Author Topic: The Crankshaft Story...  (Read 4543 times)

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Offline CrouchyUK

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The Crankshaft Story...
« on: November 16, 2012, 06:35:18 PM »
We were doing the “balancer in” or “balancer out” argument on here a while back?  Whilst we were discussing this, it just didn’t seem to be logical that some owners swore blind that they had removed the balancer and found absolutely no difference in the levels of engine vibration.  After all, whatever effect a balance weight whizzing around at crank speed might have- removing it must make a difference- pure physics.  And testers did say the original TX was a very smooth runner.

To recap, these engines are 180 degree twins- the idea being that each piston cancels out the primary vibration due to the other.  But you are left with the secondary vibration that’s attempting to “roll” around the axis of the crankshaft.  Less vibration, different frequency but still annoying.  So Yamaha then looked at how to reduce even this lesser vibration.  Pure design and no maintenance required- reduce the moment of the forces by placing the cylinders as close together as possible – it means the camchain gear ends up on one end of the engine, instead of the middle which is why the TX had this arrangement.  Then finally you dream up a counterbalance system to balance out the rest.  You don’t want to put it down in the engine like the TX750- you learnt that lesson!  So you put it up behind and above the crank out of the way.  Of course then you end up with a complex chain drive with a chain tensioning system that is also difficult to get to and has very little compensation for chain wear....

Then I read just one of the many reviews of the 500 I have, which claimed that for the XS500C Yamaha had “increased the flywheel effect by 10%” to try and cure the poor slow speed running that got slated in the TX and XS500B reviews.   Now a lot of crappy slow speed running was down to the Keihin carbs, which were just as bad in the Honda CB500T, but some of the problem must have been the very light flywheel of the original bikes.  Yamaha raved about the small 180 firing interval between the cylinders whilst completely blanking the fact that after that, it’s a long 540 degrees before you get a bang again to keep it all moving.  So you need a nice heavy flywheel and/or plenty of momentum (revs). Not exactly what we got with a lightweight crank and a bike that needs to run slow in traffic and an engine that needs to idle....

So Yamaha changed the crankshaft- almost on the quiet.  The part number for the pre-XS500C crankshaft was 371-11411-00-00, while the part number for the C, D and E models was 1A8-11411-00-00.  They even updated the parts books so only the later crank was available for any of the bikes.  But from what I can see the balancer shaft part number stayed the same throughout.  So if the crank weight only got altered- vibration levels must have changed....

Onto the big weigh in!  I have a couple of the 371 cranks out of my bikes, and I also bought a 1A8 crank and rods as a replacement as both my cranks are scrap.  Or at least scrap if I ever wanted them to drive a balancer again.  I put them both on the scales:-



So above is the 371 crank at 8610 grams- now to see what the 1A8 crank comes in at:-



..9500 grams so 890 grams heavier!  And that's 10.3% heavier than the 371 crank so the information on that was correct.  Everything else about the cranks are the same dimensionally, so they are a straight swap.  They also apparently hardened up the balancer drive cog- too late for mine unfortunately.

Even on the scales you can see the 1A8 is a lot bulkier, but apart from the main crank webs being thicker, Yamaha did seem to "increase the flywheel effect" by adding weight around the crank big ends- no need to point out which crank is which:-



So if the balancer shaft stayed the same, then the C's, D's and E's probably had slightly higher vibration levels than the early bikes.  But ran better through slow traffic and probably idled better.  And if you wanted to rip the balancer out it would not be as missed as if you pulled it out of a TX.  So nobody was wrong in the original argument and everybody was right.

BTW, in the last photo top right is a nice view of a balancer driver sprocket worn away to almost smooth (Check that chain tension!).  And you may just see that the 1A8 crank has wiped the big end journal big time.  That is 3 boat anchors I have now....  :'(
1975 Honda CB200T (2) - 1975 Yamaha XS500B (2) - 1976 Yamaha XS500C
1977 Yamaha RD400D - 1977 Yamaha XS500D - 1978 Honda CB550K - 1980 Honda CB400N
1980 Honda CB900FA - 2000 Kawasaki ZX6-RG2 - 2002 Suzuki GSX-R1000K2

Offline steve

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Re: The Crankshaft Story...
« Reply #1 on: November 16, 2012, 06:36:39 PM »
Great info, crouchy! 

Offline c3powil

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Re: The Crankshaft Story...
« Reply #2 on: July 03, 2013, 07:28:01 PM »
Maybe this is also part of why my bike has idling issues. Thanks for the info! Anyone want to help me do a crank swap?  ;D

Offline jimmer

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Re: The Crankshaft Story...
« Reply #3 on: July 03, 2013, 08:12:19 PM »
This may be an older thread,but some how I missed the pictures.Great comparison and research 8)

Offline CrouchyUK

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Re: The Crankshaft Story...
« Reply #4 on: July 03, 2013, 08:53:46 PM »
It was buried down in the topics jimmer- I thought I would make it sticky so the new members could see it easily.  We seem to be on another round of balancer discussions so almost need some of this stuff as FAQ's!
1975 Honda CB200T (2) - 1975 Yamaha XS500B (2) - 1976 Yamaha XS500C
1977 Yamaha RD400D - 1977 Yamaha XS500D - 1978 Honda CB550K - 1980 Honda CB400N
1980 Honda CB900FA - 2000 Kawasaki ZX6-RG2 - 2002 Suzuki GSX-R1000K2

Offline nigecollier

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Re: The Crankshaft Story...
« Reply #5 on: October 30, 2013, 06:13:25 PM »
balance shafts ! just more junk spinning round, ive not noticed any vibration on my xs race bike worth worrying about ? I actually stopped racing my rd430 because after 4 laps I could not feel my hands due to the high frequency vibes, must admit ive never run the xs with balance shaft fitted?

Offline TXRAT

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Re: The Crankshaft Story...
« Reply #6 on: April 23, 2014, 10:05:08 PM »
 My backup motor did not come with any starter or balancer chains on it, Just remove all the bits that might fall off from vibrating?. Hey friend of mine has a 400ex s with the 180 crank in it and no balancer and it doesn't vibrate too much.  I can't take all that stufor shor should I take out all of that I can't take all that stuff out. I can only take out the balancer shaft then when that let the right side bearing come loose or is that  held by a C clip? will some one in the know send me to the right place or let me know whats up?? ok to ditch chain and it done or pull tentioner too? it is a 371 well they are. i looked and there are the short adjust kind any how so what is exacty the deal on it? my other motor has one, of each btw. look to be in reusable shape. olny one starter thought.

Offline TXRAT

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Re: The Crankshaft Story...
« Reply #7 on: May 18, 2014, 10:40:33 AM »
i did refit my balacer chain. as much vibs as a 400 from 81 rides great at 55 in 4th gear.wont idle worth a well ya know. light crank+old style keihen chrome dome carbs!! lol!!