Brake force one: water as hydraulic fluid?

[SA_SA_SA]

http://www.brakeforceone.de/en/bfo-h2o/

Does this pass the Brucey test or is it a mad idea?

[[XAP]Bob]

Heat will be the issue I suspect...
Water tends to have dissolved gasses, and gets less soluble with higher temperature...

I would therefore suspect that even if the system doesn't boil the small volume of water in the calliper then it will certainly provoke the release of dissolved gasses -> bubbles -> compressible...


What temperature do other brake callipers get to? Why is mineral oil used?

[Brucey]

is it April 1st?

Blimey. I hardly know where to begin.

As a brake fluid, pure water can be made to work; the pressures are not that high and much of the perceived compressibility is going to bein the hoses anyway. But, outside of an emergency, why use that as a fluid, when there are plenty of other (better) options?

Some problems;

- err... it freezes solid in winter. If your Caliper freezes it will push the pistons out and then break the caliper body in half. If the MC freezes solid it will also break in some way.

- seal lubrication; water is a rather poor lubricant in this context. In fact water is the best thing to add to rubber if you wish to cut it or grind it. I would expect the seals to wear prematurely vs other fluids.

- corrosion; water is quite reactive, you will have crevices in the system and any dissolved gases will help it along nicely.

- boiling performance; water boils at 100C (at sea level) and substantially less than this as you go higher in altitude. This would make it easily the worst brake fluid available. Whatever chemistry/corrosion is going on will be accelerated at temperature.

They say 'use 20% glycol in winter'. This isn't so mad, there are good additive packages for anti-freeze that reduce corrosion etc. But I say 'why not use 100% glycol?' After all that is in essence what DOT 3/4/5.1 brake fluids are.

If this lot stay in business more than a year the inescapable conclusion will have to be that there is more than one born every minute; 600 euros a go for a brake that is in real danger of failing spontaneously (due to fluid choice) is just bonkers.

cheers

[[XAP]Bob]

Given the current mild winter I'm going to forgive myself for not thinking about the freezing point....

D'Oh...

[SA_SA_SA]

I had presumed it was a closed system hence the temperature could rise above 100 (superheated water?). Maybe not then.

[Brucey]

I had presumed it was a closed system hence the temperature could rise above 100 (superheated water?). Maybe not then.

If you have ever had a normal (car, motorcycle, etc) brake boil up it is painfully evident that, when the brake is off, the pressure inside the system should be 'one atmosphere'. If this were not the case, the pistons would be on the move!

What usually happens is that you use the brakes hard, they get hot, you come off the brakes, intending to reapply them, and then when you do, a few seconds later, there is nothing there. Whilst the brakes were off, two things happened; more heat (from the last braking effort) ran through the system, heating up more fluid, and the pressure dropped to one atmosphere. Thus the fluid can spontaneously boil when you are off the brakes; the now-surplus fluid is burped out of the compensation port into the reservoir, leaving a compressible mixture of fluid and gas in the brake line.

In the case of the system here, it is a 'closed system'; this will behave slightly differently. There is nowhere for the fluid to go when it boils, and the brake will just jam on. If there is enough dissolved gas in the system, the brake may start to drag when it gets warm, well before the system actually boils.

cheers

[Roadster]

Blimey, 600 Euros... I'd expect a drag parachute to be included at that price!

[hufty]

A few years ago there was web chatter from trials bike riders running their (closed system) Magura HS33s with water...
http://www.trials-forum.co.uk/topic/162 ... d-maguras/

[hufty]

...and having had a better look, I hope this is the beginning of a closed system revival. I'd go for pistons that retract properly over auto-adjusting ones any day. Nothing worse than that scything sound that develops half way through a ride because the tolerances are (imho) too fine.

[Vorpal]

I work with glycol / water mixtures, and I wouldn't use a 20 / 80 mixture. Maybe the other way round.

Two reasons...
Glycol does almost nothing for the boiling point at that percentage. It needs to be more than 50%, or under pressure. Glycol absorbs water from the environment (rule of thumb is 3% per year), and as little as 5% can cause the boiling point to go down 40 or 50 %. That is the biggest reason that manufacturers recommend changing brake fluid every year or two.

Glycol has different heat transfer qualities. Water or an 80 water mixture will absorb 10 or 20 percent more heat than glycol, increasing the liklihood of boiling. Closed systems also don't dissipate heat as well.

[[XAP]Bob]

I had presumed it was a closed system hence the temperature could rise above 100 (superheated water?). Maybe not then.

If you have ever had a normal (car, motorcycle, etc) brake boil up it is painfully evident that, when the brake is off, the pressure inside the system should be 'one atmosphere'. If this were not the case, the pistons would be on the move!

What usually happens is that you use the brakes hard, they get hot, you come off the brakes, intending to reapply them, and then when you do, a few seconds later, there is nothing there. Whilst the brakes were off, two things happened; more heat (from the last braking effort) ran through the system, heating up more fluid, and the pressure dropped to one atmosphere. Thus the fluid can spontaneously boil when you are off the brakes; the now-surplus fluid is burped out of the compensation port into the reservoir, leaving a compressible mixture of fluid and gas in the brake line.

In the case of the system here, it is a 'closed system'; this will behave slightly differently. There is nowhere for the fluid to go when it boils, and the brake will just jam on. If there is enough dissolved gas in the system, the brake may start to drag when it gets warm, well before the system actually boils.

cheers

And that drag will add more heat....

[Bmblbzzz]

On a comparatively minor point, they say that water is not environmentally harmful if spilled, eg in a crash, compared to brake fluid; but then they recommend you add glycol...

[Stewart H]

If I were designing a "green" system then vegetable oil would probably present fewer difficulties.

[Bmblbzzz]

If I were designing a "green" system then vegetable oil would probably present fewer difficulties.

And if you were stranded in the wilderness you could always eat it!

[Brucey]

...and having had a better look, I hope this is the beginning of a closed system revival. I'd go for pistons that retract properly over auto-adjusting ones any day. Nothing worse than that scything sound that develops half way through a ride because the tolerances are (imho) too fine.

I think that you are correct in thinking that (for any given system MA/lever stroke) you will get a slightly greater running clearance betwixt pad and rotor with a 'closed system' vs an 'open system'. This is because there is always a little dead travel in the lever in an open system, so that nothing happens when you pull the lever until the compensation port is closed off. I guess this ought to be about 10-20% of the lever stroke (although it could be more), so if all else is equal, a similar variation in running clearance would be expected.

However as the brake is released, the forces that retract the pistons are similar in both systems; atmospheric pressure is the primary force that pushes the pistons back when the pressure in the system is reduced. With either system, sticky piston(s) can cause inadequate (or at least unbalanced) piston retraction.

The difference is that should the pistons simply be slow-moving, they are more likely to retract fully in a closed system; this happens because in a closed system, once the lever is released, there is still a driving force for retraction, whereas in an open system the compensation port may re-open when the pressure in the system is low and the pistons are not yet fully retracted. Once the compensation port is open, there is no driving force to further retract the pistons, barring 'seal rotation', which I do not think is a large component of overall piston retraction in bicycle disc brakes.

[Seal rotation is a specific design feature in disc brake calipers for cars and motorcycles, without which multi-piston systems would never work properly. I do not think it is so well exploited in most bicycle brakes, because the seals are not large enough in cross section to provide much movement. The way seal rotation works is that the piston seal elastically deforms under load into a parallelogram shape (because of the shape of the seal and the way it fits into its locating groove); once the pressure is reduced, the seal recovers its square(r) shape and this draws the piston back. A typical car brake will use a seal of about 5mm x5mm section, and this will give about 0.5mm piston retraction. Bicycle systems use seals of about 2mm x 2mm cross section, which ought to provide ~0.2mm piston retraction. I have not examined this aspect in detail; to get this much movement, all the usual design features in the caliper need to be implemented perfectly. It is also questionable whether this much movement is enough in many systems, because of the likely run-out in bicycle discs.]

I've used closed system brakes for many years and I think they are OK. However I also think that closed system brakes fell out of favour in good part because of the heat/self-locking issue; the slightest amount of air in the caliper is absolutely fatal in this respect, whereas in an open system it just makes the brake spongy.

One thing that I do find a bit weird is that it is possible to trade running clearance against system MA, yet bicycle disc brakes are always struggling for running clearance. This surprises me because if it were a fundamental problem, motorcycle brakes simply wouldn't ever work properly, or would need servo-assistance. Lever strokes and disc sizes are not so far different that they alone ought to be the explanation. My suspicion is that the running clearance can be less in a motorcycle brake; perhaps rubbing is better tolerated, but also I think motorcycle discs run a lot truer than bicycle discs, and the bearings, caliper support etc are all stiffer and less prone to movement.

So in bicycle brakes, I think there is scope for improving matters by
- having better quality discs (that run truer)
- having stiffer mountings for everything (and here through-axles are a good step)
- using better/larger seals for better piston retraction
- using a better MC design that has less dead stroke before the compensation port closes
- using better hub bearing designs that provide for an improved lateral location of the disc

cheers