OzVMX Forum
Clubroom => Tech Talk => Topic started by: evo550 on December 16, 2013, 09:47:16 pm
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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?
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Evo the final line of that waffling bloated pile is the crux " 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."
Follow that as your guide and enjoy your hybridizing*. Cheers Tim754
*hybridizing yep that is the actual word :)
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If you don't have the original engine to compare how do work out if you have it in the right spot, cheers Andrew
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Putting the CS as close as possible to the swing arm pivot is a good start. This will be dictated by the rear mounting holes of the motor. When your'e building a Hybrid, its most likely that you will have to make new rear engine hanger anyway. Try to get the motor as far back as possible and keep the Counter Shaft center with the swing arm pivot.
If the CS is too high, the bike will want wheel spin everywhere and not provide any useful drive out of corners etc. If the CS is too low, the bike will want to wheel stand everywhere.
Negative travel is when you rev the bike out of a corner or from landing a jump etc. the forces of the chain will make the rear suspension compress, thereby losing some control and travel of the suspension. Most LTR bike squat simply by revving the motor without riding the bike. Try it. Start your bike and stand beside it and give the bike a good rev. Watch what happens to the rear of the bike. It will more than likely squat. This is caused by the inertia inside the engine cases. There are a lot of factors at play for getting engine placement just right. If at all possible, try your best to get the engine on the same angle from the CS position as what the original bike was. That way, you will at least be in the ball park...
And have fun "Hybridizing" I know I do.
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Yeah, I think the bloke was trying to tell you (In a convoluted way) to get the swingarm and countershaft centrelines as parallel and close to each other as possible. It's basic engine swap 101.
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Thanks Gents...
Negative travel, you learn something new everyday. Trust an American to make a short story long....
Hybridizing, why didn't I think of that.. ;D
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I know its hard to tell from photo's but am I even close to the right position, that is the best I could get the yamaha enginge to sit in the zundapp cradle without hacking up the frame to much, its all welded in so its nearly to late to turn back but it could still be done if its all wrong cheers Andrew
(http://i1058.photobucket.com/albums/t418/mavman4200p/DSCN0315_zps219ca66e.jpg)
(http://i1058.photobucket.com/albums/t418/mavman4200p/DSCN0311_zps30b356d8.jpg)
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Thanks for the Pic. It looks like the CS is about an inch (25mm) too high. You might be able to drop the rear of the motor a bit more but to me it looks like you will need to do a cut and shut on the lower frame rails.
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You can't look at just the swing arm bolt and the sprocket in isolation, you have to look at the line through the axle and swing arm bolt then look at the sprocket location, looking at the pic below if you ignore the axle sprocket looks above the swing arm bolt, but draw a line through the axle and swing arm and you quickly see the sprocket is slightly below that line.
(http://i695.photobucket.com/albums/vv319/peddrotzr/TZR%20535%20Bike%20build/2009_0705finalbuild20015-1.jpg)
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I should have put a larger picture of the full swing arm it lines up like the one in lozzas picture when I get time later tonight I'll put one up for closer scrutiny
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I'm interested in this thread, hope to see & learn more.
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At what point in the suspension travel should the CS, SA & rr axle line-up? If I look at the arrangement on my KX250A5, with unladen sag the CS is miles below the line. With my KDX/KX conversion he CS is closer to the SA pivot & closer to lining-up with the rr axle line...... Hmmm :-/
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A road bike is a totally different animal. In road applications, the manufacturer/designer want the rear of the bike to squat a bit so all that horsepower gets to the ground without wheelspin everywhere.
Imagine if you will, all the forces of the chain when the bike is under acceleration. As the chain gets pulled by the engine sprocket, it tries to straighten in the shortest line, thereby "lifting" the rear wheel as it straightens out. This is squatting. Racers control how the bike accelerates out of corners by adjusting ride height and preload on the spring. Some riders like wheelspin, others don't.
With a dirt bike however, all the forces are best kept neutral so the suspension can do its job without having to fight against what the chain wants it to do. Have a look at the BMW 450 dirt bike. The countershaft sprocket actually rotates around the swing arm pivot! That is as neutral a position as is possible.
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My cs sprocket swingarm pivot and axle are all in a straight line when the suspension is in its mid travel which is how I was told I should set it up is this correct cheers Andrew
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Mavman does state above the widely accepted engineering practice ;) Worth following for sure.
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My cs sprocket swingarm pivot and axle are all in a straight line when the suspension is in its mid travel which is how I was told I should set it up is this correct cheers Andrew
That sounds about right to me....the chain won't be fighting against the suspension on compression or rebound.