front shock fluids.

[Lordcargo]

What does everybody use. It's time for a fluid change and wondered what's available in the US.

[Tony Smith]

Whatever you use make sure that it is at least the same viscosity as the OEM fluid.

If someone tells you that you should use XYZ fluid because it is 5/10/15 or whatever weight - walk away, they do not know what they are talking about.

Viscosity is measured in Centistokes and has bugger all to do with the weight - as an easily understood comparision, cream is much, much more viscous than milk, yet it is lighter and floats on top of milk.

Also, if you have the BMW red dampers in the bottom of your forks do not use Belray of any peruasion - their secret special ingedient "seal swell" will destroy the red damper quicker than you can say "oh bugger I wish I hadn't done that".

No without lecturing you on the viscosity of various fork fluids (you can find any number of websites to do that!) I'll tell you that Castrol Fork5 is as near as dammit to the BMW OEM fluid - so to is Dexron II/III (not Dexron II/III compatible) auto trans fluid.

But unless you live in a country with billard table roads and you are 75lbs wrining wet, that fluid (and Castrol 5) is too light.

I weigh a classified amount (my wife reads this board) but let's say I admit to being North of 125kg. I use a 50/50 mix of Castrol Fork 5 and Fork 10.

Occasionally you will see bottles of Castrol fork 7.5 - do not be tempted as the mix of 5 and 10 is in fact slightly more viscous than 7.5 - go figure.

[Bob_Roller]

OEM BMW fluid is 7.5 w .
Readily available at Iron Horse BMW in Tucson .

[Barry]

Tony is correct. The nominal weight is a very poor guide not least because it's based on viscosity at 100 deg C which is not where forks operate. 40 deg C is closer at least in the UK. Years ago l produced this table for making a comparison and found that oil close to original BMW spec was best, but and this is a big but, only after overhauling the forks and reducing leakage past the valve washer to make rebound damping work properly.  Rebound damping is very sensitive to even a one thou difference in valve washer bore so it's no surprise that different oils suit different bikes best.

[Tony Smith]

With respect Barry you are confusing the "pour point" determined weight of a fluid and the viscosity.

Pour point determined fluid weight is indeed determined by measuring the rate at which a fluid moves down an inclined plane at 100 degrees centigrade.

Viscosity on the the other hand is a measure of the force required to "sheer" a fluid.

Annoyingly and confusingly oil manufacturers use weight and viscosity interchangeably, they are wrong to do so as they are two entirley different things and are not necessarily inter-related.

This video is a good explanation of viscosity and how it is measured (in units called Centistokes)

https://www.cscscientific.com/csc-cientific-blog/bid/101944/viscosity-basics-for-those-of-us-who-aren-t-scientists


And lastly some charts of relative viscosity of fork fluids.

http://www.mediaturbo.com/clients/marzocchi/forkoilviscosity.html

https://transmoto.com.au/comparative-oil-weights-table/

[Barry]

Call the manufacturers labelling what you want Tony but the one thing everyone agrees on is that it's nonsense and should be ignored.

The only important thing for everyone to remember is you need to look at the viscosity at 40 Deg C as being the closest published figure you will be able to find to actual operational temperature.  If anyone thinks their oil operates at a higher temperature and I'm sure it does in hot climates then it's possible calculate the viscosity at any temperature provided you have the figure for viscosity index.

[Tony Smith]

Call the manufacturers labelling what you want Tony but the one thing everyone agrees on is that it's nonsense and should be ignored.

The only important thing for everyone to remember is you need to look at the viscosity at 40 Deg C as being the closest published figure you will be able to find to actual operational temperature.  If anyone thinks their oil operates at a higher temperature and I'm sure it does in hot climates then it's possible calculate the viscosity at any temperature provided you have the figure for viscosity index.

Barry, we can certainly agree that using "weight" to select or compare fork fluids ( or shock oils for that matter) is a waste of time.


To accurately compare different fluids the viscosity measured in Centistokes is what you compare, and yes I agree that 40 degrees Centigrade is the relevant temperature.


The biggest problem is that few fluid manufacturers go to the trouble of quoting ther fluid's viscosity and instead quote "weight". Hence the need to consult websites such as the ones posted in my previous post.