Forks......................

I'm curious as to how they work , well ,I know how ,but still have queries on a few things .
Springs keep the bike up and returns it to a standard position after it has been compressed buy a bump .
Fork oil travels through small holes at a measured rate to stop bike pogoing up and down too fast or not fast enough .
The valves the oil passes through are towards the bottom of the fork leg .
Lets say they are 200mm from the bottom of the fork.
The height of the fork oil in the tube would be approx. be 700mm .
500 mm above the valves .
With me so far ?

I'm thinking stuff all oil passes through the valves as they are designed to slow it down on compression and rebound .
This could be all done with 50mm+- of oil above the valves , in theory .
The problem with this is the bike will nose dive under braking as the very large air gap will compress way more than a fork filled with oil , so a oil height is
recommended to keep it all under control .

Finally the question.

And then ...

What's the question?

I have been playing around with fork oil height  and there is no way I can tell any difference with 50mm (oil fork height ) of oil being added or taken out of the fork leg .
That's 100mm difference .

You will see in the manual'

Naked bandit has 472 cc of oil

The S model has 473 cc of oil

TWO LOUSY UNMEARSURALE DROPS DIFFERANT .

And I can not tell the difference with 100mm difference .

I  think there is a question in there somewhere , but not sure what .

Ive been taking fork oil out to try and use more travel but its not happening .

With less oil are you going to get more suspension travel used up ,or does the spring over-ride it and you end up with the same travel but more dive under brakes ?

gus wrote:With less oil are you going to get more suspension travel used up ,or does the spring over-ride it and you end up with the same travel but more dive under brakes ?

I know the "basics" of shock and fork control, and got a good idea on what sag setting to have for what "I" want, but as for proper tuning, as in building one to do what I want, I lack the complete science there, as its a bit more complicated than just doing this and that, but from having them built over the years and talking to the one that has built all mine, compression damping control is the main factor in dive when braking.

and as I`m sure you know, you want to try and get as much travel as possible so your bike tracks the road, not skips or bounces, so the wheel has proper contact at all time, now this is where correct valving comes in to play with low and high speed damping control, and is where it gets complicated, to get exactly the feel I want.

there`s heaps of stuff on the net on bandits from what others have done and their remedies of control (fast larry`s for one example), but until you`ve ridden the bike for a few thousand K`s, you really don`t know  what you like .

the lad that does my stuff says you should never use more than 5w oil.  I always read DIY`s putting in 15w saying it fixes a problem, but when I put this forward to my bloke , he just shook his head and said  "DON`T DO THAT" the way to fix it is by CORRECT valve control.

gus wrote:With less oil are you going to get more suspension travel used up ,or does the spring over-ride it and you end up with the same travel but more dive under brakes ?

Not really.  This just what I think I have figured out over the years; take it with a grain of salt as I am definitely still learning.

I believe the air has two purposes; firstly it is required for clearance as the fork effectively shortens during its travel, and fluids are essentially incompressible.

Second, it acts as a kind of bump stop.  Every time the air volume halves, its pressure will double.  So if you start with say 120mm, then as the fork travels 60mm its pressure will go from atmospheric (14.5 psi) to 29, then after another 30mm of travel it will be 60psi (round figures) and so on, increasing exponentially as you approach the limit of travel.  Of course, in that last itty bit the pressure will be quite high and will tend to blow seals L.  But in a more real world example it will provide a supplementary spring force at the limit of travel and so help reduce bottoming.

In the long-past days of my youth, we did use 15, 20 and even weight heavier oils.  Over time oil volumes increased (in the day a dirt bike might take 200cc per leg, now it will be around a litre), and viscosities decreased.  My guess is that heavier oils have a greater tendency to fade as they heat up in use, and as well are more susceptible to viscosity break down during service (due to shearing).

Gus, you asked why only have just enough oil to cover the valves?  Good question, my guess is that in all but the very best (race) forks, there is no separation between the air and oil.  As the fork cycles up and down the air and oil are mixed into a frothy emulsion.  Having a lot of oil probably means that this emulsion foam stays well above the valving and so does not cause the substantial fading (remember what a drama this used to be) that would occur if it were passed through the damping valves.  As well, greater oil volumes give the heat somewhere to go.  Stick your hand on a dirt bike fork leg after it has been working and you will be surprised how hot it gets.

As I have said before, I see the Bandit problem as this.  Suzuki has built the forks with a very plush comfy setup.  This is achieved by having quite light springs (believe they are around 0.7 kg/mm) and a lot of (low speed) compression damping.  When you reset the forks springs to a more appropriate (for sporty riding) value (1.0 – 1.1 kg/mm) you now have a fork that is too stiff (right springs + too much damping).  You could put thinner oil in (say 2.5 wt), but you will now have too little rebound damping as the rebound was set (by the factory) for 0.7 kg/mm springs, plus you have just reduced the viscosity and have even less rebound damping.

So the proper solution is 1. Heavier springs, 2. Less compression damping, 3. More rebound damping.

That's my 2c.

Cheers

That's good ,the more I tried to un-complicate the system the more complicated it got .I checked the cable tie today and have got used travel to about
80% so that's good enough seeing I have to allow for touring (more weight)

Gus, I found a chart in the manual for the Ohlins forks that sums up the relationship between oil level and the air spring force.