Evo550 - there are varying viewpoints on CS / SA center / rear axle center orientations, and swingarm angle set ups. A mm or two can make a massive difference in how the rear end works, and the entire handling of a bike. That's why you see adjustable SA pivots on Road Racers ( interestingly, it appears Honda, with their new "production Moto GP" bike, are not giving buyers the option of that tuning feature - well, they have tended to take quite some time till they offered it to even satellite teams, on 'lease bikes') But a good set up, sure as hell isn't the chain torque being delivered into the system with Pro Squat from the very start of the rear travel, especially on a 12" + travel dirt bike. Also, things like the positioning of the chain rollers, top and bottom, can make major differences to how a rear end works. Look to the varying positions of rollers - late model YZFs have the top roller, quite rearward of the SA pivot, but, higher up than other bikes that have it forward. For quite a few years, a good change to many Honda's was to change the sizes of the rollers - top or bottom, or both, to change the chain torque effect on the rear end. It's also the sort of thing more 'clever' mechanics / tuners / crew chiefs will experiment with. For example, you could counteract the Pro Squat of chain torque, through an earlier contact point of an top chain roller - the contacting of the chain against the upper chain roller, tends to stiffen / through extension forces , the rear end. Mind you, 'Pro Squat" through the chain pulling the swingarm 'up', 'stiffens' the suspension, through torquing forces. As a very, very rough median point, you'll generally find most dirt bikes - well most single pivot Motorcycles, which pretty much means all , other than Lawills rear end / Boyesen rear ends / concentric SA pivot rear ends - will have the CS / SA/ and rear axle being close to - and it can be either side of, depending on a designers / manufacturers idea of what's good - being 'inline when the sag, or negative travel is set.
As a side note - much like the various aftermarket engine repositioning kits most people will be aware of for the YZF 450s engine at it's contact point with the swingarm bolt - swingarm pivots are routinely 'moved' by non concentric swingarm pivot bearing housings within the swingarm ,. That is especially useful when racing rules prevent one from modifying the frame, and when teams don't want such a change to a bike to stand out like the 'proverbials'. And, it's not just Yamaha that engine positions are 'played with' by teams. Just have a squiz at the different bolt centre points on various front engine mounts of teams bikes. It, engine repositioning, and Countershaft repositioning, is a very common thing to be done, and not that hard to do. Aprilia RSV 'R's ( perhaps the wrong labelling by me), have, as a std feature, a moveable engine position. I think a few of 'R', 'RR' etc top end production Superbikes, have that as an additional feature. Many Std Superbikes, have Swingarm pivots with blocks that can be changed for adjustable / changeable ones.
As I've written here before, I learned a valuable lesson about engine positioning / chain torque effects on a bikes suspension and handling, with one of my early 'hybrid' projects, in the very early 80s - when reasonable CS/ SA pivot orientations had been 'cracked' by most designers. My engineer dad, held his tongue, and let me learn from my mistake. It was only a matter of about 2mm rise in the height of the CS center to the SA center on an RM / XR hybrid, but it was enough to 'spoil' the phenominal working of the Full Floater rear end. I rectified it quickly, and have never made the same mistake since. It's why I so encourage people to put the placing of the CS centre, as close as is possible to the original positioning of the std engine - unless you have very good reasons for doing otherwise.
So this came from another forum, where someone was critical of someone elses engine swap, and in particular the countershaft sprocket position, I asked "so where should the CS be located" and above is the response.....
Can anyone decipher it? and let me know what negative travel is?
