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Doerman
Posted on Wednesday, March 26, 2008 - 05:47 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

I was reading in the Motorcyclist magazine about the new CBR100RR where they discussed the improved frame stiffness on this model at length. How actually Honda selected to improve frame stiffness and Kawasaki on the ZX10 chose a regionally manage stiffness and let other components on the bike work in concert with the frame design to yield a better ride. The Ducati Desmoscedici was built with a 30% stiffer frame than the 1098R.

They also discussed the approach that cars take to improved handling which is to build a stiff chassis and let the suspension components do the work of yielding good ride qualities.

Buell builds bikes with intentionally (torsionally) stiff frames to get good handling.

It is all confusing to me. Why would some manufacturers go with increased stiffness and others take a different approach? Is it really such a non-linear system that they can not figure it out?

I remember my Yamaha XS650 and how I hated when the frame “noodled” in the turns at the slightest upset from a bump. I love the Buell X-frames for their stiffness and can not imagine how a flexible frame can yield any advantages in handling.

So any dynamics and suspension experts in the house that might want to shed some light on this?

Additional though:
I saw an 08 CBR1000RR in person this weekend and it is a sweet looking bike. I believe Honda will have success with it.
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Mikef5000
Posted on Wednesday, March 26, 2008 - 06:27 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

When upright, the suspension works to keep your tires in contact with the ground when going over a bump. While leaned over, your suspension can not absorb bumps like it would upright. Therefore your tires and frame will take the impact. If your frame is absolutely solid as a rock, it will not flex when you hit a bump while leaning. And would be similar to a bike with no suspension, or way to stiff suspension; the tires could loose contact with the ground. This would be BAD when leaning.
So, with some chassis flex, it can actually help keep traction while going over bumps while leaning.


Manufacturers spend massive amounts of time working to make the perfect amount of chassis flex on each bike, Too much chassis flex is bad, but too little could be worse.

I think saying "NOW WITH A STIFFER FRAME" is more marketing than anything else.

It is completely different with cars, because no matter how your driving, the suspension will always be correctly aligned (not like a bike that leans), so the suspension in a car can do all the work 100% of the time, and the frame doesn't need to do any. With cars, I don't believe there is ever a chassis that is "too stiff". Although generally stiffer means heavier (not always, but generally), and weight is always the enemy.

(Message edited by mikef5000 on March 26, 2008)
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Doerman
Posted on Wednesday, March 26, 2008 - 07:38 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

I know, at near full tilt lean, half the force is going up the direction of the shock and the other half is contributing to flexing (torsionally) the frame. Therein lies the difference between a car and a bike.

I remember when the Honda VTR 1000 came out in '97, they tried to dampen the vibrational effect from torsionally loading the frame by using a rubber bushing in the swingarm. They soon abandoned that idea. There have been many different attempted solutions to the frame torsional load problem attempted by the different manufacturers.

Chapter 1 of the "trilogy of tech" is frame rigidity. And Buell stays pretty true to that. After 7 years of adhering to a pretty stiff box frame in the XBs it became just a tad bit more stiff in the 1125R, according to the literature on the bike.

So how does the relatively stiff Buell frame solve the issue that ensues when you get forces (leaned over in a turn) that torsionally loads the frame.
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Mikef5000
Posted on Wednesday, March 26, 2008 - 07:43 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

That's the magic of Buell!

But really, they spent a massive amount of time making the chassis as stiff as possible, while still allowing it to flex when needed. (This is every manufacturer though, not just Buell)

It's not about NO FLEX, more so; NO BAD FLEX.
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Aesquire
Posted on Wednesday, March 26, 2008 - 08:57 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

With a bike leaned over, the steering head has to be able to flex in a very controlled direction. It must let the lever arm of the fork move only in one plane. You don't want the wheel to rotate in an axis that would let it steer by itself. Same in the back.

The idea is to absorb a bit of energy while not letting it feel like the frame has a hinge.

Subtle stuff, and the cutting edge of design engineering.
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Doerman
Posted on Wednesday, March 26, 2008 - 09:15 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

The thought I had and perhaps did not express it is that other manufacturers are still seeking a solution and Buell has settled on a solution and keep dialing it further in.

That's probably a great oversimplification.
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Johnnylunchbox
Posted on Wednesday, March 26, 2008 - 09:18 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

They have addressed this issue on their bikes in a unique manner.

http://www.motoczysz.com/main.php?area=home
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Xb984r
Posted on Wednesday, March 26, 2008 - 09:34 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Doerman,I think the other manufacturers have somewhat different goals than Buell,they have to worry about their bikes performance in the top levels of road racing,Buell does not.
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Etennuly
Posted on Wednesday, March 26, 2008 - 10:50 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

$$$
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Blake
Posted on Wednesday, March 26, 2008 - 11:37 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Think of the swingarm. It is incredibly stiff in the fore-aft and up-down senses, but not so much in the lateral or side-to-side sense. Lateral flexibility allows more compliance when leaned over and hitting a bump.

I can guarantee you that it is a very delicate relationship, too compliant and the bike ends up with unwanted resonances and goofy dyamic structural response to other inputs.

Same goes for the steering head and forks, which need to be stiff enough to allow the forces of turning the bike to act without the chassis getting all springy and noodled up as you say.
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M1combat
Posted on Wednesday, March 26, 2008 - 11:38 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

There's a lot of not so correct info here so I'll just throw in my two cents...


While it's correct that frame stiffness (or lack thereof) helps the suspension absorb bumps better it's also correct that REDUCED UNSPRUNG MASS helps the suspension absorb bumps : ). Even while leaned over.

If you reduce unsprung mass you can make the frame a bit stiffer because the suspension is able to do a bit more of the work.

Centralizing mass helps this too but the reaction is different of course.

Suspension geometry plays a part as well. I'd be willing to bet that Erik didn't put numbers in a hat, pull one out that read 21 degrees, and use it for the rake.


"Motorcycle Dynamics" by Vitor Cossalter will answer all of your questions (along with a bit of thought of course).
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M1combat
Posted on Wednesday, March 26, 2008 - 11:42 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

"They have addressed this issue on their bikes in a unique manner.

http://www.motoczysz.com/main.php?area=home"


As has Buell : ).
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Doerman
Posted on Thursday, March 27, 2008 - 12:27 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Thanks M1Combat for the reference to Cossalter's book. It is also interesting that you mention centralizing mass. That was also one of the design goals of the new CBR1000RR so maybe that is also the reason they could/needed to stiffen up the frame.

Not to play an all hail to East Troy or something, but thinking through these design principles and looking at what has been produced in the XB and further refined in the 1125, you can see a steady march toward a design goal that Buell set upon years ago.

Looking at other designs, the BMW has a tremendous stiffness in the driveline (motor and shaft drive). The frame is almost incidental in that it connects just the motor to the front and the backbone. What might be noteworthy is that their most race oriented bikes have left the paralever and Hossach systems and gone back to more traditional forks. I bring the BMW designs up as a comparison, because of their apparent extreme stiffness. A different way of skinning the cat.
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Court
Posted on Thursday, March 27, 2008 - 06:25 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

I just bought Cossalter's book. For some reason I am thinking it's been out of print or unobtainable for some time.

I'm not smart enough to understand all this stuff. But I am smart enough to understand how well some of the folks who DO understand understand it.

There's no "random" to it. The laws of physics are immutable.

There was a time, years ago, when folks scoffed at the Buell frame while touting the attributes of some of the other, better known, sporting motorcycles.

Many frames were crushed, bent and warped in an effort to understand how various minds approached the problem.

The Buell frame is engineering artwork . . . . that you can ride.

I'm eager to dig into the book and see if I can put some depth behind some of my intuitive assumptions.

Court
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Not_purple_s2
Posted on Thursday, March 27, 2008 - 08:54 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

There's also balance in the design. I don't have it handy but I remember reading in the first 1125R issue of Fuell that the changes to the frame and swingarm on the 25R increased stiffness to the point where the front end became a weak link. This was the reason behind the larger front forks, to stiffen the front end and balance it with the rear.
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Davegess
Posted on Thursday, March 27, 2008 - 09:15 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

It is not just stiffness. EVERY frame will oscillate at some point. It is how the frame dampens this oscillation that really matters. Uncontrolled these can be very bad, the classic tank slapper. When you hear a MotoGP rider complain of chatter it is likely an uncontrolled oscillation in the front end.

Buell understand how this all works as well as if not better than anyone in the industry. Does anyone have it solved? I don't think so, if they did GP riders would not struggle to get new bike to handle the way they want. Tires, and road surface are just two variables that can change independently form the chassis designer and upset their work.
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Loki
Posted on Thursday, March 27, 2008 - 09:22 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

There is a good source on the web that really explains this. It is old info but still pertinent to describing and understanding this concept.



check out http://www.tonyfoale.com/


Its not black magic, just doing a balancing act.
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Dhalen32
Posted on Thursday, March 27, 2008 - 09:39 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Others have written some technically accurate stuff above. Perhaps another way to think of it simplistically in one sentence is the following: When a motorcycle is leaned way over; the frame Is the suspension.
Dave
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Blake
Posted on Thursday, March 27, 2008 - 02:17 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

"There's a lot of not so correct info here"

Please elaborate.
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Jaimec
Posted on Thursday, March 27, 2008 - 03:22 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

I remember this conversation from last year when I started it under some misconceptions. The neatest thing I learned is that the Buell "Fuel-in-the-frame" design actually allows the liquid fuel to dampen out lateral oscillations when leaned over, not unlike a shock absorber.

That's something none of the other manufacturers can say.
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Doerman
Posted on Thursday, March 27, 2008 - 06:19 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

This discussion happen last year too? I must not have paid attention in class then.

dampen out lateral oscillations when leaned over
Hmm.. Not really disputing what you say since I have no knowledge to do so. Just intuitively it seems like the fuel dampening effect would be small unless the tank is filled to the rim. Fuel will dampen the vibration of individual panels and prevent them from acting like a drum skin and preventing some harmonics from occurring.

I would think that tires would be the most effective component at reducing the frequency response amplitude from road input when the bike is leaned over.
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