rear wheel; the flip side of that statement is that in first gear, the rear wheel has very little leverage against the engine. It might be possible to roll a 600-cc four-cylinder engine through compression in first gear - you're only squeezing 150cc's at a time - but on an 1100RS, you're talking about 550cc's being compressed in one compression stroke, and it takes a LOT of torque to get through that.

 

In second gear, the engine has less leverage against the rear wheel - and conversely, the wheel's leverage against the engine has improved. Now you might be able to get the engine through a compression event without skidding the rear wheel. Third gear? Even easier. Fifth gear? Much easier.

 

For the R1100RT at 8 MPH (a good running bump-start), fifth gear would spin the engine at about 400 RPM. The starter only spins the engine at about 180 RPM (listen next time you hit the starter: you hear about 3 compression events per second), and that's enough to get it to light up. So fifth gear should give you enough speed for the engine to start. The problem is that as soon as it starts to fire, things will get lurchy: you're basically lugging the engine. Trick is to clutch-in as soon as you're aware that the engine is firing and trying to run on its own.

 

One other thing to note is that a fuel-injected bike like this will require 12 volts beforehand in order to fire. This is necessary to run the fuel pump/ECU/injectors. Battery too low to power the starters, but still gives ~11-12 volts under accessory load? Should be able to bump start. Lights out when you turn the key? Don't even bother bump starting; you need to charge or get a new battery.

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In first gear, the engine has a LOT of leverage against the rear wheel; the flip side of that statement is that in first gear, the rear wheel has very little leverage against the engine.

 

There's my "AHA!" moment. Thanks guys! Also good to know about the EFI needing a little juice too. That will save me some frustration sooner or later.

 

Now, about why that dang battery was dead in the first place.... hmmmm. ;-)

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When ever I have left stuff on for a couple of days and killed the battery bump starting does not work. I have taken to carrying my small Battery Tender with me on trips. If there is NO sign of life when you turn on the key don't bother to push the bike, all you'll get is frustration. I have found that I can jump start the RT with regular jumper cables and a willing car driver.

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".....I have taken to carrying my small Battery Tender with me on trips. ...."

 

Ever been unable to start the bike on a trip and used the BT to get enough charge to start? Just wonder how long it would take. I have never been in that situation on a trip, and don't take the BT, but I'm curious. I have Good Sam (RV) Emergency Road Service and they would come out and with a cable start, and that's always been my plan on the road!

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Just wonder how long it would take.

 

My "Battery Tender Jr." puts out about 750mV, so it'd have to sit all weekend. Or maybe a whole week. Whatever it is, it's waaaaay too long. The Professor and Gilligan could start a bike with a coconut battery faster than that.

 

I don't understand the statement "bump starting doesn't work". Clearly bump starting works. Unless, the battery is totally flat, as stated earlier.

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Your Battery Tender Jr outputs up to 750 milliamps (i. e. 3/4 amp). That will charge a battery enough to start a bike in a few hours, and fully charge it in a couple of days.

 

However, if the battery's initial output voltage when you first connect the Tender is too low (like below approx 9 V?) the Tender has an internal protection circuit that prevents it from turning on at all. That way if you connect it backwards somehow, it won't wreck itself or your battery.

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right 750mA, sorry musta been a typo.

 

I've never been able to get anywhere with the BT Jr. unless i leave it overnight. Whether you wait two hours, or two days, it doesn't matter, there are faster ways to get it started.

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  • 2 weeks later...

Get yourself the 2004 RT "jumper cable" terminal kit and install it on yr bike :thumbsup:

 

Did this on my 2003 (2002 build) RT and my R1100R.

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right 750mA, sorry musta been a typo.

 

I've never been able to get anywhere with the BT Jr. unless i leave it overnight. Whether you wait two hours, or two days, it doesn't matter, there are faster ways to get it started.

 

I wonder if that is the Jr., or the condition of your battery?

I use mine all the time.

Connect after every ride, even daily commuting.

Goes green in very short time.

If I disconnect it and don't connect for some reason, it will show red when reconnecting but get green very shortly.

I got 50+ months from original battery and changed it as preventive maintenance.

Just wondering if you let the battery sit for periods of time w/out riding or charging it?

 

 

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I wonder if that is the Jr., or the condition of your battery?

 

Good point. I keep forgetting that I'm probably not the typical BMW owner. I am very reluctant to spend money replacing parts that don't absolutely have to be replaced.... So it's most definitely the battery. I use the battery tender more to nurse along the old battery, rather than to keep a fresh battery looking new. Thus, the long charge times.

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".....I have taken to carrying my small Battery Tender with me on trips. ...."

 

Ever been unable to start the bike on a trip and used the BT to get enough charge to start? Just wonder how long it would take.

BTDT -- left a light on in my trailer overnight. The answer is: in my case, a couple hours of BT put enough juice into the battery to turn it over and get going.
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I've been gone for a couple of days so I'm slow replying to this. I keep my Gel Cell on the BT all the time as it is easier to plug that in than unplug all my accessories. The gel cell will pull down enough for my ABS to not engage on start up pretty quickly. That said my experience is, the only time I have left stuff on for a few days my battery was DEAD DEAD! It would not bump start and I ultimately got a jump start with regular jumper cables off a pick up truck. Now I carry the BT with me and my reasoning is this: It is easier to borrow an extension cord than find a BT if I needed it. I haven't tried it when the battery is DEAD so maybe it wouldn't work. My big brush cutter has a similar battery on it and I have used the BT to recharge it when it was pretty discharged. I'm going to look into a jumper kit as mentioned, sounds like a good idea.

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  • 4 weeks later...

I always bumped my old 4 cylinder 750 Yamaha in first. Got a lot of frustrating skids but my 250# self overcame most of them. Once someone told me to try 2nd I never had that problem again.

 

I appreciate the knowledge beforehand about needing power for the EFI, etc. in case it ever happens with the new bike.

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I use 2nd gear.. I also stand up on the pegs a bit and drop onto the seat when I let out the clutch.. It keeps the rear from skidding

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mistercindy
I use 2nd gear.. I also stand up on the pegs a bit and drop onto the seat when I let out the clutch.. It keeps the rear from skidding

+1

 

I've done exactly that for a lot of motorcycles.

 

BTW, I'm fortunate enough to live on a cul-de-sac with a good downhill slope of about 100 yards! Its come in handy a couple of times. :grin:

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