Hello Vital MTB Visitor,
We’re conducting a survey and would appreciate your input. Your answers will help Vital and the MTB industry better understand what riders like you want. Survey results will be used to recognize top brands. Make your voice heard!
Five lucky people will be selected at random to win a Vital MTB t-shirt.
Thanks in advance,
The Vital MTB Crew
I could be wrong, but I think a moving bushing (like on a dirt bike) is actually more tolerant of misalignment.
It's just because the lowers are cast. Which is one of the worst way to make a high precision part.
Cast part + tolerance fit = big variance.
I doubt anyone has enough experience, but a fork like the carbon Dorado would have a lot better alignment due to being a carbon mould, which is just a lot more accurate in production than casting magnesium
I've bought a couple of ex WCDH bikes before, both had shims between the hub and the fork dropout. I wonder if that was for fork alignment. They were super thin, like .25mm feeler gauge stuff.
It's not quite as bad as that. The casting is more like a near-net billet, IIRC that gets clamped by the dropouts (and probably near the arch too), then a dual spindle mill cuts the bushing and seal bores for both legs together.
You can buy the dimpled bushings at industrial suppliers or bearing distributors. My guess is they aren't aren't beneficial as they aren't expensive enough that they wouldn't be on premium forks. I remember some SID forks had bushings with holes drilled through them. They were worse than plain bushings, the dimples might retain dirt at service.
Here's a spiral oil-grooved bushing, perhaps this is what CP was referring to. I've never seen these with a thin enough section to work in forks.
I'm pretty sure that the increase in friction between services at the bushing surfaces is miniscule compared with the change at the seals. I've experimented with various different bushings and always returned to the DU type.
Your right, I wasn't going for hyperbole so sorry if people misinterpreted that as casting = bad.
For the record I think the standard of engineering in forks and mountain bike products is extremely high these days and incredibly impressive.
Its just not necessary for the companies to manufacture tolerance fits for bushing and stanchions to something insane like IT3 standard, the breakin of the fit solves the fitment in 90% of cases.
Each case is different, but lately I find it much more common to have loose bushings vs tight. I've had several customers' forks from a variety of major brands develop bushing play within a few rides. Its like the previous gen of forks were too tight, people complained/developed solutions (like burnishing), and manufacturers made a tolerance adjustment for the current batch of forks but they over-corrected. I know making forks is difficult, but it seems like this issue should be dialed in by now.
I think there was a decision at some point to shoot for looser bushings so that riders (and reviewers) have a great initial impression out of the box rather than having to ride a for for a while and break it in.
Here's a fork alignment question that someone in the industry might be able to answer: how consistent are manufacturing tolerances for front hub dimensions? Seems like there would be a variety in actual hub widths in the field, for instance from 109-111mm for Boost 110 axles, which would likely impact fork bushing alignment when everything is bolted down.
Asked another way, if I go to a fork tuner and have them work their fingers to the bone re-aligning my CSU and/or lowers, what are the odds that my newly perfect fork alignment will go to shit when I bolt in a different front wheel with a slightly over-width or under-width hub?
Also, responding to an earlier comment, yes the tight hub fit of Rockshox lowers has been confirmed by Dave Camp elsewhere in the forums as a solution to production variations in manufacturer's hub widths. They expand on purpose to adapt to different width hubs. They're Rockshox' solution to the same problem Fox addressed with their floating axle mount.
That's super interesting (never heard that one before), and I'm sure your guess is right. Anything that's feeler-guage width must be for alignment. But I think we're getting into "Mike's Secret Stuff" territory here*. I'm sure it doesn't hurt small bump performance, but those micro washers are probably doing most of their work between the ears of the athlete and not in terms of actual performance limitations of the fork.
*I included a hyperlink to the relevant scene in Space Jam just in case there's anyone in these forums who wasn't alive for Bill Clinton's inauguration. Unlikely. But maybe?
Hub width? Floating axle solves that. Aside from Ohlins is Fox still using it? Anyone else?
I've found the variation is similar to disc brake tabs on hubs, when swapping wheels you might find the rotor is a little off and rubs. I've been using the syntace rotor shims to make wheel swapping easier for years, recently I picked up some 15mm ID shim washers to standardise hub spacing as I went down a rabbit hole on front end feel when swapping wheels. I've been gluing them to the inside of the hub end caps for ease of use. Getting the right hub spacing for good fork alignment was something suggested to me by a suspension service guy, I was a little sceptical at first, but it seems to make a big difference.
Pulled the PUSH 9.1 fork apart.
Total ride time is 4 hours.
Front wheel weighs 4.75lbs. I am putting it back together as is for now.
yeah I'm not sure dude, I dropped the washer in the garage and it was never seen again lol
But that bike (Dean Lucas 2014 Worlds Devinci Wilson) was like nothing else I have ever ridden (and I have had a LOT of bikes)
It had the full NSR treatment while he was wrenching for devinci, when the local suspension service shop took apart the rear shock they were like "wtf is going on in here!" was as close as I'll get to been a factory rider lol
Dorado is floating axle also. I don’t think Boxxer /zeb has it.
Sorry if I wasn't clear. My question wasn't how fork manufacturers can mitigate manufacturing variations in hub width, although that's an interesting question, too.
My questions were: 1) how wide is the variation in hub width between different hubs and wheels on the market? and 2) what affect would a wider or narrower hub width have on a fork chassis that doesn't employ a floating axle and has been carefully re-aligned using a slightly different hub width?
1) I think this would vary with the hub mfgs themselves, I have seen an assembly drawing for a front hub that had a tolerance of +- 1mm for the effective width (which is huge). In measuring maybe a dozen random boost front hubs over the years, I have never found one more than 0.5mm over or under 110mm.
2) Potentially a big effect. Increasing or decreasing effective hub width by 0.2mm can be enough of an adjustment to go from a "sticky" chassis/bushing fit, to one where the uppers drop to the bottom with gravity.
A floating axle system for the front hub does not solve alignment issues, it eliminates hub width as a factor in alignment (if done correctly). If the stanchions as they come out of the crown are skewed inward (narrower width at the ends), a floating axle system will do nothing to help this.
In the case of Fox forks with the floating axle, it can actually cause alignment issues itself as shown by Diaz Suspension.
What is the serial number on that fork?
With our inverted design, we use a film of incompressible hydraulic fluid (fork oil) that acts as a bearing between the stanchion tube and the fork bushing. We're able to achieve this as the fluid naturally gravitates towards the bushings with gravity. With the bushing at its minimum ID tolerance, and the stachion at its maximum tolerance, there is still a diametrical clearance between the two components.
If the stanchion is touching the fork bushing in a dry state, like what is shown in this video, then something is incorrect. During assembly, we have two positions where the stanchion tubes and bushing clearance are checked on 100% of the forks. One process is automated and mechanical, the other is by hand. Is it possible that one got through? While it would be extremely difficult, I would never say never. But both fork tubes in a single fork? Let's just say I'm questioning this one.
Darren
Interesting video of the effect on hub width on fork friction, second part he shows that a floating axle doesn't solve the problem
Hi Darren, the serial number for the fork is 9.101563.
No alterations, it is exactly as delivered by PUSH. Just 4 hours of ride time and a full tear down.
I've found it in our system. Are you the original purchaser?
Yes sir.
It was delivered in early April but I was occupied with other projects and am just now getting some ride time on it.
I will typically pull apart a brand new fork before riding to check things like bushing fit/chassis alignment as well as verify bath oil, etc. I decided to give this one a few rides to allow for some "break in" before pulling it apart.
How was it when riding? Could you tell it was that restricted or just felt like a stiff fork?
Perfect, we'll need to get that back for inspection. I'll shoot you an email to get that process going.
Coming directly from an Intend Edge on the same bike the PUSH 9.1 was very different in terms of damping, ride height, travel used, and feed back to the hands.
But it is hard to say how much of this is due to the bushing fit. The damper in the 9.1 also has much more support than the Intend damper provides, and I have yet to start experimenting or bracketing the settings on the PUSH fork. All riding so far has been as it came set from the factory which is basically in the middle of the range for LSC and HSC.
Interesting! Yeah maybe not a fair ride test yet. But interested after you get them figured out.
I have a dorado and really like the lean angle grip but the torsional twist in certain situations isn’t ideal.
Oh this is where I was supposed to post this!
This is a sample of one situation. I did a 200h service on my MY23 ZEB over the weekend and based on everything discussed about alignment and binding, I tested it out with the fork fully disassembled (without dust wipers and cleaned out). Each stanchion in it's respective lower hole/bushings runs fine. Both at the same time (fork assembled basically) run fine in the top bushings, but bind once they are inserted in the lower bushings. That is without a hub. Installing my wheel everything then runs smoothly.
Empty lowers fit very snugly over the hub (have to wedge the hub in) while I can easily insert the hub with the fork fully assembled - apparently the lowers by itself are bent inwards a bit, but the dimensions of the fork are such that when the CSU with stanchions is installed and putting the hub in it everything is nicely parallel and nothing is really preloaded or increases frictiin in any way out of the normal.
This mirrors my experience with RockShox forks, they seem to have a good production process in place in terms of chassis alignment and bushing fit.
It's not really a problem in terms of something I would expect them to fix - I feel like Fox and Rockshox both produce forks within an acceptable tolerance for large scale production. Some of the smaller but still mainstream companies making conventional forks tend to have more variation, usually much too tight which is definitely an issue.
Bushings need to have a certain amount of clearance for proper hydrodynamic sliding conditions, but it is tough to make every fork perfect which is why they are usually produced on the tighter side of what is ideal.
I think the problem sticks to Rockshox more than others because they have possibly been more common, but also because they are slightly more progressive and some people struggle to get full travel, they will go looking for solutions and feel like the bushing resizing helps. A bit like grease in negative air chambers - thats not a real issue but gets attributed to Fox more than Rockshox by people cosplaying as suspension guys on instagram.
"thats not a real issue but gets attributed to Fox more than Rockshox by people cosplaying as suspension guys on instagram. "
then why does it make such a difference when you remove all the excess? is physics cosplay aswell?
Post a reply to: Fork chassis, axles, friction: burnishing, chassis alignment, etc...