Changing fork oil.

[McCabe-Thiele (Ted)]

Thanks Chris for the manual page information that confirms the oil volume as 145-150 ml for changing the fork oil.
That's a job this winter/spring for me to top mine up to the correct level by adding another 40-45 ml to each side.

I'm not familiar with the 350 front suspension, had me wondering if it has smaller front forks, different internal design or is the difference in oil volume related to the different weight of the two models.

Every day is a schoolday.

[K2-K6]

Interesting job honda-san, I'm fascinated by suspension and how it can best work.

A question though with "Still best practice though to fit the close coil end of the spring into the fork leg first." Isnt that the end that's moving with input and directly linked to the unsprung mass of wheel etc ? Therefore should be the other way up, with close coils and hence mass best placed at the sprung mass supported "static" end in top fork leg and under the cap.

Interested in views, as I've got the design of forks having damper rod bolted to fork lower casting and moving with the wheel, the top of the rod acting on the spring against the fork cap nut, or is the fork on 400 different in layout.

Agree that in this instance the variation in mass at either end is unlikely to be felt in much meaningful way.

Interesting point Nigel. How would that same logic apply to a valve spring where the closed end of the spring fits closest to the cylinder head? Would you therefore say the rocker is the 'moving' unsprung end just as the front wheel would be on the front suspension in which case I see your logic. Very odd when you think about it if the closed end in the front suspension goes in first and therefore being closer would take more force to compress whereas on a valve the closed end being closest to the cylinder head is designed to reduce valve bounce.

As I understand it, the variable rate spring doesn't "see" which end is compressing it, the softest part will always move first in whichever orientation. So that doesn't matter.

In valve springs, the mass of the moving part has more importance in valve train performance and benefits any reduction available for the moving component, as honda-san notes. so completely logical in putting the higher weight of spring end at the fixed position.

In suspension, it could be speculated either way of which bit is stationary vs mobile. Suspension convention has it that the ratio of unsprung mass vs sprung mass is at advantage the lighter the unsprung is in comparison to the other.
In other words, the wheel assembly if moved lighter gives better suspension performance.  This because the ideal damping rate to control the sprung mass movement (the vehicle itself) would get more compromised if that damping rate was needed to increase for control of the unsprung mass of wheel etc. A different compromise to valve spring application.

It could be more illustrated by saying the vehicle mass is static or intended to be, with the wheels etc moving up and down to follow the road errors.  Which, if perfectly done (impossible) would give a beautiful ride. The aim of any vehicle design is to lighten the unsprung in trying to move to this scenario,  making heaviest part of spring more ideal at the fork or shock top position. 

As honda-san notes though, this would be a very, very small effect. And especially so in terms of just how little difference there is from one end of the spring to the other in grams when compared to the bike components in many kilograms.

[K2-K6]

Oil quantity isn't absolutely critical, it just has to stay reasonably abovecthe highest point that the damper valve reaches in full travel. This to avoid it causing aeration etc. Some just dip a welding rod down the centre of the spring to gauge where it sits if uncertain of what's been left in there to skew volume.

Too much though, and it'll lock the leg hydraulically against the top as the excess oil has nowhere to go if it hits the cap before full travel is complete.

A very consistent automatic transmission fluid to consider is "LT71141" specification as its a fully synthetic long life demand oil with very stable viscosity characteristics for a flat performance curve.

I do like though the Motul fork oil products, offering various viscosity range and some experiences on here for the 500 fork have been judged successful when using lower viscosity spec to get good subtle road relevant bump absorbing characteristics.

[McCabe-Thiele (Ted)]

Although I used Fork Oil a benefit of ATF's is they usually contain plasticisers to keep seals supple.

I am not familiar with LT71141 ATF, most of my automatic vehicles have been Dexron based ATF's, there is a fully synthetic Dexron 6 ATF that you can buy for under £13.00 a litre .

[Bryanj]

Originally Honda specified engine oil for forks but changed to ATF as it does not "foam" as much so damping stays more consistent.
Now companies make specific oil for forks in differeing viscosities to alter damping.
As to emptying forks we use to take the bike outside in the yard, undo both drains and pump to see how far it would go, not advisable in an encosed garage filled with your own tools, the freezer and possibly washing machine with the bosses best clothes inside!!

[McCabe-Thiele (Ted)]

One drawback with ATF is they do not show the viscosity in easy to understand units like SAE 10.

[Martin6]

Irritatingly, there isn't a standard for fork oil viscosity. Fork oil weights can really vary between manufacturer. 40W Ohlins 1316, is the same as 15W Silkolene RSF15 fork oil.
https://motoiservices.com/en/suspension-oil-equivalence-chart/