I'm Sort of Sorry...but....the tube does not do things like that....
It is Plugged at the end...and has those rings and valving are on the "big end" for a reason...The oil is in the lowers...the rings and Valving control both Compression and Rebound damping...Hell!! They Sort of Work without any oil....Look Carefully, and tell me how the Spring has anything to do with travel [unless it coil-binds]...Then...Take the screw out of the bottom of the lower, and pull down...the travel just increased....
Or...Buy into the Aftermarket Hype!!!!

 

I'm sorta following you.
True, the rings at the top are controlling oil flow into the damper rod and it comes out of the 2 or 4 holes drilled in the sides of the rod.
That doesn't limit the travel as much as the spring.
Soft spring will collapse more and hard spring will collapce less under same amount of weight. Really weak spring will cllapse and bind. Really hard spring will hardly move at all.
That, imo, is the the way to control amount of travel. Damping rod just softens (dampens) the travel by using oil flow and compressing air in air assist systems.
Aftermarket hype? You mean someone came up with a product that you can adjust daming with? How is that a hype?
Thats my understanding of it anyways.
here's a good pic of the rods with predrilled holes and gold valves that you can adjust to your liking

 

I always thought that travel was controlled by Damper length...No way a 9" damper has as much Stroke capability as an 11" one....as long as the spring don't coilbind.

 

I'm no engineer and don't even play one on TV

My thinking is that the damper is bolted internally to the lower and goes up and down with it as it absrobes bumps.
The way it functions is that it gets filled with oil on its UP stroke and forces oil out on the DOWN/compression stroke. The amount of oil inside of it and the rate of oil flowing out of the holes determines your damping.
Travel is still determined by the compression rate of the spring - the force needed to compress it and the force of rebound.
That is why it so important (to me) to have the right spring rate for my application.
I also run Gold Valve Emulators from RaceTech. That is the valve that sits on top of the damper tube to control damping. Per their instructions I drill extra holes in the damper rod to render it useless, so just the Emulator does all the work. Unlike predetrmined factory rods it is totaly adjustable.

 

Thanks guys, the fxd damper rods(9.5) are the same as used for fxr2/3/4. im wandering why moco used 2 different damper rods. I know the regular models have 23 3/8 tubes and the sport models have 25 3/8 tubes. The damper rod would make the legs 1 3/8" taller on their own. Why the need for 2 in. taller tubes. So if set up properly the forks would be nearly 4" taller. I dont think their that much taller than non sport models. Obviously it would get an inch more travel. But why that and the longer tubes.

 

I set my forks up to compress about 1" with my weight on the bike, mine has the factory 2" over tubes. I'm really not sure about extra travel or not with the longer damper tunes/forks. The maximum travel available should be the distance between the bushing in the top of the lower leg and the bushing on the bottom of the lower fork tube. That's the maximum travel, the actual travel should be slightly less than that so the bushings do not have contact causing deformation. It does vary with different tubes and lower legs,!lengths and bushing locations. I think?

 

My thinking is along the same lines as thermodyne. You can have a longer fork tube for cosmetic reasons, as many mods to Harleys are purely for looks. The longer damper rods make me think that with the longer tubes It provides more travel in the front end, providing more ground clearance and more travel for the more Agressive riding that the sport models should see. The early fxr's with the 35mm front end, had 2 different tube lengths with the"S" models having the longer forks, they sat higher than the standard models. Just an FYI that may or may not have any significance

 

that would make sense, but the travel is still determined by the main spring and the little toput spring which all apear the same
how many of us actually set up proper sag? measure travel?

 

The rods determine the stroke and the spring free length. They also are calibrated to scooter weight, and ride quality. Or lack there of, as the case usually is.

I think I would try to find a set of rods that fit the scooter I was installing them on. Or close to it.

The longer tubes from the MoCo have different internals. Aftermarket is usually done with spacers.

 

Therm has it right...again...

The holes are drilled way oversize on the dampers when you use the emulators so they (the holes) have little affect on dampening
then the emulators can take over most of that control.

No oil is meant to pass by the "rings" on the damper piston. It goes up/down through the center of the hollow tube/piston, and
in/out of the side holes.

On damper length.... for instance if you bought 2" over fork tubes, but used a 2" shorter damper, you would gain nothing in height,
but lose travel*. Upward travel is stopped by the damper piston bottom. Thats why the damper has to be installed from the top.
(there is a top out spring in between the piston bottom and fork tube bottom to soften the blow...LOL)

*(spring pre-load would be the same)

 

My guess is that longer rod will hold slightly more oil inside and the damping is slightly different. I doubt you would ever notice the difference. As long as preload on the spring is correct, you are good to go.