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Old 08-17-12, 11:28 PM   #1
sailorbenjamin 
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Proper fork rake for a Raleigh Competition?

I've built a fork alignment jig and I put my Competition fork in it to try it out. Now I think the steering axis should follow the centerline of the fork blades but on this fork the leading edge of the blade is parallel to the steering axis.
What really counts, though is the actually amount of rake at the dropout. This fork has 2 3/16" or 56mm. If I push back about 3/16" so that the blade CL is in line with the steerer tube CL I get a rake of 2" or 51mm at the dropout.
What do you guys think? Does anyone know what this should actually be? Is 5mm not enough to worry about?
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Old 08-18-12, 05:56 AM   #2
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5mm is almost .200" to me that's alot.
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Old 08-18-12, 07:15 AM   #3
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I don't personally think that 5mm is all that much when it is in the positive direction. If it was 5mm the other way I'd be a little more concerned. The amount of rake needed is dependent on head angle built into the main frame. With earlier Competitions this was more slack, while later ones were really raced-up for quicker steering (Competition GS.)

Having "too much" rake, and thus more trail, is going to make a bike more stable and less twitchy overall. But the trade-off is a slightly less maneuverable bike. Unless you are needing a really twitchy race bike I think erring on the side of a more stable bike would be the way to go here.

If you ever intend to put serious miles, or any weight on the front end by adding a handlebar bag or front panniers you'll want a more stable front end with more rake and more trail. With more weight up front quick twitchy steering becomes seriously unfun -all the more crappy when you are tired and fatigued when keeping a twitchy bike riding straight is making you even more tired. Steep head, rake, and trail numbers also leads to a greater possibility of shimmies and other handling quirks.

And one other factor is that usually when forks do get bent, it is almost always the other way due to crash damage. Usually the rake gets made too small due to a collision that pushes the bottom of the fork back, or bends the steer tube to crown area back, which precipitates a much reduced rake. About the only way to accidentally bend a fork out to increase rake (like you suspect) is by jumping off drops so that hard landings tend to bend the fork out rather than back. Odds are the previous owner probably wasn't doing this on a Raleigh Competition. The bike really doesn't lend itself to BMX-type tricks and riding it off of drops. Possible, but I doubt it.
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Old 08-18-12, 07:36 AM   #4
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I would agree, Sailor. I'm not an expert on Raleigh Comp geometry but I've measured a number of new and used good-quality forks, and have had a bike with Cinelli-style sloped-shoulder fork crown and old Brit blades.

Here's how I check it:

The c/l of the fork blade should follow the steering axis. With a decent digital angle finder I'd measure the angle of the steer tube, front of blade, and rear of blade, all from the same reference (i.e. from the horizontal). The difference between the steer tube and fork front should be equal to the difference between the steer tube and the fork rear.

I'd think twice about bending, trying to localize where the distortion is first. It would make a big difference if you are correcting a kink in a blade or steer tube, a distorted brazed joint (dunno how this can happen!), or a twist in the fork crown lateral arms. The latter is the easiest to correct as long as the seat for the fork crown race remains perpendicular to the steer tube. Or, it would need to be milled before you re-install the headset. I haven't tried to do any of these corrections, except to straighten a kinked steer tube. Matt Churches and I got a UO8 fairly well straightened. It rides ok but I ride it pretty smoothly.

I wonder if Sutherland's says anything about this level of restoration work?
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Old 08-18-12, 07:38 AM   #5
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I checked the rake of both an Asian '81 Comp and a Carlton made '78; both have a Rake of 50mm taken from a line intersecting the Center of the steer and the fork crown. Both were aligned by the same frame builder within the last two years. If you've built the jig, why not manipulate the fork back to it's original configuaration. 5mm is not much, but why not make it perfect having gone this far?
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Old 08-18-12, 07:38 AM   #6
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Here's the way I understand the relationship between rake and trail. Trail is a measurement between where the centerline of the steerer tube intersects the ground and the center of the tire contact patch( a line drawn straight down from the axle to the ground). The contact patch normally sits behind the steerer centerline. Further behind means more stable. If you draw all of this out with different amounts of rake, you will see that decreasing rake increases stability. Many people get this backwards.
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Old 08-18-12, 07:39 AM   #7
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Quote:
Originally Posted by Amesja View Post
I don't personally think that 5mm is all that much when it is in the positive direction. If it was 5mm the other way I'd be a little more concerned. The amount of rake needed is dependent on head angle built into the main frame. With earlier Competitions this was more slack, while later ones were really raced-up for quicker steering (Competition GS.)

Having "too much" rake, and thus more trail, is going to make a bike more stable and less twitchy overall. But the trade-off is a slightly less maneuverable bike. Unless you are needing a really twitchy race bike I think erring on the side of a more stable bike would be the way to go here.

If you ever intend to put serious miles, or any weight on the front end by adding a handlebar bag or front panniers you'll want a more stable front end with more rake and more trail. With more weight up front quick twitchy steering becomes seriously unfun -all the more crappy when you are tired and fatigued when keeping a twitchy bike riding straight is making you even more tired. Steep head, rake, and trail numbers also leads to a greater possibility of shimmies and other handling quirks.

And one other factor is that usually when forks do get bent, it is almost always the other way due to crash damage. Usually the rake gets made too small due to a collision that pushes the bottom of the fork back, or bends the steer tube to crown area back, which precipitates a much reduced rake. About the only way to accidentally bend a fork out to increase rake (like you suspect) is by jumping off drops so that hard landings tend to bend the fork out rather than back. Odds are the previous owner probably wasn't doing this on a Raleigh Competition. The bike really doesn't lend itself to BMX-type tricks and riding it off of drops. Possible, but I doubt it.
Yet, it is possible. Some less-than gentle riders may not have appreciated the Comp and jumped it.
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Old 08-18-12, 07:41 AM   #8
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Yup, i had it backwards.


Last edited by Amesja; 08-18-12 at 07:49 AM.
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Old 08-18-12, 08:14 AM   #9
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Thanks for the drawings. Those are keepers and I'm glad you included the effects of head angle.
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Old 08-18-12, 08:26 AM   #10
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Yoinked from Wikipedia...
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Old 08-18-12, 04:57 PM   #11
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Quote:
Originally Posted by Velognome View Post
I checked the rake of both an Asian '81 Comp and a Carlton made '78; both have a Rake of 50mm taken from a line intersecting the Center of the steer and the fork crown. Both were aligned by the same frame builder within the last two years. If you've built the jig, why not manipulate the fork back to it's original configuaration. 5mm is not much, but why not make it perfect having gone this far?
I think the forks you are talking about were from the later high-trail twitchy-style years. The OP's fork might have been from the earlier (as early as 1968, perhaps) years of more rake and less trail. I don't know. If it is an earlier fork, it's probably designed with 60 mm or so as new, delivering perhaps 40 mm trail. It seems a little high-rake for a bike sold new with 700c tubulars, however.

The fork crown has already been cold-set once. Bending it back is at least two more cold-sets, depending on how well it's done the first time. If the crown is mild steel in the 10xx or 20xx classes, it's probably ok to do this. If it's a CrMo or 531 it might not, since those stronger alloys are less ductile than the former ones.
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Old 08-18-12, 06:51 PM   #12
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Quote:
Originally Posted by Velognome
I checked the rake of both an Asian '81 Comp and a Carlton made '78; both have a Rake of 50mm taken from a line intersecting the Center of the steer and the fork crown. Both were aligned by the same frame builder within the last two years. If you've built the jig, why not manipulate the fork back to it's original configuaration. 5mm is not much, but why not make it perfect having gone this far?

Originally Posted by Road Fan

I think the forks you are talking about were from the later high-trail twitchy-style years.
Ya, the "twitchy years" I like that
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Old 08-18-12, 07:02 PM   #13
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Amesja,
As previously said, thanks much for the diagrams from Wikipedia. I am going to print these for reference and to use in explaining these terms to others. Very helpful. Good thread too, great discussion.

Bill
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Old 08-18-12, 09:07 PM   #14
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Ok, so this bike is a '75 with a 60cm frame. That's "pre-twitchy", right? I've got a '74 (or is it a '73?) in the basement with a 57 or 58 cm frame. I guess I need to dig it out and measure it.
I'm looking at that book, The Custom Bicycle by Michael J. Kolin and Denise M. de la Rosa, c. 1979. In the chapter on Paramounts they talk about using a 1 3/8" rake on track frames, 1 3/4" on road frames and 1 1/2 on crit frames. They go on to say that they had been using "a 2 inch rake on touring frames but laboratory tests confirmed that 1/4" less rake gave the tourist a more stable and safer ride." I wonder how their frame angles compare to Raleigh's Time to get out the angle meter.
It's a nice enough bike that I want it to be right.
Thanks for all the research, guys.
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