Rear Shock Upgrade - Advice/Guidance.

stevekoz

Well, recently got myself my first full suspension. Went with the well spec'd Rockrider 9.1.

The bike is everything i was hoping for and more. Its so controlled and fun ride. I've begun upgrading the bits i think it needs to make it great and with my b'day round the corner i thought why not treat myself to some new gear - namely a rear shock.

The bike has an xfusion e1 - no lock out or rebound adjustment, but honestly, i ride fairly hard and i'm fairly heavy rider (ok very heavy!) and its coped brilliantly, but i just feel if it had a few more options to tune it i'd get even more from the bike.

So, its 190mm eye to eye and 50mm stroke. First off i think i could get away with 200 x 50, but i think sticking with the same size would be best - agree/disagree?

Second, i've got a few options but only on a 2nd hand level, im limited to about 150.

I've seen for that the following

Rockshox Monarch 3.1/3.2/3.3/4.2
Rockshox Ario 2.1/RL
Fox RP23
Fox Float R
Fox DHX Air 5.0
SR Suntour Epicon LOD
Manitou spv 4 way Air/Coil

Amongst others.

I've also not sure if to change from air to coil shock - i've heard saying that coil shock is much better for a heavier rider agree/disagree?

Any thoughts on what people would recommend?

Cheers as always guys n girls

supersonic

You would need the same size, or risk upsetting the geometry. 10mm is more than it sounds here!

Fox RP23 or the best Monarch you can afford.

hainman

http://www.chainreactioncycles.com/Mode ... elID=70137
i have the very same bike and have thought about upgrading the rear shock for lock out,this looks a safe bet...and in budget.keep me posted how you get on whatever you get.
cheers
Craig

stevekoz

I saw that on CRC Hainman. It did appear to be a good bargain and i think tis the same shock the higher end rockriders FS's use unless you go for the very top then i think its the monarch as mentioned by supersonic above.

I've got my eye on a monarch 3.3 at the moment, so if i'm lucky that is what i'll end up with i think. The Fox's seem to always jump a bit out of reach at the very top of my budget and i always like to save a bit of money.

To be honest, i've been really happy with the xfusion - as a basic stock shock its been great so far. No issues with it at all, for the money i'd recommend one to anyone really.

The Rookie

Don't forget that 10mm at the shock could well be 20 or even as much as 30mm at the rear wheel as none run a 1:1 linkage, most are circa 2-2.5:1, you don't want to raise the BB over 1/2"!

mrmonkfinger

Stevekoz wrote:

I saw that on CRC Hainman. It did appear to be a good bargain and i think tis the same shock the higher end rockriders FS's use unless you go for the very top then i think its the monarch as mentioned by supersonic above.

Don't forget if you stick with xfusion you will be able to use the existing mount hardware. If you get something from another manufacturers, you'll *probably* need new hardware, so budget a few quid for that.

benpinnick

Id bid on this: http://www.ebay.co.uk/itm/FOX-RP2-REAR- ... 337d614a86

benpinnick

mrmonkfinger wrote:

Stevekoz wrote:

I saw that on CRC Hainman. It did appear to be a good bargain and i think tis the same shock the higher end rockriders FS's use unless you go for the very top then i think its the monarch as mentioned by supersonic above.

Don't forget if you stick with xfusion you will be able to use the existing mount hardware. If you get something from another manufacturers, you'll *probably* need new hardware, so budget a few quid for that.

Most manufacturers run Fox standard hardware now, so it shouldn't be an issue to swap... probably.

benpinnick

Or this! He's open to offers - http://www.ebay.co.uk/itm/Fox-Shox-RP23 ... 25770c7099

Now that would be an upgrade!

stevekoz

Well been watching those examples, missed out on a couple sadly, i am tossing between the monarch rt and fox rp23 if i can get one as cheap as possible.

Does anyone know by the way difference between normal and high volume air cans? I've heard the high volume is not so good for heavier riders like me?

garethscott

ooops, been bidding against you..........
won the rp23 on sat night.
in a similar position to you going from no lock out to rp23
tried a rp23 and a high volume one, did not like the high volume one seemed strange rp23 (standard) was a much better jump in tech.

should arrive in the post on weds for a night ride on fri

benpinnick

youre right on the HV shocks. HV is designed to allow a lower pressure at the start of the stroke, which means a smoother transition and more linear feel. The problem lies in that heavier riders will simply blow through the start of the stroke and tend to wallow. You can add air of course, but this causes the shock to feel harsh in its mid stroke. Normal volume is better for heavier guys, BUT, you can mess with the air volume really easily by adding air can reducers (which are cheap), eliminating some of the HV problems.

mrmonkfinger

Stevekoz wrote:

the way difference between normal and high volume air cans?

Normal is more progressive, high volume is more linear.

Basically, it gives a different feel to the suspension. Any given frames will tend to work better with either one or the other. And you might prefer the feel of one over the other.

benpinnick wrote:

The problem lies in that heavier riders will simply blow through the start of the stroke and tend to wallow

Not sure I understand how this works TBH, any chance you can explain?

benpinnick

Some google fu turned this up: http://www.pinkbike.com/news/tech-tuesd ... -2011.html.

Probably better than I can do.

mrmonkfinger

Cool, ta.


Still not sure I see the link with rider weight though, that's just talking about spring rate & sag?

nicklouse

for heavier riders just get them reshimmed to suit.

different can size just changes the spring progression rate.

benpinnick

OK, geek time. Let me apologise in advance if some of the terminology is wrong, and also credit Jemima from STW for some of the geekier bits:

Air cans follow the following spring formula:

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

What this actually means is a high volume (blue) and standard can (red) spring rate that looks like this:



As you can see from the graph, the HV can needs less inputted energy to reach full travel than a smaller can. It is more linear. As an aside, an infinite volume can would be perfectly linear, (like a coil) but they dont exist. So, assuming the same starting pressure in both cans, a heavy rider will likely blow through all their travel on the HV can before they would reach 2/3rds (or thereabouts) travel on a normal can.

Without changing can volume, the only way to avoid this is a) add compression damping, which has obvious draw backs or b) add air pressure. The problem with adding air pressure is the non-linear nature of the spring. Rider weight creates a linear input. A rider of 100kg exerts twice the force than a rider of 50kg. In a linear spring such as a coil, this correlates to twice the movement. i.e. if a 50kg rider uses half the stroke, a 100kg rider would use all the stroke, simples. However, air behanves differently, it is rising rate. What this means is that a shock at 200PSI needs more than double the force to move 50% of its stroke than a shock at 100PSI.

Since a rider exerts a linear force by weight, this means adding extra air pressure to the can causes the required input to accelerate at a rate faster than the rider weight adds it. You cannot double the air pressure to accomodate the double weight heavier rider and expect that your shock will behave the same (as a coil would if you double the spring rate) - it won't. What happens is that you kill the small bump sensitivity as the force needed to move the shock through the 1st 10% of its travel is like the force needed on the smaller air can's last 10% of travel. Which is bad.

Hope that helps.

benpinnick

PS - I should add that this is a bad intepretation of what I remember from a Tech Tuesday video I once watched, and in no way should be relied on to be true

PPS - Things like compression ratios, rising and falling rates caused by linkages etc. all play a part, and so its not a simple as I have it, but I think it deals with the basic question that was raised.

cooldad

You forgot to factor in height above sea level and air temperature. Half assed job.

The Rookie

Effect of tyre pressure and it's volume, stiction in linkages and the shock as well as the compression damper all have an effect.....

Just for referance the maximum force coming from the damper during a bump hit can frequantly be 4 or 5 times the force from the spring (air or coil).

mrmonkfinger

benpinnick wrote:

k = g*P*A^2/V

benpinnick wrote:

P is the pressure

benpinnick wrote:

You cannot double the air pressure to accomodate the double weight heavier rider and expect that your shock will behave the same (as a coil would if you double the spring rate) - it won't.

Your (or Jemima's) formula says otherwise - the spring rate is proportional to pressure.

I'm missing something again?

bennett_346

mrmonkfinger wrote:

benpinnick wrote:

k = g*P*A^2/V

benpinnick wrote:

P is the pressure

benpinnick wrote:

You cannot double the air pressure to accomodate the double weight heavier rider and expect that your shock will behave the same (as a coil would if you double the spring rate) - it won't.

Your (or Jemima's) formula says otherwise - the spring rate is proportional to pressure.

Yep that's how i read it.

I see what you are saying Ben but the formula must be wrong if what you say about pressure is true. I'm not an expert on shocks so i can't say which is correct, but i do know my physics and in that equation pressure is proportional to spring rate.

However your explanation was a bit "mushy" as saying "behave the same" could mean anything.

supersonic

The pressure is proportional to the spring rate - the difference in feel and shock movement is due to the way the seals behave under higher pressure, the friction increases. This is one reason why some manufacturers go for low ratio systems.

benpinnick

I think SS is correct, the pressure probably effects other things and thats what causes the stickiness. Its a little fuzzy as the video was a couple of years back I think, hence the caveat.

Well worth watching though, some interesting stuff on how negative springs work etc.

mrmonkfinger

benpinnick wrote:

I think SS is correct, the pressure probably effects other things and thats what causes the stickiness. Its a little fuzzy as the video was a couple of years back I think, hence the caveat.

Well worth watching though, some interesting stuff on how negative springs work etc.

no worries

"stiction" from the seals is a definite problem, I think dual air springs do a lot to mitigate its effects. I quite liked having the ability to tweak the air chambers on revs for instance, you could just keep slowly upping the negative chamber until the start of the stroke was just nicely smoooth - although I can see why RS have the single air option and most air forks/shocks stick to a single air adjuster.