Hypothetical Question (Car Related, similar to "will the plane take off?")

NYSpeed Automotive NYSpeed Off Topic
1

so, i listen to a lot of podcasts driving around all day. i really enjoy a lot of npr’s programs, this american life, car talk, ect

every week on car talk they do this thing called “the puzzler.” if youre not familiar with the show, they ask a question and then give you the answer the next week. sometimes car related, sometimes not.

this week’s was good, i got it after thinking about it for a few minutes. in summary -

you and a group of people are on a safari trip in africa. you are riding in a land rover when another off road vehicle wants to pass you. you pull off to some deeper sand, and let him pass. when the driver goes to pull out, he lets out the clutch and accidentally stalls the car. he pushes the clutch in, turns the key, and nothing happens. the group pops the hood, checks the connections to the starter, hits it with a rock, ect, ect. no dice. the starter has simply turned over the motor for the last time.

the driver opens the trunk to look for something he can work with, no tools, no spare parts, simply a jack and a rather long rope.

its starting to get dark, “the vultures are circling” as they said, when finally the driver says he has a plan. within a few minutes, the car is back up and running, and the group in on their merry way. how?

im not going to spoil the surprise if youre sitting there thinking about it, so scroll down to see the answer

ok the answer is, the driver jacked up the rear wheels, put the car in neutral, and coiled the rope around one of the rear tires. then, someone sat in the seat, clutch engaged, key in the on position, while someone else ran with the rope spinning the wheel. the driver popped the clutch, and the car started up.

it works out on paper i guess, but im wondering if this could really happen and work in real life. ive bump started cars more times than i can prob remember, and it always seems as if it takes a second or two after lifting the clutch pedal before the motor fires up, could a rope and two people running have the same force as the wheels on pavement?

anyone have a beater we can try it out on???

2

No way would this work in the real world. The rope wouldn’t be long enough to get the wheel spinning fast enough. Even if said rope was long enough, I HIGHLY doubt that you could pull it to get the wheel spinning fast enough to dump the clutch. Also, better hope you were not in 4wd before it died or your really gonna be fucked. You’d probabally be screwed as your in deep sand so pushing it to the mph required for a clutch pop isn’t a possibility

3

Who goes to Africa?

4

It won’t work. One word, inertia.

The reason you can pop start a car by pushing it is you have 3000+ lbs of car moving when you pop the clutch. All that weight, even going only pushing speed, has enough inertia to turn the motor over.

In NPR’s hypothetical the only thing they have turning is the axle and 2 wheels. As soon as the clutch is popped the few pounds that are turning from spinning the axle is going to be stopped dead by the motor.

5

thank you jay for saving me from typing that

… what jay said.

6

:tup: to JayS.

7

It might work if you could get enough rope wrapped around the tire and then use another truck to pull the rope. Pop the clutch while it’s still pulling.

On paper anyway. In the real world I’m picturing the truck getting yanked off the jack by the force of the rope pull, especially since it will be pulling entirely from one side.

8

I was hoping for something related to the car not being grounded, lol.

9

Yeah I think you’d have to get the wheels spinning impossibly fast to turn the motor. You’d be better off trying to fasten the rope to the idler pulley and turning the engine over directly.

10

The only reason why it works on the ground is due to the fact that you having a few thousand pounds of steel moving at a few MPH. A few hundred pounds of driveline moving at even 10 or 20 MPH is just going to dead stop.

But yes, of course it COULD work if you had a really strong rope, and dozens of people pulling on it as you let the clutch out. Its just all about getting enough force, which would be really hard.

Did the jack he have on hand, happen to be some sort of crazy sand jack? I dont think a standard jack works on deep sand, thats probably loose packed.

11

What if you set the land rover on a treadmill?

:io:

12

I would have lassoed a roving rhinocerous to the frame and hit him in the nuts using the jack

13

Well, since it’s the internet time to obnoxiously over-analyze the thing and focus in on irrelavent details so as to sound smarter than I am: the whole scenario is implausable. Deep sand = desert != safari.

14

If this was a 3 cylinder shit box with lousy compression, I would say it’s plausible. Definitely not gonna happen with a healthy V6 4-stroke engine.

With boat motors, it’s pretty difficult to even start a 2-cyl 40HP by hand, but not horrible. However, I’ve seen my dad hand-start 4 cylinder 100+ hp boat engines. With 2 people, and some good timing, it’s not THAT hard. A V6 though? not happening…Especially not on a 4 stroke motor. Hell, my parents have a little 15hp 4-stroke outboard (2 cylinders) and if it doesn’t want to start, it can get tiring to yank that cord. MAYBE just MAYBE if the gearing was VERY favorable.

The reasoning is sound, but it’s just not gonna happen in the real world.

15

Not exactly. Look up the Sahel region.

The better question to ask would be what Land Rover were they driving? And don’t all Land Rovers have full-time 4-Wheel-Drive? I’m not sure.

16

oh yeah. There is the issue of the rear diff as well. As far as I know, if the car was jacked up so that both rear tires were off the ground, they would need to spin BOTH wheels at the same time. Anytime I’ve had both rear wheels off the ground and spun one, the other side would spin the opposite direction. It’s not until both are spun forward simultaneously, that the drive-shaft would be spun.

That may depend on the actual diff and the design, but it’s definitely an issue.

17

lol so you see my douchebaggery and raise me a new haircut.

18

I would stop listening to this podcast if they are car people and claimed this plausible. lol

19

+1

20

You guys are forgetting that you have a mechanical advantage working in your favor if the truck is in high gear and depending on the rate of the final drive.

I think it’s plausible. Even more so if you pull 4 spark plugs and are able to drive the car to a hill that you can use to bump start on all 8.

If there was something overhead maybe you could wind the rope around the main pulley jack the car up and then tie the rope to it. Turn the ignition on and dump the jack. Instant pull start with madddd force.