270 degree crankshaft

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Dances with Shrapnel said:
Adding to my comments above regarding introducing other problems with a rephased crank and cam on a Norton twin, I have heard there may be traction benefits upon acceleration with a rephased crank. This would be applicable in dirt racing, dry lake bed or (playa) land speed trials or on asphalt where you are leaned way over, at the margins of your traction and pouring on as much throttle as the system will handle.
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My friend tells me that the way the rear tyre deforms and restores itself under acceleration is important. He was a top rider and has rarely crashed, so he might know about such things. All I know is that my bike has a standard crank and the Battlaxe rear tyre never lets go, even when I wick it up too much when cranked over - probably under-powered - needs a 270 degree crank .
 
mschlake21 said:
I'll throw my 2 cents in, just based on my experience with the rephase process. I bought a junk Yamaha xs650 2 years ago for like 125 USD. Got it running and used it around town during my first year of law school. Vibration at interstate speeds was very bad: 35 min and your hands were numb, an hour and you can't feel the clutch lever anymore. no isolastics obviously but it had clip ons so I'm sure that didn't help. it was fun but started running pretty badly, lots of burning oil, and i ended up rebuilding it. somewhere along the way, i decided to make the motor a project and ended up with a pretty hopped up rephased yamaha.

first thoughts: when i got back on it and year and change later, it was so much faster than before that i only really noticed the jump in power from unrelated engine modifications. first interstate ride i remember thinking the vibrations were still pretty bad and i know i was frustrated. i did notice some difference but i remember thinking that the vibrations didnt change

Last month, i did 500 miles with two friends, one with a 70' bonneville and one with another xs650. we trade off during rides often and i have to say, there is no comparison. my bike has WAY lower vibes. that being said, it is pretty expensive to do, and if i hadn't ridden a bike with a standard crank after, i would have still been thinking mine was bad. it does help, but vibes are not gone. i see people all the time on the 650 forums claiming it eliminates vibration which is total nonsense.

I've got 8500 miles on the rephrase motor without any issues, so it seems to hold up, but time will tell
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Got a question - when you rotated the XS650 crank to make it 270 degree, did you rebalance it to compensate for the reciprocating weight at a particular operating rev range ? When the crank throws are at 180 degrees, the rods and pistons tend to balance each other, so the balance factors might be quite a bit different.
 
A 270 degree crank introduces a rocking couple to the engine's balance that the isolastics are not designed to handle.

I have used both a 270 and 180 degree crank in an isolastic Commando and although the formulas said the engine was vibrating less, the vibration felt in the bars and footpegs was definitely increased. They sounded cool though!

I would say there is a benefit in a solid mount engine. Jim
 
I assume that everyone knows that Sir Eddy's 500cc LSR bike has a 270 degree cranks shaft.
I can say that at 9,800 to 10,000 RPM this motor had minimal vibration and pulled strong all the way up to redline.

The basic thought behind a "quartered" crank engine (and design advantages associated with it) is to never have both pistons at maximum or minimum velocity at the same (as you all know, this is exactly what DOES happen in a 360 degree upright twin). More specifically, the idea is to have one piston moving at MAX velocity while the other one is at MIN (zero) velocity, in order that inertia about the crankshaft is preserved. In other words, the first swiftly moving piston/conrod combination helps "pull" the second piston/conrod combo through it's deceleration & change in direction, a point at which it has little or no inertia. In effect, the quickly moving half acts as a inertial flywheel for the other, and vice versa several times per revolution, resulting in relatively constant inertia (stored or potential energy) in the rotating parts in the big end of the engine. All this allowed us to drastically reduce the counter weight and lighten the crankshaft.

Many people assume that a 90 and 270 degree are two ways of saying the same thing, but really, they are two entirely different setups. The difference is really not in the separation of the crank pins, so much as the timing of the valve train, which results in power strokes that are separated by either 270/450 degrees, or 90/630 degrees.

The true 270 degree crank defines power strokes that are separated from each other by 270, then 450 degrees. Imagine the #1 piston @ TDC at the beginning of the power stroke, while the #2 piston is 270 degrees behind it, halfway through it's own intake stroke. Follow this thing around, and 270 degrees later, the #2 piston is @ TDC at the beginning of it's power stroke. Following the progression of the #1 piston, we find it is now halfway through it's exhaust stroke, a full 450 degrees away from the beginning of it's next power stroke.

Now, take the same crank, and change the valve timing setup so that when the #1 piston is @ TDC at the beginning of the power stroke, the #2 piston is not far behind, halfway through it's compression stroke. Only 90 degrees later, the #2 piston is experiencing the beginning of it's own power stroke, while the #1 piston is only halfway through it's power stroke. Overlapping power strokes, the beginnings of which are only separated by 90 degrees this time. The next one comes 630 later. Same crank, really, only different orientation of SSBB from #1 cylinder to #2 cylinder.

I can say that Sir Eddy's Rocket set records with a 270/450 setup and we will go back next year and give it our best shot to push it up again.

270 degree crankshaft

270 degree crankshaft
 
acotrel said:
All I know is that my bike has a standard crank and the Battlaxe rear tyre never lets go, even when I wick it up too much when cranked over - probably under-powered - needs a 270 degree crank .
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Its (hopefully) redundant to point out here that a 270 degree doesn't/can't give any more power.

Its the cam/compression/ports/tuning that gives any power gains, the rephased crank just gives a different power delivery and, possibly, the potential for a few more revs - which could give a bit more power.
 
A 750cc commando motor which revs safely to 9000 RPM gives more power than one which revs dangerously to 7000 RPM - what is the potential competitive difference if the power is delivered in a way which is more usable and there is more of it ? I suggest the difference between a fast commando with a standard crank and a slow one, is probably about 5 BHP, if we don't consider torque and handling effects.
 
acotrel said:
A 750cc commando motor which revs safely to 9000 RPM gives more power than one which revs dangerously to 7000 RPM -
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Quite so.

But didn't you quote that you have the stock 850 cam in yer seeley.
Which is all done in the power dept by 6500 rpms...
 
I use the 850 cam but advanced 12 degrees to compensate for the two into one exhaust. The motor powers straight through the top of the safe rev range. It never feels as though it has run out of puff. In fact it is a worry because I often see 7,500 RPM as I change up through the close ratio box. When I originally set up the cam timing I suspected that Mr Norton might have detuned his Commandos slightly to comply with the noise laws and make the bike better as a commuter. Because they will certainly cop the inlet valves opening at 65 degrees BTDC. What is a bastard is the exhaust then opens before 90 BBDC so the exhaust is LOUD ! However the motor delivers massive torque and with good gearing and methanol fuel, is pretty fast. The paint on the top end of exhaust pipe only lasts a few minutes.
Have you ever tried changing your cam timing slightly to see what happens ? You need a way of changing it a few degrees at a time. I've got three keyways in the cam sprocket, broached at random.
(How many race cams are sold by racer guys who will tell you the very best timings to use when you set it up in your motor ? ) I suggest you always have to optimise systematically by trial and error. I'd never buy a race cam and simply set it up as recommended without moving it backwards and forwards. All motors are different from each other. Even with Speedway Jawas, you can build two identical motors and one will be faster than the other.
 
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