On Mon, 26 Oct 2020 21:24:08 +0000, newshound
wrote:

snip

p.s. Whatabout what difference it might make if I put 5W oil in a
motorcycle fork as opposed to the recommended 10W?

I ask because one of the issues that most people suffer on motorbikes
is 'dive' under braking and unless your bike is fitted with any 'anti
dive' solutions (one of mine is), I don't think going between most of
the std weight oils used in such roles would impact that much (given
how much the suspension has to move 'normally' to give a reasonably
smooth ride)?

The one anti-dive system I had that really worked was on an MZ 250 with
a leading link fork.

I had a few step though mopeds and small motorcycles that were the
same. ;-)

The torque reaction on the shoe carrier plate
(can't remember the proper name for it)

Backplate?

made a huge difference. Contrast
that with the Aerial Leader / Arrow with their trailing link suspension,
I'm convinced that even with the tiny 6 inch drums the braking effect
was deliberately weakened to reduce dive.

Like fitting narrower tyres to stop a car from rolling over. ;-)

I don't know about modern anti dive systems, is there some cleverness?

All the active suspension and electronics.

The only passive effect I can see with a telescopic fork is that the
torque reaction will increases the loads on the plain bearings.

On my GPZ550 it has hydraulic anti-dive. When you operate the front
brake it actions a piston in a bypass valve on the fork legs that
prevents the damper oil going past the damper itself and so stops the
fork from diving. Also it has 'air assisted' front suspension (you
pump it up).

It also has a complex rear suspension linkage (Uni-trak) that provides
'anti squat' (the back sinking down on acceleration) so it was quite
technically advanced tool in it's day. ;-)

(On the
MZ you could actually feel the front lift when you braked hard).

Yeah, like the bikes I mentioned above. ;-)

Putting in a thinner oil will just reduce the damping a bit. But 10 to 5
is not a very big change.

I guess as with most things it's a compromise. For me it would be
right if it keep the front wheel running smoothly on the road (no
patter or other anomalies) whilst providing the best ride for the
rider?

I know all the design theory about EHL film
thicknesses, but (within limits) gearboxes are not actually all that
sensitive to oil viscosity because a thicker oil will run a bit warmer,
and that will drop the viscosity significantly.

Yeah.

The same is probably
true of fork oil, especially on bumpy terrain.

Oh, for sure, Paris-Dakar suspension units spontaneously combusting is
proof of that!

Whilst the UK roads are bad, they aren't quite that bad so I think
some of it can be a bit suck it and see (within some constraints).

It's funny, when I thought I was going to put the 2L Pinto engine in
the kitcar (instead of the 1300 Kent) I needed to replace a broken
road spring to took the opportunity to upgrade the struts for stronger
ones with heavier springs and damping. Then I changed my mind so had
some std tension springs made (with +1" lift to offset the extra
weight over the donor vehicle) but left the h/d dampers. So, if you
sit on the front in now sags as would be right for a comfortable ride
but you don't actually get one once moving because of the stiff
dampers. ;-(

I kept the springs and may return to them if / when I convert it to
electric. ;-)

Cheers, T i m