Disc brakes vs Disc/drum brakes

[Douglas Ferrier]

My experience in using slotted rotors (front only) with appropriate pads has been a far superior outcome to the foz original rotors/disks.

To me it seems subaru underestimated the demands on the foz brakes, hence it is quite common to see the "not really satisfied" notes on foz brakes on many forums and motoring literature.

When comparing wear rates it is important to start calculating vehicle kinetic energy (linear and rotational) that needs to get bled off as heat through the brakes and the relative surface areas available to the friction surfaces, (relative pad thickness and surface area).

I made some quick back of the envelope calculations when I was fed up with the stock rotors needing skimming regularly and the pads being chewed through 3 to 4 times faster than I had experienced in other cars and at that time it made it pretty clear to me that the car was about 40 to 50% greater in mass than the other cars I had been use to and this means about an equivalent greater amount of heat energy bleed during each braking op and this was being done through surface areas that were only in the order of 10% larger thus explaining the less than happy outcome compared with previous experiences.

Before moving to the slotted rotors and pad combination I had several brake experts say nah you only need it for racing, the price variance was minor so I gave it a shot and the solution has come up trumps for decreasing brake wear while retaining performance.

Pad wear and rotor wear decreased significantly while improving braking feel and retention of feel during multiple stops.

Hence I think what I am saying is ratbag is correct the right engineering for the right job.

Keep in mind that where you use the car also impacts significantly on wear rate and perceived performance.
Comment: Melbourne traffic is a killer these days compared to 10 years ago, so the same routes take longer and induce much higher wear rates than ever before, freeways you use to be able to use cruise control on are now impossible and have you braking, accelerating 2 or 3 times every km due to traffic volumes being so high. The civic we have is now chewing brakes more rapidly than in years gone by, same setup same pads etc simply more use and I suspect more heavy brake events as it seems there are far more random acts of madness on the road than in years gone by.

So to summarise moving to slotted (vented rotors) with appropriate pads has vastly improved my foz braking experience, thus I would say is a better matching engineering solution than what came of the factory for this vehicle.

As Ratbag has alluded to -brakes may not save you, compromising on a system critical element is a dubious choice ...

And nothing is perfect, this being my first subaru it has impressed me overall (with the brakes being the only real disappointment - a solution to which I found at very reasonable cost so can not really complain given the mass consumer market it is trying to each)

Doug

 

[guzzla]

I made some quick back of the envelope calculations when I was fed up with the stock rotors needing skimming regularly and the pads being chewed through 3 to 4 times faster than I had experienced in other cars and at that time it made it pretty clear to me that the car was about 40 to 50% greater in mass than the other cars I had been use to and this means about an equivalent greater amount of heat energy bleed during each braking op and this was being done through surface areas that were only in the order of 10% larger thus explaining the less than happy outcome compared with previous experiences.

And nothing is perfect, this being my first subaru it has impressed me overall (with the brakes being the only real disappointment - a solution to which I found at very reasonable cost so can not really complain given the mass consumer market it is trying to each)

Doug

I certainly agree with the Forester's brakes not being up to the standard most people would like. However longevity of pads and rotors has certainly not been the problem with mine. The rear pads and rotors are original and never been machined at 260,000km while the original front pads were
renewed at 238,000km and the rotors machined for the first time simply to match the new pad surfaces and not because of any scoring or warping. I guess my driving must go easy on them, yet they are by far the most durable brakes of any of the dozen or so cars I owned in my life. I've owned other popular cars where pads needed renewing at less than 50,000km.

Can't say the same for their performance though. I don't like the greasy, numb feel, their lack of "bite" and their weak feel. As you say, they feel like they were made for a much smaller car. It is only brought home to me by the brilliant brakes on my new Mk7 Golf.

 

[Ratbag]

Can't say the same for their performance though. I don't like the greasy, numb feel, their lack of "bite" and their weak feel. As you say, they feel like they were made for a much smaller car. It is only brought home to me by the brilliant brakes on my new Mk7 Golf.

What do you mean by " ... greasy, numb feel ... " and "bite"?

Could you try to quantify this in some way please?

You have already agreed that the difference in actual stopping distance between a (relatively) light weight sports hatch and a considerably heavier AWD small SUV is minimal, of the order of ±3%. A difference of this quantum is as likely attributable to tyres, technique, familiarity, and/or any number of other non-random variables.

This also accords well with Wheels findings in their comparison between the Audi Quattro Bi-turbo and the twin turbo Liberty. As they stated, "the Audi brakes felt better, but it didn't stop any faster than the Liberty" and went on to say that in every other respect the Liberty was a superior car, specially when it came to price - less than half the price of the Audi.

As I have mentioned before, it's hardly the 30-35% greater stopping distances observed in the 1960s and 1970s between base/medium model Holdens, Falcons, Valiants (and similar US derived cars) and the stopping distances of similar cars from England and Europe - around 170-180 feet from 60 mph for the former, and around 128-135 feet for the latter - approximately 33% greater. Now that difference was significant. And that was only on the first stop. Forget about a second or third ...

 

[guzzla]

What do you mean by " ... greasy, numb feel ... " and "bite"?

Could you try to quantify this in some way please?

Just like I've said, it's the easiest way of describing how they feel in lay terms. The pads don't feel like they have "feel" or friction against the rotor faces. They feel slippery. And when you actually first touch the brake pedal there is none of that instant bite or clamping feel that I get on my Golf. Someone else coming from another type of car might not notice anything different (eg my daughter coming from her Corolla thinks there's nothing wrong with it) but I have the every day comparison of what great feeling brakes are.

And as far as stopping distances go although I have never done (and not in a position to do) an accurate scientific test, the Golf simply pulls up in a much shorter distance than the Forester. I know it weighs more but it also has the bigger tyres and generally it's the Golf that is carrying more load now. I'm usually alone in the Forester. On a trip back from Goondiwindi yesterday morning in the Golf a kangaroo hopped across in front of us and I really had to hit the skids big time. We pulled up short of hitting it and I know from experience that if I had been in my Forester that I would have hit it. There were 3 adults and a boot full of luggage too so it would have almost negated the weight difference between the cars.

For actual stopping distances tyres will make an incredible difference. Just look at the results of a comparison test done with the new Mazda3 and new Golf in the March edition of "Wheels" this year. Same driver, same location, same cars - the base Mazda3 with 215/60 16 tyres did a 100-0 in 44.8m while an 86kg heavier version with 215/45 18 tyres only took 39.4m. More than 5m is a huge difference. A similar result was obtained with the Golfs. One version with 195/65 15 tyres did the 100-0 in 42.1m while a 63kg heavier Golf with 225/45 17 tyres took 37.8m. What we are looking at there is well in excess of 10% shorter stopping distances in both cases. My Golf is the lightest version but with 205/55 16 tyres. I can only assume if they tested it that it would have pulled up in around 40m.

 

[Ratbag]

Just like I've said, it's the easiest way of describing how they feel in lay terms. The pads don't feel like they have "feel" or friction against the rotor faces. They feel slippery. And when you actually first touch the brake pedal there is none of that instant bite or clamping feel that I get on my Golf. Someone else coming from another type of car might not notice anything different (eg my daughter coming from her Corolla thinks there's nothing wrong with it) but I have the every day comparison of what great feeling brakes are.

That's what I thought you might mean. Have you ever considered that other people might find this kind of grabby, jerky brake feel to be the opposite of what they want or feel comfortable with?

I have driven cars with brakes that felt like this. Mostly it was due to scoring on either the discs or drums. However, sometimes it was characteristic of the car.

"Bitey" braking like this makes my wife car sick before the first corner ...

I also prefer "progressive" brakes. My Impreza had those (ventilated discs/drums), as have all the other cars I have owned. i.e. where the braking response is directly proportional to pedal pressure ...

Most American cars that I drove many decades ago had those "grabby" brakes, as they didn't have anything else going for them. Gave one a sense that something was happening when it wasn't really ...

And as far as stopping distances go although I have never done (and not in a position to do) an accurate scientific test, the Golf simply pulls up in a much shorter distance than the Forester. I know it weighs more but it also has the bigger tyres and generally it's the Golf that is carrying more load now. I'm usually alone in the Forester. On a trip back from Goondiwindi yesterday morning in the Golf a kangaroo hopped across in front of us and I really had to hit the skids big time. We pulled up short of hitting it and I know from experience that if I had been in my Forester that I would have hit it. There were 3 adults and a boot full of luggage too so it would have almost negated the weight difference between the cars.

For actual stopping distances tyres will make an incredible difference. Just look at the results of a comparison test done with the new Mazda3 and new Golf in the March edition of "Wheels" this year. Same driver, same location, same cars - the base Mazda3 with 215/60 16 tyres did a 100-0 in 44.8m while an 86kg heavier version with 215/45 18 tyres only took 39.4m. More than 5m is a huge difference. A similar result was obtained with the Golfs. One version with 195/65 15 tyres did the 100-0 in 42.1m while a 63kg heavier Golf with 225/45 17 tyres took 37.8m. What we are looking at there is well in excess of 10% shorter stopping distances in both cases. My Golf is the lightest version but with 205/55 16 tyres. I can only assume if they tested it that it would have pulled up in around 40m.

I don't know if you have considered that even in a controlled test, the precise location of the tyres of the particular car on the particular part of the road/test track surface is a non-random variable. The specific coefficient of friction of the specific strip of road surface will have as much an effect as the coefficient of friction of the tyre tread compound.

The tyre carcass is incredibly important in dry weather braking, whereas the tread and tread pattern become at least as important in wet weather braking (if not more so).

The same goes for tyre width and 'footprint' on the road. In the dry, it should be as large as possible (preferably bald ... ); in the wet, it needs a good tread pattern, with far narrower tyres. Check out what happens when it rains during a GP race ...

 

[guzzla]

I understand what you mean by grabby. But I don't find the Golf's brakes like that. They just feel like they have a good grip on the rotors and at low speeds you can actually hear the rubbing sound. I assume the Golf's brakes (like those on many other European cars) are designed for rapid response at high speeds on autobahns - and having driven on them at considerable speed I understand why. You are also right about brakes that jerk and make passengers sick, and my SWMBO would never put up with that. I even have learnt her tolerance limit

for when I try to step it up a bit in some more spirited mountainous driving we tend to do. She is rarely with me now in the Forester and that is a sort of freedom time for me. I still get great enjoyment out of driving it - it has such wide ranging abilities.

 

[Ratbag]

^ My SWMBO hasn't got a tolerance limit ... Around town, it's best if she drives us in her car. She appears to lack the ability to predict what the car is going to do. This seriously upsets her semi-circular canals and balance. Her Dad was the same. Both were OK when they were driving, but not when someone else is driving, whether me or anyone else.

I have not found the brakes in either of our two Foresters to be inadequate in any way - in the wet; in the dry; towing an un-braked trailer; not towing; loaded; just with me as the driver; whatever ... .

However, I am used to driving many kinds of vehicles (I hold a current National Heavy Vehicle licence), and therefore am used to driving cars (etc) within their limitations. When towing a tandem axle, double horse float (with horses) behind my fully loaded '68 LC, it was usual to take 500-600 feet to stop from 100 kmh. If you didn't, the horses were dead or dying by the time you stopped ... The vehicle and float would stop a lot faster than that without any problems, but the horses couldn't stay upright if you did ... This means you must think in these terms, and drive accordingly.

That having been said, I am more than happy with the brakes in both the Foresters.

They are progressive and responsive. Very few of the cars I have driven have had better brakes, and I have never run up the back of anything; or even come close.

Even the HR and my father's hot rod Torana could be driven safely in almost all circumstances, even though their brakes were worse than just marginal if one drove either of them to the limit. The same conditions apply as when driving the LC with horses on behind ... :iconwink: :lol: :rotfl:.

It also needs to be said that driving a car that's low to the ground gives a feeling of going faster and braking harder than driving one that's higher, even if there is no difference in fact. I have driven lots of low cars - Lotus Super Seven; Morgan; MGs (TC, TF, TD, MGA, MGA Twin Cam, MGB, and a couple of their sedans); Datsun 2000 (racing version); Prince Skyline 2000 (the one with the Merc engine, 3x DCOE Webers choked down to 38mm (IIRC) and funny gearshift gate), Jaguars, Aston Martins; Ferraris; Minis; etc, etc. Some of them for 50-80,000+ miles. I am very familiar with this effect.

I am very flaming glad that I cannot hear the brakes in either Forester. It would drive me nuts. That also usually indicates some kind of crap under the pads, or scoring, or both. Your VW may just be like this, I don't know, but I don't like to hear these things working ... I just like them to work properly and effectively. AFAIAC, the brakes in the Impreza and two Foresters do just that. On the very rare occasions when I have had to stop brutally fast, they have all delivered the goods, and in a straight line, regardless of the road surface, wet or dry.

I am very happy for you that the brakes in your VW suit you, noise and all. That does not make them any kind of touchstone, any more than the Forester brakes suiting me makes them one. These things are merely personal preference.

BTW, did they test the Mazda and Golf brakes (and tyres) in the wet or on a wet skid pan?
Wheels used to do this, and also published figures for multiple stops. I haven't bought their magazine since I recognised it for what it was when I was about 20 y.o.
Autocar did better reviews, IMHO, but the same caveat also applies to them, and all other motoring writers right up to the present day, IMNSHO.

My insurer will not tolerate fitting wider rims or tyres than standard (e.g. going from 215/60 16 to 225/60 16), but are quite happy with fitting higher profile tyres (e.g. going from 225/55 17 to 225/60 17, or from 215/60 16 to 215/65 16), as long as within legal limits.
They feel the same way about modifying the OEM suspension in any way other than replacing like with like.
I know why this is the case in both cases - it increases the risk of destabilising the car's handling and braking in some or all conditions, and therefore increases their risk.

Money talks. In this case, it is telling the truth ...

 

[guzzla]

I am very flaming glad that I cannot hear the brakes in either Forester. It would drive me nuts. That also usually indicates some kind of crap under the pads, or scoring, or both. Your VW may just be like this, I don't know, but I don't like to hear these things working

I am very happy for you that the brakes in your VW suit you, noise and all.

BTW, did they test the Mazda and Golf brakes (and tyres) in the wet or on a wet skid pan?

My insurer will not tolerate fitting wider rims or tyres than standard (

You can only hear the rubbing sound of the brakes with very good hearing, only a very low speed when cold and if there is no other sounds like the radio. My experiences with other Golfs tells me it's normal. I've only done 45,000km (in 13 months) and just had the 3rd service and they tell me there is minimal wear on the pads but I don't expect to get the same great war from them or the rotors like I have from the Forester. I'm told European manufacturers set their brakes up with soft pads and rotors for a different braking feel.

As far as the "Wheels" test goes I assume it was done on the same dry surface that they did the acceleration tests. They just say "Private road". I still buy "Wheels" after all these years - since I read the first one in Nov 1969 and think that it is the most credible car information available to Aussie buyers of the models being sold here. I've learnt to read past all the crap and editorial themes that they seem to push.

That's interesting about your insurer. I've never questioned mine (Suncorp) but then I continue to run the standard 215/55 17 tyres on stock rims. Golfs come with as standard 195/65 15, 205/55 16, 225/45 17, 225/40 18 and 235/35 19 tyres depending on the model and it is legal to fit any of these to mine if I wish. I chose the 205/55 16 on stock VW alloy wheels because I know what a horrible ride and the likelihood of bending rims and splitting tyres can be like on very low profile rubber. I even feel the 55 profile tyres on the Forester XT's is marginal considering the off-road use they could be expected to receive. I've split the wall on 2 of mine over the years although who is to say it wouldn't have happened on higher profile tyres anyway. The gibbers on the way out to Birdsville would be enough to wreck any tyre. And they did survive the Gibb River Road in 2010 when plenty of "real" 4WD's had tyre trouble. Just luck.

 

[Ratbag]

^ The 225/55 17" on the SH give a noticeably less comfortable ride than the 215/60 16" on the SG. On top of that, it does not handle anywhere near as well, but that could be attributable to its being a larger, higher car all round than the SG.

SWMBO likes it though, and that's all that matters. I don't drive it very often.

My local fish & chip shop has Wheels on their counter, so I get to see it from time to time. If anything, the standard of journalism and reviews has gone backwards since I stopped subscribing to it in 1968-69 ...

Getting back on topic, the noise you are hearing could be the removal of the very fine layer of rust that forms on all disc brakes overnight if the weather is humid or cold and damp/wet. That is removed the first time you touch the brakes while moving.

Back when rear discs became the selling point for all sorts of basic cars, it was very common to see cars where that layer of rust was such that it indicated that the particular brake had not worked for some considerable time. Usually only on one rear disc, but sometimes on both. I have seen this on cars that cost up to $100K at the time.

You previously compared the Wheels reviews (for want of a better term ... ) of a base model Mazda 3 costing under $16K, and the VW Golf GTI costing around $42K. Do you really believe that to be a fair and reasonable comparison, given the not inconsiderable price difference?
And "only" 86 Kgs is not an inconsiderable difference in weight for vehicles in this weight class, FTM.

Testing braking distance is basically flawed for at least the reasons I have already mentioned. It will give an indication of the particular car's braking performance, but given the variables that cannot be controlled accurately, I would expect at least ±5% to 8% in any particular test, even using the same car, on the 'same' track, under the 'same' conditions with the 'same' driver.

This is quite different from a test that shows (say) a 30 to 50% difference in the minimum braking distance after multiple runs (allowing the brakes to cool completely between runs), using both the same and different drivers.

There is also merit in testing the ability of the brakes to recover after multiple runs, and ditto on a wet surface.

However, unless these tests show gross differences between cars, they are unlikely to mean anything. Minor differences of the order of ±5 to 8% can quite properly be considered to be the result of un-controlled or poorly controlled non-random variables.

Basic testing theory dictates that there is always a "reaction range". This has nothing whatsoever to do with driver reaction times, but refers to the fact that any test of almost anything will return results within a range of values. It predicts (correctly ... ) that no two tests can ever be identical in every respect. Basically the old Buddhist (Taoist?) philosophical principle that it is impossible to step into the same river twice - firstly, the river has changed; and secondly, the person has changed ...

Now, if one were to compare the stopping distance of my old LC with that of the SG Forester, those results would almost certainly be statistically significant.

What would be interesting would be to see if the LC's results improved as the load increased (it was a deep well ute) - my own observation was that it did.

It would be equally interesting to see if the Forester's results declined. My own observations indicate that they don't - nor do they improve. They also don't decline with my fully loaded trailer on behind with two adults in the car, plus a whole lot of other crap.

To me, this indicates that the LC is optimised to have about 0.5-1 tonne in the back (this also dramatically improved its handling and stability); and that the Forester is over-braked when carrying only the driver without any other load or an un-braked trailer being towed. This also accords with my experience.

All this indicates that testing of anything, in any field of endeavour or investigation, is difficult, and can be fraught. The Placebo Effect doesn't only apply in medical environments ...

Differences in 0-100 or standing quarter mile times are statistically different between a turbo and non-turbo Forester of the same model. They are probably different between our SH and SG, but this difference would not be statistically significant; nor is it significant in any driving situation.

For just one example, the turbo SF Forester was only a little faster than the SG series II N/A Forester. The SF turbo managed 0-100 in 8.4 secs; the SG series II does it in around 9.3 seconds. Hardly an earth-shattering difference ...

Hopefully this clarifies for you why I don't place much store by the differences that Wheels magazine claims when they are writing about differences that are minimal; whether in braking performance or anything else.

 

[guzzla]

^ The 225/55 17" on the SH give a noticeably less comfortable ride than the 215/60 16" on the SG. On top of that, it does not handle anywhere near as well, but that could be attributable to its being a larger, higher car all round than the SG.


You previously compared the Wheels reviews (for want of a better term ... ) of a base model Mazda 3 costing under $16K, and the VW Golf GTI costing around $42K.

I think I have mentioned before how I have noticed that the 215/55 17 tyres on my Forester when it was stock made it feel sharper in the steering but less cushioned in the ride than my brother's stock SG Forester X with 215/60 16 tyres and that is exactly what would be expected.

The cars "Wheels" compared were a $21490 base model manual (like mine) and a $20490 (but $22190 as tested with metallic paint & safety pack - like Subaru, VW doesn't "do" safety packs. it's all there standard) base model Mazda3 Neo manual. So it's hardly the unfair comparison you're suggesting. The other two were a $31990 Highline Golf auto and a $32590 Mazda3 SP25 GT auto. There is no such thing anymore as a $16000 Mazda3 and $42000 will get you a Golf GTI (or me one day when the Forester dies, as I keep reminding SWMBO :biggrin: )

I understand what you are saying about the difficulties in testing braking but what I was really pointing out was that "Wheels" found in that test that the heavier cars in each respective model braked to a stop over a shorter distance than their lighter model and as an amateur observer I would attribute that to the wider tyres.

I remember reading a handling / braking comparison in a magazine (can't remember which but think it might have been "Wheels")years ago using a whole lot of soft roaders and off roaders including an SG Forester (which did pretty well but was let down by its dreadful stock Geolandar tyres they observed) and I was staggered that one to have some of the shortest braking distances was a Toyota Prado - that great lumbering whale of all things! That's not to say that it was stable doing it, of was directionally good or that it was progressive and controllable.

 

[Ratbag]

^ Sorry about the price.

I was looking at this page:

https://www.drive.com.au/new-car-showroom/mazda

and pulled the price for a Mazda 2, not a Mazda 3.

It does rather beg the question as to why the pricing and relative merits of VW and Mazda vehicles are being discussed on an off road Subaru forum ...

As for tyres, both the SH and SG have wider tyres than my '68 LC, and bigger diameter rims in the case of the SH.

Why heavier vehicles tend to brake better (relatively speaking) is because their weight per square inch on the tyre footprint more easily breaks the molecular bond between water and hydrophilic road grime molecules on the road when it is wet ... When dry, they have a greater weight per unit footprint than lighter cars, making them less likely to exceed the coefficient of friction between a given tyre compound and a given road surface.

However, there are so many other factors that determine how a vehicle brakes that I just can't be bothered going through them all here.

 

[Kevin]

It does rather beg the question as to why the pricing and relative merits of VW and Mazda vehicles are being discussed on an off road Subaru forum ....

Yes, wondering that myself! (but I can guess) :lol:

 

[Ratbag]

^ So can I ... :rotfl:.

 

[Scooby2]

intriguing .... considering all the variables involved!

Best regards,

 

[Ratbag]

^ Gidday S2

Yes, mate.

Without even trying to quantify the number of non-random variables involved, about 10-20 spring to mind. I could write a 500-1000 word essay on the basics of each of these ...

I have also tried to construct (in my mind) a properly controlled test environment that accounts for even these obvious factors.

In the light of my original comments, I have also tried to think of a standardised and simple way to test any given car driven by a human on a "normal" test surface such that it would return reproducible results.

I can't, not in either case ...

Even using some kind of rocket sled with the car attached and fitting an automated braking system to the car, the external surface that is applying the braking force would rapidly change its characteristics.

This surface could be conditioned by performing multiple trial runs, then cleaned using a standard cleaning process so that all subsequent tests used more or less the same surface with very similar coefficient of friction characteristics, but this would still be flawed to some degree.

Of course, the standardised cleaning process would need to be performed after each and every run so that each new test started with the same conditioned surface.

Then there is the real world, where all these variables are in a constant state of flux. Each and every one of us compensates for these variances all the time, more or less successfully. It's something to wonder at ... :iconwink:.

 

[Scooby2]

^

Then there is the real world, where all these variables are in a constant state of flux. Each and every one of us compensates for these variances all the time, more or less successfully. It's something to wonder at ... :iconwink:.

operator and machine. Who says the operator's 100% efficient, and who knows if the machine is as well? There are more questions here than you can poke a forky stick at!

Best regards,

 

[Ratbag]

^ As my brother (barrister) often remarks, given all that, it's a source of constant amazement that the gutters aren't running over with blood; and that it is a tribute to the strength of our self-preservation instincts that we have so few "body contact events"!

I have to agree with him ... :iconwink:!