[Drivingforfun]

Silly questions I guess but a family member was given a courtesy car which was a 4WD pick up truck thing. I had a go and was playing with the settings...I've never driven a car with 4 wheel drive.

Using what I knew from the sports car world I put it in 4WD mode expecting it to accelerate faster...but it seemed to drive better in 2WD mode. 4WD mode felt like I was driving on mud or ice, like the wheels spinning. Much worse going round corners

My brother who drives off road stuff a lot laughed at me for using 4WD mode on asphalt road and explained, which made perfect sense but I don't get how permanent 4WD cars with no option to switch to 2WD mode work properly on the road

I guess there's a big difference between applying 4 wheel drive for track use, and for offroad use..

[Janet729Shires]

You've stumbled upon a classic 4WD misconception! That "mud or ice" feeling when using 4WD on asphalt in a pickup truck is called drivetrain binding. Most trucks have part-time 4WD, which mechanically locks the front and rear axles together. On dry pavement, where tires can't slip, this causes stress because the front and rear wheels need to rotate at different speeds when turning. Permanent (full-time) 4WD or All-Wheel Drive (AWD) systems found in cars and some SUVs avoid this by using a center differential, which allows the front and rear axles to spin independently, making them perfectly fine for road use. Your sports car's AWD is for performance and traction on roads, while a truck's 4WD is for extreme off-road conditions where slippage is expected.

[amy598henry]

Originally Posted by Drivingforfun

Silly questions I guess but a family member was given a courtesy car which was a 4WD pick up truck thing. I had a go and was playing with the settings...I've never driven a car with 4 wheel drive.

Using what I knew from the sports car world I put it in 4WD mode expecting it to accelerate faster...but it seemed to drive better in 2WD mode. 4WD mode felt like I was driving on mud or ice, like the wheels spinning. Much worse going round corners

My brother who drives off road stuff a lot laughed at me for using 4WD mode on asphalt road and explained, which made perfect sense but I don't get how permanent 4WD cars with no option to switch to 2WD mode work properly on the road

I guess there's a big difference between applying 4 wheel drive for track use, and for offroad use..

Hello,
Totally fair question—and a great learning moment! In part-time 4WD systems, engaging 4WD on dry asphalt causes drivetrain binding since the front and rear axles can’t rotate at different speeds, making handling worse. Permanent 4WD (or AWD) systems solve this with a center differential that allows smooth power distribution and cornering on pavement. So yeah, 4WD isn’t about speed—it’s about traction, and mostly shines off-road or in slippery conditions.


Best Regards,
Amy Henry

[Rolebama]

It is all down to the setup. A sports car will, generally, have tighter suspension and, sensibly, have different tyres and tread pattern compared to an off-roader. The breed of 4WD family cars/SUVs we have remind me of how just about every motorcycle manufacturer made a 'trail' bike at some time. They were also a compromise between road and off-road, and failed at both.
I was a member of a Forum some years ago when it snowed, semi-thawed, and froze leaving road surfaces very slippery. Someone put a photo of a 4WD which had slid off the road into a ditch because the driver, assumingly, thought he could drive 'normally'. Within hours there were hundreds of pictures of a whole variety of them in ditches, wrapped around trees, lamp posts, telegraph poles etc. One thing they all had in common was that they were all running on 'sport' tyres.
Personally, I spent two days winching them onto a recovery vehicle.

[Drivingforfun]

That makes sense, thank you @Rolebama

I'm still confused as to how it handled like a normal car in 2WD mode, but pressing the 4WD button and simply sending power to the extra axle felt like the wheels were spinning uselessly when cornering slowly (i.e. parking)

All I can think, is, it almost felt like traction control on a normal car when you only have one wheel stuck, i.e. wheels seemed to spin... When turning sharply, I assume one wheel wants to travel further... perhaps 4WD mode links the axles and incorrectly distributes the power sent wheels in a similar way?

[Rolebama]

As with most parts of a car, 4WD can be designed to operate in different ways. With an offset of power sent to the axles, and some of the design ideas are questionable, to say the least. Similarly some have mechanical linkage between axles, some have an automatic gearbox type linkage. Seems to me that the one you drove was an 'automatic' type setup. Undoubtedly, to me, the best road cars I have driven, with a solid feel, would be any one of the Jensen range.

[Drivingforfun]

I suppose user error may have come into it as well... bear in mind in the same session I wondered what it was like to drive in low range mode, realised it's just like being in first gear or even lower, tried to turn it off without realising you had to stop, so had to get to a safe spot while (rightfully) feeling an idiot for making so much engine noise

Never driven a Jensen but I drove an old Porsche, I don't know Porsches well but it was a modified racing one that I kept stalling because the clutch had a tiny biting point, but that felt incredibly solid like you say of the Jensen 👍

[olduser]

Originally Posted by Drivingforfun

Silly questions I guess but a family member was given a courtesy car which was a 4WD pick up truck thing. I had a go and was playing with the settings...I've never driven a car with 4 wheel drive.

Using what I knew from the sports car world I put it in 4WD mode expecting it to accelerate faster...but it seemed to drive better in 2WD mode. 4WD mode felt like I was driving on mud or ice, like the wheels spinning. Much worse going round corners

My brother who drives off road stuff a lot laughed at me for using 4WD mode on asphalt road and explained, which made perfect sense but I don't get how permanent 4WD cars with no option to switch to 2WD mode work properly on the road

I guess there's a big difference between applying 4 wheel drive for track use, and for offroad use..

This may answer some of your questions but it is a bit outdated but it saves a lot of typing😏;

https://www.artofmanliness.com/skill...l-drive-works/

The subsequent changes are, these days there can be a limited slip differential in the transfer box enabling the front and rear wheels to revolve at different RPM this reduces the stresses involved in 4WD on a grippy surface and accommodates any difference in circumference of the tyres.

[Rolebama]

I got so used to driving Land Rovers in the 70s, where you can change from low to high and 2WD to 4WD on the move, that when I was asked to drive one of the site lorries I destroyed the transfer box. That was when I learnt that not all 4WDs are the same.

[Rolebama]

@olduser I read the first few paragraphs of that and gave up. Changing from 2 to 4WD does NOT increase power. It will allow for higher rpm at the engine for the same road speed, but his writing gives a false interpretation. I think the man's a fool.
As for a limited slip diff, the point is that if you have no traction on a wheel, instead of just sitting spinning on a non-tractable surface, when the difference in wheel rotation reaches a certain point, the LSD will kick in and effectively lock the diff, meaning you now have traction on the wheel on the tractable surface, giving you drive. It also comes in handy on road cars when cornering fast. When the inner wheel loses traction on a bend, because of weight transference, the LSD will kick in and stop that horrible screeching and giving drive to the outer wheel.

[olduser]

Originally Posted by Rolebama

@olduser I read the first few paragraphs of that and gave up. Changing from 2 to 4WD does NOT increase power. It will allow for higher rpm at the engine for the same road speed, but his writing gives a false interpretation. I think the man's a fool.
As for a limited slip diff, the point is that if you have no traction on a wheel, instead of just sitting spinning on a non-tractable surface, when the difference in wheel rotation reaches a certain point, the LSD will kick in and effectively lock the diff, meaning you now have traction on the wheel on the tractable surface, giving you drive. It also comes in handy on road cars when cornering fast. When the inner wheel loses traction on a bend, because of weight transference, the LSD will kick in and stop that horrible screeching and giving drive to the outer wheel.

I agree with you, power stays the same but that article (I think) was ignoring the difference between power and torque to not confuse the basic's 4 wheel drive stuff.

Just a point, LSD found on normal cars work continually, in effect the LSD is trying to keep both half shafts running at the same speed. Running straight ahead there is very little if any slip in the LSD clutches, the slip in the LSD clutches on the inside wheel increases as the vehicle corners with both wheels on the ground, the torque to the inside wheel will be controlled by that sides LSD clutch pack With one wheel off the ground the friction in the appropriate LSD pack will spin that wheel at the same speed as the opposite grounded wheel.
The transition through theses conditions is smooth and continuous.

Racing cars usually have adjustable LSD's the setting being adjusted by the driver to compensate for different track conditions, too stiff and the car runs out at corners, too soft = sensitive steering and poor traction.

Then at the other extreme there are lockable differentials, when locked both half shafts are fixed at the same speed as though it was a solid axle.

[Rolebama]

@olduser I have to disagree with your post above. Under 'normal' conditions, the diff works normally. The LSD only comes into effect when traction is biased. ie when the wheels or half-shafts are running at a speed difference relevant to each other. Until that time the LSD is just ballast. Working as you describe would mean high tyre wear and the possibility of a locked diff when cornering.

[olduser]

Originally Posted by Rolebama

@olduser I have to disagree with your post above. Under 'normal' conditions, the diff works normally. The LSD only comes into effect when traction is biased. ie when the wheels or half-shafts are running at a speed difference relevant to each other. Until that time the LSD is just ballast. Working as you describe would mean high tyre wear and the possibility of a locked diff when cornering.

My problem is, how do the clutches know it is time to act, the best way I can explain it is each clutch pack can transmit a finite amount of torque beyond that they will slip?

This may help to illustrate;


https://www.youtube.com › watch?v=93TT8ewtAew


Many years ago, I helped a guy who raced a Ford Escort which had a LSD fitted when he bought it.
He was sure the LSD was worn out, when we stripped it, we found the friction plates were not new but not excessively worn, he wanted new so I fitted them but lap times did not improve.
He was convinced the problem was the LSD was not effective enough, and was keen to weld the differential up solid.
We decided we could get an extra plain plate into each pack which would increase the torque before slip set in but now out on the track, he was troubled with understeer.

[Rolebama]

The LSD is disengaged when travelling with an even amount of traction. It only comes into effect when there is a difference in wheel/shaft rotation. All the LSDs I have seen are multi-plate, and quite loose. Putting an extra plate in probably partiaqlly jammed it, so it would have essentially acted as a diff lock, hence the driven wheels would only want to go in a straight line, explaining the sensation of understeer.
As to welding the diff - the mind boggles.

[olduser]

We tested our setup before and after modification - By rotating the Diff input shaft and judging the force to stop one wheel, (axel off the ground) with the extra plate in it was harder to stop one wheel. I think the clutch packs are spring loaded but I cannot clearly remember how, may be a Belleville spring first or last in each clutch stack, putting an extra plate in just loaded the spring more, increasing the slip torque. Most explanations of how LSD works suggest a spring between the diff gears pushing them towards the half shafts but I can't recall ever seeing one.

As to welding up the diff, his argument was it's only like a go-cart but in the end it would be the same as a locking diff, sometimes helps on soft ground but wrecks the transmission on made up roads.

I remember at least one competitor, in the Mini class had a locked diff (I don't remember how) but seeing as a Mini, when racing*, did not use much steering his car did not stand out. He did have to trailer the car to events, some drove their cars to events.

*High powered Minis tended to charge into corners, slightly steer into corner, lift off and or brake, tail would drift out, when front was pointing at next straight, off the brakes and power full on to the next corner, long bends became a few short straights. This lead to interest in changing the brake balance front to rear or just run with the rear tyres overinflated.

[Rolebama]

The easiest one I ever worked on had very light springs to keep the plates separated, then they had a ramp and roller affair to engage it. So when a wheel 'broke free', the virtually immediate change of speed in the slipping wheel would throw the rollers up the ramps, under centrifugal force, to compress the clutch plates . As traction was gained. the wheel would slow, centrifugal force would decrease, and the diff would return to normal. This is a bit of an over-simplification, but that was the nitty-gritty.
(Very similar to the design of a lot of Japanese 'twist and go' mopeds, and small engined motorcycles.)

[olduser]

Yes, I see with that construction the LSD would 'cut in' Rolebama but I am not sure it would feel nice to drive though?

[Rolebama]

Originally Posted by olduser

Yes, I see with that construction the LSD would 'cut in' Rolebama but I am not sure it would feel nice to drive though?

You wouldn't notice it. The ramp and rollers take the 'shock' out.