blarson

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.

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!

Taking a few pictures may be worth a thousand words. I'll have to see if I can get my girlfriend to sit at a good corner and do that. In the meantime..... When this pitch is happening my upper body is leaning to the inside of the corner but I'm keeping my rear in the seat. I'm generally holding onto the tank with my knees to take pressure off the bars. I still get the same feeling when I'm hanging all the way off as well. I set my body position before corner entry. Clutch is fully engaged as well. In fact, I'm using just a slight (maybe) 10% throttle opening all the way to around the apex. I do that because the throttle response on these bikes is horrid so I found that holding a steady 10% throttle reduced drive lash when I get on the gas around the apex. I usually apply slight pressure on the outside peg ala' the pivot steering introduced in twist of the wrist 2. In short, it doesn't feel like I'm pushing the bike underneath me.

The longer I've been riding the more I've begun to take notice that the more rapidly I change direction the faster the rear end of my bike pitches to the side. This accompanied by a sudden tightening of the line I'm taking through the corner without any additional steering input. This characteristic displays itself only when I change direction quickly and with a fair amount of lean angle and sometimes manifests with the additional feeling of the rear end tucking under (twisting feeling). None of these odd characteristics apply at modest (street legal) cornering angles. Obviously, the amount of variables that might be contributing to this steering characteristic are indescribable within the confines of this forum. However, I'd greatly appreciate if some of the more knowledgeable individuals would take a crack at some of the possible explanations. Unfortunately, the advice I receive will be the best bet I have for figuring this out as I cannot afford any riding school until I finish my bachelors degree and make some money. That being said, here are some details(and my life story) that may help with analysis. This bike in question is a 01' ex500 that is mostly stock in hardware except for the forks being filled within the last 2000mi with 20wt oil. The tires are have only about 2000 miles on them and less than a month old they do not display any cupping or deviation from the original profile. The front is a dunlop qualifier and never feels like it's pushing unlike the pirelli sport-demon tire I had on just before. The rear is a dunlop D208SM sized one up (130x70 to 140x70) from stock. I would have never up sized the rear tire except that doing so was the only way I could put a radial tire on the bike. With these tires the bike will hold at least 10 degree more lean angle without feeling upset compared to the bias ply tires on there before. I run high 20's-low 30's for pressure in the rear and changing tire pressure does not seem to change the aforementioned handling issue. Sag is set correctly. The rear shock is past it's prime leading me to believe that it is at least in some way responsible for the tucking feeling but does not explain the sideway pitching. This bike has been my daily driver for over a year but I won't rule out rider error. That being said, I have made every effort to isolate any action on the bars by holding the bike with my lower torso to the best of my ability. My body weight is around 200 pounds wearing full gear. I am also generally riding straight up and without hanging off although the problem is still there when I hang off as well. Off throttle application is as smooth as I can make it from near the apex of the turn. The bike generally reacts smoothly when I put on the gas mid corner. I don't feel the rear tire sliding nor am I laying down any rubber. My relatively (un)educated guess is that I am experiencing a high degree of chassis flex due to increased cornering loads from the modern tires. This explains to me why the line would tighten as well as why it would pitch to the side. I think the twisting or washing out feeling might be from a combination of mis sized tire and crappy shock and unrelated to the pitching. Ohh wise ones, what do you think? Sorry for the novel. Thanks for the advice.