Def

I just went to a dyno today and laid down some pretty strong numbers for a bone stock S52B32(212rwhp and 217rwTQ, HP peak at 6300RPM, rev limiter 6500RPM). Someone else there with a 3.2L M motor had an underdrive crank pulley and some other bolt-ons and put down about 240rwhp and 240rwtq, impressive for such light work to the motor. He said the crank pulley gave a huge gain(relatively speaking here), and it drops ~8-10lbs off the crank(gets rid of a useless 5.5lb big timing wheel that is used on the OBDI motors). FYI: I am talking about all these applications with respect to DEs and AutoX - no way I got the bling to be racing this car!

So I started thinking - inline 4 engines, especially those with larger displacements, tend to have nasty 2nd order vibration. The general consensus around HT.com is that a solid crank pulley and extended high RPM use will eventually cause catastrophic main bearing failure by not damping said vibrations. Twin balancer shafts are used on many engines to help damp this, but even those engines with the shafts tend to wear a bit on the main bearings due to the "rough" nature of 4 bangers.

Inline 6 engines on the other hand, have a perfectly balanced engine configuration, and the crank pulley is actually smaller on my engine than a B-series engine. It makes sense that there is not much, if any actual damping of the rotational assembly by the crank pulley on a BMW inline 6 engine. My reasoning is that the crank is BEEFY - looks almost like a marine diesel compared to most cars, and the stock flywheel is a portly 21lbs and ~280-290mm in diameter. The guy at the dyno said he has had it on for a long time now, and has experienced no ill effects.

Since my only real criticism of this car is that it feels like it is revving up through "goo" in the first two gears due to the huge inertia of the rotational assembly - this presents an interesting opportunity to drop the inertia of said system + gain some power. Lightweight flywheels for this car cost about $500(BMW tax ), so this would make the car "near perfect" in my eyes plus give me a little more HP(not that I really need it...).


I figured I would post here in the hopes that someone technical would waltz by and offer some insight on my analysis of the engine configuration's impact on vibratory damage when removing some small amount of damping from the rotational assembly. So by all means, don't be shy.

Track rat

I'd like to know the answer to this too. An automotive engineer told me that the dampeners in main pulleys not only reduce N, V, &H but also prevent main bearing wear and bolts from vibrating loose all over the motor. I'm sure this is the case on a thrashy Olds Quad 4 motor, but I wonder if a silky smooth B18C5 would eventually kill itself w/o a dampener. Anyone with many miles of hard track use minus dampener care to chime in?

bikeboy80

I can say I have a whole lot of hard track use on mine (unorthodox ultra R, alt belt only) , but with about 80 autox runs reving up to 8500 for a good bit of time, I havent had any ill effects yet.

the engine is a b18b that when i had the bottom end built I made sure to have the whole rotating assembly (including the crank pully) ballenced. I drive the car everyday and have about 10k miles on it since the rebuild and the pully was installed and there are no weird vibrations or anything like that so far.

like i said i dont have a whole lot of high rpm time on the setup but i still figured i'd give my input

Def

I did a quick and dirty analysis and it looks like there shouldn't be a problem on an I6 or V12 that is internally balanced(nearly ALL modern engines are internally balanced). An inline 4 suffers from second order vibration, so reducing the inertia and slightly lessening the damping of the rotating assembly might be rough for a longer stroke motor.

After looking at things in a little more detail, it seems that a lightweight flywheel will do just as much, if not more "harm" than a solid crankshaft pulley though. The damping characteristics of the stock Honda crankpulley aren't all that stellar, with a thin elastomer ring and only the inertia of the outer ring and accessories to act as the reference inertia(or stabilizing inertia if you will). A solid crankshaft pulley will remove this very infintesimal amount of deflection, but I doubt it is enough to really mean the difference between sparkling clean main bearings at 100k stock miles and a trashed crank at 10k with a crank pulley.


On the BMW front(since there are SOOOO many BMW owners browsing these boards) - the stock crank pulley offers almost NO damping. The ribbed grooves the belts ride in are one solid piece(just like aftermarket pullies, but made of steel here instead of aluminum). It has a VERY thin elastomer ring separating this pulley and a ~10" in diameter crank angle timing sensor that weighs about 5.5lbs total. This is not even used on 96-99 E36's(same part as on the '95 OBDI M3's) - so going to an aftermarket crank pulley nets a noticeable reduction in inertia.

The damping afforded by a piddly 5.5lb steel ring suspended by a small amount of rubber is absolutely laughable compared to the amount of power a meaningful damper would need to dissipate if the rotating assembly had very intense vibrations.

In fact, BMW stamped on the pulley that the natural frequency of the assembly is 320Hz. Doing some mucho complicated math tells us that this is aimed at reducing harmonics around 3200RPM - smack dab in the middle of day to day cruising speeds. This leads me to believe that BMW did this to maintain a large amount of inertia on the rotating assembly for that "not peaky" feeling, but needed to damp out some low level harmonics to meet their stringent NVH requirements. Namely, it is just there to reduce possible engine noise at a common cruising speed(about all that lightweight ring could hope to do) - hardly cause to say your engine will self destruct.

Lowering the inertia on the rotating assembly in any form does increase the maximum loading the main bearings will see, but a crankpulley and a lightweight flywheel(when used SEPARATELY!) don't change the inertia or damping of said assembly enough to cause any increased engine wear on a naturally balanced and harmonically neutral engine assembly.

To clarify - I wouldn't use a crankpulley on a "bad" engine design like an H22A in a Prelude, because its long stroke and large power pulses from its 550cc cylinders definitely need all the inertia and damping they can get(in addition to the stock balancer shafts, which are barely adequate).


Hope this helps show people that things aren't as "black and white" as they might appear in the Internet world.

.RJ

https://honda-tech.com/zerothread?id=343351

https://honda-tech.com/zerothread?id=219450


FWIW.

aklucsarits

Steve Dinan does not like them:
http://www.dinanbmw.com/html/d...s.htm

...short, but interesting read.

Andrew

allenp

I think we may be getting our engine vibrations confused…

Keep in mind that there are 2 distinct kind of engine vibrations that are a result of combustion and reciprocating mass: 1) translational vibrations of the engine assembly itself, which are counteracted by such things as counter-rotating balance shafts and tuned engine mounts, and 2) torsional vibrations of the rotating assembly (crank/pistons/flywheel/pulleys).

The primary cause of wear on the crank journal and bearings are the translational vibrations caused by the combustion forces and reciprocating masses. There’s not much that a pulley can do to counteract these forces, since they are contained inside the engine block itself. The inertia or tuned damper of the pulley can’t reduce the up-and-down forces on the bearings.

However, these forces also set up torsional harmonics in the crank itself, resulting in torsional bending modes (think twisting of the crank) that can result in fatigue failures of the crank itself. The crank damper pulley is typically tuned to damp out the largest of these twisting modes. Some engines even have a dual-mode damper, with 2 elastomer rings separating 3 masses.

Your note about the inline six being naturally balanced is true in the translational sense – but not torsional. Every combustion engine suffers from torsional vibration modes. You did note the beefiness of the crank, however, which significantly impacts the location (frequency) of these twisting modes, and probably has much more to do with the pulley size and tuning.

I always understood the primary benefit of lightweight underdrive pulleys as reducing the torque demand of the accessories on the engine, as opposed to the reduction of inertia. Certainly any reduction of rotating mass is beneficial to engine acceleration performance, but the relative effect of a pulley as compared to a flywheel, much less the vehicle itself, is minimal.

And one more note: since the 6-cylinder, 4-cycle engine fires 3 times per revolution (3rd order excitation,) the 320Hz tuning actually represents 6400RPM.

Def

I understand that external balance shafts are meant to control engine movement, but I was under the impression that many 4 cylinder engines had twin balancer shafts connected to the crank assembly(or was I mistaken?). I'm specifically thinking of the QR25DE in the Senta Spec V. Maybe I was getting the vibration confused, as I was cut and pasting alot of that stuff from a BMW board.

I'm extremely tired right now - but I think you are agreeing with me on the fact that the small amount of damping available in a crank pulley(of which the BMW pulley has even ldamping than a Honda one) is not going to influence the main bearing wear?

I think it is primarily there to reduce engine "roughness" to meet more stringent NVH standards.

The torsional vibration of the rotating assembly is probably going to be affected more by any reduction in inertia of the system rather than reducing the damping by a small amount.


Crank pullies do significantly reduce the inertia of the drivetrain(not as much as most lightweight flywheels though). The BMW factory pulley has a huge amount of inertia though, due to the large diameter of the timing ring. So you get some gains from underdriving accessories, and alot of gains from the reduction in inertia.

I'll see if I can find a pic.

GSpeedR

This is something you may have already considered, but the position of the crank pulley harmonic damper is also of great importance especially on an inline engine. With the flywheel connecting to the crank on the "output" shaft (I think most people call this the "back" of the engine) the crank will experience significantly less torsional deflection (and faster vibrational decay) closer to the flywheel compared to positions further away from the flywheel. This is because of the large rotational inertia of the flywheel which more efficiently dampens out torsional vibrations at closer distances along the axis. So if Piston #6 (or #4 for my car) is closest to the flywheel, then the crank/main bearing is going to experience more torsional deflection and longer sustained vibration at/around Piston #1. So the harmonic damper on the crank pulley (other side of the crankshaft) will aid in reducing the deflection at the other end, where the flywheel's inertial effect is not as great.

The harmonic damper "evens out" the stress across the crank. This should have a significant effect on reducing the failure of the crankshaft itself compared to reducing the stress seen on the main bearings/rods/etc.

I kind of did this off the top of my head, so feel free to point out errors of thought.

allenp

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">I understand that external balance shafts are meant to control engine movement, but I was under the impression that many 4 cylinder engines had twin balancer shafts connected to the crank assembly(or was I mistaken?). I'm specifically thinking of the QR25DE in the Senta Spec V.</TD></TR></TABLE>

They do have exactly such devices, the GM 2.3L Quad-4 being one extreme example. In a 4-cylinder engine, there are primary inertial forces and secondary inertial forces resulting from the reciprocating mass. The primary forces are self-balanced, the secondary are not -- this is where the balance shafts come in. They counter-rotate so as to cancel each other's effect in the horizontal axis, but add to each other in the vertical.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">I'm extremely tired right now - but I think you are agreeing with me on the fact that the small amount of damping available in a crank pulley(of which the BMW pulley has even ldamping than a Honda one) is not going to influence the main bearing wear?</TD></TR></TABLE>

Yes, I agree with this. The pulley's job is to damp torsional modes, not translational.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">I think it is primarily there to reduce engine "roughness" to meet more stringent NVH standards.</TD></TR></TABLE>

I'm sure NVH is one driver, however I would not be certain that there are not certain torsional modes that could be detrimental to the fatigue life of the crank that are damped by the pulley. But I have no data to support. My company has done extensive research on the 6.8L Ford V10, which is a big challenge. Without its damper, it would have durability issues.

Another purpose they serve is to reduce the torsional vibrations passed onto the accessory drive, thus reducing belt noise as well as preventing damage to the accessories themselves.

As far as the significance of the inertia reduction... Some quick figuring on the only vehicle I have data on: If you halved the pulley inertia, which I figure would be an extreme example, the total rotating inertia of the vehicle system (including engine, flywheel, clutch, transmission, driveshafts, brakes, wheels, tires, and vehicle mass) would be reduced by 0.47% in 1st gear, 0.19% in 2nd gear, with continually diminishing returns as the reflected vehicle inertia increases with each gear. Undoubtedly it would improve free response, such as throttle blipping and rev-matching, but I have a hard time that it would significantly improve in-gear performance.

Regular Series

So why do inline fours create so much more vibration than sixes? It seems like the engine would still be able to damp itself because at least one piston will be at BDC when another is at TDC (firing).

Don't flame me, I really don't know.

allenp

No flames here! I'll get to your question in a moment.

After reading Dinan's article, I feel even more confident in my stance. While replacing the damper pulley with an underdrive pulley will not pose a threat to your bearings, it does pose a threat to the crank itself -- an even bigger concern.

Def, I think the bottom line is that your potential gains simply will not outweigh the risks. And while the lighter flywheel is a more expensive proposition, it is also much more significant in terms of inertia reduction (see my numbers in the previous post.) Additionally, it will not result in the loss of damping for the crankshaft natural frequencies. Our research suggests that a lighter flywheel might actually reduce the amplitude of the crank torsional vibrations.

As to the question about why 4-cylinders are worse balance-wise than 6-cylinders... It's actually much more complicated than that. And I would have to dig deep into my engine vibrations lecture notes to go too far into it. But, there are a couple of things I can say...

4-cylinders have greater combustion-related vibrations than 6 due to the fact that there are fewer firings per revolution than a 6-cylinder: the 4 fires twice per rev, the 6 3 times. Since the firings are closer-spaced on the 6, it is smoother.

Then there's the question of inertial balance. When piston engines are spinning, there are pistons, crank throws, con rods, etc flying around in a complex motion. Each of these has mass, so when they change direction, they cause a force to be exterted on the engine block. It just so happens that in an inline-6 cylinder configuration, almost all of these forces naturally cancel each other out. However, on a 4-banger, this isn't the case, so external balance devices are required. V6 engines also suffer from imbalance, but of a different sort than 4's. Even V8s aren't perfect. The type of imbalance also depends on the V-angle - 60 degree V6s require different balance shafts than 90 degree.

Wow, that's quite enough from me.

Def

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">As far as the significance of the inertia reduction... Some quick figuring on the only vehicle I have data on: If you halved the pulley inertia, which I figure would be an extreme example, the total rotating inertia of the vehicle system (including engine, flywheel, clutch, transmission, driveshafts, brakes, wheels, tires, and vehicle mass) would be reduced by 0.47% in 1st gear, 0.19% in 2nd gear, with continually diminishing returns as the reflected vehicle inertia increases with each gear. Undoubtedly it would improve free response, such as throttle blipping and rev-matching, but I have a hard time that it would significantly improve in-gear performance.</TD></TR></TABLE>

The inertia of the pulley is MUCH less than half on my specific application. I'll dig up a pic.



The stock pulley weighs around 9lbs(with 5.5lbs of it being the unused timing ring). The aluminum pulley weighs about 8-9oz. The big inertia drop comes from eliminating the timing ring, and the fact that the aluminum pulley weighs MUCH less. I rode in a car with this pulley and the reduction in inertia was extremely noticeable. Not as much as going from a 21lb flywheel to a 8.5lb flywheel, but quite a bit. For power purposes, a car with an intake, crank pulley, track(test) pipe and exhaust put down 30ft-lbs more TQ than me from 3k RPM to 6.5k RPM. Needless to say, his car was way faster than mine, and he said his biggest gains came from the pulley.

I'm only concerned about the bearings really, because quite honestly - the crank on this motor is ridiculous. I've *never* heard of anyone breaking a crank, and people have run 600+rwhp through these motors with no problems whatsoever. It is forged, and I seem to recall the crank weighing in at almost 30lbs. I understand vibrations can increase the stress on it, but at only ~260bhp I doubt it will be snapping anytime soon.

The lightweight flywheel does offer bigger gains, but this pulley costs $200, and a flywheel costs $500. The pulley will actually make the car faster, which is my end goal. Plus lightweight flywheels on these cars make the transmission chatter EXTREMELY loudly. This can't be good for all those components.

Another thing I forgot to mention, the stock flywheel in my car is termed a "dual mass" unit. It has a ~16-18lb main unit, and a smaller 3-4lb mass suspended by springs. This has to provide most of the damping on a rotating assembly, since I still refuse to believe a freely rotating 5.5lb steel ring with ~2mm of rubber mounting is going to significantly damp a comparatively much more massive rotating assembly.

I think the most miles I have seen on an S52 motor like mine with a solid crank pulley is about 50k with lots of DE's in that time period. So if things were going to fatigue, I think they would have done so by then.

I'm not ruling out that it is dangerous, just trying to get to the bottom of the situation.

allenp

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">I'm only concerned about the bearings really, because quite honestly - the crank on this motor is ridiculous. I've *never* heard of anyone breaking a crank, and people have run 600+rwhp through these motors with no problems whatsoever. It is forged, and I seem to recall the crank weighing in at almost 30lbs. I understand vibrations can increase the stress on it, but at only ~260bhp I doubt it will be snapping anytime soon.</TD></TR></TABLE>

Well, I have no data with which to debate you on this, other than my seeing the measurements of crankshaft torsional vibration with and without such a device. However, this was on a completely different engine. I can only cite Dinan's assertions as being somewhat relevant to your application.

And even if your pulley inertia is 1/10 the original... As you can see by my numbers, the percentage reduction in OVERALL inertia that the engine torque must overcome to accelerate the vehicle is very small. I have a 7.5lb aluminum flyhweel in my GSR, and assuming a reasonably linear inertia to mass ratio, that's a 55% reduction (OEM is 13.5lb). I noticed very little difference in in-gear acceleration. The only appreciable difference is throttle-blipping.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">Plus lightweight flywheels on these cars make the transmission chatter EXTREMELY loudly. This can't be good for all those components.

Another thing I forgot to mention, the stock flywheel in my car is termed a "dual mass" unit. It has a ~16-18lb main unit, and a smaller 3-4lb mass suspended by springs. This has to provide most of the damping on a rotating assembly, since I still refuse to believe a freely rotating 5.5lb steel ring with ~2mm of rubber mounting is going to significantly damp a comparatively much more massive rotating assembly.</TD></TR></TABLE>

Now you're speaking my language. My day job (i.e. what I should be doing right now) is developing these dual-mass flywheels and reducing the gear rattle (what you called "chatter") in the transmission.

First, a note on gear rattle: It is an NVH concern, little more. It is caused by the torsional fluctuations of the crankshaft lighting off natural frequencies in the transmission, causing the free gears to rattle against each other. Only under EXTREME circumstances (i.e. diesel trucks) is this noise of a durability concern as opposed to simply a driver annoyance. So, long story short, if you can put up with this noise when you have a lightweight flywheel, it's no harm to your car.

Secondly, the purpose and function of the dual-mass flywheel. I am 95% certain that your clutch system is manufactured in Germany by my company -- LuK. What it does is this: To reduce gear rattle, one has to reduce the amplitude of the vibrations going from the engine to the transmission. Basic vibes theory tells us we have 2 ways of doing this: Add damping to attenuate the vibration, or move the natural frequency of the system out of the engine's operating range. The dual-mass flywheel does just that -- by splitting the inertia of the flywheel, you basically add the 2nd inertia to the transmission, thus lowering its frequency out of the driving range.

The dual-mass flywheel has little effect on the crankshaft vibrations attenuated by the damper pulley. Its tuned natural frequency is in the 10-20Hz range, far too low to affect the crank vibrations.

maxQ

This looks like a well-explained (and I assume pretty accurate) primer on engine vibrations.

Bow to the big Z

Def

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Well, I have no data with which to debate you on this, other than my seeing the measurements of crankshaft torsional vibration with and without such a device. However, this was on a completely different engine. I can only cite Dinan's assertions as being somewhat relevant to your application.

And even if your pulley inertia is 1/10 the original... As you can see by my numbers, the percentage reduction in OVERALL inertia that the engine torque must overcome to accelerate the vehicle is very small. I have a 7.5lb aluminum flyhweel in my GSR, and assuming a reasonably linear inertia to mass ratio, that's a 55% reduction (OEM is 13.5lb). I noticed very little difference in in-gear acceleration. The only appreciable difference is throttle-blipping.</TD></TR></TABLE>

I don't have much time, so I'll just briefly respond to this.

I believe Steve Dinan later retracted that statement(frankly, you can't apply statics to a system with vibrations), but left it up to sell more lightweight flywheels for $900 a piece. I don't trust what he says one bit, as I have *ALWAYS* seen financial motivation for whatever he says as "scientific proof." Not to mention his "tuning packages" for BMWs are a joke. 40rwhp routinely turns out to be more like 5rwhp. Sounds honest to me...

As for the inertia change, the only thing I have to go by is seeing a crankpulley in action on the same motor I have. It is not a huge difference, but very noticeable in first and second gear. Even though the car makes just as much torque as mine below 3k RPM(~200ft-lbs to the wheels), when he laid on the throttle - it snapped your head back, which my stock car doesn't do at all. Rev matching was much easier on his car as well.

I'm still undecided, as I keep hearing reports of a post someone from Unorthodox Racing made back in ~2001 that had ACTUAL data that supported that their crank pullies do little to no damage to the crank and bearings based on harmonic vibrations. Everything else I see is alot of speculation, which is fine, but as a hopeful engineer - I know that *facts* based on speculation can be largely colored by any preconceived notions one might have on the subject. It's easy to look at "one side" of the picture then come to a "conclusion."

I'm just trying to forget everything I "know" and see where it takes me.

JoelG

This subject was covered very well in a recent issue of GRM (May 2003, P-122). It's also been hammered pretty hard in an online bmw community that has a number of automotive engineers as members. The short of it is that torsional vibrations are a serious issue with the long crank on the bmw I6s and removing the harmonic damper is a bad idea. Removing the harmonic damper and switching to a lightweight flywheel is an even worse idea. Believe me, if running w/o a harmonic damper was a good idea, Tom Milner would do it. On the other hand, the PTG cars do run a VERY light flywheel.

joel
many bmws


Modified by JoelG at 4:33 AM 8/28/2003

allenp

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">Everything else I see is alot of speculation, which is fine, but as a hopeful engineer...</TD></TR></TABLE>

I AM an engineer, and my company HAS done research on the effects of EXACTLY such a device. No speculation here. Some extrapolation, surely, but NOT speculation. My assertions are grounded in fact.

Dinan's simple illustration of the crankshaft torsional vibration is right on - I have similar but much less simple simulations of the multi-DOF system that we did the study on. It is NOT based in statics, but actual harmonic analysis. If the data that I had was not confidential, I would post it for you.

I'm not sure how you take my comments to be speculation - I have repeatedly stated that I have real data, just for a different application. But the mechanics of the IC engine are the same.

Def

I hate to be quoting the dictionary, but I think some people need a little lesson in "perspective."

Speculation: -Contemplation or consideration of a subject; meditation.
-A conclusion, opinion, or theory reached by conjecture.
-Reasoning based on inconclusive evidence; conjecture or supposition.

Looks like we've been making a great deal of conjecture without ACTUALLY sitting there and thoroughly analyzing the engine in question. Note, that by my saying there is speculation in this thread(or others on the Internet) does not devalue your experience or opinions. In fact, I'm quite enjoying this thread and am trying to learn as much as I can. Don't get all pissy over semantics, this is an open debate - and we have not stripped down an S52B32 and started doing detailed measurements and building a sophistocated model. So calm down, it's just a discussion on a message board.

I understand you have experience with this, that's why I keep asking you questions and stating anything that occurs to me that might change the situation between the engine you're familar with(of which I have no clue what it's like really, just pointing out 'different' features of my engine from Hondas as a reference), and looking for more clarification on the subject.

Ultimately I just want to grasp a firm understanding of this situation. My intent was never to flame or cast doubt upon anybody's abilities or knowledge.


So I guess I have one more question, feel free to ignore it if you feel you still feel offended.

When thinking of vibration, and a dynamic vibration absorber(not sure if it applies in this circumstance) - specifically, those on the end of a professional level bow. You either shoot the bow and get lots of vibration, or you have two masses on flexible rods with move violently after you shoot the bow, thus removing all the vibration from it.

Now, the mass of the dynamic vib. absorbers are a large fraction of the weight of the bow, and they must move around quite vigorously(since any vibratory force must be dissipated in their movement, and F=MA). So how can a seemingly large amount of energy be dissipated with an almost infentesimal displacement of a comparatively light timing ring like on the BMW crank? The pulley itself is solid steel, so only the 5.5lb timing ring can damp harmonic vibrations.

allenp

Thanks for the lecture. I know what speculation is, and I know that what I'm doing is not speculation. I am giving you information, grounded in actual research that I possess, and making assertions based on reasonable extrapolation that it probably applies to your situation as well -- being that if a potentially life-shortening mode did not exist within your crankshaft, then BMW would not have wasted the time or money developing and manufacturing one. I suppose you can call that conjecture, but it's not far-fetched.

But before we let a useful discussion completely disintegrate:

If I have time today, I will try to simulate some acceleration times with and without the pulley inertia on a vehicle whose data I possess. I can state with some reasonable confidence that you won't see much difference because the percentage difference is very small. I think most of the gains that you see from the pulley change are from the reduction in accessory drive torque due to the diameter change.

And in regards to your bow analogy... You're correct, the damper pulley is exactly that - a dynamic vibration absorber, or DVA. Now the bow example is one that is ideal for a DVA, in that the bow operates at a fixed frequency, so the absorber can be optimized for that particular frequency. Unfortunately, engines operate over a wide range of frequencies, so there must be a compromise. The inertia of that ring is likely just enough to attenuate the resonance to an acceptable level. Unlike the bow, there is still excitation in the crank at the critical frequency, but it is down to a level deemed acceptable for fatigue life.

And to address the question of mass ratios more directly: Since this is a torsional vibration system as opposed to a linear one, the key are the rotational inertias in question. You'd be suprised how significant that 5.5 lb mass is, rotationally, compared to the rest of the system.

GSpeedR

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by maxQ &raquo;</TD></TR><TR><TD CLASS="quote">This looks like a well-explained (and I assume pretty accurate) primer on engine vibrations.

Bow to the big Z</TD></TR></TABLE>

Pretty good thread.

I had never considered the "yawing" axis of rotation an engine experiences.

Def

Just a little case of miscommunication, I didn't intend for my comments to come across negatively. So let's have a and keep being geeks.


Back to the timing ring. I realize inertia and angular acceleration are the issues at stake here, but the distance from the axis of rotation out to the edge of the ring isn't much farther than going out to the journals of the crank. So I basically assumed comparing weights would get you a "close enough" approximation. Again, the cranks on these motors are FAR heavier for the number of cylinders than your average Honda B/D/H series motors. It seems that since the power pulses take a relatively "long" time even at redline, and the rubber offers little to no deflection, that the angular acceleration of the ring would simply not be enough to make the difference between a "happy" crank and one that is on the verge of snapping. This is just intuition, so tell me if I'm wrong here. I'm wondering that even after the crank is fatigued to the point to where its critical stress has levelled off - would it be capable of cracking with such a low powerlevel and some increased vibrations. It still appears to me that the large amount of inertia afforded by the 21lb stock flywheel(which, btw, a stock Integra GSR flywheel is 18lbs, so your 7.5lb flywheel is a quite significant reduction in inertia assuming a linear distribution of mass) is more of a factor in the "roughness of the ride" the crank experiences for lack of a better term.

I've been reading up on threads, and some people report a slight "hum" or "whining" above 5-5.5k RPM with a solid crank pulley. Your thoughts on this? Just small vibes giving off noise, or potentially harmful vibes?

On the subject of why BMW put it in there, with OBDI they had to have a timing ring on the pulley for the crank angle sensor. I'm sure they used the opportunity to put some damping in the system by isolating it with rubber(probably half my car is made from elastomer bonded metal ) - and when they went to OBDII, the timing ring wasn't used anymore, but the production costs aren't worth it to tool for a completely different pulley that eliminates it. I'll check around and see if the newer iteration of this engine still has the timing ring.

On this same subject, Honda went with solid crankpullies in ALOT of its engines, and then later went to rubber-ringed pullies on the same engine. I think the first generation B16A, which still redlined at 8000RPM, had a solid crank pulley. While the 2nd and 3rd generations of this motor had a rubber pulley. Some of the older D motors(D15B7 and some others, maybe D16A1?) also had solid crank pullies. This was my line of reasoning in that it could just be noise driving the manufacturers to put more damping on the rotational assembly, instead the vibes down to a low enough level to keep the crank alive.

Mista Bone

Well, D16Z6 with a lightweight pulley and 10,000 miles.............



Threw in new main bearings (all green) and put the stock pulley back on.

30,000-35,000 miles later, 25 bottles of nitrous.......




I'll stay with the stock pulley from now on.

Def

Mista Bone,

That's some nasty bearing wear. It's hard to tell, but are they rusted? Guess all the lead, tin and nickel has been scored off. How did the crank journals look(I'm guessing fine if you just put new bearings in)?

allenp,

What's your take on the bearing wear? Was this most likely caused by the crank pulley or something else? If it was caused by the pulley, what specifically caused it since you said increased torsional vibrations shouldn't cause this?

Arf - this is a confusing subject... Everytime I see some new information the waters seem to get murkier.

allenp

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">Just a little case of miscommunication, I didn't intend for my comments to come across negatively. So let's have a and keep being geeks.</TD></TR></TABLE>

Sounds good. No hard feelings.

Okay, as for the size of the motor: The specific engine that I have data for is the Ford 6.8L V10. The crank for this motor is friggin' huge, as are the pistons. The crank damper pulley inertia and rubber spring is tuned to attenuate the largest natural frequency to acceptable levels... I really really wish I could share some of the specifics with you, as the difference between "acceptable" and "dangerous" really is small in these situations. It's amazing how much of a difference a seemingly small device can make, between as you say a happy crank and a sad one.

I imagine some Honda motors did not need a crank pulley because, simply stated, the crank natural frequencies were out of the operating range of the engine, or that the excitation amplitudes at the frequencies were lower than their bogey at the time. Why they later went with dampers I am not sure, perhaps they experienced more warranty than they wanted to, and felt that it could be reduced by adding the damper. Or perhaps the next Engine Engineering Manager wanted the amplitude bogey reduced for his own personal stimulation. Who knows...

Okay... Back to flywheel inertia. Now.. You are correct, in a way, when you say that the crank goes for a rougher ride with a lightened flywheel. You have significantly reduced the inertia of the rotating system while not touching the engine torque. And since the rotational corollary to F=MA is T=I(alpha), where T=torque, I=moment of inertia and alpha=angular acceleration, rearranging to solve for alpha we see that it has gone up. But, you have at the same time rearranged the vibration system entirely, changing its natural modes. It's very possible that by reducing the flywheel inertia, the natural frequency of the crank that is being absorbed by the pulley damper may now be much higher -- perhaps out of the driving range. Also, since there's less inertia vibrating about the crank, the amount of energy stored in the crank dropped, dropping the amplitude of the resonance. But since I don't have the data to analyze this system, the specifics truly are speculation.

As for the noise at high engine speed... My calculations centered the 320Hz damper at 6400RPM.. So it's possible that the observations of the noise are indicative of approaching the peak of that resonace. It's also possible that, since the end of the crank that's attached to the accessory belt is also where the largest angle amplitude is, the noise is created by the rubber belt being excited by the pulley.

Now the bearing wear shown by Mista Bone. I will stick by my earlier assertion that the crank pulley damper should not have an implication on bearing wear since it strictly acts in a torsional sense, whereas bearing wear would be exacerbated by increased translational vibration. I don't know much about tribology, but my first question about those bearings would be lubrication?

What remains in the back of my head is this: No matter whether any of my data is true for your engine, I simply cannot believe that the reduction in inertia afforded you by the replacement by the solid pulley will be worth the possible risk involved. I still owe that simulation... I'll try to do that.