Fork Bath Oils

Fork Bath OilsBath oil is the few cc (usually between 5 cc and 40 cc) which splashes around in the bottom of each fork leg and keeps the bushings lubricated so it all slides smoothly.

Background

It began in 2003 when Manitou jumped all their forks from grease lubrication to around 16 cc of "semi bath" oil in each lower leg.  Until then (and even many years past) other manufacturers were using fork oils.

From then and for the whole next decade (until Shockcraft started investigating) it used to be simple, but awful.  Manufacturer specific oils could be hard to find and were often of dubious quality.  Mechanics ended up using whatever they had lying around, usually fork oil.  It worked, but it didn't work that well.  Forks were sticky and just didn't slide nicely.  Thicker oils which lubricated better were sometimes available, but many of the seals back then weren't good enough to hold them in, especially in the cold.

Then fork manufacturers wised up.  DT Swiss started using a new fluid called Motorex Supergliss 100k.  Fox introduced a new fluid called 20 wt Gold with new SKF seals that could hold it in.  Rockshox introduced a new 0W30 bath oil a few years later.

Then SKF released a whole range of fork seals that could hold back the good thicker oils.  Game on.

At Shockcraft we began selling two oils bath which fit most uses:

  1. Motorex Supergliss 100K - an almost magical lube oil which slid better than anything else we had ever found.
  2. Motorex Fully Synthetic - a multigrade synthetic which was almost as slippery as Supergliss 100K in the warm, but flowed much better in the cold.  This was a close relation to the original Manitou Semi-Bath oil.

What Makes a Good Lube Oil?

There are three main factors involved in how good a lube oil is:

  1. Viscosity - Too thick and it slows the fork down.  Too thin and there's not enough oil film strength for part protection.

  2. Slipperiness - This is especially important to reduce stiction (static friction).  It is affected by viscosity and film thickness.

  3. Cling - The oil has to stay on bushings and stanchions.  It is also affected by viscosity.

What Else Matters?

There are two main factors involved in the application.

  1. Bushing Tightness - A looser fit allows more oil film and requires a thicker oil.

  2. Riding Temperature -  Because all oils get thicker in the cold and thinner in the warm.

In Search of the Best Bath Oil - Viscosity-Temperature Testing

After initial cold and hot test riding we designed and built a viscosity test rig that could be temperature controlled, allowing manufacturers' oils to be baselined and data points to be collected where there were none.  Oil manufacturers usually only publish 40°C and 100°C data points.  The results got very interesting.  We found errors in manufacturers data sheets, we found manufacturers using conservative place-holder data instead of real test data.  Their fluids were doing better than the numbers said.

Results

From our bath oil viscosity-temperature testing we've produced a graph of 8 different lube oils and their viscosity trend from 40°C down to 0°C.


Graph of bath oil viscosity

Recommendations

All this testing has enabled us to produce the nice chart below to show which lubricant works best based on temperature.

Chart of bath oil type vs temperature

Supergliss
oils slide better than anything else, but they do have a narrower temperature range.  We now have three grades to cover almost all applications.
Fully Synthetic oils have a wider useful temperature range but don't slide as well as Supergliss.
Fork Oil 10 wt is needed for forks with wiper seals not capable of dealing with the thicker and tackier Supergliss and Synthetic Oils.

For temperate conditions (i.e. NZ North Island) you can run Supergliss 100K year round with great results.

For wide ranging conditions (i.e. Central Otago) you can run Supergliss all year and put up with a slower fork in winter, or run Fully Synthetic all year round but will get better winter performance by switching to a Polar Synthetic.