Balancing question!

[Mazdabater]

This has been bugging me for a while I see alot of guys that get rebuilds bragging about "rotating assembly balanced" what I want to know is how far out of balance could a rotary possibly be? Anything that spins in such an odd way at such a high RPM would eat itself if it was slightly out of balance.

Someone enlighten me!

[t_g_farrell]

If you intend to take the rebuilt engine beyond its designed max rpm, like
say in a PP or BP that will see 9 or 10K rpms, you need to have it balanced
to make sure its good up to the new rev limit. Theres other mods needed as
well like pinned and hardened stationary gears, oil mods on the eshaft,
extra dowel pins between housings and plates etc.

Also if your mix/matching some rotating assembly parts you may need it
rebalanced as well.

Normally the rotating assemblys are good to 8K rpm in a stock engine I think.

[j9fd3s]

basically T_g is correct.

the rotary doesn't reciprocate like a piston, and also the rotors are iron and comparatively heavy (piston is 500 grams, rotor is 5000 grams, give or take).

what mazda did with the engines prior to the Rx8 engine, is to cast a batch of rotors, and then grade them PER BATCH, lightest to heaviest. then they will match up pairs of rotors that are close, and balance them to the counterweights. mazda's production tolerance is something like +/- 50grams, a piston engine would explode but the rotary is fine up to 8000-8500*

when mazda does a high performance engine, like they list in the competition book, they will balance the rotors closer than the normal production, and then its good to 9k*

so what does this mean? this means if you have a factory pair of rotors and counterweights, that you are in good shape. since they grade rotors per batch, this also means your rotor from 1987 and your rotor from 2007 are different weights (2004+ rotors are lighter, and they added the side clearance cut too).

it also means that counter weights are slightly matched to the engine.

it means that if you want to build a high rpm engine, you wanna either use a matched assembly, or balance it in some fashion.

i had my P port balanced, and it is very smooth

*depends on the year/model/apex seal choice

[FC Zach]

Also if your mix/matching some rotating assembly parts you may need it
rebalanced as well.

So true, cause no matter how sure you are, shit does happen. My first rebuild I did back in 2003/04 I was almost positive I was using the correct counter weights, I was wrong (i had a S5 front weight). Throughout the years of driving the piss out of it and going to unheard of RPMs eventually lead to this disaster.

As you can see here the bearing toasted, eventually causing the rotor to smack the irons from excess play, not one part or this motor was reusable.
http://www.rotarycarclub.com/rotary_forum/attachment.php?attachmentid=9395&d=1292475429

Because of this I made sure I had my new motor balanced and even though I did have matching weights and rotors there was a noticeable amount of material removed from each part.
http://www.rotarycarclub.com/rotary_forum/attachment.php?attachmentid=9654&d=1297144063


On my new motor I had the side faces clearanced as well and if you look closely you can see a small drilling on the lower right end of the rotor. Very precise, I believe Racing Beat uses a dynamic balancing machine.

[TitaniumTT]

mazda's production tolerance is something like +/- 50grams, a piston engine would explode but the rotary is fine up to 8000-8500*


Excellent post!

My theory for this is that they're still matched within about 1%, and the rotor is rotating at 1/3 eccentric shaft speed. So while you may be spinning the engine to 8500 rpm's the rotors are only spinning @ 2833rpms.

[Mazdabater]

Ah well that all makes sense then. Thanks alot guys! I now feel less dumber lol

[j9fd3s]

Excellent post!

My theory for this is that they're still matched within about 1%, and the rotor is rotating at 1/3 eccentric shaft speed. So while you may be spinning the engine to 8500 rpm's the rotors are only spinning @ 2833rpms.

lol, i didn't even think about that, but you're totally right.

http://www.rotarycarclub.com/rotary_forum/attachment.php?attachmentid=10773&d=1311357897

that's the mazdaspeed motor sports page. the 20B is the 13G based engine, NOT the cosmo engine, they are TOTALLY different.

i'm also not sure which 13B and 12A rotors they are using either....

[NoDOHC]

The rotor balance is not important.

What you are really balancing is the eccentric shaft.

Without the counter weights, when the front rotor is accelerating upward, the rear rotor is accelerating downward. The reactive force will make the engine want to pitch forward in the car. The counter weights offset this tendency by working opposite the rotor, because the weight is offset more than the rotor from the rotating center of the E-shaft and they are also offset more from the pitch axis (which lies approximately in the center of the offset in the E-shaft and parallel with the front axle), the counterweights can be significantly lighter than the rotors.

Balancing a rotary is much easier than a piston engine, as there are no strange factors to multiply the piston-rod assembly weight by (which are actually slightly dependent on rod to stroke ratio) but only the weight of the rotor, seals and some oil (I use 200g for the oil).

[j9fd3s]

The rotor balance is not important.

What you are really balancing is the eccentric shaft.

Without the counter weights, when the front rotor is accelerating upward, the rear rotor is accelerating downward. The reactive force will make the engine want to pitch forward in the car. The counter weights offset this tendency by working opposite the rotor, because the weight is offset more than the rotor from the rotating center of the E-shaft and they are also offset more from the pitch axis (which lies approximately in the center of the offset in the E-shaft and parallel with the front axle), the counterweights can be significantly lighter than the rotors.

Balancing a rotary is much easier than a piston engine, as there are no strange factors to multiply the piston-rod assembly weight by (which are actually slightly dependent on rod to stroke ratio) but only the weight of the rotor, seals and some oil (I use 200g for the oil).

ive seen an engine run out of balance and its WEIRD, it does almost exactly what you say, except there is a side to side bit too.

i'd do the out of balance rotor dance if we were in the same room

-mike

[NoDOHC]

ive seen an engine run out of balance and its WEIRD, it does almost exactly what you say, except there is a side to side bit too.

i'd do the out of balance rotor dance if we were in the same room

-mike


Yes, the side-to-side (or yaw) is caused by the rotors accelerating sideways. This is corrected in the same way as the pitch (I left this out for simplicity sake).