Hill decent and slippery stuff

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Originally Posted by gghaggis

The one main advantage I can see with HDC is that it can apply braking to select wheels depending on lock-up - ie it will release/partially release a wheel that has locked up. A driver-controlled braking technique could never enforce that fast enough to make a difference.

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Which is precisely what ABS does, and in fact HDC is part of ABS. I do not see any difference between the computer applying a given brake pressure, and the ABS system then releasing wheels as required, and a human applying the same pressure, and ABS doing exactly the same. If ABS works differently when in HDC mode I agree HDC then has a point, and it may do for Land Rovers. I don't know but I doubt it, and I know Jeep's system does not differentiate between a human or their HDC.

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Originally Posted by gghaggis

You can 'drive through' the HDC too, balancing the car between braking and forward momentum with the throttle. This is a technique best done in GGS mode, so that you have greater control over the throttle and diff lock-up.

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Interesting tip for GGS, I shall try that. I usually use RC mode on hills as I like my centre coupling locked up nice and tight on gradients.

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Originally Posted by gghaggis

Note that the HDC in the 2010 models is much smoother and quite unobtrusive.

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Agreed, although I would say a driver can still do it better.

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Originally Posted by rmp

If you are descending a 4WD track chances are the gradient varies and, therefore, so can the braking force applied. The example I gave was a spoon drain where you may be descending quite quickly, but as you come to the relative flat part you can slow down effectively. HDC keeps the vehicle at one speed, like cruise control, and doesn't factor in the changes in terrain. Now yes the HDC speed can be varied, but what's quicker -- changing foot brake pressure or pressing buttons to increment and decrement speed? And if it all goes wrong and you need to come right off the brakes what do you do then? Disengage HDC, re-engage it...change the speed....it's all easier with simply modulating the brake pressure.
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I agree! I generally found HDC to be more effort in such situations and creaqted clumsier reactions trying to change it's speed and turn it off/on.

HDC might be able to quickly apply the brakes, but it can't read the ground.

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Originally Posted by rmp

No. HDC brakes wheels. It does not force them to turn. It sets a maximum speed, not a minimum.

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Umm. Read what I said again. I said just that ;)

"Engine turn wheel, brake control slippage."

Robert,

The difference is that when you release brake pressure (say a wheel has locked up), you release pressure on _all_ wheels. The HDC releases pressure only on the wheel locked up. I believe this is different to the Jeep system.

Once you've commenced a hill descent with HDC on, you shouldn't need to disengage it. You can still override it with either the brakes or the accelerator. Of course, some descents (the rear-clenching type ones) require slower progress than the HDC can provide, and in those instances you can't just rely on the computer. You need to know how to drive.

Cheers,

Gordon

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Originally Posted by rmp

Interesting tip for GGS, I shall try that. I usually use RC mode on hills as I like my centre coupling locked up nice and tight on gradients.

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Now you see there is something I cannot agree with. If you are ascending an incline then yes you want to lock diffs so that you do not lose power on individual wheels when they start to lose traction. But when descending an incline you actually want to disperse power in a controlled fashion by allowing individual wheels to turn and find traction as they roll. On a vehicle without HDC you use engine braking and no lockers so that wheels can turn at whatever speed they need to. By locking diffs you prevent that from happening. With HDC, as we have agreed, the brakes are applied individually on the hubs so with lockers on all you are doing is stressing the transmission and excessively wearing brakes.

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Originally Posted by rmp

Agreed, although I would say a driver can still do it better.

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As John Dewey said, you cannot win an argument through debating points of difference when one or the other have already made up their mind.

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Originally Posted by disco2hse

Umm. Read what I said again. I said just that ;)

"Engine turn wheel, brake control slippage."

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That's not my point. With HDC to slow the car the brakes are applied. Another way to get the car to move as slowly as it would with HDC is not to brake but to drive the wheels at that speed. This method is less likely to induce a skid.

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Originally Posted by gghaggis

Robert,

The difference is that when you release brake pressure (say a wheel has locked up), you release pressure on _all_ wheels. The HDC releases pressure only on the wheel locked up. I believe this is different to the Jeep system.

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It's the same on the Jeep and all other HDCs I've seen. Yes, when the driver releases footbrake pressure all four wheels get less brake pressure but again -- that's not the point. As ABS works in low range, when a wheel slips the computer will reduce brake pressure on that wheel anyway, exactly as it does for HDC. Therefore, I'm not seeing any value in HDC unless the ABS calibration is somehow different HDC vs non-HDC. Jeep's isn't, I don't know for certain about LR.

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Originally Posted by gghaggis

Once you've commenced a hill descent with HDC on, you shouldn't need to disengage it. You can still override it with either the brakes or the accelerator. Of course, some descents (the rear-clenching type ones) require slower progress than the HDC can provide, and in those instances you can't just rely on the computer. You need to know how to drive.

Cheers,

Gordon

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Yep agreed and I do think HDC systems descend too quickly for many descents, rocky ones in particular. An exception is Toyota's Crawl Control which goes down to 1km/h.

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Originally Posted by disco2hse

Now you see there is something I cannot agree with. If you are ascending an incline then yes you want to lock diffs so that you do not lose power on individual wheels when they start to lose traction. But when descending an incline you actually want to disperse power in a controlled fashion by allowing individual wheels to turn and find traction as they roll. On a vehicle without HDC you use engine braking and no lockers so that wheels can turn at whatever speed they need to. By locking diffs you prevent that from happening. With HDC, as we have agreed, the brakes are applied individually on the hubs so with lockers on all you are doing is stressing the transmission and excessively wearing brakes.

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For those interested in why I suggest lockers in downhill try this exercise; find a steep slope, one so steep you gain a little speed even when locked in first low. Make sure on this slope the vehicle will be cross-axled, ie diagonal wheels in the air. See what happens as the vehicle passes the cross-axle section...it'll take off due the effect of the differential. Then try the same again, but with locked diffs. The difference is remarkable and that is why using lockers when descending is a good idea. Disco2hse, sorry I do not understand your explanation, particuarly "disperse power in a controlled fashion by allowing individual wheels to turn and find traction as they roll."

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Originally Posted by disco2hse

As John Dewey said, you cannot win an argument through debating points of difference when one or the other have already made up their mind.

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I'm not here to win or lose (go to General Chat if you want to score points), I'm here to further my understanding and I like having my views challenged. Gordon's always up for that and it's great to have an intelligent debate without it descending into ill humour. Actually I rather think Gordon and I agree on most things but have a habit of approaching any given topic from entirely different angles so it looks like we're disagreeing ;-)

Robert,

Not sure I follow you - ABS doesn't normally activate in low range at the speeds we're talking about? You'd feel it in the brake pedal if it did.

Agree with the diff scenario - RC will give you better control in the situation you're talking about (more-so if you have the rear eLocker). But as it auto-engages/auto-disengages, it will also avoid the loss of control that disco2hse is referring to - best of both worlds :BigThumb:

Cheers,

Gordon

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Originally Posted by gghaggis

Robert,

Not sure I follow you - ABS doesn't normally activate in low range at the speeds we're talking about? You'd feel it in the brake pedal if it did.

Agree with the diff scenario - RC will give you better control in the situation you're talking about (more-so if you have the rear eLocker). But as it auto-engages/auto-disengages, it will also avoid the loss of control that disco2hse is referring to - best of both worlds :BigThumb:

Cheers,

Gordon

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ABS does work in low range, I've had it activate and LR tells me should work in low range. What I don't know is whether the activation calibration is different in HDC, I've never got an answer to that question. That for me is the big question about the effectiveness of HDC.

The rear locker isn't in my experience smart enough to engage in the situation I described and even if it was it'd need to pre-lock, not react, and until LR come out with terrain sensing technology it is not as good in that situation as a manually operated locked. That said I do think the auto locker is wonderful and it allows you to focus more on the line than the diff, just sometimes I'd like it to be manually operable.