Hydraulic Brakes for a tandem

[greyingbeard]

Maguras use Citroen LHM hydraulic fluid, which is different to ornery brake fluid.

[tim-b]

Hi

Both types of brake fluid are susceptible to water causing problems. The DOT fluid used in most cars is hygroscopic; it absorbs water which lowers its boiling point. Mineral oil fluids aren't hygroscopic, however water will pool below the oil (the oil is lighter) which has a lower boiling point than the oil

In the case of mineral oil, you probably won't see a change of appearance due to water ingress because the two stay separate

This is worth a read

Regards
tim-b

[Brucey]

The Magura fluid appears not to be identical to LHM; it has a different boiling point. However there are plenty of people who happily use LHM in systems that use proprietary mineral oils eg.

-Magura
-Shimano
-Tektro
-some Giant

with no reported ill effects. However the fluids are different (in name and appearance at least) so you should be aware that any warranty will be voided if LHM is used in place of the manufacturer's specified fluid.

cheers

[greyingbeard]

Im sure the instructions for my old maguras mention LHM. If I ever find them that is. They are in German, but then they would be as I bought them in a sale in Germany

[Brucey]

these (rim brake) instructions

http://www.magura.com/uploads/media/downloads/rim_brakes__09_E.pdf

mention the use of vegetable oil in an emergency , but also indicate that 'damage arising from....modifications...non-factory changes or improper service' will void the warranty.

LHM is not mentioned in this document. Nor is it in this one

http://www.magura.com/uploads/media/downloads/MARTA_09_E_01.pdf

for Marta disc brakes. In fact they don't mention any alternate fluids at all in this one; probably the fluid runs a lot hotter in a disc brake than in a rim brake as noted earlier in this thread.

cheers

[bensonboo]

FWIW pwa's experience is not at all unusual, even with the most expensive hydraulic brakes that are allegedly suitable for tandem use.

A tandem (a loaded tandem especially) is about the same weight as a motorcycle plus rider. Look at motorcycle brakes, and then ask what miracle of engineering allows hydraulic tandem disc brakes to work as well as that whilst being about 1/4 of the weight...

Basically there is no 'miracle' at work here; if you want hydraulic tandem disc brakes that are going to resist fading/boiling under all conditions then you would pretty much end up with something more like motorcycle brakes.

cheers

Equally, those same motorcycle brakes are designed for stopping said bike from possibly 150mph plus repeatedly. that's some going on a fully laden tandem!

[Brucey]

sure, and the energy in the bike at that speed is obviously huge. But then again so is the air cooling potential; the net result is that brakes on high performance machines used for racing do have design issues, but the brakes don't quite need to get bigger and heavier pro-rata with (say) the engine power.

I was thinking of a lighter weight motorcycle TBH; but even the slowest and most feeble motorcycle still needs to have brakes that will allow a safe descent of an alpine pass and it is the same deal as with a tandem; lots of energy being chucked into the brakes, poor air cooling (lowish speeds) and plenty of time for heat soak.

cheers

[reohn2]

Motorcycles have engine braking,a very useful addition and something bicycles don't have,so everything is on the brakes.
And as bicycles need to be light there's no heat sink either in the rotor or the calliper so the heat has to go somewhere if it can't dissipate into the air,that somewhere is the fluid.

[bensonboo]

Motorcycles have engine braking,a very useful addition and something bicycles don't have,so everything is on the brakes.
And as bicycles need to be light there's no heat sink either in the rotor or the calliper so the heat has to go somewhere if it can't dissipate into the air,that somewhere is the fluid.

There are calipers and rotors with heat sinks of sorts, although aimed at downhill racing at present. Scroll down to the rotors and caliper underneath them at the link below.

http://www.bikerumor.com/2012/04/18/shi ... ke-groups/

[DaveP]

That page won't load properly for me - I can see some of the top but it won't scroll down. Do you have any other references?

As its a holiday weekend here's a (probably very silly) idea that came to me a while back. How about cooling the whole thing down with a little spray of water? Easy enough to contrive, with a Heath Robinson anthology and existing garden sprayer technology, but would it be a sensible or a suicidal thing to try?
I've never really understood why disc brakes are better than others in wet conditions. Much would depend on this detail!

[reohn2]

....I've never really understood why disc brakes are better than others in wet conditions. Much would depend on this detail!

They don't get as wet in the same way rims do,they heat up and burn off water quickly,and aren't near the tyres and road surface where all the muck and water spray come from.

[reohn2]

Motorcycles have engine braking,a very useful addition and something bicycles don't have,so everything is on the brakes.
And as bicycles need to be light there's no heat sink either in the rotor or the calliper so the heat has to go somewhere if it can't dissipate into the air,that somewhere is the fluid.

There are calipers and rotors with heat sinks of sorts, although aimed at downhill racing at present. Scroll down to the rotors and caliper underneath them at the link below.

http://www.bikerumor.com/2012/04/18/shi ... ke-groups/

DH racing is like other racing,it's about getting from A to B as fast as possible,which means using brakes intermittently and as little as possible.
I've no doubt the calipers and rotors in the link are improved for what they're intended for,but tandems are a completely different kettle of fish.The weight is far heavier with tandem all up weights being around 50% more compared to solos,and method of riding and slowing very different to racing,especially on steep and or long descents.

The article mentions sandwich rotor construction,something which has proved inadequate for tandem use,I've seen photos of a sandwich rotor with the 'meat'(aluminium) melted through tandem use.

[Brucey]

even so, some disc brakes are very poor indeed when the discs are properly wet. I think this can be mitigated in some cases by using sintered pads; they seem less prone to this behaviour. But if it is bad, it is very bad; I would not use some disc brakes for urban commuting because there is a 1-2s delay before damp brakes dry out and come on strong; no problem in other types of riding but incredibly dangerous if you are trying to do an emergency stop; it will basically double your stopping distance from about 15-20 mph. If the brakes are affected in this way (not all are) then there will also be a situation where the rain/spray is so torrential that the brakes will just stop working because they are wet all time. This is rather less likely to happen on a bike with mudguards and relatively narrow tyres, but on an MTB with chunky tyres and no mudguards, rather more so.

I have owned and used bikes with disc brakes that were without reproach when dry, but were completely useless when wet.

The 'water-cooled disc' approach is fine but obviously you will run out of water at some point and then you just have the brakes as normal. They used this system in F1 years ago, and because the car's pre-race weight was measured with the water in the car, but the brakes needed to work well without water anyway (it would run out) the MO was to leave the pit lane with a tank full of water and then to dump it on the formation lap. This gave a lighter car (under the minimum weight once the water was gone) and this gave a better performance boost than the extra brake cooling did. The law of unintended consequences....

BTW the thermal shock of water cooling can affect very hot parts; if you were to cool a brake disc with a water mist it would be necessary to do it on both sides because otherwise the disc could eventually end up dished.

cheers

[reohn2]

Contrary to Brucey's experiences,I've never had problems with cable disc brakes(BB7 road,operated by Shimano STI's and Tektro RL530's).
I/we(tandem) have ridden some very wet rides in the past,in fact it's the consistent and reliable braking wet weather or dry,that sold me on discs,something rim brakes could never offer.
Other than an Arai drag,I've never used drum brakes.

In the short time I've had an MTB and ridden some really muddy/snowy/wet rides I've not had any problems with the Spyke discs on the Genesis Longitude either.

[Cunobelin]

Back to basics.

The power to the brake is determined by the cable. If the cable run is contorted, long and the cable flexible, then the cable outer flexes and absorbs the energy you apply at the lever.

There is a similar problem on some recumbents

I use the Nokon modular system as this allows tighter runs, and is stiffer than a standard cable outer the difference in performance on my BB7s was amazing