[Closed] dot 4 or 5.1

just know that 5.1 has a higher boiling point

not always true (although is often the case)

a good dot4 can be higher boiling point than a cheep dot5.1

dot4 simply means glycol-ether (a water soluable chemical that has a very high bopiling point and gud lubricating properties, not too disimilar to sex lubes except you wouldnt war this on your skin! 😛 )

dot5 is silicone based (in laymans terms, like oil, but with silicone not carbon)

dot 5.1 has a higher boiling point than dot4, but is still glycol-ether based.

The standards define a minimum boiling point, theres nothing to stop you labeling something as dot4 when in fact it may meet the dot5.1 spec.

Dot5 (not to be confused with 5.1) can be used in some systems, but requires the right seals, and the system to be properly flushed. Its advantage is it doesnt absorb water (but does absorb air), and is fine on most paints.

Mineral oil (citoren, shimano, magura) is a whole other kettle of aquatic vertibrates.

[i]not always true (although is often the case)
a good dot4 can be higher boiling point than a cheep dot5.1 [/i]

That's rubbish! They're made to a standard, both 4 and 5.1 have to meet their respective minimum standards. As to price, they're all the same - if you're spending £££ on DOT fluid you've got more money than sense.

Re what to use - it doesn't particularly matter, nor does it matter if you mix 4 with 5.1 (you just end up with a lower boiling point).

That's rubbish! They're made to a standard

Calm down dear!

The standard defines minimum wet and dry boiling points and what the main components must be. The clue is when manufacturers state "exceeds specification xxxxxxxx"

n.b. there are also SAE spec on which the dot spec is based. Also dot spec specifies where in the world a fluid is intended for use (normal temp range, humitidy).

e.g if i make soemthing thats 90something percent glycol and boils at 300deg then theres isnt any reason not to market it as a dot4 or a dot 5.1 fluid.

Now pick up your toys and put them back in the pram please.

but does the increased boiling pt of 5.1 make it better than 4 for mountain bike use, or am i unlikey to reach the limits of dot 4 on the bike?

Your very unlikely to boil the fluid in modern brakes, the plastic pistons pretty much insulate the fluid form the pads. You'd see more of an effect in older brakes with metal pistons.

Your more likely to notice the fluid expanding under braking which is unavoidable and perfectly normal. If you managed to boil brakes the lever would lock completely out, you'd probably blow a few seals, and the whole lot would need re-building. Unless this happens to you regulalry I wouldn't worry one way or the other. Just buy fresh fluid every time, fluid in an unsealed container for a year is the worst type, regardless of what it says on the spec.

lakesrider,

use 5.1 DOT. It's ability to work at higher temperatures is advantageous as there is onlt a small amount of fluid in mtb disc brake systems - say 10ml.

As stated previously there are no problems with mixing of 4 and 5.1 DOT fluids so just flushing through when bleeding is all you need to do.

5.1 is just a newer version with bugs ironed out,

Sorry to interrupt the nerdfight... 😉

I noticed a definite improvement when I flushed out the dot4 and replaced with fresh 5.1 - whether that was down to the bleeding or the different fluid I couldn't say.

5.1 is just a newer version with bugs ironed out,

kinda, but its more like a better version of dot4, dot5 as I said earlier is completely different.

Just buy very small bottles of dot5.1 (or dot4), as I said your very unlikely to boil the fluid in modern MTB brakes anyway.

Any difference in feel is either because its bled better or a placebo effect.

Also 5.1 is more Hydroscopic than dot 4, so although initially dot 5.1 will have a higher boiling point initially, but the longer you leave it in the system the more water it'll absorption and thus lower its boiling point overtime. Which would be why 5.1 is more for performance as it's higher boiling point and needs to be flushed more regularly, where's 4 boils at a lower temperature and will stay at that more consistently...

The difference would be be of greater significance when disc brakes first came on the scene but now you won't notice much difference due to the systems being better sealed and dissipating heat better...

However I do just sit behind a keyboard all day so I'm probably waaay off!