Author Topic: Crankshaft counterweights related to secondary balance  (Read 454 times)

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Offline corky

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Crankshaft counterweights related to secondary balance
« on: November 06, 2020, 10:32:59 am »
hey guys,

I currently have Lindskog Dynamic Balancing doing a crankshaft for me (cb750), and I was going to have him lighten it 10%.  They asked me about the rod and piston weight compared to stock, and how lightening the crank would affect vibration at higher rpm.  Before this phone call, i was always under the understanding that on an inline 4 cylinder (2 up, 2 down) crankshaft, bob weight didn't matter so much as long as they balanced each other out.  This is true for the primary balance, which can be balanced.  Then i started looking into what secondary balance is, and how the counterweights have an effect on it.  Also noted is that inline 4's have more secondary imbalance than an engine where the pistons are out of phase with each other.


i would assume different engines are effected differently with this issue, and wondered for anyone that has lightened crankshafts, if they have noticed vibrations at higher rpm's, and if this is something that can be hazardous to the crankshaft.  I seem to recall that a lot of the performance cb750's typically used factory crankshaft weight.  My primary basis for lightening the crank was to have a motor that rev's a little more freely.

Lindskog recommended I weight the original rod/piston setup and compare it to what I am going to use.  I will figure this out over the weekend, and am thinking I will do whatever Linkskog recommends, as they have a lot more experience than what I have.

I always appreciate this forums input and experiences.

Thanks,
Daniel

Offline Old Scrambler

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Re: Crankshaft counterweights related to secondary balance
« Reply #1 on: November 06, 2020, 12:17:47 pm »
Crank-weight vs. piston/rod weight is controllable to the degree of application; changing the ratio will change how the motor performs throughout the rpm-range.  Do you want more pull at lower and mid-range.......usually a lighter crank, but not too light.......top-end performance, stay close to stock with lighter rods/pistons.......hint, aftermarket piston-pins may be significantly heavier than OEM or performance-minded aftermarket units. Back in the '60s one of the original HRC motor-builders recommended a 71/29 ratio for good corner-pull in upper mid-range of rpms..........Today ???
Dennis in Wisconsin
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Offline Captain

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Re: Crankshaft counterweights related to secondary balance
« Reply #2 on: November 07, 2020, 01:11:20 am »
 You have nothing to be concerned about as I've been modifying the SOHC & DOHC cranks for 40 years and supply to others around the world including endurance race teams.   Current customer cranks are 8kg finished (OEM is 12+ kg) that's over a 33% reduction and these will out-perform and are more durable than the heavy cranks they begin life as. One of the reasons is that as you lighten a crank you push the naturally occurring harmonic higher up the RPM range as well as reducing its intensity. Go light enough and you position "above" your operating ceiling so there are benefits here that many do not realise exist. In the case of the SOHC cranks you will be rewarded across the entire operating range (other than idle) with increased acceleration and quicker handling due to the much lower moment of inertia.
 All that is required is to make sure that you maintain 100% (sometimes in special conditions 101%) primary balance of your bare big end/counterweight and the secondary coupling is handled by the adjacent cyl and the structural integrity of the webs and interconnecting main.
 This is all well proven in the real world and has been for decades.
 Feel free to be more aggressive with your attempts on your crank but avoid the area around the drive gear.   

 Captain 
« Last Edit: November 07, 2020, 01:17:46 am by Captain »

Offline MRieck

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Re: Crankshaft counterweights related to secondary balance
« Reply #3 on: November 07, 2020, 01:53:13 pm »
hey guys,

I currently have Lindskog Dynamic Balancing doing a crankshaft for me (cb750), and I was going to have him lighten it 10%.  They asked me about the rod and piston weight compared to stock, and how lightening the crank would affect vibration at higher rpm.  Before this phone call, i was always under the understanding that on an inline 4 cylinder (2 up, 2 down) crankshaft, bob weight didn't matter so much as long as they balanced each other out.  This is true for the primary balance, which can be balanced.  Then i started looking into what secondary balance is, and how the counterweights have an effect on it.  Also noted is that inline 4's have more secondary imbalance than an engine where the pistons are out of phase with each other.


i would assume different engines are effected differently with this issue, and wondered for anyone that has lightened crankshafts, if they have noticed vibrations at higher rpm's, and if this is something that can be hazardous to the crankshaft.  I seem to recall that a lot of the performance cb750's typically used factory crankshaft weight.  My primary basis for lightening the crank was to have a motor that rev's a little more freely.

Lindskog recommended I weight the original rod/piston setup and compare it to what I am going to use.  I will figure this out over the weekend, and am thinking I will do whatever Linkskog recommends, as they have a lot more experience than what I have.

I always appreciate this forums input and experiences.

Thanks,
Daniel
Daniel
How did you find out about Lindskog? Are you working with Mike?I have used them for 30 years. Funny thing is we never lightened modern 600cc cranks used for roadracing. Torque was better using a stock weight balance crank. The 750 can stand some lightening because of the heavy rotor hanging off the end of the crank (which also helps dampen vibration). I installed a lightened crank in my FJ11200 and pulled it after a couple of months. Midrange torque wasn't good forcing downshifting. Stock, balanced crank produced much better roll on at highway speeds.
Owner of the "Million Dollar CB"

Offline simon#42

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Re: Crankshaft counterweights related to secondary balance
« Reply #4 on: November 08, 2020, 02:42:32 am »
not a great fan of lightening cranks . the results look good on the dyno but lap times seemed to suffer . when we ran 250 honda gp bikes we managed to get , at great expense what is called an A  kit which makes the production racers closer to the works bikes . as well as an ignition , cylinders , heads and pipes there was a new much heavier crank . the bike was much better so i guess honda did it for a reason .

Offline corky

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Re: Crankshaft counterweights related to secondary balance
« Reply #5 on: November 09, 2020, 05:56:55 am »
hey guys,

I currently have Lindskog Dynamic Balancing doing a crankshaft for me (cb750), and I was going to have him lighten it 10%.  They asked me about the rod and piston weight compared to stock, and how lightening the crank would affect vibration at higher rpm.  Before this phone call, i was always under the understanding that on an inline 4 cylinder (2 up, 2 down) crankshaft, bob weight didn't matter so much as long as they balanced each other out.  This is true for the primary balance, which can be balanced.  Then i started looking into what secondary balance is, and how the counterweights have an effect on it.  Also noted is that inline 4's have more secondary imbalance than an engine where the pistons are out of phase with each other.


i would assume different engines are effected differently with this issue, and wondered for anyone that has lightened crankshafts, if they have noticed vibrations at higher rpm's, and if this is something that can be hazardous to the crankshaft.  I seem to recall that a lot of the performance cb750's typically used factory crankshaft weight.  My primary basis for lightening the crank was to have a motor that rev's a little more freely.

Lindskog recommended I weight the original rod/piston setup and compare it to what I am going to use.  I will figure this out over the weekend, and am thinking I will do whatever Linkskog recommends, as they have a lot more experience than what I have.

I always appreciate this forums input and experiences.

Thanks,
Daniel
Daniel
How did you find out about Lindskog? Are you working with Mike?I have used them for 30 years. Funny thing is we never lightened modern 600cc cranks used for roadracing. Torque was better using a stock weight balance crank. The 750 can stand some lightening because of the heavy rotor hanging off the end of the crank (which also helps dampen vibration). I installed a lightened crank in my FJ11200 and pulled it after a couple of months. Midrange torque wasn't good forcing downshifting. Stock, balanced crank produced much better roll on at highway speeds.

Good morning, Mike!

Rick Stetson recommended them to me is how I found out about Lindskog.  I do not think the person I am working with at Lindskog is Mike, but he seems like a great guy.

I appreciate all the advice about this topic, and will let you guys know what I end up doing.  He had me weigh the rods and pistons, and here is what I got for weight, which I found interesting.

Pistons with pins: Factory Honda - 201 grams.  J/E (dynoman) 71mm - 233 grams.
Rods : Factory Honda - 362 grams. Carrillo - 318 grams

Thanks,
Daniel

Offline MRieck

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Re: Crankshaft counterweights related to secondary balance
« Reply #6 on: November 09, 2020, 06:53:02 am »
hey guys,

I currently have Lindskog Dynamic Balancing doing a crankshaft for me (cb750), and I was going to have him lighten it 10%.  They asked me about the rod and piston weight compared to stock, and how lightening the crank would affect vibration at higher rpm.  Before this phone call, i was always under the understanding that on an inline 4 cylinder (2 up, 2 down) crankshaft, bob weight didn't matter so much as long as they balanced each other out.  This is true for the primary balance, which can be balanced.  Then i started looking into what secondary balance is, and how the counterweights have an effect on it.  Also noted is that inline 4's have more secondary imbalance than an engine where the pistons are out of phase with each other.


i would assume different engines are effected differently with this issue, and wondered for anyone that has lightened crankshafts, if they have noticed vibrations at higher rpm's, and if this is something that can be hazardous to the crankshaft.  I seem to recall that a lot of the performance cb750's typically used factory crankshaft weight.  My primary basis for lightening the crank was to have a motor that rev's a little more freely.

Lindskog recommended I weight the original rod/piston setup and compare it to what I am going to use.  I will figure this out over the weekend, and am thinking I will do whatever Linkskog recommends, as they have a lot more experience than what I have.

I always appreciate this forums input and experiences.

Thanks,
Daniel
Daniel
How did you find out about Lindskog? Are you working with Mike?I have used them for 30 years. Funny thing is we never lightened modern 600cc cranks used for roadracing. Torque was better using a stock weight balance crank. The 750 can stand some lightening because of the heavy rotor hanging off the end of the crank (which also helps dampen vibration). I installed a lightened crank in my FJ11200 and pulled it after a couple of months. Midrange torque wasn't good forcing downshifting. Stock, balanced crank produced much better roll on at highway speeds.

Good morning, Mike!

Rick Stetson recommended them to me is how I found out about Lindskog.  I do not think the person I am working with at Lindskog is Mike, but he seems like a great guy.

I appreciate all the advice about this topic, and will let you guys know what I end up doing.  He had me weigh the rods and pistons, and here is what I got for weight, which I found interesting.

Pistons with pins: Factory Honda - 201 grams.  J/E (dynoman) 71mm - 233 grams.
Rods : Factory Honda - 362 grams. Carrillo - 318 grams

Thanks,
Daniel
Now that makes sense. I know Rick very well.
Owner of the "Million Dollar CB"