New Differential or Rear End for heavy snow in 2wd

[RonD]

They don't need to put up signs, people that need to know this already do know this

Same as using the E-brake to get RWD OPEN differential to "act like" a limited slip to get unstuck

 

[MikeG]

I'm not convinced that there is any practical effect. Nothing in my high school or college physics backs up that claim as being of any real, practical, or measurable effect.

 

[don4331]

Sand and mud, I agree that tires can end up pushing a lot of crap in front of them.... but that isn't going to be anywhere as near as much of an issue in snow. With 2wd, one set of tires is going to be pushing 'stuff' in front, regardless, so I really don't think that is the issue or relevant in the least. Drag is drag.

FWD cars have more weight over the drive wheels, that is going to equal more friction via the contact patch of the tires. Coefficient of friction x contact patch x downward force due to gravity. That's traction, period, doesn't change whether front or rear of car. Like you say basic physics...... more weight on the tire, or better friction (due to tread compound being appropriate) or more contact patch due to airing down. Yeah all of those will help no matter what direction you are going.

Think it's an old wive's tale that reversing a 2wd drive pickup will make it up a hill that it couldn't do going forward. If anything the situation gets slightly worse up an incline, as far as the weight distribution, trying to go up in reverse. Yeah you might be able to reverse your way out of getting stuck on fairly level ground, but that's because the tires already have a path, if they didn't dig in too bad when you got stuck. Only way to test is to do it WITHOUT driving in the 'tracks' made earlier. I've had to take multiple passes up a slope in both FWD and RWD, each time 'plowing' a bit more snow out of the way.

Also, any suspension 'squat' will work AGAINST you trying to back a RWD vehicle up a slope.

FWD typically 60% of the weight on the front tires. Empty pickups have very little weigh on the rear tires. That's the problem with rear wheel drive, not whether it is 'pushed' or 'pulled.' Vehicle frames are stiff enough and don't compress, the vehicle doesn't care, except that yeah the back end can go sideways on RWD vehicles if the rear end looses traction. By the way, FWD cars can kick out the back end too, if they lose lateral grip. Easier to control trying to back a RWD up a slope? Yeah probably, but it does not change the traction available.

FWD can get stuck if you run out of traction, been there, done that. All the 'pulling' vs 'pushing' doesn't do a bit of good, once the traction from the tires, is less than the force needed to move up an incline.

Someone from Texas telling the Cdns how to drive in snow

Coefficient of friction is a dimensionless number. You don't multiply it by contact patch; traction is just coefficient of friction * downward force due to gravity. Now, contact patch will define friction coefficient but increased contact patch does NOT equal increased traction in snow. 215/75R15 will beat 295/55R15s in snow 99 days out of 100 on a Ranger.

The driven axle (FWD going forwards/RWD going backwards) "climbing" thorough the snow (or mud or sand) takes less force and provides a packed surface for the non-drive axle to roll on is far more effect. Successfully reversing up a hill with RWD isn't an "old wives tale". The requirement of the non-driven tire to pack the snow before it can be rolled over may seem like a non-trivial force but I can assure you it makes a difference,

 

[franklin2]

One factor I haven't seen anyone mention. Having the steer tires contributing to the forward motion is a big deal. Besides the weight of the engine, I think that is the one other big advantage of a front wheel drive car or a 4x4 car or truck. Steering the wheels while the back 2wd is trying to push kills any forward motion you may have gained. But if you point the power to the direction you want to go by steering the powered wheels, that is an advantage that a regular 2wd rear drive car or truck can't match, no matter how good the tires are.

 

[MikeG]

Actually the contact patch DOES affect the traction, a great deal. Otherwise racing slicks would be no more effective than bicycle tires.... think about it 'Splain how the size of the contact patch has nothing to do with it......

I grew up in the Ozarks. While we didn't have a lot of snow, we had some of the WORST conditions to drive in. Hills, curvy roads, and a climate that would often cycle between being above freezing during the day, and below freezing at night. Can you say ice? At least one bad ice storm a year.

It's actually easier to drive when it's frozen all the time, trust me.

Anyway - there are theoretical nuances, and then there's the real world. All explained by physics, but there are some factors that are simply of no practical matter. Tread compound, for example, means VASTLY more than where the drive axle is. Learned that once when I bought some GREAT rain tires in the Florida summer.... an inch of the first snowfall of the year back at college, and it was like driving on skis.... Barely made it to the tire store on roads that had the tiniest incline, to get some reasonable tires for the weather.

I've had plenty of FWD and RWD vehicles in snow/ice, and once in a while, mud.

 

[rusty ol ranger]

Speaking from experence....

In a RWD truck going backwards actually is a hinderence up a slick hill.

 

[RonD]

Speaking from experence....

In a RWD truck going backwards actually is a hinderence up a slick hill.

Then you were doing it wrong, joking
The difference is slight but not non-existant, and as MikeG said traction is traction, if there wasn't enough going forward then there may or may not be enough going backwards but pulling a load vs push a load can matter

 

[Bgunner]

Actually the contact patch DOES affect the traction, a great deal. Otherwise racing slicks would be no more effective than bicycle tires.... think about it 'Splain how the size of the contact patch has nothing to do with it......

For on road a large contact patch will hinder traction in snow since a large patch will try to ride on top of the snow where a thinner patch will cut deeper and get closer to the road for traction. A larger contact patch on snow is more favorable when you need flotation like you have in off road scenarios. Then an extremely large patch will help keep the vehicle on top of the snow and help prevent it sinking.

Contact patch has a lot to do with traction but it is also very case specific like in your example versus driving on the road versus off road in snow or sand. It all depends on the ground surface and what type of flotation is needed.

To u can throw as many formulas and theorys around as you want but in practice is where you find what does and does not work well.

 

[RonD]

From the physics point of view
The tire rotation has a upward force, so if drive axle is in the front it lifts tires over resistance, so pulling vector is inclined
If drive axle is in the rear this upward vector pushes the front down slightly, which increase resistance

If you were to put a 12" long weight on a flat surface and put a string on both front sides and then pulled at a slightly inclined angle
Then did the same but with strings on the rear sides you would find it would take slightly more force to pull the load from the rear than the front, because of the inclined angle

 

[scotts90ranger]

There are MANY possible scenarios and types of snow, some takes nothing to drive through, others are just a pain in the rear... sometimes you can just plow through bumper deep (and on the '90, the bumper is up there...) snow in 2wd and others axle deep snow is just ridiculous and a complete pain... but when I go snow wheeling sometimes we're breaking trail in 2' of snow, going until you can't, back up and just ease into it to get on top again... last year we found some fun snow, it was high enough to go through the grille and plug my air filter, pulled that and banged it out on the bumper and kept going . That whole area is closed this year though, it was part of the wildfires by Detroit Lake...

 

[racsan]

When I had a 4wd I would always start out in 2wd first, just so I would know how slick it really was, as has been said: you can go pretty well with a 4x, but it aint helping it stop. Theres really not been enough snow around me for years to need a 4wd, those time you do need it though youre glad when you have it.

 

[Jim Oaks]

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This was last year in Colorado. Going up the hill in 4WD. Open front diff. Left wheel under 4WD power digging through. Right wheel (not under 4wd power) getting to much resistance from the wet snow. Truck started sliding left and basically pivoting around the snow pushed up in front of the right front tire.

 

[MikeG]

From the physics point of view
The tire rotation has a upward force, so if drive axle is in the front it lifts tires over resistance, so pulling vector is inclined
If drive axle is in the rear this upward vector pushes the front down slightly, which increase resistance

If you were to put a 12" long weight on a flat surface and put a string on both front sides and then pulled at a slightly inclined angle
Then did the same but with strings on the rear sides you would find it would take slightly more force to pull the load from the rear than the front, because of the inclined angle

No. The tire does not have either upward or downward force, while rotating. It has angular momentum. Any upward or downward force (between the tire and the road) is due to gravity, and also suspension movement under acceleration / decceleration. How can there be 'upward force' on SNOW? Step on it and it squashes out. Force of the tire, on the (packed) surface, is going to be parallel to the road. Likewise, any drag due to pushing the tire through snow (or mud or whatever) will be parallel to the road. At least if the road is reasonably smooth.

ANY force pushing the vehicle forward (acceleration) will cause the back suspension to squat. ANY force retarding acceleration (ie. braking) WILL cause the rear suspension to lift. Weight transfer follows the end that goes down. That's suspension 101. The reason why is that the center of mass of the vehicle is above the contact patch of the tires and road. If you want to mock this up with a weight or whatever, then the strings have to be EXACTLY on the centerline of the contact patch, and EXACTLY parallel to the surface that the weight is being pulled across.

There might be the eensiest advantage to backing up when on a perfectly level road, with a RWD vehicle. All that goes out the window when going up a slope. And the only difference, even then, is the slight force to overcome compression of snow. Which, if you think about it, is next to nothing. After all we can kick snow out of the way. The cross-sectional area of the front tires, times the depth and density of the snow, is all the force that the driven (or undriven) wheels on the front of the vehicle have to overcome. If that little bit of drag is holding you back... pretty well screwed at that point.

 

[MikeG]

For on road a large contact patch will hinder traction in snow since a large patch will try to ride on top of the snow where a thinner patch will cut deeper and get closer to the road for traction. A larger contact patch on snow is more favorable when you need flotation like you have in off road scenarios. Then an extremely large patch will help keep the vehicle on top of the snow and help prevent it sinking.

Contact patch has a lot to do with traction but it is also very case specific like in your example versus driving on the road versus off road in snow or sand. It all depends on the ground surface and what type of flotation is needed.

To u can throw as many formulas and theorys around as you want but in practice is where you find what does and does not work well.

Agreed that there are times that you want to stay on top of mud/sand/snow, and other times dig in. My post was referring to a claim that the contact patch size did not affect traction, which it clearly does. When there are multiple layers of 'stuff' then yeah sometimes... the answer is not entirely clear cut. But contact patch size DOES affect traction, when on / in a consistent medium.

 

[MikeG]

View attachment 51784
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This was last year in Colorado. Going up the hill in 4WD. Open front diff. Left wheel under 4WD power digging through. Right wheel (not under 4wd power) getting to much resistance from the wet snow. Truck started sliding left and basically pivoting around the snow pushed up in front of the right front tire.

Perfect example of the stability problem illustrated. Three wheels at the limit of traction, and additional resistance from non-driven right front wheel imparts drag. It would likely have kept the back end in line with an open diff, because one of the back wheels would be 'tracking' in a rut from the front wheel..... but you'd have lost a third of the driven wheel traction.

Would have been an interesting test of the idea of backing up the hill to see if there would have been any difference. Were you able to keep going? What kind of tires did the truck have?

One thing to keep in mind is that not only is there resistance of the tires to rolling through the snow, but also force has to be applied to the truck to go uphill, to overcome gravity. Steeper the slope, the more force needed to move the vehicle uphill. So the equation changes as both the snow gets deeper / denser (wetter), but also as the angle of the slope increases. The road is an inclined plane, lifting the truck.