AWD Tyre Wear Misconception?

[Hol]

Gress said:

kambites said:

I think this myth has roots in the fact that using the engine to provide some of the braking force keeps the brake temperatures lower and hence makes brake fade less likely; and means that if the brakes fail, the car will continue to slow, at least to an extent.

There may also have been more truth to it when most cars were RWD and had drums on the back? Drums brakes are pretty good these days, but there was a time when they were pretty awful.

Edited by kambites on Friday 12th July 07:36

He's quite old, so back when he was young he was probably right, when everything was small drum brakes, being able to use the engine to break kept temps lower so helped stopping distances on longer drives, or just outright made cars of that era stop faster since cars back then sometimes had undersized breaks.

Most people who come out with opinions like this have just made an assumption and simply stick to it, because they have never actually investigated the matter any further to confirm they were right.

In your father’s case, we probably notices that his rear tyres wear quicker than the fronts, because that’s normal on a RWD car where the rear tyres are transmitting power and assume that an AWD car just doubles the scenario.

The reality is that an asymmetrical AWD drivetrain in the same body shell is handling a lot less than the original weight on each driveshaft and there is a lot less chance of one tyre losing traction and prematurely wearing.


But, his mind was probably made up years ago.

[Panamax]

As regards brake efficiency and discs/drums it's worth remembering cars had drums both front and rear back in the day. Typically "twin leading shoe" at the front and "single leading shoe" at the back, all operating on a single hydraulic circuit. This set-up gave more braking at the front than the rear as has continued through disc/drum and disc/disc systems.

Back in the day engine braking would be useful in two situations,
To add more braking than the brakes could apply on their own, or
Since all/most cars were RWD it enhanced stability through retardation at the rear wheels with no risk of a front lock-up.

The old expression "standing on the brakes" was literally that. With drum brakes and no vacuum servo it will often have been impossible for a driver to brake hard enough to lock the wheels or even to achieve particularly effective retardation. So literally the driver's whole weight on the brake pedal. Drum brakes have some "self servo" effect at the leading shoe but not as powerful as a vacuum boosting.

[LeoSayer]

Panamax said:

LeoSayer said:

True, but in my experience the engine braking you get from permanent 4WD does help to slow a car down much better in icy conditions than 2WD.

Unless the differentials get involved in some way the effect of engine braking in a 4WD vehicle will be to apply the same amount of retardation to each wheel. i.e. the same rate of rpm reduction.

When you press the brake pedal the amount of retardation at each wheel depends on the performance of its calliper/pad/disc. i.e. although the fluid pressure will be the same each side of the car the actual brake may perform differently. There may also be pressure differences front/rear or diagonally in a dual circuit system. This may make little difference under grippy conditions but can become significant on ice/snow, causing an individual wheel to spin or lock lock up.

All of the above will change if an ABS system chooses to intervene.

My experience was in an 2004 Audi A6 with ABS and permanent 4WD on summer tyres in a snowy slope.

Going down the hill in neutral the car would hardly slow on the brakes because the ABS would intervene even with minimal pedal pressure.

The same slope taken in a low gear with no braking would slow noticeably better.

If ABS didn't intervene, would it have made a difference? I don't know but my assumption is that 4WD worked better than brakes in the specific situation because the engine braking was distributed equally between all 4 wheels unlike brakes which would work individual wheels and be front biased.

Anyway, this is drifting slightly off topic.

[GreenV8S]

LeoSayer said:

Going down the hill in neutral the car would hardly slow on the brakes because the ABS would intervene even with minimal pedal pressure.

The same slope taken in a low gear with no braking would slow noticeably better.

My Landrover offroad training emphasized being in 4wd in the right gear when going downhill in slippery conditions and the most import part which was repeated before every descent was: stay OFF the brakes.

I imagine this would be particularly important for vehicles without ABS, but the training vehicles did have ABS.

[Smint]

The full time 4WD system on Subarus tends to wear all 4 tyres at a similar rate, certainly the summer sets on our previous Outback and the current Forester haven't worn especially well, though to counter that the Winter set have seen some 7 or 8 winter's use (on both cars) and still have around 6mm tread left.

In any case tyre replacements on lots of AWD cars can be expensive, its best to have all 4 tyres of similar tread depth, hence if you wreck one of a half worn set you then have the quandry of do i replace all 4 or are the warnings about certain AWD system being made of cheese valid and will saving money on tyres by replacing one only cost me a prematurely worn transmission?

[ATG]

My first car was a 4wd Celica and it ate tyres. Because I was young and drove like an idiot chasing my house mate in his MR2 to and from the place we worked. Happy days!

[jimmyjimjim]

Gress said:

My father is the same. Shouts at me every time I drive him anywhere since I don't use the engine brake, because he thinks i'll won't stop as fast. I tried explaining to him that if your brakes can lock up the tyres then it doesn't matter where the braking force comes from but he just doesn't understand it

I read the best response to this a while back, it was along the lines of "we have disk brakes now, and they can lock the wheels completely, unlike the drums in your old mg where engine breaking and prayer was mandatory to stop in the same county."

[hellorent]

It used to happen years ago in early 4wd's, google "4wd wind up" it could cause tyre wear

[E-bmw]

I think that basically there is no hard & fast precise answer to your question.

Is it possible in modern cars? - Yes it is.

Is it possible to drive in such a manner to make the wear less? - Absolutely it is.

Are there modern cars where it is not the case? - Yes there are.

There are 4wd cars & 4wd cars, there as many types of 4wd as there are car manufacturers & each one will have a different effect on tyre wear.

So is it possible to answer your question accurately? - No, you would need to tell us exactly which models you want to compare to even stand a chance of an accurate answer & even then driving style/journey profile could dramatically change the outcome.

[stevieturbo]

E-bmw said:

I think that basically there is no hard & fast precise answer to your question.

Is it possible in modern cars? - Yes it is.

It is easy to answer, and it is not possible.

Gress said:

He swore up and down that AWD cars wear through tyres basically twice as fast.

Because this is his comment, twice as fast. Which in no way is a reality, ever, anywhere.

Might they wear a little bit more ? possibly. But twice as fast ? Never Not even remotely close to twice, not even 1.5x, not even 1.25x.

Perhaps 5% ? 10% tops ? If even.

[E-bmw]

stevieturbo said:

E-bmw said:

I think that basically there is no hard & fast precise answer to your question.

Is it possible in modern cars? - Yes it is.

It is easy to answer, and it is not possible.

Given a level playing field, as in same car, same driving, same roads etc I agree with you, however that isn't quite what I was eluding to.

Consider this, which is what I meant.

Is it possible to wear tyres twice as fast by driving differently?

Yes it is possible.

Is it likely that, that is what you would do? No it isn't, hence my answer.

[Sheepshanks]

Gress said:

...as I took his word and thought that I would eat through tyres if I were to track the car :')

I would think it's highly likely that any car would eat through tyres if you were to track it.

[FMOB]

Well which ever axle wears its tyres out first, tyres just don't last enough..

[E-bmw]

FMOB said:

Well which ever axle wears its tyres out first, tyres just don't last enough..

?

[Smint]

E-bmw said:

FMOB said:

Well which ever axle wears its tyres out first, tyres just don't last enough..

?

It helps if you refuse to buy tyres that come with less than 7mm new tread, a trend in tyre manufacture i hadn't twigged was as common as it is untill i bought my last set, 7mm, won't be buying Falken again for that reason but they're not alone, ie i understand some Contis come with just over 6mm and Mich not far off, just no, the only makers i know offering over 8mm new tread on car tyres are Goodyear with the Efficient Grip, Uniroyals and some Vredesteins and until the other makers start offering some value for money again those makes it will be.

Not easy to find new tyre treads depth specs, Camskill note the depth on most, TyreReviews on the tyres physically tested, other than those sources don't know where else to find the info.