What is the standard front fork oil viscosity?

[MrMogensen]

What is the standard front fork oil viscosity, and how often should it be changed?...

I just got my FZ6 (not S2) back from the local repairshop. I wanted a harder front suspension (tired of front end diving deep down while braking etc.)...
The boss said that for about 420$ he would put in Öhlins springs + flush out old oil and pour in a heavier/thicker oil.

When I came to pick up the bike the bill was suddently only 300$ and I immediately thought wtf did they NOT do? Turns out the mechanic only pulled out the standard springs, dropped in Öhlins and topped up with a bit of oil to standard level (a bit of oil got pulled out along with the standard springs).
The explanation was that the mechanic said the oil was fine and he didn't believe it needed to be changed. Fork oil would last a long long time he said.

I took the bike for a ride and honestly unsure if I feel any difference... it does feel a bit more firm perhaps but I am not sure if there is much diference in the nose dive during braking.

The boss whom I made the deal with wasn't there but I have just recieved an E-mail from him because I wrote that I wanted to have the front fork oil changed to a heavier type and pay no more than a total of 420$.
Boss wrote that the mechanic had checked if my fork would work with a thicker oil and the answer was no! He wrote that my bike has got SAE10W and at first he thought it would have been 5 or 7,5. Also he wrote that 15 would be way to thick because valve in the fork leg is too small to handle 15 and therefore the fork wouldn't work very well...

He hoped I could live with my bike as it it... otherwise I could come back and he could take it for a testrun... But he is just going to say the same thing I bet...

I had the bike for 2 years and never changed the front fork oil. I doubt the previous owner who had it for 2 years aswell ever did it!? So I really have to doubt that the front fork oil is really 10W... or maybe it's too old?... I am pretty sure that I have read other posts from people in here who has got 15W in the front fork without problems?

Question is... are they $%#& me over because he is too lazy to do the job?
The mechanic asked me if I wanted the old springs with me so at least I know they actually changed them!

 

[bd43]

This is my understanding of front fork suspension, just having gone through a R6 fork mod. All front suspension dive under braking to some extent. The spring and its rate provide the force necessary to counter-act things like braking. The oil weight does not but controls the rate at which the fork compresses and the rebounds.

One thing you can check to see if the springs are right for you is check the static sag of the fork. It should be around the 25% mark of full travel. Full travel as I measured it a while ago under this thread is around 5". If you are sagging more than that, your springs are not stiff enough. This is why Ohlins typically will ask the weight of the rider and the bike to provide the proper spring. The R6 has the capability to fine tune the sag with the pre-loader adjustment.

Yamaha recommends their Suspension Oil 01 which told by my dealership, and if you Google around, is 5Wt. When I changed out the oil on my R6 fork, I put in 5Wt. In your case, you cannot adjust the valving, therefore moving to a different weight of oil will not only affect compression, but rebound which may not be what you want. I personally want the rebound to be a little quicker so the front wheel will track the ground better. Again, the R6 fork gives you the option to adjust valving on compression and rebound independantly.

These are just my opinions on the matter. Hope that helps.

Edit: One other thing I should point out, under your extreme riding conditions, the fork should reach near or at full travel. A zip tie on one of the tubes will mark how far it has travelled. Going to a stiffer spring might help your diving issue, but suffer on the full travel suspension.

 

[MrMogensen]

Thank you very much for that long post... lots of interesting reading!

Yes I have to check the sag... !

... really annoying the part about that I might have 10W in my fork (or not?)... but I guess they have checket that in some way?

 

[kawwikid]

I have Racetech 1.0 springs in my forks, and im running 15wt oil at 467ml per fork. The ride vs. stock is night and day. First off, im kind of a big guy (about 280 with all my gear on) so the stock spring and valving was nowhere near what I needed. The pre-load can be adjusted on the stock forks, but its a bit of work to do. There's a spacer on the top of the spring in each fork, by increasing or shorting that spacer you can change the rate of pre-load on the spring, which will change the rate of sag. By determine the amount of sag you have now, a good tech (theres the key) would know how much you would need to add or take out of the spacer to get the sag right..As for the 15wt oil. I have had it in my bike for 1500 miles now, and haven't run into a problem at all. The compression feels a lot better, but the rebound stroke is a bit on the slower side. On most roads and bumps you wont notice it. But on really big bumps, or pot holes it can make the bike it hard on the next bump if theres more than one. But the upside is the bike is super stable, and the faster you ride the better it gets. Its a good change for the money, but not as good as going to the R6s forks...

Sean

 

[MrMogensen]

Sweet! I always slow way down then riding over bumps - especially when I have got my girlfriend on the back, so the "rebound-problem" shouldn't bother me that much.

Right now my bike is having a regular service done in Malmö/Sweden (drove it right over the bridge between Copenhagen and Malmö...
When I pick up the bike after the weekend I'll have a chat with them about 15 oil in my fork and see what they say... perhaps they are more willing than that other shop

 

[Bikebiz]

The stock oil is Yamaha 01, which is equivalent to 7.5 weight. They could have at least drained them and used 10w. If you feel no difference I'd be insisting they do this.

 

[MrMogensen]

Bikebiz>>>

The guy claimed I already had 10w in the fork... I don't know how to argue with that, but I simply do not understand why he didn't just flush/drain it out and pour in new 10w... I don't know how they "measured" how thick the oil is and the mechanic just said it "looked fresh"... I don't know how to argue with that... I at least paid less than expected.
I do feel difference but it's small - new springs AND new oil would surely make more of a difference.

I'll see what the Swedish Yamaha shop says in 1-2 days.

 

[RJ2112]

Bikebiz>>>

The guy claimed I already had 10w in the fork... I don't know how to argue with that, but I simply do not understand why he didn't just flush/drain it out and pour in new 10w... I don't know how they "measured" how thick the oil is and the mechanic just said it "looked fresh"... I don't know how to argue with that... I at least paid less than expected.
I do feel difference but it's small - new springs AND new oil would surely make more of a difference.

I'll see what the Swedish Yamaha shop says in 1-2 days.

I don't recall the MOM requirements for changing out the fork oil..... I know it's listed there.

Telling the difference between 5, 7.5, and 15 wt oil is not that easy.

The very basic design of the FZ6 fork makes oil weight and fork spring 'strength' the only two changes you can make easily.

I happen to dislike the OEM dual rate springs intensely. Too much 'dive' when you hit the brakes. That is due to the design of the spring. One section is wound at a different angle than the other. If you were to look at the OEM spring from the side, it is clearly visible. The coils with tighter spacing, are the ones that give the softer 'feel'. That part compresses more easily.

The stiffer section doesn't start working until the softer part is consumed. The fork dives 3 inches or more, before that starts happening. Too much, for me personally.

As there are two distinct spring rates, and the oil in the fork controls both compression and rebound damping...... at best it is a compromise between solid performance, and soft ride.

Going to single rate springs allows a better match between the damping and the spring rate.

In that circumstance, going to a stiffer spring will reduce the dive you experience when you use the brakes. It should not affect the day to day 'feel' of the bike, if you get ones that are not as stiff as 'race' rated units.

 

[bd43]

Probably the reason they didn't change the oil is the forks need to be removed from the bike and inverted to do it properly.

That takes a little bit of time, at least another hour of shop time.