How do I measure fork rake??

[onomic]

Hi,

I just bought a frameset that I need to replace the fork. I have several spares sitting around but I'm not sure what the rake is on the fork on the new frame. Is there an easy way to measure rake or do a side by side evaluation? Any thoughts help, thanks!

[kramnnim]

I've always wondered this...

[lovebird]

There must be an easier way, but I've done it by laying the fork flat on a table. I used a block under the steered so that the steerer measured level. Use a skewer in the fork dropouts. Then just measure from the top of the steerer column and the top of the skewer, in each case to the top of the table. Then just subtract half the width of the column ( and of the skewer if you want to be really exact), again subtracting in each case from the measurement to the table. Voila, the difference between those two adjusted measurements is fork rake. Hope that helps!

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[Black Dog]

There must be an easier way, but I've done it by laying the fork flat on a table. I used a block under the steered so that the steerer measured level. Use a skewer in the fork dropouts. Then just measure from the top of the steerer column and the top of the skewer, in each case to the top of the table. Then just subtract half the width of the column ( and of the skewer if you want to be really exact), again subtracting in each case from the measurement to the table. Voila, the difference between those two adjusted measurements is fork rake. Hope that helps!

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This works but you have to subtract ½ the width of the skewer. Rake is measure on mm and there is a difference between, say, 43mm and 45mm rake in terms of handling.

[onomic]

Thanks so much, I'll give that a shot. I was trying to visual something like that but having it written out fills in the gaps of my thought process.

[lovebird]

Good luck. One other tip is to be careful locating the skewer. It's the hub that centers the wheel so when you put just a skewer in there without a hub there is some wiggle room and you just have to try as best you can to get the skewer roof centered.

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[ORMojo]

Good luck. One other tip is to be careful locating the skewer. It's the hub that centers the wheel so when you put just a skewer in there without a hub there is some wiggle room and you just have to try as best you can to get the skewer roof centered.

Or what I do is mount an unbuilt hub...

I've done it exactly the way lovebird described, and the way illustrated here in a borrowed photo.
http://i668.photobucket.com/albums/vv48/ORMojo1/MeasuringForkRake_zps444dc0a6.jpg (http://s668.photobucket.com/user/ORMojo1/media/MeasuringForkRake_zps444dc0a6.jpg.html)

I've also done essentially the same thing in a vertical orientation. Clamp the steerer into a workstand so that the steerer is perfectly vertical. Then, there are two variations...

1. If the inside of the steerer tube is not obstructed, I've rigged up a plumb line dropped through the very center of the steerer tube, down past the horizontal plane intersecting the dropouts. Then, measure from that plumb line to the center of the skewer (I'm abbreviating by leaving out details, but I think you'll get the idea). (Of course this won't work on carbon or other forks where the steerer tube is not open through the fork crown, but it works fine on steel forks and others.)

2. If the inside of the steerer tube is obstructed, I've done it as a 2-person operation, holding (or clamping, to make it a one person operation) two long (4-foot, in my case) levels vertical, one next to the fork crown, one at the centerline of the skewer installed in the dropouts with an unbuilt hub. Again, I'm leaving out details, and you obviously have to do some additional math (subtracting the distance from the center of the steerer tube to the edge of the level held vertical next to the crown, etc.), but hopefully you get the idea.

Finally, if you want to determine the rake of a fork while it is still installed on the frame, here is a quick and dirty way. Measure the distance from a fixed reference point on the front face of the bottom bracket shell (or even the center of the crank bolt) to the centerline of the skewer with the fork facing perfectly forward. Then make the exact same measurement with the fork facing perfectly backward (yes, if the bike is completely built up, you will need to loosen the stem from the steerer tube). Subtract the smaller (backward facing) measurement from the larger measurement and divide by two. (Note that the most accurate version of this will use a fixed reference point on the frame that is on the same horizontal plane as the skewer.)

So . . . lots of options for you!

[tuscanyswe]

Does the 2nd alternative for measuring fork rake in the example above really work? My initial thoughts tell me (perhaps wrongly) that one would need to know the hta angle and apply some formula for this to work proper?

[Scooper]

Does the 2nd alternative for measuring fork rake in the example above really work? My initial thoughts tell me (perhaps wrongly) that one would need to know the hta angle and apply some formula for this to work proper?

Fork rake is the offset of the axle from the steering axis. The head tube angle, the fork rake, and the wheel radius are all used to derive the trail.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/BikeCAD/BikeCADForkRakeDef.jpg

Former framebuilder Dave Moulton has a nice blog entry about rake and trail:

Trail, fork rake, and a little bit of history (http://davesbikeblog.squarespace.com/blog/2007/5/4/trail-fork-rake-and-a-little-bit-of-history.html)

This is a pretty foolproof way to measure rake.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Forks/DeterminingForkRake_zpsc83dacfa.jpg

[Peter P.]

Here's what I've done to measure fork rake.

Remove the front wheel and the front brake. Loosen the stem bolts at the steerer so the fork can swing 360 degrees.

Measure from a point on the crankset near where the axle is. Doesn't have to be the axle center, somewhere around the crankbolt is good.

Swivel the fork 180 degrees and measure to the same 2 points. Subtract the larger number from the smaller number and divide by 2.

That's your rake.

In the OP's case, since he HAS to replace the fork, the methods described above may be just as easy.

[Mark McM]

What we are talking about here is measuring the fork 'offset', not the 'rake'. Rake is an angle, not a distance, so the fork rake is the angle of the fork, which will be same as the head angle.

From Merriam-Webster dictionary definition of rake (http://www.merriam-webster.com/dictionary/rake):

rake: noun

Definition of RAKE

1: inclination from the perpendicular; especially : the overhang of a ship's bow or stern

2: inclination from the horizontal : slope

3: the angle between the top cutting surface of a tool and a plane perpendicular to the surface of the work



Motorcyclists have this right, and when they refer to rake, the are referring to the angle of the steering axis.

http://www.flattrackaccessories.com/product/hondadef487x319.jpg

[Scooper]

What we are talking about here is measuring the fork 'offset', not the 'rake'. Rake is an angle, not a distance, so the fork rake is the angle of the fork, which will be same as the head angle.

From Merriam-Webster dictionary definition of rake (http://www.merriam-webster.com/dictionary/rake):





Motorcyclists have this right, and when they refer to rake, the are referring to the angle of the steering axis.

http://www.flattrackaccessories.com/product/hondadef487x319.jpg

There seems to be some confusion over the term “Fork rake,” which I can understand. The dictionary definition for rake is “The angle of inclination from the vertical.” However, when referring to bicycles, rake and offset are different terms for the same thing. Both are the term for the distance the wheel center is set from the steering axis, regardless of the head angle.

This probably came about because early framebuilders were artisans, not scholars. To add to the confusion, a bicycle head angle is measured from the horizontal, not the vertical. Back in the day when all bicycles had perfectly level top tubes, it was the angle measured from above the top tube to an imaginary extension of the head tube.

Universally, for bicycles, fork rake and fork offset are the same.

[tuscanyswe]

Fork rake is the offset of the axle from the steering axis. The head tube angle, the fork rake, and the wheel radius are all used to derive the trail.


Former framebuilder Dave Moulton has a nice blog entry about rake and trail:


This is a pretty foolproof way to measure rake.

Snip**


Seems like a good way.

My original question tho was rather about the particular way of measuring fork rake from a fixed point on a bike. Since the hta will effect the distance measured but perhaps its such a small difference that it wont matter much if any.

But as an example. A fork on a bike with a hta of 25 degrees (yeah i know but this fork will move more up and down than back and forward when facing one way or the other) will not show as much difference in length from crankset to hub axle when fork is facing forward or backwards as it would on a bike with say 73 degrees hta.

Wow im bad at explaining but thats how i c it anyways :)

[Scooper]

Seems like a good way.

My original question tho was rather about the particular way of measuring fork rake from a fixed point on a bike. Since the hta will effect the distance measured but perhaps its such a small difference that it wont matter much if any.

But as an example. A fork on a bike with a hta of 25 degrees (yeah i know but this fork will move more up and down than back and forward when facing one way or the other) will not show as much difference in length from crankset to hub axle when fork is facing forward or backwards as it would on a bike with say 73 degrees hta.

Wow im bad at explaining but thats how i c it anyways :)

Well, one easy way to measure fork rake while the fork is installed on the frame is to mount the frame in a workstand so that the head tube is as close to vertical as possible, remove the front wheel and install a wooden dowel in the dropouts as shown in my post above, set the workstand up so that the frame is at a right angle to a nearby wall, and perform the measurements from the wall to the centerline of the dowel first with the fork dropouts pointed toward the wall then with the fork rotated 180° so the dropouts are pointed away from the wall (as shown in my post above) and divide the difference by 2. With the head tube in a vertical position, HTA won't be a factor.

[ORMojo]

Well, one easy way to measure fork rake while the fork is installed on the frame is to mount the frame in a workstand so that the head tube is as close to vertical as possible, remove the front wheel and install a wooden dowel in the dropouts as shown in my post above, set the workstand up so that the frame is at a right angle to a nearby wall, and perform the measurements from the wall to the centerline of the dowel first with the fork dropouts pointed toward the wall then with the fork rotated 180° so the dropouts are pointed away from the wall (as shown in my post above) and divide the difference by 2. With the head tube in a vertical position, HTA won't be a factor.

Yes, and this way you describe here, Moulton's way, and the final way I describe in my post above are all just variations on the same approach. The main difference is whether fork is installed or not, and whether you use a wall as a fixed point to measure from, or, as I suggested, a fixed point on the bike frame itself to measure from.

[onomic]

This is great! Thanks for all the replies, advice, pics. Armed with all these techniques I should be able to figure this out and get a correct replacement installed. Now if I could just get home from my work trip to get done. I'll let you know how it all works out. Thanks again for the wealth of knowledge.