Warping brake discs.....again!

[DrFarkoff]

As for his assumption that high performance vehicles should not use silicone based brake fluids is complete rubbish Castrol SRF is silicone based, and formula 1/Indie car teams etc use it, I would be tempted to try but £50 per Ltr is a bit more than I want to spend. In addition most European sports car / Hyper car manufacturers use DOT 5.

But for some reason our friends across the pond seem to think that, the older dot 3/4 stuff is better

I did a bit of researach a while ago and put an article together It may be of interest to anyone thinkimg of using DOT5 Silicone fluid in their DOT4 braking systems

articles for the e zine

Well I choose Brake fluid! “Why” you say? That would be because for the average daily commuting motorcyclist or a Sales rep or a Mum taking kids to school it’s a total Mystery!!!

Well here’s a try the definitive low down!

DOT = Dept Of Transport (in the Good Old U S Of A) The Higher the number following DOT then the higher the specification (or Boiling Point) DOT 2 – DOT5.1 (Please Note DOT5 spec fluid is silicone based more info on the shortly!)
If you look at your brake fluid reservoir on the top or nearby you’ll see “Use DOT3 fluid only” That will probably be on an older bike (like an 80’s model).
What it is actually saying is:- “That the minimum specification for brake fluid in this system should be DOT3” in reality You may in this instance top up the fluid with DOT3 or 4 OR change the fluid for DOT4 or DOT5.1 Note that is DOT five point one, each of these are higher specification than the original DOT3 fluid

Now All of these brake fluids are what are called Polyethylene Glycol esters basically they are made from a substance very like concentrated antifreeze!

Perhaps that is a clue to why Mechanics (or should I call us technicians these days) and manufacturers’ request that you should change your brake fluid every 2 years (earlier in Servo ABS systems)
No, not enough of a clue? Well read on, the fact is that antifreeze mixes very well with water, that’s its purpose, for the brake fluid to replenish the space that’s left as your pads wear down there must be an allowance for air to enter.
If, Air can get in? Then so too can water (carried as vapour in the air) water can also enter the system by a process known as diffusion where water creeps in through rubber seals and hoses (this happens over a long time)

As the amount of absorbed water increases over a time period your clean and fresh “golden” coloured brake fluid will turn to a brown contaminated liquid that’s is not very good at dealing with the high temperatures created whilst braking, bear in mind the difference in efficiency shown for just 3% of water absorbed (wet boiling point)

...........Dry Boiling Point..................Wet Boiling Point (i.e. 3% water)

DOT 3............205ºC ...............................140ºC

DOT 4 ............230ºC ..............................155ºC

DOT 5 ..........260ºC ...............................180ºC

DOT 5.1 .........270ºC ..............................190ºC

Also the brake fluid develops slightly corrosive tendencies’ and your systems seals will degrade!

So folks now you understand why we ask that you change fluids etc every 2 years!

It is Not because we get more money from you instead it is the simple fact the fluid doesn’t do it’s job anymore!




Now back to DOT5 spec This fluid is a silicone based brake fluid and under NO circumstances should it be used in a Standard DOT2, 3, 4 or 5.1 system. Or mixed with any of these Brake fluids) AN agreement was reached that silicone brake fluid be coloured “Purple” to prevent mixing with standard “non silicone” brake fluids
A few people are probably thinking “That is great! If I use silicone fluid in my bike it can’t absorb water and I’ll have good brakes!”

Wrong for the following reasons

Silicone based brake fluid is ONLY half as resistant to compression as Polyethylene gycol Hence you will experience longer lever or pedal travel to gain the same brake effort! In other words your brakes will be spongy! ALL of the time! Another note Some Harleys use this stuff as standard Need I say more?

Silicone Brake fluid will NOT absorb water under any circumstance; the water therefore must collect at the lowest point as it is more dense than the silicone. The lowest point is your calliper.

Water boils at 100ºC, the temperatures that can be created by a long drawn out braking manoeuvre will exceed that 100ºC quite dramatically so the water turns to steam and further reduces your braking capabilities.

In fact they will probably not work at all!!

Never Ever use silicone brake fluid in a braking system that it is not specified for! The silicone used attacks the standard seals used in a regular Polyethylene glycol system but at least it’s purple and you will see it leaking out??

 

[Johnny Davies]

Warped discs

FACT - 1200 GS discs DO WARP....FACT. You don't need to be a GP, WSB, BSB or road rider or other to find this out. I would challenge anyone here to prove they were above this!!! The brakes are not set out right... I'm a brake engineer by training/job and things ain't right! Disc's aren't full floating, strength is altered via servo's etc. This is a good example of guinea pigging - they don't know what is really right and will find out in time.

 

[Steptoe]

They are designed to take extreme heat differentials - besides in the scenario you suggest above the pads would in fact act as a heat sink and help dissapate heat energy.

So whats the special design in a flat piece of round metal/alloy that makes it behave differently .

And why do brake discs warp on cars when parked up with the handbrake on after being used on a track. ? Life is full of mysteries. I'll die before i know all the answers.

Oh, and it's spelt dissipate

 

[cookie]

can i refer you lot back to my post (no. 2). if this is true, then surely that is an admission that the old design was not right?

perhaps someone who has warped discs could make the relevant enquiries with a dealer.

mine are not warped, and so i can't be arsed

 

[DrFarkoff]

Oh, and it's spelt dissipate

Pedant !!!

 

[capt geeze a five]

So whats the special design in a flat piece of round metal/alloy that makes it behave differently .

And why do brake discs warp on cars when parked up with the handbrake on after being used on a track. ? Life is full of mysteries. I'll die before i know all the answers.

Oh, and it's spelt dissipate

Hi steptoe,
after a car has been used on a track, if you pull in and stop right away,and use the handbrake (handy to stop the car roll away) the brake pad can stick to the very hot disc,it can happen to the front too, so what you get is the brake pad melting (part) and sticking to the disc, then you get the a judder under braking,
so if you can, the best thing to do is have a slow lap, to let the brakes cool, if not then just drive about the pits slowly to do the same,
hope this helps.
capt.

 

[Barleymo]

Just returned from a trip to Morocco, but whilst in Spain manager to warp my front disc on the hairpin's round Ronda, the bike is out of warranty so BMW wanted something like £145 per disc.........ye right, replaced them with some from stealthproducts in Sheffield, £106 the pair. Stuck some EBC HH's in the front as well, Perfect, better than the BM setup, good initial bite, strong and progressive.

 

[Owen Snell]

There are a lot of possible reasons for warped brake discs, apart from them simply being manufactured incorrectly from new.

Excessive heat build up in the disc can cause warping, and this can be due to inadequate design or simply a worn disc - the thinner the disc, the less heat absorbtion capacity and so the heat builds up faster during braking.

Poor maintenance on a sliding calliper (or a floating disc) means that the braking load is not applied evenly on each side of the disc, effectively bending the disc every time the brakes are applied. This can result in a warped brake disc. The sliding calliper can seize or stick due to build up of road crud or corrosion, and this problem is made worse when the brakes are normally lightly used. Heavy brake application tends to free the calliper before it actually seizes, although it will not stop the problem in the end.

The most common reason for warped discs is holding the vehicle on the brakes when stopped at traffic lights / junctions. The disc and pads are all hot after braking and if the pads are held onto the disc when stopped, that part of the disc cannot cool down. The difference in heat expansion between the cool and hot parts of the disc causes warping. Since the front brake does most work on both cars and bikes, the discs are hotter and more prone to warping. The best option is to hold the bike with the rear brake / car with the handbrake so that the front discs are not clamped by the pads. Also, it makes sense (where possible / safe) to move the bike slightly when at lights, to allow more even cooling on front and rear brakes. A small movement will be enough to move the pad contact point on the disc for this purpose.

Finally, using any type of sand / emery / garnet paper on a disc will not sort out a warped disc. The material removal will be minimal and since your hand will follow the warp of the disc as it moves, it will not change the profile. The only option is to skim the disc on a lathe so that it is completely true. However, this is only possible if there is enough material left after skimming so that the disc is still above the manufacturer's minimum thickness. Also, some types of disc may not be suitable for skimming at all, so check with BMW before attempting to machine the disc in any way. In any case the cost of skimming may be almost as much as new replacement discs.

 

[bustupbiker]

**Seems’ some people on the Adventure Rider forum have ‘sanded’ there discs with abrasive paper to remove residual brake pad material and fixed the problem completely.**


When i were a lad i worked for a Triumph/Rover dealer in Brisol.
Triumph used to have blocks of abrasive stuff that we had to fit in place of the pads on major services, we used to run the wheels up on the old wheel
balancer machine and stick your foot on the brake, this was to remove the glaze and crap from the discs....glazed disc will not (disipate) nother one for you Steptoe... the heat...Well thats what we were told in them thar days.

 

[Barleymo]

its funny though that every bike i have owned since i was 16 has had a front disc or two, i haven't changed my riding style at all in all these years, and i have ridden some bikes one hell of a lot harder and faster that the GS, yet this is the first time i have ever experienced a warped disc on a bike. I have had then warp on cars before but like i say never on a bike. Makes you wonder why BM have altered the design for the new GS!!!