Not really sure what you mean. The shock differs in travel from the front because of shock linkage. Therefore a shock stroke may be only 2 inches but the rear wheel will move 5inches (just some rough numbers). The front fork stroke is what you get. If the fork has a 5inch stroke then you get five inches of travel. In the rear the rear wheel my move 2 inches because of a bump you hit, but the shock may only move 1/2-3/4 inch. Most linkage designs have a 3 to 1 or 4 to 1 ratio. The Intense bikes have a 2 to 1 ratio which allows for better small bump feel since the shock is moving farther than on a bike with a 3 to 1 ratio. Hope this helps.

 

You are very wrong about linkage ratios.

First, the ratios fall between 2:1 and 3:1. Having a ratio higher then 3:1 would need extreme damping due to very high shaft speeds. Also, 2:1 ratios would give the least amount of small bump sensitivity. With a 2:1 ratio, the rear wheel moves 2mm for every 1mm the shock moves, compared to a 3:1 ration where the rear wheel would move 3mm for every 1mm the shock moves. So if you hit a small bump that needs 20mm of suspension movement to smooth it out, the 2:1 ratio will need to move the shock 33% farther into its stroke to get the 20mm of travel then a 3:1 ratio. Since rear shocks become harder to compress the deeper into their stroke, that extra 33% means you would need to hit it harder. This means lower ratios give you less sensitive feel.

also, very few frame designs give a linear ratio. Most frame designs have a rising rate to try to give a bottomless feel.

OP, Linkage ratios have very little to do with rider weight. The compression damping and spring rate are when weight comes into play:thumbsup:

 

I was reading about Fox offering volume spacers for their Float air shocks and they showed a compression ratio chart that showed varying compression ratio's for each volume can and with 3 different size spacers that changed the air cans volume. I was wondering what the differences in those ratio's meant in real world applications. Guess I didn't explain myself well. Oops.

 

All of you are wrong in one way or another.

The OP was asking about compression ratio, which is the difference in air pressure between a shock that is fully extended vs fully compressed. (Not counting negative air space, etc) This tells how quickly the force ramps up as the shock nears full compression. If the spacers make the air volume smaller, that means the shock has a high compression ratio -> higher pressure when fully compressed -> more force -> shock is more progressive.

U guys are talking about linkage ratio. This does not say anything at all about whether a suspension will be harsh or not. The harshness depends on how much wheel travel you have and how the shock is valved. A high linkage ratio means that the shock moves very little and has to create high damping forces at slow shaft speeds. It is slightly more technically difficult to do this due to [various shock non-idealities blah blah] but a good shock properly valved should have no problems.

 

You are right about slow shaft speeds. I confused myself for a second.

But there is a direct link between shock linkage ratios and how progressive you would want your spring. In normal situation (a straight rate linkage design) You do not want your shock to be progressive, no matter what your weight is. This is why air shocks always get compared to coils. The words "coil like feel" get tossed around in the air fork and shock world all the time. Where low volume air shocks come into play is when frame designs have a falling rate linkage design. The ramp up of the shock is used as a crutch to counter the falling rate design of the frame. Put a low volume shock and a bike with a rising rate design and it will be nearly impossible to use full travel.

For the OP, Air can volume has very little to do with rider weight. Spring rate in general does though. Adding air to a proper sized air can is the way to go. If doing so makes the ride harsh, you most likely dont have a shock that is valved correctly and gives you to much compression damping at low shaft speeds.

 

Thanks Beanbag thank makes some sense of it all.

 

I found this thread and thought I better get help myself. I am still learning about the compresiion ratios and where I need the ratio to be for me. Let me tell you my specs and problems. I am 255 lbs riding weight. I have an Ibis Mojo HD 140 with the 2012 RP23 on the rear and TALAS 150 on the front. I have about 107 psi in my fork. The fork feels pretty good other than a little too much fork dive and I don't use my full travel. I have the low speed compression clicked all the way over to prevent the fork dive, but doesn't help as much I would like. Any advise for my fork? Now my shock. What difference does a bigger air can do for you? I have a standard air can as far as I know. I have about 210 psi in my shock. My shock feels a little harsh through mid travel and I don't use my full travel (about 3/4 of an inch left on the shock.) Would the Fox Air Volume Spacer Kit help me? Any other advise? Thanks for any help or advise.:thumbsup:

 

Can't help on the fork sounds like normal air fork characteristics for a heavier rider - too much air volume. Rear shock try a larger air can it needs more air volume.

 

neblackb, I "think" that the mid stroke harshness you feel is a design characteristic of the DW link. Again, not 100% sure. HOWEVER, I am commenting because almost every rider's complaint with their air shock is that they blow through the mid stroke. Most people who send their shock to Push are looking for this characteristic to be added/enhanced on their shock by custom valving. IMO, just lower your rear shock pressure and see if that helps. Otherwise, post in the IBIS forum and see what other HD riders are experiencing. I only wish my Nomad had more mid-stroke support.

 

Do you actually use the on-the-fly travel adjust feature of your fork?

 

Looking for info on the consequences of reducing travel on an air fork brought me here. On a shock, it's simple enough that I understand it. Beanbag pretty much nailed it, save for his use of the word 'difference', which may confuse some people who think subtraction. Just remember we are talking about ratios, which means division.

Here is a graphical representation:

I have a Fox volume tuner in my own RP23 and it has worked wonders. The spacer subtracts it's volume equally from the uncompressed air volume and from the compressed volume. But changing the divisor (the compressed volume) has proportionately more effect than the same change in dividend (the uncompressed volume).

In this example with a 50 cc volume reduction, the total volume is reduced only by 5%. But the compressed volume is reduced by 50%. and the compression ratio is almost double (10 vs. 19).

Pressure, volume, and temperature are linked together by the nature of gasses (like those which make up air). This way of looking at a shock focuses only on volume, but you can work out what is going on w/ pressures and temps to look at the system from those prespectives, too.

Fox briefly explains spring progressiveness tuning

 

Hi,
I've bought the spacers now bui I really have doubts in which one to install. How can I know which spacer is the recommended for me, just try the 3?
Many thanks for your help
Edgar

 

I would just play around with them, Really only takes a minute to install, largest spacer being the most progressive of course. Good luck with it. I used the large spacer on my RP23.

Tech Tuesday - How to Install a Fox Float Air-Volume Spacer by RichardCunningham - Pinkbike

 

Thx, I'll do that

 

Cool find! I'm doing some math relating the shock and my frame to find these ratios, but I ran into a question.

Ok when we talk about 2:1 / 3:1....we are talking about wheel travel versus shock travel....I got that.

Using this...I found some numbers for my setup wish is a frame Scott Spark 10 L 26" with a Fox CTD 6.5 (165.1mm) eye-to-eye/1.5 stroke (38.1mm). Wheel versus ground travel is 104.1mm, so I found the Ratio of 2.73:1.

Now my question...or perhaps statement... remains: To determine if 2:1 is actually less sensitive to small bumps than for example 3:1...the air chamber volume would also need to be consider correct?

So lets say for my setup; the same shock with 6.5/1.5 but with smaller shaft and cannister diameters...would have a different curve Force vs travel than the bigger cylinder Fox CTD.

In my understanding I would need to put much more pressure in the shock (to the small chamber one) to clear the same bumps from the FOX, if the pressure was the same from the bigger FOX it would be more saggy....but the ratio would still be the same at 2.73:1???

...then...does the added pressure would compromise sensitivity??? since the ratio is still the same...I guess not??

Intriguing relationship.