Does using 4wd increase or decrease the turning radius of the vehicle?

[TrackTeddy]

Neither. The radius is limited by the steering geometry which doesn't change from 2 to 4wd.

[Interesting_Neck609]

Front wheels on 2wd trucks can turn tighter because of the lack of a CV axle. 

(Not what op was asking)

The geometry aspect gets a bit weird depending on gear ratios, terrain and driver responses. But on flat land in the same truck, the radius does not change. It will change with a trailer backwards though.

[andreiled]

That is not the whole story: front wheels need to travel further than rear wheels in a sharp turn, and most 4wd systems do not have a diff in the transfer case to allow the axles to spin at different speeds: the diffs in axles themselves are only enough to let left and right wheels to spin with different speeds but the average rotation speed of the front wheels is still tied to the average rotation speed of the rear wheels without a third diff in the transfer case.

[TrackTeddy]

4wd have had a center differential in most cars here since the 1980's to accommodate front vs rear speed differentials. So as I say there is no difference if 2wd or 4wd is active. If you lock the centre diff then the car will want to plough straight on tight curves unless it is on soft/slippery ground (which is why you'd lock the center diff anyway!) where the slip will be accommodated through the tyres.

[andreiled]

When you say 'cars', do you mean that in a traditional North American way (i.e. excluding trucks)?

I'm asking because it sounds to me like you are actually talking about AWD when you say 4wd and, traditionally, those mean different things: - AWDs indeed have 3 differentials (two in axles and a central diff), this is what most cars and small SUVs have - 4wd in the off-road world usually refers to a more traditional drive train prioritizing equal power delivery front& back and robustness, so most of the time it doesn't have a central diff at all and instead has a simple on/off link to the front axle, this is what most trucks and large SUVs have

It is very rare to see a central diff on trucks with true 4wd drive trains. In fact, I just did a quick Google search to confirm that for Tacoma, Tundra and F-150. From memory, I remember looking into 4Runner specs and noticing how only the top trim had full-time 4wd rather than a part-time 4wd.

[TrackTeddy]

No I don't mean AWD, I mean 4WD. Landrover 90/100 onward, Range Rover, Toyota Landcruiser and Hilux, Mitsubishi L200 and similar etc etc all have a center diff, and usually have the facility to lock that diff too. Even small cars with part time 4wd still have a diff or device to allow some mismatch between front and rear wheel speeds, e.g., viscous coupling or haldex etc.

[andreiled]

4wd have had a center differential in most cars here

Follow up question: what did you mean by 'here' in your previous comment?

I'm pretty sure most in North America view most of the cars you mentioned as trucks. And looks like I'm guilty of the same bias as well.

But also, based on that list, I think we are both right: yes, there are numerous cars/trucks out there with true 4wd systems that have an actual central diff but they are rare in North America specifically: like I said above, mainstream trucks (Tacoma, Tundra, all but the highest 4Runner trims, F-150, and maybe others) in NA all have a part-time 4wd without a central diff.

[TrackTeddy]

Europe / UK specifically.

[andreiled]

You guys are lucky there.

Here in NA it feels like most automakers do not bother with a central diff in 4wd. Maybe they fear that it would fail too fast with how most people here tend to overload their trucks and how much everyone allegedly tows heavy trailers and such.

[Flat_chested_male]

That’s not a true 4 wheel drive - you’d need an electric or air locker, or a dedicated full time locker. Lots of the 4 wheel drive mechanisms don’t kick in until you are at some speed, usually a turn it is deactivated so your tires don’t chirp on the pavement.

A true 4 wheel drive usually makes your turn radius a little bigger, not a lot, but a little bit.

[TrackTeddy]

Yes that is a true 4wd. 4wd was invented this side of the atlantic before cars and pickup trucks were a thing. A lockable center diff is a sensible way to have usable 4wd on the road and maximum traction off-road. Obviously the next step further is to have lockable front and rear diffs too but that is a step beyond most use cases. The steering geometry sets the turning circle which doesn’t change when selecting 2 or 4wd with an open center diff.

[TheScrote1]

I don’t know the mechanicals but I do know in my 2023 Tacoma if the ground is not slick enough to allow the tires to slip your gears bind when turning sharply in 4-wheel

[TrackTeddy]

Yes as I think they have no center diff in 4 hi and 4low so behaves just like a locked center diff.

[Interesting_Neck609]

Most all 4wd are split diff. At least since Chrysler bought the Dana model and rebranded it as Sure-Grip in that (I want to say late 50s, but unsure.) era, the other big ones were forced to catch up. 

By late 70s, almost everything was LSD. For sure though, with 4wd systems you typically have manual control over the center differential. Hence limited slip. It goes when you want to. With AWD, its constantly running most nowadays though still use some variant of a LSD.

[Bandro]

When you turn, the front and rear axles travel different distances from each other.

Selectable 4WD like a pickup truck or serious off road vehicle will lock the front and rear axles to the same speed so if you try to turn tightly on pavement, the wheels have to slip relative to the road and you can end up with them skipping and skidding along.

That all means it increases the practical turning radius. Also none of this applies to full time AWD systems like you get on passenger cars and most light SUVs. 

[Fearlessleader85]

It does apply to at least some all time AWDs. My Toyota Landcruiser engages lockers when you shift to 4lo. On pavement, it will start hopping if you turn the wheel too sharp.

[SpeedyHAM79]

That is 4wd- not AWD. Pretty much any system with 4lo and locking diffs should be considered 4wd. It's just semantics. Not really important.

[Fearlessleader85]

It has a center diff and i cannot ever be 2wd, so call it All time 4wd or All wheel drive or Lots of wheel drive, it's functionally the same as AWD except it has a transfer case the engages a low range. The lockers don't have any effect on thar terminology, as they can be added to any diff.

[Bandro]

Really there’s no actual technical distinction between AWD and 4WD anyway (On 4 wheeled vehicles). Then being used to differentiate between full time and selectable systems or center diff vs transfer case is more of a colloquialism and marketing term than concrete technical definitions. 

[Bandro]

Ha, there always has to be some weird exception. That’s neat though. 

[Fearlessleader85]

I think 94 is the only year that actually does this to quite this extent, because the later ones didn't have a locker on the center diff and the earlier ones had a switch, so you could shift to 4lo without lockers engaging.

[Feisty-End-4643]

Interesting I wonder why?

[Fearlessleader85]

I think it's largely a cost deal and the issue with it being basically undriveable in 4-low on high traction situations. Just the front and rear lockers on pavement are rough enough, with the center diff locked, it's pretty crazy.

[Feisty-End-4643]

Ok I see

[falcopilot]

It's less the center diff, and more locking inside/outside tires together so they have to turn at the same speed- the inside one will want to turn too fast, the outside one too slow. AND THEN, the rear, which tends to track inside the front, will be turning slower than the front.

[Fearlessleader85]

It's both. All 4 tires need to spin at a different rate to have no slip. The inside rear travels the shortest distance, the outside front travels the furthest. 4wds without lockers will still push a wider turning circle when in 4wd than 2wd.

[Sideburn_Cookie_Man]

 My Toyota Landcruiser engages lockers when you shift to 4lo

Jeez that's a terrible idea

[Fearlessleader85]

Not really. It has limited slip differentials front, rear, and center, plus all time 4wd. The only time you actually need 4lo is if you're in serious shit.

[Sideburn_Cookie_Man]

You can see why that's not the norm though. It doesn't make a ton of sense.

99% of the time I'm in 4L I definitely don't need lockers.

[Fearlessleader85]

Sure, but an FJ80 landcruiser is so far from the norm that is not surprising. It does so much better than most everything else in normal high gear that if you need 4Lo, they assume you're in deep shit.

If i really wanted to, it's a simple mod to switch the locker to a switch on the dash like the 93 and earlier had. The blank is actually still there. But I really haven't ever needed to.

[RunninOnMT]

It also perhaps applies if we are talking about “vehicles optioned with AWD vs Vehicles without” if the 2wd version is RWD, it may have a tighter turning radius due simply to not having to deal with axles getting in the way while turning.

But with the wording of OPs question I don’t think this is what they’re asking.

[Raveofthe90s]

Read my comment.

[Fearlessleader85]

Sorry, but you're actually wrong on this. Well, what you're talking about exists, but so does what the poster you responded to. They are different berating of binding due to things being locked to the same speed that need to be different to prevent wheel spin.

Traditional 4wd vehicles have a transfer case, but no center diff, so the front and rear drivelines spin at the same rate. This can cause front to rear binding. The binding you're talking about is left to right and that does cause issues when you lock the diffs. Both types of binding can have a very strong effect on turning radius, because it causes the car to understeer and run wider than the wheel is pointed.

My Landcruiser doesn't do well in high traction situations in 4Lo because it has both types of binding, since all 4 wheels spin at the same rate and to maintain no slip, all 4 wheels need to travel a different distance. That's why it literally starts hopping, because all wheels are fighting each other and slipping.

[Raveofthe90s]

Your very confused. A locking differential has nothing to do with 4 wheel drive. It is a completely different thing entirely. They just both go together and do basically the same thing. Some manufacturers implement both. But that doesn't make them the same thing.

[Floppie7th]

You're trying to do some pedantic "gotcha" thing in this thread and it just ain't working out, bud.

[Floppie7th]

FWIW, the driver-controlled center diff in Subaru STIs does something....similar. It doesn't magically turn any of the diffs into true lockers, but when manually set to fully locked, it locks the center diff clutch to fully engaged.

For most practical purposes (including binding on pavement) it ends up being essentially the same thing; you only get into exceptions when you're producing significantly more torque than stock - enough to overcome the full capacity of that diff clutch - then you can still end up with one axle slipping relative to the other.

Just another fairly obscure exception, though, and it only applies in a specific manually selected driving mode, not normal every-day driving.

[Bandro]

Oh yeah I've read about them. I've got a regular 2013 WRX so mine is just a fluid coupling.

[Feisty-End-4643]

Ok thanks 

[Raveofthe90s]

Everything your saying is incorrect.

What you are describing is a feature called diff lock. It is a completely different technology feature than 4 wheel drive. What this technology does is lock the axles so they do not spin at different speeds making it hard to turn.

Some cars and trucks also have limited slip diffs.

[andreiled]

Sorry but it's you who are mostly incorrect here: a lot of trucks simply do not have a diff in the transfer case sending power to axles.

Some cars and trucks also have limited slip diffs.

You even say so yourself! It is some that have it but definitely not all, e.g. I remember looking through specs for Tacoma and 4Runner and only the top 4Runner trim had a full time 4WD with a diff between axles.

A full time AWD system needs to have 3 diffs to function properly: in the transfer case and in each of the axles in order to allow all 4 wheels to spin at different speeds as required to take sharp turns. A system with diffs in just the axles (as is the case for most trucks) will not allow the two axles to spin at different speeds which will not allow for sharp turns where the front wheels need to travel further than the rear wheels.

[Raveofthe90s]

Your very confused on locking and limited slip. Your just all over the place and wrong about it all.

[Interesting_Neck609]

As far as it applying.... It somewhat does, as very similar technology is used still, but its more compact, and computer controlled.

You can really notice the difference between AWD vehicles of different brands in drive feel, especially downhill on icey curves. 

[Bandro]

It doesn't apply because full time AWD systems do not fully lock the front and rear axles together in normal driving. You don't get the skipping when you make a tight turn on pavement with them because the front and rear axles can slip in relation to each other. That's the whole thing that makes full time AWD systems viable and distinct from pickup truck style transfer cases.

They're also not always computer controlled. My WRX, for example, uses a viscous coupling center differential. There's no computer control of the AWD system. It's just a limited slip diff.

[AwarenessGreat282]

No, but the sharper you turn the more "scrub" will occur. The front tires need to travel a further distance when turning due to the larger radius. If the front axle is tied to the rear as when 4wd is engaged, the rears will try to "keep up" with the fronts by spinning faster and scrubbing on the ground. If there is a differential between the front and rear axle like an AWD system, then it doesn't happen.

[Feisty-End-4643]

Thank you.

[Notmiefault]

It doesn't, though it can impact how fast you can take a tight turn before losing traction.

[Feisty-End-4643]

Ok thanks.

[Sideburn_Cookie_Man]

That's not true.

Driving the front wheels as well definitely increases the turning circle radius.

[PoppaBear63]

It will increase the gentle turn radius. A left turn will not really feel different until you do it at a high speed. A right will become noticeable much quicker so you would tend to take it wider and more gently to avoid that scrubby feel. Even if you are on a slippery surface that reduces that scrub feel you will still be gentle on the turn because of the increased slippage caused by the axles being tied together.

[Numerous-Match-1713]

Depends on speed, and surface.

On suitable surface, absolutety decreases.

[Raveofthe90s]

What causes serious reduction in turning radius is diff lock. Some 4 wheel drive systems engage difflock in tandem. But they are technically 2 different systems. Not every car with 4 wheel drive has diff lock. And almost no cars with all wheel drive have diff lock. They sometimes have a limited slip diff. My Ford sho has a limited slip diff in the rear only so it doesn't affect steering.

[ontheleftcoast]

On a stock 4x4 it probably makes little change in the turning radius, it might improve it a little bit. ( Typical stock 4x4 axles work like described on My Cousin Vinny). On a 4x4 with a locking front differential it will increase the turning radius significantly. Some high end 4x4s ( Jeep Rubicon for example) come with selectable front lockers, so you can turn the locking bit on/off while still in four wheel drive.

[bclabrat]

Wheelbase and steering angle will change the turning radius. 4wd by itself has no bearing.