Grinding the underside of the fork crown

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  • I've got this builder making me a fork, and since the available crowns were a little taller than the original one that came with the frame, he suggested to grind the underside of the crown in order to get the tire closer to the frame and thus achieve the geometry that was originally designed for the given frame. I wonder how safe is it to grind the material in that spot, and does it weaken the fork crown? Here are some images as an illustration. It's his actual work. He also mentioned that he actually cut of more material than shown in the picture.

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    • IMG-20200626-WA0007.jpg
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  • Don't know about strength but it looks a mess. Why not file it flush with the crown or at least follow the curve of it?

    How much do you care about matching the original geometry?

  • Yes, I agree it looks awkward, putting it mildly. I'll probably revise all this with the builder or look for someone else. I very much care about matching the original geometry. Using a crown with a larger height gives the top tube a slight slope, very annoying to me.

    Here's what the original looked like, approximately:

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  • Would that have been cast like that or was it cut away too?

  • I believe that was cut away. The cut might look more severe from that angle. It should be this fork crown:

  • I very much care about matching the original geometry. Using a crown with a larger height gives the top tube a slight slope, very annoying to me.

    how much extra length are we talking here? Because the toptube angle change from even a large fork length adjustment will be barely perceptible.

    as a general rule, 20mm of fork length equates to around 1 degree of angle-change on the toptube.

    I would argue that even a 1 degree angle is imperceptible by eye and this is using our extreme example of fork length change.

    In this situation, realistically you'll be grinding out the fork to achieve somewhere in the region of 5mm fork-length change. This is a quarter of a degree of angle change on the toptube.

    You should listen to your builder and trust them. Because right now you're focussing on something that is inconsequential and irrelevant

  • Ceeway sell that fork crown don’t they?

    I am not an engineer but it seems removing material from the area in question would be unlikely to be structurally detrimental.

    In the photos in the first post it looks like the builder has added brass to the lug presumably to strengthen it, though in don’t know that that has anything to do with removing material from the underside.

  • Thanks everybody for your input. I will hopefully get some update from the builder soon and possibly post it here.

    The builder is from Russia, so he was apparently unable to source the specific crown although he had used the same one before. Well, this was probably the first alarm that I should stop.

    So several things worry me here: strength of the crown - if too much material is removed (he apparently removed more than pictured in the first post). Inward-sloping shoulders give me the impression that it possibly could lead to greater forces being distributed to that area, but I'm no engineer, so I can only guess.

    Aesthetics - I didn't expect it to look that awkward. It could look even more so if more material is removed, so we'll have to see.

    The benefits of shorter fork in this case - not sure about this after retrodirect/glowbike's advice. I have strong attention to such details, and this detail is giving me headache, knowing that the fork is not as originally designed for the frame. Even knowing that the front of the top tube will go down 5 mm would probably give me some peace of mind. I appreciate your advice very much, though.

    Well, if this doesn't look too good after the builder sends me the progress, I will have to propose him some other solutions - such as acquiring the original crown (sending it to him somehow) and doing it like he originally did. After all, he built the frame too. Otherwise I'll have to find another builder or just give up on this.

    I admit that this was a rushed decision, going with this version of crown, since he lead me to believe that he can achieve the same geometry with it, but it turns out that it won't be so straightforward, and some compromises would be included.

    EDIT: Well, here what he sent me. How does this look, everyone? I don't like this at all, so will bail out of this option. We'll see if the builder is willing to reconsider other options.

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  • Excuse my ignorance..why not use a different fork crown loads are available readily? Id feel uncomfortable with taking out that much material.
    That said I am not a engineer.

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Grinding the underside of the fork crown

Posted by Avatar for pop.zr @pop.zr