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Stanchion diameter


stewie911
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What is the significance of fork stanchion diameter?

I see a lot of guys asking the diameter, but why does it matter?

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Bigger is better, up to a point.

 

In the old days most stanchions were 28mm and made of steel. Then they went to 32mm alu. By going bigger, you can make the walls of the tubes thinner, yet the tubes will still be stronger.

 

Most X-country bikes are 32mm.

 

For freeride some manufacturers have a 36mm (alu) or 32mm (steel) and downhill stancions by the king of forks, Marzocchi, are a whopping 40mm. They look like motorbike forks.

 

Manitou, always odd, has some odd sizes like 30mm.\

 

Speaking of Marzocchi, it died a sad death in this country. No agent, no spares, no hope.

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JB is correct.

 

It is about the EI value of a tube which is the bending stiffness. E is fixed for a specific material, while I (2nd moment of area, if I remember correctly from varsity) is dependant on the cross sectional geometry of the tube.

 

Since I is a function of (outer radius minus inner radius)^4 you can increase the stiffness of a tube by increasing the outer diameter and having a smaller wall thickness.

 

This will also result in less weight because the weight of the tube is a function of (outer radius minus inner radius)^2.

 

So the stiffnes increase by a x^4, the weight increase by x^2.

 

As JB said it only works up to a certain point, because with thin wall thickness comes problems like fatigue strength and weldability etc.

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Speaking of Marzocchi, it died a sad death in this country. No agent, no spares, no hope.

A gap in the market perhaps?

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What Stephan explained is correct, and I also believe that "bigger is better" up to a point. Other than the strenght / weight issue, there's also more surface area = larger wear surface = less wear / pressure on stanchions and seals. However, I've been in a cycle shop a few weeks ago and there was a top of the range KTM bike fitted with a DT Swiss Fork with 28mm stanchions.

 

Don't know the reason, but it seems that DT Swiss is going for 28mm stanchions on XC bikes while Rock Shox and Fox have pretty much standardized on 32mm.

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