Cost of repair Audi BMW Saab...(crossposting)

I replace the front and rear tires on my Saab 9-5 at the same time. You have to apply some form of wear management so the front and rear tires wear close to the same rate.

Imad Al-Ghouleh schrieb:

Because it is. Especially compared to a FWD saab or a quattro Audi. :-)

BTDT.

The rearwheel drive is fun and with all the electronic gimmicks it will really do it's job. However at a certain climb angle or even slipperyness of the road, the rearwheel drive gives in, then the FWD and then the quattro.

Fair enough. In my experience, handling is one thing. In dry conditions I couldnt argue which is better on the edge of friction as I haven't driven all drive concepts in this condition. Maybe I should make it clear that IMO the risk of getting stuck with a rearwheel drive is higher than with a FWD or quattro.

Ok, so I live in Austria, we also have quite a bit of snow and it's always the RWD vehicles that get stuck first.

You are comparing apples and oranges. Using the same high quality winter tires FWD's do get you further and quattro even more so.

Regards

Wolfgang

fbloogyudsr schrieb:

Good advice. Quattro does not negate the laws of physics. Friction still counts :-)

Regards

Wolfgang

If it still lags after chipping and related improvements, you bought the wrong chip. If you think a "chip" by itself is the way to increase performance in a motor, you need some engineering advice.

Really ? Better engineered ? Do we need to have a "peak HP" pissing contest ?

You evidently don't understand how a turbo charger works if you think lag should "disappear". In addition, I think you are confusing "significant horsepower under boost" with "lag". I had a non-turbo Nissan that cranked out wonderful HP above 3500 RPM - That doesn't mean that it had "lag" below that. The fact that an engine is not generating peak HP (yet) does not mean that the turbo is "lagging".

Again, you don't even know what lag is. Let me know how many engines (turbo charged, supercharged, or normally aspirated) you find that generate peak HP off the line.

I switch between winter tires and summer tires when appropriate, and what was on the front last year, goes to the back this year. They're all directional, so that's as much tire rotation as is possible, but you're right, it makes the sets ready for replacement all at once.

All cars wear tires, and unless something is drastically wrong, it's not going to differentiate between manufacturers.

Dave Hinz

Sorry, no. This is contrary to the laws of physics. If you assume equal axle weights, as the car climbs it places more weight over the rear axle and less over the front. So a rear wheel drive car would have an advantage over a FWD in climbing. Obviously, an AWD car with the same weight and tires would be better than either.

I have never found a FWD car is better than a RWD car in the snow in general. The reason people think that is because at the point that they

*do* lose traction (and they all will eventually), it is easier for the inexperienced troglodyte driver to control the FWD's inherent front end plowing understeer than the RWD, which can be made to either under or over steer with judicious input on the fun pedal.

-Fred W

They sell more, and these are the cars that last for 300k miles or more, while the Opels, Fords and VW's are in the scrap heap.

-Fred W

For an anecdotal data

I owned an early SAAB 900 Turbo and it was without a doubt the most troublesome car I've ever owned. Let's take a look:

--front calipers with integral emergency brakes cost a bundle and rot in a few years (40K)

--front seat springs (actually rubber straps) snap due to seat heaters (50K)

--transmission (60K)

--key breaks off in switch on floor (60K, middle of January)

--head liner falls down (65K)

--fuel gauge breaks (50K)

--brake master cylinder (55K)

--fuel smell all the time (>40K)

--cracked head (80K)

--another transmission (90K)

--turbo (90K)

--clutch slave and master cylinders (60K)

--sunroof leaks

--door seals fail

--anything plastic starts to crumble

I used synthetic oil every 3000 miles so the engine ran great. The maintenance just cost too much to keep. I bought a VW and relaxed on the weekends.

Matthew

00 BMW 528i (best overall car so far)

Just exactly the point I wanted to make!

You are now forgetting the most important point here, i.e., as it is impossible to keep perfectly straight steering - for one thing nothing is perfect, for another lateral slopes will spoil the rest of any good attempt at this. This, in practice, in the real world, zillion light years from where you live, means that when slippery enough and from certain climbing angles on the RWD will start swinging its butt so badly that no amount of wheel work will be able to compensate in order to keep the car on the right path. If you don't acknowledge this, it is only because you've never experienced that before. Actually, it does happen even with Quattro, given a sufficient amount of torque and pressure on the accelerator, and that's precisely because of your explanation above.

When climbing under those conditions Quattro has no FWD rivals, RWD is totally out of the question, but FWD is the easiest to handle.

The reason FWD is way more effective on slippery ground than RWD can be read in my previous point. Any attempt at countering this would suggest a tremendous lack of hands-on experience.

Well, Floyd, my argument does not hold true for your 330xi, but the point is that RWD will render their rear tyres unusable much faster than Quattro will render either front or rear or both, that's simply because any burst of acceleration is evenly distributed. You know the worst for tyre life is drift spinning, and that's the only thing I envy from those driving BMW RWDs. Of course I must concede RWD on the dry is way better fun than either FWD or Quattro.

He certainly wouldn't if he'd been driving an RWD.

JP Roberts

You almost got me there :-)

This is going to be a bit longer:

There's sort of a thinking error in your statement. It took me a while to do the math (i.e. mechanics) but the outcome is, that the ratio front/rear with regard to the friction force does _not_ change.

Let me elaborate:

The friction is depending on two parameters (yes, this is a simplification for tires, but it's valid in all cases so bear with with me): the friction coefficient µ and the force _orthogonal_ to the surface. The formula for the friction force is Ff = Fn x µ.

The force pressing the car down onto the tarmac in this case is the mass of the car x g (the earht acceleration 9,81) so you got Fn = mass x 9,81

Now if you have the car on a level surface and assume a 50/50 distribution then the orthogonal force per tire is basically a quarter of the Fn. So the result would be Fn/4 x µ.

The | V indicates the direction of Fn

____ __/ | \__ |_ __V___ _| ____U______U_____

So far so good.

Now the worst case example:

Tilt the road and car 90° (don't sit in the car).

__ | | | |C \ | | | | | | |C / | |_|

In this case, the car would have to be held by something else, because for Fr = µ x 0. I.e. there is no acceleration towards the tarmac and so there is no resulting orthogonal force pressing the tires to the tarmac and therefore no Friction. The car would slide.

So if you choose increasing angles between 0 and 90°, the orthogonal force down on the tarmac slowly decreases on all four tires and is gradually "converted" into a force wanting to push the car "backwards".

But again, for all tires.

The core message is that the friction force is slowly reduced but equally on both front and rear tires as long as you don't change the center of gravity.

Ok, now most likely I have made a complete fool out of myself, but if you are in doubt, then imagine a 90° sloped road. You'd need to support the car on the trunk because there is absolutely no way the tires would be able to hold the car in that position :-)

In your theory, there would be a 100% load on the rear wheels and the car could still go.

I'd be curious to learn if I am really wrong. Mathematically and physically I mean.

I agree. But getting away from a standstill is easier with the FWD because the RWD just slips sideways if it looses traction and you can't steer the direction vector.

Regards

Wolfgang

This is true.

I think you're right. The best handling snow car I ever had or drove was my Alfa GTV6 -- w/ RWD. I liked it even better than the original Audi Quattro -- which had gobs of traction, but wasn't particularly nimble. The only problems the Alfa had in winter were low ground clearance, and a poor defroster.

My old 2002 never kept me from getting first tracks on a powder day, or home in time afterward. I drove right past plenty of 4WD cars stuck in snowbanks and ditches.

Around here we have ice storms, which are so bad it's dangerous to walk. Yet somehow the old farmers manage to get by in their old pickup trucks, without yuppie 4WD or highfalutin' Finnish winter tires. We're talking bargain basement

1982 Ford Rangers and Toyotas. Geez, how did people get around before Quattros and Xi-s?

Matt O.

Not a bad try, but you're missing the key factor: Torque. Since the center of gravity is NOT on the road, it has a torque arm to the point of contact of the tires. The SUM of the forces on the contact area is as you worked out, but it doesn't remain 50/50 front/rear since the rear axle is providing a counter-clockwise (if viewed as in your drawing) torque while the front axle can only provide a clockwise torque. To reach rotational equilibrium more of the weight force in on the rear axle. It's the same reason that your car will nosedive under braking and lift the front end under acceleration.

If FWD slips you can't steer either, it's just that most FWD cars have a front weight bias (due to having the engine, transmission, and other such bits up front) so you have more traction all other things being equal. Bill

Momentum, forces... one should forbid phsyics students to ever read the internet and see the ways their science is abused for biased arguments... In my opinion, the most important driving aspect of FWD in the snow is that traction and steering are intimately connected, which makes the car very intuitive to drive. You can make either concept drive relaitvely well in the snow, and there are also examples for FWD that are undriveable in the snow. From anecdotical experience, while I lived in Germany, when a lot of snow fell I would never drive the BMW 320ci, it was very hard to drive, and mpossible to drive of summer tires. We also owned a cheap FWD Fiat Uno, and that car was a darling in the snow, you always felt what it was doing because the steering would feel connected to your hands, the BMW would regularly totally lose steering feel and you felt like you were just helpless. Downright scary, and twice when surprise by snow it was a miracle I made the journey from work to home (both in the city, 8 miles apart) in one piece.

There is no dount in my mindthe Saab is a very soothing bad weather car. It's my number one choice for being caught in a bad storm, and we also have a 4x4 SUV. The more it rains, the more it feels like it steers on tramlines. And don't be misled by the sunny California thing, we have some pretty awful storms here every once in a while that invariably hit when you're away from home and have no choice but heading back.

...pablo

The point exactly - and the reason FWD has an advantage over RWD (in general, you can always find exceptions). Most FWD cars have less than optimal weight distribution with a front bias. This is a feature in the snow (probably for acceleration too as it helps fight the FWD front end lift issue).

Give me a RWD car for *limit* handling. Give me a FWD car for snow.

You mean someone outside of France buys those Frenchy cars? Wow.

Or a viscous AWD car for both. ;-)

-Russ. '88 iX

You have to be careful how you use the term "advantage." If you mean "doesn't get stuck in snow" then FWD _can_ have an advantage with their weight bias, BUT that's not the whole story. For one thing, how many FWD cars come with limited slip or a locking differential? Not a lot. A RWD with limited slip or locker will be at an advantage getting started in many low traction conditions (of course AWD with LSD or lockers trumps both, but "AWD" with open diffs often loses). As a second point you've just stated that a RWD will *handle* better in the snow. How's that you may ask? The "limit" of handling applies no matter what the coefficient of friction. The same factors that make a RWD corner better at speed make it corner better in poor traction. The recommendation for where to put the "good" set of tires (if you have one better set) for winter is on the rear tires... to prevent skidding. That same front weight bias makes FWD more likely to spin out when cornering or braking. I won't call that an "exception" just a trade-off. I play both sides of this issue myself, I have a Saab 900SPG and a E30 BMW 325 and I've been in situations where either one would have been the better choice (Saab loses goes up slippery hills with an open diff and FWD, BMW gets stuck when you need traction on the _front_ wheels to turn out of a tight space even when the rear tires are moving the car)

Bill