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How does a thicker shaft affect damping?

ChrisRobin

Turbo Monkey
Jan 30, 2002
3,215
124
Vancouver
I know people used to say you should avoid the thicker shaft Fox shocks.

Cane Creek had 8mm shafts on most of their coil shocks but I think for the Specialized Demo, they made them with 9.5mm shafts.

How does the larger shaft affect damping?
 

englertracing

you owe me a sandwich
Mar 5, 2012
1,053
634
La Verne
Wellllll........
First we must separate the two damper designs.
Traditional vs twin tube.

On a traditional the additional shaft volume must pass through the base valve, stiffening the contribution of the base valve when no other changes are made. It also Reduces the contribution of the midvalve to compression and rebound damping, as on compression the flow across the piston is lessened due to the large shaft volume, the rebound flow is also less due to the shaft volume.
So just stepping to a larger shaft you CAN end up with too much HSC and less rebound. Also more seal friction.....

A twin tube operates entirely differently and the shaft volume won't change compression damping much but will affect rod force (the contribution to spring rate and the progression of the spring rate if accommodations aren't made in the reservoir with regards to gas volume and pressure) rebound damping would also be reduced with a larger shaft as a lower total volume will pass through the rebound valve.
 

Kanye West

220# bag of hacktastic
Aug 31, 2006
3,548
314
Yup, more emphasis on the base (or bridge) valve. Stick your finger into a glass of water, and the amount that the water level rises is what the bridge valves acts on. How thick the fluid feels to your finger is sort of what the midvalve or main piston represents.

The compression ratio behind the piston for the rebound stroke is also changed (assuming the body diameter doesn't change and only the shaft diameter does). Also the gas pressure requirements to keep that zone from cavitating on compression change.

On the structural side, stiffness, seal drag, additional bushing length needed a proper L/d to be stable, etc.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
18,028
2,183
01776
I know people used to say you should avoid the thicker shaft Fox shocks.

Cane Creek had 8mm shafts on most of their coil shocks but I think for the Specialized Demo, they made them with 9.5mm shafts.

How does the larger shaft affect damping?
Just a comment on the fox shock thing...I think a thicker shaft was indicative of a boost valve equipped shock, vs. thin shaft/non-boost air assist valving. So, the thin shaft is simply more consistent across the damping range by nature of the damper, whereas the thick shaft had the gimmicky boost valve that ramped up damping at end stroke and is only good for regressive end strokes. I'm sure it has some impact on other shocks, which you'll totally be able to tell out on the trail.

tl;dr it wasn't the shaft that mattered.
 

Kanye West

220# bag of hacktastic
Aug 31, 2006
3,548
314
tl;dr it wasn't the shaft that mattered.
That's what your mom said.

Those Boost Valve shocks were great. The architecture was sound. I think they needed iterating once or twice more, but I still think they should have gone that route than this double-barrel shit.
 

Jm_

sled dog's bollocks
Jan 14, 2002
13,216
4,982
AK
The boost valve RC4 was crap. The large shaft severely limited tuning options. I think it was something about piston flow, very little space left for the piston/shim arrangement and very difficult to get the kind of damping support you need. It could be made to work decent on some bikes by junking the boost valve and re-valving the main piston, but other bikes were hopeless. The thin-shaft RC4 was a good shock and THAT is what they should have went with instead of the twin tube, but you know, "one shock for all people and tunes"...
 

dovbush66

Monkey
Aug 27, 2018
188
211
Ireland
I think big shaft RC4 works decent depending on the bike and if you like a stiff setup. It turned my aul cove from a sofa to a pop machine that feels like a DJ while still feeling decent on tracks. The 2012+ ones after they fixed the explodey compression stacks are dead on reliable too and super easy to service.
 

daisycutter

Turbo Monkey
Apr 8, 2006
1,566
63
New York City
I think big shaft RC4 works decent depending on the bike and if you like a stiff setup. It turned my aul cove from a sofa to a pop machine that feels like a DJ while still feeling decent on tracks. The 2012+ ones after they fixed the explodey compression stacks are dead on reliable too and super easy to service.
I have them on all my ancient downhill 26 bikes and they have been really solid and reliable. Every time i tried different shocks the performance was never as good. The Fox DHX2 leaked after a month's use.
 

mykel

closer to Periwinkle
Apr 19, 2013
3,383
2,021
sw ontario canada
Has anybody every played with a J damper (Inerter) on a mountain bike?
With the changes to F1 and them being banned for the new season - just made me wonder.
 

Lelandjt

Turbo Monkey
Apr 4, 2008
2,020
404
Breckenridge, CO/Lahaina,HI
I don't think anyone has mentioned that bigger shaft, means more oil displacement into the reservoir, so more compression of the gas behind the IFP or bladder. So, a little more spring progression. Not a damping difference, but maybe enough extra spring progression to feel?
 

Westy

the teste
Nov 22, 2002
44,767
10,417
Sleazattle
Sounds like they are good in situations where you want to lower the resonance of the suspension but need a higher spring rate for downforce and roll stability, not a bike problem.
 

HAB

Chelsea from Seattle
Apr 28, 2007
11,235
1,622
Seattle
Sounds like they are good in situations where you want to lower the resonance of the suspension but need a higher spring rate for downforce and roll stability, not a bike problem.
Yup. A super digressive rebound curve is also useful for road-going applications where you really care about chassis stability, but then also want the wheels to drop into holes quickly to maintain contact. Again, not applicable to MTBs.