# Frame built for rigid 468 mm axle-to-crown, fork is 483 mm. Okayish?



## mikael_on_wheels (Jun 10, 2019)

So, I want a boost (110/15) rigid fork for this frame. Original rigid fork has 468 mm axle-to-crown and offset seems to be 42 mm according to geo charts. Head tube angle is 71.

468 mm forks are usually 100 mm (Salsa's offering) and 483 mm rigid forks are mostly 110 mm.

I'm currently considering a custom fork or getting a Krampus fork which is 483 mm and has a 47 mm offset. Will the 15 mm difference in a-c matter? For handling I'm not that worried, but I don't want to stress the frame. Head tube is not oversized.

Also, there's a 5 mm difference in offset (Krampus 47 mm and stock fork 42 mm). If my mind is straight, I don't think this should affect the head tube angle?


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## looks easy from here (Apr 16, 2019)

Rule of thumb is every 10mm increase results in slacking the hta by roughly 0.5°. It will also increase stack, decrease reach, slack the sta, and raise the bb height.


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## mack_turtle (Jan 6, 2009)

This might help. geometryCalc


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## Fleas (Jan 19, 2006)

Just by seat-of-the-pants measurements, I ran several different rigid forks over the years on various bikes with varying geometries. Going with a slightly longer fork _always_ seemed to be an improvement.
The best improvement was realized on our mountain tandem, which is still rigid. Obviously, there is a limit, but +15mm is not it. I think you'll be fine.

-F


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## shortnangry (Nov 16, 2014)

mikael_on_wheels said:


> Also, there's a 5 mm difference in offset (Krampus 47 mm and stock fork 42 mm). If my mind is straight, I don't think this should affect the head tube angle?


The difference in offset will not affect head tube angle. What it does affect is trail (basically the distance between the tire contact and steering axis). Larger offset results in in shorter trail making the handling less stable at speed but also more precise in steering. The effect is similar to changes in HTA: steeper HTA results in less stable handling at speed but more precise steering. In isolation, by lengthening the a-c by 15mm you're slackening the HTA by roughly 3/4* and by increasing offset, decreasing trail. How that nets out for handling or geo numbers by changing both at once, I have no idea (especially since HTA effects trail and a slacker HTA lengthens trail) but the geo calc that was provided is a good resource that should show the relative changes.


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## mikael_on_wheels (Jun 10, 2019)

Fleas said:


> Obviously, there is a limit, but +15mm is not it. I think you'll be fine.


Thanks!


shortnangry said:


> The difference in offset will not affect head tube angle. What it does affect is trail (basically the distance between the tire contact and steering axis). Larger offset results in in shorter trail making the handling less stable at speed but also more precise in steering. The effect is similar to changes in HTA: steeper HTA results in less stable handling at speed but more precise steering. In isolation, by lengthening the a-c by 15mm you're slackening the HTA by roughly 3/4* and by increasing offset, decreasing trail. How that nets out for handling or geo numbers by changing both at once, I have no idea (especially since HTA effects trail and a slacker HTA lengthens trail) but the geo calc that was provided is a good resource that should show the relative changes.


Thanks, that's good input. My biggest gripe with the Krampus fork now is the short steerer (26 cm). The head tube is 18 cm, so with headset installed it's only about 6 cm of steerer for spacers and stem. For some people it might sound like a lot, but I have a stack problem.


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