4WD isn't always brilliant ...

[peasant]

Had a scary experience this morning. On my way to work there is a small lane with a steep downhill bit. This morning the road was wet and greasy, but not icy. While driving down slowly (it IS narrow) I noticed another car coming up and so I slammed on the brakes to make it into the next driveway to make passing easier. What happened next was a bit scary

Due to the gradient being rather steep, the road rather slippery and my slamming on rather hard, my rear wheels blocked and started to slide. No surprise there.

That came a millisecond later, when the VC decided that as the rear wheels weren't turning, the front wheels needn't turn either and dutifully locked them up as well
As I was going slow initially, I had also pressed the clutch with the brake ...and I tell you, it feels very strange indeed to uncontrollably slide down a hill with four blocked wheels, slowly going sideways ...even though you've already let go of the brake.

I actually had to let the clutch out, blip the gas, get the wheels turning again and then start braking (more gently this time) all over again.

I was theoretically aware that this could potentially happen (especially off-road on tricky surfaces) but to feel it happen on the road ...that was a lesson and a half.

Thankfully nothing serious happend, I still came to a standstill (if not at the envisaged spot) and the other car managed to squeeze past.

[Syncro G]

Wouldn't a 2wd lockup like that in simular conditions? As the brakes could lock the rears and front brakes are stronger than rear ones it probubly didn't take much VC input to make them lock too, the brakes would do most of it. If you only locked the rears it could have helped it spin ala handbrake turn so that wouldn't be much help. Being 4wd your method of corecting it worked on all 4 wheels so should have brought it all back in control quicker than something with unpowered axles might - still better off in a syncro I recon.

I once made the mistake of dabing the brakes on the landy when driving down a steep hill on snow - grip was very low and so the brakeing broke traction and stalled the engine! I would have expected it to slide if I'd have used the clutch but didn't expect the brakes to overpower the engine which usally resists change quite strongly! In retrospect I surpose I might have used the pedal harder than planned as I wouldn't have felt any feedback of slowing sensation as grip was lossed, would have been cadanceing it though (again trying to avoid locking up!). I restarted it quick and gave it a lot of revs (it was in low box at the time so needed a fair welly to get the wheels to match the rapidly increaseing slide speed). To my releaf it actully worked and I got it slowly back to near walking speed, I could feal the front axle regaining grip/steering through the steering wheel as I increased the revs, don't think that'd have happened if I couldn't have powered the axle as there just wasn't the grip avalable for the ground to do it (snow on compressed ice it turned out, pritty extreme really) - had to have a little break after that to carm nerves - would have put it through a drystone wall at 20mph+ if it carried on sliding - that'd hurt without belts I recon, I did contomplate jumping out the door but couldn't bare to watch it crash!

[type2nut2]

Don't put your foot on the clutch until you are nearly stationary - it'll help a lot.
It is actually good driving practice to brake with the running gear engaged anyway.

[Andy syncro-nutz]

Operator Error!

[HarryMann]

Don't put your foot on the clutch until you are nearly stationary - it'll help a lot.
It is actually good driving practice to brake with the running gear engaged anyway

Yup, easy mistake to make when going slowly, but maybe was caught in one gear too high, so thought clutch needed quickly...

I'd actually leave the clutch and risk stalling it to a standstill to get that VC anti-lock effect...


Think both examples there might also illustrate the 'whiff of gas' theory is so often a powerful way of regaining traction & steering control momentarily, then brake again when pointing in right direction... (preferably up someone's drive in thos esituations!)

[eric]

Very very interesting, except that what you blokes are on about is way beyond my comprehension.

For me, coming down the steep lane, then slamming on the anchors would be enough for me to think about. The analytical info described is like discussing how a female thinks. Ie.---- impossible! Well, for me anyway.

Great post, thank you.

Eric.

[peasant]

What happens is the following (I think )

When you lock up the wheels, friction is drastically reduced (in comparison to rolling wheels)

Sliding down a steep hill with locked wheels and pressed clutch, you will keep sliding for a long time, because of the low friction and gravity+momentum pushing you on.

In a 2WD when you release the brakes , the little bit of friction on the wheels may be just enough to get the wheels turning again (so you can steer again and possibly brake more gently).

In a 4WD, there are situations where the inertia of the locked drivetrain is bigger than the friction of the tyres ...releasing the brakes does nothing, you keep on sliding.

In this case you need to get the drivetrain turning via the engine (engage the clutch) to turn the wheels to regain traction