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Opinions on steel fork dimple?

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Opinions on steel fork dimple?

Old 12-19-23, 06:36 AM
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Opinions on steel fork dimple?

*sorry cross post* i couldn't figure out how to delete the CV post.

i have a friend out in socal who recently dimpled the inside of a steel fork of a mid level bridgestone about 1-2mm per fork blade to gain enough clearance to go a up a tire size. frame already had enough space on the chainstays. i'm curious to ask any frame builders : do you think this would have any adverse effect or since it's steel and the dimple was 1-2mm it not really an issue? not sure i'd be open to doing something like this in the future but it is interesting. he went from 1.8" width tires to 2.3". i've seen people dimple chainstays but i hadn't heard of fork blades - thanks for you feedback !


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Old 12-19-23, 08:25 AM
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This is one of those "I'd do it on my fork but not for a customer's fork" things. The worst case should be pretty obvious and one that we should all never be a part of for another. Andy
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Old 12-19-23, 12:13 PM
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I would have to see the fork. I wouldn't do it on one of my bikes either, I would just build a new fork. I would hope people that are doing this for a living would avoid doing this.
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Old 12-19-23, 12:19 PM
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Originally Posted by unterhausen
I would have to see the fork. I wouldn't do it on one of my bikes either, I would just build a new fork. I would hope people that are doing this for a living would avoid doing this.
juvela mentioned in the OP's other thread on this topic that, at one time, some European manufacturers used to dimple the inside of some fork blades. My guess, as I noted in that thread after reading juvela's post, is that any manufacturer who dimpled a fork would likely do it before brazing, since the dimple would be very close to the heat-affected zone.
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Old 12-19-23, 01:46 PM
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Schwinn used to do it on some models, but not on lightweight forks. I have seen it on a World, which was a high-quality touring bike in the '50s, and also on a tandem. Can't remember if it was a Paramount or maybe a Town and Country, which was also a high quality hand-made frame, but heavier than a Paramount. Also probably '50s era. I have not seen it on any more recent forks.

I put slight indents in a MTB fork I made in '81. It was really a hybrid "mountain-trials", 26" wheels but made for Observed Trials. I rode over a lot of picnic tables and natural obstacles that size or larger, often coming down on the front wheel which sort of simulates a frontal collision in terms of the forces on the fork, pushing the blades back. Other times I lauched off a dropoff and landed both wheels at the same time, which pushes the fork forward. Although these events were not as frequent as say pedal strokes, which cause high-cycle fatigue, I think a series of crashes and near-crashes might qualify as low-cycle fatigue. Anyway, whatever you call it, this fork withstood it just fine for some number of years, never yielded or cracked. When I sold the bike maybe 10 years later I put a different for in it, so I still have the '81 fork, waiting for "just the right bike" (which may never happen).

That's just one data point, and also the blades I used might be stronger than what your friend used. I used Reynolds Tandem, the oversized "Jack Taylor" blades. In a tandem crown made by Angel Rodriguez, intended for road bikes, but almost wide enough to fit 2.125" balloon tires (54 mm). They did clear without indents but with no mud room, so I gave 'em a skosh more room. If your friend's blades are weaker and/or if his indents are more severe, then it might be crazy to ride it.

Like Andrew said, those slight indents are something I'm willing to do on a bike for me, but I wouldn't do it for a customer.

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Old 12-19-23, 06:00 PM
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thanks for the feedback everybody : sounds like crimping your own bike could be considered but not a bike being passed to someone else. some bikes with thicker tubing it's been done and is prob ok and dimpling steel before brazing is less risky than after bike is complete
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Old 12-19-23, 06:29 PM
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From now on, respondents in this forum need to rate the perceived danger of a modification/repair/design from 1 to "too dangerous for Mr. Bulgier."
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Old 12-19-23, 06:30 PM
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It would be interesting to know which direction the dimple was oriented. My non-expert, seat-of-the-pants guess is that a dimple that runs across the fork blade (which is the way the pictured dimpling tool is now oriented) would be more likely to cause problems than a dimple that runs longitudinally as is common with chain stays.
A photo of the dimples in question would be helpful.
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Old 12-19-23, 06:59 PM
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Originally Posted by bulgie
Schwinn used to do it on some models, but not on lightweight forks. I have seen it on a World, which was a high-quality touring bike in the '50s, and also on a tandem. Can't remember if it was a Paramount or maybe a Town and Country, which was also a high quality hand-made frame, but heavier than a Paramount. Also probably '50s era. I have not seen it on any more recent forks.

I put slight indents in a MTB fork I made in '81. It was really a hybrid "mountain-trials", 26" wheels but made for Observed Trials. I rode over a lot of picnic tables and natural obstacles that size or larger, often coming down on the front wheel which sort of simulates a frontal collision in terms of the forces on the fork, pushing the blades back. Other times I lauched off a dropoff and landed both wheels at the same time, which pushes the fork forward. Although these events were not as frequent as say pedal strokes, which cause high-cycle fatigue, I think a series of crashes and near-crashes might qualify as low-cycle fatigue. Anyway, whatever you call it, this fork withstood it just fine for some number of years, never yielded or cracked. When I sold the bike maybe 10 years later I put a different for in it, so I still have the '81 fork, waiting for "just the right bike" (which may never happen).

That's just one data point, and also the blades I used might be stronger than what your friend used. I used Reynolds Tandem, the oversized "Jack Taylor" blades. In a tandem crown made by Angel Rodriguez, intended for road bikes, but almost wide enough to fit 2.125" balloon tires (54 mm). They did clear without indents but with no mud room, so I gave 'em a skosh more room. If your friend's blades are weaker and/or if his indents are more severe, then it might be crazy to ride it.

Like Andrew said, those slight indents are something I'm willing to do on a bike for me, but I wouldn't do it for a customer.
bulgie, did you dimple the fork blades before or after brazing?
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Old 12-19-23, 09:00 PM
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Originally Posted by obrentharris
It would be interesting to know which direction the dimple was oriented. My non-expert, seat-of-the-pants guess is that a dimple that runs across the fork blade (which is the way the pictured dimpling tool is now oriented) would be more likely to cause problems than a dimple that runs longitudinally as is common with chain stays.
A photo of the dimples in question would be helpful.
Brent
^ this is a good point i'll ask my friend to snap a photo or a description . he's a bit of an eccentric technology luddite tho - type of guy who will draw you something on a napkin
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Old 12-20-23, 04:27 AM
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Originally Posted by Trakhak
bulgie, did you dimple the fork blades before or after brazing?
Don't remember, but it doesn't matter. The indent is well below the HAZ.
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