Straight or curved blade forks???
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  1. #1

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    Straight or curved blade forks???

    I am buuilding a race bike and would like to know the difference between the 2. I am buildiung a CAAD 8 R1000 frame. I am looking at the easton sl or slx forks as well as the look hc3 and cannondale fork. I know all of cannondales forks are all curved blades. Is there a reason for one or the other??? Still learning?!?!?
    THANKS ! HOOV

  2. #2
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    Simple answer

    Quote Originally Posted by hoovypedals
    I am buuilding a race bike and would like to know the difference between the 2.
    The difference is that one is curved and one is straight. A bit facetious, but there is no intrinsic reason to choose one type over the other unless you prefer the look. There are many design "knobs" which can be turned to change fork performance, but straight vs. curved is not really a significant one.

  3. #3
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    Mr. Kerry Irons is probably right.

    But one thing you should know is that regardless of whether or not you get a straight or curved fork (both have "rake" by the way) that you get the proper rake for your size bike and head angle. Use the same rake as what came with the bike as it was designed around that.

    Getting back to whether one is "better" than the other.....

    I think that there is no difference. However, I think a case can be made that a curved fork will be slightly more comfortable (in metal at least) than a carbon fork because any vibration or shock damping will probably take place over the length of the curve rather than at one place.

    At least that is my mind imagining it.

    I have also heard this, or something like it, from a pretty well known builder of steel bikes.

    Anyway, my steel bikes have straight forks and curved forks. I can tell no difference at all.

    I am not trying to start curved vs straight war here, but in my minds eye I can see a curved fork being theoretically better, at least in terms of vibration damping.

  4. #4

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    hmm... my two cents' worth...

    being from an engineering background, my hunch on the subject is that curved forks offer better damping qualities and straight forks should give better handling/cornering rigidity... I might be wrong tho...

  5. #5
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    Strange conclusion

    Quote Originally Posted by domo
    being from an engineering background, my hunch on the subject is that curved forks offer better damping qualities and straight forks should give better handling/cornering rigidity.
    Curious as to how you reached this conclusion. Whether a beam is curved or straight does not (inherently) change its load carrying capacity, damping characteristics, etc. What affects these things is the construction of the beam - its dimensions, geometry, and the material of construction (the properties thereof).

  6. #6
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    I think you need to take a good look at the Reynolds Ouzo Pro.

  7. #7

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    I disagree

    Quote Originally Posted by Kerry Irons
    Whether a beam is curved or straight does not (inherently) change its load carrying capacity, damping characteristics, etc.
    I don't agree with that particular statement. I remember Serotta doing extensive lab testing (and rider testing as well) and found different vibration damping and bump absorbtion characteristics with steel forks (that were noticeable to most of the testers). Now with carbon fiber....I don't know. I would say that if there is no noticeable difference, it is because carbon fiber is just too stiff of a material to tell. So you may be correct in this particular case (never ridden both to tell myself) but I think that overall generalization is incorrect.

    -R

  8. #8
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    Why?

    Quote Originally Posted by Anti-gravity
    I don't agree with that particular statement. I remember Serotta doing extensive lab testing (and rider testing as well) and found different vibration damping and bump absorbtion characteristics with steel forks (that were noticeable to most of the testers). Now with carbon fiber....I don't know. I would say that if there is no noticeable difference, it is because carbon fiber is just too stiff of a material to tell. So you may be correct in this particular case (never ridden both to tell myself) but I think that overall generalization is incorrect.
    Are you saying that forks made from identical metal alloys or CF fabrication, wall thicknesses, and tube diameters will behave differently if one is curved and the other straight? That is the only valid comparison. I highly doubt that the Serotta testing to which you refer was on that level ground. On what basis do you "think that overall generalization is incorrect"?

  9. #9
    yellow is mellow
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    weird science??

    I harbor a theory that I never heard anywhere else though maybe there's a reason for that and judging from the tone of the conversation I might find out why real quick.
    My theory is curved is more comfortable because it's locked to the hub at a less vertical angle than straight forks. So the less vertical the angle of the fork the more vertically compliant it is. does that make any sense??

  10. #10
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    Quote Originally Posted by Kerry Irons
    Are you saying that forks made from identical metal alloys or CF fabrication, wall thicknesses, and tube diameters will behave differently if one is curved and the other straight? That is the only valid comparison. I highly doubt that the Serotta testing to which you refer was on that level ground.

    In this thread, Dave Kirk, formerly from Serotta, states that a straight blade fork is stiffer than curved and when at Serotta he performed testing which confirms this.

    http://forums.roadbikereview.com/showthread.php?t=30463

    After reading what Dave wrote, along with some personel experience with different steel forks, using the same Dedacciai blades, I believe that a straight blade fork IS slightly stiffer all things being equal. As a builder it's an easy matter to adjust for this though so it still comes down to the builders skill in determining the ride quality on any given fork.

    Ed

  11. #11

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    Quote Originally Posted by Kerry Irons
    Are you saying that forks made from identical metal alloys or CF fabrication, wall thicknesses, and tube diameters will behave differently if one is curved and the other straight?
    Yes.

    Quote Originally Posted by Kerry Irons
    That is the only valid comparison. I highly doubt that the Serotta testing to which you refer was on that level ground. On what basis do you "think that overall generalization is incorrect"?
    Well, Serotta did the tests and found the differences and hence incorporated that into the design of their steel forks, so it must have some significance.

    Quote Originally Posted by Kerry Irons
    Curious as to how you reached this conclusion. Whether a beam is curved or straight does not (inherently) change its load carrying capacity, damping characteristics, etc.
    You're original statement certainly doesn't make any intuitive sense to me. Granted, not all engineering (or physics for that matter) is entirely intuitive. But a curved beam will certainly handle load differently than a straight beam, it's a simple matter of allowing extra degrees of freedom through torque and rotation rather than simple compression (or less of the former). It may not affect damping characteristics, but it will certainly affect load handling (or whatever the proper term is). You ask for proof of this conclusion, which I think Nessism's post provides somewhat, yet I see no proof of your statement. Sounds like pure assumption and psuedo-science to me.

    -R

  12. #12
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    Wow, what a leap

    Quote Originally Posted by Anti-gravity
    Well, Serotta did the tests and found the differences and hence incorporated that into the design of their steel forks, so it must have some significance.
    Here's a quote from Kirk's post: "All the lab stuff is cool but it really matters how it feels on the road. We did blind tests on the road with the forks covered so you couldn't see them (not easy to do by the way) and compared the curved vs, straight fork. Some folks could tell the difference , others couldn't. We also asked the subjects which fork they would rather ride - A or B. As I recall no one liked A ( straight) better."

    You make quite the leap from Serotta's actual result to your conclusion!

    Quote Originally Posted by Anti-gravity
    You're original statement certainly doesn't make any intuitive sense to me. You ask for proof of this conclusion, which I think Nessism's post provides somewhat, yet I see no proof of your statement. Sounds like pure assumption and psuedo-science to me.
    It seems to me that David Kirk's post says it all. I go back to my original statement: "There are many design "knobs" which can be turned to change fork performance, but straight vs. curved is not really a significant one." How you get from this statement to "pure assumption and pseudo-science" is a mystery to me.

    We could have a long discussion about the finite element analysis and bending loads, but it's pretty pointless in light of David Kirk's post.

  13. #13

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    Arguing the wrong point

    Quote Originally Posted by Kerry Irons
    Here's a quote from Kirk's post: "All the lab stuff is cool but it really matters how it feels on the road. We did blind tests on the road with the forks covered so you couldn't see them (not easy to do by the way) and compared the curved vs, straight fork. Some folks could tell the difference , others couldn't. We also asked the subjects which fork they would rather ride - A or B. As I recall no one liked A ( straight) better."

    You make quite the leap from Serotta's actual result to your conclusion!



    It seems to me that David Kirk's post says it all. I go back to my original statement: "There are many design "knobs" which can be turned to change fork performance, but straight vs. curved is not really a significant one." How you get from this statement to "pure assumption and pseudo-science" is a mystery to me.

    We could have a long discussion about the finite element analysis and bending loads, but it's pretty pointless in light of David Kirk's post.
    I never said that curved vs. straight was the best way to adjust the ride characteristics of a fork. I argued your statement:

    "Whether a beam is curved or straight does not (inherently) change its load carrying capacity, damping characteristics, etc. What affects these things is the construction of the beam - its dimensions, geometry, and the material of construction (the properties thereof)."

    Which I think is incorrect, Kirk says:

    "There's a good reason the pillars that hold up your porch aren't curved.

    I fully realize that if you think that it doesn't make a difference then nothing I can say will change your mind. I fully respect that. But I can tell you that I've had the opportunity to lab and road test countless forks of all configurations. With a steel fork there is both a measurable and feelable difference. The key is having otherwise identical forks with the exception of how the rake is attained. All other things of course need to be identical....wheels, tires, frame etc......"

    You originally doubted their consistency and, well there you go.

    You blanket statement that the shape of the fork doesn't change it's properties is what I was arguing. There are plenty of other design "knobs" to turn other than that, but the shape can be significant (the results you quoted say something about this). Is it the best way to change the properties of a fork? I have no idea, but that's not what I was trying to prove. Not everyone could tell the difference between the straight and curved forks in Serotta's test, but who's to say that varying the wall thickness in two straight-blade forks will produce better results? You cannot throw out this factor in fork design, it does make a difference to some degree.

    If you look back at my previous posts, I never made a black and white arguement as whether this was the key factor in fork "feel" or whether straight or curved was better. I was simply arguing that point. There was some bad science in some of your posts and a lot of assumptions.

    -R

  14. #14
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    Ok

    Quote Originally Posted by Anti-gravity
    I never said that curved vs. straight was the best way to adjust the ride characteristics of a fork. I argued your statement: "Whether a beam is curved or straight does not (inherently) change its load carrying capacity, damping characteristics, etc. What affects these things is the construction of the beam - its dimensions, geometry, and the material of construction (the properties thereof)."
    I agree that I overstated that. My point was that in the context of bike forks, any differences are not significant. Kirk proved this with his blind tests: "Some folks could tell the difference , others couldn't." It's not what I think that matters, it's what objective tests like this one tell us that matter.

    The one key difference in straight vs. curved bladed forks is that you can define an impact vector that goes straight up a straight blade fork, whereas there is no such vector for a curved blade - the impact forces will always impart some bending moment on the curved fork.

    However, when you say that there is a "feelable difference" you are disagreeing with Kirks objective data. IME, when you have a "some can feel it, some can't" situation, then you are most often dealing with noise or at least insignificant differences.

    Quote Originally Posted by Anti-gravity
    If you look back at my previous posts, I never made a black and white arguement as whether this was the key factor in fork "feel" or whether straight or curved was better. I was simply arguing that point. There was some bad science in some of your posts and a lot of assumptions.
    You say bad science, and I agree that I was wrong to say "no difference." A lot of assumptions? How so? The point is "no significant difference" which is what the OP asked about.

  15. #15
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    I think that ...

    Everyone is twisting what Kerry is trying to say.

    To analyse how the fork behaves, we need to first examine the system as a rigid body - like the good engineers we are. Based on a rigid body analysis of the fork, it makes no difference what the shape of the fork is - only the length and angle of the vector that joins the crown and the axle is important. So now we have a FBD, with a fixed boundary condition at the crown end, and a vertical Force vector at the axle end. From these two vectors, we can calculate a bending moment, and plot a moment/shear diagram along the length of the fork vector. Based on this shear and moment diagram, we design the fork cross-section along this vector, to carry the loads as need be. We can design a fork (within manufacturing limitations) that has almost any shape along its x-section, to have strength, stiffness, and damping characteristics that we want.

    If we look at steel forks, they are curved for a reason. Because of the properties of steel, and the manufacturing methods used to make the fork. I guarantee that if you make 2 forks with the same tubing cross section, the only difference being one is straight, and the other is curved..... there will be a difference. I wont be bold enough to say which direction this will go. Kerry's point is just that. You can't seperate the two. The curved fork is designed to be a curved fork. The straight fork is designed to be a straight fork. In all likliehood, the difference between the two is mostly cosmetic.
    "I become a happier man each time I suffer" - Lance Armstrong

  16. #16

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    Quote Originally Posted by Kerry Irons
    I agree that I overstated that. My point was that in the context of bike forks, any differences are not significant. Kirk proved this with his blind tests: "Some folks could tell the difference , others couldn't." It's not what I think that matters, it's what objective tests like this one tell us that matter.

    The one key difference in straight vs. curved bladed forks is that you can define an impact vector that goes straight up a straight blade fork, whereas there is no such vector for a curved blade - the impact forces will always impart some bending moment on the curved fork.

    However, when you say that there is a "feelable difference" you are disagreeing with Kirks objective data. IME, when you have a "some can feel it, some can't" situation, then you are most often dealing with noise or at least insignificant differences.



    You say bad science, and I agree that I was wrong to say "no difference." A lot of assumptions? How so? The point is "no significant difference" which is what the OP asked about.
    Well I guess the results are up for interpretation. I don't see the results he briefly described as entirely plagued with noise (I guess I'd need to see the actual data to say for sure). As with any qualitative experiment, you're bound to get some non-biased data. The fact that he said all the testers chose one particular fork (the curved one) says to me that the experiment proved that some feelable difference can be created merely with the shape of the fork. All the subjects are humans with different ranges of sensitivity and different ways of interpreting what they are feeling, so it's unlikely they will produce an overall bias as a whole.

    I'll admit I exaggerated with the comments about bad science and assumptions, it just annoys me when generalizations are made when their is more to the picture. I did take this argument out of context, and suffice to say didn't provide the OP with any helpful information. I was really arguing the significance of the design method in certain applications, and that it was not a useless concept, but in the case of carbon forks I don't really know (never said I did). I suppose there could be no difference in the feel of carbon forks whether curved or not, but that lack of difference in feel could be attributed to other properties of the design. So, yeah, not a big help in this thread, good arguement though.

    -R

  17. #17
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    [QUOTE=Trek_envy]Everyone is twisting what Kerry is trying to say.

    Yup. Its not just the curve or lack of that defines a fork's ride. Curved forks by Surly ride like bricks, stiff and precise and weigh a ton. Good forks but not the smoothest. I have a couple of lighter straight forks in steel (one Kelly, one Steelman) that are not as stiff as the curved Surly and ride much smoother. Aside from my direct experience with these I have no mathematic evidence. I think straight blades are pretty with straight frame tubes too.

    Its hard to find a curved bladed high end steel fork these days anyway because adding curves adds cost and with the right legs it is not needed. Not that curvy legs aren't nice.

  18. #18

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    Well said. .

    Quote Originally Posted by Trek_envy
    So now we have a FBD, with a fixed boundary condition at the crown end, and a vertical Force vector at the axle end. From these two vectors, we can calculate a bending moment, and plot a moment/shear diagram along the length of the fork vector. Based on this shear and moment diagram, we design the fork cross-section along this vector, to carry the loads as need be. In all likliehood, the difference between the two is mostly cosmetic.
    Engineering Statics/Strength of Materials class 201. On the cosmetic end, every fork (road bike) I've ever had was straight. I love the look of a straight blade fork. . So does Ernesto Colnago too.
    ;)

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