Need help from expert: Fox 38 Factory Fork Setup Issues - Seeking Advice for EMTB Descending Performance

[PowerExecutor]

Setup: I'm around 123 kg, and I've set up my Fox 38 Factory fork to achieve 25% sag as recommended by Fox's guidelines.

Problem Description: When descending at speeds around 25 km/h or higher, I experience an issue where the fork feels overly stiff when the front wheel encounters bumps or obstacles. This results in significant vibrations and forces on the handlebars, making it challenging to maintain control. However, on long, uneven sections, the suspension seems to perform better and absorbs the terrain effectively.

In summary, it seems I’m facing an issue with high-speed compression. The adjustment knobs don’t seem to make a noticeable difference. I’m also uncertain about the number of volume spacers (tokens) currently installed. Do you think it would make sense to remove some tokens to improve the fork's response to fast compression?

Any advice or insights on fine-tuning the setup for high-speed performance would be greatly appreciated. Thank you!

 

[steve_sordy]

Firstly, I never claim to be an expert!

There is a section on this Forum for suspension queries and it has several responses fixed at the top of it. Sorry, you are going to have to look for it, but one of them is this below. It's very good.

Bikerumor Suspension Setup Series: Full Series PDF - Free Download!

I too set up my Fox 38 Elite 160mm travel as recommended by Fox and it was crap. I was being shaken to bits! So, I sorted it out myself. It took a while.

I'm a bit lighter than you at 90kg + clothes and riding gear., say 98kg total.

I'm not too bothered about what the % sag is, it's a set up guide not a riding target. I started with sufficient air pressure to give me 30% sag. I put all the settings at mid-way and then rode then bike to see how it felt. I started altering everything until I was happy. The fork started with 4 tokens factory fitted out of a maximum 6 that could be fitted.

68psi gives me 26.3% sag (this figure was a result not a target, after the last token change)
All damping settings are fully closed CW, settings are ACW from closed.
LSR 5 clicks back out of 14 clicks range.
HSR 5/9
LSC 15/16
HSC 3/7

During the process, I altered the number of tokens in stages, one at a time until I got the ride I wanted. I ended up with two tokens.

The process was iterative. Alter this, ride, keep or adjust. Move on to the next one. If you adjust the number of tokens (up or down), each time you must inflate the fork in steps, cycling the fork through as a big a range of travel as you can each time.
Be aware that some new Fox forks have too much grease in them and this can block the ports from one chamber to another. One way to fix this quickly without having to dismantle and clean the fork is to deflate the fork, turn the bike upside down, remove the wheel, reinsert the axle, put your foot on the handle bar, grasp the axle and jerk the lowers up as far as you can. Repeat several times. If you are lucky, you will feel and hear a squelch. That would be the blocked ports clearing. It doesn't always work.

You are heavier than me and have different ride priorities, needs and wants. You will end up with something different. If you ride somewhere radically different, you may need to alter the settings by 1 or 2 clicks at the most.

 

[whitymon]

Hard to give you accurate guideline but I do not really follow any Fox recommendation for the psi, that make no sense for me.

For example I run 88 psi on my fork and I am 73kg all equipped, so cf post above it also really depends on your bike geo, your position and what you ride.

I would also be careful about how you tune your fork. LSC and HSC are somehow related, the numbers have to correlate somehow.

Second thing that comes to my mind, you need to play with rebound too, not having enough rebound does not allow the fork to do its job. You need enough to use travel and reset the fork otherwise you will feel the bumps.

Tons of people have different tuning so as @steve_sordy explain, you need to find yours by test & try.

Last words on that, I definitely have totally different settings for a same bike depending on where I ride and its weather, from tire pressure (which should be your first action - their role is as important as suspension) to the fork/shock.

NB: I am very low at the moment on mud/leaves like 23psi max.

 

[steve_sordy]

@whitymon Good call on the tyre pressure, I should have remembered that. It's not just pressure but the tyres themselves too. Both can dramatically alter the ride. I ride tubeless with usually 17psi front and 20psi rear. Thats a 29x2.5 Maxxis Assegai on the front and a 27.5 x 2.6 WTB Trail Boss on the rear.

This is a good guide to setting your tyre pressure.

Tech Tuesday - Find Your Tire Pressure Sweet Spot - Pinkbike

www.pinkbike.com

 

[RustyIron]

Problem Description: When descending at speeds around 25 km/h or higher, I experience an issue where the fork feels overly stiff when the front wheel encounters bumps or obstacles. This results in significant vibrations and forces on the handlebars, making it challenging to maintain control. However, on long, uneven sections, the suspension seems to perform better and absorbs the terrain effectively.

In summary, it seems I’m facing an issue with high-speed compression.

Hey, PE.
As you might imagine, it's difficult to provide great advice with just a brief exchange of words over the internet. But we can try. First, high speed compression has nothing to do with how fast your bike is going. The "speed" we're talking about is how fast the fork/shock is compressing and rebounding. Fox suspension components are notoriously over damped, so we'll assume that your concerns are a result of that. You didn't state the exact details of your fork, but that's ok. My suggestions will be rather generic.

First, you asked about volume spacers. Take them all out. I'm going to go out on a limb and assume you're not a Rampage rider, and that in your weight class, you'll benefit from having the extra volume and plusher overall ride. So do that and reset your sag, noting the pressure.

Now get out the Fox recommendations for your exact fork. Find the line that shows the pressure you just set up. It may not match your weight, but that's ok. It's the actual pressure that we care about. You can see the recommended settings on that line. But we don't care about those recommendations because Fox overdamps everything. Go UP three lines on the chart. That line will show lower rider weight and air pressure. That's the line we want. Set up the compression and rebound adjustments with the recommendations from THAT line. This is your new starting point from which you can make minor adjustments. Now go ride. Give it a couple rides before you make any other adjustments.

You want your rebound adjustments to be fast, but not so much that the fork "clunks" when it extends. If it's too fast, it will feel like a pogo stick and want to buck you. Compression, if too slow, will feel harsher than it needs to. If it's too fast, then your bike might feel lethargic and "muddy."

Give that a shot and let us know if it feels any better.

 

[Mikerb]

My first thought re your set up is that 25% SAG is probably too much for your weight...meaning your air pressure is too low. There are so many variables however.
Firstly Fox changed the damper tune on the Fox 38 some time a round 2022 to make the fork perform much more in line with their previous "e Bike" tune. ( plusher on top and greater ramp up). So your fork model year is relevant.
Second, the same fork settings for the same rider will change on different bikes mostly due to different head angles and the bikes weight balance front to back. In short, copying other rider's settings is a waste of time.
Third, you need to know how many tokens are being used, so deflate the fork and start from scratch getting the spring rate right first. At your weight you likely need at least 2, possibly 3 token for a 160mm fork.......maybe up to 4 for 170/180mm.

Make a decision how many tokens to start with.
Set SAG ( I suggest no more than 20%) with all compression and rebound settings fully open/fast. Set the SAG in the ready position and with the fork mobile ( ie compress the fork a few times so stiction is overcome and set the Oring on the last upwards movement of the fork). Repeat a few times to ensure you are getting a consistent result.
Your spring rate is the baseline for set up. It is naturally progressive and the more tokens used the more progressive it becomes......fewer tokens= less progressive. Note the air pressure.

Next adjust low speed rebound to a middle position.....everything else left open/fast. Test ride whilst adjusting that up and down a couple of steps to decide which feels best.
Next set low speed compression to a middle position and repeat as a bove.
You should now have a fork that performs the way you want for most riding except bigger and harder hits, drop and jump landings since those settings largely impact on the top third to a half of the forks stroke.
Finally repeat the above first with high speed rebound, then high speed compression.
Lastly check what proportion of fork travel you are using on your hardest drop/jump landing etc. It should be a bout 95% whereas a complete ride without anything too extreme should still see your o ring at 80 to 85%...meaning you are using most of the travel available with some in reserve for the extreme bits.
Based on those findings you may then need to adjust your spring rate up or down by a few psi.

 

[Bones]

Without reading all the replies, my understanding and setup is this.
On the 38s all the damping oil does through the slow speed rebound circuit first and this part of the system is chocked by only having something like a 2mm hole for the oil to pass through.
So I have my forks with the slow speed rebound set fully open and high speed about half way.
Also ignore fox recommendations for sag at 20% and go closer to 30%.
On my 170mm forks about 34mm was fox setting but I'm now running about 48mm.
I also run fast compression fully open with slow comp about half way.
Now I'm about 75k in riding gear so my settings are no good for you but they should give you some better understanding of how this fork seems to work.

 

[Mikerb]

If you are happy with your settings then fine.....but just to provide a different perspective, and one that I realized after months of trying different set ups........
The spring rate dominates! Set your air pressure too low and the rebound forces are so low that damper settings are all to heavy unless left virtually wide open. It was not until I abandoned the idea that using lower air spring pressure makes the fork more plush, that I was able to balance rebound and compression settings....and actual feel the difference each couple of clicks on each setting made to its performance.
I should also mention that prior to somewhere around 2022 Fox changed the damper settings...for the better. I have identical bikes, one with a 21 Fox 38 performance elite and the other with 23 fork..same travel, same spec. I set both the same......the 23 fork is amazing on all terrain. The 21 fork is fine for fast enduro type descents with drops and jumps but does not deal well with repetitive small chatter.

 

[irie]

If you wanna go faster downhill, don't brake unless you think you're gonna die.

 

[PowerExecutor]

Hey, can somebody from experts explain to me how slow-fast compression works? Like, what spring or valve starts first and all that stuff? I'm trying to get a better understanding of what I'm dealing with.

 

[Bones]

It's relevant to shaft speed, not how fast you are traveling.
Slow speed mainly slows down how much the fork dives when braking and it keeps the front end level when going around berms etc.
Fast compression controls how the forks react to bottoming out on landings, and multiple hits like rock gardens.
It also affects how harsh the forks feels.
So if you set them up for a plush feeling and still bottom out, then you need to fit some tokens in the air side.
Again that's my understanding of what's happening.

 

[Bones]

Here is some interesting reading

Spring rate vs. compression damping – fork comfort for sustained rocky descents

Home › Forums › Bike Forum › Spring rate vs. compression damping – fork comfort for sustained rocky descents Search Overview Chat Bike Members News Women This topic has 4…

singletrackworld.com

 

[Mikerb]

Hey, can somebody from experts explain to me how slow-fast compression works? Like, what spring or valve starts first and all that stuff? I'm trying to get a better understanding of what I'm dealing with.

I think you are better off thinking a bout what effect both low and high speed compression have on the forks performane rather than worrying about how the effects are achieved. As stated a bove low and high speed relate to the speed of the fork travel.
So going back to the basis of the fork set up.......spring rate/ PSI in the air spring........the resistance to fork travel is progressive. That means at the beginning of the forks travel the resistance is at its lowest and if the air pressure is set quite low ( greater SAG) the fork will push through the first part of its travel very easilly. Setting SAG prevents that first part being too easy since the bike plus rider weight will compress some of that initial travel all the time=SAG. But for terrain where the fork is being compressed by a steady force, a very good example given a bove being when negotiating a berm, you may want the fork to be more supportive in that next area of fork progression after SAG. Low speed compression will do that. Berms are not however the only example of where the fork is typically compressed in a more constant way.
As far as high speed compression is concerned this is typically when a lot of fork travel is used in a single hard hit. That could be a landing from a jump or drop, or fair sized square edge obstacles. Now it may be that the combination of your chosen spring rate and rebound settings provide sufficient support even on the hardest hits you encounter on your riding terrain, in which case you probably need little or no high speed compression. The opposite is true if despite your spring rate and rebound settings being mostly ideal, you bottom out on the harder hits, high speed compression might need to be set higher.
Personally I set low speed compression for best overall fork performance but leave high speed compression open for most terrain. If I then expect to be doing a run with the bigger hits I add a couple of clicks of high speed compression.