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ok so whats the deal here. In cars you want as little flex in the frame as possible for handeling and suspention reasons. Meanwhile Ducati touts the 'stiff but flexible' nature of their frame, and Yamaha is putting the '05 frame back into their MotoGP bike's frame to get rid of the front end chatter by gaining back the flex that they lost in their '06 frame.

Woulden't it be better to make a bikes frame as stiff as possible and then enginear whatever you need into the far easier to change and adjust suspention system? Or am I missing why flex is good?
 

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Some flex is good because as motorcycles lean, their suspensions become less and less efficient. Forks and swingarms simply can't react to inputs coming at an angle as the bike is leaned over. Too stiff is where chatter starts. So some flex is desirable.

Just like mountain bike frames. A totally rigid and stiff frame would be horrible to ride as it would transmit every jolt straight to the rider.

In both cases, too much flex isn't cool either. Basically you want the steering head and swingarm pivot area of the frame to have a little torsional give, and the swingarm as well, wile the frame spars or center section has to keep everything under control.
It goes way deeper than this, but that's the Readers Digest version. I'm not an engineer, just a decent roadrace chassis and set-up guy who got to pick some very talented designers brains.
 

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in an example of too much flex....from what I recall...that was one of the major complaints with alot of riders of the 998's with the Single Sided Swingarm....when the bike was a 916 and as it developed...the flex amount wasn't as big an issue...but as HP increased...the bikes were going faster and were being pushed harder showing more and more flex in the swingarms....some riders just bloody couldn't get used to it...some riders never gave it a single thought...some tuned their suspensions counting on that flex and some just couldn't get their suspensions tuned right because of miscalculation of that flex....so in an answer somewhat to your question....flex is left somewhat in the comforting hands of the rider and what feedback they can give to the engineers...but there is a certain amount of expected flex per some of the circumstances that Chuckracer was siting.
 

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[/QUOTE]Woulden't it be better to make a bikes frame as stiff as possible and then enginear whatever you need into the far easier to change and adjust suspention system? Or am I missing why flex is good?[/QUOTE]

Motoczysz is doing just that. Check out the 6x-flex fork link here:
http://www.motoczysz.com/main.php?area=bike.

Basically a super stiff carbon frame with adjustable flex in the forks.
 

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I don't think it's a question that anyone can give a definitive answer to (even the factories). Personally, I try to concentrate on giving the head tube the greatest amount of rigidity & go from there. If other areas seem like they'll have too much flex, I'll put an external brace or internal sleeve there. In the end, that's why we test bikes, to see if they REALLY work the way we intended them to.
Without testing, you're a genius only in your own mind.
 

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Holy thread resurrection Batman. The theory is about designing a certain amount of flex/blending for a specific application. As an example, on a tight, twisty track, if you had the rear wheel pivot (bend) to allow a tighter turning radius (like AWS) you'd be able to turn quicker and then apply full power sooner. I think there is a recent article in Sport Rider about it.
 

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Nichols engine bolts are the best thing you can do for you DUC...trust me it is the best bang for buck you will ever spend
 

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CKvBP said:
ok so whats the deal here. In cars you want as little flex in the frame as possible for handeling and suspention reasons. Meanwhile Ducati touts the 'stiff but flexible' nature of their frame, and Yamaha is putting the '05 frame back into their MotoGP bike's frame to get rid of the front end chatter by gaining back the flex that they lost in their '06 frame.

Woulden't it be better to make a bikes frame as stiff as possible and then enginear whatever you need into the far easier to change and adjust suspention system? Or am I missing why flex is good?
Cars don't lean much so what works for them doesn't necessarily apply to bikes. The suspension systems on motorcycles are excellent for absorbing bumps or enhancing power delivery and braking as long as the bike is straight up and down. Once you're leaned over the suspension doesn't work as well. Hence frame flex is intentional and becomes part of the suspension. Also, frame flex can contribute to rider feedback. Too stiff and riders complain of little or no feel.
 

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Anyone remember Rainey's ROC chassis that was so stiff, they had to go to something else in the middle of the season? or Edwards removing the upper motor mount bolt in his VTR in his last year of WSB?

Keep in mind, as some of these guys have said, it's about desired flex in the desired direction. I believe you'd want torsional ridgidity so the frame doesn't twist (think about an old chevelle ss that's been highly modded and the twist will be obvious) but you would also want some lateral flexibity without allowing so much flex that the frame is like a wet noodle. A good example of that concept is a racing swingarm - tall and thin so it doesn't twist or flex front to back, but has a slight bit of float side to side for when the bike is leaned over. a problem with allowing these things to flex is how do you control their motion without dampening? some of it can be done with shape, but some of these things don't liked to be flexed over and over or you can get work hardened parts, right? not an easy dilemma to solve and often, stiffening one part of the bike, causes another to flex or softening one spot might do the same. i believe if you have the correct input force and just the right amount of flex, you can induce natural frequency oscillations and i'm guessing that that is the chatter the hondas and yamahas have had.

to answer your question, a car doesn't lean over as much when it turns so your desire is to maintain as much down-force on all four tires by limiting the over-turing moment created by the CG - Lower the CG, reduce the amount it moves rotationally
 

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I figure that if the big 4 struggle with it, who am I to declare where, how much, etc. I just take my best guess & test it but I've never designed or tested 200MPH bikes. My knowledge & experience ends around 130. The repeated flex of an aluminum part scares me though - I stick with 4130 which I feel is more predictable & with which I have more flex experience. I just found this thread at the bottom of another thead I responded to & it seemed like an interesting subject.
BTW: I won't build a bike without Nichols motor mount bolts.
 

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jack said:
Holy thread resurrection Batman. The theory is about designing a certain amount of flex/blending for a specific application. As an example, on a tight, twisty track, if you had the rear wheel pivot (bend) to allow a tighter turning radius (like AWS) you'd be able to turn quicker and then apply full power sooner. I think there is a recent article in Sport Rider about it.
There was a counter-point article in Motorcyclist by their resident engineer who pointed out the flaws of such thinking. Basically the flex of the chassis will not keep the wheels in line. He compares it to the four-wheel steering setups that were all the rage 10-15 years ago, that have now fallen out of favour.
 

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there is a whole lot to it...way too much to try and explain in a thread here. One of the authorities, whose book to get is Tony Foale. His book, Motorcycle Handling and Chassis Design: The Art and Science, has one paperback for sale on Amazon for $68. that says a lot.

Last year someone in the SF Bay area got enough people to go in on flying Tony Foale to CA to put on a seminar.

and there are others with other theories.
 

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Not to throw the thread off topic, but in the Northeast there are Dirt Modified racecars in which the chassis builders specifically build in flex to the chassis.

While I'm not absolutely certain, there is speculation when the chassis is "exhausted" (worn out) and we see race teams selling their chassis after 10-15 races or so.

Dana
 

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Dana said:
Not to throw the thread off topic, but in the Northeast there are Dirt Modified racecars in which the chassis builders specifically build in flex to the chassis.

While I'm not absolutely certain, there is speculation when the chassis is "exhausted" (worn out) and we see race teams selling their chassis after 10-15 races or so.

Dana
On a similar note, connecting a sidecar to a motorbike will actually warp the chassis in such a way that you can never return it to a solo bike without the handling being affected. Not many people know that.
 

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After the superbikes of the '80s and early '90s, the motors were so powerful & the chassis & tires so bad, that most top teams looked for dirt track riders to try & handle the beasts. Bimoto never had a problem using the same motors, so they figured out something. The big 4 were bragging about the increased stiffness in their frame each year & then in about '97(?) Honda announced that the new 900 had a "tuned flex" frame and as far as I'm concerned, it's been a crap shoot from then on.
 

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PS: Nice to see you here RockStore, stick around.
PPS: I've ready as many books on chassis design as I could find & Tony Foale is the one that I would deem as todays current Guru on the subject but even he readily admits that there is LOT'S to learn.
 

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Norm said:
PS: Nice to see you here RockStore, stick around.
PPS: I've ready as many books on chassis design as I could find & Tony Foale is the one that I would deem as todays current Guru on the subject but even he readily admits that there is LOT'S to learn.

Chassis design can't be easy or Yamaha wouldn't have had all the problems with chatter last year in MotoGP. Kawasaki's always made bikes with plenty of power but handling has traditionally been one of their strong suits. For a while the Team Roberts bike was reported to be the "best handling bike in the field." When they got the Honda motors and an instant 30 more HP the handling wasn't so stellar anymore. Funny how that works...
 

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All indications of whether chassis flex, if any, is good, is a complex issue with more questions than available answers. I think the only thing we can conclude it that if anyone posts up an absolute answer, they are to be ignored at all costs!
 

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JEC said:
On a similar note, connecting a sidecar to a motorbike will actually warp the chassis in such a way that you can never return it to a solo bike without the handling being affected. Not many people know that.
I want what you're smoking :rolleyes:
My old URAL had sh*tty steel tubed frame and you could drop the sidecar and ride it solo just fine anytime you wanted...another wife's tale
 
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