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Since the advent of disc brakes, ABS and non-hydroscopic brake fluid, is 'brake fade' a term to be confined to history?
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If the discs are worn below suggested manufactuers tollerance, then you will still experience brake fade.
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Marcus,
As far as I understand it brake fade is mainly due to the friction material and stopping surface heating up well beyond the anticipated design temperature for the vehicle. The friction material itself loses the property of grip.
This can happen with perfect brake fluid and disc brakes. ABS doesn't help because you are experience the opposite of locking up.
It is true you experience fade rarely these days because brakes on normal family cars are quite capable. Large diameter and/or vented discs with adequate cooling airflow arrangements plus a suitable pad material sees to this.
Sometimes a cheap/unsuitable pad material will edge towards fade with repeated hard use on the road, a track day with your own car may well find them out as well.
I have little to do with aftermarket performance parts but guess this is where all those expensive "sporty" brake pads come in.
David W
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Marcus
The other manifestation of brake fade can occur with old, water contaminated, brake fluid. As fluid ages it collects water from the atmosphere (it's hygroscopic) and its boiling point drops. Under hard brake use this fluid can boil, producing vapour in the system which results in the pedal going to the floor, whilst having no braking effect. When the fluid cools the vapour condenses and the system feels normal. That's why regular brake fluid changes are recommended.
This particuler problem is made worse if the brake pads are worn thin, as the thinner pads allow more heat to transfer to the pistons and hence to the brake fluid.
Regards
John S
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John,
As I expect you know, silicone brake fluid is non-hygroscopic and I think this is what Marcus was referring to.
However, even with silicone brake fluid, water will still enter the braking system (at each end of the system and, I believe, via the walls of the hoses). Since the silicone brake fluid will not absorb water, this collects at the lowest point of the system which is exactly where you don't want it - i.e. in the wheel cylinders where it will turn to steam under severe braking and impair braking efficiency.
Silicone fluid is also slightly compressible so it gives a softer pedal than the old fashioned Dot 3 or Dot 4 fluid. Its only advantage is that it doesn't melt paintwork (been there, done that!). It is definitely not recommended for systems with ABS which could be upset by the compressibility.
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Dizzy
Yes, I use silicone fluid in my Minor, for the non-hygroscopic properties, as it does few miles. With a basically non-hygroscopic fluid I doubt that water ingress is any sort of problem. I've heard this about 'soft' pedals and compressibility, but again I have my doubts. Frankly any difference in compressibility of liquids is, I believe, unlikely to be noticeable in as crude a testing apparatus as a brake system. I also believe it's used by the US Military, but there must be, I agree, some reason why it's not more common in Europe. (Couldn't be the service work for fluid changes of course). It may be that ABS is sensitive to the fluid, but I would have thought that sensitivity could have been designed out. Even so I wouldn't worry about using it in either of my ABS equipped cars. I've not seen any 'don't use DOT 5' warnings and, for safety, by definition, all DOT aproved fluids are supposed to be compatible. You would lose the benefits of DOT 5 by mixing, but it shouldn't be unsafe.
Certainly one of the correspondents here had his Audi refilled with silicone fluid by an Audi specialist, who apparently always used it in customers cars, many of which have ABS.
Regards
John S
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Some interesting stuff there, John.
I must admit to having no personal experience of silicone brake fluid, and I am not a chemist, so I have relied on the statements of 'experts' whose opinions I have judged at the time to be reliable.
It has also been said by experts that Dot 5 is not compatible with Dot3 and/or Dot4 and that the whole system needs dismantling and cleaning before changing over. However, a friend uses Dot5 in his Standard Vanguard and I am quite sure he didn't dismantle it beforehand, so perhaps you are correct in suggesting no problems with mixing. However I would want to see this in writing on the can first.
I think I'll still play safe and steer clear of using Dot5 in my BMW's ABS braking system but I suspect that the other objections to Dot5 that I passed on may be over-stated.
That's what I like about this forum - there's always someone willing to give us the benefit of their own experience (and common sense!) and keep us on the right track.
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Dizzy,
I understood that the whole point of using silicone brake fluid was because of its non-hygroscopic properties which prevented it boiling under repeated severe braking. If you are correct about water collecting at the lowest point and that "Its only advantage is that it doesn't melt paintwork" why would anyone use it?
C
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Brake fade is not a thing of the past. It’s just that modern cars have much better brakes than cars of old. Brake fade occurs for the reasons stated when some part of the system is overheated beyond its capabilities. The main variable over time is that the brake fluids boiling point falls very rapidly as it absorbs small amounts of water and air.
Dizzy is quite correct that water vapour passes into the fluid through the brake hoses. Again, modern material formulations for hoses are more resistant to this than some years ago, but the effect does still occur.
I think you may be confusing DoT5 with Dot5.1, however. DoT5 fluid is silicone based, DoT5.1 is non-silicone based.
DoT 5.1 fluid is a conventional glycol based fluid like DoTs 3 & 4 and can be mixed with these safely. It does have superior wet & dry boiling points to DoTs 3 & 4 and about half the cold temperature viscosity. This makes DoT5.1 especially suitable for ABS in cold climates and a superior performing fluid in hot climates.
DoT5 silicone based fluid SHOULD NOT be mixed with non-silicone DoTs 3, 4 & 5.1. If you use silicone fluid you should flush it through to get rid of the old fluid.
The main reason that manufacturers will not use silicone brake fluid is that ALL silicone-based fluids (including greases) are banned from their sites because they have an adverse reaction with many other conventional fluids in use. If any silicone should ever get into the paint shop the consequences don’t bear thinking about (mainly from a cost point of view – we would be talking 6 or 7 figure sums easily).
Silicone fluids are also slightly more compressible than conventional fluids, which is why they are unsuitable for a wide range of hydraulic applications.
Personally, I have never used silicone fluid even in the rally car and I never will. I do, however, have DoT5.1 in my ABS equiped Rover Vitesse Sport.
Hope this helps.
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Cyd,
You seem to know the subject well. I wonder if you can answer Cardew's question, i.e. just why do people use silicone rather than conventional brake fluids? Do the rubber seals last longer, for instance?
Certainly the (Ferodo) rear wheel cylinder seals in my Triumph are very short-lived but I put that down to them being made of nitrile rubber instead of a more suitable rubber (butyl?). Perhaps using DoT5 would help prolong their life?
One last comment - I wonder what idiot decided to designate silicone fluid as 'DoT5' when DoT3, DoT4, and DoT5.1 are glycols. Seems a case of blindly following formalities rather than using the brain.
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Silicone fluids to DoT5 have been available for longer than DoT5.1 and certainly have superior boiling point properties to DoT4 which I suspect leads to their use in motorsport. The conventional fluids to DoT5.1 are a more recent development hence NHTSAs addition of the .1. I think.
Personally I never had any cooling problems on the rally car because rather than using silicone fluid I simply used tubing to vent air from the front of the car to the brakes. The brakes were of course kept in tip top condition.
The weak point on most older calipers was the area where the fluid passed over the disc to reach the outer side of the caliper. Sliding calipers remove this problem by only having their cylinder(s) on the inside.
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Cyd/Dizzy,
Thanks for the interesting information.
Possibly I am being thick but I am not sure if the above replies answers my question.
I am clear now that the fluid itself has a higher boiling point. However the contention of Dizzy(which I have heard before)is that the non -hydroscopic properties of silicone causes water to be present at the calipers rather than being absorbed into the fluid. It is this water that boils, rather than the silicon fluid itself.
C
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Cardew, you are not being thick! The problem with all this brake fluid business is that we don't seem to have ready access to back-to-back test results, though what Cyd has given us must come quite close.
I think you will have gathered that I have no first-hand experience of DoT5 silicone fluid but I have come across claims by more than one motor sport activist that water can make its way to the calipers and collect there. A Google search on the subject yesterday brought up several such claims. Perhaps this is one of those things that is theoretically possible but uncommon in practice - I just don't know.
I would still like to learn of any benefits of DoT5 to the 'ordinary' motorist apart from its non-melting of paint.
That reminds me - if anyone does spill glycol brake fluid on their paintwork, copious amounts of water applied very quickly might prevent any damage. However, I can tell you from bitter experience that you need to be very quick!
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Dizzy
Like you I've never seen any definitive information on this. I'm confused by what mechanism the water 'makes it's way to the calipers'. With a standard, hygroscopic, fluid there is a definite attraction for water to enter the fluid. That is absent with silicone - indeed it positively repels water. So, given the relatively small exchange of atmospheric air in a braking system, there is surely less opportunity for water to enter. The 'coming in through the hose walls' option too must be reduced, as the fluid is not attracting the water, although that route must be miniscule anyway. I wonder if the instances of water collection follow a fluid change, and residual accumulated water is driven out? My view is that if the fluid is DOT approved it is safe.
Regards
John S
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Quite correct Cardew. Once water gets in, it is the water which boils. This is because the water is not absorbed by the fluid but remains in its free state - therefore its boiling point remains 100C rather than reducing the boiling point of the fluid by 10 - 20 degrees.
see my further essay further down, where I make some quotes from the Bosch automotive handbook (the automotive engineers bible virtually).
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Cyd
Interesting information! I hadn't thought of impact on other processes in respect of silicone fluids.
My can of Dot 5 silicone definitely says it can be used with other conventional brake fluids. This should be the case, as it couldn't meet the DOT standards if it couldn't be mixed. All DOT fluids are, I believe, required to be compatible. Therefore, say, putting DOT 4 in an older system is quite safe, as you've done with the DOT 5.1 in your car. Equally, whilst putting DOT 5 silicone into a system with non-silicone fluids wouldn't be unsafe there would be no point as the benefits of the silicone would be lost. When it comes to installing later fluids, then the question of how to remove the old fluid comes up. Not easy, and I'm wary of the advice to use different fluids as flushing media. On my car I filled a fully replaced system with DOT 5 from the first fill.
Regards
John S
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Yes, FMVSS 116 requires that DoT brake fluids are compatible and specifies a test for this. However, the bottle of DoT5.1 I have clearly states #do not mix with silicone based brake fluids# on its label. Evidently this is given as #good advice# rather than any technical requirement.
I can understand why anyone who had a fluid boiling problem in the past may have chosen to use DoT5 silicone fluid. Its minimum boiling point is 260C compared to 230C for DoT4. These users would most likely have been motorsport competitors. However these users would still have to contend with the water problem. Here's what the Bosch Automotive Handbook says about that: #Testing of the wet boiling point is directed toward determining the characteristics of used brake fluid. Brake fluid can absorb moisture through the systems hoses, mostly through diffusion. This is the main reason for it having to be replaced every 1...2 years#
Diffusion of water into the brake system will occur whatever brake fluid one uses. This fact is not pointed out to the purchaser when the fluid is being sold on the basis that its non-hygroscopic and therefore never needs replacing. This is factually correct, but does not mean that water cannot present itself in the brake system. Even motorsport users would probably
flush through fresh fluid occasionally.
Here's what the Bosch Automotive Handbook has to say about silicone fluids once water is present: #A critical factor for brake fluids based on silicone or mineral oils is the absorption of free water in a fluid state, as the water forms vapour bubbles when it heats up to more than 100C and freezes when it cools to less than 0C.#
Now that DoT5.1 fluids are readily available with a (dry) boiling point performance that matches that of silicone, it seems to me that the use of silicone no longer carries any advantage. Ok so the boiling point falls with water absorption (much slower than it does with DoT3 or 4), but if this is a
problem the fluid can be changed more frequently (and you should be doing something to improve brake cooling). Further, since glycol fluids absorb the water you can be certain the water has been removed when the fluid is replaced, which may not be the case if free water exists in the system.
For ME the final word goes to an old university buddy (and motorsport pal) who is now a senior engineer in chassis systems at a Midlands luxury auto manufacturer. When asked why manufacturers do not use silicone fluid he replied #because its really bad#.
You pays your money and you takes your choice.
Apologies to Marcus. Your original question about whether brake fade can still occur in a modern car has been hijacked into a discussion on the merits (or not) of various brake fluids. I believe the answer to your question is here, but to summarise:
Brake fade can occur in any car if you get the brake system hot enough. There are a number of possible causes for fade occurring and hence a number of possible solutions. Your supposition that modern brake fluids are non-hygroscopic is incorrect (except for silicone based). Did you ask the question out of curiosity or do you have a problem you need help with?
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Cyd
This gets interesting. Certainly a collection of water at the caliper end would boil at 100C and give problems. From that view point the fact that water is miscible in conventonal brake fluid may actually be a benefit, as reduces the risk of it collecting at one point. I know silicone is non-hygroscopic, but I wonder how much water it can absorb - more or less than conventional fluid?
I've not used silicone for its boiling point (the Minor is hardly a rally car) but the non-hygroscopic properties should reduce water ingress and thus reduce braking system corrosion. I've always been cautious of the 'never needs changing' slogans. Perhaps I should annually pump some fluid through each of the bleed screws. I have to say the cylinders are still free after 4 years, and I've had cylinder sieze in less time than that on conventional fluid.
Regards
John S
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Cyd,
Thanks for all this - it would make a good subject for a MSc thesis.
I'll do without silicone and SWMBO says she will too - but in a different context! Now that would make an interesting thread if only I can think of a motoring connection.
C
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Cyd,
Certainly no apologies needed, It has been an interesting discussion.
I really asked out of curiosity in the first place. However I did wonder if silicone could be used and eliminate the need for changing brake fluid every 2 years. Seems not!
Marcus
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I remember doing my Racing License test at Donington in the mid 90's in a standard Astra GTE. I was amazed how quickly the brakes faded when put under serious stress. (i.e. by lap 2). I assume the car was reasonably maintained.
I also remember getting brake fade in my well-maintained company Cavalier by the end of a brisk drive across the Snake Pass.
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PB,
I wonder if you were as terrified as I was when I last got brake fade. This was in my uncle's nearly new Vauxhall Velox in 1960 as I raced down a winding and rather steep country road in Essex and the car just seemed to go on and on without slowing however hard I pressed on the brake pedal.
When I told my uncle (a garage proprietor) that there was something wrong with the brakes on his car, he looked at them, felt them and smelled them and then rather impolitely explained to me the phenomenon of brake fade. I'd never heard him swear before!
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NO!
The Mondewoe had it in big quantities last month, a couple of fast slow downs on a fast run and then a very big slow down on a not very steep down hill caused enough fade to get a little worrying. Never terminal, I am not going to stop, rather when is it going to stop. It did, but I wouldn't have been in Mr Ripleys' good books. Had the pads and disks checked by my local independent who found now't wrong as far as he could see. Just to much heat in too short a time.
Bill
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I once felt some fade on the standard ventilated disks on my Xsara, but now I am using ATE Power Disks which are grooved, and they are great. www.contiteves-am.com/english/indexmit.htm?produkt...m has some more info.
Ben
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