B6 shocks

Post Sat Jan 31, 2015 9:32 pm

Hi, are these oil or gas?
Many thanks,

Post Sat Jan 31, 2015 9:38 pm

Post Sat Jan 31, 2015 9:54 pm

Thanks Kos,
no oil needed in strut housing upon reassembly then.

Cheers.

Post Sun Feb 01, 2015 7:52 pm

Yes, you should always put some oil in the strut housing, im sure BMW say it is to aid cooling, but it also helps to prevent corrosion.

Post Sun Feb 01, 2015 8:38 pm

Antifreeze

Not needed for cooling on bilstein as the bit that gets hot is at the top, but will stop corrosion .

Post Mon Feb 02, 2015 9:53 am

What about antifreeze?

Post Mon Feb 02, 2015 9:58 am

Surely antifreeze evaporates leaving nothing but oil won't.

Post Mon Feb 02, 2015 12:47 pm

Either neat glycol based anitfreeze or 5wt suspension oil.

Either will do a similar job of stopping corrosion, neither should dry up.

My preference is 5wt fork oil as I always have a supply.

With nothing in them the damper will quickly over heat when driving making the ride bouncy. (oil and air gets thinner in the damper).

Post Mon Feb 02, 2015 2:26 pm

Thanks for all the advice, I've got some motorcycle fork oil I can use.

Post Mon Feb 02, 2015 2:54 pm

HairyScreech wrote:
With nothing in them the damper will quickly over heat when driving making the ride bouncy. (oil and air gets thinner in the damper).

Wrong!

An increase in heat in a cylindrical tube of fixed volume/stroke will result in a higher pressure, in accordance with Charles' Law and Boyle's Law.

A cylinder containing a higher pressure substance/gas will require more work to compress it/change it's form.

This is similar to having a stiffer spring when compared to a cylinder with an up/down motion.

Therefore, the ride will get stiffer, and LESS BOUNCY.

Post Mon Feb 02, 2015 3:30 pm

Dampers fade when they overheat,
In the real world........

Post Tue Feb 03, 2015 11:54 am

You're right, dampers do fade when they overheat, but that's dependent on the type of damper in question.

The shear modulus of a spring decreases with an increase in temperature.
This will change the value of the spring rate

F = -kx

An increase in temperature will therefore result in a lower force required for compression, leading to a "softer" spring.

Compressed gas dampers follow and behave totally different to springs with an increase in temperature, in "theory" they should stiffen, but that's all dependent on the design and what they've been designed to do under different conditions.
The perfect damper would be one that behaves the same under all temperature conditions and extremes, and it's very possible to to make one which changes and adjusts depending on the changes of temperature.

Post Tue Feb 03, 2015 1:06 pm

There are dampers that do this, the one I use on the downhill bike has a compensator system for the low speed rebound circuit, the adjustment rod for the orifice has a piece of polymer in it that expands to close the orifice with rising heat.

Does seem to work, the old one always began to get a bit "choppy" near the end of a really hard run due to heat if set up to feel right when cold, this seems to stay consistent.

Road car dampers are really pretty crude when you look at what's out there. (excluding expensive race stuff obviously)