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steering change from new headset with more stack height? (slightly raising front end)

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steering change from new headset with more stack height? (slightly raising front end)

Old 02-20-10, 10:14 AM
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TallRider
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steering change from new headset with more stack height? (slightly raising front end)

So I bought a new, properly-sized replacement fork for my buddy's commuting bike. (Here is the thread about the previous fork, and the thread about the first used replacement fork I'd grabbed.)

Let's assume for the moment that the new fork has the same approximate dimensions (crown-to-dropout, and rake) as the original fork on my buddy's bike before it got totally messed-up.

What I'm wondering is: if I install a new sealed-cartridge 1" threaded headset, that has a significantly increased stack height compared to the traditional steel-races with caged-bearings style, how much will steering be affected? Just modeling it in my head, seems like it will slightly decrease trail, but not much, since the front end will be raised by at most 5mm.
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Old 02-20-10, 10:32 AM
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I can't imagine that any headset will have 5 mm of additional stack height below the head tube. Additional stack height above the head tube won't affect steering, but may result in a steer tube that's too short. Maybe I'm not understanding what you're trying to say.
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Old 02-20-10, 10:41 AM
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There won't be any change. Most of the difference in stack height is at the top of the headset where it doesn't affect geometry at all. What little might be chalked up to a taller bottom bearing, no more than a few millimeters, will be too little to make any kind of meaningful difference.

If you want to know exactly how much the angle will change divide the difference in lower bearing stack by your wheelbase, and go to a sine table to find the angle with a sine equal to that ratio.

I ran a sample calculation and assuming a lower bearing stack height difference of 4mm, the change in headtube angle would be about 0.2 degrees, probably within the working tolerances of framebuilders.
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Old 02-20-10, 11:01 AM
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I agree - very little difference. Probably about the same as running slightly less pressure in the back tire or slightly more in the front. Basically it will be almost completely imperceptible.
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Old 02-20-10, 11:48 AM
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It's true that most diff in stack height is results from above the headset (locknuts, lock-washers and the like) but there is some difference below - in this case it's from relatively-modern sealed-bearing units being a fair bit thicker than traditional caged bearings.

the new fork has a longer steerer tube so I'll be okay there.

but y'all are right that the actual change in steerer tube angle will be very slight. I guess I could calculate that (good old trig) and it would probably be minuscule. sorry to bother.
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