[Reinhard]

Originally Posted by alpha1

I don't recall observing this hence my questions:
When one wheel is jacked up, and in gear, does it spin freely (like how the rear wheel of FWD cars does?
If it is indeed spinning freely then how can it transmit the torque to the engine?

Oh no - I misunderstood your question then. I thought you were asking about it spinning freely when the engine was running in gear while the other wheel is stuck on ground.

So - when in gear and engine is "off" - the wheel does not spin freely. And that is why the gentleman in the video has looped the rope over it & pulled with a force. He has basically created a simple pulley mechanism in which he is able to apply an amplified force to pull the rope horizontally and from a distance on the wheel edge. This makes it easier. If one tries to turn the wheel by bare hands - it would not be this effortless.

Originally Posted by alpha1

In case of vehicle on ground and push started, both wheels touch the ground. In order to start pushing the vehicle has to have the clutch engaged, and once it picks momentum, you disengage the clutch. The torque from the wheels is transmitter to engine and cranks it to life.

Actually you have to disengage the clutch to push the vehicle in gear. When you engage the clutch - the engine is connected with the gearbox. When you disengage - the two are isolated.

And yes - once you engage the clutch the wheels torque the engine and crank it like you said.

[SS-Traveller]

The TUV3OO video - was the engine also cranked with the starter motor at the same time as the chap was pulling on the rope? It sounded like that...

I don't see how a heavy common rail diesel can start so effortlessly by yanking on a rope around the wheel, that too by a chap who doesn't appear to be well-built. The 800cc engine of the Renault can be started this way, I'm sure.

[Reinhard]

Originally Posted by SS-Traveller

The TUV3OO video - was the engine also cranked with the starter motor at the same time as the chap was pulling on the rope? It sounded like that...
I don't see how a heavy common rail diesel can start so effortlessly by yanking on a rope around the wheel, that too by a chap who doesn't appear to be well-built. The 800cc engine of the Renault can be started this way, I'm sure.

Yep sounded so. Also - near the end of it - I think I saw him quickly shift into gear by left hand and then revving to show the wheel turning faster with it. Smart chap! He'd have to pull with much more force if the clutch is engaged.

[Sutripta]

Originally Posted by amit_purohit20

Its because the torque required to turn a wheel which is in air is very very less and so the whole torque gets transferred to the free spinning wheel.

Same torque on both wheels.

Sutripta

[amit_purohit20]

Originally Posted by Sutripta

Same torque on both wheels.

Sutripta

My bad! Thanks for correcting me Sutripta Da it should be same torque on both wheels.

Off-topic but why would a car which is on a jack, try to lunge forward due to sudden throttle inputs. Its something to do with the momentary reactions but your views from the physics stand point would be well appreciated!

Thanks

[Sutripta]

Ever noticed the whole car shaking on engine startup (less so on stop).
Accelerating wheel (due to sudden burst of engine speed) => torque => same torque to other wheel + shake on engine start + these jacks are totally unstable.

I think the video is a sham. Ever tried starting to push a car when in gear. And remember when you are working on one wheel, you are introducing a factor of two, almost dropping two gears.

You can point some of the others in this thread to the relevant threads on differentials.

Sutripta

[Slickshift99]

I once had my car battery dead while i had gone shopping for groceries, all it took me was to stop an e-rick and jump start my car , it fared me 10 bucks. Most of the cities in india are getting e-ricks and they are happy to help for a quick buck!

[WhiteKnight]

Originally Posted by Sutripta

I think the video is a sham. Ever tried starting to push a car when in gear. And remember when you are working on one wheel, you are introducing a factor of two, almost dropping two gears.
Sutripta

Thank you, couldn't agree more. I was surprised we're discussing it at tbhp.

With dead battery, a modern car will not start, because the fuel pump will not work and neither ecm will give any directions.
Only, and safe way is to use a jumper cable.

Old CJ series Mahindra is one of the vehicles that I remember having pump not driven by battery. We could start that without a key just by pushing it. It will fire on the moment you release the clutch. It was a fine art of releasing the clutch and pressing it the moment jeep starts. Else, the 3 gear CJ can go on without any accerator input.

[Sutripta]

^^^
It is almost impossible for humanpower to turn over a diesel from standstill. Which is why when pushstarting we disengage clutch, build up momentum and then pop the clutch. In this case clutch is always engaged.

Jack up a driven wheel, keep the car in gear, grab the wheel with both hands and try turning it. And remember in a CRDI one is likely to need an average of one full engine rotation (worst case two) for the ECU to sync.

What one is most likely to do is topple the car from the jack.

Sutripta

[puneetakhouri]

I have been aware of this approach for quite a long time. I have primarily seen truck drivers use this technique when their heavily loaded trucks run low on battery and obviously they cannot push it. So they tie a rope to the wheel, jack it up, the driver on the wheels, the cleaner rotates the rope and simulates pushing the vehicle and the driver releases the clutch and voila!

Attaching the video for this.

https://www.youtube.com/watch?v=zcBE8HEAyi4

[govindremesh]

Originally Posted by Sutripta

I think the video is a sham. Ever tried starting to push a car when in gear. And remember when you are working on one wheel, you are introducing a factor of two, almost dropping two gears.

Originally Posted by MegaWhat

I'm not sure if this is a stupid query.
But what's stopping the other front wheel of the kwid (the one that's on the ground) from rotating once the engine is running? When the engine is running and the reverse gear is engaged, torque would be transmitted to both wheels wouldn't it?

Originally Posted by alpha1

I don't recall observing this hence my questions:
When one wheel is jacked up, and in gear, does it spin freely (like how the rear wheel of FWD cars does?
If it is indeed spinning freely then how can it transmit the torque to the engine?

Originally Posted by SS-Traveller

I don't see how a heavy common rail diesel can start so effortlessly by yanking on a rope around the wheel, that too by a chap who doesn't appear to be well-built. The 800cc engine of the Renault can be started this way, I'm sure.

Sorry to quote multiple posts, as I have the same doubt and still didn't get the trick in the video. How is it possible that the Kwid could start with the drivetrain engaged in reverse gear, one wheel spinning and the other firmly on the ground?

The way the engine started in Kwid (so smoothly) makes me feel that there was someone in the Kwid who started the engine manually with the transmission in Neutral. The person can be sitting in a crouched position to avoid appearing in the camera.

Originally Posted by Reinhard

Why only one wheel spins - is the open differential indeed.

It still confuses me on how can one wheel spin and the other wheel be stationary when the transmission is engaged in reverse and the clutch is couples the engine to the transmission. How I understood it is: the open slip differential allows slipping of wheels, but not independent rotation when it is transmitting power from the engine.
Did I understand wrong or does the open differential scenario only come into picture when the clutch is decoupled?

[Sutripta]

Originally Posted by govindremesh

Sorry to quote multiple posts, as I have the same doubt and still didn't get the trick in the video. How is it possible that the Kwid could start with the drivetrain engaged in reverse gear, one wheel spinning and the other firmly on the ground?

That's OK. That's how a differential works.
For the diesel, the 'trick' (I feel) is that there is a person inside manipulating the clutch, if not the starter.

The way the engine started in Kwid (so smoothly) makes me feel that there was someone in the Kwid who started the engine manually with the transmission in Neutral. The person can be sitting in a crouched position to avoid appearing in the camera.

A small petrol engine can be started this way. In this way it is no different from say rope start (recoil starter) engines. Or motorcycle engines which you kickstart.

It still confuses me on how can one wheel spin and the other wheel be stationary when the transmission is engaged in reverse and the clutch is couples the engine to the transmission. How I understood it is: the open slip differential allows slipping of wheels, but not independent rotation when it is transmitting power from the engine.
Did I understand wrong or does the open differential scenario only come into picture when the clutch is decoupled?

The wheels don't 'slip'. Fully geared, but shall we say 'differentially'!
There is a pretty long thread on differentials in the 4x4 technical section. Do go through it.

Sutripta

[amit_purohit20]

Originally Posted by govindremesh

It still confuses me on how can one wheel spin and the other wheel be stationary ...

For basic understanding:
https://www.team-bhp.com/forum/techn...ml#post1425400 (How does your car take a turn smoothly - The Differential)

For more complicated details refer:
https://www.team-bhp.com/forum/4x4-t...tribution.html (Torque generation and distribution)

[Tgo]

Question: How does rolling a wheel help prime the fuel lines when the battery is dead?

One can do this if the battery is low, but surely not when the battery is dead. The car’s immobilizer would also not be working on a dead battery.

[amit_purohit20]

Originally Posted by Tgo

Question: How does rolling a wheel help prime the fuel lines when the battery is dead?

One can do this if the battery is low, but surely not when the battery is dead. The car’s immobilizer would also not be working on a dead battery.

When you switch off an engine fuel which is there in the fuel lines does not return back to fuel tank. It means that your primary fuel pump which is in the fuel tank need not function for the engine to start.

As soon as the engine starts it will rotate the alternator too and it will start operating all the devices which work on electricity.