RoadBikeReview.com's Forum Archives - Components


Archive Home >> Components(1 2 3 4 5 6 7 8 9 10 )


straight bladed forks... why?(24 posts)

straight bladed forks... why????
Mar 15, 2001 2:05 PM
what's the deal with straight bladed forks on road bikes? how does it affect the handling of the bike and the comfort level?
just something different...dave
Mar 15, 2001 2:17 PM
Colnago was one of the first major builders to begin using straight bladed forks. It's primarily done for a different look.

The handling isn't affected by straight blades. The amount of rake will affect handling, but the same rakes 40, 43 and 45mm are available in either style.

Contrary to popular belief, a curved blade fork doesn't flex at the curve in the blade to improve ride. If any siginificant flexing occurs, it will be at the crown area. Straight blades, by themselves won't deteriorate comfort level.
Dave, I'll buy that. Although I sort of assumed that thebill
Mar 15, 2001 2:21 PM
slight angle in my former Look fork accounted for some of the difference, I can easily believe that the vast difference in crown between the Look and the Wound-Up accounted for the difference far more than the loss of the slight angle. I never measured the relative rake, which may account fully for the rest.
I can speak only from my own experience with Look to Wound-upbill
Mar 15, 2001 2:17 PM
and I realize the limited applicability of this observation, particularly since the Wound-up isn't exactly a straight-blade and the Look wasn't exactly not a straight-blade. BUT, when I went from the slightly less than straight Look to the straight pipes of the Wound-up, the alertness and responsiveness of the ride improved DRAMATICALLY with, to my perception, no loss of comfort. A great improvement.
Bottom line, I think that, as a general matter, you pretty much can figure out self-evidently that a straight fork is going to give a more positive response and feel. That said, there are going to be significant differences among forks that transcend the straight/not-straight distinction, based on other engineering choices.
Absolutely wrong!!!Kerry Irons
Mar 15, 2001 3:51 PM
I have no doubt that you experienced a dramatic improvement going from the Look to the Woundup - from a middle of the pack fork to one of the top 3 forks made. However, it had ABSOLUTELY NOTHING to do with whether the fork was straight blade or curved! Forks have a certain degree of flex in each plane. That, and the fork offset, are what determine the ride. You can get any flex you want with either straight blade or curved forks. Steel forks flex most near the crown, and so do CF forks, although with CF you can more easily tailor the flex along the length of the fork. In either case, the forks are mostly flexing where any fork would be straight. Straight blade forks vs. curved is strictly a style/appearance decision. I can't imagine how "you pretty much can figure out self-evidently that a straight fork is going to give a more positive response and feel". This is in no way evident from any known materials or physics principles.
Absolutely wrong???DCP
Mar 15, 2001 7:02 PM
It may be wrong in the sense that with an unlimited budget you could make a fork with specified flex in any plane, but its does seem pretty obvious that a straight fork is going to be less vertically compliant than a similar curved fork. Thus your opening observation is, IMHO, a little severe, even if you are right in the context of high end composite forks. Now whether less vertical compliance is a good or bad thing; that depends on the rider and the purpose of the ride.
Kerry is rightScott Parker
Mar 16, 2001 5:38 AM
There are some basic engineering priciples going on here, and one of them is not vertical compliance. I question your statement that "its does seem pretty obvious that a straight fork is going to be less vertically compliant than a similar curved fork". I trust you are not an engineer; otherwise you would know that it is not obvious and that forks don't move up and down, they move back and forth.

Kerry observation was not severe. It was based in fact and known measurements. What was your observation based on?
Splitting Hairsgrz mnky
Mar 16, 2001 7:39 AM
Well, as a mechanical engineer, we could probably have a little discussion about the ability of a curved vs. straight fork to move in the vertical plane. Even if you want to ignore the idea that the blades of a curved section (out of column), can them selves flex upward more easily than a straight section (in column), you still have to admitt that both blades move in an arc and therfore have an upward component. Truth of the matter is we're splitting hairs on a small scale and other factors are pretty significant - like how much material is used in the lay-up schedule, and whether the guy running the autoclave had a fight with his wife that morning.
Convince me that Kerry is rightDCP
Mar 16, 2001 8:07 AM
Not an engineer - so enlighten me. The bend in a traditional fork appears to my admittedly untrained eye to act as a spring under a vertical load, which means it provides vertical movement. Not saying you are wrong because I don't really know, but I just find it flat hard to belief that such a fork won't move vertically when a vertical load is placed on it, e.g hitting a hole in the road. If you are trying to say that a traditional fork with a bend does not move vertically, how can it avoid moving vertically? Perhaps more fundamentally, why is the bend there?
Convince me that Kerry is rightScott Parker
Mar 16, 2001 11:08 AM
Why is the bend there? To provide the offset which you need to get the correct trail for the bike to handle properly. Straight bladed forks have offset as well, but it is provided by the fork blades being at an angle relative to the steerer tube.

Many analysis of frames and forks have been done. Forks bend most nearest the crown because it is furthest away from the point of force (the hub). (A further explanation will take a long time...). When you hit a bump, the fork actually goes FORWARD, unless you hit something very tall. This forward motion will result in a vertical motion, on the order of 1/4 of the forward motion; however, because the fork bends most near the crown where the blades are straight for both a straight fork and a curved, the forks will feel indistinguishly different.
I'm convincedbill
Mar 16, 2001 12:10 PM
The fork is a lever, with the fulcrum at the crown, sort of, right? The energy from the bump that causes the fork to want to rotate in an arc at the hub, the point of force, becomes concentrated in the fork at the crown. Relatively speaking, because the hub can move (a little) and the crown can't rotate around the fulcrum (the inertia of the bike and your body, etc, won't allow it to move much at all, really, for a sudden bump), the energy is imparted to flexing the fork at the crown.
Actually, this comports with my paper clip/desk flex model, referenced below, now that I look at it again. And, if you hold a pencil in a vise and push the end of the pencil around in an arc, the pencil will break at the vise, not at the end in your hand. That at least is intuitive.
Still, Kerry, no need to shout.
Not So Fast.....grz mnky
Mar 16, 2001 1:04 PM
First, I'm not saying I agree with Kerry on this one, although I do on most everything else.

To turn things around on you you can't blindly say that a fork bends most at the crown because that's the point furthest away from the point of force. To do so would neglect any effect of the section through wich the force is being applied and the entire field of Statics of Materials. The moment is higest at that point relative from the view point of the hub. Now turn things around and pretend that your frame of reference is the _bike_frame_. From this view and your statement is would appear that the _hub_ moves the most and therefore the force must be highest there. What"s happening here? Sir Issac Newton had a little bit to say about equal and opposite forces a while back. Youu can't have a force at one end of the fork without one on the other end. A free body diagram would clear this up.

So maybe the fork acts like a beam and there is deflection along the _entire_ length. Think of standing at the end of a diving board or holding a fishing rod. How much deflection depends upon position and the cross section of the member (did I say member?). The fork isn't modeled as an infinitely rigid member with a hinge point at the crown with a spring and damper. To do so would be a gross over simplification. It's the skill of the designer/builder to achieve the proper flex characteristics. One of the reasons why carbon is becoming popular is that it's easier to "dial-in" the bend. It's also has a great strength to weight ratio and it's frequency response can be quicker and still have superior dampening.

Witha small bump the fork will move forward and vertically (up). Make the bump large enough, in the extreme, and the fork will move aft and vertically (down).
Not So Fast.....Scott Parker
Mar 16, 2001 1:41 PM
"To turn things around on you you can't blindly say that a fork bends most at the crown because that's the point furthest away from the point of force. "

I'm not saying it blindly, I'm saying it based on the analysis I have heard of.

"To do so would neglect any effect of the section through wich the force is being applied and the entire field of Statics of Materials. "

It isn't being neglected. Remember your beam equations. A load at the end (hub) will produce the highest forces at the other end (crown). Sure the cross section changes, but not enough to compensate for the forces involved.

"So maybe the fork acts like a beam and there is deflection along the _entire_ length. "

Yes, but most of it at the crown end due to the forces involved.
Mr. Completelygrz mnky
Mar 19, 2001 12:59 PM
So based upon hearsay you reject my arguement and the basis of fact?

It isn't "mostly at the crown due to the forces invloved." What about looking at things from the frame of reference of the bicycle frame: wouldn't your postion be reveresed? Obviously the diving board and fishing rod analogy didn't work for you. If all the bending takes place at the fork crown area how come it doesn't fatigue and fail here?

You have to be a bit more careful and rigorous when you're talking about forces and mechanical components. Bending moment is highest in the center of the section while shear forces are higher in the ends. If you had the skills to do the analysis yourself you would see the folly of your arguement.
Kerry, three !!!'s?bill
Mar 16, 2001 10:38 AM
You may be correct that the bend in a fork, given the other design qualities, doesn't act as a spring at the bend, and I believe you. But it's not stoopid. Is it?
Well, maybe it is. I just took out a paper clip from my drawer, bent it into the shape of fork (more or less), bounced it on my desk, and damn if the whole thing didn't flex pretty much evenly, across the whole structure.
Well I'll be. Chalk one up for counterintuitive.
Yupgrz mnky
Mar 16, 2001 1:09 PM
Your intuition is pretty good. ;-)
!!!'s?Kerry Irons
Mar 16, 2001 4:30 PM
Sorry, but there's enough things we don't understand well. I have a hard time when I see statements that are no way based on fact or fundamentals. They hit me like "the weight of a hub is supported by the bottom spokes in a wheel" and if they aren't challenged, they get repeated. Any time I see one of these kinds of comments, you'll see !!! from me. Nothing personal.
questionDog
Mar 19, 2001 3:08 PM
Wouldn't the fork tend to flex most where it is thinnest (smallest cross section)? Doesn't the diameter of a tube affect its stiffness?

Doug
Ease of design - build?DCP
Mar 15, 2001 3:09 PM
I'm just guessing, but as I understand it carbon fiber design and construction is as much art as it is science. It must be easier to predict response to loads with a simple straight piece than one than curves. It must also be easier to manufacture.

Of course, ease of manufacture would also have been true with metal forks, and so if the bend didn't do much, why is it still common?
Ease of design - build?grz mnky
Mar 15, 2001 3:46 PM
Yup, the folks in Sandy, Utah that make the tubes for the Wound-Up unit also make tubes for tons of other applications, one of them being the PowerX windsurfing masts (I'm spacing the name of the parent co. and am too lazy to look it up). Straight, symetrical, even tapered things, are a lot easier to make than curves.

The bend (and taper) does do something and is along a continuum as a function of it's location and cross section. Anyone who says it doesn't probably never passed Statics of Materials. You'll get a bit more verical compliance with a bend over a straight tube, all other things being held equal. You also have a greater lenght of material which also affects the response - what's easier to bend a long fishing rod or a short one? You can crunch the numbers all day long, but it comes down to a but of an art to truly get things right and be asthetically pleasing. To me the Wound-Up unit looks clunky and linear, but it certainly works and has it's fans. Still, my opinion is only worth what you paid for it.

Ultimately engineering is all about trade offs and the NFL principle (No Free Lunch). It's a question of ballancing priorities and hitting the objective. It helps to be creative and come up with a new mouse trap. Then it's up to the marketing types to hype the crap out of something and sell it. It helps to pay attention to trends, but you can also buck them and knock the pundits on their collective ear.
Ease of design - build?eddy
Mar 17, 2001 11:11 PM
Just an artist butting in on all of you engineers- If you want to know why the curved fork blade is still being used, instead of looking at the physics of it all, take a look at just how sexy a curved fork is! Cyclists enjoy their sleek machines, and could all be damned to hell for thier relative vanity. History and tradition also has an effect on some buyers. The curved fork blade has been in development for over a century, and was and is still implemented for a variety of reasons. Shock absorbtion included, even if it really doesn't work that way. Mine's chromed and curved.
I can't tell the differenceDog
Mar 15, 2001 7:28 PM
Maybe I'm not sensitive, but I can't tell the difference between my Star Carbon straight fork on the Colnago and the curved carbon forks on my EV2's (3 of them in the last year). All are around the same weight, even thought the Colnago fork appears to be much beefier. I think tjeanloz said the Bianchi forks I had, based upon my description of them, were made by Advanced Composites (AC). Honestly, I just can't tell any difference.

Doug
Handling characteristics (not technical).nigel
Mar 19, 2001 11:06 AM
I've got a wide, bladed, carbon fork on my Giant TCR. Seems very comfortable to me, soaking up bumps on the often-crappy New York City streets I ride on.

One thing I've noticed, however: On decently windy days, I've noticed the wind drag from crosswinds "pushing" me over on the road a bit. I'm getting used to it, and am compensating, but I was initially perplexed, having ridden only "round" steel forks in the past. If you live in a part of the country where it's often very windy/breezy, you may want to consider a rounder/less aero fork for handling purposes.

Giant's fork is aerodynamic like few others, and is wider and beefier, so this may explain the crosswind situation. Still, the ONCE boys have raced Giants (and these forks) for the past few seasons with good success.

Just a little input. Don't think this would be as severe with most bladed forks (Look, Colnago, Time, etc.) though, since they're not as wide and flat as the Giant's is.
what about stiffness under braking?Dog
Mar 19, 2001 3:02 PM
We normally think of stiffness of forks with respect to vertical compliance and laterally. But after reading something Sheldon Brown wrote about the difference between track forks (round, not set up for brakes) and road forks, it made me think.

Does the curve assist in fore/aft stiffness under braking loads?

Doug