Changing fork oil.

[Bradders]

Am going to put fresh oil in my front forks over the winter. Not done this job before. Have watched various YouTube videos and some say to take the springs out while others leave them in. What’s the best thing to do?
Also some talk about an air gap and some don’t. Do I need to do that as well?

Thanks.

[McCabe-Thiele (Ted)]

When I did my 400 earlier this year the first lesson I learnt was do one side at a time.
Undo the top cap bolt, undo the drain plug, leave it to drain, I left mine overnight.
Re-fit the drain plug, ideally with a new washer, add the correct amount of oil for a top up by adding it to the top through  the center of the spring, I used a syringe.
Fit the cap back being careful not to cross thread it, tighten up, repeat on the other side. Don't worry about any air gap, it will settle down during standing or after a short couple of stops as the front forks compress.

[TrickyMicky]

My workshop manual is not to hand at present, but I seem to remember that it gave a figure for the amount of oil needed, can't remember if it was 300 or 330ml.
   I don't know if I've got this arse upwards or not, but if you brim fill the fork, as you cannot compress a liquid, what happens when the fork compresses?  Chance of popping the oil sael?

[McCabe-Thiele (Ted)]

It's 105 ml for changing fork oil, 125 ml after full strip down as per downloadable Honda Manual here.

Haynes manual shows larger quantities 145/150.

[TrickyMicky]

Well done Ted, thanks.  If it's true that confession is good for the soul, I have to admit that hundreds of years ago, when I changed my fork oil for the first ever time, I thought I would speed up the slow dribble of oil out of the drain plug. I held the front brake on and gave the handlebars a damn good downward thrust.  I never realised that such a small amount of oil could cover so much equipment on the far side of the garage!!!
     As it's supposed to be the time of good cheer, maybe a thread could be started allowing people to admit to unbelievable cock-ups they (we) have made in the past.

[Bradders]

Thanks Ted and Micky. Do I need to take the springs out at all?

[McCabe-Thiele (Ted)]

When I did my 400 fork oil change I removed both cap nuts, then  drained the oil. Consequence was both springs then pushed upwards making it much more difficult to get the first cap back on.

[McCabe-Thiele (Ted)]

No need to take the springs out, if you do for some reason make sure you put them back the right way round.

Don't ask which way round, was it the coil spacing or is that the valve springs?

[Bradders]

Thanks Ted. I will leave the springs in. Thanks for your advice.

[honda-san]

The genuine Honda shop manual needs to be read carefully as it covers both the 350/4 and 400/4.

400/4 takes 145 /150cc of oil per fork leg at oil change, 160/165cc if completely stripped (the 105 and 125 cc figures suggested previously are for the 350/4).

I have always (well, every 6000 miles over a total of 250,000) simply drained one leg at a time and re-filled with 145cc, and not worried about removing the spring nor about the air gap - just put the correct amount in.

As to which way round springs go, variable pitch springs should always be fitted with the closely spaced coils at the non-moving end. In the case of valve springs this means against the cylinder head to minimise the spring mass that is moving as the cam operates, and therefore to minimise inertia effects at very high rpm's which contribute to possible valve bounce. For fork springs it is much less important as the change in moving mass due to spring orientation pales into insignificance against the mass of the rest of the bike and rider which is going up and down. Still best practice though to fit the close coil end of the spring into the fork leg first.
FWIW, my day job for 35 years was as a design / test / calibration engineer at a company that provided consultancy services relating to spring technology.

[McCabe-Thiele (Ted)]

I can't see your figures in the Honda Manual P 51 shows 105/125 cc in the Honda manual do you have a page number that shows the higher volume?

My Haynes manual 1990 version  shows 145-150 cc
My Cycleserve manual P101 shows 145-150 cc.

[honda-san]

As I suggested, the manual does need to be read carefully as it covers both the 350/4 and 400/4.
Data specific to the 400/4 starts on page 87 in the "CB400F supplement" part of the manual.
Page 111 contains the relevant specifications. The Haynes manual is also correct in this respect as it covers only the 400/4.

[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.

[McCabe-Thiele (Ted)]

Thanks Chris, I'll check that out, when I drained my 400 forks there was hardly any oil in one side so I felt the benefit when I put 105ml in, I might add another 40 odd ml over this winter. That will be a 38% increase so worth the effort.

These were the answers I received in December 2023?

.http://www.sohc.co.uk/index.php/topic,30113.0.html

[Laverda Dave]

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.