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Originally Posted by Piff
(Post 20050915)
Well, the 72ish degree HTA comes from a relatively long replacement fork, versus a likely short original fork as the 1200 has tight clearances. Another commenter in the thread equaled 20mm fork length equal to 1 degree for a HTA, so it seems likely that the HTA change is at least half a degree. Yes, no?
And yes, I thought there was too much rake originally by looking at the fork, but, based on others commenting on how an increased fork length leads to more trail, I'd have to agree that what I previously called "twitchy handling" should actually be called floppy. And since there's too much trail, adding rake would increase the HTA and lower trail. https://www.sheldonbrown.com/rinard/forklengths.htm Wheel flop changes only very small amounts even with a full degree change in HTA. 16mm of flop to 18mm. Did you measure your wheelbase? Is your top tube no longer level with the wheels in? What is your experience with bikes of this geometry? Does it fit you correctly with a normal stem length? I'm thinking that either the bike is fine and just seems weird to you, or the downtube is buckled behind the headset from a front end collision, which you should be able to see or measure via a big change in wheelbase. But there is no combination of common replacement forks that should make your handling much different than common production bikes which have neutral to long trail. Since you have the trail number, you can measure the fork axle to crown length by using 45mm as the vertex and the distance to the axle as the hypotenuse. |
to your specifics IDK..
People seeking the Bike Quarterly magazine promoted lower trail, increase the offset to achieve it.. |
Originally Posted by Piff
(Post 20050750)
I do appreciate your help thus far, but that isn't exactly the answer to the question I was asking. Considering that the bike is handling badly with too much flop, and that the trail is about 62mm, would increasing the rake by 4-5mm be a good decision?
Over the years, Ive built a couple forks with adjustable dropouts that let you change the offset/rake between 50-80mm. Ive found that it takes at least a 10mm change in offset/rake to produce any appreciable difference in handling, and even then you adjust and it feels no different after an hour of riding. A 5mm change isn't really going to change much, if anything. |
Ah, well I'll take yours guys' advice and leave it as is. Perhaps in time when I've got some saved up I'll bring it in to a framebuilder to have the frame's alignment checked, as well as having a new fork made for it. We'll see.
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Originally Posted by Piff
(Post 20053393)
Ah, well I'll take yours guys' advice and leave it as is. Perhaps in time when I've got some saved up I'll bring it in to a framebuilder to have the frame's alignment checked, as well as having a new fork made for it. We'll see.
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Originally Posted by Kontact
(Post 20053467)
Did you check the wheelbase to rule out any frame weirdness?
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Originally Posted by Piff
(Post 20053486)
Yeah, I measured the wheelbase and it's about 1/2 an inch longer than what the spec sheet shows. Within normal boundaries considering a new fork, I imagine?
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With short reach brakes, I'd expect the original fork's ATC measurement to be closer to 365 and according to 'Fork Lengths' on Sheldon's site, it could be even shorter (Tange 7B @ 363.5). This would steepen up the HTA and reduce flop and is probably the best replacement dimensions. As has been said, rake of anything between 43-50 should be magic.
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Originally Posted by duanedr
(Post 20056333)
With short reach brakes, I'd expect the original fork's ATC measurement to be closer to 365 and according to 'Fork Lengths' on Sheldon's site, it could be even shorter (Tange 7B @ 363.5). This would steepen up the HTA and reduce flop and is probably the best replacement dimensions. As has been said, rake of anything between 43-50 should be magic.
Agreed, as I mentioned in my first posting. Andy |
Originally Posted by Andrew R Stewart
(Post 20056602)
Agreed, ...
Seriously, I should read more closely. I saw someone based their calcs on a different number so didn't realize we'd stated the correct answer. |
I have a 912 fork from that model year and it measures 385mm.
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what size tires was the 912 built for? I took the OP's catalog page to mean he has a 1200
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