Advice on a bent design

Discussion in 'Recumbent bicycles' started by Sticker Jim, Mar 16, 2004.

  1. Sticker Jim

    Sticker Jim Guest

    After a couple more trikes, I'm going to try my first bent.
    From reading sites, checking dimensions and looking at
    commercial bents, I came up with a pretty standard version
    of a semi-low racer (I guess it would be called) made out of
    2" box tube. One thing I realized is that the front end
    design is pretty dependent on how much foot/wheel overlap
    you decide is acceptable. At any rate, the main question is
    about rake and trail. I have the design set to 70 degrees
    and about 3" of trail. I tried 60 degrees, but it looked far
    too laid back and the boom had to be very long to reduce
    heel/wheel overlap, or I had to raise the pilot position
    higher than I wanted. I remember reading that the optimum
    place to start was about 65 degrees for rake, and to aim for
    4" to 4.5" for trail. The wheel base is 50" and I'm not sure
    of the weight (likely around the 30 LB mark) but I'm not
    worried about the frame flexing or the swing arm. Based on
    experienced bent riders, does the basic design at least look
    rideable and will the 60 deg/3" trail work ok with the 50"
    wheel base?

    Thanks.

    http://bikes.jkcc.com/cads/1st%20bent%202.jpg
     
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  2. Tom Sherman

    Tom Sherman Guest

    Sticker Jim wrote:

    > After a couple more trikes, I'm going to try my first
    > bent. From reading sites, checking dimensions and
    > looking at commercial bents, I came up with a pretty
    > standard version of a semi-low racer (I guess it would
    > be called) made out of 2" box tube. One thing I
    > realized is that the front end design is pretty
    > dependent on how much foot/wheel overlap you decide is
    > acceptable....

    Moving the BB higher relative to the seat will also reduce
    foot/wheel interference. The only reason not to use a higher
    BB (6+ inches/16+ cm) on a SWB bike is foot numbness. The
    shorter boom will also improve weight distribution. Others
    may disagree, but I have found the best handling SWB bikes
    to be those with a slight rearward weight bias.

    --
    Tom Sherman - Quad Cities (Illinois Side)
     
  3. 2" box tube home built you say? How about this one?

    http://home.earthlink.net/~rehaston/

    The angles and trail are identical to a road bike,
    around 1" of trail. This is also similar to my M5. That
    is if I am defining trail correctly as the distance from
    where the steering axis intercepts the ground to where
    the tire contacts.

    The tiller available with the handlebars tilted down
    desensitizes the steering, so it handled well at low speed.

    "Sticker Jim" <[email protected]> wrote in message news:%pQ5c.305-
    [email protected]
    >After a couple more trikes, I'm going to try my first bent.
    > From
    > reading sites, checking dimensions and looking at
    > commercial bents, I came
    > up with a pretty standard version of a semi-low racer (I
    > guess it would be
    > called) made out of 2" box tube. One thing I realized is
    > that the front
    end
    > design is pretty dependent on how much foot/wheel overlap
    > you decide is acceptable. At any rate, the main question
    > is about rake and trail. I have the design set to 70
    > degrees and about 3" of trail. I tried 60 degrees, but
    it
    > looked far too laid back and the boom had to be very long
    > to reduce heel/wheel overlap, or I had to raise the pilot
    > position higher than I wanted. I remember reading that the
    > optimum place to start was about 65 degrees for rake, and
    > to aim for 4" to 4.5" for trail. The wheel base is 50" and
    > I'm not sure of the weight (likely around the 30 LB mark)
    > but I'm not worried about the frame flexing or the swing
    > arm. Based on
    experienced
    > bent riders, does the basic design at least look
    > rideable and will the 60 deg/3" trail work ok with the
    > 50" wheel base?
    >
    > Thanks.
    >
    >
    > http://bikes.jkcc.com/cads/1st%20bent%202.jpg
     
  4. Sticker Jim

    Sticker Jim Guest

    > > One thing I realized is that the front end design is
    > > pretty dependent on how much foot/wheel overlap you
    > > decide is acceptable....
    >
    > Moving the BB higher relative to the seat will also reduce
    > foot/wheel interference. The only reason not to use a
    > higher BB (6+ inches/16+ cm) on a SWB bike is foot
    > numbness. The shorter boom will also improve weight
    > distribution. Others may disagree, but I have found the
    > best handling SWB bikes to be those with a slight rearward
    > weight bias.
    >
    > --
    > Tom Sherman - Quad Cities (Illinois Side)

    I wanted to try to keep the BB as low as possible for the
    reasons you mentioned - foot problems. I broke my back and
    have numbness problems and occasional hotfoot when I ride my
    trikes as it is, but the problem is slowly taking longer and
    longer to present itself the more I train on the trikes. I
    decided with the bent I would try to start out with only
    about 4 to 5" between the seat bottom and the BB and try it
    from there. So, while trying to minimize seat to BB
    distance, keep the seat as low to the ground as possible and
    minimize wheel/heel interference, the only option seemed to
    be stretch the boom a little. It'll be about 19" from head
    to BB, a little long I think, so for stiffness, that's why I
    went with 2".

    As for weight distribution, with the size of my butt, as my
    pedaling technique improves, I'm hoping both weight
    distribution and pedal steer will not be an issue. If it is,
    I take the saw to it and try again :)
     
  5. Sticker Jim

    Sticker Jim Guest

    Uhmmm, I should have said 2" steel (not Chro Mo, just mild),
    vs aluminum which is what yours appears to be made out of? I
    was going to use 1 1/2" x .064 box steel but thought there
    would be too much boom flex at the length I was going to
    make it, so switched to 2".

    I think that's a cool looking bent too :) Nice short boom
    and looks as if it has a little more wheel/foot overlap than
    mine will. I think your seat solution is excellent. I might
    "borrow" a slightly modified version of that cool little
    triangle for my seat as well. The actual seat I'm going to
    use will have more of a "C" shape to it, but I haven't
    rendered it out yet, so used a trike seat instead. I really
    like the nice straight chain line on yours as well. I wanted
    to get my seat a bit lower than yours so I had to sacrifice
    chain line to do it.

    I will decide what type of steering I'll try once the wheels
    seat and cranks are on it, but I'll make provisions for it
    to be able to have both OSS and/or USS.

    "Robert Haston" <[email protected]> wrote in message
    news:[email protected]...
    > 2" box tube home built you say? How about this one?
    >
    > http://home.earthlink.net/~rehaston/
    >
    > The angles and trail are identical to a road bike, around
    > 1" of trail.
    This
    > is also similar to my M5. That is if I am defining trail
    > correctly as the distance from where the steering axis
    > intercepts the ground to where the tire contacts.
    >
    > The tiller available with the handlebars tilted down
    > desensitizes the steering, so it handled well at low
    > speed.
     
  6. Jeff Wills

    Jeff Wills Guest

    "Sticker Jim" <[email protected]> wrote in message news:<%pQ5c-
    [email protected]
    >... <snip>
    > At any rate, the main question is about rake and
    > trail. I have the design set to 70 degrees and about
    > 3" of trail. I tried 60 degrees, but it looked far too
    > laid back and the boom had to be very long to reduce
    > heel/wheel overlap, or I had to raise the pilot
    > position higher than I wanted. I remember reading that
    > the optimum place to start was about 65 degrees for
    > rake, and to aim for 4" to 4.5" for trail. The wheel
    > base is 50" and I'm not sure of the weight (likely
    > around the 30 LB mark) but I'm not worried about the
    > frame flexing or the swing arm. Based on experienced
    > bent riders, does the basic design at least look
    > rideable and will the 60 deg/3" trail work ok with the
    > 50" wheel base?

    It sounds like a recipe for fork flop. The low racers that
    I've seen use about a 73 degree head angle and almost no
    offset on the fork. The OHPV Varna gang built their chassis
    around these dimensions and all were ridden very
    successfully: http://www.ohpv.org/pir2003/varnas/index.htm .
    All the riders were comfortable on their bikes with or
    without the fairings.

    BTW: careful with the terminology. "Rake" to bicycle people
    means "distance from the front axle to the steering axis"
    aka "offset". "Rake" to motorcycle people means "angle of
    the steering axis from vertical" aka "head angle". I prefer
    to avoid the term altogether.

    Jeff
     
  7. Tom Sherman

    Tom Sherman Guest

    Sticker Jim wrote:

    >>>One thing I realized is that the front end design is
    >>>pretty dependent on how much foot/wheel overlap you
    >>>decide is acceptable....
    >>
    >>Moving the BB higher relative to the seat will also reduce
    >>foot/wheel interference. The only reason not to use a
    >>higher BB (6+ inches/16+ cm) on a SWB bike is foot
    >>numbness. The shorter boom will also improve weight
    >>distribution. Others may disagree, but I have found the
    >>best handling SWB bikes to be those with a slight rearward
    >>weight bias.
    >>
    >>--
    >>Tom Sherman - Quad Cities (Illinois Side)
    >
    >
    > I wanted to try to keep the BB as low as possible for the
    > reasons you mentioned - foot problems. I broke my back and
    > have numbness problems and occasional hotfoot when I ride
    > my trikes as it is, but the problem is slowly taking
    > longer and longer to present itself the more I train on
    > the trikes. I decided with the bent I would try to start
    > out with only about 4 to 5" between the seat bottom and
    > the BB and try it from there. So, while trying to minimize
    > seat to BB distance, keep the seat as low to the ground as
    > possible and minimize wheel/heel interference, the only
    > option seemed to be stretch the boom a little. It'll be
    > about 19" from head to BB, a little long I think, so for
    > stiffness, that's why I went with 2"....

    The other option to reduce boom length while keeping
    foot/wheel interference to a reasonable level is to use a
    smaller front wheel. There is decent selection of ISO 305-mm
    tires, as long as one uses cantilever, direct pull
    cantilever (V-brakes), or hub brakes (drum and disc).

    --
    Tom Sherman - Quad Cities (Illinois Side)
     
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