New Bike for Meemaw - or: Bend 'til it Breaks?- Mtbr.com
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  1. #1
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    New Bike for Meemaw - or: Bend 'til it Breaks?

    Hi guys,

    I hope I attracted a lot of experienced frame builders and tube benders with this catchy title. So here is what this thread is gonna be about. My Meemaw (well, actually my girlfriends ) has a old bike from Hercules. Steel, brazed, pretty and simple.
    Since she is getting in her "second" best years slow by slow she'd like to have a frame with a lower entry. Well, looking at her actual bike i totally can get that:
    New Bike for Meemaw - or: Bend 'til it Breaks?-omi_original.jpg

    After long back and forth and some wicked, wobbly moments with a full basket she decided to get a new bike. We visited almost every bike dealer near Munich but had no luck. She didn't felt comfortable on any of those bikes and often said that the bikes (mostly the steering) felt somehow awkward and shaky. Additionally there isn't a lot of choice with a really low entry and 28" wheels. The thick aluminium-pipes aren't her taste either. Last but not least, she often says that her bike runns incredibly smooth. While others are pedaling she often can hold their pace without having to pedal herself. So no new bike.

    or maybe..?
    Well, I thought, why shouldn't I build her a custom frame? I quickly measured her frame, took a possible good picture, threw everything in CAD and started drawing.
    Here is my first draft:
    New Bike for Meemaw - or: Bend 'til it Breaks?-omi_rahmen.jpg

    and the geo chart with a comparison to her old bike:
    geo low_entry Hercules low_entry_imp Hercules_imp
    Reach 420mm 410mm 16.54" 16.14"
    Stack 575mm 550mm 22.64" 21.65"
    "Reach" bar height 390mm 380mm 15.35" 14.96"
    "Stack" bar height 670mm 650mm 26.38" 25.59"
    steering tube angle 72 73 72 73
    bb height 250mm 270mm 9.84" 10.63"
    bb drop 90mm 70mm 3.54" 2.76"
    chainstay length 440mm 450mm 17.32" 17.72"
    seat tube angle 76 74 76 74
    seat tube length 480mm 520mm 18.9" 20.47"
    wheelbase 1060mm 1063mm 41.73" 41.85"
    crankarm length 175mm 175mm 6.89" 6.89"

    - steering angle is a little shallower, the front a little longer. New scool!
    - BB-height is a lot lower. That should make the bike feel a little more stable and render it more easily to put her feet on the ground
    - seat tube angle is a little steeper for more pressure on the pedal
    - seat tube is shorter for being able to lower the saddle
    - despite all changes the major contact points are still about there where they used to be

    My biggest worry (apart from her doesn't getting along with the frame) is that the design with the heavily bent down tube could not be strong enough. I contacted a tube supplier in Germany and the only tube they had fitting my needs has a outer diameter of 35mm (1.38") with a wall thickness of .8mm (0.031").
    I have to confess that I neither have any experience with sterss/strength analysis of bended tubes nor with building a low-entry frame.
    The tightest bending radius is about 140mm (5.5") which should be fine(?).
    The additional pice of tube between the steering tube and down tube is intended to introduce the forces more evenly into the down tube and to relief the weld a little.
    Since the bike is intended to be a city bike the occuring forces will be much lower than on a mtb. Never the less it would ease my mind if someone could say something to my draft.

    edit: I'm going to add a reinforcement between down tube and seat tube as well.

  2. #2
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    I'd use something like 1.5-2mm wall if it's going to just be a cantilevered design like that.

    .8mm is a disaster waiting to happen, for sure.

    I'd also look at foot-forward (very slack seat angle/combo of upright and recumbent) if her balance is suspect. They are very comfortable/stable to ride and you can put your feet down flat when needed with your butt still in the saddle.

    -Walt

  3. #3
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    Something that you have not documented is fork offset and therefore unable to establish the trail measurement. These bikes are usually designed as low-speed bikes and often have trail numbers at the low end for easy steering/balance reasons. They can feel twitchy as a consequence. Also, the single downtube needs to be large diameter to resist flex. A combination low trail and small diameter DT can work to accentuate a noodly feel. Try riding these bikes along with conventional triangular framed bikes of a similar design and give it a head shake treatment. This is where you ride slow and deliberately and very rapidly pull the handlebars left/right to invoke a frame flex. A noodly frame will exagerate a feel of lack of control whereas a stiff frame will snap back into the straight ahead immediately and feel secure. It is an inherent nature of this type of design to do so. Also agree on the wall thickness as Walt suggests.

    Eric

    edit: It looks like that Mixte frame has a tube type sticker on it. Some Reynolds 531 Mixte tubesets made beautiful riding bikes and will be hard to replicate.
    If I don't make an attempt, how will I know if it will work?

  4. #4
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    thank you Walt and Eric for your reply!
    I hope I made a good progress since my last post here.

    Walt, I exchanged the down tube for one with 38mm (1.5") and 1.2mm wall thickness (0.050"), because that seems to be a standard size. A 38mm tube with 1.2mm walls should be very similar to a 35mm tube with 1mm wall thickness.

    I tried doing some simulation in Fusion360 but apart from the fact that I only know very little about simulation it doesn't seem to have the best solver for something like that. Or maybe it's just me (which perhaps is even more likely).

    I also implemented the reinforcement between the seat tube and the down tube. I made it fairly small so that one could still pass between the down tube, the reinfocement and the chainring. In fact I'm going to look at the gear ratio she has on her bike later on today in order to estimate the minimum size of the chainring.


    I'm not sure about the slack seat angle you proposed.. I don't have much experience with those bikes but speaking from bare instinct I would have guessed that a steeper seat angle and by that a position more central over the bike would be more stable? I also don't want to change to much of the original geometry since she has no problems with that bike but didn't come along with the ones she tested (also having pretty slack seat angles). I hoped that the extremely low BB (250mm, 90mm drop) should be sufficient for having a short way from the pedal to the ground, also being able to do while still sitting on the bike. Of course that would result in a not optimal saddle height...



    Eric,
    concerning the fork offset, I didn't documented it because I am still doing some research about the impact of steering angle, trailing and offset. If you have any suggestions I'd love to hear them!
    I planned on buying a fork and not building one by myself. Additionally I intended to use a front disc brake. Doing so will increase the forces I have to deal with but the first simulation in Fusion looked very promising.

    I checked the frame and the sticker you saw tells no tube type. It is just something about the manufacturer.

    Here you have a picture of the current state of the prototype!
    New Bike for Meemaw - or: Bend 'til it Breaks?-low_entry_mk2_v2.jpg

  5. #5
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    Lengthening the chainstays would add some stability and smoothness.
    http://www.bikingtoplay.blogspot.com/
    RIGID, not "ridged" or "ridgid"
    PEDAL, not "peddle." Unless you're selling stuff

  6. #6
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    Re: Fork. I use this principle when dealing with type of situation - if it ain't broke, don't fix it. It applies here in this way: you are effectively only re-configuring the way the frame functions as a step through, the BB is lower, seat will drop with that and the handlbars will remain as is unless you lower the HT height as well. With the females, I generally find that if they are comfortable with the present situation and there is no reason to change, leave the whole area alone. Your finished bike will will look different but have the old familiar feel that she is accustomed to.

    Eric
    If I don't make an attempt, how will I know if it will work?

  7. #7
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    What are you going to use to bend the tube?

  8. #8
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    Not sure if it was mentioned but, I would raise that 2nd tube up to the top of the gear. Stronger and you have to step over the gear anyway.

  9. #9
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    In this scenario, I too like the 'ain't broke, don't fix it" concept. I think I'd simply cut the dual TT's out of the mixte frame and then add some back in that give lower standover, but yet still enough support to the front triangle. Afterwards, I'd have memaww choose her favorite color and I'd get it PC'd for her. IMHO this would be the most conservative approach and the path of least resistance.

    Later,
    CJB

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