CSU rebuilding

link1896

link1896

Mr Greenfield
During lockdown I bought myself a 20 ton press and taught myself how to rebuild CSU’s following Oliver Majewski’s (Blue Liquid Labs) method.

Crowns make hideous creaking noises due to corrosion forming on the mating surfaces between stanchions and crown, and steerer and crown. Locktite 609 is the go to compound, but dougal over at Shockcraft has started using PVA. I’ll be trying pva soon, as fox’s paint is pretty delicate, 609 damages it very quickly.



Oliver is a dead set legend. We’ve had many technical conversations. We’ve both enhanced the method further.

A 20 ton press is marginal, 30 or beyond is really needed. And pneumatically controlled is desirable, you need three hands to do this.

Rockshox stanchions need the best part of 20T to remove. Steerers are barely an interference fit, I’ve had some forks a rubber mallet can get the steerer out. Stanchions seem to be a good fit.

Fox steerers need at least 20t to remove. Sometimes I’ve gotten stuck and had to redo as I didn’t have enough force to drive home.

This process is very tooling heavy. Everything is subtly unique. You need a lathe, a mill, and a very good eye for accurately measuring and checking tooling fit. One wrong move and it’s good night $600 csu. And with availability being an utter nightmare, this is a high risk game.

Every generation and year of fork is different. 2019/2020/2021 fox36 for example, just to get stanchions back into the crown, requires 6 unique drifts for the bottom end of the stanchions. Madness.


To press the stanchions out, you need a delrin or ultraHDPE block to engage the bottom face of the crown and a drift to thread into the top cap position. Thanks to summitfever for the generously donated UltraHDPE off cuts. You press the stanchion out via a drift engaged into the top cap thread, drift fit is critical when you’re passing the best part of 20t though 8-10 turns of fine thread!. Again, fox is up to their usual shinnanigans with odd ball threads. (The latest generation of Fox36 forks, foot bolt threads are a unique to fox thing)


And then there are steerer removal tools. Every bloody crown is different. Tooling again is a huge challenge.







 
beeb

beeb

Dr. Beebenson, PhD HA, ST, Offset (hons)
link1896 said:
Click to expand...
Is that final pic an example of the corrosion that occurs under the stanchions?

Very impressed mate. Pulling apart such complex interfaces were clearly not on the manufacturer’s list of priorities!
 
link1896

link1896

Mr Greenfield
Without moisture and air, the two dissimilar metals shouldn’t oxidise. I believe this is Douglas approach. I will try this soon. The interference fit is the mechanic connection.


Here is a better shot of a fox crown with oxidisation. This was with Kashima stanchions. The oxidise is like sand, I remove with a wire wheel on the Dremel. The kashima really is tough, can clean it up with the wire wheel with no signs of any abrasion.

 
S

SummitFever

Eats Squid
Have you been able to measure what the ID/OD difference is of the interference fit? A bit of heating and freezing might make things go a lot easier.

The last pic is not conclusive but it looks a bit like when Fox press the stanchions into the crown, material is being picked up by the stanchion and then forced along and in-between the crown/stanchion interface. The stanchion itself will probably not be strong enough to resist inwards deformation and the "galling" for want of a better word can result in a gap between stanchion and crown. A gap will result in dirt/moisture ingress and corrosion.

The design of the "lead in" on both parts of a press fit interface is not rocket science, but is critical to avoiding galling particularly with dissimilar materials (like a soft crown and a hard coat stanchion). The relative deformation properties of the stanchion and crown will dictate what sort of lead-in for part mating is required. Maybe Fox assemble using cryo techniques that involve no real interference.

If its not a cryo-fit assembly, a 45 deg chamfer with sharp inner edge/outer edges on both parts is definitely NOT the thing to do if you want to press these parts together where the other part is softer than the inner (if the inner were softer eg. the stanchion, then it would abrade but the material would end up outside the finished part).
 
link1896

link1896

Mr Greenfield
SummitFever said:
Have you been able to measure what the ID/OD difference is of the interference fit? A bit of heating and freezing might make things go a lot easier.

The last pic is not conclusive but it looks a bit like when Fox press the stanchions into the crown, material is being picked up by the stanchion and then forced along and in-between the crown/stanchion interface. The stanchion itself will probably not be strong enough to resist inwards deformation and the "galling" for want of a better word can result in a gap between stanchion and crown. A gap will result in dirt/moisture ingress and corrosion.

The design of the "lead in" on both parts of a press fit interface is not rocket science, but is critical to avoiding galling particularly with dissimilar materials (like a soft crown and a hard coat stanchion). The relative deformation properties of the stanchion and crown will dictate what sort of lead-in for part mating is required. Maybe Fox assemble using cryo techniques that involve no real interference.

If its not a cryo-fit assembly, a 45 deg chamfer with sharp inner edge/outer edges on both parts is definitely NOT the thing to do if you want to press these parts together where the other part is softer than the inner (if the inner were softer eg. the stanchion, then it would abrade but the material would end up outside the finished part).
Click to expand...

Indeed I have been able to measure. Fox's stanchion OD's are all over the shop so I won't comment for now, but RS runs a tight ship here. about 0.06 to 0.08mm interference fit. Using online interference fit calculators, this comes out at about 14 Ton of press fit force, so in the ballpark, my gaugeless press might be under rated too. But calculators show for a friction free shrink fit, need a temperature differential of 151c. Best I've achieved is 68 degrees, -18, plus 50, which does help.

45 degree chamfer on everyone's stanchions, guessing 15-20 degrees on crowns. Pulling apart a freshly assembled crown and stanchion, I see no evidence of galling when I take a look with a good magnifying glass. Possibly assembled at the factory with some cryo-fit and/or lubrication?
 
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