?Straight bladed forks verses Curved bladed forks?

Discussion in 'Cycling Equipment' started by Bigbananabike, Jan 25, 2007.

  1. Bigbananabike

    Bigbananabike Member

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    Hi.
    I have the opportunity to buy some straight bladed Look full carbon forks to replace my existing curved bladed, alloy steerer forks.
    It would lighten my bike up - a bit.
    Are the straight bladed forks going to make the ride harsher?
    Is the effective rake(if the rake noted is the same or similar) the same?

    Cheers, Paul :)
     
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  2. John M

    John M New Member

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    There is no real functional difference between straight or curved forks in ride. Whereas a curved fork could theoretically be a slight bit more compliant than a straight fork of otherwise identical construction, any difference would be negligible compared to the compliance of the tire.

    The measurement of rake is independent of the shape of the blade.
     
  3. bobbyOCR

    bobbyOCR New Member

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    that's about right. Straight forks can also be more aesthaetically pleasing on certain bikes. curved fork=narrower tubed traditional frame straight fork=compact race frame.
     
  4. CAMPYBOB

    CAMPYBOB Well-Known Member

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    Fork stiffness is not soley dependant upon the only the profile shape of the blades, but the cross-section of the blades, crown, internal or external stiffeners, wall thickness, material, etc.

    Tire compliance be damned, there's a world of difference in riding (especially sprinting/out-of the saddle climbing/descending) a bike with a stout fork as opposed to a spaghetti fork.
     
  5. j.r.hawkins

    j.r.hawkins New Member

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    What straight vs curved-blade forks will affect is the amount of axle trail.

    Curved forks put the wheel further ahead, reducing the axle trail and (I'm assuming) increasing the responsiveness. Straight forks will increase the axle trail unless the dropouts are on the leading edge of the fork, making the front wheel more likely to flop from side to side at slow speeds (eg climing steep hills out fo the saddle) but making high speed handling smoother and more predictable.
     
  6. bobbyOCR

    bobbyOCR New Member

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    I thought trail was completely a component of the rake on the fork. So, logically, a straight and a curved 43mm rake fork would have identical trail. the straight may be stiffer and less compliant, but still have the same trail.
     
  7. j.r.hawkins

    j.r.hawkins New Member

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    Specified that way, yes, you're at least partly right. Axle trail will be a function of forward rake measured at the axle, and caster (head tube) angle:

    The more rake for a given caster angle, the less axle trail.

    The more caster for a given rake, the more axle trail.

    I'd be interested to know from somebody who's well informed what these variables do to bike handling.

    I'm reasonably sure that less trail means sharper bike responsiveness (definitely does for cars), but what the interplay is between high and low speed effects, and understeer / oversteer on bikes I'm relatively clueless. I've only got comments I've read from race cycling magazines as a guide - hardly authoritative.

    Looking forward to hearing knowledgeable views... :cool:
     
  8. alienator

    alienator Well-Known Member

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    Rake on a bicycle doesn't matter. All that you need to know is the offset. With offset, headtube angle, axle to crown distance of the fork, and tire size you can figure trail. Specifically trail is this: it is the distance from where the steerer's axis of rotation intersects the ground to the center of the contact patch. Whether you've got a straight bladed fork or a curved bladed fork. it doesn't matter, so long as the offset as well as the crown to axle distance are the same between forks.

    Two forks, one straight bladed and one curved, with the same axle to crown length and with the same offset, will have the same trail.
     
  9. sugaken

    sugaken New Member

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    Well, apparently, some people call what you call the "offset" the "rake":
    http://www.velonews.com/tech/report/articles/7322.0.html

    I prefer the term "offset" like you, though.
     
  10. bobbyOCR

    bobbyOCR New Member

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    Thanks. I will adopt the term offset instead of rake then, it seems to be a more accurate term than rake.

    I am also interested as to how the fork's offset/rake affects handling. I assumed that a longer trail/smaller offset resulted in shaper handling.
     
  11. 9.8mps2

    9.8mps2 New Member

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    It is little wonder there is confusion on this. Even the "authority" in the referenced velonews is only partially correct. But to use his diagram :
    ( diagram depicts a motorcycle or MTB setup )
    Rake is an angle - not a dimension - formed between steering head and vertical
    It DOES matter hugely affecting steering quickness and stability. Bigger rake angles give more stability and slower corner response - look at a cruiser m/c alongside a sportbike.
    Setback is what the author refers to as offset
    Offset would be a variance between steering head and fork angle ( using triple clamps of different setback) bicycles are not set up this way so fugittaboutitt.
    Trail is exactly as defined. It yields stability - the turned backwards fork demo does not consider gyroscopic stability of the wheel . A curved fork blade will provide a responsive turn with a steeper rake, yet preserve stability with the added trail .
    Understeer and oversteer ( pushing and sliding ) do not apply to bicycles - that falls to the realm of motorsports.
    Campybob is right. Stiffness is better than "Land'oLakes" forks. Different stiffness degrees are optimal in different axis -longitudinal, lateral, torsional - throughout the chassis.
    I think the guys at Cervelo have a very good handle on this.
    Going to the straight forks may improve or disimprove your bikes handling in varying the chassis dimensions. The factory guys usually are better at setup and arriving at the "sweet" numbers than we give them credit for.
     
  12. mikesbytes

    mikesbytes New Member

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    9.8 has corrected the terminology confusion.

    This would fall into the "if it ain't broke, don't fix it". If there isn't anything wrong with the handling of the bike as it is, then why bother fiddling with it? You could end up with a bike that no longer handles properly, particually at speed.
     
  13. j.r.hawkins

    j.r.hawkins New Member

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    Found an interesting link from a bikebuilder here:

    http://www.calfeedesign.com/frontendterms.htm

    Summary:
    Rake and offset are the same thing. Rake is not the head tube (steerer tube) angle.

    He says that "The steeper head angle bikes are a little more agile, or require less effort to steer", but I'm unclear if he means that the head tube angle number is higher (which would be closer to vertical -steeper - and therefore less caster and axle trail) or the steerer is angled back more.
     
  14. 9.8mps2

    9.8mps2 New Member

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    I wish I had ,my man .
    More research reveals more contradictions. Reliable sources give both perceptions of rake as a dimension and an angle - and again, will sometimes contradict themselves in the same breath. They do the same regarding rake vs offset. http://en.wikipedia.org/wiki/Rake_and_trail#Steering_axis_angle - will mention the variance in rake as a term in the bicycling and motorcycling theaters. rats. Their illustrations of the relationships of parameters are helpful in gaining insight.
    I reckon a lot of this confusion results from the lack of an engineering standard of terminology that SHOULD be able to encompass all aspects of the two wheeled world. eg: SAE, NACA, ASTM etc...
    Of course if one governing body states "This is what it is", one discipline will state " We have always called it 'this', and will continue to do so".
    This don't help the poor mechanic who ,when working on a global vechicle will encounter a mix of SAE and metric ( and , God forbid even Whitworth) fasteners. ( SAE is called "American" , or "regular" in much of the US.) Why we remain so ego/ethnocentric and will not join the rest of the planet is beyond me.
    So to get (way) back to Paul BBB 's delimma - The real world stance of Mikesbytes vs. the fact that Paul wants the look ( pun intended) of these forks. Then the fork salesman will have his own slant on terminology complete with eye rolling and head shaking . Paul brings these trick goodies home , and his eval will predictably lack objectability as he's got his ego and need to justify his expense in the way. ( This is all hypothetical, Paul) A sort of double blind test with a bunch of forks , riders perceptions, and the time clock would yield some viable data. But then Paul would be a factory rider , complete with hotties competing for the privelege of applying his chamois cream for him, and he would be signing for a bazillion dollars to endorse a fork that he would attest as his secret for success - but the production version would bear only a passing resemblance to his personal setup.
    Hey, the aftermarket folks gotta eat too.
    Clear as mud ??
    I think I picked a bad week to give up sniffing glue...

    Mike S.
     
  15. alienator

    alienator Well-Known Member

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    Technically, Calfee is wrong. Rake is the angle of the steerer tube. Bicycling co-opted the motorcycling term and used it wrong. Offset is the correct term to use because it describes exactly what it is: how much the axle is offset from the steerer's axis of rotation. That people use the term "rake" to describe offset is just an incorrect use of the term.

    As far as trail goes, more trail means that the re-centering force on the front tire is greater, which translates into greater stability.
     
  16. mikesbytes

    mikesbytes New Member

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    My background knowledge on steering geometry comes from motorcycling.

    Unless the new forks are slightly longer or shorter, the significant change when changing forks is going to be the trail. Whenever I've seen concern about speed wobbles, its usually been on bicycles that have very little trail.
     
  17. alienator

    alienator Well-Known Member

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    Well, there's no change in trail so long as the offset is the same, as well as the axle-crown distance.

    As for "speed wobble," that's a system issue, i.e. the total bike system including the rider, that is a result of system harmonics. Speed wobble isn't just something that can happen with short trail. It can happen with long trail, too.
     
  18. j.r.hawkins

    j.r.hawkins New Member

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    I'd be prepared to consider that if Zinn and other cycling sources weren't in agreement with him and disagreement with your usage of the term. You seem to be using the term the way motorcyclists do.

    A quick search using the two words rake cycling on Google turned up the following:

    From: http://www.sheldonbrown.com/gloss_ra-e.html
    "Rake
    The "rake" or "offset" of a fork is the distance between the wheel axle and the extension of the steering axis. This may be accomplished by bending the fork blades, or by attaching the fork ends to the front of the blades, or by tilting the blades where they attach to the crown.
    Rake is one of the three factors that affect the trail of the bicycle, which has a considerable influence on the handling qualities. "

    From: http://www.sheldonbrown.com/gloss_tp-z.html#trail
    "Trail
    Trail is the distance from the contact point of the front wheel with the riding surface to the intersection of the steering axis (head tube) with the surface. The trail is a function of the head angle, the fork rake, and the tire diameter. Trail has a major effect on the handling of a bicycle. More trail increases the bicycle's tendency to steer straight ahead. A bicycle with a largish trail dimension will be very stable, and easy to ride "no hands". A bicycle with a smaller trail dimension will be more manuverable and responsive."

    From: http://www.phred.org/~josh/bike/trail.html
    "For a given steering angle, offsetting the hub forward reduces trail, while offestting the hub backward increases trail. This may seem counterintuitive, since very stable cruiser bikes usually have more fork rake than twitchy track bikes. But the other factor at work is the angle of the steerer -- cruiser bikes have very slack head tubes, so they have more trail despite their fork rake, not because of it. "

    Finally, for an interesting and amusing "discussion" involving Lennard Zinn clarifying the confusion of terms between the motorcycle, motor racing, and cycling world, see:
    http://www.velonews.com/tech/report/articles/7322.0.html

    I attach a copy of an extract from his cycling primer that fits it all together re: the effects on bike handling: (from: http://www.velonews.com/media/Block40.pdf)
     
  19. mikesbytes

    mikesbytes New Member

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    Absolutely, there are a lot of factors that can cause speed wobbles.

    Seems the cycling world has conflicting terminology for the word "rake". I'll stick to the motorcycle terminology which has rake as the angle of the head and offset as the distance between the head and the forkes, which is zero on bikes that don't have front suspension, ie all road bikes. Anyway its just terminology, if the bike doesn't wobble or do some other dodgy thing, then its ok.
     
  20. alienator

    alienator Well-Known Member

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    Yup w.r.t. speed wobble. As for "rake," I'll stick with the motorcycle definition, too, especially since "offset" accurately describes what is actually going on with a bicycle fork. And while Zinn and Sheldon are quite knowledgeable on some things, they aren't--to the best of my knowledge--scientists or engineers. Zinn has been known to write a technical gaff or two in his articles. FWIW, motorcycle terminology also uses offset, but that offset is the distance from the steering axis to a line drawn between the longitudinal axes of the forks. So in that light, using offset for bicycles is a better, uniform use.

    Whatever the terminology that someone chooses to use, there is no black magic in bicycle fork geometry, and that geometry has been clearly explained here.
     
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