Ask Pinkbike: Fox 38 Damper Tuning, Short Cranks for eMTBs, Axle to Crown Heights, & How to Avoid Going Over the Bars
Here at Pinkbike, we get inundated with all kinds of questions, ranging from the basic "Can I have stickers?" to more in-depth, soul-searching types of queries like if you should pop the question or what to name your first child. Ask Pinkbike is an occasional column where we'll be hand-picking and answering questions that have been keeping readers up at night, although we'll likely steer clear of those last two and keep it more tech-oriented.
Question: @dazza1 in the Mechanic's Lounge: I'm looking to get my Fox 38s custom tuned. I would like some recommendations of what are the best things to do option wise and any company recommendations. I'm in the UK so would need to be a UK company.
A few issues I don't like are the small bump sensitivity isn't the best, they also dive a lot in the travel almost like pedal bob on a rear shock if that makes sense and it would be nice if they were a bit plusher on the medium size slower hits as that's whats on one of the local trails I ride. It's very steep and rooty blown out and rough.
I could run less compression but then my forks dive more. I could also take out a volume spacer as I rarely use full travel unless I mess up but I was wondering if that would make the diving worse. I'm definitely not an expert rider by any means, but progressing every time I ride. I bought the bike because it had Fox factory suspension with lots of adjustability; coming from my old 2013 Trek Slash I quickly outgrew that suspension, particularly the rear shock. Any advice would be great.
 | Suspension setup can be a tricky one to achieve given all of its complex internals. Thankfully, you don’t have to worry about how it works, just where it works. Most brands have a detailed setup chart that relates spring rate to body weight, and the corresponding damper controls, to help you find a happy place relatively quickly. Carving out some time to do repetitive tests where you systematically make changes to the suspension on a short track that reflects your favorite, or most commonly ridden, terrain will expedite this process. A few more details about what bike you’re riding, and how much you weigh would help us further, but we can get started regardless. Jordi Cortes from Fox went through the setup on a 36 fork in their Dialled series and the 38 would basically be the same. You can follow along with our main setup tips here. Remember that the set up of the bike, including the rear shock can affect the weight balance. Some frames don’t always require the general 30% rear shock sag rule, so check with the manufacturer on that. From what I can read into your concerns, it sounds like you could actually benefit from running a higher spring rate. That may seem counterintuitive, however, you could be sitting too deep in the travel, frequently hitting the ramped portion of the air spring curve which will lead to the harsh feeling you described.  Regarding the volume spacers, they do affect the 38's air spring curve throughout, but in different amounts, as seen in the Fox 38 Tuning Guide. The largest change happens at the end of the stroke. Sticking with the stock number of volume spacers and adding low-speed compression could help with controlling the front end height under braking. Secondly, opening the rebound control will help the fork recover quicker on repetitive bumps. I’d start by checking the advised spring pressures and rebound settings, but don't be afraid to move up a weight bracket or open the rebound further. Keep track of your settings, make one change at a time, and write down your qualitative findings. After you’ve done a bit more setup homework, think about where you feel most confident in the suspension. The best thing to find is predictability. That will build confidence in how you expect the fork, and rear shock, to react to each bump. Did you purchase the bike used? If the fork hasn't hasn't been serviced in a while that could explain the poor small bump sensitivity. A basic lower leg service is something that you could do at home with a bit of mechanical know-how, or any reputable should be able to help with that. I'd start by focusing on your setup and the overall condition of the fork before trying to have it fully re-tuned. Unless there are some other parameters that haven’t been announced here, I’m confident that re-tuning the damper isn’t necessary. The only case I’ve experienced where the rider needed the damper re-tuned was due to the weight being less than 48 kg (106 lb). You could always reach out to the Fox service center in the U.K., Silverfish for more assistance. |
The Fox 38 Factory Grip 2 damper has a wide range of adjustments for riders of all skill levels.
What's the right crank arm length?
Question: @BilalKasks in the eMTB forum: What is the optimum crank arm length for you? I am interested in 155mm crank arms. The stock length is 165mm but the shock travel is 180mm and sometimes the pedals are hitting the rocks/roots. My frame size is XL. Do you think 155mm crank are too short for XL frame? Does anyone use the same length?
| Crank length is one of those parameters that doesn’t have a “correct” dimension for a mountain bike. There are strengths and weaknesses that depend on the type of riding you’re doing, what terrain you ride in most frequently and what feels natural to your body. Before we go further though, check out the suspension setup tips mentioned in the section above to ensure your sag is set accordingly. If you're sitting at 50% sag, your search for shorter cranks could stem from this misjudgment. Longer cranks do not necessarily produce more power. In fact, shorter cranks are more effective at spinning due to their ground clearance, which is the focus of your question. No one else has broken down this misconception and then followed up with real-world comparisons as in depth as Seb Stott broke down Why Shorter Cranks Are Better According to Science. Standing at 191cm / 6'3" tall, Seb is by no means small, however, he found more benefits from stubby cranks, even when coasting downhill. There are some caveats that come with shorter cranks though, such as factoring in addition dropper post travel. You can read all about his on-trail experiences comparing 155mm cranks to 170mm cranks on an enduro bike here. I’ve always found it interesting that smaller bikes are generally spec’d with short cranks, while extra large bikes receive longer cranks. Sometimes that range can be as much as 10mm (165 to 175mm), however, the bottom bracket height between frame sizes doesn’t change, making the break-over angle less and crank strikes (and chainrings) more frequent on the larger size frames. Neither 155 or or 175mm crank arm lengths would be inappropriate on an XL bike, but there’s been a trend towards shorter cranks. That’s especially true for e-bikes which benefit most by keeping the cranks, and therefore the wheels, turning at higher RPMs. Surprise, surprise; the answer isn’t as clear as you’d hoped. It depends on what suits you. Luckily, crank arms can be changed rather quickly and aluminum ones are fairly inexpensive if you fancy experimenting yourself. |
Shorter cranks are growing in popularity, and not just for eMTBs either. The misconception that longer cranks produce more power isn't as straightforward as it sounds.
Learning to fly without getting bucked.
Question: @Tchai8 in the Downhill forum: Almost go OTB on many jumps, especially with larger lip. If I preload it is not as bad. Any thoughts or techniques to better approach this?
| After spending enough time around bike parks and jumps, I’ve witnessed this scenario far too often. Learning to jump is no small task - remember, we all had to start somewhere. There are a few basic guidelines to start with here, but practice and progression are the keys here. Start small, really small. A pump track is a great place to begin or revisit. Don’t worry about what bike you have - you can pump up the tires and suspension on an enduro bike to increase the efficiency. Keep the wheels on the ground and work on your timing of body inputs to build speed. This will help you learn when to push the bike into the face of the jump, or absorb it. A simple “fly-out” jump or step-up style of jump is also encouraged. There are fewer consequences surrounding these types of jumps since your vertical speed is much lower upon landing. It’s also worth investigating your bike setup. The general rule of thumb for balanced suspension is 15-20% sag (total travel used while standing stationary on flat ground) at the front and 30% for the rear suspension. Similarly, the front suspension should return faster than the rear, since we’re riding downhill to gain speed. We recently created a video to help you set up your suspension properly, because a balanced bike will eliminate irregularities and build confidence across a variety of jump shapes. Taking a lesson from a certified coach would also be a great move. They’ll be able to identify your body position, balance, and suggest ways to improve. Moreover, you’ll improve much faster with their help. If that’s not feasible, hang out at the local jumps or skatepark where you can absorb information from other higher-level riders. Filming yourself and looking back on what went right or wrong is also a worthwhile practice. |
Talus Turk didn't start with gaps or moves of this magnitude, but step-up jumps are a safer place to dial in your bike setup and air awareness.
Does changing fork travel affect the axle to crown length?
Question: @fiveminuterides in the Mechanic's Lounge: I know most people want to ask questions about how to increase fork travel. In this case, my Canyon Neuron 6 came with 130mm travel front and rear with a RockShox Judy. About two years ago, I changed the Judy for the Rockshox 35 Gold with 150mm travel. I’ve actually been really happy with this change. I like how it rides, climbing is still fine, etc. and of course, this changed the head angle by 1° But since learning that a 20mm upgrade to the fork travel may be putting stress on the frame, I also wonder if it’s putting stress on the rear shock. The Rockshox 35 Gold has the ability to change the travel by inserting a travel spacer, which would reduce the travel to 140mm, but I wonder whether or not that will actually change the overall length of the fork, and therefore reduce the head angle by half a degree or not? Is anyone able to advise?
| These are great questions and it sounds like you’re very in tune with your bike’s handling. Although some manufacturers allow for the fork travel, and therefore the axle to crown (ATC) height to change, that number usually isn't more than 10mm from the stock setup. Product development teams have put effort into building each bike with a specific character. Changing the travel beyond that scope can have major drawbacks on the bike’s handling by raising the bottom bracket and slackening the head angle too much, not to mention posing the risk of stressing the frame. With that said, the same is not always true for rear shocks. The eye-to-eye, or overall length of the shock can remain the same when internal travel spacers are added to limit the stroke. It’s unclear which model year Canyon Neuron 6 you have, so to find out the maximum axle to crown fork length for your bike, I’d reach out to Canyon with your serial number handy. You can find the axle-to-crown (ATC) measurements for all RockShox forks in their well laid out technical specifications webpage (page 15). The ATC for the Gold 35 29” fork at 140mm depends on the offset as well - that’s the horizontal distance from the steering axis to the axle - which means it could be 551 or 557mm. Page 31 of the 2023 Rockshox fork service manual shows you how to change the travel step by step. And yes, the axle-to-crown height will change when you adjust the travel - the spacers change how much the fork is able to extend. |
Not all frames can handle the increased forces that come with extending the travel beyond the recommended length. On top of that, it can also throw off the handling.
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it should be the same as the left one.
Some forks/shocks are massively overdamped. Some are massively underdamped. Some are harsh no matter where you have damper settings.
Then lets not forget the confusion with HSR/LSR on Fox forks and shocks. It's basically impossible for the average rider to dial them in correctly. Even trying to use other riders settings don't work as they all dyno differently.
A good tuner can sort all that out. But most of them are just charging for volume spacers.
So perfectly put! I want it super plush, no bob, lot of midstroke support, compliant on small chatter, not too much ramp up at end stroke and absolutely no harsh bottom out. “Have you set your sag?” …crickets
Also, if you have an X2 post 2021, just send it back to Fox and they will give you the new updated version. It's a great shock when functioning as intended and it is painless to service with the proper tools. Just pay for shipping one way. Cheaper than buying anything else
also I didn't mean sell an x2 because of the reliability, but rather the fact that to do an aircan service you have to rebleed the damper, plus the special tools needed to take it apart in the first place. If you ride a lot it's a shock that's kinda a pita or expensive to maintain.
I only service my damper like once a year or probably more accurately when it gets a little squelchy, but I drop my lowers and pop off my aircan every 2 to 3 months.
I once saw a comment on here that summed it up perfectly which I'll paraphrase: the really good amateur riders just run whatever is cheaper/on sale/reliable etc and make it work with skill. It's only the less good riders that need the crutch of riding the newest and shiniest stuff. Run what you've got, just set it up properly and service it!
I know longer travel means more flex, but again I very much doubt that is genuinely a limiting factor for anyone on here. In fact I very much doubt 160mm is either, honestly if unless you're racing I don't think people are being honest with themselves about what is actually holding their riding back, their skill. I'm nothing special and I used to ride revolution bike park on a 120mm Scott Spark with 32s on it before I got my Slash, sure I was not the fastest on the hill and it was sketchy at times but I would ride all the tracks except the 50 to 01 line. I was the limiting factor, not my bike
Where? and Why? are questions that immediately spring to mind!
You can also ride DH track with hardtail bike, but it doesn't mean it's the right tool for that purpose.
www.shockcraft.co.nz/technical-support/setup-suspension/1-page-suspension-setup-guide
Using sag puts all the emphasis on the first few mm. Which change massively between spring types and have little relation to mid-stroke. Compare RS Debonair B1 vs C1 and you'll get very different mid-stroke for the same sag.
Oh I get it now, it’s an ad.
@HankHank: This is where a fly-out jump or a quarter-pipe at a skatepark will be safer to adjust to. It might be worth double checking that your suspension is balanced and you're running the appropriate spring rate too.
-pinkbike comments
The total compression damping available is low even with HSC and LSC fully closed. But the harshness from the mid-valve choking leads everyone to run compression more open and try to compensate with more air pressure.
To fix it requires significant modifications (i.e. cutting metal and adding new parts). I think I'm the only one worldwide doing this.
The GRIP2VVC fully closed can produce about the right compression forces, but the damper curves at that point are all wrong and it feels terrible. So you end up running it softer and overspringing air to survive. Which compromises everything.
@panaphonic: Same harshness issue, even less support. I mod and revalve those and it's a royal PITA as 0.02mm stack thickness chnage will double/halve your compression damping.
@bikesjim: Is that the one where they change the IFP spring and not much else?
One thing you learn as a racer is that the clock doesnt lie, doesnt matter how much you dont like the feeling of the bike, the clock tells the truth.
How many people ride a trail bike with a double down or DH casing rear anchor on the back, but would ride faster with a EXO+ trail tyre....
How often does a bike review complain because there isnt a 200mm dropper, which would put the saddle lower than Jackson rides his Dh bike!
Just because you like to do it different, doesnt mean its faster, going slower might feel better, but its still you going slower!
If you ever want to post a damper over for my 38's, Lyiks (I run them with a push coil, its not any faster but feels nicer than the air) or even my boxxers (I have coil and air, neither is faster but I like the feel of the coil better for most things) and I will back to back test it for you, feel free, I would only give the honest outcome of the test.
I would be faster right up untill the flat.
awesome for your customers, we all know so many riders who get the latest, greatest and spend a fortune.
love it when I pass them on my inferior bike
Sent one over, I will give you some real data and real feedback from someone who wont go on what I think but will give you test data on multiple tracks and conditions and has years of test data to go off too from doing race setup.
unless you dont think your test output will match your typing output...
I really don't like fox forks and wasn't loving the 2022 RS. Mainly could never tune out the high speed braking bump sharp jabs. They were killing my old broken wrists. This fork now has incredible support and soaks up everything. The thinnk that always makes me laugh is anyone that pushes on the fork comments, "Wow, you run your fork really stiff!" then i have to explain to them its actually extremely plush and thats what support feels like.
You don't need to know who I am Mr Dougal, I am just a random old boy joey who rides bikes for fun.
I don't need your tuning, but would be interesting to let you know how good it is or isn't and post it back to you afterwards.
If you don't want to send something over for some good testing and I would publish the results on this conversation then that's all good.
Ask the question on mtbr and see who pops up.
It's ok, you are obviously not going to send anything over to be tested.
I am out with the local EEB Enduro champion just now, picked a shock up from the guy who was 2nd and none of us know who you are.
I am the Scottish and National downhill champion.
Have never heard of you, but I have only been racing for over 15 years and have multiple Scottish titles and a couple of national ones (in the old and slow category). Would have been fun to test between tunes as racers we are always looking for every advantage and the off season is the ideal time for testing. If it worked well I could have fed some info to a couple of world cup boys.
Oh well. Maybe next time.
Bike geometry also changes fork loading a lot so that needs taken into account for a whole view. The difference between a slack bike with long reach and short stays vs a steeper bike with long stays and moderate reach can be 20-30% in fork loading.
There is a sick world of riding out there once you take a wee peak over that rock.
Enjoy.
You would need soft vice grips (and a vice) for the diameter of the shaft (which I forgot, but I believe it's 10mm?) a 12mm wrench to undo the head, some small pick to pry away the loctite and a bullet like thingy (I've seen people use a object similar to a ballpen) to put it back in again, to make sure you dont damage the o-ring when sliding it back in. And a little loctite 277 for when you are thightening the head back on the shaft.
I would use a mix of slick-honey with a dash of Fox 20 to lube up the pistonhead and tube, adding the 3ml Fox 20 on top later.
SX and NX for example, "exist" but then are nowhere to be bought. Even my bikeshop can't find them.
Considering the growing interest in short cranks, there is a surprising void here
Man, this felt so weird for the first 20 minutes, I could barely stand on the pedals comfortably, then after a few rides it was natural. When I went back to a bike with 175mm cranks for the first 10km it felt like I was John Cleese in the Ministry of Silly Walks sketch. Rode gravel and MTB with 175mm cranks for a few rides then went back to 145mm cranks on that road bike. Weird again for a few minutes but I got used to them faster this time.
Now I need to find a nice loop with a bit of flat and a long climb, not too long, not too small and do back to back test with exact same bike and gearing in the same day and check on the gps record. Repeat 4 to 5 other days, alternating which crank I am using for the first ride, then study the gpx records and see if I am statistically faster with one length over the other.
Goldix are very bad. I got some for my son.
the axle was too long
the thread for the pedal needed to be redone
the so called sram interface for the chainring did not fit a sram chainring
the alloy of the crank arms is very soft, my son at barely 50kg bend them...
I use the Shockwiz for my bikes, because i'm just an average 50s bimbler on Reds... so don't need complete setup, it works well. But on my lads DH bike, we go to RSRBikeworks as he's good enough to notice a difference.
Here's a simple and reliable setup guide that takes you through all the adjustments in order and uses frequency to set spring-rates: www.shockcraft.co.nz/technical-support/setup-suspension/1-page-suspension-setup-guide
To the guy with the question that got dodged - get an avalanche damper. Because you cam actually have supportive fork that sucks up square edges,
I'd be interested inseeing PB buy some aftermarket dampers for 38,s amd Zebs and write up a comparison of performance avalanche and Fast both offer aftermarket custom tuned.
I mean, isn't an influencer just another name for a person who advertises stuff for pay?
www.rsrbikeworks.co.uk
hardtail bike by using a 118mm ISIS BB? Hope E-bike cranks are way cheaper than the standard MTB axle version.
www.hopetech.com/_repository/1/documents/E-BIKECrankFitmentDiagram_web.pdf
"If I don't preload. I get bucked." You just answered it yourself.
Short cranks don't promote spinning because of ground clearance, they promote spinning because your foot doesn't have as far to move per rev so it can more easily make more frequent the direction changes required of higher cadences. Like an oversquare racecar engine that can rev higher. Ground clearance just lets you pedal, period, even in rough terrain.
Axle to crown does not change by 7 mm when the offset changes by 7mm. That makes no sense at all, is it not what that chart says. The longer A2C with the (F) is for a model with fender compatibility. At least give correct answers when you put questions on the front page.
My recommendation would be, take your weight bracket, follow the PSI's indicated and add around another 7% to 10% on top of that, and then follow the indicated rebound settings for this level of air-pressure and figure out if you like to be at the slow or the fast side. This setup mostly works when you are getting up to the speedy kinda things and like some support.
If you are not happy start playing around with the tokens.
(edited)
Something i will agree on though, The price is stupid, I paid similar pricing for both at around 15k NZD each.
Just to make you really mad, i also have 2 surrons.
I bought a bike with 165mm cranks and it absolutely sucked. Less torque and less power. Swapped it to 175mm and I was able to pedal up the same hills in a 36T rear instead of 42T.
A shorter crank reduces the pedalling circumference, thus increasing velocity for each pedal stroke. It happens naturally.
I also had about 10% of the pedal strikes I had before.
For a given cadence, your legs need to actually move faster (not in RPM, but lineal rate). I find this to be a much bigger factor than how many revolutions I am making. My body has had no issues turning shorter cranks at a speed necessary for my max output.
Since we are basing science off of personal anecdotes now: long cranks suck.
Reducing the circumference reduces the velocity for the same cadence. This is pretty simple physics.
Looking at the lever arm of the crank for peak torque is only part of the story. The longer your cranks, the higher your foot needs to be at TDC, and the more you need to bend your knee. With shorter cranks, you can keep your legs straighter and generate more consistent power throughout each stroke.
For me, shorter cranks are more comfortable and result in fewer pedal strikes. The only small downside to shorter cranks is the need to run the saddle higher to get back to the correct pedaling position, which probably also means a change to the height of my handlebars too.
By reducing the crank length, your RPM naturally increases in order to keep the same speed, which is what you're defining as natural cadence, which is incorrect.