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4L Question

McGilli

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Here's a newbie question:

If You're in 4L and you're going up a hill - and it is covered in 12" of snow - and the hill starts getting steeper and steeper - so the Jeep is going slower and slower - and then it keeps getting steeper (Now it's at about 20% grade for the last kick)--> Is there a point you should just take your foot off the gas and retreat - or is it ok to keep the gas down until the Jeep just won't move anymore even with the pedal down in 4L, cuz you are almost near the top and just want to try and make it?


This happened to me today - a hill I've made it up before without snow - but just couldn't do it today with the snow - but the engine just didn't sound healthy near the end..
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chevymitchell

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Here's a newbie question:

If You're in 4L and you're going up a hill - and it is covered in 12" of snow - and the hill starts getting steeper and steeper - so the Jeep is going slower and slower - and then it keeps getting steeper (Now it's at about 20% grade for the last kick)--> Is there a point you should just take your foot off the gas and retreat - or is it ok to keep the gas down until the Jeep just won't move anymore even with the pedal down in 4L, cuz you are almost near the top and just want to try and make it?


This happened to me today - a hill I've made it up before without snow - but just couldn't do it today with the snow - but the engine just didn't sound healthy near the end..
Keep her to the floor until it's unsafe. You're not going to hurt the Jeep by trying to make it. Might suggest 4H though if you need momentum over grip.
 
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McGilli

McGilli

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Keep her to the floor until it's unsafe. You're not going to hurt the Jeep by trying to make it. Might suggest 4H though if you need momentum over grip.
I'm so glad you said that LOL

1. I thought I was grinding something to death lol

2. I had to retreat - then went back up in 4H and with momentum I was able to get 10 feet farther lol but it was a good learning experience for me :)
 

#diesel

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Coming up on my second winter, waiting anxiously on that first snowfall.
 

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I disagree with the theory of more raw power. The art of off-roading independent of any obstacle is traction. With even a small amount, momentum can be maintained. Without it, your momentum will decrease independent of the drag resistance encountered, including hills.

Certainly with an increasing slope, this increases resistance in the form of gravitational drag. Without either sufficient inertia or motive force in the form of traction, your momentum will decay to zero, or worse, loss of control.

The art of managing momentum is to keep motive traction (period). Spinning wheels, while sounding either cool or scary is your worst enemy!

I have only open diffs (owning a Sport S), thus only having the maximum of only two working wheels, one on each axle. Still I've made many difficult obstacles, including some hills by focusing primarily on not spinning anything. Modulating power with an emphasis of using the grip of the tires, rather than shear acceleration, sometimes even using my parking break to slow the angular momentum of just one axle actually improves tractive grip.

This phenomena is actually the exact nature of limited slip differentials. The internal axle clutching creates differing amount of breaking force between the two wheels with an emphasis on decreasing slip between the two. Hence it's name.

The secret is to never spin the wheels under any circumstance. Of course this is easier said than done. Especially when gravity is working harder due to an increased slope.

Next time combine your inertia with careful application of your throttle, with a lighter foot perhaps, and look for the sweet spot and just focus on maintaining momentum, rather than just increasing power in getting to the top. I think you'll be rather surprised at your success rate.

Outside of drag racing, pure acceleration and unbridled torque is not the off-roader's friend. In fact, it's his worst enemy. Remember, it's never about raw power, but rather just enough torque to maintain motive traction. This always means a lighter rather than heavier touch on the accelerator pedal.

Jay
 

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Bruce Willys

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Like Chevymitchell mentioned, safety is a concern here. Maintaining control of the vehicle is key, using the right amount of wheel spin necessary to get up over something is the way to do it which is an acquired skill. Recklessly flooring it up a steep snowy hill can end with the vehicle sliding sideways and flipping. Make sure you're able to stop and come down the way you came in reverse if you don't make it all the way up, too much speed and gusto without being in control can end badly.

Spinning out all the way up a steep hill, pedal to the floor in 4lo will definitely build up heat in the transfer case and cause additional wear. Done on a regular basis and with excessive (unnecessary amounts of) wheel spin is not great for the driveline. Yes, you may need some wheel spin to get over slippery and steep obstacles but always in a controlled manner. I've seen plenty of damaged/stuck vehicles because someone thought 'if I only hit it faster, I'll make it up' and then they lose control of the situation.

Also, I bet a lot of the crunching that you heard was the brakes doing their thing trying to act like lockers.
 

MichaelAnthony

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Well last week when I was up home in the foothills of the appalachian I went for a night ride. @jeepoch are times when it is necessary to do as stated above 4 hi and smash it... this is due to loose terrain and there is no way to possibly 4 lo your way up a hill... below at my pic 33 degree slope loose shale, roots, clay, not a very good scenario. So when you 4 lo it as I showed the person with me, the only thing that happens is you just sit and spin, albeit slowly and make no progress up the hill. Sometime while hill climbing up those kind of conditions, speed is your friend and you need that momentum with the limited amount of traction going up a hill in those conditions. So 4 hi and smash it and thats when I found out my 3.6 needs more umpf... there was mud, dirt, rocks and leaves flying everywhere.... yeee ha
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At some point, on snow and ice, you reach a point where your tires are still rotating forward and you Jeep is moving backward. Not fun. Only experience and judgement will dictate when it's time to just say no.
 

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Michael,

All right, I gave some pretty generic common sense physics behind the most typical off-road approach. Of course there are case by case exceptions. Your experience and skill should dictate exactly how you tackle any tough situation.

When you are just starting to learn you need to start somewhere. Start with how to handle the vast majority of situations. (Maybe even way more than the vast majority.)

How many situations can you think of where speed solved the obstacle? You quoted one. I'll bet you take that approach as the rule rather than the exception you'll wind up with a damaged Jeep or worse almost every time.

I'd rather attempt anything (almost everything, whether unknown or not) slow and puzzle out a solution via higher velocity only when all other factors and risks have been taken into account. Losing control at anywhere near or above the lowest minimum velocity possible to keep momentum is both foolish and dangerous.

In my previous post, please tell me how many times I used either of the words speed or velocity? I'll give you a hint: ZERO.

Sorry but that's advice I'll NEVER advocate. Especially to someone new to off-roading and asking for it.

Jay
 
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McGilli

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Thanks everyone - always good to get different view points.

Ultimately I'm glad you can't do any damage to the gears etc when in 4L if your tires cannot rotate and you have the gas down!
 

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word302

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Michael,

All right, I gave some pretty generic common sense physics behind the most typical off-road approach. Of course there are case by case exceptions. Your experience and skill should dictate exactly how you tackle any tough situation.

When you are just starting to learn you need to start somewhere. Start with how to handle the vast majority of situations. (Maybe even way more than the vast majority.)

How many situations can you think of where speed solved the obstacle? You quoted one. I'll bet you take that approach as the rule rather than the exception you'll wind up with a damaged Jeep or worse almost every time.

I'd rather attempt anything (almost everything, whether unknown or not) slow and puzzle out a solution via higher velocity only when all other factors and risks have been taken into account. Losing control at anywhere near or above the lowest minimum velocity possible to keep momentum is both foolish and dangerous.

In my previous post, please tell me how many times I used either of the words speed or velocity? I'll give you a hint: ZERO.

Sorry but that's advice I'll NEVER advocate. Especially to someone new to off-roading and asking for it.

Jay
Well the reality lies somewhere in the middle of both of your approaches. In the real world your tires are almost always slipping and some amount of spin is generally desired. I Don't advocate for hitting everything fast by any means but momentum is almost always your friend. Wheel spin is a necessity when you're in any kind of terrain that requires you to clean your tires.
 

BlackGenesis

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Thanks everyone - always good to get different view points.

Ultimately I'm glad you can't do any damage to the gears etc when in 4L if your tires cannot rotate and you have the gas down!
I have no doubt that your tires were actually rotating with Jeep stationary and gas down.
If you have automatic...
In the unlikely even that your tires were not rotating (have to be stocky tires on pavement scenario with gradually gas input) then you are just wearing out your clutches.
If you have manual...
Clutch slip kid gear - replace clutch and resurface flywheel (
 

jeepoch

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Well the reality lies somewhere in the middle of both of your approaches. In the real world your tires are almost always slipping and some amount of spin is generally desired. I Don't advocate for hitting everything fast by any means but momentum is almost always your friend. Wheel spin is a necessity when you're in any kind of terrain that requires you to clean your tires.
@word302,

Let's apply some science here shall we. Not totally comprehensive but enough to help the discussion.

By definition a slipping wheel produces the least amount of tractive effort in all circumstances. The amount of physical motive work on any wheel, decreases dramatically with tire slippage. The maximum force than can be applied to a moving vehicle therefore is one with absolutely no slipping wheels.

In physics this can be expressed (as simply as possible):

P = vF or F = P/v
where:
P = power
v = velocity
F = force

Force is inversely proportional to speed within the applied power of the system. In other words, the greater the rotational velocity (of the wheel) the smaller the force acting on the wheel (neglecting all other variables). During wheel slip, the tires may spin at a higher rotational velocity, but less force is actually being transfered to the ground.

In a real application however, such as vehicle motion, many variables can't be ignored such as tractive ability (the type and quality of tires), drivetrain efficiency and engine output (available power), rolling and contact resistance (coefficient of friction), as well as the total mass and geometry of the vehicle.

Regardless of the above, clearly a tire will slip as it's contact resistance diminishes at a specific amount of applied force (both its torque and speed). Let's refer to just the above equation to keep this as simple as possible. The math can get rather complicated if we let it.

So from this equation (derived from Newton's second law), both the contact resistance and speed of the wheel will work against the applied force at a fixed amount of power (torque).

While power is rarely ever fixed it is typically the only real control variable available to the driver at the time of wheel slip. Noting that very skilled drivers will also use small steering adjustments to also potentially increase contact ground resistance with the tires.

The overall goal in any off-roading condition is to find ways to always increase the contact resistance with the tires in order to apply the maximum force with the available torque at the lowest possible instantaneous wheel (rotational) speed. Again, I need to point out that any applied force is inverse to the velocity. If you can't accept that fact, go complain to Sir Isaac but please give him my compliments.

Unfortunately I also believe too many drivers confuse this instanteous rotational wheel speed with forward momentum. Momentum is built up with the power of the system with sufficient contact resistance which increases over time (not instanteous). In other words it is the amount of pent up kinetic energy of the entire system and is directly proportional to both the vehicle's mass and its velocity. While this is a factor to the overall motion, it is only a factor of the circumstances relating to the amount of tractive effort occuring instantaneously at the powered wheels. You can think of these two things as almost independent variables but not completely. Momentum does have a dynamic impact on contact resistance. This is where the math can get really complicated. Has anyone here studied tensor theory?

Still, the art of maintaining enough momentum (not just speed) is to ensure that the tires are contributing, not slipping, to the overall tractive effort. This means using both momentum and only that amount of instanteous force that can continue to provide motive effort (with minimal wheel slip).

So in almost all low friction circumstances this equation shows that less torque, in order to lower (but not entirely remove), the amount of applied force at the wheel for the specific contact resistance at any particular instant in real time is required to keep the wheels from spinning. Please also recall in all cases a non-slipping wheel is a contributing wheel. A slipping wheel is wasting the applied power with the potential side effect of diminished or loss of vehicle control.

The art then is to use both the vehicle's previously built up kinetic momentum PLUS as much instantaneous force as necessary (not possibly available), to keep contributing to the highest possible tractive effort to continue forward motion. This should show you that a lighter foot is almost always better in nearly every situation to minimize wheel slip. Any amount of positive traction (no matter how small) is better than none. It's a Jeepers job to maximize this traction, not lose it with slipping wheels.

Another addige to the phrase - less is more.

[Edited]
Please also realize I'm having some fun while providing some physics here. I realize that off-roading experts have both skill and experience to know when they can push the envelope. Some may even cite numerous corner case scenarios were they can bend the rules. Fine. Expertise comes with a certain amount of trial and error experience.

However, my discussion has always been primarily focused for the novice beginner. Am I an off-roading expert? Definitely not. Do I attempt challenging obstacles? You bet. However, I don't do so recklessly. I take my time and think things through. Has this strategy failed me? Not yet.

However, it's your Jeep. Man-handle it any way you want. I have no right or desire to say or dictate otherwise. Your adrenaline rush is up to you.

But the tenderfoot has to start somewhere. Personally advising anyone to consider applying less rather than more power to the generic situation is sound.

I'm hopefully just giving the newbie the background physics of some of the things involved, whether off-roading or leisurely driving to the mall.

Jay
 
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McGilli

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I have no doubt that your tires were actually rotating with Jeep stationary and gas down.
If you have automatic...

Clutch slip kid gear - replace clutch and resurface flywheel (
Alright.

I drove 100km today to get to another mountain road that was too steep with too much snow just to verify and you, and anyone else who mentioned that is correct.

I was able to pay more attention and yes it's the feeling like when the ABS kicks in, that grinding kind of stuttery (I made that word up) feel in the wheels as they spin.

So - solved :rock:
 
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McGilli

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Oh and Based on the great and lively comments here in this thread - I was also able to test - repeatedly LOL - this hill I could not get up today - and trying it with pedal down, spinning away and climbing... But then also - just start from zero with barely any gas, letting traction do its thing...

It was enlightening. In the end... I think I need snow tires with big knobs on them :CWL: for just a little extra traction maybe... I don't know.. I'm sure that's another topic haha!
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