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I ride in the northern suburbs of New York where road surfaces can be unpredictable. Sunken gas & water shut of valve covers and the like. The 585 has Campy Erus tubulars that seem proven to be completely bullet proof. The issue is when hitting these potholes I'll get a really loud THWACK sound from the front of the bike that scares the bejeuses out of me . The first couple of times this happened I was sure the next thing would be me hitting the deck. But I'd find myself just be going along fine except for sore wrists and palms. Luckily this does not happen to often.
My question is is this maybe the sound of the fork bottoming against the frame?
The headset seems correctly adjusted, but I was wondering is the distance between the fork & frame something that should be measured?
Any ideas Chas?

Thanks
fred g.
 

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loose headset...

The top cap should be tightened until you get some additional drag as the fork is turned, then backed off 1/8 to 1/4 turn. If the headset is too tight, the fork will not return to a straight ahead position after a turn, without some help. The old advice to clamp the front brake and rock the bike to feel for play does not work with integrated headsets. You won;t be able to feel any play, but the top cap will not be tight enough. You may need as much as 5Nm of torque to adequately preload the bearings.

Also check the gaps between the fork and frame and the headset top section and head tube. Thin .25mm shims are included with the headset to insure that a small gap can be maintained between the top section and head tube. Without some gap, the bearings will never adjust properly. You''ll be pressing the top section against the head tube instead of preloading the bearings.
 

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A thought

could be, if Shimano shifters, the levers activating slightly and thwacking back? I know sort of pothole you mean, more like a post hole...I've hit our potholes hard enough and I can see the lever pop open and slam shut.

This would be after confirming headset ok, and front skewer tight, and hub not a little loose, etc.
 

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In my case the headset definitely loosens up. Last night I had tightened up the headset before a group ride where they took me over a big pothole (I was in the back) and I heard the big crack. When I got home the headset was loose, I can tell by how easy you can turn the spacers by hand. Without touching the top cap, I loosened the stem, and the headset tightened up, so I am pretty sure that it is the stem slipping on the steerer tube when there is a big impact. Maybe I need to tighten the stem more, it is a Thomson X4.

The 555 also came with the cheaper FSA cap that does not have the separate bolt for the expanding part, the cap both expands and pulls down on the stem at the same time, not the best design. The one that came with my 585 is much better with a separate expander bolt.
 
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