motorbike fork questions

[Random13]

Hi,

I am trying to change my fork oil and want to do it on the bike .

I have purchased the fork oil but have the following questions:

I have been reading around on forums and for my bike the oil level is 92mm and capacity 490mm.
Does the level mean where the oil should go up to assuming there is a sign or something on the fork having numbers on it ?

re capacity , does it matter if I put it 500mm or 480 , in another word if it is not dead accurate ?


what is air gap on fork ?

re construction of fork , you have the cap on the top, the spring in the inner tube and then oil in the bottom or outer tube , is this correct ? any other parts in construction of fork ?

are there any spacers or washer in the tube ?

Is 15w fork oil too stiff in your opinion ?

many thanks

[Snod Blatter]

The oil level or air gap is usually the amount of distance you have inside the stanchion (shiny tube) from the top to where the oil starts, with the spring(s) removed and the fork collapsed onto its stop. Of course there are other ways you could measure this air gap too, so the best way is to follow the factory manual for your bike. You will have to measure this level yourself, no way you'd see down the stanchion for a mark even if there was one.. Which there never is.

You do need to be accurate with the oil level, an extra centimetre of oil (and a cm less of air gap) roughly equals an extra 10Nm of spring though of course this depends on the bike. The less air you have in there the less gas there is to spring around, so the harder the fork is. If you have just drained the oil you cannot be sure how much of the old oil is left in there so putting in the same amount of new oil into each fork may mean you have different total amounts and different air gaps, which can lead to odd handling - this is where the air gap measurement is useful instead.

There are more parts to a fork, some have a "damper rod" under the spring which controls oil flow through some holes, others have a "cartridge" which is a bunch of little discs that act as a valve, these tend to perform better. For a basic oil change you shouldn't have to mess with any of this though, it's all held in at the bottom by a sizeable allen bolt which you should leave alone!

There may well be a spacer and some washers in the stanchion as the spring rotates slightly as it compresses, which would wear the fork innards if a washer wasn't used either end. Whether a spacer or even a second smaller spring is used depends on your particular forks and how the manufacturer felt that day.

Hard to say if 15w is too much, what does the manual recommend?

[Teflon-Mike]

would help to know what bike it was.
As to 'accuracy'.. the qty/level of fork oil is pretty critical, but more so between each leg...
As to weight? Depends what's recommended; the the bike; use you put it to; whether your a fat git or a flyweight, whether you 'like' a firmer ride, or preffer it a it more comfy etc etc etc...
So stabbing in the dark offering answer's really.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[bikenut]

as mike says..........

find your bike on cmsnl and print off the exploded view of the fork and internals......does your fork have a dedicated drain hole or is it drained via a concealed allen bolt at the bottom ( wheel out ) ??

take lots of pics and how are the seals at the top??
____________________
nuts about bikes

[Kickstart]

Hi

Be careful with the fork oil weight. Personally I would just stick to standard grade oil unless there was a specific issue I was trying to fix.

The springs, and the amount of oil (hence the air gap above the fork) controls how far the forks compress. The viscosity of the fork oil controls how quickly the forks compress. So using thicker oil will still have the forks compress just as far just they will take longer to do it (and longer to uncompress afterwards). Use much thicker oil than standard and the forks won't react quickly enough to absorb bumps (holding the bars will be like holding a pneumatic drill), and when you brake for a corner the forks will compress but not extend quickly as you come off the brakes. Had this with the ZZR600 when we first got it and the handling was awful until the fork oil was put back to standard (although I have no idea what oil had been used in there).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Random13]

i am about 18.2 stone and may use the bike for track days , would you use the 10w oil then ?

[mentalboy]

[Random13]

cbr600

[Kickstart]

[Random13]

it keeps bottoming out , do you reckon it is the spring or oil ?

for instance if I am going on speed bumps it does bottom out and goes bang

[Kickstart]