Trading 43 rake for a 50 rake fork?

[Mark Kelly]

specs_img.gif

The only fork manufacturer spec I could find (Wound Up) - note use of orthogonal rake dimension.

[GrantM]

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".

And it's safe to say the given specifications for an Enve fork, they use a 73 degree angle, not 90.

-g

[EricKeller]

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

the traditional way of making a fork is that there is one fork, one frame and never the two shall part. In fact, when I order a tube set I have to remind myself to order the fork parts since it's supposed to come with that stuff. AFAIK, A/C is something that came to be with commodity forks, I don't think that it's a traditional measurement. But I will admit I have never had a reason to understand what they are measuring since I have never used a commodity fork.

I am pretty sure rake is something that was defined as the measurement perpendicular to the steerer rotational axis, and that's a vehicle dynamics definition, not a bicycle frame builder definition.

I have tried to talk Brent into adding a fork design function to bikecad. The way it is now seems to be based on the assumption that the fork comes to the builder as a part, which just isn't the way the great framebuilder in the sky intended things to be done.

And it's safe to say the given specifications for an Enve fork, they use a 73 degree angle, not 90.

-g

I can't imagine that Enve specs anything that can't be measured on the fork itself. I like your definition because it's the one that is useful to an effbuilder, but I can't imagine a fork manufacturer using it.

[GrantM]

I can't imagine that Enve specs anything that can't be measured on the fork itself. I like your definition because it's the one that is useful to an effbuilder, but I can't imagine a fork manufacturer using it.

It does seem clear that Enve uses the direct axle to crown method.

Specs for forks are not the "fork length" method (parallel to head tube, 90 degree then rake to hub)
they would be too (correction: LONG) if 367mm was plugged into that formula.

-g

[progetto]

I measure every aftermarket fork before drawing my full scale plan. Not sure how some manufacturers come at their measurements but if I measure it my way then transfer to my plan then I know it's right. Having said that, a few mm one way or another won't be noticeable when ridden.

[e-RICHIE]

<cut> The way it is now seems to be based on the assumption that the fork comes to the builder as a part, which just isn't the way the great framebuilder in the sky intended things to be done.

Sadly, this ^ is true and also explains any inability to grasp the situation atmo. The span (or length of blades, or the real estate taken up...) under the lower end of the head tube is a fixed position based on the design of the frame. If the rake called for in a given situation is 4.3CM, then the span is chosen according to the plan. If you hypothesize about what may happen IF the rake morphed into 5.0CM, you don't get to that point only by moving the axle point forward by the difference, you also have to ensure that the span is considered so that the top tube stays in the same plane (and doesn't rise, or fall, as a result of changing the fork spec).

I feel badly for those of you so tethered to math, or cad, or these design programs. For a production run, I understand. For a simple frame, a simple frame design, or to fab a single and simple frame, you really need to use some hand's on experience, a ruler, some cutting tools, and a decent fixture - these, plus some sweat equity, will enable the issue being discussed to become intuitive. I've yet to read a discussion similar to this one in which the maker could come to a conclusion about what he's doing without having a laptop or a power source handy.

Go out and hit your thumb with a hammer once in a while. After some repetition, you'll learn where to leave your finger and well as how to swing the tool.

[NYCfixie]

I am sorry I headed in early last night and missed the "live" conversation. No matter what side you are on (lowers the head tube or does not lower the head tube), I really appreciate the thoughtful and well explained responses. There are obviously a lot of really smart people on this board who also happen to know a great deal about bike design.

As I have moved from a 54cm top tube frame to a 53cm frame (knee injury, short arms for my height, Shimano creating shifters that are longer 5600/6600 versus 5700/6700, better fit at Signature, etc.) I have moved towards the dreaded (IMHO) 71.5 head tube angle on a custom or high-end-production bike. It is no fun not being able to fit on a 56 or 58 because that is what seems to be the sweet spot in terms of design for production bikes. Maybe custom is the only way to go if you fall outside of these standard parameters.

I am amazed the production builders like Soma and Surly spec different fork rakes on their framesets as head tube angles change but some higher end production frame builders do not. It was mentioned earlier that these higher-end production companies are probably lazy - could this be true? I am going with it is a cost/stock issue. If they have taken the time to design smaller frames (53, 52, 49), and even the larger ones (60, 62) with different HT angles why not supply the forks with the appropriate rake to match?

I read through all of the posts and what I have gathered is the following:
- I should change the 43 rake fork to a 50 because it will decrease the trail, increase the front center, and thus improve the handling (quicker and more stable)
- It seems like most people agree that the head tube will not be lowered by changing the fork (assuming I am swapping a 43 Enve for a 50 Enve)


I also think that the real problem here is that I am making assumptions. If the high end production builder decided to spec a 43 rake fork even on smaller frames with different HT angles then maybe they decided the shorter front center and larger trail numbers are just a part of production bikes. If so, it seems very short sighted.


Again, thanks for all of the great information.

[e-RICHIE]

If the high end production builder decided to spec a 43 rake fork even on smaller frames with different HT angles then maybe they decided the shorter front center and larger trail numbers are just a part of production bikes. If so, it seems very short sighted.

Not to me ^ .
They are not making bicycles. They are making money atmo.

[EricKeller]

I feel badly for those of you so tethered to math, or cad, or these design programs.

I learned 30 years ago that math will lie to you about frames, so I have never used it. I do like to be able to draw something up in cad and make changes. Bikecad doesn't help much with forks, I would like to be able to look at clearances.

My jig is set up with the fork measurements determining the location of the bottom of the head tube. Stole that idea from Bill Grove, but I'm sure I've seen it elsewhere. In fact, some of the European fixturing incorporates a fork in the setup. Mostly for production bikes, I guess.

[zank]

This isn't rocket surgery. The terms may not be consistent from company to company (I measure "axle to crown" along the hypotenuse of the right triangle created by the fork), but the worst case scenario is that the bottom of the head tube will be a mm lower with the 50 if you go from 43 to 50 mm rake and keep what I call "axle to crown" constant. That 1 mm difference won't be felt. You do theoretically need a little more head tube (an extra 2-3 mm) than that though as the 50 mm rake fork effectively drops the right angle of the fork, but again, I don't think you would notice.

43 mm rake
367 mm axle to crown
364.5 mm length along steering axis

50 mm rake
367 mm axle to crown
363.6 mm length along steering axis

[Curt Goodrich]

Whether or not there's discrepancy in the math, the forks in question vary in actual length vs. stated length easily by 2mm. So at a certain point this whole discussion seems a little odd. On my Marchetti frame jig, the bottom of the head tube as a point in space is located via a built in dummy fork. The rake and bb drop are set and the fork length gets measured from that. The lower headset stack is added on to create effective fork length is created. No trig is harmed in the finding of this number. The Bike Machinery arrives at the same number using a similar method.

[Mark Kelly]

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

No, they don't. I asked ENVE directly and they confirmed that they measure fork length and rake by the standard method.

Given that both US based fork manufacturers, all the CAD programs, Sheldon Brown and every other reference I can find agree that this is the standard method, I think we can now answer the OP's question.

To the O.P: The answer is Yes.

[Mark Kelly]

...the bottom of the head tube will be a mm lower with the 50 if you go from 43 to 50 mm rake and keep what I call "axle to crown" constant. That 1 mm difference won't be felt. You do theoretically need a little more head tube (an extra 2-3 mm) than that though as the 50 mm rake fork effectively drops the right angle of the fork, but again, I don't think you would notice.

43 mm rake
367 mm axle to crown
364.5 mm length along steering axis

50 mm rake
367 mm axle to crown
363.6 mm length along steering axis

This is case 2 in post #8.

[cilantro]

For whatever it's worth, wikipedia says "The fork offset is the perpendicular distance from the steering axis to the center of the front wheel." (not that it can't be wrong, but often it reflects consensus).
Online calculators use the same concept.
This re-raking expert says "Fork rake is measured on a line perpendicular to the steering axis."
Looks like Mark Kelly has plenty of support. I found no source for the idea that rake is measured parallel to the ground.

[NYCfixie]

This re-raking expert says "Fork rake is measured on a line perpendicular to the steering axis."

IMHO this article might support both arguments or at least the marketing behind production forks. Your average bicycle consumer is not going to get into all of this detail. They want to know "is that cool fork going to fit my bike" and maybe even "is it built for short reach or long reach brakes". I would even assume very few people at the companies producing these forks really understand all the ramifications of the design. Let's be fair, the engineer/designer is probably not the person answering the phone when you call and ask about a to c, rake, etc. It is someone reading off the marketing sheet.

If I go with the marketing hypothesis, then I would say the FLA (length on axis) measurement is what they are sharing with the public (i.e. 367mm so it works with short reach brakes) but the actual length measurement FL (a to c) differs slightly (longer or shorter) based on the actual rake of the fork. Even with all of the math, I find it hard to argue with the picture Grant shared in post#39 which shows real world data.

Could both arguments be correct?
- The custom builders (and mathematicians) are dealing with building forks from materials so they can control all elements of design and in this scenario they are correct?
- The production forks are more about generalized fit for many frames and marketing your product so it does not confuse the end user so you limit (or normalize) the data you provide?

Again, thanks for all the thoughtful, and helpful, responses.

[GrantM]

If I go with the marketing hypothesis, then I would say the FLA (length on axis) measurement is what they are sharing with the public (i.e. 367mm so it works with short reach brakes) but the actual length measurement FL (a to c) differs slightly (longer or shorter) based on the actual rake of the fork. Even with all of the math, I find it hard to argue with the picture Grant shared in post#39 which shows real world data.

No, what i've learned is that fork "length" (as given in fork manufacturers specifications) is the direct line, axle to crown,

fuck the rest, it's too hard to meaure 3D accurately.


-g

[e-RICHIE]

No, what i've learned is that fork "length" (as given in fork manufacturers specifications) is the direct line, axle to crown.

-g

Go out to the shed and hit the hammer with your thumb a few times. After several Malcolm Gladwell moments
you'll learn where the fingers can hide in the event of an emergency. PS Editorial comment follows: I pity the
who didn't spend at least one session at the Outward Bound School of Framebuilding, for these are the ones
who will take the longest to accumulate a knowledge base. Asshat me - I can take it.

[Mike Mcdermid]

can we just confirm which is correct all my drawings have the rake perpindicular to the actual fork length so it looks like a right angle triangle

is this not right

caveat is that these are suspension forks and are set sometimes sagged

[GrantM]

can we just confirm which is correct all my drawings have the rake perpindicular to the actual fork length so it looks like a right angle triangle

is this not right

Yes, that's correct.

The two commonly published measurements are rake (parallel to steerer, 90 degrees) and Axle to Crown,
(direct distance from the center point of dropouts to center-line of the fork crown, at the race.)

-g

[blasdelf]

Frankly, it surprised me greatly that rake and length are conflated into one number by using axle to crown.

if they did it your way, a range of the same road fork design with different offsets would have varying brake clearances

and it's thankfully gotten quite a bit harder to sell road bikes where the brake pads are near the top of the slots

(also it's really easy to measure the hypotenuse of the fork's right triangle, and a pain in the ass to measure the offset or "grant length" without clamping shit down)

But then again, this really shouldn't have been so surprising to me, because the industry standard measure
of front center does exactly the same thing - it assumes BB drop is not part of the relevant measurement
when it clearly is. Two bikes with the same front center measurement can have significantly different
wheelbases if the BB drop is not the same. When I use bike cad to design custom frames, I check the
horizontal center-line, not just chainstay length, and not just front center, which are subject to bb drop.

oh you got this all upside down, the standard measures include the BB drop *because* it plays a role in what the rider feels

you don't ride the wheelbase alone, your balance is over the triangle formed by it with chainstay length and front center

a higher BB will make the bike feel shorter, and lower feels longer, thankfully the lines from the BB on the bike are shorter and longer when that happens so we can all play along instead of inventing new impractical measures from invisible points in space

chopping the horizontal wheelbase into two measurements doesn't make sense to me, it doesn't mean anything divided up without the BB drop included