What’s the difference/benefits?

High Volume = More linear spring rate
Low Volume = Spring rate ramps up significantly through its travel

Each is desirable in different situations. On a bike with a naturally rising spring rate, a High Volume can would be desirable (or preferably a totally linear coil spring!). On a bike with a linear rate, or even a falling rate (a la Cannondale Prophet, or Santa Cruz Heckler), a low volume can is desirable so that the bike doesn’t blow through its travel too easily…

Thanks.

One more question. My bike takes a 200×50 shock, could I use a 200x57mm?

mboy – why does a heckler come with a high volume air can then? The heckler has a linear spring rate that requires a HV can to achieve all its travel at the same rate otherwise it would ramp up at the end ergo not achieving all travel or even close (source for this is the santa cruz site btw)

One more question. My bike takes a 200×50 shock, could I use a 200x57mm

Depends, lower the air in the shock / remove the spring in coil, compress suspension till you get full travel and see if the shock moved an extra 7mm if tyre will hit seat tube, pedal will hit the ground, something will catch somewhere, the earth will exploded causing a horrible death etc. etc.

mboy – why does a heckler come with a high volume air can then? The heckler has a linear spring rate that requires a HV can to achieve all its travel at the same rate otherwise it would ramp up at the end ergo not achieving all travel or even close (source for this is the santa cruz site btw)

I am guessing, but if a frame has ramp up designed into the suspension movement and you have a LV can then the 2 ramp up forces could mean that you’d never really get full travel as the force required to get to that point is to high.
Whereas a HV air can requires less force to get to full travel so when you hit that drop or big rock there is enough force to more the shock through it’s complete shock stroke.

racing_ralph – Member

“mboy – why does a heckler come with a high volume air can then?”

So that riders can replace it and then glory in how much better it rides.

why replace? just fill outer air can with grease? oh and you know better than SC? because…..?

My orange 5 was blowing through it’s travel with a Hv can, had it swopped for a small one deliberately to increase the ramp up. Helped with bottoming out issues but in the end I went with a coil van rc. Best move I made.

I was running the shock pressures at upper limits so also had to have the damping changed.

Mojo were great at chatting through the options and giving free advice.

mboy in yet another false information / utter bullshit shocker 😉

High volume in my DHX Air was shocking on the Enduro but fill the air can with grease and its fine, no more blowing through the travel. Infact its quite the opposite now.

racing_ralph – Member

why replace? just fill outer air can with grease? oh and you know better than SC? because…..?

That’s the other approach of course (or add a shim/bit of inner tube/whatever else to bodge it). Do I know better than SC? Never claimed to, but TF know suspension I reckon… and there’s an awful lot of Heckler owners that complain of how they blow through travel. It put me off the thing after about 10 minutes, I thought there was something wrong with the demo bike but no.

mboy in yet another false information / utter bullshit shocker

LOL, you are a d*ckhead when you want to be! 😉

Just looked at the SC site… I’m that out of touch with their bikes, I was referring to the old Heckler. The current bike looks to have a lower front mount on the shock, combined with a longer shock length. The older Heckler (I’m thinking like 5 years ago here) had a shorter shock, and a higher front shock mount, like on a Cannondale Prophet.

You must remember that ANY air shock is going to have a rising spring rate as it compresses. So for true suspension performance, a coil spring will always be better, especially where a linear rate is desirable. But for many applications, a modern high volume air can air shock will perform very well. And for some designs (less prevalent these days, but bike such as the Cannondale Prophet), a shock with a pronounced rising rate is desired so that the bike is less susceptible to blowing through its travel.

That sorted it? 😕

Oh, and should you feel your shock is too linear, of course you can put some grease in the air can to reduce its volume slightly.

why replace? just fill outer air can with grease? oh and you know better than SC? because…..?

To be fair, thats hardly a basis for making a point, as SC have been known to send out the same frame from different build runs, some with HV shocks & others with LV, depending on whast shocks they had in stock at that time.

How exactly is a high volume shock more linear? Someone please explain the science in that? To my mind, the gas inside high and low volume shocks will ramp up just the same.

balfa… they will if you do the calcs by halving the volume etc…

Say for example that the low volume can is 10mm3 and the high is 20mm3 in volume.

Now, if the shock compresses, reducing the volume by 5mm3, that’s halving the volume of the low volume can doubling the pressure, but only reducing the volume by 25% with the high volume can which gives 1.5 times the pressure.

So, as the force required to compress the shock further is related to the pressure, then compressing the low volume shock is now harder than compressing the high volume shock. They both ramp up but the hv one ramps up slower.

The formula for stiffness of an air spring is:

k = g*P*A^2/V

where

k is the stiffness (N/m)
g is the ratio of specific heats for air (1.4)
P is the pressure
A is the area of the piston and
V is the volume of air in the spring

Therefore, all other things being equal you can see that increasing the volume of air in an air spring reduces the stiffness. Also, if you consider compressing the spring then the ‘new’ stiffness results from the pressure increasing and the volume decreasing thus increasing the stiffness compared to the uncompressed position. Reducing the volume of air in the system increases the stiffening with compression.

I could show you a graph I’ve just done but I’m worried I’ll be branded a turbo-geek… oh, can’t help it. Here’s the graph.

Both springs are set with the same inital stiffness but the blue line has 50% more air volume in the system. You can see its less stiff at end of stroke…

The graph above is an excellent explanation. I’ve had 2 Hecklers one with a LV can and the current has a HV can.

The LV can was by far a better ride feeling when set up for my ride weight without any messing about at all, set the sag and ride. The HV on the other hand took me ages to try to sort it out just how I liked it, setting the correct sag meant blowing through the travel and increasing the pressure meant loosing out on the small bump sensitivity. Spoke to TF and it wasn’t the first time they’d come across the issue so packed it off to them so they could work thier magic (in my eyes anyway).

I was given the extra grease in the air can advice and whilst I understand the theory wasn’t going to go down that route, nor was I going to shim it myself.

Sure the shock’s are great but very different in my opinion

All I know is that I was advised not to go for a HV can on my Nicolai Helius as it’d do ‘funny things’ to the suspension feel and a LV can was much better

Had same problem with Heckler (current) with rp23. Tried packing HV shock with grease with no joy. Changed to DHX air and problem solved. Different bike.

in what way “different”

I had a Fox Float RL AVA (AVA aka Adjustable Air Volume)

You let the air out, screw the air can in or out and re-pressurise. It made a bit of a different, but overall the bike still felt a bit bobby, and was a little trapdoor-ish over fast compressions.

Lummox – Member
My orange 5 was blowing through it’s travel with a Hv can, had it swopped for a small one deliberately to increase the ramp up. Helped with bottoming out issues but in the end I went with a coil van rc. Best move I made.

But a coil would surely be more linear than even a high volume air shock?

Damper could be tuned quite differently I spose

hmm – love all these “educated” musings

FWIW, I’d use shock oil instead of grease if you want to reduce the effective size of an aircan – it’s easier to measure accurately (so you can gradually add more to make it work how you want) and it has the advantage that it’ll nicely lubricate everything.

“large volume cans”

*sniggers*

So, does this mean there are loads of hv cans around not being used? Wifes specialised fsr never uses bottom third of travel even with generous sag. Was considering trying one the other day but I’m wondering if it will clear the pierced seat tube.

I have a spare RP HV can, email in profile.

Oh, and should you feel your shock is too linear, of course you can put some grease in the air can to reduce its volume slightly.

Would it make sense to fill the entire body of the shock with grease? I guess it would mean everything was well lubricated plus ramp up the rising rate making it ideal for heavier STW riders. Have I missed anything though, are there any downsides to this solution? Would it make sense to use a lighter grease to reduce the spring rate slightly for lighter riders?

Grease is incompressable it’d lock the shock solid if the air can was full of it.

mboy and racing_ralph – regarding your initial postings, bike frames don’t have spring rates – what you are refering to is leverage ratios.

Just so you don’t confuse other people!

Grease is incompressable it’d lock the shock solid if the air can was full of it.

Even if the grease were low viscosity stuff?

Fluids are incompressable.

BadlyWiredDog – The air chamber of an air shock needs to have some air in it in order to act as a spring. It contains very little oil and only for lubrication. I think you’re confusing it with the oil used for damping, where viscosity does affect the characteristics.

Loco – I did send you an e-mail recently about the amount of oil to put in the air chamber of a shock when servicing but didn’t get a reply.

The hv can is an lv can with a sleeve round it with a hole for the extra air. Grease goes in the extra air space between the normal can and extended outer can. Easy way to try a lv can if you have a hv

longwayhome, really what shock was it in regard too? Resend and I’ll have a look.

BadlyWiredDog – The air chamber of an air shock needs to have some air in it in order to act as a spring. It contains very little oil and only for lubrication. I think you’re confusing it with the oil used for damping, where viscosity does affect the characteristics.

Thanks – I know the difference between oil and grease btw, one comes in tubs and tubes, the other is in bottles as a general rule. So you’re saying that oil is compressible and would be a better medium to use?

I think that BWD was taking the pee 😉

So you’re saying that oil is compressible and would be a better medium to use?

No, I’m saying you don’t seem to know the difference between an air chamber and a damping circuit, and that an air chamber should have mainly air in it. I think that was clear. It should be now.

I think that BWD was taking the pee

Initially when writing my post I did put “are you trolling?” but then I thought BWD might find it offensive if he wasn’t, so I didn’t. See where being nice to people gets you!

So BWD are you trolling?

Loco – will resend (in a bit, got a meeting) the e-mail to save you hunting for it.

Anyway, there’s quite a good thread on mtbr about shimming HV air cans using a strip of plastic or similar that might be useful for anyone who’s blowing through the travel on their rear Fox with an HV can. Fwiw, running an RP23 on a Pace RC405 – not the standard shock – I found a Push tune ultimately sorted the issues I had where if the shock was set to correct sag it blew through the mid-stroke and if it were set to not do that, it was simply too hard.