Fett Bro's Air Cannister 2's

[ralger]

IMO - 2-10% doesn't help where it is needed (the first 50% of shock travel). There is a reason Fett made theirs larger.

A variable volume means you will be changing the pressure when you change the volume and will need to adjust both volume and pressure to maintain ride height.

 

[ahicks]

IMO - 2-10% doesn't help where it is needed (the first 50% of shock travel). There is a reason Fett made theirs larger.

A variable volume means you will be changing the pressure when you change the volume and will need to adjust both volume and pressure to maintain ride height.

Ride height is what you are trying to work around - that's the priority. Then you try to get comfortable?

Edit: I don't see the air resi as having much of an effect on the first half of the travel. FWIW

 

[ralger]

ahicks - I see you have the NexTech Resi's. Any idea how much volume they can add (I know they are variable)? They told me 100% when I asked, which, I don't believe. Do you adjust them when you ride or just have them set at max. volume?

 

[ralger]

[I haven't seen a resi with a removable end or something. Kinda surprised at that.... but maybe it's just the limited market?]

The ones I am building will have that. I am starting at 80% volume increase with a flow control. The removable ends will allow me to decrease the fixed volume if needed.

 

[arteeex]

Looking at the 10% graph the max calculated pressure is 770 psi with 70 psi initial, which is roughly 25% lower than the working max of the stock volume. My take on this is; if you add too much volume there won't be enough end-of-stroke pressure to resist bottoming and the energy available for effective rebound will be reduced. Is this not the case?

 

[ahicks]

ahicks - I see you have the NexTech Resi's. Any idea how much volume they can add (I know they are variable)? They told me 100% when I asked, which, I don't believe. Do you adjust them when you ride or just have them set at max. volume?

Before this morning, I'd never even considered what the volume might be? No, not set at max, maybe something closer to 1/2, but I keep the front end down kinda low for play in the twisties. Likely a lot softer front end than most will run here. That's what actually got me interested in the resi's. I was playing up in the top half of the compression stroke frequently - getting into that harsher range supplied by the stock shock. I was originally running them in the 40 - 45 lb range. With the resi's that went up to 55 - 60 which did raise the ride height up a little, maybe 1/4", but the harshness is gone. I dinked with the volume settings frequently when I first got them, but settled in to where they are set now after a couple hundred miles? I've had the sled for a while now, pretty much settled in on the setup.

This may be a good read as well. We got into this topic a while back:
http://www.ty4stroke.com/viewtopic.php? ... ht=nextech

 

[ahicks]

<<<<My take on this is; if you add too much volume there won't be enough end-of-stroke pressure to resist bottoming and the energy available for effective rebound will be reduced. Is this not the case?>>>>

I couldn't agree more. That's the downside regarding an oversized resi. You're suddenly worse off than you were when you started.

<<<I am starting at 80% volume increase with a flow control. The removable ends will allow me to decrease the fixed volume if needed>>>

Assuming they were priced reasonably and I needed a set, that sounds really attractive from where I'm siting!

 

[ralger]

Looking at the 10% graph the max calculated pressure is 770 psi with 70 psi initial, which is roughly 25% lower than the working max of the stock volume. My take on this is; if you add too much volume there won't be enough end-of-stroke pressure to resist bottoming and the energy available for effective rebound will be reduced. Is this not the case?

Actually, is it more than 25% lower because the stock volume pressure will go to infinity (in theory, can't be bottomed out). By adding a resi., it now will have a finite pressure where it will bottom out.

However, even at 770 psi with an A=2.4 sq. in, is roughly 1,848 lbs. per shock meaning there would need to be 3,696 lbs. of total force required to bottom out the shocks.

The other thing to remember is that the calculations assume the air has enough time to flow from the shock to the resi. and not restricted by the orifice. It is likely the pressure in the shock will be higher than in the resi.

 

[ralger]

<<<<My take on this is; if you add too much volume there won't be enough end-of-stroke pressure to resist bottoming and the energy available for effective rebound will be reduced. Is this not the case?>>>>

{I couldn't agree more. That's the downside regarding an oversized resi. You're suddenly worse off than you were when you started.}

That is very true assuming no flow restrictions from the shock to the resi. That is why I am using a flow control that will restrict the flow from the shock to the resi; but will NOT restrict the flow from the resi back to the shock as to not restrict the rebound.

 

[arteeex]

Oh, you can bottom an air shock. And, it will be followed by a wave of gas at a hypersonic velocity as it escapes the ruptured tubing.

I would tend to believe there's meaning in the working pressures (somewhere below infinity) derived from the range of initial preload of 50 -150 psi in the stock shock design. I’d like to believe someone smarter than me did this for a reason.

On the force to resist bottoming I think the more relevant factor would be the resultant impulse (F = p/t). Sharp spikes of short duration are going to walk right by 3700 pounds because you have a lot of mass that needs to change direction in a short time.

Regardless, I'll be interested to see your results. My gut says too much volume is not good for most conditions and getting the right flow rate between the shock and reservoir will be a challenge. But I'm okay putting the theory to work and being proven wrong.

 

[ahicks]

From more of an "in use" perspective (engineering is too much like work!) you can darn sure bottom one of these shocks. The question will be how hard is that to accomplish, as well as how often does it happen?

Regarding the flow thing, between the shock and the resi. With the schrader valve removed, and nothing that appears to impede the airflow otherwise, there's nothing noticable going on with my setup? Would it help anything to control that flow somehow? I think the answer to that confuses the original problem - coming up with the correct resi volume. Once that's been established for you particular circumstances, then dinking with the flow to see if you can make a noticable (practical?) difference in damping (or whatever) might be in order? Documenting the findings regarding the changes in flow, to somebody not familair, may be a bit challenging though? "It works" may be a difficult sale.

Exellent conversation with you guys. Thanks!

 

[ralger]

When I talked to Nextech, they said the flow was controlled by a specific orifice size they used to help repvent bottoming out; don't know anymore than that.

Hygear recommends drilling out the shock orifice when the schrader valve is removed (yikes!) as to not restrict the flow before the flow control.

If you have concerns with bottoming out, then a flow control would help create a steeper curve near the end of travel.

I don't believe there is an answer to "the correct resi volume" that would work for all riders in all conditions.

I know I'm not smart enough to figure it all out and just need some snow to do some testing!

 

[ahicks]

When I talked to Nextech, they said the flow was controlled by a specific orifice size they used to help repvent bottoming out; don't know anymore than that.

Could be OFD, but I don't remember any orifice regarding my install, or mention of such in the directions, but maybe they've gone to that in later production?

Hygear recommends drilling out the shock orifice when the schrader valve is removed (yikes!) as to not restrict the flow before the flow control.

Same thought here.... I think he may be refering to a restriction on the back side of the schrader valve though? Maybe that's what Nextech is refering to as well. I didn't think to look doing my install.

If you have concerns with bottoming out, then a flow control would help create a steeper curve near the end of travel.

Bottoming no longer much of an issue for me. Kinda my point? Yes, it will on occasion, but that's usually the result of an "oops!".

I don't believe there is an answer to "the correct resi volume" that would work for all riders in all conditions.

We're in agreement there, but thinking your "tunable" solution is as close as I've heard of regarding something affordable - as well as tuneable. People need to understand the need for tuning going into something like this? Maybe some suggestions regarding a few different pressure ranges/ride heights (eg. low/medium/high)?

I know I'm not smart enough to figure it all out and just need some snow to do some testing!

Amen.

 

[woolyviper]

according to the graphs, it looks like by adding 12-15% air volume, for my riding style, will give me exactly what i'm looking for - i can run a little higher pressure than I currently do, and compress the shock a little more - the biggest difference i noticed going from an fx to an rtx was the inability to compress the front suspension on bigger bumps - adding the fb canisters should make this better.

 

[woolyviper]

Here are a couple of curves showing what adding 10% volume does to the spring rate vs. stock and what adding 100% volume does vs. stock.

Keep in mind - the curves assume there is no flow restriction between the shock and reservoir.

The amount of volume to add is strictly a rider preference and depends on what type of riding you do. It is impossible to pick a fixed volume that would work for all riders in all conditions.

i'm not questioning your graphs (you're obviously smarter than i am), but wouldn't your x axis have to show the resultant volume with the intial volume built in (example start at 13 and add 10% your resultant would be 14.3), thus moving the curve further out vs. the stock volumes?