Is 4wd more stable than 2wd?

[otherclive]

Given the number of cars with botha 2wd variant and a 4wd variant (audi, skoda, Kia, VW, Nissan, Volvo) have there been any comparative tests to see if the 4wd variant has better stabilty compared to the 2wd models when towing at 85% or higher?

Other Clive

 

[Lutz]

If you are talking about the same models with the only difference being one with and the other without 4WD, then there is no difference in stability. The marginal increase in weight for 4WD is not going to be noticeable. The only advantage of the 4WD will be its better traction.

 

[JTQ]

I would agree.

My experience re towing stability points more to how stiff in the lateral plane is the vehicles rear end.

Thus I would check if the cheaper 2 wheel drive vehicle also features "higher aspect ratio" tyres on smaller diameter rims. The walls of these tend to be more compliant than large wheels with low aspect ratio tyres and this compliance degrades the towing stability.

A generalisation I know but in my experience a pointer nevertheless.

 

[Lutz]

Agreed, JTQ. Very soft off-road tyres on small diameter wheels, as found on many 4x4's, are not conducive to stability, but 4WD versions of saloon cars or estates tend to be fitted with low aspect ratio tyres and therefore do not normally suffer from this disadvantage.

 

[Garfield]

There was a series of posts some time ago concerning the volvo V70 awd vs the XC70 both 4wd estates based on the same car but the XC70 has greater ground clearance? to give better offroad capability. The general consensus seemed to be that the purely road going V70 awd was the more stable of the two cars.

Back to the original question I would say that if you give a car 4wd and change nothing else then the benefit of engine braking applied to all four wheels might make a tiny difference to stability and give a big improvement to traction but at the expense of fuel economy. If you then raise the suspension, fit softer dampers and fit higher profile tyres to give the car more offroad ability then stability will suffer.

 

[RogerL]

4wd gives better traction - and better stability when cornering as sideslip from drive forces are spread over both axles - but they're even for straight line stability (when most snakes occur).

V70 will be more stable than XC70 because of it's lower centre of gravity.

4x4s are stable towing because of their weight, not because of 4wd or high ground clearance.

 

[Frank_374237154]

Don't think there is a simple answer.Most 4x4 these days are part time ie driven at one end and only engaging the other when traction is lost.Can't see 4 wheel drive making much difference in this case.

Some cars are permanent 4 wheel drive ie drive to all four wheels is always engaged.My 2003 Audi A4 falls into this category and I think it does make a difference in this case.If the outfit pitches and the front goes light,the rear is still pushing which quickly damps out the movement.Any momentary loss of traction on any corner or side quickly has the excess power directed to whichever wheel(s) still have traction at both ends.

Ultimately I cannot be sure whether the excellent stability I enjoy is down to the 4 wheel drive or to the sports suspension,which is stiffer,lower and has wider tyres (235/45 17's)than standard.

I was always led to believe that rear wheel drive was best for towing as the noseweight pressing onto the rear wheels maintained traction.A proper four wheel drive has this advantage while still pulling from the front.In my case tyre wear was equal all round,until I started towing,since when rear tyre wear has been greater than front tyre wear,which perhaps shows just how much work is being done at the rear controlling the caravan.

Incidently it is amazing how many "4 wheel drive" vehicles are not permanently engaged,and this includes a number of proper offroaders.This especially applies to soft roaders.My father's 2003 Toyota Rav4 is permanent,but its replacement is part time,which has proved a deal breaker at replacement time for my father.

 

[RogerL]

Front or rear wheel drive - "best" depends on the conditions - rear wheel drive gives more traction at irrelevant traffic light grand prix and important hill starts - front wheel drive give more stability as it tends to straighten outfits.

Part-time 4wd, in all their forms, are the best of all worlds - front drive stability under normal conditions, with power/torque transferred to rear wheels to maintain traction.

For stability alone, a 2wd car with lowered sports suspension is better than a 4wd car with raised centre of gravity and soft suspension IF, but only if, you're comparing cars of the same weight.

 

[spj]

Hi all,

One difference between some like for like comparisons are the Audi A4, the 2wd has a simple beam type axle rear suspension set up and the Quattro has a multi link set up, the Passat will be similar as will the A3 and A6. The main contributor to stability in these cases will be the extra kerbweight, although in extreme conditions (very extreme) the 4wd fully independant suspension will give better grip on an uneven surface.

I looked at the Audi Allroad for towing but its kerbweight was a bit light, I have no doubts to its ability as I previously towed with an A4 2.5 tdi Quattro which was a fantastic tow car but with a bigger van and a dog now we needed a heavy estate shaped car, then with a heavy nose weight the list of cars gets shorter.

spj.

 

[Lutz]

The extra kerbweight of a 4WD version compared to its 2WD counterpart is going to make negligible, if any, difference to stability. I would challenge anyone to be able to distinguish any improvement based on kerbweight alone. If there is going to be a difference, then one can only put it down to the suspension setup, although even there, one cannot say whether, in principle, one is better than the other, and if so, which of the two.

 

[Garfield]

Hi Roger,

Why "only when comparing cars of the same weight"?

My experience is that a light car with sports suspension gives better stability than much heavier cars with offroad suspension.

My sportsboat at 1550kg was towed by a real variety of vehicles.

A 1989 Golf GTI (970kg!) was used to collect it from the dealer 70 miles away, Despite the weight ratio somewhere near 160% the outfit handled ok, I kept to 55mph but occasionally hit 60 with no tendency for snaking.

An RWD BMW 530 quickly replaced the golf but at 1499kg I was still over 100% but at least under manufacturers max tow limit (1600)

A FWD VW Sharan and a Ducato van gave better stability than the BMW due to a shorter rear overhang and having suspension designed to carry weight.

A freinds 1992 Discovery started snaking as soon as it exceeded 55mph.

Obviously, regularly hauling a heavy boat up a slipway was a severe test of traction which the Disco won hands down of course

Next was the rear wheel drive BMW which was well ahead of the FWD Sharan despite being 250kg lighter, the Ducato was no heavier than the Sharan but it's lower 1st gear gave it the edge.

 

[RogerL]

Garfield - "Why "only when comparing cars of the same weight"?

My experience is that a light car with sports suspension gives better stability than much heavier cars with offroad suspension"

Up to a point you are correct but eventually towcar weight tells. No-one could suggest that a Lotus Elise would give the same towing stability as a Toyota Amazon!

 

[Rodone]

I tow with either a Honda Accord 2.2 diesel estate or a current Honda CR-V 2.2 diesel. Both have the same engine- though the CR-V uses a 6 speed gearbox. Both are excellent tow cars, but I think that apart from a slight weight advantage and, in my view, being the more comfortable to drive, the CR-V has two distinct other advantages. The nose weight allowance is 100kg's to the Accord's 75, and the occasional 4 wheel drive overcomes the front wheel scrabble that can occur with the Accord, not helped by the rather long body overhang. On a straight flat road, keeping within the legal speed limits, there is little to choose between the two when towing, but, bearing in mind my comments above, I would choose the CR-V over the similiar powered Accord every time.

 

[Garfield]

Your'e quite right Roger I was a bit sweeping with that statement! and of course a heavy car with good suspension is by far the best.

If it were not for the fuel economy penalty and purchase price, none of us would choose 2wd over 4wd for towing but when you compare for example the peugeot 4007 2.2hdi 4x4 to the BMW 530d touring, I would expect both to have similar cabin space and weight but the 235hp BMW is actually more economical than the peugeot according to the manufacturers websites and if you go for the 520d which still has more power than the Pug it uses about 27% less fuel, which, for the 20k miles we do each year amounts to

 

[RogerL]

Your'e quite right Roger I was a bit sweeping with that statement! and of course a heavy car with good suspension is by far the best.

If it were not for the fuel economy penalty and purchase price, none of us would choose 2wd over 4wd for towing but when you compare for example the peugeot 4007 2.2hdi 4x4 to the BMW 530d touring, I would expect both to have similar cabin space and weight but the 235hp BMW is actually more economical than the peugeot according to the manufacturers websites and if you go for the 520d which still has more power than the Pug it uses about 27% less fuel, which, for the 20k miles we do each year amounts to

 

[David_from_Ashby]

Time for a few more pennies worth...

If you take three otherwise identical cars (same weight, inertia, wheels, tyres etc) except one is fwd, one rwd and the other 4wd. It is fair to say that under poor traction conditions, the 4wd (followed by rwd) will be better. However, when considering the "snake", the fwd will be marginally better and the rwd the worst. This is because of the distribution of lateral grip on the towcar. When a tyre is generating longidtudinal grip (traction/braking) its ability to generate lateral (cornering) grip is reduced. For good stability the front axle should have less lateral grip than the rear which is the case for a front wheel drive car. With a 4wd car, the loss of lateral grip under traction conditions will be roughly equal, and therefore less stable than fwd. Obviously a rwd car will offer the worst case for stability.

In reality, the above example is rarely applicable, because the weight distribution between fwd, rwd and 4wd is different, tyres can be different betwen axles and variants and suspensions will invariable be very different between the three types. It is therefore very difficult to predict (accurately) the relative stability and "snake resistance" offered by different types of car, 4x4 and MPV without conducting either road trials or advanced simulation.

However it is possible to make a few generalisations:

1 cars with low profile wide tyres will be more stable than the same car with high profile narrow tyres (on dry roads at least)

2 Heavy cars with a high yaw inertia will be more stable than light cars or cars with low yaw inertia

3 Cars with short distance from the rear axle to the towball will be more stable than cars with a long distance

4 4wd and rwd will have better traction on low grip surfaces and up hills

5 Fwd will be more stable in a straight line at high speeds

6 All car/caravan combinations will have a critical speed, above which instability is a certainty

7 Skillful driving (reading the road conditions ahead, being aware of what ALL the vehicles around and ahead of you are doing, etc) can always prevent a car/caravan combination from becoming unstable by avoiding destabilising events!

There, that is sure to spark some debate.....

David

BTW I can back up all of the above statements with maths, and may well publish some of it in the future if I get the chance to do so.

 

[RogerL]

Time for a few more pennies worth...

If you take three otherwise identical cars (same weight, inertia, wheels, tyres etc) except one is fwd, one rwd and the other 4wd. It is fair to say that under poor traction conditions, the 4wd (followed by rwd) will be better. However, when considering the "snake", the fwd will be marginally better and the rwd the worst. This is because of the distribution of lateral grip on the towcar. When a tyre is generating longidtudinal grip (traction/braking) its ability to generate lateral (cornering) grip is reduced. For good stability the front axle should have less lateral grip than the rear which is the case for a front wheel drive car. With a 4wd car, the loss of lateral grip under traction conditions will be roughly equal, and therefore less stable than fwd. Obviously a rwd car will offer the worst case for stability.

In reality, the above example is rarely applicable, because the weight distribution between fwd, rwd and 4wd is different, tyres can be different betwen axles and variants and suspensions will invariable be very different between the three types. It is therefore very difficult to predict (accurately) the relative stability and "snake resistance" offered by different types of car, 4x4 and MPV without conducting either road trials or advanced simulation.

However it is possible to make a few generalisations:

1 cars with low profile wide tyres will be more stable than the same car with high profile narrow tyres (on dry roads at least)

2 Heavy cars with a high yaw inertia will be more stable than light cars or cars with low yaw inertia

3 Cars with short distance from the rear axle to the towball will be more stable than cars with a long distance

4 4wd and rwd will have better traction on low grip surfaces and up hills

5 Fwd will be more stable in a straight line at high speeds

6 All car/caravan combinations will have a critical speed, above which instability is a certainty

7 Skillful driving (reading the road conditions ahead, being aware of what ALL the vehicles around and ahead of you are doing, etc) can always prevent a car/caravan combination from becoming unstable by avoiding destabilising events!

There, that is sure to spark some debate.....

David

BTW I can back up all of the above statements with maths, and may well publish some of it in the future if I get the chance to do so.

Experienced caravanners with mechanical knowledge and numeracy have known that 1-7 are true for a long time - perhaps we should have published!