78E front fork question

[1fasttrucker]

Hi I bought a 78E 1100 basket case for $250 bucks first standered I have ever owned the front forks at the top of the fork have a big nut with a straight slot screw resesed down inside, are these forks air & oil forks?

Because my 79 special has a top that screws into the main tube , but the top has an air nipple to air up the fork , any wisdom would be awesome !!
Thanks
Ken

[skids]

There are three preload positions for the older standards and that is what that thing is for. Use a big flathead screwdriver and push it while turning a little. As I remember it, there is a pin that positions into notches ... kind of like the rear shocks.

[skids]

PS Those are not air assisted forks. I didn't know the 79 specials had the air forks...

[1fasttrucker]

PS Those are not air assisted forks. I didn't know the 79 specials had the air forks...

Thanks man Great Help

[bikerphil]

Hi I bought a 78E 1100 basket case for $250 bucks first standered I have ever owned the front forks at the top of the fork have a big nut with a straight slot screw resesed down inside, are these forks air & oil forks?

Yes they are air and oil forks. Above the oil inside the fork is a pocket of air which is compressed each time the fork is compressed. Adjusting the oil level higher will make the air pocket smaller and the air will compress more quickly giving a firmer suspension feel and less dive upon braking. The air is not adjustable from the outside of the fork like the 80-81 Standard models are.

[Eveready1100]

Err, Phil. They are straight oil forks. If there is no way of pressurising the air while in a static position (sitting), they are not air forks.
Yes, there is a bit of air in them when assembled (at atmospheric pressure) but it does nothing relating to the stiffness of the ride etc. That's what the springs do.
All that raising the oil level in the forks does, is overwhelms the valve assembly in the leg which makes the front end stiffer, not the best way to go about it as the forks may behave very badly (read dangerously) if ridden on a very bumpy bit of road.
Just put a thicker grade of fork oil in it, for them to work properly. Not recommended to overfill them.

[bikerphil]

Yep you're right, I was just trying to explain that there is some air inside the fork and it is not just filled with only oil.

[crazy steve]

They are straight oil forks. If there is no way of pressurising the air while in a static position (sitting), they are not air forks.
Yes, there is a bit of air in them when assembled (at atmospheric pressure) but it does nothing relating to the stiffness of the ride etc. That's what the springs do.
All that raising the oil level in the forks does, is overwhelms the valve assembly in the leg which makes the front end stiffer, not the best way to go about it as the forks may behave very badly (read dangerously) if ridden on a very bumpy bit of road.
Just put a thicker grade of fork oil in it, for them to work properly. Not recommended to overfill them.

I have to disagree. How much air is in there can make a big difference in how the forks perform. That's why the factory specifies a certain amount of oil; it's not the oil volume, it's the air volume that determine how the fork works to some degree.

Changing the oil viscosity only will affect the damping across all points in the forks operation. Oil in not 'compressable', i.e. it doesn't change volume under pressure. Air is compressable, and the larger the volume, the more you need to compress it before it reaches a given 'viscosity' or pressure. This is how air-assisted forks work; more pressure, they get stiffer, less, softer. You can do the same thing by adjusting the oil level in forks; more oil = less air, and vice-versa. That why leaking forks that are low on oil have so much brake dive; too much air. But you can 'tune' forks by adjusting the oil level either up or down. You can't get carried away, usually a variance of 10CC is about the biggest change you want to make, but a bit of playing with oil levels can pay dividends in handling. My personal experience is adding about 8CC to OEM XS standard forks will reduce brake dive and make them more responsive with a slight decrease in ride quality while 'cruising'. A worthwhile tradeoff for me....

[GLoweVA]

I love it when you gurus get into these engineering philosophy discussions about how the parts are designed to work.
It makes me really think about the parts and the possible symptoms when they don't work they way they're supposed to.

Too bad 1fasttrucker's question has already been answered. LOL!

[1fasttrucker]

just a thought

I thank all you folks for all the answers , i have put a 20 weight oil in my forks on my standerd havent rode it but I will see what happens !!thanks ken

[skids]

Thats heavy oil. I hope it works for you.

[DGXSER]

Oil in not 'compressable', i.e. it doesn't change volume under pressure.

Sorry Steve, but that is incorrect. The only truly incompressible is water. Oils are compressible, it just takes ALOT more pressure to compress it than air. So for the forks, only the air is being compressed, but that is because there is not enough pressure to compress the oil, not that it can not be compressed.

The way I understand the fork system to work, it is really a combination of compressing the air, the spring force, and the fluid movement in the damping rod of the fork. Each plays a part in how the fork operates. Changing any of them changes how the fork will react. One of the mods I have heard of for our forks is drilling out the holes in the damping rod, giving more fluid movement with less pressure. I believe that is done when changing springs to progressive or part of the Race Tech Emulators upgrade. On the later model forks, they added the compressed air. They also changed the damping rod for the compressed air forks. And they are different from the special models to the standard models when both are compressed air.

So it is a system, each part, air, oil type, oil quantity, and spring rate have an effect.

[Laptop_geek]

Sorry Steve, but that is incorrect. The only truly incompressible is water. Oils are compressible, it just takes ALOT more pressure to compress it than air. So for the forks, only the air is being compressed, but that is because there is not enough pressure to compress the oil, not that it can not be compressed.

Well, in actuality your also incorrect. Water will compress, any liquid will, solids will too. Also it doesn't take a lot of pressure to compress it, as soon as any force (or pressure in this case) is applied it will start to compress, the reason why we call them "incompressible" is because, when compared to gases, they don't compress very much under the same amount of pressure. For all practical purposes, most of the liquids we deal with may be thought of as "incompressible", I just wanted to clear that up.

The way I understand the fork system to work, it is really a combination of compressing the air, the spring force, and the fluid movement in the damping rod of the fork. Each plays a part in how the fork operates. Changing any of them changes how the fork will react. One of the mods I have heard of for our forks is drilling out the holes in the damping rod, giving more fluid movement with less pressure. I believe that is done when changing springs to progressive or part of the Race Tech Emulators upgrade. On the later model forks, they added the compressed air. They also changed the damping rod for the compressed air forks. And they are different from the special models to the standard models when both are compressed air.

So it is a system, each part, air, oil type, oil quantity, and spring rate have an effect.

This is very true, any time you change one component of the system, all of the others will be affected, its important to remember that when working on these types of systems.