Fox DHX 5.0 Questions

[pechelman]

My background is racecar engineering and Im very familiar with Vanilla RCs as Ive rebuilt them probably a couple hundred times and dyno tested them at least as much. For all practical purposes, Im specialized enough with dampers of all sorts to be considered a damper engineer. So here are my questions;

Is the "Pro Pedal" adjuster simply the Low Speed Compression Bleed Adjustment? (same as the Vanilla RC Blue knob)

Is the "Bottom-Out" adjuster some sort of a High Speed Compression Adjustment or just a supplemental spring rate adjustement for bottoming (aka nothing to do with damping)?
Ive got no idea what this dial is adjusting exactly, so if anyones taken one apart or can point me to a service manual that shows what exactly its doing that would be fantabulous. Right now all I can think of is that its has an elastomer type bottoming bumper or spring at the end of the resevoir that acts as a physical spring\stop against the floating piston.

fyi, if you think my reference to High \ Low Speed adjustments has anything to do with how fast you're going on the bike thats not what Im talking about.
I am however talking about piston velocities and the damping characteristics before and after the dampers transition when the shims open (knee).


I would normally call the manf when I have a question like this, but after many many many phone calls to them in the past Ive since discovered its a complete waste of my time trying to get one of their "engineers" to tell me what I need to know.

any insight or info would be great.

[crashnburn'd]

From riding, adjusting, etc here's my take on things... I can't really tell you what is actually inside though. Maybe try calling www.pushindustries.com instead of Fox, they might be a little more willing to talk to you.

The bottom out doesn't seem to be the same as high speed compression. I when I turn the dial to increase the bottom out resistance, it seems to only affect how quickly the end of the travel ramps up. Whether they're actually using a bumper or somehow changing chamber volume, I'm really not sure.

Also with the Pro Pedal, I'm not sure that I would call it slow speed compression. It gets rid of pedal bob if you adjust it right, but from my experience it makes the suspension feel stiffer for every little, quick bump you hit.

If you find out exactly what the adjustments do you should post something here.

[funkendrenchman]

Pro pedal is supposed to give the shock a platform, so that it does not bob as much as you are pedaling it, but it will still be able to absorb small bumps.

Bottom out control, I believe, should make the shock more progressive so as you get close to the end of the travel is becomes stiffer making it harder to bottom out.

Of course, I don't own the shock, so I'm not 100% sure.

[pechelman]

Ill definitely give them a call sometime this week, thanks.

From what you described that sounds exactly like a low speed compression adjustment. Closing off a bleed will generally damp out slower piston movements akin to pedaling. It will also shift the entire Velocity-Force damper curve up by that same amount. If you increase it more than a few clicks, you get an added extra spring rate due to higher pressures seen inside the damper (ie you're flexing the components\shock body enough to contribute an additional spring rate).

Ill post a follow up of the phone call soon.

[crashnburn'd]

You know anything about Swinger 6-Ways?

If you do, maybe you can give me some insight on setting up my bike a little better. I think I've been adjusting the wrong thing.

During the week, most of the trails that I ride are rooty and have a lot of smallish rocks and I want to make the suspension more sensitive to that. I've been trying to play with the high speed compression because my thinking was hitting the roots and small rocks would be more of a quick motion. Should I be really be adjusting the low speed compression? The shock has an SPV adjustment which only complicates things more because it seems to take away the small bump sensitivity...

[Shepherd Wong]

From my brief experience on the shock this is what I've felt. The pro pedal is just low speed compression. I believe the boost valve controls how fast the piston has to move before it overcomes the low speed compression and moves into the high speed dampening. The bottom out is just position sensative and not speed. It's all pretty confusing to me and would also like to hear for sure how it is all related.

[pechelman]

Well if I were setting up a racecar heres what I would do first.

Ignore the whole sag stuff its complete garbage to some extent. Bike manf need to give the end user a bike RIDE HEIGHT to set their bike to such that the suspension geometry is at its optimal location...or the location to which it was optimized and designed. So having said that, do 2 things.

1. Try a lower spring rate.
2. Decrease your dampers air pressure.

If you cant get a spring, Id imagine you can fool with backing off the preload a bit, although that wont really be the same. Adjusting pressure will be easy.
Essentially, everyone in all industries that use dampers try to solve their problems with the damping adjustments, since theyre so easy to change, rather than adjusting spring rate. Id also suggest going to something lower as you can add spring rate very easily by adding extra resevoir pressure or adding helper springs around the actual damper shaft. PM me about that if you have a question. Also FYI, dont go too low on resevoir pressure or youll end up cavitating a bit more which can make damper characteristics....well, uncharacteristic as well as cause erosion damage to orifices, shims, needles, or bleeds. 50PSI the bare minimum IMO.

On a somewhat funny quirky note, youd be surprised how much spring rate your dampers produce without airpressure and springs just due to FRICTION.

In my experience with racecars, 95% are oversprung, although thats due to many additional reasons as well.

Damping wise, I always like to start full soft or no damping or fast, whatever your manual \ shock says. Generally small bump sensitivity is controlled by your low speed bleed adjustment. High speed impacts \ damping is generally classified as big hits such as potholes, dropping stuff, or anything that is characteristically out of place on the terrain (ie something that will upset the damper\tire). Roots COULD be in the realm of high speed, but here in lies the problem with trying to tune dampers to feel and my severe dislike for dealing with fox's so-called damper engineers.

The low-high speed transition velocity is UNKNOWN unless you have access to a damper dynomemeter. Furthermore, without adequate data aquisition to know exactly what piston velocities you see when riding over roots, you have no idea what regime you need to tune for. Generally speaking, dampers transition from low-high speed somewhere in the neighborhood of 2-5 in/s.
It all has to do with the piston designs and the shim stack, as well as any other high speed adjustments you may have.

Now, having also said that, and knowing you have high speed adjustments, youre at a huge advantage. Start with your high speeds turned all the way down so they impart no extra added damping force. This will be like tuning a normal 2 way vanilla rc. Dial in rebound then compression. Then slowly increase your rebound high speed to get it where you want it and then the same with compression. If it helps at all, 2-wheeled bikes generally preform best and fastest while their dampers spend 70% of their time in rebound an d 30% of their time in compression as opposed to 4-wheeled vehicles being best at 50 / 50. This is primarially driven by the fact youre dealing with just two wheels and two dampers and you need to keep the damper from packing down in sucessive bumps unlike in a car where you turn a lot giving the dampers time to "breathe" and extend to rebound. There are other things at play, but thats really what the driving factor is there...at least as far as I know.

If youre really interested in how dampers work, I can help answer questions, but perhaps the best source of this is a shock manual made by a real damper company. Look up Ohlines, and tell them to send you the Ohlins TT44 manual (~14$ I think). If you want a lot of quantitative jargon, buy Dixon's Shock Handbook. If you want all around good damper knowledge AND vehicle dynamics including bicycles get Milikens Racecar Vehicle Dynamics. Finally, if you want to know everything, sign up for one of Claude Rouelle's OptimumG seminars. I think hes having a 2 or 3 days seminar for students in california somewhere for ~300$. Alot of this information one really apply to bikes, but its damned interesting and Claude is a great guy with lots of great stories and information from his 20+ years in motorsport.

ok so Ive officially written an essay, sorry for the length, hope I helped, should get workin again.

[pechelman]

From my brief experience on the shock this is what I've felt. The pro pedal is just low speed compression. I believe the boost valve controls how fast the piston has to move before it overcomes the low speed compression and moves into the high speed dampening. The bottom out is just position sensative and not speed. It's all pretty confusing to me and would also like to hear for sure how it is all related.

Thanks for the info about the propedal and bottom out adjustment. I kinda figured thats what they were, but still going to call to verify this with the push people.

Re Boost Valve.
If they didnt change anything, the "Boost Valve" is just air pressure for the resevoir. I fvcking hate this stupid terminology. Anyway, the purpose of having an air charge in a resevoir first and foremost is to provide EXPANSION room for the added volume of the damper shaft as it enters the damper body during compression. If you didnt have this expansion room, youd have a solid locked damper incapable of extension or compression. Furthermore, as noted above, the air pressure within the resevoir aids in mitigating cavitation or the boiling of damper fluid. As we all know, Tboiling increases with pressure. Finally, it can also be used to tune a shock by adding pressure and therefore adding an additional quantifiable spring rate. Then theres something called Hysteresis, but we wont go there.

Interestingly enough, I know of some professional racecar teams in the IRL who dont use ANY resevoir pressure for certain tracks.

[Shepherd Wong]

Thanks! It's pretty fun learning about shock because it's pretty easy to notice the changes on the trail. The thing that I noticed is that adjusting the pro pedal changes the pressure in the air chamber. If you dial less pro pedal that will take the pressure down in the air chamber.

[pechelman]

wow thats interesting, im going to have to experiment with that and my shock pump later on. That brings up another reason I dont totally care for Fox, is that on the shocks Ive dyno'd, Vanilla R and RCs from 2000-2004, each models rebound and compression damping adjustments were NOT independent.

I may have some old dyno F-V and F-D plots hangin around somewhere that I could post on here as a reference as to how what effects what on a Stock Vanilla RC from 2003-2004.

[pechelman]

heres an old dyno graph of a Fox Vanilla RC.

Ive removed the scales due to its proprietary nature, if you really want to know what forces and velocities those are PM me and I can talk.

Each set was keeping the rebound damping full out and incrementing the compression damping by 3 clicks from none to all.
Notice how that adjustment does NOTHING until roughly the last 2 runs, ie 12-15 clicks.
Also notice that the rebound damping force INCREASES just by changing compression. Its also interesting to note that unless you're running a lot of freaking compression damping, you dont even utilize one of the compression shim stacks. In the autoracing world this shock is complete garbage, but it seems to do the trick quite nicely on a bike. Weird.

edit: it would be waaay clearer how much rebound force changes if I spent the 10 seconds to shift those 2 curves back to the others, but it was obvious we didnt want to use this damper so we tried another configuration.

Anyway, here it is;

[jibco]

Milikens Racecar Vehicle Dynamics.

Ahh, the bible.
Good discussion here. I've tuned with good results using the 30/70 ratio as well.

[flowtron]

I'm pretty far out of my league here, but what kind of race cars were these? Road, off-road, rally? The damping on an off road vehicle would be significantly different than a vehicle raced on road. Or is high speed/low speed simialr through everything....now I'm thinking about position sensitive damping (ie: Marzocchi)....aahhhugggggg.
Are car suspension systems similar or are there variations in axle (hub?) path? Different mtn bike suspension designs dictate different damping tunes, and Fox works with alot of companies to tune OEM shocks for different bike designs.
What about Curnutt? How are they received in the world of auto shock/damper design?

[pechelman]

I'm pretty far out of my league here, but what kind of race cars were these? Road, off-road, rally? The damping on an off road vehicle would be significantly different than a vehicle raced on road. Or is high speed/low speed simialr through everything....now I'm thinking about position sensitive damping (ie: Marzocchi)....aahhhugggggg.
Are car suspension systems similar or are there variations in axle (hub?) path? Different mtn bike suspension designs dictate different damping tunes, and Fox works with alot of companies to tune OEM shocks for different bike designs.
What about Curnutt? How are they received in the world of auto shock/damper design?

They were a formula style on-road autocross type of racecar. The damping required is different, and thats why we reshimmed them to suit our damping forces. The high-low speed transition point doesnt matter what youre driving\riding over, its totally defined by the shocks internals. The transition occurs when pressures inside the damper build up, due to restricted flow through the orifices, and open the shims. Basically theyre just round spring steel washers that cover up holes in the faces of the piston that flex open under these high pressures.

To be honest, theres nothing unique really about any suspension design. Everythings been done in one way or another or has been though of. What is different however is its intended purpose and how its designed. For example a lot of attention in biking seems to be on maintaining a very low and forawrd IC to minimize chain induced suspension compression and also to minimize wheelbase changes. In autoracing, generally speaking, we like to keep our ICs far out to minimize camber variation, although we also like to maintain stable Roll Centers, Pitch Centers, Yaw Axis, and SVSA (anti\pro squat geometries) Its all about getting the best compromise based off of your tire data, which is another wonder in and of itself.

Position sensitive damping is kinda similar to this high-low speed shim opening stuff, except the position determines how many bleeds\orifices are in the compression\rebound hydraulic circuit.

Im honestly surprised Fox works with OEM people to custom tune their shocks. As far as I know, they have no real damper engineers and they do NOT own a damper dyno. How the F you design a shock without a dyno is beyond me. Maybe theyve grown up in the past few years and hired a real engineer and gotten a dyno. I guess they have someone who has screwed with them enough hes just got the feel for it. Although if they had a dyno, I know they could produce an even better product as they wouldnt have shocks that produce FV curves like above.

Re Curnutt, I never heard of them before but I just read a short blurb about them on mba. Anything thats actively dynamically controlled always peaks my interests. From the looks of it however, it doesnt look like a shock that would interest professional racers, primarially because it doesnt look like it can handle the damping loads and packaging it would suck ass.
Although it should be noted that progressive rates on racecars generally do NOT work and actually make the cars slower and handle like ass. Also in the series I competed, our wheel rates were much lower than on a mtn bike and had much less travel. Everyone generally ran an inboard\rocker suspension with a motion ratio of roughly 1 and linear. It looks that most bikes run around a MR of 3 (6in wheel / 2in shock travel) and anything but linear. Infact Ive seen some graphs that are actually digressive -> linear -> progressive which seems crackheaded.

For a SERIOUS damper, checkout Ohlins TT44. That thing is PURE SEX. Its absolutely ridiculous as far as adjustability, different pistons out the wazoo, TONS of manufacturer support as to changing shims etc, I dont even know where to begin talking about it. Of course for how much they cost Id expect it, but it is the best off the shelf damper you can buy for motorsport...at least as far as I know.

let me know if you have any other questions
I know Ive said a lot here.