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Discussion Starter · #1 · (Edited)
Do you think your fork feels harsh over small bumps?

Chances are your fork lowers may have tight bushings, which is quite common! Especially on brand new forks.

I knew mines were not great ... I had to apply way too much force and wiggle the stanchions almost all the way to make the uppers slide into the lowers while servicing the fork. How could they be great at dampening small vibrations that don't generate enough force?? Once I realised that ... I ordered a tool to resize them.

Fork Bushing Sizing Tool



Once received I measured the "head" on the tool to use with 35mm stanchions at 35.1mm.



Fox say correct bushing tolerance is a diametric clearance of

min: 0.0015" (0.0381 mm)
max: 0.0090" (0.2286 mm)

source: https://www.ridefox.com/fox_tech_center/owners_manuals/07/eng/bushing_technology_and_inspection.htm

That +0.1mm (0.003937") to resize/widen the bushings is perfect.

If you do the math and take Fox's recommandation to see what the result should be, you get:

(0.0090"-0.0015")/2 = 0,00375" (0.95mm)

Which is right in the middle!

RS doesn't even list the bushings on their "Spare Parts Catalog" because they are made to last forever (basically if the fork is well maintained, you won't have to worry about it), so I guess they make them a bit tighter now to backup that claim

Truth is, most of the time they are too tight and when people scratch their head and try to improve small bump compliance by tweaking x or y they miss the main culprit (tight bushings), it generates a ton of frictions and from there it's an uphill battle.

Do a test next time you service your fork.

1 - Drop the lowers
2 - Remove the damper/spring
3 - Remove the seals
4 - Cycle the uppers into the lowers by hand (with nothing else)

You'll be surprised (or shocked. LOL) by how much force you need to cycle the uppers ... I can almost guarantee they won't fall down like mine! If it's the case then your bushings are too tight/narrow.



Out of 3 forks I serviced recently (Pike/Lyrik/Yari), including my own they were all sticky and hard to cycle, you had to slightly move the uppers side to side to push them down ... and apply force at the same time!

Once I resized the bushings, well ... have a look at the gif, the uppers now drop under their own weigh into the lowers.

It's very easy to use the tool, just force it through the bushings with a twisting motion and you'll have perfectly honed out bushings in your fork. (apply some Motorex Supergliss on the head and on the bushings with a long paint brush before)

If you want to do it, you've got 3 options.

1) - Make it done by a proper suspension specialist/tuner (it's nice to do it when you need servicing or tuning)
2) - You could buy the Fox tool for about $1000, yeah you read that right (ref: 803-00-813), that includes all the diameters they make (32/34/36/40)
3) - Or you could ask Oliver at Blue Liquid Labs | An Oliver Majewski Project to make you one for 140USD with any head diameter you want/need (that's what I did)

And don't forget to lube them with a nice slippery oil such as the Motorex Supergliss 100k (Big thanks to Dougal for testing and releasing that info)



Resizing your bushes will give you slick, super smooth, stiction free action ... IMO that's the number 1 thing to verify and correct if necessary before modifying anything else. You can definitely feel the difference over small bumps and chatter it's incredibly supple off the top now.

because the fork doesn't need a lot of force to start moving/reacting and make those stanchions slide (they are not micro-stuck like before). it's a bit like when you have seized bearings on your frame, because of the weight/leverage AND it probably happened over a long period of time you can't really tell ... until you swap them for fresh ones and all of a sudden it feels like a brand new bike and your shock can do it's job to dampen small vibrations and chatter, NOW you can feel it ... well it's the same feel :)

I hope it will be helpful for some of you!

Some comments about tight bushings ...

TF-TUNED (MTB suspension tuners in the UK) say:

They simply won't "bed in" as is commonly believed & the bushes need re-sizing. This process involves changing the shape of the bush within the lower leg, to fit the stanchions better and prevent binding when the leg is passed through.
source: https://www.tftuned.com/tech-help/22-rockshox

Fox about Race Prepping a Fox 40

With the stanchion carefully set aside, Kolja does one of the things that he says makes the biggest difference to the feel of the fork-he works the bushings. With mass manufacturing the tolerances for the bushings is tight, opening the bushings does have an effect on the life of the fork, which would mean the fork would need servicing much sooner. By opening the bushing the fork moves more freely in its travel.
Source: https://www.pinkbike.com/news/race-prepping-a-fox-40.html
 

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If your fork bushings are tight, the fork can feel great after a service but gets sticky really quickly as the tight bushings plow the lube oil off the stanchions.

Bushing clearance for ideal sliding is about as much clearance as you can run without creating noticable play or knocking. The clearance is needed for lubrication film.

RS and Fox have tightened up their fork bushings a lot in the last few years. Fox went from ridiculously sloppy to almost Ohlins tight.
 

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Discussion Starter · #4 ·
If your fork bushings are tight, the fork can feel great after a service but gets sticky really quickly as the tight bushings plow the lube oil off the stanchions.

Bushing clearance for ideal sliding is about as much clearance as you can run without creating noticable play or knocking. The clearance is needed for lubrication film.

RS and Fox have tightened up their fork bushings a lot in the last few years. Fox went from ridiculously sloppy to almost Ohlins tight.
Thanks Dougal, it's always nice to hear about your experience and feedback

The most ridiculous was an almost brand new Pike ultimate, I've had to use the tool about 10/15 times in each leg before it went smooth ... shockingly tight!

BTW: thanks a lot for the Supergliss 100k, it's ridiculously good, I'm hooked :)

Using a small paint brush to lube the bushings, it was not even dripping, it's sticking to the surface but it's super slippery at the same time ... perfect for the job. So glad to have 10cc of this amazing oil in each leg now!
 

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Great stuff. I had the exact same problem. Brand new 2019 RS Rev fork. TERRIBLE stiction off the top and just notchy/harsh throughout the entire stroke. Sent it back to SRAM for warranty. Of course they didn't do anything other than service the lowers and call it 'good' for the very reasons you listed above. So I sent it off to Ryan @ Fluid Focus.

Tight bushings. He honed them, rebuilt the lowers and sent it back and said enjoy...and he was right. It is smooth as glass now.
 

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Great post!

I recently did the 200 hr service on my Recon RL and while I did lightly wet sand the bushings, I am now kicking myself for not checking fit with the guts removed.

On a similar note, when I tore my rear suspension apart for inspection on my Marin Rift Zone; four out of the six bearings felt crunchy. But once I pressed them out of the frame and rocker they felt fine.

Calipers came out to confirm my suspicion. All the bearing bores were way too small. I'd say about 0.001" interference fit is good for a bearing of this size, and they were all -0.006"!

I ended up designing and making a custom tool to resize these bores,while retaining cylindricity. New Enduro Max bearings run beautifully installed and the bike is much better at ironing out the small stuff.

Sent from my 2PZC5 using Tapatalk
 

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Pictures or it didn't happen.


I'm actually looking into patent protection for it right now. I'm a mechanical engineer who works in CNC Manufacturing and don't think this tool exists.

The design is scalable for essentially any size and depth. It works on bores with a flange, which prevents the use of normal cylinder honing tools. It requires no special fixturing. Works on anodized bores. It can be made quickly and relatively cheaply and can be used by a novice with a hand drill.

With so much manufacturing out of China, I'm guessing undersized bores aren't that rare.

Sent from my Pixel 4 using Tapatalk
 

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Discussion Starter · #10 ·
Great post!

I recently did the 200 hr service on my Recon RL and while I did lightly wet sand the bushings, I am now kicking myself for not checking fit with the guts removed.

On a similar note, when I tore my rear suspension apart for inspection on my Marin Rift Zone; four out of the six bearings felt crunchy. But once I pressed them out of the frame and rocker they felt fine.

Calipers came out to confirm my suspicion. All the bearing bores were way too small. I'd say about 0.001" interference fit is good for a bearing of this size, and they were all -0.006"!

I ended up designing and making a custom tool to resize these bores,while retaining cylindricity. New Enduro Max bearings run beautifully installed and the bike is much better at ironing out the small stuff.
Thank you! Woow, you're quit the mechanic.... that was risky, what a story! Well it proves that tolerance variations are pretty common when building stuff at the industrial level and while it kinda works you could also find yourself at the very limit of what's acceptable for QC. I guess that's what happened with your bearing/frame story, and it's definitely the case with fork bushings! Your frame was alloy right? Do you mind sharing more info, pics of the tool? I love those stuff AND happy ending stories. LOL :)

FYI: I don't think wet sanding bushings are a good idea, (I could be wrong). You seem to be the kind of guy who can do better and make his own resizing tool!
 

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Discussion Starter · #11 ·
Great stuff. I had the exact same problem. Brand new 2019 RS Rev fork. TERRIBLE stiction off the top and just notchy/harsh throughout the entire stroke. Sent it back to SRAM for warranty. Of course they didn't do anything other than service the lowers and call it 'good' for the very reasons you listed above. So I sent it off to Ryan @ Fluid Focus.

Tight bushings. He honed them, rebuilt the lowers and sent it back and said enjoy...and he was right. It is smooth as glass now.
Glad he did a proper job! It's too often overlooked. Hence why you should go to a REAL suspension tuner ... easiest and safest way to enjoy the benefit of a slick suspension!
 

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Discussion Starter · #12 ·


I'm actually looking into patent protection for it right now. I'm a mechanical engineer who works in CNC Manufacturing and don't think this tool exists.

The design is scalable for essentially any size and depth. It works on bores with a flange, which prevents the use of normal cylinder honing tools. It requires no special fixturing. Works on anodized bores. It can be made quickly and relatively cheaply and can be used by a novice with a hand drill.

With so much manufacturing out of China, I'm guessing undersized bores aren't that rare.
That's super cool! Good on you
 

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Discussion Starter · #13 ·
Could the stock bushings be replaced by those?

https://www.igus.com/product/3



Do the mechanical properties match what's needed for a MTB fork?

Example:
------
Maximum running speed, linear, continous
1,575 fpm (8.0 m/s)
Maximum running speed, linear, short-term
1,969 fpm (10.0 m/s)
Coefficient of sliding friction, dynamic, against steel
0.06 - 0.18 μ
------

They make them in:

35mm ID - 39mm OD
36mm ID - 40mm OD

ID is fine but what about the OD?

And you can request a free sample ;)
 

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Glad he did a proper job! It's too often overlooked. Hence why you should go to a REAL suspension tuner ... easiest and safest way to enjoy the benefit of a slick suspension!
Def. Truth be told, after SRAM but before sending off to Fluid Focus, we even installed the Charger 2.1 RCT3 damper upgrade which effectively turned this fork into a Pike Ultimate thinking it would solve the problem. The sealed cartridge being smaller did mitigate some of the problem but it was just so obvious something else was going on. That's when I sent it off to FF. Point is...yes, if you are experiencing similar issues, no need to waste a lot of time and $$ on needless work....it's almost certainly under sized bushings. Send it off to someone who knows what they are doing ;)
 

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Thank you! Woow, you're quit the mechanic.... that was risky, what a story! Well it proves that tolerance variations are pretty common when building stuff at the industrial level and while it kinda works you could also find yourself at the very limit of what's acceptable for QC. I guess that's what happened with your bearing/frame story, and it's definitely the case with fork bushings! Your frame was alloy right? Do you mind sharing more info, pics of the tool? I love those stuff AND happy ending stories. LOL :)

FYI: I don't think wet sanding bushings are a good idea, (I could be wrong). You seem to be the kind of guy who can do better and make his own resizing tool!
Yes, frame was alloy, and a committed move! I worked on the rocker arm bores first to prove the concept. After I worked on the first bore, I kept measuring it over and over, not believing I was already up to perfect size so easily. It actually improved the roundness of the bore, as evidenced by uneven removal of the anodization.

I'm not 100% on whether wet sanding the bushings is a good idea or not. I used 800 grit bathed in oil, and removed all grit afterwards. I have worked with Igus bushings before, and don't think surface finish is super critical. Even if it is, you can go all the way up to optically clear on plastic by increasing the grit of the compound. A proper tool to cut to size is most definitely a better option.
 

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Discussion Starter · #16 ·
Def. Truth be told, after SRAM but before sending off to Fluid Focus, we even installed the Charger 2.1 RCT3 damper upgrade which effectively turned this fork into a Pike Ultimate thinking it would solve the problem. The sealed cartridge being smaller did mitigate some of the problem but it was just so obvious something else was going on. That's when I sent it off to FF. Point is...yes, if you are experiencing similar issues, no need to waste a lot of time and $$ on needless work....it's almost certainly under sized bushings. Send it off to someone who knows what they are doing ;)
Amen to that!
 

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Could the stock bushings be replaced by those?

https://www.igus.com/product/3

And you can request a free sample ;)
I have worked with Igus before and they make a great product. They sell direct to consumer and their technical sales people are super helpful. You could reach out to them with the details of the application and they will help you select the correct bushing material.

I'm pretty sure they will even do custom sizes, obviously at a higher price point.
 

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Discussion Starter · #18 ·
Yes, frame was alloy, and a committed move! I worked on the rocker arm bores first to prove the concept. After I worked on the first bore, I kept measuring it over and over, not believing I was already up to perfect size so easily. It actually improved the roundness of the bore, as evidenced by uneven removal of the anodization.

I'm not 100% on whether wet sanding the bushings is a good idea or not. I used 800 grit bathed in oil, and removed all grit afterwards. I have worked with Igus bushings before, and don't think surface finish is super critical. Even if it is, you can go all the way up to optically clear on plastic by increasing the grit of the compound. A proper tool to cut to size is most definitely a better option.
You did a great job! Thanks for sharing that story.

You're probably right about the surface finish, as long you have enough clearance and lubrification it should be OK! Well it does feel ok NOW HeyHey.
 

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Discussion Starter · #19 ·
I have worked with Igus before and they make a great product. They sell direct to consumer and their technical sales people are super helpful. You could reach out to them with the details of the application and they will help you select the correct bushing material.

I'm pretty sure they will even do custom sizes, obviously at a higher price point.
You know what? I'll send them an email out of curiosity and see what they have to say. We never know ...
 

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What's stopping them uppers from dropping completely in? Looks like you are dropping them through the first set of bushings and then they are stopping on the second set of bushings? If that's the case, seems like there may be more gains to be had with figuring out a way to align the bushing bores better?

Aren't the stock bushings lined with PTFE, basically making them a metal backed IGUS bushing?

How consistent in sizing are the stanchions?

0.0015" to 0.009" is wider then what I thought it would have been. Wonder how open you can go before it starts making noise.

Anywhere in the USA selling supergliss?
 
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