Topic: Another fork oil thread...short forks

[budman]

I'm getting close to building the front end for my 74' CB750.  I'm using CycleX 2" under forks, which are actually only 1" shorter than the stock forks that came off the bike.  I've searched and read a bunch of info on fork oil capacity, but I can't really find anything on what to do with shorter forks.  Any suggestions/recommendations?

[Trad]

Just put in the same amount per the manual. A little over filling isn't going to do any damage.

[NobleHops]

One of the operative things at play with the amount of fork oil used is the amount of air that remains at the top. This air is highly compressible and creates an "air spring" that is a big component in the suspension action and tuning along with the viscosity of the oil used. For this reason I personally think "distance from the top" is a more useful way to gauge and refer to oil volume than the amount of oil you pour in. 

EDIT 6 inches from the top (or ~140mm if you prefer) is a good rule of thumb for these old damper rod forks, and this measurement is normally made with the fork assembly fully compressed, with the spring and spacers removed. If you have a mityvac or similar, just put a mark 6 inches up the suction tube, overfill it slightly, put the suction tube 6 inches down the tube and Hoover out the excess.

If you find you'd like it a little firmer, try going to 5 inches, etc.

N.

Edited for accuracy, with thanks to PeWe for catching my mistake.

[Kenzo]

I'm in agreement with NobleHops, the air gap acts as a progressive spring.

I also read once about a suggestion to install Schrader Valves on the fork cap and running 5-10psi or so pressure like the XR650's use to...yet I don't really see anyone using them. What gives?

Cheers,
Kenzo

[seanbarney41]

I'm in agreement with NobleHops, the air gap acts as a progressive spring.

I also read once about a suggestion to install Schrader Valves on the fork cap and running 5-10psi or so pressure like the XR650's use to...yet I don't really see anyone using them. What gives?

Cheers,
Kenzo

it is an old school mod that helps stiffen up the front end a bit, pressure needs to be adjusted every couple hundred miles though with a low pressure hand pump.  New springs and modern fork oil in the correct weight is just better, and then there are things like cartridge emulators...the air caps are a noticeable improvement though if that is all you got

[bwaller]

Adding air, progressive springs, oils, all sorts of attempts at improving old suspension. First if there are signs of heavy wear it will be difficult to make the old like new again. Find the best components you can to start.

Simply from one guys perspective... the best solution I have found is straight weight spring suited to rider/bike weight. (only one example is RaceTech) These springs are shorter and allow for proper length pre load spacers. Then oil viscosity (I use 15W oil) for damping rate and oil level (which determines total fork travel) are next items to deal with. You want to use as much travel as possible without bottoming. Adding emulators and the tuning they allow (emulator spring rate and settings) give additional fine tuning.

Ideally under rider weight you want +-1/3 of total travel in static sag, then this allows suspension travel, for both compression & rebound. Simple principal but it takes a little work to get right.  There are many good articles online for tuning older suspensions, inform yourself before starting. Good Luck, the results can be rewarding.

[budman]

Thanks for all the input.

My thinking is along the lines of what NobleHops described.  I was thinking the air volume left at the top of the short tube should remain the same as a stock tube.  The manual supplement states 5.3 ~ 5.5 oz. of oil.  I probably do some math for grins and calculate what an inch of inner fork tube volume converts to in ounces, and subtract that from what the manual says.  Then I'll see what kind of measurement I get from the top of the tube.

[budman]

The fork tube ID is 1.065", so (0.5325"sq. x 3.1416) x 1" = 0.89 cubic inches.
0.9 CI = 0.5 oz. or 15 cc.
5.3 ~ 5.5 oz. - 0.5 oz. = 4.8 ~ 5.0 oz. of fluid per fork tube.

That amount should keep the fork oil the same distance from the top of a standard length tube.

[PeWe]

I filled my fork totally collapsed with no springs in. Oil level 140mm from top.
According to recommendation from the spring manufacturer of my progressive springs. There is a link in attached post
http://forums.sohc4.net/index.php/topic,123278.msg1399249.html#msg1399249

[NobleHops]

I filled my fork totally collapsed with no springs in. Oil level 140mm from top.
According to recommendation from the spring manufacturer of my progressive springs. There is a link in attached post
http://forums.sohc4.net/index.php/topic,123278.msg1399249.html#msg1399249

This is more correct, I am very sorry, I should not have written my post saying 100mm from memory. 140mm (or ~6 inches, if you will) is closer to ideal for a starting point. Thanks for speaking up PeWe. I'm gonna go back and edit my post just for accuracy, so I don't lead someone astray.

[budman]

I filled my fork totally collapsed with no springs in. Oil level 140mm from top.
According to recommendation from the spring manufacturer of my progressive springs. There is a link in attached post
http://forums.sohc4.net/index.php/topic,123278.msg1399249.html#msg1399249

Thanks for the info.  I'll fill the tubes with 5oz of fork oil and see how close to I am to 140mm.

[MCRider]

I'm in agreement with NobleHops, the air gap acts as a progressive spring.

I also read once about a suggestion to install Schrader Valves on the fork cap and running 5-10psi or so pressure like the XR650's use to...yet I don't really see anyone using them. What gives?

Cheers,
Kenzo

it is an old school mod that helps stiffen up the front end a bit, pressure needs to be adjusted every couple hundred miles though with a low pressure hand pump.  New springs and modern fork oil in the correct weight is just better, and then there are things like cartridge emulators...the air caps are a noticeable improvement though if that is all you got

+1 on the emulators. They won't work n the early CB750 forks with the groves and clips and pistons, though.
Here's a pic of my DIY air caps. I, or rather the machine shop, drilled the caps and threaded them for fittings with compression rings. Got the parts from a suspension kit for a car. The hose plugs into each cap, and gracefully meets in the center, where the 2 ends are attached to a compression T. This eliminates the need to struggle getting them synched. The T has a automotive air filler which I bracket in the center and topped off with a skull and crossbones cap.

True, it takes very little air to make a difference. 5-8lbs at ride height is great. Easy to do. If you open the system and the  valve core, extend the forks all the way, replace the valve core, then let it settle back into its sag, you'll likely be where you want it.

I also use the top of oil measurement as that's the way the racer guys do it. A little clumsy with the tubes on the bike as you have to collapse them all the way without springs. Better to do in the vice. Put your highest amount in. If that's too much, you can siphon some out to suit, while assembled.

[budman]

It's been a few weeks since I got the forks built.  I ended up using something in the neighborhood of about 7 oz.  per fork, which is a significant amount more than what the factory manual says.  I used the 140mm Progressive method, but I went from the top of the threads instead of the top of the tubes as described in their instructions.  The Progressive instructions illustrate a fork tube with the threads starting at the very top of the tube vs. the CB750 which have the threads starting about 1/2" from the top of the tube.