Reservoir outlet? Top vs. bottom?

[sirhk100]

I just did a couple different searches to try and find info but came up pretty much empty handed.

What's the pro's and con's of the location of the reservoir outlet at the top vs at the bottom?

Valving differences?
Cavitation?
Rebuild procedures as far as air in the reservoir?
etc...


Thanks,
Khris

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[picone]

The shaft displaces fluid on compression, you want the fluid to be going thru the shims and the bypasses but some of it is going into the res. At bump you want it to all go thru the shims for lots of resistance. In other words, it increases the efficiency of the shock by putting the res out the bottom. You would have to ask Kritter but I believe that you also get more cavitation with all the compression pressure directly against the air reservior (increase pressure on the air and high side of the oil but consequently lose pressure on the low side of the piston).

[Samco Fab]

I think King shock offers some good explanations of reasons to go with res. out the bottom. There is no "better" or "worse", just advantages and disadvantages.

In general, if you develop more compression dampening forces and less rebound forces like most off road vehicles do, a resivor out the bottom is advantageous.

I have worked with vehicles with very high spring rates, and with resivor out the bottom, and was not able to develop enough rebound dampening due to cavation, so bumping up the nitro. charge was the "fix" in that case, but resivor out the top would have been possibly a better choice. That was a Hummer H1 with super stiff payload springs, and had very little compression. Most off road cars dont have those kind of spring rates.

One possible advantage of resivor out the top is more consistant rebound as the shock heats up and cools down. I have heard that some cars will start to kick, or ride bad when the shock heats up, and that is due to temp induced pressure changes in the resivor changing the valving. A large resivor can help this, but if the res. out the top was used, only compression forces would change, and that could be better or worse depending on the setup. This is just an idea of mine, take it with a grain of salt

One could argue the best setup is to use a coilover as the primary rebound shock, and the res out the bottom bypass shock for primary compression. You may have the best of both resivor worlds

Mid position resivor outlets could also be possible, I have seen them, but not heard any cons of the mid travel res. outlet.

[Kritter]

I have heard that some cars will start to kick, or ride bad when the shock heats up, and that is due to temp induced pressure changes in the resivor changing the valving.

The increased heat which increases the pressure increases the gas force which a lot of people like to confuse with valving changes...ala "add more nitrogen to make your shock stiffer"... it has nothing to do with valving. Gas force is gas force, valving is valving. Run the smallest shaft diameter possible to get the least amount of change.

[Samco Fab]

The increased heat which increases the pressure increases the gas force which a lot of people like to confuse with valving changes...ala "add more nitrogen to make your shock stiffer"... it has nothing to do with valving. Gas force is gas force, valving is valving. Run the smallest shaft diameter possible to get the least amount of change.

From what I have been told by King, as the shock temp increases , the oil expands, and the nitrogen gets hot, both things increase the pressure in the resivor and somehow change the valving. With the res. out the bottom, it seems to change the rebound valving with extreme shock temps.

Mabye with the resivor out the bottom, there is still some cavation under rebound, and that pressure increase would add rebound dampening. I dont know what the exact definition of valving is, but the gas pressure seems to change the effective dampening of the shock. It does not change the pistion or bypass tube adjustments, but if it reduces cavation I would suppose it changes the "valving".

Kings solution was to themally isolate the res. by adding a second resivor that just holds nitrogen, which would also decrease the pressure change from shaft volume displacement, even if you had a big shaft. I think the pressure change changes more than force on the shock rod.

King said that guys at really rough races like Primm would note that the cars felt harsh and rough, like they had too much rebound, and when they changed a flat or made a drivers change, the shock would cool, and the car felt normal again. That is when they implemented the dual res. They said it seemed to influence the rebound side more than the compresion side.

I think in general, one of the problems with large shock shafts is that when the tire comes off the ground, when it starts to compress it has to overcome the force of the nitrogen on the shaft, and in a large shock shaft that could lead to ride quality problems with lightweight cars. I have noticed that Fox seems to always run the smallest shaft that they can get away with, for this reason.

Interesting subject for sure. I dont know if King's adding a second resivor is the best solution to the problem, but I guess it is a step in the right direction.

[Beat98TJ]

It depends. Low port shocks have huge valving lag on rebound, where as high port (near eyelet) can have some cavitation in compression.

[WannaB-class5]

Bottom res means almost no chance of cavitation. An advantage for most people. As for adding nitrogen to "stiffen" the valving, here is what happens. With low pressure, low speed shaft movement often compresses the nitrogen gas instead of moving the shims at the desired rate. So to get to your "real" rate instantly, use high psi. For a race vehicle you want to be on your rate asap, for say a daily driver/pre-runner it might be nice to have the low speed buffer. Not sure I explained that well but whatever!

[motochris]

OK, so what's the answer.....res. on top or bottom of a bypass?

[Kritter]

bottom

Food for thought on temp.

I can put a damper on the dyno and dyno it every 10 degrees from 100 to 300 and the only thing that changes is the rod force. The curves will all lay on top of each other +/- 5%.


As far as shaft dia., smallest diameter shaft that gets the job done is the one you should use like I said above.

[Samco Fab]

bottom

Food for thought on temp.

I can put a damper on the dyno and dyno it every 10 degrees from 100 to 300 and the only thing that changes is the rod force. The curves will all lay on top of each other +/- 5%.


As far as shaft dia., smallest diameter shaft that gets the job done is the one you should use like I said above.

That is very interesting, have you done that test? Kritter what do you think about the King double resivor setup that guys are running on some class 1 cars?

I wonder what Bilstiens argument for res out the top is on the Blackhawk?