# What is the extra cylindar I see on some rear shocks?



## GotoDengo (Aug 6, 2010)

Can anyone enlighten me as to what the extra cylindar is I that sometimes see on rear shocks? Seems like they're usually on downhill/AM bikes. Like in this pic


----------



## Mr. Blonde (May 18, 2008)

It's where the oil inside goes when the shock compresses. The valves that control the shock movement are between the two chambers.


----------



## GotoDengo (Aug 6, 2010)

Thanks.


----------



## smilinsteve (Jul 21, 2009)

It's called a piggyback reservoir. It's supposed to have some advantage over making room inside the main shock body for the oil during compression, because air mixes with the oil, and foams and damping is not as good. I think.


----------



## Mr. Blonde (May 18, 2008)

smilinsteve said:


> It's called a piggyback reservoir. It's supposed to have some advantage over making room inside the main shock body for the oil during compression, because air mixes with the oil, and foams and damping is not as good. I think.


No

Most longer travel shock designs have significantly more oil in them and the oil flows thru the valves at considerably higher volumes. Since there's so much oil and it moves more quickly there's just not enough room in the main body for it to go. With the higher oil volumes most manufacturers found they were compromising oil flow to cram everything inside a single tube. There is one notable contradiction to this in the Avalanche Woodie but Avy is on a whole nother level in engineering. Another function is to gradually (or suddenly) increase resistance pressure to the flow of the oil by changing the volume or pressure of a little air chamber on the piggyback. This controls the characteristics of the suspension travel at or near bottom out. The reservoir in the piggyback has a piston that floats inside it and separates the oil and air chamber. Some shocks have a rubber bladder instead of a piston but that's a different topic completely.


----------



## smilinsteve (Jul 21, 2009)

Mr. Blonde said:


> No
> 
> Most longer travel shock designs have significantly more oil in them and the oil flows thru the valves at considerably higher volumes. Since there's so much oil and it moves more quickly there's just not enough room in the main body for it to go. With the higher oil volumes most manufacturers found they were compromising oil flow to cram everything inside a single tube. There is one notable contradiction to this in the Avalanche Woodie but Avy is on a whole nother level in engineering. Another function is to gradually (or suddenly) increase resistance pressure to the flow of the oil by changing the volume or pressure of a little air chamber on the piggyback. This controls the characteristics of the suspension travel at or near bottom out. The reservoir in the piggyback has a piston that floats inside it and separates the oil and air chamber. Some shocks have a rubber bladder instead of a piston but that's a different topic completely.


Right. That's what I meant to say.


----------



## Carraig042 (Nov 12, 2009)

In addition, I know with some types of piggy back shocks (may not be the same for mtb), but the extra oil is also good as it will not warm up and get thinner as easily, which would change dampening characteristics.

-Brett


----------



## Mr. Blonde (May 18, 2008)

Carraig042 said:


> In addition, I know with some types of piggy back shocks (may not be the same for mtb), but the extra oil is also good as it will not warm up and get thinner as easily, which would change dampening characteristics.
> 
> -Brett


The heat control characteristics are a function of the higher volume not the reservoir itself. There are numerous other benefits to higher oil volumes. I didn't feel it necessary to list them all. If OP would like I can delve into that too.


----------



## GotoDengo (Aug 6, 2010)

Mr. Blonde said:


> If OP would like I can delve into that too.


Ha, no... thanks, though. There's not too much downhill or AM in my local area, so it's not like I'm considering one of these bikes. Was just curious what it was. As a FS noob I don't think I understand the dynamics of fluid-based damping enough to even appreciate more details. I was just confused b/c (unlike the above pic) it seemed like most of those AM bikes had coil socks, so I didn't know what that piggyback was. I understand now that there's a fluid shock inside of the coil.

Thanks again.


----------



## smilinsteve (Jul 21, 2009)

GotoDengo said:


> I didn't know what that piggyback was. I understand now that there's a fluid shock inside of the coil.


Kind of. 
The typical "shock absorber" consists of a spring, along with a damper to control how fast the spring returns after compression. So for a coil over, the spring is obvious, and the damper is the fluid inside the shock that has to flow through an orifice as the spring returns to its unsprung length. You can imagine a smaller oriifice, or more viscous oil, would result in slower return of the spring. etc.


----------



## smilinsteve (Jul 21, 2009)

What kind of saddle/post is on that bike in the OP?


----------



## GotoDengo (Aug 6, 2010)

smilinsteve said:


> What kind of saddle/post is on that bike in the OP?


Not sure. I just saw that in some other post and copied it as an example of the piggyback shock I was asking about. I don't even have the link I used to get that pic from...sorry.


----------



## fastale (Jul 2, 2007)

smilinsteve said:


> What kind of saddle/post is on that bike in the OP?


Ibeam - proprietary, cannot use with standard rail type clamp

http://www.pricepoint.com/detail/15...s-67-Saddles/SDG-Bel-Air-SL-I-Beam-Saddle.htm


----------

