Ive been on a mission to learn more about suspension / shock tuning. I’ve got a fox X2 on my Hightower, under advice from TF Tuned that it’s better suited to larger riders (I’m 6’4″ and 230lbs) than the monarch it came with, which I never got on with.

There’s lots of scope for tinkering with the shock – pressure, tokens, HSC, LSC, HSR, LSR. Most of it I can broadly get my head around but am baffled by one bit – the relationship between rider weight, pressure and compression damping.

The fox base tuning guides basically show that all forms of damping (compression and rebound) should increase as the shock pressure is increased.

For rebound damping I understand that the rebound force is nearly exclusively driven by the shock pressure – so a heavier rider would run a higher pressure, which causes the shock to rebound faster, hence (generalising) you’d expect heavier riders to run with higher rebound damping.

But for compression the force is proportionate to the weight of the rider. A heavier rider would run higher pressure and it is this increased pressure that resists the increased forces under compression. I don’t understand then why compression damping should also increase at higher pressures – surely the air pressure itself is already addressing the increased forces?

I run the shock at 250psi but like the feel with HSC way more open than the tuning guide suggests.

It’s a miracle if that makes sense. If it does, anyone care to educate me on why compression damping should increase as pressure increases?

Thanks!

Adjust ride repeat.

It’s a simple method understand what the changes feel like.

Thanks. I’m doing that and coming to a different conclusion than what fox recommend. Just want to understand the theory of why higher pressure requires higher compression damping.

Is the tuning guide specific to your bike?

Bike frame design will have a big impact on the way a shock behaves.

Taking the idea and running, I don’t know anyone running anything like as much air as Rockshox suggest in their fork.

Once frame is out of the way as a variable, rider weight and rider riding style are probably the next two big variables, but maybe look at it this way, of everyone agreed with the manufacturers, there would be no aftermarket tuning, no alternative cartridges, no PUSH…

Of how you like the shock is different than Fox say, so long as you aren’t exceeding design tolerances I wouldn’t worry too much.

Heavier rider – more energy to dissipate

Kinetic energy = (mv^2)/2

Therefore heavier the rider, the more kinetic energy.

Riders kinetic energy is either converted to strain energy in the spring (in the case of a conventional spring for simplicity)or converted into heat by the damper, well in reality a combination of the two.

So if we’re starting with more energy, where should this extra energy go?

It can go into the spring displacing it further (more sag), run a greater spring rate so the more kinetic energy can be converted into strain energy per unit displacement, it can be converted to heat by the damper or any combination of the above.

In practise we first increase the spring rate to maintain the bikes geometry, supporting the riders weight as it were.

Lets for arguments sake a light rider puts 50% of kinetic energy into the spring and 50% converted into heat by the damper, if we want to keep this ratio for a heavier rider, since damping is proportional coefficient multiplied by shaft velocity, we want to maintain the same shaft velocity (feel), we need to increase the coefficient of damping, hence the compression damping needs to be increased.

Well this is how I understand it in my head.

Can look at it from the POV of force too –

Rider force = damping force + spring force
ma = cv + kx
Riders mass x by riders acceleration = damping coefficient x shaft velocity + spring stiffness x spring displacement

If we want to maintain that ratio of forces and keep the same feel (shaft velocity) you have to bump up the compression damping.

I have my first man-crush on deanfbm.

If it’s anything to consider – Mojo recommended me to run my HSC / LSC way out (20 and 22 from closed) and my HSR is nearly 3/4 open too

LSR is heavily damped and I’m running close to my body weight (but not quite) in pressure

Dunno if that helps you but they’re nothing like the standard settings Fox recommended.

Air spring force is position sensitive, you use this first to get your sag correct. Imagine the two scenarios below which could both be considered correct setups in terms of using full travel occasionally:

Too high air pressure and too little sag (combined with very low compression damping to still allow full travel) and you will lose grip as the wheel cannot extend to meet the ground.

Too low air pressure (combined with very high compression damping to prevent over-use of travel) will sit the bike too deep into the travel, giving a low BB, slack seat/head angles and potentially poor pedalling performance (depending on the design).

Then you can get into leverage curve to mix it up, if your bike is linear/regressive, you may want to use lower pressure and more damping to lessen the natural progression in air spring rate, and the opposite for a progressive curve.

On top of all that is personal preference and riding style! Personally I would get the sag right, then decide between volume spacers or compression damping with trial and error to achieve the correct ride.

use the cane creek app for tuning the shock.

I brought a X2 recently and although it felt amazing with a bit of tinkering, I stuck a Shock wiz on it. Wow, it’s now way better than I could have imagined.

Well worth the £35 hire for the day.

My sag was set correctly, but I needed more HSC and more LSC. Rebound was already spot on. I went For the PLaY tune as I have the climb switch for climbing.

where was the shock brought from lol

Not the slickest videos, but this guy really knows his stuff and manages to sort of explain it…

Spring Rate vs Damping:

[video]https://www.youtube.com/watch?v=EJELsCkYEDI[/video]

High and Low Speed Compression:

[video]https://www.youtube.com/watch?v=JEBNlHXUGdE[/video]

Several others in a similar vein on his channel

Folks, thanks for all the comments – really appreciated. Starting to make a bit more sense to now. I did wonder about hiring a shockwiz, one for a free weekend sometime.

deanfbm and bacondoublechee – kudos for the in depth explanations!!