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Thread: Crankshaft Stress Analysis

  1. #1
    Team Z8 ZMates's Avatar
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    Crankshaft Stress Analysis

    Still a lot of work to do modeling the crankshaft: drilling holes, shaping webs, etc. The modal analysis is also a shortcut: fixed the flywheel end rather than attaching a movable flywheel, no weights on the journals or front end of the crank (weights of pistons, rods, damper, etc.). However, like this, the first mode is at 500Hz and second mode at 1900Hz.
     
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  2. #2
    Wow, that's really going down the rabbit hole! :-)
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  3. #3
    Z8 Ate My Homework! Norcal's Avatar
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    What's the goal? A new, higher performance crank? Longevity? Smoothness? Are you making one to be installed in your car?

  4. #4
    Team Z8 ZMates's Avatar
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    As usual, the goal is more power/less weight. I'm just seeing what can be done with the S62 if you're not subject to the cost/NVH/regulatory constraints BMW faced in designing the engine.

    The modal analysis is important because a rubber harmonic balancer will dampen the first torsional vibration mode, but not any higher modes. The second mode vibration peaks when 4*rpm is equal to the resonance frequency. The modal analysis indicates whether or not raising the rev limit will get you into the operating range where the second mode is a problem. You can then address the 2nd mode either by designing a stiffer crank that pushes the second mode higher and out of range again or by using a centrifugal pendulum absorber that would absorb the first and second mode resonances.

    More stress analysis to come besides the modal analysis. I'm trying to find the weak points of the stock engine and make sure any changes don't make the problem worse.
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    Dinan S2 package: headers, throttle bodies, oversized MAFs, airfilters, anti-roll bars, lightweight flywheel
    Dinan by Brembo brake kit and monoball control arm bushings
    BBS forged individual wheels
    Quaife differential, 3.64:1 final drive
    Meisterschaft GT titanium mufflers
    K&W 3 way adjustable competition monotube dampers and monoball adjustable mounts
    CDV delete

  5. #5
    Team Z8 JoshB's Avatar
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    I imagine a lot of this analysis has already been done by Dinan and others in building racing versions of the S62. Since our engine is not cutting edge for racing development any more I wonder if you could not persuade someone to give you the info and not reinvent the wheel!
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  6. #6
    I thought I was somewhat technical, but now I'm not so sure after reviewing the above picture . Is this topic related to the earlier flat plane crankshaft discussion?
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  7. #7
    Team Z8 ZMates's Avatar
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    Quote Originally Posted by JoshB View Post
    I imagine a lot of this analysis has already been done by Dinan and others in building racing versions of the S62. Since our engine is not cutting edge for racing development any more I wonder if you could not persuade someone to give you the info and not reinvent the wheel!
    Good idea! I have had few email exchanges with Dusty Renteria at Dinan and my S62 buddy over on M5board, Peter, has had more in depth discussions (I didn't want to bother Dusty with the same questions). Dusty was kind enough to set me up with the bits I needed to install the dry sump Dinan uses on their Daytona prototype engines. Dinan gets a nice power bump from the dry sump because the improved oil control allows them to run a lower tension oil ring and no second compression ring, which reduces frictional losses.

    However, the Daytona prototypes engines are otherwise pretty close to stock. They do run lighter pistons, smaller piston pins, and a billet crank. Otherwise, though, no changes to heads, cams, or compression ratio are allowed and the engine is limited to 6800rpm. They are using stock main caps, bolts, and harmonic balancer.

    According to Dusty, the S62 won't last long above 8k rpm. Unfortunately, from the simulations I have done, peak NA power from ported heads and more aggressive cams (I have flow numbers for heads from 4 different shops) comes between 8-8.5k rpm and you should have some headroom above this range if you are going to actually use this power. So there is a lot to be gained from sorting this out. In addition, I am installing a transaxle, so I need to worry about resonances and stresses of the driveshaft too. You can be sure that this project will be well engineered! And I'm having a blast doing it.

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    Quote Originally Posted by EDEZ8 View Post
    I thought I was somewhat technical, but now I'm not so sure after reviewing the above picture . Is this topic related to the earlier flat plane crankshaft discussion?


    Good memory! The answer is "possibly". There are a lot of advantages, including stress on the engine. Plus it sounds sooooooo much better to my ears.
    Silver/black
    Dinan S2 package: headers, throttle bodies, oversized MAFs, airfilters, anti-roll bars, lightweight flywheel
    Dinan by Brembo brake kit and monoball control arm bushings
    BBS forged individual wheels
    Quaife differential, 3.64:1 final drive
    Meisterschaft GT titanium mufflers
    K&W 3 way adjustable competition monotube dampers and monoball adjustable mounts
    CDV delete

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