How thick should a fork stanchion be

[danson67]

A good quality steel MTB unicrown is butted up to about 1.3mm thick at the top, so with a decent aluminium alloy like 6061-T6 I'd reckon on you needing 3-4x that, so 4mm at least, plus some to take account of the proportionately lower fatigue strength of aluminium.

Yield strengths:
6061-T6 265 N/mm2
531/cro-mo/4130 etc. 700 N/mm2
Prestige/Nivacrom/753 etc 1200+N/mm2

Fatigue strengths:
6061-T6 95 N/mm2
531/cro-mo/4130 etc. 420 N/mm2
Prestige/Nivacrom/753 etc600+N/mm2

The Easton stanchions might only be around 1mm wall at the top clamps, but they'll be seriously butted around the lower clamps, then thin out again down to the bottom of the stanchions.

All the best,

 

[Chute55uk]

The Easton stanchions might only be around 1mm wall at the top clamps, but they'll be seriously butted around the lower clamps, then thin out again down to the bottom of the stanchions.

All the best,

would you know if i could lower the sliders in the clamps? as you see in the pic they are sticking out by 10mm or so at the top, i realise this would raise the nose up a but but would it be safe to do?
sorry about the thread hijack

 

[velomaniac]

Hijack away, I've got the info I need

 

[danson67]

Chute:

would you know if i could lower the sliders in the clamps?

I'm not too familiar with the internal butting in there, but I suspect the official answer's no...

The Marzocchi manual in the link above has zero tolerances on the dropout to lower crown dimension (454mm). This would suggest that the butting is short. If you can get inside the stanchions from the top, you should be able to see (and maybe measure) the butt. If there's plenty of butt above the lower clamp, you might be OK, but I wouldn't want to call it myself.

The manual suggests the solution of putting more headset spacers in to raise the upper clamp:

"The stanchions edge must be aligned with or slightly lower than the upper plate.
If fork legs overprotrude, fit some [headset]spacers to the plate close to the steer tube."

It would seem that the top of the stanchions should only be clamped where the threaded fitting overlaps to strengthen it. So how you've got it at the moment probably isn't recommended either.

All the best,

 

[02gf74]

I'm confused - I tought the OP was about fitting lighter stanchions but it seems now that the OP has made a set of rigid forks using 2 tubes bolted to the suspension fork crown - right?

If that is the cas,e the surface treatment is irrelavent since theree is no slideyness going on - but what are the tubes from? Presumable they are for forks since they must have drop outs for the wheel and disc brake maount, or were these added to the tubes by the OP?

confused. :?

 

[velomaniac]

I have a set of DNM Comet forks. The stanchions are clamped to the crown as is the steerer in the fashion of older marzocchi forks with bolts.
I have removed the upper stanchions and springs/elastomers from the lower magnesium outers and chucked them. I have fitted 2 x aluminium round tubes of the length needed to reach from the crown clamp to the bottom of the fork lowers to make a rigid fork of about 440mm A-C.
Result a rigid fork purely because the suspension parts have been removed and replaced.
Original question was concerning if the thickness of the walls of the alloy tubes I replaced the original stantions with. It seems I have not chosen thick enough tubing and will rethink problem.
I have replaced internal telescopic parts with rigid tubes so that I can use these now lighter suspension adjusted forks for my home made 29er. In original format they were horrible, heavy pogo sticking forks with too much travel for the frame.
Hopefully you are less confused

 

[stevet1]

I *think* that the fork uses the existing dropouts, and in fact external stanchions, just replaces the inner stanchions with longer tubing. I'm now wondering whats keeping the tubing in place when the front wheel is lifted... I'm starting to see a dental appointment in the near future.

 

[velomaniac]

I thought of that

Steerer style AHead star nut in bottom of each leg tube with bolt through bottom of each leg.

Anyhoo this idea is shelved for the moment as I'll use a slightly suspension corrected rigid 26er steel fork with a U-Brake utilising a bolt on U-Brake plate so that i can get brake blocks onto the 29er rim. Doesn't look nearly as aesthetically pleasing as the converted sus forks but I know it'll be safer.

Basically I want to explore 29er bikes before maybe buying one. I can do this as my mass of old bike bits allows me to throw something together that covers the basics but is not purpose built.

 

[02gf74]

maybe getting a bit more of it now - so what is the wall thcickness of the stanchions that were removed?

if the replacement are the same material, then there is the answer, if one is steel and other aluminuum then the Al will be at least 2.5 thicker than the steel ones.