AWD Transmission: DSTC on or off - XC90

[ianu]

Taken from the Front Park Assist thread....
"Originally Posted by ianu
...I can see why that would make you nervous Roy. I reckon if you can walk down it and stay upright - the car will cope with it coming back up.
You'll need to turn off the DST though so it doesn't try and brake a wheel if it spins and keep you foot on the gas pedal.
I'd love to see it have a go though - but it's your car Roy and I'd be nervous too. Perhaps if you had someone with a tractor on standby....

Cheers
Ian

Ian, won't turning the DST off stop the AWD reaportioning more drive to back wheels if the front start spinning, as you'd expect, with the weight being transfered to the rear on a 45 degree slope?"


If you knew you were going to need all the grip you could get would you turn the DSTC off ?
On two occassions when I've been in a demonstration with a car that has AWD (not 4WD with diff locks) once in an XC90 an second in a new ML320 - when faced with a slope that was going to challenge the grip of all the wheels - the drivers instructed to turn OFF the DSTC.
The way I believe this works is because the DSTC will either cut power or apply the brakes to a spinning wheel with it on but continue to pass drive to the remaining to a point at which it runs out of wheels to pass it to and you come to a stop.
With it off I believe - even if a wheel spins it will still pass drive to the other wheels and keep drive applied. I think the XC90 will bias drive to the rear anyway with all the weight leaning on it.....
Maybe only one way to test the theory - but it definitley worked for me on a pretty steep slippery wet muddy hill on a Freedom drive day in Northamptonshire (Lamport House I think).
Anyone have a similar experience or am I talking rubbish ?

Cheers
Ian.

[Arianne]

Ianu & Co.

I'm interested in this one...........

We go to the Austrian Alps every Xmas / New Year. Last year was our first trip with a 4x4 in the XC90. But the ski slopes were almost bare of snow so the roads were clear.

But there was one day upon which it snowed heavily. And even with my winter tyres mounted I thought the XC90 could have done better on the sloping track leading up to our appartment.

I had the DSTC switched on. That's because my dealer told me never to disable it when we bought the car new. But I wonder.............

You have a point about the dynamics of the wheels on snow.

So let's see what others think. Perhaps Ry can help as he seems to have more off-road experience than us and those winter tyres he uses are a good clue?

OK lads.............

Arianne.

[RoyMacDonald]

Originally Posted by ianu

Taken from the Front Park Assist thread....
"Originally Posted by ianu
...I can see why that would make you nervous Roy. I reckon if you can walk down it and stay upright - the car will cope with it coming back up.
You'll need to turn off the DST though so it doesn't try and brake a wheel if it spins and keep you foot on the gas pedal.
I'd love to see it have a go though - but it's your car Roy and I'd be nervous too. Perhaps if you had someone with a tractor on standby....

Cheers
Ian

Ian, won't turning the DST off stop the AWD reaportioning more drive to back wheels if the front start spinning, as you'd expect, with the weight being transfered to the rear on a 45 degree slope?"


If you knew you were going to need all the grip you could get would you turn the DSTC off ?
On two occassions when I've been in a demonstration with a car that has AWD (not 4WD with diff locks) once in an XC90 an second in a new ML320 - when faced with a slope that was going to challenge the grip of all the wheels - the drivers instructed to turn OFF the DSTC.
The way I believe this works is because the DSTC will either cut power or apply the brakes to a spinning wheel with it on but continue to pass drive to the remaining to a point at which it runs out of wheels to pass it to and you come to a stop.
With it off I believe - even if a wheel spins it will still pass drive to the other wheels and keep drive applied. I think the XC90 will bias drive to the rear anyway with all the weight leaning on it.....
Maybe only one way to test the theory - but it definitley worked for me on a pretty steep slippery wet muddy hill on a Freedom drive day in Northamptonshire (Lamport House I think).
Anyone have a similar experience or am I talking rubbish ?

Cheers
Ian.

I've never reached the limits of the AWD with it's standard setting yet Ian but in the section called DSTC in my manual says the Traction Control (TC), Active Yaw Control (AYC) and Roll Stability Control (RSC) cannot be turned off. The only part of the DSTC that can be turned off is the Stability Control (SC) which reduces torque to prevent slipping during acceleration on slippery surfaces. The manual says it is benificial to turn it off in deep snow or sand or when driving with snow chains.

The information in the manual makes me thing the demonstraters turned the DSTC off (Actually it just reduces the function of the SC, the rest of the DSTC stayed working.) to gain more torque for the steep climb. The AWD would have stayed working exactly as normal, moving the drive to the wheels with most grip.

However I believe the working of the DTSC is slightly different in the latest 185 bhp AWD so the latest manual may say something else.

All the best.
Roy

[ianu]

Thanks Roy - that would make sense as i realise there is more than one system in play here.
Rather timely - for those that live across the midlands we had some snow here Sunday night which I unexpectedly found myself having to drive through travelling from Lincolnshire back to Oxfordshire. Around Northampton was a nightmare for visibility and the main roads were struggling to keep just one lane open.
In line with the dealer recommendation - I'd never advocate turning this off even in what may seem the most adverse conditions especially when the road surface is changing constantly underneath your tyres between grip and no grip.
I took the opportunity to push the car on Sunday night in the lanes that were effectively virgin slush and snow and I can't commend highly enough how well it coped when it was obviously struggling for a wheel with grip. The DSTC lamp was permanently flickering in some cases where it wasn't apparent grip woud be a problem. BUT - there was one point where the car slowed dramatically to a point where I don't think it had any grip and the DSTC lamp was solid on - momentum kept us rolling and we got grip back.
This is potentially a point at which you'd turn it off if you got stuck I think - or you were already lining up for a run that you knew would potentially stop you dead and would be the last thing you needed.
Just to add finally that as Roy mentions - the other systems do not switch off and I still think will over-ride if it looks like getting drastic.
I proved this last year in a deserted car park covered in about 2 inches of fresh snow attempting to demonstrate to my son what that button on the dash does.
If you get the opportunity try it - a figure of eight with it switched on in second gear - it'll keep the car checked. Try it with it off and yes it will slip - but as soon as there's any hint it's getting too sideways it intervenes - fascinating.
I'll caveat all this by saying that the laws of physics are still valid - and these systems do not replace the need for plain old safe common sense when conditions are against you.
If you get the chance - try for one of the freedom drives with an off road course next time Volvo have them available.

Cheers
Ian.

[RoyMacDonald]

Originally Posted by ianu

Thanks Roy - that would make sense as i realise there is more than one system in play here.
Rather timely - for those that live across the midlands we had some snow here Sunday night which I unexpectedly found myself having to drive through travelling from Lincolnshire back to Oxfordshire. Around Northampton was a nightmare for visibility and the main roads were struggling to keep just one lane open.
In line with the dealer recommendation - I'd never advocate turning this off even in what may seem the most adverse conditions especially when the road surface is changing constantly underneath your tyres between grip and no grip.
I took the opportunity to push the car on Sunday night in the lanes that were effectively virgin slush and snow and I can't commend highly enough how well it coped when it was obviously struggling for a wheel with grip. The DSTC lamp was permanently flickering in some cases where it wasn't apparent grip woud be a problem. BUT - there was one point where the car slowed dramatically to a point where I don't think it had any grip and the DSTC lamp was solid on - momentum kept us rolling and we got grip back.
This is potentially a point at which you'd turn it off if you got stuck I think - or you were already lining up for a run that you knew would potentially stop you dead and would be the last thing you needed.
Just to add finally that as Roy mentions - the other systems do not switch off and I still think will over-ride if it looks like getting drastic.
I proved this last year in a deserted car park covered in about 2 inches of fresh snow attempting to demonstrate to my son what that button on the dash does.
If you get the opportunity try it - a figure of eight with it switched on in second gear - it'll keep the car checked. Try it with it off and yes it will slip - but as soon as there's any hint it's getting too sideways it intervenes - fascinating.
I'll caveat all this by saying that the laws of physics are still valid - and these systems do not replace the need for plain old safe common sense when conditions are against you.
If you get the chance - try for one of the freedom drives with an off road course next time Volvo have them available.

Cheers
Ian.

Was your test with snow tyres or without Ian? I was stunned at how much better grip the Vredstien Wintrac 4 Extremes gave over that the original Conti's. in slippery conditions. Do you have a manua gearboxl, I notice you mention being in second gear? I have an auto which is less likly to slip according to the Icelanders.

The thing about turning the DSTC off is that you only increases the torque hence giving the rest of the DSTC system that can't be turned off more work to do, so you're more likey to be able to see it at work as the increased torque would cause the wheels to skid and slip more in very slippery conditions.

I wouldn't turn the DSTC off unless the snow was very deep, where you would need more power to push through it.
Roy

[Engineer]

Hi guys I don’t have your AWD system but have driven a few and IMHO it works basically like this.

TC transfers torque to the wheel that needs it most for ultimate traction (no brakes involved), AYC will cut in whatever and operate the brakes (can’t be turned off), but only SC can be turned off (which also operates the brakes).

Which is handy, quote manual “while the vehicle is accelerating under certain circumstances, such as when driving with snow chains, or driving in deep snow or loose sand, it may be advisable to temporarily switch off this function (SC) for maximum tractive force.

Also the SC & AYC will not work if the brakes are overheated hence the DSTC light.

Cheers

[Chaddie]

I thought whats best to do if you want to get up a snowy slope, is to get as far back as possible, turn off the traction control, and gradually accelerate as much as you can without too much wheelspin. then when you get to the hill, slip it into a fairly high gear for your speed and floor it-this will give you the power, and because of the high gear, you wont get much wheel spin.


A sideways question; will Hill Decent Control help going down hill on Snow or Iccy roads or should it not be used in this conditions?

[RoyMacDonald]

Originally Posted by Chaddie

I thought whats best to do if you want to get up a snowy slope, is to get as far back as possible, turn off the traction control, and gradually accelerate as much as you can without too much wheelspin. then when you get to the hill, slip it into a fairly high gear for your speed and floor it-this will give you the power, and because of the high gear, you wont get much wheel spin.


A sideways question; will Hill Decent Control help going down hill on Snow or Iccy roads or should it not be used in this conditions?

The higher gear would reduce torque. Do you really want to have as much speed as possible if you suddenly find there's no grip at all on the hill? At speed with no grip! Power isn't what you want in very slippery conditions. The bit of the DSTC that you can turn off reduces torque (power) and gives the same type of reduction as using a higher gear. Turning it off would make the car more likely to slip. It's if the snow is deep (where you'll have reasonable grip anyway) you want to turn it off.

Hill Decent Control uses the ABS braking and engine breaking to reduce the speed. I haven't heard of it being used in snowy conditions yet. LandRover are using it on the new Freelander. (and the Haldex AWD)

Some off road experts think HDC is too fast for safe controled descent and is more for emergency use if you're losing control on a steep descent. I'd be interested to hear more about how good Volvo's version is.

In very slippery condition the laws of physics are still going to apply though and I'd be looking to fit the most suitable tyres, and chains if necessary, for the conditions rather than rely on Hill Control Descent.

Roy

[ianu]

Originally Posted by RoyMacDonald

Was your test with snow tyres or without Ian? I was stunned at how much better grip the Vredstien Wintrac 4 Extremes gave over that the original Conti's. in slippery conditions. Do you have a manua gearboxl, I notice you mention being in second gear? I have an auto which is less likly to slip according to the Icelanders.

The thing about turning the DSTC off is that you only increases the torque hence giving the rest of the DSTC system that can't be turned off more work to do, so you're more likey to be able to see it at work as the increased torque would cause the wheels to skid and slip more in very slippery conditions.

I wouldn't turn the DSTC off unless the snow was very deep, where you would need more power to push through it.
Roy

Hello Roy,
I actually specified the 17" rims on mine which came with Michelin Synchrone Road/Snow tyres and they were excellent. Mine is also manual and these tyres gave me 42k miles and I changed them at just under 3mm all round.
I had them replaced with Michelin MXV 4 Energy (which was not my first choice but it's a long story) and these are truly awful even though they coped OK last week in the snow. In the wet they skip all over the place and bounce and tramline like crazy - I'd never recommend them and I'll be glad to change them in the next few months.
I'm interested that the Vredesteins are good - I've just put a complete set on my V6 Omega on the strength of the Auto Express tyre test this year and was seriously considering them on the Volvo.
They look good price wise too - the Synchrones are nearly £180 quid each which although brilliant tyres makes me wince a bit.
The XC90's I've been in at the two motorshows at NEC and Excel off road courses and the freedom drive going up the muddy 'mound' were on standard Conti's and didn't miss a beat - but I think ther is more choice in this size available now anyway.

Back to DSTC - I know one thing for certain - when it's on and it does what it's supposed to when it's supposed to - it's bl**dy brilliant. I reckon that if you turn it off when it's sensible to do so - it'll also be bl**dy brilliant - and it'll be great fun experimenting (try not to damage anything else though !!) - take care.

Cheers
Ian.

[Rooster]

I've driven a xc90 off road on one of them driving things and have to say i was very impressed,

I had it going up hills with a full load and stopping half way up and setting off again and it did it easy, it just gripped and gripped

I had the DSTC on altho it seemed just as good with it turned off