Rear tires slipping in snow/slush

[Dude]

Thanks Point. Yep, that makes sense.

I am still curious about Ford's warning not to use SLIPPERY mode on dry, hard pavement though.

I'm not doubting your observation, but rather, questioning Ford's warning. Since SLIPPERY mode is essentially just a more sensitive NORMAL mode (four-wheel drive mode is NOT locked) the road surface should not matter.

Well the way I read Ford’s note is slippery mode “could” have the listed issues “depending on the four-wheel drive mode selection”.

“ Note: Do not use slippery mode on dry, hard
pavement. This could produce some
vibration, driveline bind up, and potential
excessive tire and vehicle wear depending
on the four-wheel drive mode selection. “

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[sajohnson]

Found this video, helped me to understand a little more as well.

Good video!

Their animated videos showing how the various AWD/4WD systems work are helpful.

As the narrator says, the system in the BS/BS BL is more accurately called AWD. There is a lot of confusion over what constitutes AWD vs 4WD. Almost all of the systems in passenger cars are AWD, but that's where the similarity ends. There are dozens of different systems. Individual mfrs often use more than one. Some are much more capable than others.

Some -- like my 2002 WRX -- are/were really ONE (1) wheel drive, because they have all open differentials with no way to control torque. The WRX has viscous coupling units (VCUs) but they are ineffective. Power follows the path of least resistance.

Newer cars typically at least have traction control/torque vectoring, as mentioned in the video. That clearly helps, but it is less than ideal because the brakes must be applied while trying to move forward. Also, brakes can overheat when too much torque vectoring is needed.

Some form of locking is much better than torque vectoring.

Regardless of the technical details, all new vehicles should have a basic diagram on the window that indicates how many tires must have traction for it to move up a (say) 10% slope. Because AWD systems are far from equal and people should know what they are paying for.

 

[sajohnson]

Well the way I read Ford’s note is slippery mode “could” have the listed issues “depending on the four-wheel drive mode selection”.

“ Note: Do not use slippery mode on dry, hard
pavement. This could produce some
vibration, driveline bind up, and potential
excessive tire and vehicle wear depending
on the four-wheel drive mode selection. “

Good point.

Of course Ford uses the same language with the other modes -- see SAND above (post #97).

Ford could have been more precise in their warnings. For example ROCK CRAWL enables "four-wheel drive lock" and the rear diff lock. Using ROCK CRAWL on dry pavement definitely will produce some vibration, driveline bind up, and potential excessive tire and vehicle wear -- unless the driver never turns.

 

[SandDawg]

Thanks Point. Yep, that makes sense.

I am still curious about Ford's warning not to use SLIPPERY mode on dry, hard pavement though.

I'm not doubting your observation, but rather, questioning Ford's warning. Since SLIPPERY mode is essentially just a more sensitive NORMAL mode (four-wheel drive mode is NOT locked) the road surface should not matter.

Easy. While the 4wd isn’t locked in slippery it does engage as needed. At points during your route you encounter a stretch of dry pavement while the 4wd is still engaged it could cause a number of things to if you were turning, stopping or accelerating, all of which could damage the drive train if done too often. For example, in 2001 I bought a new Gwagen that had been test driven by the dealer’s daughter. She apparently drove it on dry pavement with all three diffs locked. The whole front suspension was bent by the torque produced by the driveline. Thankfully, I didn’t have to pay for all the bent parts.

 

[sajohnson]

Easy. While the 4wd isn’t locked in slippery it does engage as needed. At points during your route you encounter a stretch of dry pavement while the 4wd is still engaged it could cause a number of things to if you were turning, stopping or accelerating, all of which could damage the drive train if done too often. For example, in 2001 I bought a new Gwagen that had been test driven by the dealer’s daughter. She apparently drove it on dry pavement with all three diffs locked. The whole front suspension was bent by the torque produced by the driveline. Thankfully, I didn’t have to pay for all the bent parts.

Ugly story about the Gwagen. Nice vehicle. The right 'boxy' look, serious drive train, but just a tad pricy.

That 4WD drive train bind-up is very obvious. You'd think any driver would instantly know there is something wrong.

I understand what you're saying -- I'm a tech, and have been working on cars for almost 50 years. What I'm wondering about is: Given that SLIPPERY mode is essentially a more sensitive version of NORMAL (but with lower throttle response and lower rpm shift points), both will switch to four-wheel drive locked automatically, and rear diff lock is "available" in both modes -- what is the difference that allows NORMAL to be used in all conditions?

It's been established by many testers that NORMAL mode is very capable off-road -- so presumably it can fully lock 4WD, and the rear diff can be manually locked, so there could be situations where (say) a secondary road is very slick and hilly -- both the center and rear are locked) but then the highway is bone dry. Apparently that's OK in NORMAL mode, no harm done, but it SLIPPERY mode that would be a problem.

Dude posted this in #106 above. From the manual:

“ Note: Do not use slippery mode on dry, hard
pavement. This could produce some
vibration, driveline bind up, and potential
excessive tire and vehicle wear depending
on the four-wheel drive mode selection. “

~~~

Unlike SAND, ROCK CRAWL, and MUD & RUTS modes -- but like NORMAL mode -- SLIPPERY does NOT say it engages four-wheel drive lock. So I'm left wondering what the problem is.

Needless to say, there are more important issues -- I'm just curious.

 

[Mark S.]

all of which could damage the drive train if done too often.

Unlike the vehicle in your example, the power transfer unit and rear differential in the Bronco Sport are clutch-based, i.e. not hard gears. I think it would be more appropriate to say using SLIPPERY mode on dry pavement could result in increased wear rather than damage.

So I'm left wondering what the problem is.

We'll likely never get the full technical answer to this unless someone on the 4WD system design team is willing to spill the beans. My personal belief is similar to yours, in that SLIPPERY mode makes the system more sensitive to wheel slippage and engages all the tools it has available to stop it, including torque vectoring. Indeed, the whole point of SLIPPERY mode is to PREVENT wheel spin and loss of traction. The system senses wheel slippage as a difference in speed from one wheel to the next, which occurs any time you turn. If SLIPPERY mode is indeed more sensitive--as we suspect--it may "fooled" into employing the 4WD lock AND torque vectoring inappropriately, i.e. when turning on dry pavement. That will certainly increase wear on driveline components, tires, etc. It could also result in untoward vibrations. That's my best guess at this point.

 

[Mwittke5857]

I'm too old to be nice. Compare the tread pattern on the Assurance to the Trail Terrain. My Crap Conti's are mud and snow rated. Not in your wildest dreams. Might as well be Potenza tires. Ford Motor Company does like all other manufacturers. They use the minimum, "Read CHEAPEST" they can. They're not in the game for you but for them. It's called PROFIT. If they can save a nickle on every vehicle, it means more money and bonuses for the execs. Let the flaming begin.

Tires were my first mod and I went with MT Baja Boss tires. Closest one to a mud tire you can get w/o it being an actual mud tire. They eat through the snow

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