What makes a hardtail frame "supple"?- Mtbr.com
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  1. #1
    hardtail partier
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    What makes a hardtail frame "supple"?

    Hi friends. I'm not a builder (yet), but I've been riding a lot of hardtails lately, and I'm surprised how few builders factor in ride feel when designing a hardtail.

    I'm trying to learn what makes a frame "supple." I have my theories, but it's amazing how one frame feels as supple as an anvil, while another feels like a wet noodle.

    I know the holy grail for many is a supple frame that stays stiff under pedalling, but I'm hoping to learn more from the masters here. I assume the following affects how stiff a frame is:

    - tubing thickness
    - tubing metallurgy (chemical meakeup)
    - gussets
    - joining type (weld vs braze)
    - how tight fitting the joints are
    - tube shape
    - frame shape

    In your opinion, who has the holy grail of chainstay, dropout, and seatstay design for a compliant ride that isn't noodly?

  2. #2
    hardtail partier
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    What do you guys think contributes best to a comfortable MTB frame on the trails? Seatstay, chainstay, or where they meet the seat tube?
    Last edited by hardtail party; 06-09-2020 at 12:11 PM.

  3. #3
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    IsoSpeed and the thinner seatstays on my Trek Procaliber 9.7. 😁

  4. #4
    pvd
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    Generally, it comes down to good engineering. Sizing the tube diameters correctly for the rider,use, and load.

    Typically, we shrink the top tubes and seat stays to soften the bike and add to the down tube and chainstays to prevent twist.

    Good engineering of yokes will be a big factor in all this.

    Also, what feels right in one environment won't feel good in another. My Warbird hardtail feels amazing in Marin where high speed singletrack and loamy, rocky, rooty chutes are the game and you need a flexy bike. In Gooseberry or Sedona, it's a bit too flexy when ridden hard on the rocks and you need a far stiffer bike. Horses for courses.

    I don't make bicycles. I make weapons systems. | Peter Verdone Designs

  5. #5
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    Yeah, I think you missed the number one factor: tubing diameter.

    For the front, it's pretty basically going to be the diameter of the tubes (unless you're getting into shaped profiles), and you need to choose that based on rider weight, style, and things like fork length.

    For the back, one thing I've found to make a really big difference is how much of an S-bend the seatstays have. Deeply S-bent stays definitely comply more than single bend fairly straight stays. I try to find somewhere in the middle that isn't too far in either direction, and that's what's ridden the best for me.

    The other factor that can contribute to this is wheel flex. Wheels can be completely different in terms of stiffness, so unless you're testing all these hardtails with the same wheelset, that's an additional factor that may be affecting your experience. I find flexy wheels to be a noticeably unpleasant feel as opposed to steel frame flex.

    And this is all based on one material. You move to carbon or aluminum or titanium and it's back to square one.

    If by "I'm surprised how few builders factor in ride feel when designing a hardtail" you mean any of the big brands made is Asia, I'm not really surprised. If you mean boutique builders here in the states, it could come back to what Peter said about horses for courses. Different terrain definitely dictates different bike design, flex included, and with builders all over the country, they may just be building what rides best where they're from.
    Myth Cycles handbuilt bike frames
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  6. #6
    hardtail partier
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    I'm usually testing all these hardtails with the same wheelset. I'm quite familiar with all my review wheelsets, and I'm able to feel the frame separate from the wheels. Thanks for the input.


    Ironically, I prefer stiffer bikes in Marin/Bay Area and softer bikes in gooseberry/sedona.

    Is it fair to assume that seatstays affect a "supple" ride more than chainstays?

  7. #7
    pvd
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    It depends on how fast you are trying to ride and how narrow the trail is. Stiff bikes track extremely poorly at speed.

    The frame is a system. Just one part isn't a silver bullet. Still, the chainstays aren't much of the load support component as they will be in tension.

  8. #8
    hardtail partier
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    Great info, thank you. I'm not the fastest rider on the planet.

    So, to make sure I understand, chainstays are under tension and seatstays are under compression? I hadn't thought of it that way before. Is it more the stay, or the joint/yoke/connection area that provides flex?

  9. #9
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    Personally, i can't feel a noteworthy difference with mtb rear triangles unless something weird is going on, like elevated, or a poorly executed yolk, or English style. The exception is seat tubes- an enormous seat tube makes a stiffer pedaling bike, which still isn't a personality trait i feel strongly about. I think it's easy to notice something and attribute it to the rear triangle, when the front triangle is where it's happening.

    Erich covered the front triangle perfectly (whole reply was excellent), but anyone with an interest here should experiment with a beam deflection calculator. IMO- from a design perspective, an excessively short head tube can make the front end transition from compliant to vague (and be problematic for durability), and a design where TT and DT are more parallel makes a frame ride more softly without much adverse consequence.


    I think builders who geek-out over frame deflection internalize good design practice pretty quickly. It's not very complicated once you're familiar with how it all interacts and what to avoid.

    - Chainstay/seatstay braces-- don't do anything; they're for style and an attachment point
    - tubing thickness-- A thick tube is almost as stiff as the thinnest in the next diameter (not useful for MTB tuning)
    - tubing metallurgy (chemical makeup)-- Doesn't matter
    - gussets-- I don't use them, i would expect plate gussets are the only ones that matter, and they're dumb
    - joining type (weld vs braze)-- Doesn't matter
    - how tight fitting the joints are-- Only matters for durability
    - tube shape-- If ovalizing round tubes, figure it's the diameter that's seeing the force. Small tweak.
    - frame shape-- Matters.


    I have 2 hardtails currently, built with almost identical tubes. One rides like a sawhorse (experiment) and the other is damn close to optimal. The differences are +35mm reach and a 90mm shorter seat tube.

    As someone who enjoys your videos, i'd expect you'd be initially looking at a 28.6tt and a 34.9dt, in 9/6/9. Maybe like 16x.8 seat stays.


    I'm confused why you think your builder doesn't already know this stuff. Based on your question, it sounds like there may have been a miscommunication?



    (hobby builder who hasn't made a frame in 2 years)
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  10. #10
    hardtail partier
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    Great discussion guys, thanks for teaching me. There's so much more to learn about the topic.

    @pvd, looking at your pink frame (is it a warbird?) is your seatstay bridge designed with compliance in mind? Are those hollow round bits providing any compliance, or is it more for artistry? I love how you're pushing the boundary and challenging the status quo. It looks like a very interesting riding bike.

  11. #11
    pvd
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    The seatstay bridge was designed to provide the maximum positioning support to the top of the seat stays while minimizing intrusion into the rider envelope. Compliant flex is left to the tube.

    The upper seatstay connection to the frame is a big problem in modern geometry and design, especially given my fat legs. This yoke design allows for an extremely compact package even with room for a large tire and mud.

    It also has support to reduce distortion that took place with the Red Five and Blackbird yokes during fabrication. These work out pretty well now. Future versions will include just a little more room for welding as well as a dedicated 0.500" stay version. Those are 0.625".

  12. #12
    hardtail partier
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    Thanks Peter, it just goes to show how off I was on my assumptions. I appreciate the education. Sounds like I should have studied engineering instead of education.

  13. #13
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    I'd be curious to see a list of the hardtails you've ridden/reviewed and then your flex impressions (front or rear or both) next to each one.

    Given the range of bikes you've probably ridden, my first guess is that it's going to be pretty hard to nail it down to one or two design criteria. Different materials are going to be difficult to compare to each other because the profiles used in the tubing between aluminum, titanium and steel are quite different. But if you limit it to one frame material, it's easier to nail down some parallels.
    Myth Cycles handbuilt bike frames
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  14. #14
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    When seated your seatpost is going to flex much sooner than your rear triangle for any vertical compliance. Rear end is a triangle and is a naturally rigid element.

    https://www.instagram.com/albatross_bikes/

    Have a look at what these guys are working on to achieve vertical compliance in their concept.

  15. #15
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    I subscribe to the beefy Downtube and Chainstay school of thought as described by Peter and Erich. You have to have pedal efficiency and directional control. You manipulate the Toptube and Seat Stays by reducing the tube diameter as also described.
    Don't focus on the gusset/bridges idea. They contribute nothing to the building of a supple frame. You can in fact not use bridges at all. If you are not requiring to attach fittings like guards or brakes - which are not likely. S-bends are great and still keep the rear end functioning properly yet give a little softness. A key area is ensuring the BB shell is strongly supported.

    Suppleness is a function of the tube itself. Jointing methods do not contribute.

    I have ventured into alternative frame designs including elevated Chainstay, Truss frames plus others, and I will make comment about 'fitted' Seat tubes, ultra stiff but little give. Not a likely design element in your case but worth mentioning as to how you treat your seat stem as it does also contribute.

    I have also used different profile tubes, Oval and asymmetrical and these are for the experienced to play with. They offer different characteristics to a build but suppleness is not one of their strong points.

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

  16. #16
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    Since we have ridden together, I thought I would add my two cents. One of the big factors for you will be your height. I noticed you have your dropper post slammed all the way to the seat clamp. The more post you have sticking out of the frame the more supple the ride will feel when seated. This is where a custom frame build could benefit you-- you can get the reach/front center you're after but not have to ride a "taller" frame to get there. This is precisely the reason I started down the custom frame road- more front center and more dropper in the same package.
    Also keep in mind that if we built two frames from the same tubing, one to fit you and one to fit me, that your frame would be stiffer simply because it's smaller. Shorter span between joints and smaller triangles. The rear end obviously doesn't change as much, so it is to a lesser degree responsible for the ride.
    Most manufacturers aren't changing tube sizes and wall thicknesses between frame sizes-- they still have to cater to the least common denominator, like when a three hundred pound, 5 foot tall person jumps on one of their bikes.

    That's why custom is custom. The tubes that are going into my 15 y.o., 120 lb son's bike are very different than the tubes I put in my frame, even though he's almost as tall as I am now and the frame sizes are similarish. I outweigh him by 90 lbs. We tried to tune it for him as his riding style. Anyway, I hope you get the idea.

  17. #17
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    This thread piqued my curiosity due to my history of rigid bikes. I'm no frame builder.

    Since 2007 I've ridden several steel bikes from springy to downright harsh.
    The longer tubes of the first one seemed to play into the springy-ness of the frame. It was very nice to ride and I enjoyed it on almost all terrain. However, I felt I was bordering on abuse, so I bought something stiffer.
    That bike was darn near perfect (note the curved seat stays), but it broke. They sent me a 2013, which had a triangular section hydroformed top tube, a head tube gusset, and weighed even more. And, with more standover clearance, the main triangle was slightly more compact. It was very harsh. I hadda buy bigger tires.
    When the 2017/2018 came out, it was lighter and slacker without being too much lower and I went and found a used one. It still has the curved seat stays, but it has been "tuned" to a much more pleasant demeanor overall, and feels like the 2011 did, just with more progressive geometry (which has already saved my face once).

    Not sure what all that means to you, but the frame builders who have commented here have described almost exactly what I have experienced through frame geometry and tube thickness and diameter.

    BTW - it's crazy how laterally flexy each individual seat-/chain-stay combo is without a rear wheel installed compared to how perfectly rigid it is all assembled.

    Um...carry on.

    -F
    It's never easier - you just go faster.

  18. #18
    hardtail partier
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    Quote Originally Posted by jsudar View Post
    Since we have ridden together, I thought I would add my two cents. One of the big factors for you will be your height. I noticed you have your dropper post slammed all the way to the seat clamp. The more post you have sticking out of the frame the more supple the ride will feel when seated. This is where a custom frame build could benefit you-- you can get the reach/front center you're after but not have to ride a "taller" frame to get there. This is precisely the reason I started down the custom frame road- more front center and more dropper in the same package.
    Also keep in mind that if we built two frames from the same tubing, one to fit you and one to fit me, that your frame would be stiffer simply because it's smaller. Shorter span between joints and smaller triangles. The rear end obviously doesn't change as much, so it is to a lesser degree responsible for the ride.
    Most manufacturers aren't changing tube sizes and wall thicknesses between frame sizes-- they still have to cater to the least common denominator, like when a three hundred pound, 5 foot tall person jumps on one of their bikes.

    That's why custom is custom. The tubes that are going into my 15 y.o., 120 lb son's bike are very different than the tubes I put in my frame, even though he's almost as tall as I am now and the frame sizes are similarish. I outweigh him by 90 lbs. We tried to tune it for him as his riding style. Anyway, I hope you get the idea.
    Thanks John, great to hear from you. I always had a hunch that the smaller triangles in my frames made for a stiffer ride. I appreciate your input.

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