I have in mind to revive my 26 hardtail by using a niner fork instead of its cheap Rock Shox. Will this work?

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My 6530gr Wilier Cento Uno
My 8618gr Colnago Master X-light

Niner Carbon fork is equivalent to a 29er 80mm fork with standard sag. Are you going to use a 26" wheel or a 29" wheel. If you use a 26" wheel it would be roughly ~4 mm taller than average 100 mm 26" fork unsaged. So it would be about the same as a 125mm 26" fork saged.

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For certain parts stiffer is more important than lighter.

I am going to use a 26inch wheel. I have a 80mm fork now but I would prefer a 100mm one.
So - if I understand well - in terms of geometry it will be like having a 100mm suspension fork?

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My 6530gr Wilier Cento Uno
My 8618gr Colnago Master X-light

The niner fork has an axle to crown of 470mm.

Your 80mm Rock Shox fork will have an axle to crown of ~455mm (without sag) depending on the model you have.

However much sag you run your axle to crown will decrease by that much.

A 100mm fork is around 470mm (without sag). So this niner fork will ride slightly taller than a 100mm fork with correct sag.

Essentially you have have a slacker head tube angle by say 1 maybe 1.5 degrees. The exact value depends on your frame and current fork.. The taller your current fork the less of a difference...

Whether or not that is acceptable to you is a choice you will have to make.

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Like I said, I will be equivalent to using 125mm fork with normal sag.

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For certain parts stiffer is more important than lighter.

What frame do you have?

I have a Wilier but I think of getting this NOS Commencal vip nuts for less than 190 euros.

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My 6530gr Wilier Cento Uno
My 8618gr Colnago Master X-light

I've been using a Niner carbon 29er fork in my winter bike, 7.6kg 26er Scott Scale for soon two years now. It replaced a 100mm R7 MRD, which had an a-c of 476mm. Niner has it at 470mm, so with sag it is maybe about 1,5cm higher. For me, this has been no problem at all and I've got no intentions of searching a shorter replacement.

If you've got too much time in your hands, here's two videos with the very bike and fork:
http://www.youtube.com/watch?v=svjla9z-AzY
http://www.youtube.com/watch?v=9JVrcphLc38

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-px | pxx.fi | Spec Epic S-Works 8934g

Thanks px!

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My 6530gr Wilier Cento Uno
My 8618gr Colnago Master X-light