Calculating fork rake

[Dead Man]

Hola

So I figured I'd just measure from the BB shell to the center of the skewer, flip the fork around and repeat, subtract the one from the other and divide by 2

I got 43mm.... which is obviously a very common fork rake.. so I kinda feel like I did right.. but I also know what a difference a couple MM can make - I had a road bike with a 42mm fork that was absolutely unruly... almost unrideable, really. Stuck a 45mm fork on there and suddenly she rode like a DREAM. For instance.

Is there a better/established method for accurately calculating rake when it's not otherwise known?

Thanks brus

-B

[Peter P.]

Nope; there's no more accurate method except looking at the stickers on the fork or using a surface plate, and you'd still get the same answers.

[marciero]

That would give you the horizontal distance forward of the axle. To get the rake, which is the perpendicular offset from the steering axis, you would need to multiply by cosecant of the head angle. For a 73 head angle and 45 rake fork this is approx. 2mm difference. On the other hand, if you were interested in the contribution to trail you would want the horizontal distance.

[marciero]

...so assuming your measurement is accurate you have about a 45 rake (43 x 1.0457), assuming 73 ht angle. All this neglects the slight deviation from horizontal of the line from the bb to the skewer center, which would be negligible.

[David Kirk]

It's super easy to measure rake if you have the fork out of the frame. If you want to double check your number by putting the fork on a bench I can show you how to measure it.

dave

[Mark McM]

That would give you the horizontal distance forward of the axle. To get the rake, which is the perpendicular offset from the steering axis, you would need to multiply by cosecant of the head angle. For a 73 head angle and 45 rake fork this is approx. 2mm difference. On the other hand, if you were interested in the contribution to trail you would want the horizontal distance.

You forgot to consider that the line between the BB and the front axle is not horizontal, it is at an angle (the distance that the BB is below the axles is called the Bottom Bracket Drop), so this angle must be added to the head angle to calculate the sine error (cosecant). On a typical road bike, the BB drop is roughly 6 cm, and the front center (distance from the BB to the front axle) is roughly 60 cm, so the angle of the line from BB to axle is roughly arcsin( 6 cm/ 60 cm), or about 6 degrees. For a 73 degree head angle and a 43 mm rake, this will result in a measurement error of only about 0.8 mm.

[John H.]

What fork? Often times it is printed on the steerer tube or stamped on a dropout-

[marciero]

You forgot to consider that the line between the BB and the front axle is not horizontal, it is at an angle (the distance that the BB is below the axles is called the Bottom Bracket Drop), so this angle must be added to the head angle to calculate the sine error (cosecant). On a typical road bike, the BB drop is roughly 6 cm, and the front center (distance from the BB to the front axle) is roughly 60 cm, so the angle of the line from BB to axle is roughly arcsin( 6 cm/ 60 cm), or about 6 degrees. For a 73 degree head angle and a 43 mm rake, this will result in a measurement error of only about 0.8 mm.

Well, I called it "negligible" above. But as you point out, it's close to 1mm-more than I would have guessed. This still neglects the effect of reversing the fork, which lessens the bottom bracket drop. Now that has got to be negligible.

[mistermo]

It's super easy to measure rake if you have the fork out of the frame. If you want to double check your number by putting the fork on a bench I can show you how to measure it.

dave

Inquiring minds want to know too!

[Vientomas]

http://forums.thepaceline.net/showthread.php?t=151278

[David Kirk]

Inquiring minds want to know too!

Cool - it's super simple. You need two blocks that are the same thickness (I used 2x4's here) and a 3rd block that could be the same or a bit thicker or thinner.

Put the two same-size blocks on the surface and place the fork steerer on the blocks. Now slide the 3rd block back under the blades until it just contacts each blade (keeping it perpendicular to the centerline of the fork).

Now the fork is laying parallel to the bench (or floor or counter top or....etc) and you've made a measuring jig (you're more clever than you thought!).

Now measure from the upper surface of the steerer to the surface and then subtract 1/2 the diameter of the steerer from that number. This tells you how far the centerline of the steerer is from the table.

Lastly measure from the center of the dropout axle slot to the table and subtract the steerer-to-the-table number from it and you have the rake.




The key things here are making sure that the two blacks under the steerer are the same thickness and that the block under the blades is square to the fork.

FWIW - I've found of the years that the rake number marked on carbon forks is often just wrong. Most of the time is right or very close but I've seen plenty of forks that have a sticker reading 48 mm and it's actually a 43 mm. Wrong sticker on the fork? Poor quality control? Sun spots? Who knows. But if you want to be 100% sure measure it and see what it really is and not just rely on what the sticker says.

dave

[thwart]

Cool - it's super simple. You need two blocks that are the same thickness (I used 2x4's here) and a 3rd block that could be the same or a bit thicker or thinner.

Put the two same-size blocks on the surface and place the fork steerer on the blocks. Now slide the 3rd block back under the blades until it just contacts each blade (keeping it perpendicular to the centerline of the fork).

Now the fork is laying parallel to the bench (or floor or counter top or....etc) and you've made a measuring jig (you're more clever than you thought!).

Now measure from the upper surface of the steerer to the surface and then subtract 1/2 the diameter of the steerer from that number. This tells you how far the centerline of the steerer is from the table.

Lastly measure from the center of the dropout axle slot to the table and subtract the steerer-to-the-table number from it and you have the rake.




The key things here are making sure that the two blacks under the steerer are the same thickness and that the block under the blades is square to the fork.

FWIW - I've found of the years that the rake number marked on carbon forks is often just wrong. Most of the time is right or very close but I've seen plenty of forks that have a sticker reading 48 mm and it's actually a 43 mm. Wrong sticker on the fork? Poor quality control? Sun spots? Who knows. But if you want to be 100% sure measure it and see what it really is and not just rely on what the sticker says.

dave

Thanks, Dave.

This sort of post always make me wish the Forum had a 'tech stuff' reference area for these pearls of wisdom...

[monkeybanana86]

Easy peasy! Thanks for sharing.

[cachagua]

Incidentally, a cheap way to get a very flat surface for measurements like this is to visit a countertop installer and get a castoff piece of marble or granite countertop. Now and then the new guy breaks something trying to install it, or there's just an extra few feet of this or that, and there you are.