Trading 43 rake for a 50 rake fork?

[Mark Kelly]

I was certain the lower XY headtube axis should remain constant. To prove it I plugged it into bike cad. Oops. I'm surprised. Fork Length below remains constant, rake changes as does axle to crown but the xy point of the bottom of the headtube moves. hmmmm

The difference is 2mm, as previously calculated*. As I said, it's simple geometry.

* Since the difference calculated is 2.04mm, the HTA in Jonathan's example is 73 degrees.

[Mark Kelly]

What is "effective heights"? We're taking about this:

http://www.velocipedesalon.com/forum...tml#post521166

See Jonathan's drawing.

[e-RICHIE]

If you want to find a way to lower the head tube by 2mm all you have to do is replace the fork
with one that has a shorter span OR simply change tires, or have less air in the front, or more in
the rear atmo.

[Jonathan]

See Jonathan's drawing.

the drawing does show why it changes that x/y point if you squint just right at the axle area :) The plane that rake is measured is not parrallel with the ground.

[GrantM]

See Jonathan's drawing.

Yeah, it has the same error in the formula that you're making. There isn't a 90 degree
angle between the length and rake, it's the head tube angle.

Every framebuilder who draws a full scale blueprint knows that you can change
the length and rake independently. Production forks of different rakes are designed
for the same headtube position on an X-Y axis.

Correctly measured, fork rake only moves along the Y-axis.


-g

[Mark Kelly]

Yeah, it has the same error in the formula that you're making.

Every framebuilder who draws a full scale blueprint knows that you can change
the length and rake independently. Production forks of different rakes are designed
for the same headtube position on an X-Y axis.

-g

That is mathematically impossible with different HTAs, and it isn't what the OP asked about.

BTW, there is no error in my formula, it is a simple geometric consequence of the definition you gave. You can't simply gainsay this by apealling to authority ("every framebuilder... knows"). You need to do the maths.

[GrantM]

That is mathematically impossible with different HTAs.

We're talking at cross purposes. Sub 72 or 73 degees instead of 90.

This is about an example of a single head tube angle, two different rakes.

Of course changing the head angle moves the other variables.

People refer to the wrong measurements all the time.
"Chainstay length" is the other example, where BB drop is included
in what is commonly referred to in the measurement, instead of the centerline
beween the rear hub, parallel with the ground, intersecting the BB.

thanks for playing, i'm done.


-g

[Jonathan]

Yeah, it has the same error in the formula that you're making. There isn't a 90 degree
angle between the length and rake, it's the head tube angle.

Every framebuilder who draws a full scale blueprint knows that you can change
the length and rake independently. Production forks of different rakes are designed
for the same headtube position on an X-Y axis.

Correctly measured, fork rake only moves along the Y-axis.


-g

i'm agnostic on the hole thing because everything moves anyway, but the implications of what youre saying is that every frame built using bike cad and a point for X/Y has been built using the wrong measurement?

[GrantM]

but the implications of what youre saying is that every frame built using bike cad and a point for X/Y has been built using the wrong measurement?

The implications are only confused by different defintions of what "fork length" is.

A builder doing a full scale drawing won't assume a 90 degree angle between the head angle
and where they measure rake. They would draw a line parallel with the ground through the
front hub, and extend a line from the head tube until it intersects the line where the hub is.

-g

[The Bridge Guy]

Your logic is faulty, the hub is not a fixed point, it moves horizontally as rake changes.

Take a hockey stick and trim some length off the tip of the blade, does the handle get shorter? No it does not.
The hockey stick handle length is the fork length, independent of the length of the blade.


-g

I completely agree with you. The height of the axle is fixed, of course it moves horizontally (this is why the wheelbase changes). So as the rake gets longer, it does so at an angle, the change in length has to create a similar change in height of the top of fork. Put the bottom of your hockey stick blade (pretend that it is 90° to the handle) on the edge of a table. Now keeping the handle at the same angle (head tube angle), change the blade length (rake) by sliding the blade up and down. As you stated, the length of the handle doesn't change, so the top end of the handle (top of fork) has to go up and down too. Unless you are e-richie making a fork and change the span to compensate.

[Jonathan]

The implications are only confused by different defintions of what "fork length" is.

A builder doing a full scale drawing won't assume a 90 degree angle between the head angle
and where they measure rake. They would draw a line parallel with the ground through the
front hub, and extend a line from the head tube until it intersects the line where the hub is.

-g

First let me say none of this matters, the error doesn't change how the bike feels.

I think we all build forks using what you describe as fork length, but if you're using a Anvil fixture or some other fixture that uses a xy coordinate on the lower ht things don't match up if you're pulling that XY off bikecad. I may shoot an email off to Brett Curry and see how he's programmed bikecad.

[Mark Kelly]

Grant

As far as I am aware none of the software packages calculate the way you suggest, they all use the rake measurement as orthogonal to the axle to crown measurement.

Every other resource I can find agrees with this, from Wikipedia to Sheldon Brown, so I think it's safe to call it the common usage.

Under this usage, the formula I posted is correct.

For your usage to be adopted, we would need three measurements: AC, rake and the angle at which the rake was measured. We could then collapse these to two measurements, namely AC and rake' where rake' = rake * cos (rake angle). My formula would then be correct if used with rake'.

[The Bridge Guy]

Grant, just realized this too, that we are not thinking of how the rake is measured in the same way. Peace.

[GrantM]

Grant

As far as I am aware none of the software packages calculate the way you suggest, they all use the rake measurement as orthogonal to the axle to crown measurement.
.

Then it would be interesting to poll the builders here,
because this discussion has come up before.

As I learned from Richard years ago, the traditional measurement
treats the variables independently, because there would be no desire
for the head tube postion on the xy axis to move - the top tube wouldn't
remain level.

-g

[Mark Kelly]

Agreed, this is clearly the implication of Richard's post #16.

On the other hand, when you purchase a fork from, say, Enve, I think it's safe to assume that the specifications given are according to the "common usage".

This matters to me more than most because I'm in the throes of designing a jig and I like to know what's being measured.

[CyclesNoir]

This is simple geometry, and one has to do the math.

If rake is measured parallel to the ground, a fork rake couldn't be specified without also noting HTA. That's not how its done. Trail, on the other hand, is measured parallel to the ground.

In the O.P.s example, the front end drops, but a miserly amount. This drop (or droop?) steepens the effective HTA, and thus reduces trail (as does the actual increase in rake). However, because the change in front end height and HTA are so small, it would take the princess and the pea to feel the difference.

Balance front to back, and wheelbase, are also important to the overall feel/responsiveness.

I've been experimenting with different trails on bikes with decently (590mm - 600mm) long front centers. What I find is that too little trail makes the front end feel light, as if the center of rider gravity has moved backwards. I need to move my body forward to carve a line secure-feeling line through a corner. But, when trail is increased, (which minimally shortens the front center), this weight shift isn't required.

Small frames often have short front centers. Moving to a fork with larger rake will also increase the front center. This combination may lead to good results for the O.P. But, without more data, I wouldn't promise as much.

[happycampyer]

On the other hand, when you purchase a fork from, say, Enve, I think it's safe to assume that the specifications given are according to the "common usage".

it would be interesting to know if this is true—if not, what angle do they assume (e.g., 73*) to keep the offsets horizontal. Another person who would be interesting to ping is Mike Lopez (who designed the Ouzo Pro and all of Serotta's forks).

[Mark Kelly]

fork_rake_measurement.png

From BikeCAD

frame_stability_sketch.jpg

From Calfee

[GrantM]

it would be interesting to know if this is true—if not, what angle do they assume (e.g., 73*) to keep the offsets horizontal.

I just measured my 45mm Enve fork, it's specs are 367mm (axle to crown) x 45 rake.
It's obvious they measure the rake parallel to the ground, or near enough to
some low 70's degree head angle number, and not 90 degrees.
If you measure at 90 degrees from the hub, 367mm length barely reaches the center
of the brake bolt, the crown is about 1 cm higher.

If you think about it from a fork maker's perspective, they would normalize the
height of the lower headset cup. It makes no sense to have the height of the
front end change on every different fork rake.

-g

[Chauncey Matthews]

No need for all the formulas...you increase the rake, you make the fork blades longer and the ht doesn't move.