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A question on counterweights..

Old 01-22-19, 07:13 AM
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A question on counterweights..

Ok, so this is a kind of random situation..

I have asked this question of individuals in the past but I didn't get a clear response. I am getting very close to sending my FD off to a shop to finish up getting her back on the road and I'm slightly concerned that my engine setup may not be fully compatible.

I'm running a streetported 13B-rew with an rx-8 e-shaft, 2mm superseals and S4 NA rotors that have been lightened, clearanced, and balanced. I went with the S4 NA rotors solely due to the thicker casting. Initially the plan was to use the S4 counterweights because of the S4 rotors, which I did along with the appropriate front stack. However, prior to my last engine suffering an untimely death I noticed that there was a decent amount of vibration at idle. My motor died due to oil starvation because the t-stat mounted to the motor worked itself loose and dumped all my oil on the highway. I think the vibration played a part in this. It was a bad way to end DGRR

Anyway! When my rotors were lightened they were brought down to almost an identical weight as the stock FD rotors, which leads to my question... Are the correct counterweights for an engine based on rotor weight or something different? If they're based on the weight of the rotors, as opposed to how the rotors are individually balanced, then choosing the correct counterweights would no longer depend on series once you get into aftermarket lightening, correct?

I want to ensure that I'm doing this correctly BEFORE I drop the motor into my car and have it towed off to the shop, it would be far easier for me to redo the front stack now, as opposed to once I'm paying shop time..

Thanks for humoring me and I'd love to hear peoples opinions and the basis for them!

Cheers,

Levi
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Old 01-23-19, 10:16 AM
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i'm under the impression that the S4 front CW doesn't fit under the FD front cover? easy to check!

since its not a factory assembly, you should balance it.
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Old 01-23-19, 01:13 PM
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Originally Posted by fendamonky View Post
I am getting very close to sending my FD off to a shop to finish up getting her back on the road.
It's about time Levi!

Originally Posted by fendamonky View Post
I'm running a streetported 13B-rew with an rx-8 e-shaft, 2mm superseals and S4 NA rotors that have been lightened, clearanced, and balanced. I went with the S4 NA rotors solely due to the thicker casting. Initially the plan was to use the S4 counterweights because of the S4 rotors, which I did along with the appropriate front stack. However, prior to my last engine suffering an untimely death I noticed that there was a decent amount of vibration at idle. My motor died due to oil starvation because the t-stat mounted to the motor worked itself loose and dumped all my oil on the highway. I think the vibration played a part in this. It was a bad way to end DGRR

Anyway! When my rotors were lightened they were brought down to almost an identical weight as the stock FD rotors, which leads to my question... Are the correct counterweights for an engine based on rotor weight or something different? If they're based on the weight of the rotors, as opposed to how the rotors are individually balanced, then choosing the correct counterweights would no longer depend on series once you get into aftermarket lightening, correct?
I'm no authority on this, but my understanding is you want to keep the whole rotating assembly balanced, so for a simple stock build, you'll just use the CW part that matches up with the series rotors being used (i.e., S4 with S4) and trust Mazda's engineers. BUT since you had your rotors lightened, when you say you had it "balanced" in that first paragraph above, were you referring to the whole rotating assembly (i.e., rotors, shaft, CW, etc.) or just the rotors? What actually got balanced when you had this job done? Anyway, since your lightened rotors weigh in close to the stock FD rotors, I'm guessing the FD CW would be the right one to use.

For reference, attached is the checklist from MazdaTrix that lists all the parts they would ask you to ship them if you wanted them to balance the entire rotating assembly. That's for a race motor and probably overkill for a street motor that won't rev much beyond 9K, but it gives you an idea of what all the spinny bits are.

Also, I agree with J9FD3S about getting the assembly balanced, since it isn't a factory setup anymore.
Attached Files
File Type: pdf
Balancing_Check__List.pdf (17.0 KB, 7 views)

Last edited by Pete_89T2; 01-23-19 at 01:17 PM.
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Old 01-24-19, 06:11 AM
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Originally Posted by j9fd3s View Post
i'm under the impression that the S4 front CW doesn't fit under the FD front cover? easy to check!

since its not a factory assembly, you should balance it.
Physically the Counterweights fit just fine, the main difference between them (other than weight) is the size of the needle bearings and other components in the front stack. So you MUST have a front stack that matches the counterweight, if you try to mix an FD front stack with an FC counterweight or visa versa then bad things will ensue.

The assembly was definitely balanced, it's just been so long now that I can't remember if I sent the S4 counterweights off for the initial balancing or not.

Originally Posted by Pete_89T2 View Post
It's about time Levi!
I know right! The wife has finally graduated, I've gotten a grown-up job, and I've paid off all my reoccurring debt (minus my mortgage, which is manageable) and so now I can finally focus funds on getting the car running again BEFORE we have a brood of crotch-goblins.. So yeah, I'm going to strike while the iron is hot and I can still pull the guilt trip of "I paid for all your college, so let me prioritize my car now" to get permission from the wife...

Originally Posted by Pete
I'm no authority on this, but my understanding is you want to keep the whole rotating assembly balanced, so for a simple stock build, you'll just use the CW part that matches up with the series rotors being used (i.e., S4 with S4) and trust Mazda's engineers. BUT since you had your rotors lightened, when you say you had it "balanced" in that first paragraph above, were you referring to the whole rotating assembly (i.e., rotors, shaft, CW, etc.) or just the rotors? What actually got balanced when you had this job done? Anyway, since your lightened rotors weigh in close to the stock FD rotors, I'm guessing the FD CW would be the right one to use.

For reference, attached is the checklist from MazdaTrix that lists all the parts they would ask you to ship them if you wanted them to balance the entire rotating assembly. That's for a race motor and probably overkill for a street motor that won't rev much beyond 9K, but it gives you an idea of what all the spinny bits are.

Also, I agree with J9FD3S about getting the assembly balanced, since it isn't a factory setup anymore.
Yeah, I'm definitely onboard with the need to keep everything balanced. I'm just second guessing myself about which counterweights the rotating assembly needs to be balanced with. I put a call in and spoke with Chip yesterday afternoon, I figured since he's the one that originally did the rotor lightening and clearancing before balancing the RA than he'd be the best person to ask. He said that during the balancing he'd adjust the CW weights for the assembly and mark them for the weight in grams. I take this to imply that it doesn't really matter WHAT series counterweight you provide, if it gets balanced then they'll be adjusted to match.. Unfortunately I think my stuff was done before he began scribing the weight on the rotors/CW's so I don't have that as an easy reference.

I think I'll just have to trust that I'm being hypersensitive and that everything is done correctly. If the balancing is off then I'll find out when we start the car because their will be a catastrophic failure which will make any errors self-evident.. At that point all the "hard" work will have been completed and stripping the motor down to correct the issues will be a straightforward task
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Old 01-24-19, 06:21 AM
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If the rotors were lightened, you need to have the assembly balanced. That's pretty much end of discussion.
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Old 01-24-19, 08:53 AM
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Originally Posted by peejay View Post
If the rotors were lightened, you need to have the assembly balanced. That's pretty much end of discussion.
100% agreed! My question is not whether or not it was necessary, but rather if there are requirements in place that would identify what counterweights should be used in the balancing.

From what I'm gathering it really doesn't matter, whatever CW's you send in to have the assembly balanced with will be modified to make it all work together. So in this situation I'm pretty much stuck with the S4 CWs and front stack, not because my rotors were originally from an S4 but because they are what were provided with the original balancing. If I had just sent in my FD CWs when I sent the rest of the rotating assembly in then I would need to keep using my FD counterweights for the S4 rotors.

Ultimately it should make little difference with regards to the engine being balanced, the main difference will now be in the front stack used.

At least this is how I'm now understanding it. I'd love to be corrected if I have it wrong though
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Old 01-24-19, 09:01 AM
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Also, I should clarify: While I was talking with Chip yesterday he said that any vibration at idle or low rpm would likely be caused by something unrelated to the RA. Since the RA isn't rotating at a high enough speed at idle it wouldn't really cause vibrations.

I inferred that if it WAS out of balance enough to cause noticeable vibrations at idle then it would get progressively/significantly worse as the car is driving and run up the rev range.
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Old 01-24-19, 09:27 AM
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Originally Posted by fendamonky View Post
Also, I should clarify: While I was talking with Chip yesterday he said that any vibration at idle or low rpm would likely be caused by something unrelated to the RA. Since the RA isn't rotating at a high enough speed at idle it wouldn't really cause vibrations.

I inferred that if it WAS out of balance enough to cause noticeable vibrations at idle then it would get progressively/significantly worse as the car is driving and run up the rev range.
^That's all very true... Remember, your rotors spin at 1/3 the E-shaft RPMs, so at say 1K RPM idle on the tach, they are only spinning at 333.3 RPMs. Are you using stock or Mazda competition motor mounts, or something much stiffer (i.e., delrin mounts)? If it's something stiffer, I bet that's why the vibes were noticeable.
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Old 01-24-19, 11:21 AM
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its really easy to mill the front CW to use the bigger needle bearings, its probably a good idea to do it
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Old 01-24-19, 12:05 PM
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Originally Posted by j9fd3s View Post
its really easy to mill the front CW to use the bigger needle bearings, its probably a good idea to do it
At this point I'm not sure the juice is worth the squeeze though. I think in the future I'd just re-balance with the correct counterweights since it would probably be a decent idea to have everything balanced again anyway. For my application I have the competition thrust bearing(s) and everything is already stacked up and ready to drop. If/when the engine comes apart again I'll certainly consider it, but since I'd have to purchase a completely new front stack for now I think it'd just be a lot of squeeze for not much juice

I don't plan on going nuts and abusing the car in any sort of seriously competitive driving, so it'll most likely be fine with the front stack it has currently.
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Old 01-24-19, 12:09 PM
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Originally Posted by Pete_89T2 View Post
^That's all very true... Remember, your rotors spin at 1/3 the E-shaft RPMs, so at say 1K RPM idle on the tach, they are only spinning at 333.3 RPMs. Are you using stock or Mazda competition motor mounts, or something much stiffer (i.e., delrin mounts)? If it's something stiffer, I bet that's why the vibes were noticeable.
Hmm... At that point I *think* I had gone back to a pair of good condition stock mounts, though I'm fairly certain I did have a transmission brace hard mounted to either side of the chasis. My memory is most likely just the result of a reverse placebo effect

I'll be running the FFE engine mounts this go around, and I'm strongly tempted to get rid of my old tranny brace in favor of one of the ones that the guys at Banzai have put together (or just nothing extra at all!)
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