Figuring out fork rake...

[SoCalSteve]

Hi all,

I have an Ouzo Pro thats a bit "older" and the sticker on the front that has all the stats is kind of rubbed out.

I need to figure out what the rake is.

Please tell me what the best/easiest way to do this is.

As always, thank you all in advance,

Steve

[Too Tall]

From my little black book of horrors:

RAKE (offset) AND TRAIL
Rake (offset) is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) – (rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = 33.65/tan73 – 4.0/sin73. This calculates to 6.1cm or 2.4 inches.

http://www.kreuzotter.de/english/elenk.htm
40mm rake = Trail of 60.0
43mm rake= Trail of 57.1
45mm rake= Trail of 55.1

[RPS]

Steve, are you asking for an easy way to measure the rake on your existing fork? I think that's what you asked, but now I'm not sure.

[Louis]

Please tell me what the best/easiest way to do this is.

This is how I would do it:

I have one of those laser level deals that shoots out two lines perpendicular to each other. Use that. I would mount the fork with the steering axis in the vertical direction and shoot the lines at the side of the fork such that the vertical line matches the center of the steering axis as it shines on the side of the steerer tube. I would adjust the height of the fork and laser so that the horzontal line passed though the center of the fork end. I would then measure the distance between the intersection of the two lines and the center of the fork end.

Louis

[bobscott]

I am as much fishing for a critique of this fork on the bike method as much passing it on.
I have a Sears lazer level on a tripod which projects a line which can be rotated. I place a carpenter's square on the floor with one side parallel to the axis of the front wheel. I use the square to assure that I am shooting a line perpendicular to the bike. I then shoot a line down the center of the head tube which projects down the fork and on to the floor. The line on the floor is aligned with the square to assure a perpendicular to the fork shot. Then I measure from the center of the dropout/axle to my lazer line.
I imagine you could do something similar with the fork off the bike.
bobscott

[SoCalSteve]

Steve, are you asking for an easy way to measure the rake on your existing fork? I think that's what you asked, but now I'm not sure.

Yes, thats exactly what I was asking...

Not a geometry-trigonometry-calculas lesson.

I actually did the most obvious thing and called Reynolds. They are great with customer service!

Its quite simple (no laser pointers needed) if you have a second fork that you know what the rake is (I happen to). Put both down next to each other on a level surface. From there you can tell visually which one has more rake (or less). Done!

Thanks all!

Steve

[RPS]

Great, I’m glad you solved the mystery. And even if you didn’t have an extra fork, there is another simple way to measure it with some sense of accuracy that has worked for me. Good luck.

[SoCalSteve]

Great, I’m glad you solved the mystery. And even if you didn’t have an extra fork, there is another simple way to measure it with some sense of accuracy that has worked for me. Good luck.

Do tell!

The next time I may not be so lucky as to have a "spare" fork lying around.

Thanks,

Steve

[RPS]

Do tell!

The next time I may not be so lucky as to have a "spare" fork lying around.

Thanks,

SteveSteve, what I found to be easiest and most accurate without the use of a dedicated fixture is to use the bike’s frame as an improvised jig/fixture. I removed the front brake and disconnected the bars so I could rotate the fork 180 degrees. All that is needed is a stationary reference point off the frame to measure to, but the idea is to measure the rake to the front, then spin the fork 180 degrees so the rake is to the rear, and measure again. The difference between the two measurements (normally in the 3-plus-inch range) is twice the rake. I found this to be easiest and most accurate. Just make sure the headset is properly adjusted without play.

I clamped (very lightly) a 9MM fork-mount skewer on the fork end to better define the centerline of the wheel axle and to mark the center between fork blades, and gently clamped a point of reference off the down tube near the BBKT so that it was very close to 90 degrees from the steering axis relative to the skewer (a little off won’t affect the accuracy). With a little care, you can easily get accuracy of +/- 1 MM.

When I tried it in similar manner as described by others above it didn’t work for me because I couldn’t line-up accurately enough to the head tube, or even to the fork steerer when off the bike. Even a small fraction of one degree of misalignment will throw the measurements way off when trying to distinguish between say 4.3 and 4.5 CM of rake – I just couldn’t get any repeatability. I also tried lightly clamping the fork steerer on a flat and very straight surface and that too was difficult, in part because one of my forks was not entirely round at the steerer, and the crown area made it necessary to use spacers to lift the steerer off the surface – it became a pain. Also, for accuracy of less than 1 MM or less, the surface has to be perfectly straight and not deformed by the clamping action.