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Bike geometry question

This is a discussion on Bike geometry question within the Technical forums, part of the Caferacer.net Forums category; Ok, so i have been trying to figure out the basics of the bike geometry and how it affects handling. So for example lets say ...

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    Member JasonMi's Avatar
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    Bike geometry question

    Ok, so i have been trying to figure out the basics of the bike geometry and how it affects handling. So for example lets say i have this bike with two different wheel setup. Whitch setup would you prefer as a cafe racer and why? Would like to hear an opinion from experienced rider.

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    Banned Witworth's Avatar
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    Not the 16R / 18F. 16" rear tires feel weird on sport bikes. They don't like turning in at speed much.

    I'd prefer 18's front and rear on an older sportbike. Or 17's F and R if it was feasible.

    16" fronts can be ok, but they feel unsure and weird compared to bigger wheels.

    Danger, is my business."

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    Senior Member Geeto67's Avatar
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    Let's assume rake and trail stays the same because your overall tire and wheel height stays the same. If that were the case then the 16" wheel is going to have a massive sidewall and deflection prone. This means, a 16" wheel in either situation is not ideal.

    Here is how I would approach it:
    Look at the WERA and AHRMA race book and see what wheel sizes in my bike would be limited to in the various classes
    then I would look at what tires are available for those wheel sizes and settle on a brand for each.
    then I would measure the actual overall height of the different tires
    I would then figure out the suspension sag for the bike
    then I would figure out the distance from the axle to the bottom of the tire for each combo and set the bike up so the axle was at the correct height for each combo.
    I would then take rake and trail measurements for each frame setup. Why would they be different? well even rims the same diameter will have different sidewall tire heights if the widths are different. A 120/60/18 is taller than a 110/60/18 because the aspect ratio (middle number) is a percentage of the width (first number) so 60% of 120 is greater than 60% of 110.
    now with the bike setup this way you can also check for crazy combos like your 16" rear 18" front or your 18" rear 16" front.

    but there is a quicker way. Find a bike with those wheel sizes already and figure out what it's rake and trail numbers are and see how close you are to them. An early 80's honda vf750F honda interceptor uses a 16" front and an 18" rear, and all the measurements should be online. similarly some HD models use an 18" front 16" rear (although most use a 19" front).

    also here is a rake and trail calculator to help you along:
    https://www.rbracing-rsr.com/rakeandtrail.html

    remember, lowering the front of the bike (even by decreasing the tire height) steepens the rake and raising it increases the rake.
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    jcw
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    Senior Member jcw's Avatar
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    What are your wheel widths. What are your tire options. Unless these are completely custom wheels, it may not be feasible to run them both ways.

    Like geeto said, unless the tire diameters are identical, your change the rake.

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    Senior Member o1marc's Avatar
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    When you say the rake is the same between the 2 set ups, that would require different rakes for each set up. You couldn't use the same frame and make the wheel changes without changing rake and trail.
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    Senior Member Tanshanomi's Avatar
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    Ok, so i have been trying to figure out the basics of pet ownership and how it affects agility. So for example lets say i have a two animals, a brown fuzzy one and a gray smooth one. Which would you prefer to train to run agility courses and why? Would like to hear an opinion from experienced veterinarians.
    Last edited by Tanshanomi; 07-21-2015 at 02:20 PM.
    raven and kerosene like this.

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    Banned Witworth's Avatar
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    Quote Originally Posted by Geeto67 View Post
    remember, lowering the front of the bike (even by decreasing the tire height) steepens the rake and raising it increases the rake.
    Probably more important than rake, is how adjusting the ride height, front or rear, or a combination of both, affects the front ends static trail measurement.

    BTW, it's funny how supposed experts always talk about trail figures. Hardly anyone ever actually measures them. They just quote a figure from a magazine or something. A pro race fabricator once told me the actual figures usually aren't exactly what factories say they are in factory spec sheets.

    Danger, is my business."

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    Member JasonMi's Avatar
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    This was a theoretical question... Everybody here are always mentioning rake/trail/wheelbase when speaking about geometry and handling but as i see it theres should be lot more factors to be considered. O1mark theoretically you can setup the same bike with different wheels (16/18; 18/16 or 17/17) so that rake and trail will remain the same- you must change the angle of swingarm and fork offset. In my example the numbers are from cbr600f3 specs and it uses 17/17 wheel setup.
    Tanshanomi whats your point? Just verbal masturbation in public??

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    Member JasonMi's Avatar
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    Quote Originally Posted by Witworth View Post

    BTW, it's funny how supposed experts always talk about trail figures. Hardly anyone ever actually measures them. They just quote a figure from a magazine or something.....

    .,.Everybody talks abot it, nobody knows about it.....

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    Banned Witworth's Avatar
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    Tanomatic's comments are the motorcycle forum version of this:

    The quick brown fox jumps over the lazy dog.

    Then his hand slipped off.

    Gotta get a grip.

    Danger, is my business."

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