Is your member stressed?

Discussion in 'The Perfect Line and Other Riding Myths' started by Drop_Center, Jun 5, 2018.

  1. Drop_Center

    Drop_Center Long timer

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    In regards to road bikes, what are the advantages of mounting an engine to the frame vs the old engine in cradle design in real world applications? Obviously stressed member frame engines are stronger and lighter and more advanced but stiffer usually means less feel in my experience, so why is this applied on streetbikes that are not raced or used even to a fraction of their potential?

    Is it me or do I feel much more comfortable on a steel cradle frame than a big aluminum stiffy reinforced with the engine?
    #1
  2. Drop_Center

    Drop_Center Long timer

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    I like this:

    upload_2018-6-5_13-0-13.jpeg

    and this:

    upload_2018-6-5_13-0-55.jpeg

    Not this:

    upload_2018-6-5_13-1-24.jpeg

    Or this:

    [​IMG]
    #2
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  3. Vertical C

    Vertical C Long timer

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    Advantage is weight isn't it?
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  4. abhibeckert

    abhibeckert Long timer

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    A cradle design means lifting the engine up off the ground by whatever the thickness of the frame is. The closer the engine is to the ground the shorter the distance it has to move when you change the lean angle of the bike, making the engine feel lighter without actually being lighter. I think everyone can agree this is an improvement (except perhaps for aesthetics and cost reasons).

    I've never been on a bike with a thin sheet metal frame instead of metal tubes, so I can't compare how they feel. As far as I know a design like that will allow less metal (they can be extremely thin sheets) and more strength. Also I think the metal alloys they're using are significantly more rigid than what was used on older bikes like the one I ride.

    With how much we know about metal these days, I'm pretty sure the bike is exactly how rigid or flexible as the designers want it to be. They would consider it perfect (within the budget constraints they're targeting).

    Are you sure being stiff is actually worse, or is it just what you're used to? Could the bikes you've ridden with rigid frames have been improved with suspension adjustments? Again I've never ridden one properly (a few short rides on other peoples' bike is all I've done).
    #4
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  5. Snowbird

    Snowbird Cereal Killer

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    Frame number four of your examples has the arms that go down and connect to the engine often are designed to allow flex while leaned to extremes and thereby absorb bumps without breaking traction.

    Steel has more flex as a material than aluminum. (Gross simplification, but I'm no Kevin Cameron.)
    #5
  6. tomo8r

    tomo8r Been here awhile

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    You're on a production line, you need to combine the engine and frame. Which approach is simpler and quicker? (I have no experience in Moto construction.)
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  7. foxtrapper

    foxtrapper Long timer

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    You're welcome to take my old spaghetti noodle bike out for a ride to enjoy it's feedback and handling. ;-)

    In my experience, a wiggly flexing bike is not what I'd consider a good thing. Not for handling nor for control. Now a flexible suspension...
    #7
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  8. st3ryder

    st3ryder Been here awhile

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    Direct connection from steering head to swing arm pivot. Better handling, and lighter too.
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  9. Snowbird

    Snowbird Cereal Killer

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    Frames are pretty in a form-follows-function sort of way.
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  10. High Country Herb

    High Country Herb Adventure Connoiseur

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    Weight is the reason it's done. Stiffness is a result of improved design, and is intentional.

    Once you spend some time on a rigid frame bike, you come to appreciate the predictability. A rubbery bike tends to wallow. Sure, you get feel, but the frame isn't where it was when it sent that message to you. I would rather the frame be stiff, and the suspension move.

    [​IMG]
    #10
  11. Anders-

    Anders- 690R

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    Frame flex is necessary. Think about it, how well does the suspension work when the bike is leaned over?
    #11
  12. Anders-

    Anders- 690R

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    And here's an example of how a frame should not work :lol3

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  13. PaulTim2000

    PaulTim2000 Been here awhile

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    Having started riding on mid 70's Japanese 4's I recently borrowed a restored Honda 750 from the early 80's. I had forgotten how bad they handled even on modern tyres, it's not that it's trying to kill you in the way the old Laverda Jota did it's the vagueness and underlying unpredictability of what it will do when road conditions change, it's not just the chassis but the whole package (fitting a fork brace to help prevent twist can make a massive improvement). In comparison a modern middleweight is far more predictable in real world situations.
    If we're not talking about racing then having a stiff, predictable, structure is far more important than the rather vague 'flex' you experience on some machines.
    Thinking about this I wonder how much impact chassis flex has compared to the deformation of the tyres when riding.

    All of this is just my, uneducated, opinion based on riding, not racing.
    #13
  14. Drop_Center

    Drop_Center Long timer

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    I agree, which is why I don't understand how super stiff frames are praised by motojournalists and ones that flex are dismissed as crap. It's totally predictable too, the budget bikes are always too flexy as if that has something to do with the MSRP.

    Then the internet gets a hold of the concept and all of a sudden the Honda 919
    frame flexes when you sit on it. Not kidding, people do think that.
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  15. High Country Herb

    High Country Herb Adventure Connoiseur

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    Mine works very well. My center of gravity moves parallel to suspension travel regardless of the lean angle.

    When the frame flexes, rake and suspension alignment vary, which is bad. Ever ride a bike with weak forks? The handlebars are straight, and the front wheel is all over the place. It's like riding is sand all the time.
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  16. Anders-

    Anders- 690R

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    Unfortunately, bumps on the asphalt aren't aligned to the lean angle :lol3

    Weak forks? Heck yes, not so nice. The thing with frame flex is that it's calculated to flex in a very specific way, not just being "flimsy".
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  17. Tripped1

    Tripped1 Bitch called me a feminist.

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    Quite the opposite to me, there is a distinct increase in feel with rigidity. Boxed swingarms, fork braces, radial brakes, sheathed brake lines....all more rigid, all supply more feel.

    That being said there is such a thing as too rigid, just ask any 800cc Ducati rider not named Stoner.
    #17
  18. Tripped1

    Tripped1 Bitch called me a feminist.

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    Its not the frame, the swing arm and forks the frame NEEDS to be a stable unit or the whole system fall apart....this is exactly what I ran into with my 2000 Speed Triple, I upgraded the suspension to the point that I ran face first into a frame/swingarm combo that wasn't up to task.
    #18
  19. Andyvh1959

    Andyvh1959 Cheesehead Klompen

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    I recall some years back that Honda or perhaps Yamaha had made race bike frames so stiff in lateral deflection that the riders complained of loss of feel and traction when heeled way over. The frames had to be redesigned to account for predetermined flex when leaned way over since the suspension has little to do at that point.
    #19
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  20. High Country Herb

    High Country Herb Adventure Connoiseur

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    Rigid frame bikes dealing with uneven pavement:

    #20