[adc]

Originally Posted by Samurai

So the conclusion is there is a Y rpm

Originally Posted by Sutripta

No such thing as a "Y" rpm.
Sutripta

There is no Y rpm, a slope steep enough with traction will lead to redline and huge stress - which you can even feel sitting all windows up.

Originally Posted by tsk1979

In the Himalayas there are slopes so steep, that even in 1st gear, you need gentle brake tapping.

For a tourer and if needing to keep a car for long term use [ie beyond the 4 years warranty], only engine braking with minimal brake input puts huge stress. Have felt the stress of the engine, even to redline, in the very first journey in the hills. Came back home and read about it, engine braking alone is hugely harmful as a tourer, brake pads are replaceable. So, if a tourer, a judicious mix of engine braking and brake pads, - changed brake pads of the Safari 2wd at 42k km. Thats good enough!

[headers]

Originally Posted by adc

There is no Y rpm

You pulled the words out of me adc

[Red Liner]

Originally Posted by Red Liner

If wheels spin faster, then the RPM goes up? So steeper the incline, faster the wheels spin, and more the RPM.

Originally Posted by adc

There is no Y rpm

Originally Posted by headers

You pulled the words out of me adc

As I mentioned much earlier in my very country-brutish way of conversation

[Samurai]

Originally Posted by adc

There is no Y rpm, a slope steep enough with traction will lead to redline and huge stress - which you can even feel sitting all windows up.

But Sutripta and Wolf specifically mentioned that simply going down the slope in gear will never lead to redlining. BTW, that is my observation too.

Why do you think they are wrong?

Since it is not hitting redline, then it is some rpm between idle and redline. If you don't want to call it Y, call it something else.

[The Wolf]

Originally Posted by Samurai

Since it is not hitting redline, then it is some rpm between idle and redline. If you don't want to call it Y, call it something else.

That Y is the point where gravity is fighting against the engine braking. Increase the decline angle extremely steeply, the gravity will win and the Y rpm will go closer/touch redline. But if you decrease the slope angle the engine braking will win and the Y will stay close to momentum/speed at idle rpm.

p.s: 'Traction' and 'distance' are the variable components hence misleading us to believe that the redline is unachievable in the first scenario. If the traction is made a constant, say 100% and distance as 'unrestricted' in both scenarios, the Y will eventually be the redline in scenario number one.

Hope this clarifies...

[ex670c]

Hi Guys,

While we talk about Y-RPM and Engine Braking.

What role does the entire gearing of a vehicle have to play in this scenario?

Regards,

Arka

[The Wolf]

Originally Posted by ex670c

Hi Guys,

While we talk about Y-RPM and Engine Braking.

What role does the entire gearing of a vehicle have to play in this scenario?

Regards,

Arka

Hey arka, refer to my post below...let me know if there is anything you would like to add or delete in there...

Originally Posted by The Wolf

Lovely topic, love the technical ambiguity that is being created here...hehe!

To start, there are two things in this whole deal that is being dealt as one hence the confusion. The surprise is that there is a difference between Engine braking and Gear braking.. but both, when applied together, causes retardation of kinetic energy transfer to the axle or drive shaft through the gears causing the vehicle to slow down

Let me explain the first phenomenon;
Engine braking: This is when you make the piston run against or toward the engine head with the valves/throttle body valves shut causing subtle vacuum inside the cylinder. Simpler example, in your right hand, take an empty injection syringe without the needle in it, shut the needle side orifice with the fore finger of your left hand and now try pressing the plunger into the barrel with your right hand thumb. What happens? Either the air inside the barrel tries to escape forcefully through the gap it creates between the orifice and your left fore finger skin or simply the plunger wouldnt budge. Repeat the process inversely, now try pulling the plunger from the bottom most point with the orifice shut, the plunger wouldnt budge again. Well this is what happens exactly during engine braking. The piston is made to pull or push against the presence of free available air or combusted incylinder gas respectively.

Note: In case of diesel engines where there are no throttlebodies which in our context work as secondary intake valves the vacuum created is relatively lower that the petrol engines which necessarily has one. The space in between the throttlebody valve and the intake valve works as secondary vacuum chamber when the piston is made to work against keeping them shut.

Gear braking: We all know that the torque produced at the crankshaft, due to the piston travel, is transfered to the wheels through gears used in the gearbox or torque converter/multiplier as we call it and the transfercase via an axle or driveshaft. Each gear has a certain size in diameter and the corresponding number of teeth or cogs right? Now, why is one gear different from another? Simply coz of their difference in running diameter and teeth count. For every power stroke there is a certain amount of torque/rotational force produced at the crank shaft, now when this torque requires to be transmitted to the wheels appropriate gears are applied starting with the one that can produce the maximum among all the gears, usually the 1st gear...then the 2nd gear and so on.

Now the important bit, each gear has its organic limitation as explained above due to which that can rotate only up to a certain speed for the given input hence one gear allows you to go upto one certain speed. The Low gearbox has a higher final reduction/final drive ratio than the High gearbox hence they allow only lower speeds for the given rpms at the flywheel.

@Samurai: Does the above answer you question "Why it gains speed in H while it doesn't in L"?

When Engine braking and gear braking both are applied at tandem or together one can imagine the retardation that can occur at the wheels. Hence the 2kmph descends at 45deg without throttle, almost antigravity phenomenon

[ex670c]

Originally Posted by ex670c

Hi Guys,

While we talk about Y-RPM and Engine Braking.

What role does the entire gearing of a vehicle have to play in this scenario?

Regards,

Arka

Hi Guys,

Gearing has 2 Obvious Roles
i) Reduction of Speed
ii) Increase Torque

If a JEEP is going down a Slope in 1st Low i.e 52:1 Reduction.
i) The Engine is generating Torque to Rotate the Wheels
ii) The Gearing is Slowing Down the Engine Speed

Gravity is Acting on the Wheels along with The Engine Power and encouraging the Wheels to rotate faster much against the gearing.

For 52 Engine RPM the Wheel Does 1 RPM, with Gravity Acting on the Wheels 1 Wheel RPM Force the Engine to do 52 RPM.

The Effect of the Engine is through the Clutch and the Effect of Gravity is through the Gearbox and Again at the Clutch.

One of the main reasons for Engines to accelerate by themselves is the effect of Gravity.

Hence it is advised not to ride the clutch in any running situation, as it can suddenly increase the mechanical force on the Clutch, due to the reverse effect of gearing.

On a Climb Engine Fights Gravity and usually wins. On a Descent Engine and Gravity work together resulting in the Engine Speeding up, this can be corrected by giving minor accelerator inputs.

This is also an indication of weak compression, leaking valves, worn head gasket..

Regards,

Arka

[Samurai]

Arka, from a practical point of view, have you seen engine redlining when coming down a seriously steep long incline in say 1st low?

I am saying 1st low because 1st low will put lot more force on the engine compared to 2nd low, thanks to the higher multiplication factor.

[ex670c]

Originally Posted by Samurai

Arka, from a practical point of view, have you seen engine redlining when coming down a seriously steep long incline in say 1st low?

I am saying 1st low because 1st low will put lot more force on the engine compared to 2nd low, thanks to the higher multiplication factor.

Hi Sharath,

I don't have a Tachometer in my JEEP, but this time in the EXAMM/AKC trip, my JEEP was Screaming on all the Descents (Highway & OTR) because of a Blow Head Gasket. (Insufficient Compression).

I have faced the Same on my Petrol CJ3B, when the engine had become Weak.

If the Engine is perfectly health then its RPM increases, but not Redlining it.

Regards,

Arka

[Samurai]

So that explains why your Jeep was struggling in some places.

For this discussion we have to consider healthy engine.

For a given slope, force of gravity on the Jeep will be same. I guess everybody can agree on that.

But the force exerted on the engine will differ based on the gears. It will be highest in lowest gear and gets less as we move higher in gear. This is due to decreasing multiplication factor in higher gears. That means Y rpm, where engine braking equalizes the force exerted by the wheels due to gravity will be highest at 1st gear, and gets lower at higher gears.

But there is a twist, the effect of higher momentum as the Jeep moves faster in higher gears. At this point mathematics has to take over and most of us are not capable of calculating all the forces involved here.

[SPIKE ARRESTOR]

Originally Posted by ex670c

Hi Sharath,

I don't have a Tachometer in my JEEP, but this time in the EXAMM/AKC trip, my JEEP was Screaming on all the Descents (Highway & OTR) because of a Blow Head Gasket. (Insufficient Compression).

Arka

Hi Arka,

I had noticed the smokes from your Jeep and knew right away something was wrong with your steed. Didn't expect faulty FIP settings as you had your trusted mechanic ALI alongside you. My doubt on the head gasket stands correct now. BTW did you check the compression, are there any bubbles near the radiator cap when the engine is idling?

Spike

[ex670c]

Originally Posted by Samurai

So that explains why your Jeep was struggling in some places.

For this discussion we have to consider healthy engine.

For a given slope, force of gravity on the Jeep will be same. I guess everybody can agree on that.

But the force exerted on the engine will differ based on the gears. It will be highest in lowest gear and gets less as we move higher in gear. This is due to decreasing multiplication factor in higher gears. That means Y rpm, where engine braking equalizes the force exerted by the wheels due to gravity will be highest at 1st gear, and gets lower at higher gears.

But there is a twist, the effect of higher momentum as the Jeep moves faster in higher gears. At this point mathematics has to take over and most of us are not capable of calculating all the forces involved here.

Hi Sharath,

Have I ever mentioned, the blown head gasket, as the reason for struggling off-road?

I drove it to Mumbai and Back, besides where was it struggling? The first obstacle in Lonavala, 2 different attempts with 2 different drivers was successful.

Where else was it struggling, any of the climbs? Pits? Even the Longest Climb after Lunch at EXAMM, I did it in one shot, not many people managed it in One Shot.

I think you have something personal here.

The Force exerted on the Engine will be from 2 Sides the Weight of the vehicle and the Reverse Effect of Gearing through the wheels.

The Engine Alone cannot hold the weight of the Vehicle it has to be in conjunction of the gears.

The interesting bit is in 1st Gear Low is the closest to equilibrium after that the effect of reverse gearing becomes obvious.

Regards,

Arka

[Samurai]

Originally Posted by ex670c

Have I ever mentioned, the blown head gasket, as the reason for struggling off-road?

I drove it to Mumbai and Back, besides where was it struggling? The first obstacle in Lonavala, 2 different attempts with 2 different drivers was successful.

Where else was it struggling, any of the climbs? Pits? Even the Longest Climb after Lunch at EXAMM, I did it in one shot, not many people managed it in One Shot.

I think you have something personal here.

Man, you are touchy... I didn't say when you were driving. I am not used to seeing it struggle under anybody's driving. But I did see it stuck in couple places, hence the remark. Why would I have anything against a Jeep in which I learnt offroading in the first place?

The Engine Alone cannot hold the weight of the Vehicle it has to be in conjunction of the gears.

The interesting bit is in 1st Gear Low is the closest to equilibrium after that the effect of reverse gearing becomes obvious.

This is the part that confuses me, isn't the reverse effect maximum at 1st gear?

[ex670c]

Originally Posted by Samurai

Man, you are touchy... I didn't say when you were driving. I am not used to seeing it struggle under anybody's driving. But I did see it stuck in couple places, hence the remark. Why would I have anything against a Jeep in which I learnt offroading in the first place?

This is the part that confuses me, isn't the reverse effect maximum at 1st gear?

Hi Sharath,

Getting stuck is part of driving, infact it I was winched out thrice during the EXAMM/AKC but that is again driver error.

Actually what you see is the driver struggling with the Steering, off-road.

You are right about the Reverse Gearing Effect, being Maximum, in 1st Low, but that is the point of equilibrium (Crawl Speed hence Torque), in 2nd Lo or after that the vehicle gains a lot of momentum and the reverse gearing starts acting

Regards,

Arka