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Chassis Flex


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It seems that manufacturers are constantly pimping their new "improved" sportbike chassis with every model year. The trouble is that not much has been written about the issue of chassis flex leaving riders like myself questioning what the feelings they are getting from the bike truly are. Obviously, the issue is very complicated due to different components duplicating the feel of another component. The best example of this miscommunication with the bike I can think of was Loris Capirossi telling his crew he was losing the rear only to discover that the problem was actually overly flexible foot pegs sometime last year. Breaking down the movements of the chassis we are left with lateral flex, torsional flex, and vertical flex (If I remember correctly). In both overly stiff and overly flexible chassis configurations what is the rider feeling with respect to each axis of chassis flex assuming all the other components are working perfectly? Also, what is happening to the bike? What chassis rigidity factors influence the handling of a bike to make it handle with precision?

 

So far I've been able to figure out that a chassis with a high degree of vertical rigidity maintains the steering geometry and allows for the suspension to operate as intended. I have no idea what that makes the rider feel or what a chassis that had zero vertical flex would handle like. I've also gathered that lateral rigidity reduces the amount of feedback available to a rider in a progressive manner to the point that a bike with too little lateral flex would lose traction unexpectedly. Will too much lateral flex make the bike feel like it's pitching side to side much like losing rear traction? What torsional flex feels like is a mystery to me. Don't suppose anyone knows of good engineering oriented resources for this topic?

 

Thanks for helping me break this issue down!

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It seems that manufacturers are constantly pimping their new "improved" sportbike chassis with every model year. The trouble is that not much has been written about the issue of chassis flex leaving riders like myself questioning what the feelings they are getting from the bike truly are. Obviously, the issue is very complicated due to different components duplicating the feel of another component. The best example of this miscommunication with the bike I can think of was Loris Capirossi telling his crew he was losing the rear only to discover that the problem was actually overly flexible foot pegs sometime last year. Breaking down the movements of the chassis we are left with lateral flex, torsional flex, and vertical flex (If I remember correctly). In both overly stiff and overly flexible chassis configurations what is the rider feeling with respect to each axis of chassis flex assuming all the other components are working perfectly? Also, what is happening to the bike? What chassis rigidity factors influence the handling of a bike to make it handle with precision?

 

So far I've been able to figure out that a chassis with a high degree of vertical rigidity maintains the steering geometry and allows for the suspension to operate as intended. I have no idea what that makes the rider feel or what a chassis that had zero vertical flex would handle like. I've also gathered that lateral rigidity reduces the amount of feedback available to a rider in a progressive manner to the point that a bike with too little lateral flex would lose traction unexpectedly. Will too much lateral flex make the bike feel like it's pitching side to side much like losing rear traction? What torsional flex feels like is a mystery to me. Don't suppose anyone knows of good engineering oriented resources for this topic?

 

Thanks for helping me break this issue down!

 

Wow. OK. Cool. This will be a welcome challenge... lol. This isn't a subject I know much about or that I've given much thought to, although I have read that even the major factories chase their tails over it. I will tell you what I know and try to do some research on it to form some discussion. From what I understand of it though, it is something of a black art.

 

In a nutshell, although a perfectly stiff frame/chassis would seem to be desirable, ultimately, unlike a race car, there is such a thing as "too stiff" for the motorcycle chassis. As I understand it, part of this fact is due to the shortcomings of the telescopic front fork suspension. And moreover the inability of a motorcycle suspension in general to operate efficiently while leaned over, ie. it can't react to bumps in the road when it is sideways. Obviously, the details of where and why the chassis needs to flex, ie. near the headstock and swingarm pivot, as opposed to the engine cradle or spars which need to be stiffer, or how to brace the swingarm for torsional rigidity while allowing some lateral or horizontal flex is a huge subject far beyond my own experience or knowledge at the moment. However...

 

If I recall correctly, chassis flex first became a big issue in the seventies when big motors (like the Kawasaki Z1, 900/1000/1100 or the H1/H2 two-stroke triples) became so powerful that they literally twisted the old tubular frame bikes up in knots. Chain pull would cause the swing arm to actually twist in its pivot and the frame with it. Sometimes the chassis would actually bend and stay bent, but, more often, it would wind itself up like a spring and release that energy in an uncontrolled fashion. (Just ask the guys who raced them... like Eddie Lawson, Wes Cooley or... ahem.. Keith Code). So, stiffer frame/chassis became the goal. However, frame technology, materials and design have come so far that today's chassis/frames can be so light/stiff that they actually start to vibrate or oscillate allegedly causing front end "chatter" or perhaps masking feedback of what is happening at the pavement. Or whatever else riders complain about before they begin removing engine mounting bolts from their frames... like Colin Edwards allegedly did his last year of WSB.

 

Using Google, I found some interesting articles like this blog:

 

http://firstsynn.blogspot.com/2007/04/moto...ame-primer.html

 

Here is an interesting "engineering oriented resource":

 

http://www.dim.unipd.it/lot/HTML%20flexmot...%20Madrid1.html

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It seems that manufacturers are constantly pimping their new "improved" sportbike chassis with every model year. The trouble is that not much has been written about the issue of chassis flex leaving riders like myself questioning what the feelings they are getting from the bike truly are. Obviously, the issue is very complicated due to different components duplicating the feel of another component. The best example of this miscommunication with the bike I can think of was Loris Capirossi telling his crew he was losing the rear only to discover that the problem was actually overly flexible foot pegs sometime last year. Breaking down the movements of the chassis we are left with lateral flex, torsional flex, and vertical flex (If I remember correctly). In both overly stiff and overly flexible chassis configurations what is the rider feeling with respect to each axis of chassis flex assuming all the other components are working perfectly? Also, what is happening to the bike? What chassis rigidity factors influence the handling of a bike to make it handle with precision?

 

So far I've been able to figure out that a chassis with a high degree of vertical rigidity maintains the steering geometry and allows for the suspension to operate as intended. I have no idea what that makes the rider feel or what a chassis that had zero vertical flex would handle like. I've also gathered that lateral rigidity reduces the amount of feedback available to a rider in a progressive manner to the point that a bike with too little lateral flex would lose traction unexpectedly. Will too much lateral flex make the bike feel like it's pitching side to side much like losing rear traction? What torsional flex feels like is a mystery to me. Don't suppose anyone knows of good engineering oriented resources for this topic?

 

Thanks for helping me break this issue down!

 

Wow. OK. Cool. This will be a welcome challenge... lol. This isn't a subject I know much about or that I've given much thought to, although I have read that even the major factories chase their tails over it. I will tell you what I know and try to do some research on it to form some discussion. From what I understand of it though, it is something of a black art.

 

In a nutshell, although a perfectly stiff frame/chassis would seem to be desirable, ultimately, unlike a race car, there is such a thing as "too stiff" for the motorcycle chassis. As I understand it, part of this fact is due to the shortcomings of the telescopic front fork suspension. And moreover the inability of a motorcycle suspension in general to operate efficiently while leaned over, ie. it can't react to bumps in the road when it is sideways. Obviously, the details of where and why the chassis needs to flex, ie. near the headstock and swingarm pivot, as opposed to the engine cradle or spars which need to be stiffer, or how to brace the swingarm for torsional rigidity while allowing some lateral or horizontal flex is a huge subject far beyond my own experience or knowledge at the moment. However...

 

If I recall correctly, chassis flex first became a big issue in the seventies when big motors (like the Kawasaki Z1, 900/1000/1100 or the H1/H2 two-stroke triples) became so powerful that they literally twisted the old tubular frame bikes up in knots. Chain pull would cause the swing arm to actually twist in its pivot and the frame with it. Sometimes the chassis would actually bend and stay bent, but, more often, it would wind itself up like a spring and release that energy in an uncontrolled fashion. (Just ask the guys who raced them... like Eddie Lawson, Wes Cooley or... ahem.. Keith Code). So, stiffer frame/chassis became the goal. However, frame technology, materials and design have come so far that today's chassis/frames can be so light/stiff that they actually start to vibrate or oscillate allegedly causing front end "chatter" or perhaps masking feedback of what is happening at the pavement. Or whatever else riders complain about before they begin removing engine mounting bolts from their frames... like Colin Edwards allegedly did his last year of WSB.

 

Using Google, I found some interesting articles like this blog:

 

http://firstsynn.blogspot.com/2007/04/moto...ame-primer.html

 

Here is an interesting "engineering oriented resource":

 

http://www.dim.unipd.it/lot/HTML%20flexmot...%20Madrid1.html

 

Thanks for taking a stab at it! This topic is obviously pretty complicated and I'm curious how one begins learning about these things without the benefits of an engineering degree or the desire to get one for the sake of a hobby. I found another resource for this in case you were interested. I'm not sure about the credentials of the guy who wrote the book because he throws out some really wild ideas but it seems a pretty good place to start.

 

The link to his book is as follows: http://www.tonyfoale.com/reviews.htm

 

I would be happy to hear any additional comments if other people care to chime in. The subject just plain fascinates me.

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Thanks for the link. Tony's name is certainly well respected when it comes to information on motorcycle chassis. Unfortunately I have not had much time to devote to anything outside work and sleep lately. Perhaps I can do some reading over the holiday weekend.

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Very interesting. Now I understand why Ducati is testing a Carbon Fibre frame on the GP09 bike and talking about laying the fibres to make it stiffer in one direction and more flexibile in another.

 

SHHH!!!

 

(haha)

 

that's great. i had no idea ducati were doing that, but, it certainly makes sense from that point of view now, eh?

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