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A little while ago, a year or so, I changed the cheapish steel fork on my commuter for a cheapish carbon bladed fork I had hanging around. The headset on the bike is a little non-standard but the shop that did the work for me (moved the crown race over) assured me that it would be fine.

Since then the handling of the bike has been atrocious. The new fork has a greater rake and a longer steerer but the handling of the bike is now really unsteady and twitchy. The other issue is that it's suddenly really difficult to get the loading right on the headset, a sixteenth of a turn in either direction and it's far too loose or far too tight.

closed as unclear what you're asking by Grigory Rechistov, David Richerby, Criggie Feb 19 at 9:54

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    What's the question? – dmb Feb 14 at 12:06
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    If the geometry of the new fork is different, the steering of the bike will be changed, You have proved you don't like the new steering so change the fork back to the steel one. – Argenti Apparatus Feb 14 at 12:18
  • @seedhe why edit link-spam into your question? Are you human or are you a spammer ? – Criggie Feb 18 at 10:01
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The new fork has a greater rake ... but the handling of the bike is now really unsteady and twitchy.

The greater rake reduced the trail. The trail the distance between where the pivot axis of the fork meets the ground and where the tire meets the ground. The greater rake moved the front wheel forward, towards the pivot axis of the fork.

It's like the front wheels on a shopping cart. The wheel points in the right direction because of the force from having the contact point behind the pivot axis. A smaller trail reduces the forces that keep the bike going straight.

See the Wikipedia article on bicycle and motorcycle geometry: https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Trail

The other issue is that it's suddenly really difficult to get the loading right on the headset, a sixteenth of a turn in either direction and it's far too loose or far too tight.

That sounds like the bearings might not have been properly installed - an incorrect crown race (wrong angle?) or something like that. How do you know they moved the old crown race over? They're not hard to get off with the right tools, but a little impatience during removal can result in a bent crown race. Maybe they cleaned off all the old grease and didn't re-lube it when installing the new fork?

Without details, it's almost impossible to do anything other than guess at the cause of this problem.

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    Agreed, something fishy about headset installation. I opened one up last week as it had similar symptoms and the cartridge bearings were in upside down, ie the list of possibilities is a long one! – Swifty Feb 14 at 17:10
  • @Swifty the cartridge bearings were in upside down LOL. I was actually going to put that in, but decided no one could possibly do that. – Andrew Henle Feb 14 at 18:44
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    I learnt a long time ago to never be surprised or under estimate the ingenuity of idiots – mattnz Feb 14 at 19:02
  • @mattnz I write software for a living, some of the code I currently support is used by supposedly-technically knowledgeable users. I've tried to make the app foolproof, but I am continually surprised at how God can create more ingenious fools so quickly. Over and over. "You did what? Umm, the app told you not to do that, but you did it anyway?" (App has some weird corner cases it has to handle) One place I worked: "Another stupid user error?" "So a S.U.E?" It became our code word. Next thing you know, "Jack, Susan is on the phone for you." Jack: "Oh no..." I should have known better. – Andrew Henle Feb 14 at 19:11
  • None of which is to suggest that seedhe has done anything wrong in this case ;) indeed, having one’s own cautionary tale about fork compatibility should even be a right of passage for the experienced cyclist – Swifty Feb 14 at 20:16

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