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I am going to renew the clutch fluid on my r1100s 2004 could you advise me as to whether DOT 5 is ok as I have read that this may have a detrimental effect on the seals , the manual states DOT 4.Have no issue with buying this but I have a new bottle of DOT 5 so if it is def ok to use this I have it.Any thoughts as not sure as to the compatability,Regards Markwin.

DOT 5 or DOT 5.1?

That's a very small but very important detail.

5.1 is interchangeable with DOT 4, but DOT 5 is silicone based and really should only be used in a clean system built for silicone fluids (usually race cars / bikes - it's not really a road fluid).

Going to have to double check what I actually have , thought it was DOT5 and checked on the label that it was compatible with DOT 4 -so if it is dOT 5.1 there will be no issues with using this ? The safest option is to get some DOT4 and would have if I didn't have some (possibly!) compatible fluid.

If it's DOT 5.1, then it's completely interchangeable / compatible with DOT 4.

Thanks for the responses Twinspark , checked I have DOT5.1 and it is apparently compatible as you say , wouldn't have given another thought to using this but saw a thread that implied poss concerns with DOT5.1 and the seals , cannot find the thread again as I wanted to double check I had not misread it and was about using DOT5-a BIG no no ! Kind Regards Markwin.

What Wikipedia says.

http://en.wikipedia.org/wiki/DOT_5.1

No need to use Dot 5.1 for everyday use, it makes more sense if you are an occasional track day goer. It only raises the boiling point like 30degC. Dot 5 requires the replacement of all seals in the braking system. You are more likely to use this in a racing application where a higher boiling point is desirable.

I wait till after winter to change brake fluid because the bikes don't get much use.

Just to be clear I was not implying that I could take any advantage of the slightly higher boiling point of DOT 5.1 over DOT4 it was just that I had DOT 5.1 on my shelf.There are of course sometimes advantages of using an "upgraded" specification but you need to ask yourself is it merely a marketing ploy or a real world improvement ,like everything else in life look at what the angles are and don't just assume.

I spec. 5.1 for my MX5 and Alfa 75 - but that's because they both venture out on track days.

Not convinced you'd need it on a road bike - or even a track bike, given the better ventilation the braking system has on a bike.

my mountain bike uses DOT 5.1 - but there again, it needs it for the speed I get up to

Dot 5, avoid like the plague unless the system already uses it.

Dot 5.1, also non hydroscopic, that alone makes it worthwhile.
Should have near infinite shelf life and in a sealed and working system should never need changing.

Manfred wrote:Dot 5, avoid like the plague unless the system already uses it.

Dot 5.1, also non hydroscopic, that alone makes it worthwhile.
Should have near infinite shelf life and in a sealed and working system should never need changing.

Dot 5.1 is hygroscopic. In road systems this is a desirable property as it means water is absorbed. Although performance is degraded over time, the water does not collect in the system the way it can with dot 5 which is silicon and non hygroscopic.

"5.0 is silicon based. 5.1 is a synthetic fluid. If you mix 5.0 with any other fluid, the silicon precipitates out and blocks the lines. 5.1 mixes with other fluids like 4.0 with no problem (bit like mixing full synth oil with mineral oil).

5.1 is seal friendly. It doesn't absorb water, so you don't get water is the system at all, unlike other fluids which draw water from the atmosphere (hygroscopic). I've used it for years and NEVER had spongy brakes"

This is a quote from a race tech who also uses it in his own road bikes.

Dot 5.1 does absorb moisture. Dot 5 does not.

Herb wrote:

Manfred wrote:Dot 5, avoid like the plague unless the system already uses it.

Dot 5.1, also non hydroscopic, that alone makes it worthwhile.
Should have near infinite shelf life and in a sealed and working system should never need changing.

Dot 5.1 is hygroscopic. In road systems this is a desirable property as it means water is absorbed. Although performance is degraded over time, the water does not collect in the system the way it can with dot 5 which is silicon and non hygroscopic.

Hi herb. How does the water get in there in the first place, with dot5? Even if it did (which I doubt), it would be separate and therefore easy to bleed out, no?

I've read that lever return issues are the main reason why dot5 is not widespread?

You sure that hygroscopic is good? Doesn't sound it!

http://en.wikipedia.org/wiki/DOT_5.1

This bears out what herb was saying.

In essence, water is bad in all cases, but it if it HAS to be in there, it is better suspended in the brake fluid and not separate, it seems?

But how does water get into a system filled with dot5?