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Thread: Frame Stiffening

  1. #1
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    Default Frame Stiffening

    This isn't a frame BUILDING issue, per se, but rather a question about frame modification.

    I have two frames, a 1987 Schwinn High Sierra (yellow), which I ride just about everywhere, and a 1987 Schwinn Sierra (red). Both are 21" frames, are made of "4130 Chrome-Moly", and appear to have the same geometry. The main difference is that the High Sierra has a fillet-brazed head tube and triple butted main tubes, while the Sierra has a welded head tube and double butted main tubes. I do not know what the butting profiles are.
    The Sierra frame is being built with a Staton Friction Drive gasoline motor to allow me to travel long distances at 25-30mph. I would like to carry a decent load front and rear (and in the main triangle or wherever else I can put bags), and I would like to address the frame stiffness issue I currently have with the High Sierra.
    Last summer I did a self-supported trip on the High Sierra. I never weighed it, but I believe the bike and gear together were about 100lbs. The frame did not feel stiff enough for that load, and I could feel more movement in the front triangle as I pedaled than I would have liked to.
    Another time while getting groceries I loaded my front panniers kind of heavy, with a moderate load in the Jandd Rear Rack Pack 2 on the back. I felt the same thing, with the main triangle flexing more than I would have liked it to. The load was much lighter, but I probably had more weight in my front panniers than I did on the trip.

    So, without doing any welding or brazing to the Sierra frame, I would like to stiffen the main triangle to resist these loads better. I have access to a university's worth of machinery, and would like to machine aluminum clamps to add tubular 4130 members to the frame. I have a couple of ideas on how I might do this, but I'm not entirely sure which methods would have the desired result, hence my posting here.

    Method 1 (Green): Add Mixte tubes from the head tube to the dropouts. They can be spread up to 3" apart at the head tube if need be
    Method 2 (Yellow): Same as green, but bent and attaching to the seat tube lower down
    Method 3 (Blue): Add what is essentially a continuation of the seat stays (like on a cantilever frame such as the Schwinn Heavi Duty), only the tubes are straight and not extensions of the seat stays. Probably spread about 3" wide or more to be out of the plane of the main triangle and to allow space for a frame bag
    Method 4 (Orange): Add tubes alongside the top and down tubes to make it as if the top and down tubes were ovalized, the additional tubes being in contact with the original tubes

    Attachment 121013Attachment 121014Attachment 121015Attachment 121016Attachment 121017Attachment 121018Attachment 121019Attachment 121020Attachment 121021Attachment 121022Attachment 121023Attachment 121024Attachment 121025

    (Black rectangles represent locations where clamps will be added)

    Top tube is 28.6, and 23" long center to center.
    Down tube is 31.8, 25" center to center

    **I am also adding disc brakes to the back, hence the brackets (not yet finished) to brace the stays and strengthen the dropouts.
    Ignore the 24" rear wheel. It's just in there because I was using the hub and qr to hold everything together, and it was nearby**

    Part of the reason I am uncertain which method would work best is that I do not know exactly what the problem is. I know that the front triangle is deforming elastically, but I do not know whether it is bending, twisting, or a combination of the two.

    If any of you framebuilders could chime in on which of these methods might (or might not) work, I would be very grateful.

    I realize that I could find a stiffer frame and not have to do make any modifications, but I am committed to using this one.
    I don't care too much about any added weight. I do care some, but not very much.
    I am committed to only bolting things to the existing frame. I do not want to bond, braze, or weld.
    This is the problem I am trying to solve, and the constraints I have given myself. It is as much an exercise for me in learning about post production modification and frame design as it is about getting the frame characteristics that I want.
    Thank you in advance for any help.

  2. #2
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    Default Re: Frame Stiffening

    My apologies for the rotated photos. I am unsure of how to fix them.

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    Default Re: Frame Stiffening

    My first thought is where the flex you disliked was coming from. I suspect the 1" steerer of the fork has a lot to do with it. I have toured thousands of miles on 3 self made frames. The first 2 were of classic diameter tubes, the 2nd had the thickest tubing walls I could find (1/.7/1). Both were very shimmy prone and I could see the front axle inch worming as I went over bumps and when I applied the front brakes. With #3 I went to a 1.125" steerer and thick walls. The difference was striking in how much less flex I could feel.

    I had long wondered about perceived flex and where any flex really came from and have come to believe the fork contributes far more to the total (one doesn't ride un forked frames after all) flex then has been written about. My experience with #3 seems to confirm my belief.

    Given what I have found I would not expect dramatic stiffening to be the result, if the bike still has the OEM steerer diameter. Andy (whose cross the US rig weighed over 115lbs and is planning to have about 85 total lbs for the upcoming tour around Lake Ontario)
    Andy Stewart
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    Default Re: Frame Stiffening

    Interesting. Do you think the problem might be more in the head tube than in the steerer? As in, if a bike had a 1 1/8" sized head tube but (with shims) used a 1" steerer, do you think it would have the same flexing problem?

    The flex I have experienced is not something I would refer to as "shimmy". It's definitely not anything like a speed wobble, but more something I feel when I push the front wheel into a corner, or push down hard on the pedals (not mashing, but anything harder than a nice easy spin).

    I know a lot of bike tours were done decades ago (in the '70s, for instance), where riders used heavy front and rear panniers on bikes with non-oversized tube sets and 1" steerers. Did they have the same flexing problem and just live with it? Did lugged construction maybe add appreciable stiffness at the joints? Or were the tubing walls thicker, and that took care of everything?

    Thanks,
    HopHornbeam

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    Default Re: Frame Stiffening

    Why don't you just buy a small motorcycle where the frame is designed for the duty?

    Putting a petrol motor on a bicycle is just plain dangerous not to mention noisy, ugly and environmentally suspect.

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    Default Re: Frame Stiffening

    Quote Originally Posted by HopHornbeam View Post
    Interesting. Do you think the problem might be more in the head tube than in the steerer?
    NO as the headtube is both larger in diameter and is supported at both ends compared to the steerer's smaller diameter and only being attached at the crown.
    As in, if a bike had a 1 1/8" sized head tube but (with shims) used a 1" steerer, do you think it would have the same flexing problem?
    No, I don't think this at all.

    The flex I have experienced is not something I would refer to as "shimmy". It's definitely not anything like a speed wobble, but more something I feel when I push the front wheel into a corner, or push down hard on the pedals (not mashing, but anything harder than a nice easy spin).
    This what I called "inchworming" the fork/front wheel moving about WRT the frame/rear wheel.


    I know a lot of bike tours were done decades ago (in the '70s, for instance), where riders used heavy front and rear panniers on bikes with non-oversized tube sets and 1" steerers. Did they have the same flexing problem and just live with it?
    Yes as at that time/era we knew no different.
    Did lugged construction maybe add appreciable stiffness at the joints?
    No, lugs add very little to the bike's flexing or strength. Lugs were used more to ease construction then change a bike's ride.
    Or were the tubing walls thicker, and that took care of everything?
    Thicker walled tubes (to the point of becoming solid rods) were available and used. I commented on this already with my 2nd touring frame. What I didn't mention is that this 2nd frame got a second fork with a different crown and thicker walled blades after a year, yet the inchworming was the same as the steerer was still a 1" size. This on a smaller frame size (52cm) that is already stiffer then larger sizes would be. Andy

    Thanks,
    HopHornbeam
    .
    Andy Stewart
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    Default Re: Frame Stiffening

    With all the modifications you have planned, I'd suggest walking away from the Schwinn and getting a frame more suited for the task, or as suggested an E-bike or small moto.

    It's not what you want to hear, but I question the safety of these mods and the risks of breaking a heavy, loaded vehicle not designed for it at speed.
    Last edited by Eric Estlund; 05-05-2022 at 06:04 PM.

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