steel forks-does how you get to the rake matter?

[eddief]

So steel fork nuts I was just wondering...if you had two fine steel forks made out of the same good stuff and both had a rake, let's say, of 43. If one had straight blades and achieved the rake from being bent right above the crown and the other achieved the rake by nicely curving the blades toward the dropouts---do they ride the same? Does the curvey bladed fork feel springier?

[e-RICHIE]

they should ride the same.

straight blades eliminate the charming hands-on
technology of mating like-raked blades to get a
perfect fork. offsetting the straight blades "forward"
to get the rake produces the same rake. period?
make 10,000 forks a year and labor savings add
up. they are an accountant's dream. when they
appeared in the early 90s under the guise of "new
and improved" folks either dug it or they didn't.
some thought they looked too wierd; others thought
they looked sexist. there's nothing wrong with sexist.

[eddief]

I appreciate the dialogue on this subject. Now, granted I do not own the high priced spread in available steel framesets. But the current small stable includes the cost effective Kogswell P58 and the lovely orange Rambouillet, both with steel forks. I can't help thinking that the subtle curve at the end of the Rambouillet fork blade does not act as more of a spring than straight or straighter bladed forks.

Are you just quoting, do you believe it to be true, what would science suggest?

Sorry to be so uninformed but who the heck is nigel tufnell?

[eddief]

In ancient times, hundred of years before the dawn of history..."

This is Spinal Tap (1984) Nigel Tufnell

[e-RICHIE]

i believe it to be true.

[eddief]

do you get the same ride characteristics?

[e-RICHIE]

imo, there can't be any perceptible difference.
otoh, you can't ignore the psychological effect
of seeing one vs the other and forming a bias
as to what might occur if you go down the road.

[eddief]

those curvy fork ends certainly appear to be more of a spring. For some reason my brain suggests all the curved stays and swaging would make not a speck of difference in the feel, but the curved fork blades with the wheels at the end just suggest more spring. I'll rest my case. Where's Jobst Brandt when you need him? Who the heck is Jobst Brandt?

[columbusslx]

This seems to be one for the ages. Some claim straight is stiffer. Some claim curved is stiffer. Some claim straight tracks better. Some claim curved are more supple. Some claim they ride better no handed with one or the other. Others will chime in on the crown design, saying that the crown determines the feel. I think I have heard it every way the last dozen years, esp. when Colnagos were hot with the Precisa (how soon we forget the twin downtube...) I cannot tell a difference riding with my eyes closed on two identical bikes save fork design so I think pick the one you like the look of best.

[David Kirk]

two simple words.......load path

Dave

[jerk]

huh? load path? then why not make a stupid looking fork like this? you could have a really long weird load path then. the jerk doesn't get it. they all seem the same to him. (although some forks are noodles and don't feel to nice in a sprint or when your cornering hard....don't think it has anything to do with the path the blades take to get the wheel in the right place....)

as an aside while that fork does look foolish....the bike it goes on looks really tough, especially when it' blue and under this guy: (by the way that's a 54x58.5 with a 150mm stem. same size as 93legendti!)

[Climb01742]

i'll caveat what follows with: i'm stupid but...this is probably an inaccurate analogy but say you jump off a table...landing with your legs straight gives you a very harsh landing...while landing with knees bent softens the landing...is there no parallel to a fork absorbing shock? BTW, i like how the wavy fork looks...like it was designed on acid. :rolleyes:

[Ray]

i'll caveat what follows with: i'm stupid but...this is probably an inaccurate analogy but say you jump off a table...landing with your legs straight gives you a very harsh landing...while landing with knees bent softens the landing...is there no parallel to a fork absorbing shock? BTW, i like how the wavy fork looks...like it was designed on acid. :rolleyes:
Shouldn't be the same by a long shot unless your fork has a hinge in the middle (your knee), which I suspect would cause it's own set of pretty serious problems. When you jump off the table straight legged and manage to stay that way, you effectively lock the hinge. The difference between a straight and curved fork would be somewhere between zip and almost zip in comparison.

-Ray "who's curved forks LOOK like they flex more on a ride, but sure can't FEEL a difference" Sachs

[e-RICHIE]

i agree with the previous lad with the wonderful initials.

[kidamaro]

Technically it is dependant on the forks moment of inertia, that is dependant on not only the shape of the fork, but thickness' and cross sections as well. Would be very hard to get one to one comparisons. That is most likely why there are so many opinions.

[OldDog]

e-RITCHIE - when ya goning to get with the times and start bending your forks like those Onda's? You can try them out on climbs new rig.....

[csb]

what abouts the amount of material (slag!) in the forks.
wouldnt the ever so slightly more metal in the curvaceous
tined item add some deadening properties?

[David Kirk]

The way I think of it is as a pillar....like the big pillars holding up the White House porch roof. The weight of the roof is pressing straight down onto the pillars and they don't compress much. They have a straight load path.

What would happen if the pillars were curved? As snow mounted on the roof the pillars would start to bow or flex from end to end. A crappy design for a pillar but a great way to build a fork.

The same thing happens with a fork. Straight blades send all the shcok straight up to the crown. A curved blade can compress end to end and absorb a small amount of shock. The load path is indirect.

I like load paths.

Dave

[e-RICHIE]

sounds plausible, but pillars are vertical,
head angles and forks aren't. shouldn't
that be part of the rationale?

[Climb01742]

The same thing happens with a fork. Straight blades send all the shcok straight up to the crown. A curved blade can compress end to end and absorb a small amount of shock. The load path is indirect.

so it is sorta :rolleyes: like a bent leg vs straight.

[kidamaro]

The fact that the fork is not vertical is why it is a moment of inertia thing. Becasue the fork is not vertical, it is in bending. Stress of members in bending, are dependant on the moment of inertia, that is calculated differently for differnet shapes. If cross section and thickness are equal down the length of the fork, and the only thing different is a slight curve, I would suspect the moments of inertia are very close, although I haven't calculated any and don't plan on it.

[eddief]

Remember my original question was about two forks made out of exactly the same stuff with exactly the same rake; one bent at the blade ends and the other bent at the crown/steer tube interface. Those lovely little curves at the end of my Rambouillet fork sure seem to resemble the David Kirk remarks. Where are the scientists among us? There must be a formula. Maybe
Sheldon Brown KNOWS.

[David Kirk]

It doesn't matter that it isnt vertical. In fact it would be less effective it it were.

The load doesn't need to be vertical it just needs to be in line with the head angle........which not so coincidentially it is. The load the fork sees is for the most part an up and back load. When the front wheel hits a bump it is pushed up and back. The exact angle of that load is dependent of the hieght of the riders center of mass, the wheelbase of the bike, and the fore/aft location of the rider's center of mass between the axles...........BlahBlah.

This, for the most part, makes the fork act of more or less simple compression. A straight blade for deals with that compression by move the load up the load path to your poor little hands. A curved blade fork acts as a leaf spring in end to end compression and can absorb some of the shock.

Let's look an extreme for a better example. Lets say we have the aforementioned fork with 43mm of rake. But lets say that the rake is put in the fork right at the bottom with a very radical 90° bend. It's easy to see how that would flex.......a real fork does the same thing but less.

It's 50° and I'm getting on the bike now.

Later,

Dave

Kind of like a bent knee.......kinda

[Chief]

OK eddif you asked for it. The answer lies in simple mechanicc of materials.

The vertical compliance or deflection of a fork is due primarily to flexure. The bending moment induced in the fork is equal to the product of the vertical load on the front wheel and the horizontal distance from the wheel axle to the point in question on the fork. The ultimate deflection of the fork depends upon the distribution of this moment along the fork, the modulus elasiticity (E) of the fork material and the geometry of the fork cross section (i.e., its moment of inertia (I) of the fork cross section). So if the two forks are exactly the same; i.e., they have the same material and cross sectional distribution or EI distribution, then the vertical deflection will be the same since the bending moment in each is the same. That is the compliance of the two forks will be the same.

Chief be riding instead of responding to this except it's raining in Texas.

[Chief hav'em PHD in engineering mechanics and is retired engineering professor. :cool: ]

[vandeda]

ahhh ... nevermind ... Chief got it best!

dan

[e-RICHIE]

imo, there can't be any perceptible difference.
otoh, you can't ignore the psychological effect
of seeing one vs the other and forming a bias
as to what might occur if you go down the road.




perceptible :cool:
perceptible :)
perceptible :rolleyes:
perceptible ;)
perceptible :D
perceptible :confused:
perceptible :no:
perceptible :butt:

[DWF]

OK eddif you asked for it. The answer lies in simple mechanicc of materials.

The vertical compliance or deflection of a fork is due primarily to flexure. The bending moment induced in the fork is equal to the product of the vertical load on the front wheel and the horizontal distance from the wheel axle to the point in question on the fork. The ultimate deflection of the fork depends upon the distribution of this moment along the fork, the modulus elasiticity (E) of the fork material and the geometry of the fork cross section (i.e., its moment of inertia (I) of the fork cross section). So if the two forks are exactly the same; i.e., they have the same material and cross sectional distribution or EI distribution, then the vertical deflection will be the same since the bending moment in each is the same. That is the compliance of the two forks will be the same.

Chief be riding instead of responding to this except it's raining in Texas.

[Chief hav'em PHD in engineering mechanics and is retired engineering professor. :cool: ]
And there you have it. To add to Cheif's explanation, the fact is that almost all fork's deflection takes place in the steerer tube and the crown. Fork blades themselvs are not designed to flex and though you may see the end of your front dropouts "dance" with the road's irregularities, rest assured it is not from the fork blade flexing but from the steerer tube/crown. An easy way to verify this for yourself is to just think back on every bike you've seen that's had a front end collision; those forks didn't straighten out the rake in their blades, they bent at the crown and steerer tube.

[David Kirk]

I'm certainly not looking for an arguement as your math and reasoning appear sound to me. I can tell you that oh so many years ago when straight steel forks were thought cool and we at Serotta were getting asked about why we didn't offer them I wanted an answer that was better than "they look stupid".

I set up a test to see how the different type of forks reacted. The forks were secured by the steerer and a piston pushed on an axle mounted in the dropouts directly toward the crown. The force was very easy to measure as was the displacement.

I don't recall the numbers but it was obvious as one stood watching the fork compress that the curved blades actually allowed for some movement while the straight blades showed little to no movement at the set testing force.

I can imagine that if the steerer was allowed to flex ( as normal ) that it may be more flexable than the pair of blades. My simple test isolated the blades from the steerer so that didn't enter as a factor.

Does that make sense?

Dave

[Climb01742]

i love it when guys start talking all technical and engineering. ;)

ps: dave, stick to your guns. the dang thing has GOT to deflect. of course i flunked every science course i ever took. doh. :crap:

[zap]

of course i flunked every science course i ever took.

Climb, I don't think it matters.

[Sandy]

Why are you a retired engineering professor? No one smart enough to understand what you were saying?? :) :) I think I need to start at first grade and do it all over again. :)

Still trying to put a square peg in a triangular hole,

Square Sandbox Square Sandy

[e-RICHIE]

Climb-o
of course they deflect, but the op asked:

"...do they ride the same? Does the curvey bladed fork feel springier?"

is someone able to account for this?
i doubt it; it's too conceptual imo.

yours truly,
e-RICHIE

[Climb01742]

i agree, richie, the difference in road feel is almost surely imperceptible. i'm just trying to imagine/understand it "conceptually". something curved just seems like it would deflect more easily than something straight, no?

[Chief]

Dave,

My dicussion was limited to small deformation or linear behavior and vertical loading. As I understand your test the load line was directed along the axis of the straight fork so that the blade is in direct compression without any bending. The curved fork must resist the same line of action of the load by direct compression (nearly the same but not exactly equal to that in the straight fork) and bending moment, the later being equal to the product of the load and the prependicular distance between the line of action of the load and the fork. This moment causes the fork to flex and deform.

Question: In your test were the cross sections of the two forks that you tested the same as in the question posed by eddif? If not, this is another variable that was not controlled in the tests for the issue at stake here. However, your test reflects the real world--straight and curved forks of the same material do not typically have the same cross sections. Paraphrasing e-Richie "technology doesn't replace experience" or some such phrase. My supper awaits me.

[Chief]

Why are you a retired engineering professor? No one smart enough to understand what you were saying?? :) :) I think I need to start at first grade and do it all over again. :)

Still trying to put a square peg in a triangular hole,

Square Sandbox Square Sandy

Age Sandy, age and the freedom to be a kid again without someone telling you what to do.

[David Kirk]

Hi Chief,

Yes the blades were in fact exactly the same. The only difference was that one crown had angled sockets for the blades and the other didn't. As I recall they were Hank crowns but I can't be sure of that.

So the forks were the same with the exception of the method of rake.

This has been fun.........

Dave

[vandeda]

The forks were secured by the steerer and a piston pushed on an axle mounted in the dropouts directly toward the crown. The force was very easy to measure as was the displacement.

Dave

Dave/Chief,

OK, I could be out to town here ... but this test doesn't sound completely accurate. Force directly towards the crown? Is that quite right. Here's why I think that. You're not moving, just sitting static ... lets say the rear tire is mounted in a trainer. Right now, there is a bending moment on the fork. You will have a perfectly vertical force from the weight of your body ... which will then be multiplied to the horizontal distance between the crown & the dropouts ... which will want to bend the fork. This vertical force from your body weight will change as you hit bumps and such, changing this bending moment. This force from your body wasn't taken into account in your test.

Yes/No ... am I right or a total moron?

Dan

[Climb01742]

chief, now...can you help me program my VCR? :rolleyes: