Crank Phasing - Wrist Twisters
 
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post #1 of 15 Old 05-05-2011, 04:27 PM Thread Starter
(Quintus) Pilus Prior
 
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Crank Phasing

This fascinates me and it has to be the single thing that makes the triumph trips so powerful.

Am I right in assuming that the rod journals on the trip engines are phased 120 deg apart?

Also, The f-800 beemers have no crank phasing on their twins?

And wasn't yamaha playing with the phasing on their latest R-1

Talk to me oh great guru's of the strongarm.

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post #2 of 15 Old 05-05-2011, 05:52 PM
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What the hell is crank phasing? I take it that its more involved than just the degree spread?

Well, fire the engines! Spur this iron space-pony on!

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post #3 of 15 Old 05-05-2011, 10:16 PM
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Quote:
Originally Posted by The Shadow View Post
What the hell is crank phasing? I take it that its more involved than just the degree spread?
+1, i wasnt sure what it was either but didnt want to be the only one asking. so thanks for asking for me.

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post #4 of 15 Old 05-06-2011, 08:17 AM
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Quote:
Originally Posted by Lemonhead View Post
This fascinates me and it has to be the single thing that makes the triumph trips so powerful.

Am I right in assuming that the rod journals on the trip engines are phased 120 deg apart?

Also, The f-800 beemers have no crank phasing on their twins?

And wasn't yamaha playing with the phasing on their latest R-1

Talk to me oh great guru's of the strongarm.
I'm definitely not a guru ... but I found some info to the questions you asked ...

Crank phasing is the spread of the degrees of rotation of the firing pistons on a crankshaft .... it's how they make a smooth power delivery by spacing the power pulses out evenly by making the pistons balance their firing ... Every motor with more than one cylinder has a certain degrees of crank phasing (whether it be 180 degrees for straight twins or 90 degrees apart for a production v8)

Back in the old days, most v8 engines were alot smoother than the v6 engines .... because the v8 engines crankshafts had their pistons correctly phased out which led to a smooth power delivery ... some of the older v6 engines were just v8's with 2 cylinders lopped off the end which left the engines with a noticeable pulse in the power when they were running from missing two extra cylinders to balance the other 6 out (they didn't rephase the v6 crankshafts to smooth them out) ... todays v6's are phased to 120 degrees apart in rotation ...

The BMW F800S's motor had both pistons moving up and down and firing at the exact same time ... which made alot of vibration in the motor ... so BMW put a third connecting rod on the crankshaft that was connected to a balancer ...

And on the triumph triple motors, it would make sense to me that they are phased 120 degrees apart ... as the firing of three different cylinders 120 degrees apart would make power the whole 360 degrees of rotation of the crankshaft .... on a v6 motor you just have two triple cylinder motors running the same crankshaft (hence 120 degrees of crank phasing) ....

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post #5 of 15 Old 05-06-2011, 08:44 AM
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Basically what the the new Yammie crossplane crank does. Makes it a 1/2 V8. 1 cylinder firing every 90 degrees. Makes for a bitchin sound too. Wikipedia I think has some good internal drawings of the crossplane. Youtube has some good vids for the sound. Best sound since the V4.



and the new triumph twins have 270 phasing and sound awesome.

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post #6 of 15 Old 05-06-2011, 08:48 AM
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The newer edition R1's have had the crank phasing played with to make the power delivery smoother and more usable (even tho alot of ppl now hate the newer R1's because of how they sound and feel) ... the older R1 motors had a perfect 180 degree crank phase with the power pulses coming in regularly spaced .... the newer one has an irregular crank phasing ... it has a firing order of 270 degrees followed by 180 degrees followed by 90 degrees followed by 180 degrees which makes a longer pause between piston 1 and 2 firing and maked the 3rd and 4th pistons fire closer together than usual ...

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post #7 of 15 Old 05-06-2011, 02:12 PM Thread Starter
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YouTube - Crossplane Crankshaft tech specs

It's a little corny until it gets to the graphing of the power pulses.


BMW must have decided on zero phasing to help inertia carry the compressing piston up whereas if they're 180 out, one is firing while the other is compressing thereby negating some of the firing power.

Which would also explain why they chose to do the back and forth thing in their boxers which makes sense because you have gravity working against you more in a flat arrangement ? Does that sound right to you guys?

Where's JB when you need him.

CORRECTION: of my lingo. the f800 would have a 360 crank. which wierds me out even further. why cant I get my head around this?

of course the only engine I've actually been inside is an air cooled vw.

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post #8 of 15 Old 05-06-2011, 02:17 PM Thread Starter
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post #9 of 15 Old 05-06-2011, 07:51 PM
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Yea ... when the number one piston fires, it spins all the way around till it's at 270 degrees on the crankshaft rotation to fire the other piston ... it's supposed to try to sound like the 90 degree v-twins, ya know that "potato potato" sound .... but has the added benefit of not having each piston slinging out against each other ...

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post #10 of 15 Old 05-07-2011, 02:31 PM Thread Starter
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I would think a 90 deg twin would be smoother and more powerful (slightly) than a 60 or 45.

Thanks for talking with me, steve. I'm trying to understand why mfgrs choose their piston arrangements

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post #11 of 15 Old 05-07-2011, 07:33 PM
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My Super Duke has a 75 degree twin. That's all I got.

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post #12 of 15 Old 05-07-2011, 07:50 PM
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Quote:
Originally Posted by Lemonhead View Post
I would think a 90 deg twin would be smoother and more powerful (slightly) than a 60 or 45.

Thanks for talking with me, steve. I'm trying to understand why mfgrs choose their piston arrangements
Hey no problem man ... and technically the angle of the pistons on a "v" motor can range anywhere from 1 degree to 179 degrees (otherwise it would be an "inline" design at 0 degrees of angle or an "opposed" design at 180 degrees) ... but normally the smallest angle used on a vtwin would be around 42 degrees and ranges up to 90 degrees ...

On Vtwins that share a common crankpin, the smaller the angle of the pistons the better firing interval of the pistons but the also get poorer mechanical balancing with each other ... the larger the angle the better mechanical balancing due to the pistons acting as a counterweight for each other but they also end up with more uneven firing ... 90 degrees is a happy medium between inline cylinders and opposed cylinder designs ... Ducati uses a 90 degree vtwin, they just call it an "L" twin because of it's orientation ... this was done so the rear piston will be less obstructed to airflow from the front pistion ...

Quote:
Originally Posted by sbeau1960 View Post
My Super Duke has a 75 degree twin. That's all I got.
And a freaking awesome sounding one at that !!!! ....

... Never take life too seriously ... Nobody gets out alive anyway ...
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post #13 of 15 Old 05-08-2011, 10:34 AM
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very cool thread! learned alot here.

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post #14 of 15 Old 05-09-2011, 10:05 PM
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To add further confusion,
In four strokes each crankshaft journal does 720 degrees rotation to fire once. The balance of the motor is worked out on eveness of firing position depending on the number of cylinders and piston speeds.
Pistons actually slow and come to a complete stop at top and bottom dead centers, this means some pistons are stopped whilst others are at maximum velocity, not the best thing for perfect balance, or minimum stress on the motor.
Think of it this way, a 2 cylinder motor goes bang, bang, pause, bang, bang, pause etc etc
Now lets hear from an expert.

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post #15 of 15 Old 05-10-2011, 12:27 PM Thread Starter
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Good point, Rodger. That may be the reason not all cranks are phased like the yammie. Too much torsional loading, the death of the straight eight.

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