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Thread: Wheel Flop?????

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    Wheel Flop?????

    Can anyone provide an explanation of Wheel Flop? I've been reading several articles on touring bike frames and load carrying/handling etc. and there's frequent reference to wheel flop. Bicycle Quarterly's glossary offers this definition:

    "Wheel flop: Gravity reinforces handlebar deviations from center of a bicycle with positive geometric tail. The amount of wheel flop is determined by the factor "f". "

    Huh?

    Thanks

    Rick

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    Not sure what factor "F" is but wheel flop is caused by having an inappropriate amount of trail.

    Dave

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    Senior Member Nessism's Avatar
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    I have no idea what that article is talking about regarding "positive geometric trail" and "factor "f"".

    Maybe I'm not using the word correctly but I always thought wheel flop is when riding at parking lot speeds and making a turn, the front wheel can flop sideways so to speak causing an oversteer situation. This often occurs on laden touring bikes where weight (luggage) is carried over the front wheel. It also occurs more often on bikes with slack head angles and lots of fork rake. One of my early homebuilt frames used a slack head angle 72deg. and lots of rake - 52cm. That bike steered nicely at speed but the wheel would definetly flop a parking lot speeds. For that reason touring bikes typically use a steeper head angle than one might think is necessary considering the desire for stable handling.
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    Thanks very much for the responses.

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    Senior Member PaPa's Avatar
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    Quote Originally Posted by Nessism
    It also occurs more often on bikes with slack head angles and lots of fork rake.
    Only if trail is present. "Flop" is a by-product of "trail"

    Trail is the distance between the center of the contact patch of the front wheel on the ground, and the point where the "virtually" elongated steering axis meets the ground. Since the contact patch is (usually) aft of the steering axis (establishing positive trail), whenever the laden bike is leaned, the weight of the bike sitting on the contact patch, effectively turns (or flops) the front wheel in the leaned direction. Eliminate trail (for illustrative purposes only) and flop disappears because the contact patch (now centered along the steering axis) offers no leverage to turn the wheel.

    Additional weight over the front wheel intensifies "flop" on trail laden bikes, but is not the cause.
    Last edited by PaPa; 06-02-07 at 07:02 PM.

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    Senior Member Nessism's Avatar
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    With the possible exception of some odd one off creations, all bike have trail these days. So are you suggesting that more trail leads to more flop?
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    Senior Member PaPa's Avatar
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    Quote Originally Posted by Nessism
    With the possible exception of some odd one off creations, all bike have trail these days.
    "Trail" is generally used to fine-tune the bike's handling after wheelbase, head tube angle, wheel diameter, CoG and other parameters have been established. Although beneficial in moderate amounts on typical uprights, "trail" is a considered a seasoning among gourmet frame builders and therefor, not always a requirement or even desired in some cases.

    Quote Originally Posted by Nessism
    So are you suggesting that more trail leads to more flop?.
    Yes.

  8. #8
    Senior Member Nessism's Avatar
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    Quote Originally Posted by PaPa
    "Trail" is generally used to fine-tune the bike's handling after wheelbase, head tube angle, wheel diameter, CoG and other parameters have been established. Although beneficial in moderate amounts on typical uprights, "trail" is a considered a seasoning among gourmet frame builders and therefor, not always a requirement or even desired in some cases.
    99.9% of all frames made in the last...forever have positive trail.

    Trail on modern frames/forks commonly fall in the 50 - 65mm range -65mm is pretty darn slow and 50mm is quick. Builders most commonly use a head angle between 72 -75 degrees and fork rakes typically fall between 38 - 52mm. Mix and match frame head angle and fork rake to get the desired TRAIL to suit the use.

    Back to the subject of flop again, this is mostly an issue with laden touring bikes but as stated before, it can be noticed depending on how the geometry of the bike is laid out. For example, it's possible to have two different bikes with the same trail: one with a shallow head angle and lots of fork rake and another with a steep head angle and minimal rake. Of the two bikes the one with the shallow head angle and lots of fork rake will tend to flop more than the other bike, at least that's been my personal experience.
    Last edited by Nessism; 06-03-07 at 01:31 PM.
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