On Jun 12, 3:13*pm, "frogliver" <f_gli...@nospam.com> wrote:
> Does anyone know what Honda meant by "use ATF" back in '79? You know, like
> was it a Dexron thing, or some other standard?
Automotive transmission engineers formulated different ATF's because
of problems inherent to automtive transmissions.
A motorcycle fork doesn't *have* those problems.
Most riders don't know that a fork seal works like a one-way air valve
when the forks are stroking rapidly over rough pavement.
This traps air, which effectively increases the spring rate by adding
an unwanted air spring.
My 1968 Yamaha 250 Single Enduro had special air valves in the fork
caps that could be used to "burp" excess air trapped pressure.
Air also gets entrained in the fork oil as the fork strokes up and
down.
Also, remember that a damper rod fork is velocity sensitive because of
the rebound orifice holes limiting flow exponentially at higher
stroking speeds.
The Japanese motorcycle industry used the thinnest oil they could use
and still get rebound damping.
Maybe it was 5 weight, 10 weight at the maximum.
The motorcycle magazines accused Japanese motorcycle manufacturers of
using "fish oil" in their forks, and the motomavens would recommend
switching to an American brand of fork oil, which helped out the
aftermarket lubricant industry.
Unaware of the orifice flow limitations, American riders would install
15, 20, or even 30 weight oil in their forks and they got a very harsh
ride over sudden sharp-edged bumps.
Some American suspension "experts" started talking about air getting
trapped in the forks and entraining in the fork oil because they
didn't know that fixed orifices were so velocity sensitive,
An article in one of the magazines (Cycle?) during the late 1960's/
early 1970's involved filling up forks with various fork oils and
stroking them on a machine while measuring the damping curves.
Spikes in the damping curves were noted, and oil consistency was
questioned.
The series of tests was performed with a certain laboratory oil and
interested riders demanded to know where they could get that oil,
which was unavailable at a price that riders would be willing to pay.
The motomavens suggested using ATF as a consistently formulated
suspension oil, even though there were definitely differences in the
consistency from one manufacturer to another.
The ATF recommendation got back to Japanese engineers, so they adopted
the
suggestion, just as they adopted many other ideas which came from the
Southern California hot rodding community, where the enthusiast was
often working in the *movie industry* and, if they had a degree, it
was in *cinematography*!
The California hot rodders simply had more practical experience than
the Japanese engineers!
As S'mee said, 10 weight fork oil will probably give you a baseline
feel and what you want to do is go out and ride your typical road and
see if you feel like the motorcycle controllable over undulating bumps
and that the fork doesn't feel harsh over sudden, smaller, sharp-edged
bumps.
If your wrists are getting tired and it feels like you're riding a
jackhammer, the oil weight is too heavy to flow consistently through
the orifices in the damper roads.
Race Tech (
www.racetech.com) has developed the adjustable Gold Valve
Emulator which makes a damper rod fork behave like a more modern
cartridge fork by blowing off excessive oil pressure on rapid
compression.
Maybe there's an emulator kit for your machine. Also, look up the
recommended fork oil weight on that website.
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