Rigid carbon MTB forks - so light can they be safe....??

[LikeClockwork]

Picked up some rigid carbon forks, geometry corrected for equivalent of 80mm travel sus, they've even got a carbon steerer by the looks.

Can these things possibly be safe off road...??! They are so light they're scaring me, and if I get hold of the drop outs they can be flexed quite easily - or is that the point, greater shock absorption??

Any tips or hints for what I should or should not do, there's already a SFN in them , do carbon steerers require different ones, and is any stem OK or do you have to be careful there too?

 

[widowmaker]

Re:

Cant help with the technical questions, but i recently built up a roadbike with some insanely light carbon forks and i just could not ride it, i winced and shut my eyes every time i hit a bump/pot hole

 

[LikeClockwork]

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Lol, that's really reassuring

Frankly I'm starting to wonder if I should just hang them from the beam in my place as modern art... :facepalm:

 

[Osella]

Re:

Got a picture, or a link etc..?
There's several different varieties, and different layups of carbon so it's a bit like asking 'is an Aluminium frame safe?'

I've got 3 sets of forks with carbon steerers (2 rigid, one SID blackbox) and no issues with them cracking etc; tho I have only used them for XC/singletrack-type riding, but that does include (slow) rockgarden stuff, and no cracks or creaks at all.

Also depends what it light.. The Merida forks I have (there's a similar set on sale here)

feel light at about 760g, but in reality are quite an average weight. The others I have are much higher a2c (485mm) so will fit a 650b wheel, yet weigh only 570g, so...
These are commonly sold with Java branding on:

Things to take care of are 1) the sfn, to ensure that as you tighten up the headset you're not pulling the sfn up the steerer tube, as sometimes they can slide up inside the steerer. 2) Don't hit or push down on the steerer if the dropouts are on a hard surface as you don't want to damage them. 3) Take care, even use a bit of light grease, when fitting spacers and stem if the steerer has been cut. You can delaminate and strip the steerer if you get these angled wrongly, like stripping bark off a tree, so make sure everything's lined up right. Remove the grease after fitting though! Also best if you can to use a 2-bolt stem and I always avoid using stems which don't wrap fully round the steerer i.e those without material filling in the gap between the clamps. Probably more psychological than anything!

The legs should flex (that is the point) particularly fore/aft. Hopefully they don't bend/flex inward towards one another though.. better forks with a good layup won't bend/flex inwards.

 

[LikeClockwork]

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Thank you for the comprehensive response! Much appreciated....

It is the...ummmm....Merida ones off here.....

So looks like they should be safe then! This is what happens when modern technology is allowed into my life, turns out they're actually quite heavy for a carbon fork....

 

[mattr]

If they have a carbon steerer and an sfn the chances are that they are knackered. Not a huge number of carbon steerers are rated for use with sfns. They should really use an expanding wedge type arrangement. Nothing I've fitted in the last 15 years has been sfn in carbon. Probably 50+ sets?
Check with the manufacturer.
And my training "chuck it down a rock face and never clean it" bike has some carbon forks on it. 700 something grams with an uncut steerer. They've been on there 6 or 7 years with not a squeak or creak of complaint.

 

[mattr]

Oh, rather than greasing, put a tiny chamfer on the OD of the steerer to stop delamination. Not sure that grease is a great idea on a carbon composite. A bit of grip paste will work to stop it moving round. (As long as it is a carbon steerer of course)

 

[LikeClockwork]

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My mistake...hadn't even checked, just assumed as seller mentioned nut was already fitted...

They have indeed got some kind of wedge type...thing...not a SFN :facepalm:

You see what I mean now about not allowing me near modern technology

Thanks for response!

 

[Fatal Swan]

This thread reminds me of when I bought a Schmolke carbon flat bar I bought off here... it was about 65g! The knowledge that they were crazy expensive wasn't quite reassuring enough and I couldn't bring myself to install it in the end.

BTW, The Trigon forks I had were sub-500g... :shock:

 

[Osella]

That's better if they have a wedge then! If they already had the sfn fitted then it possibly wasn't too much of a big deal, but expanding wedges are much better.
I, umm, edited my above post too to say if you use a lubricant to fit the stem/spacers, clean it off afterwards! They're really nice forks those Merida ones! Not heavy, not too light but very sturdy. Mine are fitted to a 1993 Cadex

I was worried about some Syntace carbon bars at 116g! Until I got them.. Don't see how you could ever break the things in normal XC riding even in a crash!
I am a huge motor racing fan though, so have seen a lot of carbon stuffs, how it's made, material properties etc.. Good cf stuff is just unbeatable for quality & strength, but cf done wrong is dangerous.