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  1. #1
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    650B vs B9er, affects on trail

    Okay, I love my 650B OS bike. But now I have some questions for anyone who wants to help.

    I rode my bike with 650B front and rear for a while, then I put in a 29er wheel up front and have been riding that for a while now. I figured it would just roll better, since it was larger. I obviously need to go back to 650B both wheels and try that again for a while to see if I notice a difference.

    I don't remember a noticeable difference when I put the 29er wheel in the front. I just assumed it would be better. Maybe I'll notice more when I go back to 650B front.

    What I want to know is how I affected the trail figure of the bike by using a 29er wheel vs a 650B wheel. I know it raises the front end about 3/4", so I know it makes the front end a little slacker. I'm just curious how that change affects the trail figure.

    (There are questions in there somewhere...)

    Comments, facts, opinions, suggestions, smack talk?

  2. #2
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    Quote Originally Posted by Wish I Were Riding
    Okay, I love my 650B OS bike. But now I have some questions for anyone who wants to help.

    I rode my bike with 650B front and rear for a while, then I put in a 29er wheel up front and have been riding that for a while now. I figured it would just roll better, since it was larger. I obviously need to go back to 650B both wheels and try that again for a while to see if I notice a difference.

    I don't remember a noticeable difference when I put the 29er wheel in the front. I just assumed it would be better. Maybe I'll notice more when I go back to 650B front.

    What I want to know is how I affected the trail figure of the bike by using a 29er wheel vs a 650B wheel. I know it raises the front end about 3/4", so I know it makes the front end a little slacker. I'm just curious how that change affects the trail figure.

    (There are questions in there somewhere...)

    Comments, facts, opinions, suggestions, smack talk?
    We'd need a little more information to calc the steering trail differences.

    Here is a steering trail calculator that helps me figure these things out: http://www.kreuzotter.de/english/elenk.htm

    The head tube angle changes about .85 to .9 degrees per 1 inch change in fork crown height. (The casual rule of thumb is 1 degree change for every inch change of fork crown height from the ground.)

    26 inch forks typically have a 35mm to 37mm offset (rake), 29'er forks I think are 45mm offset.

    Head tube angles on frames designed for 26 inch wheels are typically much to slack for 29 inch front wheel conversion.

    Are you using the same fork for both sized wheels? Is it rigid?

    Do you know the head tube angle with your fork(s)?

  3. #3
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    Quote Originally Posted by derby
    We'd need a little more information to calc the steering trail differences.

    Are you using the same fork for both sized wheels? Is it rigid?

    Do you know the head tube angle with your fork(s)?
    It's a rigid OS frame running a 435mm A-2-C fork with 51mm of rake. The HTA on the bike with that fork and a 650B wheel is 71*. And yes, I'm only talking about swapping out the wheels.

    I haven't even begun to figure out if there is a suspension fork that would work...

  4. #4
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    Quote Originally Posted by derby
    Here is a steering trail calculator that helps me figure these things out: http://www.kreuzotter.de/english/elenk.htm
    Using that I get:

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 29"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 73.66mm

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 27.5"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 66.04mm


    So I guess 650B in front would be "quicker".

  5. #5
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    Quote Originally Posted by Wish I Were Riding
    Using that I get:

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 29"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 73.66mm

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 27.5"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 66.04mm


    So I guess 650B in front would be "quicker".
    Yes. But remember the taller 29 wheel with 650b rear wheel raises the head tube from below the axle to the ground the height of the radius, about .75 inch higher compared to 650b at both ends. That would slack the head angle about 1/2 degree to 70.5 degrees, which shows the 650b front wheel difference to be even greater.

    It sounds like a 29'er fork. The steering trail number seems overly quick turning using the 650b wheels front and rear with that fork, but appropriate with if 29'er was used front and rear with head angle remaining the same at 71 degrees. A 26 inch wheeler fork with 1.5 inch offset and same axle to crown (435mm A-2-C) with 650b front and rear would relax the steering to be about the same as what you have with your current fork with 29'er wheel in front and 650b rear.

    Steering quickness is mostly a personal confidence thing, more about what you are used to than any "best" steering geometry. Slower steering (longer trail) is usually easier for less experienced riders and for any rider on rougher terrain because it's harder to endo.

    With a slacker head angle using the 29'er front 650b rear the calculator I get:

    Head Tube Angle = 70.5 degrees
    Outside Diameter of the wheel = 29"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 76.2 mm (3")

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 27.5"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 66.04mm (2.6")

  6. #6
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    Before I saw your reply, I figured out I had done it wrong. I was thinking about it when I was out riding and realized my error.

    I've heard that 20mm is about equal to 1 degree. So .75 is closer to 19.05mm right? So maybe the change is closer to 1 full degree?

    It is a 29er fork, albeit a short one.

    Here's my latest attempt at the geo using 1 full degree for the 29er wheel:

    Head Tube Angle = 70 degrees
    Outside Diameter of the wheel = 29"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 78.74 mm (3.1")

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 27.5"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 66.04mm (2.6")


    Quote Originally Posted by derby
    Yes. But remember the taller 29 wheel with 650b rear wheel raises the head tube from below the axle to the ground the height of the radius, about .75 inch higher compared to 650b at both ends. That would slack the head angle about 1/2 degree to 70.5 degrees, which shows the 650b front wheel difference to be even greater.

    It sounds like a 29'er fork. The steering trail number seems overly quick turning using the 650b wheels front and rear with that fork, but appropriate with if 29'er was used front and rear with head angle remaining the same at 71 degrees. A 26 inch wheeler fork with 1.5 inch offset and same axle to crown (435mm A-2-C) with 650b front and rear would relax the steering to be about the same as what you have with your current fork with 29'er wheel in front and 650b rear.

    Steering quickness is mostly a personal confidence thing, more about what you are used to than any "best" steering geometry. Slower steering (longer trail) is usually easier for less experienced riders and for any rider on rougher terrain because it's harder to endo.

    With a slacker head angle using the 29'er front 650b rear the calculator I get:

    Head Tube Angle = 70.5 degrees
    Outside Diameter of the wheel = 29"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 76.2 mm (3")

    Head Tube Angle = 71 degrees
    Outside Diameter of the wheel = 27.5"
    Fork Offset ( Fork Rake ) = 51mm (2")
    Trail = 66.04mm (2.6")

  7. #7
    TNC
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    Interesting WIWR...I would have thought you'd get a more noticeable feedback going back to the 29'er also. I'm not a good match to full 29'ers, but I have been impressed with the 69'er or 29/650B mix. A 29'er in the front seems to corner like mad on the setup I tried. I just wasn't as thrilled with the necessary loss of travel, and that's probably the only reason I don't ride a 69'er or 29/650B'er. I think the cornering prowess of the 29 front was even more impressive than its ability to roll over things...which surprised me. It's when a 29 wheel/tire goes on the rear that kills me...but I'm an old and lazy guy...LOL!

  8. #8
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    Quote Originally Posted by TNC
    Interesting WIWR...I would have thought you'd get a more noticeable feedback going back to the 29'er also. I'm not a good match to full 29'ers, but I have been impressed with the 69'er or 29/650B mix. A 29'er in the front seems to corner like mad on the setup I tried. I just wasn't as thrilled with the necessary loss of travel, and that's probably the only reason I don't ride a 69'er or 29/650B'er. I think the cornering prowess of the 29 front was even more impressive than its ability to roll over things...which surprised me. It's when a 29 wheel/tire goes on the rear that kills me...but I'm an old and lazy guy...LOL!
    I rode last night with the 29er wheel. It's been a while in this config, so I'm going back to the 650B in front on my next few rides. I might have more input after that...

  9. #9
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    Where's Yogiprofit? ;)

    Quote Originally Posted by Wish I Were Riding
    I've heard that 20mm is about equal to 1 degree. So .75 is closer to 19.05mm right? So maybe the change is closer to 1 full degree?
    I'm not absolutely sure. The commonly used rule of thumb I've often heard is 1 degree for 1 inch (25.4mm) change in crown race height. And I'm pretty sure I've heard it's slightly less than 1 degree per inch. But I'm often dyslexic and it could by the other way, 1 degree for slightly less than one inch. I don't know the math to be exact.

    Where's Yogiprofit? , he knows everything.

  10. #10
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    Was this fork built specifically for 650b wheels or is it the same fork that is used on the Black Buck 29er???

  11. #11
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    Quote Originally Posted by MMcG
    Was this fork built specifically for 650b wheels or is it the same fork that is used on the Black Buck 29er???
    It is I believe a Dimmension fork, 435mm A-2-C, and yes, the same fork as used on the Black Buck OS 29er.

  12. #12
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    Is "trail" always "trail"?

    I came up with a new question, which I think I asked elsewhere, but I'm not sure. Given the numbers above for the differences in trail, does the size of the wheel affect it? I mean is say 70mm of trial, derived through geo and different sized wheels, always going to feel the same?

    Anybody? Yogiprophet?

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