[solved]Fork oil - what weight do people recommend?

Hi,

I need to replace my fork oil - I will also be adding a little pre-load to the existing spring and I weight 90kg. I am a moderately quick road rider but by no means a racer.

What do you all think?

Frog

personally i use 20w in all my bikes but i do like hard suspension 10-15 is usually fine for road riding

looks like standard oil is 10w,I would go down the 15w route,it should be a good compromise

I use 20W which is very hard (I like it). Assume it had 10W before which was like mush. Maybe try 15W and see how it goes.

Essentially, I think everyone goes for 15W or 20W ie. thicker than the stock 10W.

If your fork oil is knackered (old) then you will noticed an improvement with almost *any* weight of new oil, just becuase it is new.

Amusingly there isn't any standardisation of fork oil viscosities, so a 15W from one manufacturer can be the same as 10W or 20W from another manufacturer, so if you don't like the result then it is worth experimenting a bit until you find what you like.

Thanks everyone - I'll go for 15w and see how it feels - can not be anything but an improvement!

Cheers

Frog

Ye my forks seem to be a bit soft! I'm new to the CB5 so not sure if this is the norm?

When the bike is sat on its wheels and i pull on the bars upwards, the stanchions extend some. so just under its own weight the forks are being compressed. Am i loosing travel with this?

Hi

this is called 'sag' and happens on all bikes - there will be specs for the sag on the CB500 somewhere but around 25% of total travel is normal with the rider on board I think - others may know better what the specific value should be.

Cheers
Frog

Cheers for that Frog.
Wonder if we can reduce this 'sag by using slightly more oil than recommended?
Think I want to change the oil as when i brake hard the front end dips massively and i don't like it doing that.

When I got my bike the suspension was mega mushy and the fork would bottom out with a clang if I hit bumps in the road. I bought 20W oil thinking to perk it up a bit, the seals were fine so I used the drain bolts at the bottom of the fork to minimise hassle. When I drained the oil there was barely any left in it!

Anyway, the new (and sufficient) oil worked wonders. No idea where the old stuff went, the seals weren't leaking (then). Since I've changed seals and another oil change, 20W again, works much better, the suspension is rock solid, the dive under braking only happens with hard braking, which is a lot better. Never grounds out now either.

Anyway, basically if you drain the oil you may find, as I did, that there just wasn't much left. Try 15W, 20W is rock hard if you really want it, but use the recommended ammount. Then if you want it even harder maybe top it up a tad, but take care as too much and you risk blowing the seals. You can also spacer the spring to stiffen it up a bit.

changing the oil weight will affect the compression and rebound, i.e. how fast the fork will compress and bounce back afterwards. All that is happening in the fork is that the oil is being forced through holes, the thinner the oil the faster it moves.


If you want to adjust the sag (how much the fork compresses with the weight of the bike) or race sag (how much it compresses with you and the bike) you can do it by adding or removing preload. In the forks that means changing the length of the spacer above the spring inside the fork.


Something else you can fiddle with a bit is the oil level in the fork. As the air left above the fork acts as a cushion and compresses, while oil will not compress. you can change the feel of the bike by adjusting the level a bit. You can also use a higher oil level to prevent a fork bottoming out, although that is really a lazy/cheap way of dealing with a spring that is too soft for your weight.


I would recommend getting a fork oil syringe like this:



This will let you be much more accurate when doing a fork oil change, and shouldn't set you back much more than a tenner.


If you just drain and refill using the bolts on the legs, or pouring in the top you have very little chance of getting the air gap/oil level correct. And that gap makes a great deal of difference to the feel. It also means you can be sure both forks are exactly the same which is also important.


One final thing is that you can generally mix different weight oils of the same brand (but check the bottle!). so if you want a bit harder than 10wt but a bit softer than 15wt, buy 500ml of each and use half and half.


Some of the above is a bit simplified, suspension is a big complex subject, so if you want any elaboration let me know

How many ml of oil is recommended for standard springs?

Hi

it best to gauge the amount by level rather than volume - the oil level should be 150mm for R ,W,X,Y models and 160 for V and 165mm for T.
Measured from top of stantion to oil with the fork fully compressed and the spring removed. Volume is 310cc +/- depending on model.

Frog

Yup, the volumes in the maunal are definatley a guideline, worry more about the level/air gap.


Its also important to "pump" the fork in and out when draining and refilling the fork, otherwise you wont get all of the oil out. About 20 pumps should get them totally dry/filled.