Pintsized
journeyman
Reged: 02/27/05
Posts: 90
Loc: Corvallis OR
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My Luna Pro frame is on its way to being a real bike. First thing, though is picking a fork. As it happens, the only Alpha Q for that size (this is the 650c, size 46 frame) has a rake of 37 mm. Holy mackerel. The 650 Ouzo Pro is a more normal 40mm and I have been advised by the LBS to go that way. I am happy to take their advice. The thing is, why would anyone make a fork as short as 37? Even for a 650 c bike. Are they assuming this is being used for some purpose other than road racing?
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BikerDoug
friend
Reged: 12/29/03
Posts: 88
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Others will probably be better able to explain this than I... but fork rake by itself doesn't tell you much. It's just a factor in determining fork trail. The shorter the trail the more unstable the bike is.
That doesn't mean that's a bad thing. It the bike is too stable you can't turn it!
Using the calculator at http://www.kreuzotter.de/english/elenk.htm
My 64cm Calfee has a 74 degree headtube and a fork with 41mm of rake. With 700c wheels that makes my fork trail 5.3cm.
Your 46cm Calfee has a 72 degree headtube and with a 43mm fork rake and 650c wheels, you'd have 5.5cm of trail. With 37mm rake you'd have 6.13cm of trail... much more stable, maybe too much.
If you get a fork with 45mm of rake, you'd get 52.9cm of trail... making it very similar to what I've got on my 64cm bike.
Now... I don't know if any other adjustments have to be made for a small frame, Lennard Zinn has a good chapter on the whole principle in Zinn's Cycling Primer.
If I've screwed anything up, someone let me know.
But with all that said, I'd recommend you call Calfee on the toll free line and get a recommendation.
HTH
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flythebike
captain
Reged: 08/26/04
Posts: 272
Loc: N. Virginia, USA
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Yes, call or better email Craig. That is what I did when I had a question about my Luna fixie's fork. I ended up with the Alpha Q cross fork, which has 47mm of rake. You'd think that would make it hard to turn, but it really doesn't. It is real stable when climbing and hammering and such, a bit less aggressive than the D-fly with a road Alpha Q.
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BikerDoug
friend
Reged: 12/29/03
Posts: 88
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When you stated harder to steer, did you mean harder to move the bike off of its line or harder to control because it's unstable?
Increasing the fork rake will decrease the fork trail. A smaller trail makes the bike less stable.
I'm not sure what size your Luna Cross is, but I'll assume it's a 56cm the headtube angle is 71.5 degrees (a Tetra Pro in the same size is 73.5). Assuming you have 700c wheels, a fork with 47mm of rake gives you a trail of 6.2cm (very stable). If you had gone with a 43mm rake your fork trail would have increased to 6.6cm making it much harder to move the bike off of its straight line. (By comparison the trail on my Tetra Pro is 5.3cm)
I assume the Luna Cross frames have the shallower headtube angle for that very reason... it make them more stable while riding through the muck.
HTH
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Insightdriver
captain
Reged: 03/07/04
Posts: 472
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I was browsing this forum and found this is a very good explanation on the effects of trail on Tom Kellog's site:
http://www.spectrum-cycles.com/612.htm 
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BikerDoug
friend
Reged: 12/29/03
Posts: 88
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Thanks for the link. A much better explanation of the effects of changing fork trail than anything I've written.
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flythebike
captain
Reged: 08/26/04
Posts: 272
Loc: N. Virginia, USA
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Maybe I didn't know that much about fork trail. Guess I need to study. So if I had a road fork it would be slower to steer, and the slack angles are done to allow for a long rake fork so the bike isn't twitchy. Got it!
It is nice that the bike doesn't really have "toe clip" overlap.
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superunleaded
captain
Reged: 12/22/03
Posts: 223
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More about fork rakes here...
http://www.bikefanclub.com/gallery/showphoto.php?photo=1657&size=big&password=&sort=1&cat=3009&page=1
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ChrisVT
friend
Reged: 01/08/04
Posts: 38
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This is fun. I have Michelin Pro Race clinchers and Conti Comp tubulars. Anyone know the wheel diameters for those tires?
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flythebike
captain
Reged: 08/26/04
Posts: 272
Loc: N. Virginia, USA
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Quote:
BikerDoug wrote:
When you stated harder to steer, did you mean harder to move the bike off of its line or harder to control because it's unstable?
Increasing the fork rake will decrease the fork trail. A smaller trail makes the bike less stable.
I'm not sure what size your Luna Cross is, but I'll assume it's a 56cm the headtube angle is 71.5 degrees (a Tetra Pro in the same size is 73.5). Assuming you have 700c wheels, a fork with 47mm of rake gives you a trail of 6.2cm (very stable). If you had gone with a 43mm rake your fork trail would have increased to 6.6cm making it much harder to move the bike off of its straight line. (By comparison the trail on my Tetra Pro is 5.3cm)
I assume the Luna Cross frames have the shallower headtube angle for that very reason... it make them more stable while riding through the muck.
HTH
I just put a 38mm trail fork on there. I decided the bike was too stable. It handles quicker now, turns in faster. Still having a bit of trouble wrapping my head around trail.
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