Frame geometry question
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  1. #1
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    Frame geometry question

    In terms of handling in the front end, if 2 bikes have the same trail but different fork offset and head angle, will the handling be the same?

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    'brifter' is f'ing stupid
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    I would think they'd feel the same, if not it would be super close.
    #promechaniclife

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    No.

    Steeper head angles result in a quicker change in direction. Trail will need to increase to result in an equal perceived sense of straight-line stability that trail provides.

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    If both bikes had the same fork offset (often called "rake") and the same headtube angle, the ONLY way they could have different trail numbers would be by having wheels of different diameters. If you are talking about a tiny difference like the difference between using a 21mm tire or a 25mm tire, that would be too small to matter.
    "L'enfer, c'est les autres"

  5. #5
    Haint
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    You're going to be talking about fractional isolation of the many variables of terrain; Road or Gravel will still be about energy-reserve conservation and stability so Steep HTA/short offset will - by default - want to track straight and have a 'dampened' arc to rotation.

    The Slacker HTA/longer offset will break from center only a bit easier, will invite a little more pulling steering input from the inside-arm in cornering as opposed to pushing from the outside-arm.

    The only time Fork Offset is called Fork Rake is when someone has laid down their Motorcycle too many times. Rake has always described the angle away from the handlebar and, on Motorbikes; 'Rake' even has connotation as a verb, whereas Offset is a variable.

    YMMV

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    Quote Originally Posted by Haint View Post
    The only time Fork Offset is called Fork Rake is when someone has laid down their Motorcycle too many times.
    Actually, while it is correct to say that rake is the angle and offset is the amount that the fork is either bent or angled relative to the steerer tube, for some reason in bicycling, offset has often been incorrectly referred to as rake. Right or wrong (wrong!) this has become common usage for many, including bike magazines and bike shops.

  7. #7
    Haint
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    Quote Originally Posted by Kerry Irons View Post
    Actually, while it is correct to say that rake is the angle and offset is the amount that the fork is either bent or angled relative to the steerer tube, for some reason in bicycling, offset has often been incorrectly referred to as rake. Right or wrong (wrong!) this has become common usage for many, including bike magazines and bike shops.
    Axle-from-fork-planal distance gives a length of an offset. Longer a measurement moves Trail closer to a fork from its HTA relationship, which only can be found if continued the length which includes any distance to the tire contact patch.

    Rake, it's Mo'Cycle Spec for HTA, is just top-half of the mechanical trail angle. Both Motorcycle and Bicycle - even full size Adult Tricycle - have a vertical line at center axle creating the angle meeting underneath by bisected the fork legs (or Leg on those Cannondale) planal arrangement w/ HTA. This distance at ground forward to this distance from fork at ground give trail length.

    Short is light.

    Long is heavy.

    Somewhere is a Mobius Strip of people not reading this Reply...

    Rotation of the forks legs around the steerer also can create offset. Offset Crowns are options, but road riding now only finds crown forks at NHBBS for the most part. Nostalgia factor vs. shitting-bull factor.

  8. #8
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    Mountain bikers know the relationship well. With the transition from 26" wheels to 29" the head tube angle was slacker, and offset had to be adjusted so the trail was not so long to produce a very slow-handling bike. Yet a 26" and a 29" bike with the exact same trail will still have different handling characteristics, give the rotating weight difference and the difference in wheel flop. In between is the 27.56 wheelset, calling again for a slightly different HTA and Offset.

    Calfee describes the relationship pretty well.
    https://calfeedesign.com/geometry-of-bike-handling/

  9. #9
    Haint
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    Quote Originally Posted by redcon1 View Post
    Calfee describes the relationship pretty well.
    https://calfeedesign.com/geometry-of-bike-handling/
    MotorCycle.com uses a Fork which also can Steam a Cheeseburger on the go.
    Must be an Albany expression...


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    Quote Originally Posted by Haint View Post
    .......

    The only time Fork Offset is called Fork Rake is when someone has laid down their Motorcycle too many times. Rake has always described the angle away from the handlebar and, on Motorbikes; 'Rake' even has connotation as a verb, whereas Offset is a variable.

    YMMV
    Your ignorance about the very basics of vehicle dynamics tells me that you've never attended an engineering school. In my case, I did, and have the degree to prove it. "Rake" has ALWAYS meant the offset from the rotational axis of the fork to the rotational axis of the wheel. This hasn't changed in over 200 years.

    Trail is merely the linear distance between the intersection of that fork rotational axis to the groundplane, and the point directly below the wheel rotational axis on the groundplane. The longer it is, the more stable a bike is at speed, but the more difficult to do low speed maneuvering.
    "L'enfer, c'est les autres"

  11. #11
    Haint
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    Quote Originally Posted by No Time Toulouse View Post
    Your ignorance about the very basics of vehicle dynamics tells me that you've never attended an engineering school. In my case, I did, and have the degree to prove it. "Rake" has ALWAYS meant the offset from the rotational axis of the fork to the rotational axis of the wheel. This hasn't changed in over 200 years.

    Trail is merely the linear distance between the intersection of that fork rotational axis to the groundplane, and the point directly below the wheel rotational axis on the groundplane. The longer it is, the more stable a bike is at speed, but the more difficult to do low speed maneuvering.
    Merry Christmas.

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    Quote Originally Posted by No Time Toulouse View Post
    Your ignorance about the very basics of vehicle dynamics tells me that you've never attended an engineering school. In my case, I did, and have the degree to prove it. "Rake" has ALWAYS meant the offset from the rotational axis of the fork to the rotational axis of the wheel. This hasn't changed in over 200 years.

    Trail is merely the linear distance between the intersection of that fork rotational axis to the groundplane, and the point directly below the wheel rotational axis on the groundplane. The longer it is, the more stable a bike is at speed, but the more difficult to do low speed maneuvering.
    So if two road bikes have the same trail, but one has a slightly slacker and the other a steeper headtube, with correspondingly differing fork offsets to achieve that same trail, would the handling be the same?

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    Quote Originally Posted by 5DII View Post
    So if two road bikes have the same trail, but one has a slightly slacker and the other a steeper headtube, with correspondingly differing fork offsets to achieve that same trail, would the handling be the same?
    No, because there would be more gravitational input on the one with a slacker headtube angle, making it quicker to respond. Having the same amount of trail would mean that lateral inputs and dampening would be the same. Don't forget that "handling" encompasses numerous inputs, trail just being one of them.
    "L'enfer, c'est les autres"

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    Quote Originally Posted by No Time Toulouse View Post
    No, because there would be more gravitational input on the one with a slacker headtube angle, making it quicker to respond. Having the same amount of trail would mean that lateral inputs and dampening would be the same. Don't forget that "handling" encompasses numerous inputs, trail just being one of them.
    Damping.
    #promechaniclife

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    Quote Originally Posted by cxwrench View Post
    Damping.
    Damn spelling nazis.....

    But seriously, "damping" is one of those words in transition, like "gauge" or "floutist" where the generally accepted spelling seems to be in flux. So, go ahead, smack me with one of your collection of improper apostrophes. I'm still going to say "dampening", the number "tree", and even sometimes "warsh"....
    "L'enfer, c'est les autres"

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    Quote Originally Posted by No Time Toulouse View Post
    Damn spelling nazis.....

    But seriously, "damping" is one of those words in transition, like "gauge" or "floutist" where the generally accepted spelling seems to be in flux. So, go ahead, smack me with one of your collection of improper apostrophes. I'm still going to say "dampening", the number "tree", and even sometimes "warsh"....
    Dampening is hitting something w/ a hose. A damper 'damps' movement. Your shocks aren't 'dampeners', they're 'dampers'.
    My dad always pronounced it that way...warsh. For some reason he always said 'southmore' instead of sophomore. He had a few weird ones! But then again he was an engineer...of the chemical variety.
    #promechaniclife

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    Quote Originally Posted by cxwrench View Post
    Dampening is hitting something w/ a hose. A damper 'damps' movement. Your shocks aren't 'dampeners', they're 'dampers'.
    My dad always pronounced it that way...warsh. For some reason he always said 'southmore' instead of sophomore. He had a few weird ones! But then again he was an engineer...of the chemical variety.
    A friends sister always said Winsconsin, but she weren't no engineer.
    Too old to ride plastic

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    Quote Originally Posted by velodog View Post
    A friends sister always said Winsconsin, but she weren't no engineer.
    I also pronounce the word "coyote" as KY-oat. But then, I grew up in Colorado, where we learned the correct pronunciation....
    "L'enfer, c'est les autres"

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    Quote Originally Posted by cxwrench View Post
    Dampening is hitting something w/ a hose. A damper 'damps' movement. Your shocks aren't 'dampeners', they're 'dampers'.
    My dad always pronounced it that way...warsh. For some reason he always said 'southmore' instead of sophomore. He had a few weird ones! But then again he was an engineer...of the chemical variety.
    Exactly. Damping means the vibration is suppressed. Dampening means something got wet. It is not a difference in spelling. BTW, us chemical engineers pronounce general words like we learned as kids, not because we are chemical engineers.

    To clarify for example. People wonder where the "r's" went from Boston when they talk about cahs and pahks. The answer is they all float up into the air where the combine with the clouds and water droplets and fall as rain in Maryland, where they all come out in the warsh.

  20. #20
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    Quote Originally Posted by No Time Toulouse View Post
    Damn spelling nazis.....

    But seriously, "damping" is one of those words in transition, like "gauge" or "floutist" where the generally accepted spelling seems to be in flux. So, go ahead, smack me with one of your collection of improper apostrophes. I'm still going to say "dampening", the number "tree", and even sometimes "warsh"....
    It's spelled "flautist."
    More Americans wanted Hillary Clinton to be President than wanted Donald Trump.

    Donald Trump has never had a wife he didn't cheat on.

    There are over 6.2 million covid cases in the United States (as of September 6th), eight months after Donald Trump said it was "totally under control," and that "it's gonna be just fine."

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    Quote Originally Posted by No Time Toulouse View Post
    No, because there would be more gravitational input on the one with a slacker headtube angle, making it quicker to respond. Having the same amount of trail would mean that lateral inputs and dampening would be the same. Don't forget that "handling" encompasses numerous inputs, trail just being one of them.
    Thanks. Can you help me understand the handling differences in these 2 scenarios? Assume all other variables are constant, with 25mm tires

    Bike A: Head tube angle 72.5, fork offset 45mm, trail 60mm
    Bike B: Head tube angle 73.5, fork offset 40mm, trail 59mm

  22. #22
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    Bike A would be a tiny bit slower to turn, might be a tiny bit more stable on a high speed descent, but not enough to really notice.

    In your scenario, the difference is so small, that just changing tires will probably the affect handling 10x more than the geometry differences do.
    "L'enfer, c'est les autres"

  23. #23
    dcb
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    Quote Originally Posted by 5DII View Post
    Thanks. Can you help me understand the handling differences in these 2 scenarios? Assume all other variables are constant, with 25mm tires

    Bike A: Head tube angle 72.5, fork offset 45mm, trail 60mm
    Bike B: Head tube angle 73.5, fork offset 40mm, trail 59mm
    As NTT said, probably not a big difference. But I'd expect bike A to have a longer wheelbase which might along with other factors make it more stable.

  24. #24
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    Quote Originally Posted by No Time Toulouse View Post
    I also pronounce the word "coyote" as KY-oat. But then, I grew up in Colorado, where we learned the correct pronunciation....
    I go w/ ky-oat as well despite living all my life in CA.
    #promechaniclife

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    If 'english' has a set of rules, why does it always break them?
    I'm going with it's not, it's just how you want things and you ain't go'ta git it!
    ... and a flautist is one who eats a lot of flautas, & I'm one!
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