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Flywheel Design?

  1. Nov 3, 2009 #1
    right basically i have decided to design a flywheel for my car engine and i would like to know what factors will determine the shape and weight of it?is there a way of working out what is the minimum weight that it could be
  2. jcsd
  3. Nov 3, 2009 #2
    Re: rotating objects?

    Minimum weight will depend on quite a few things. You will also want to look at the flywheels moment of inertia (which is probably more important if you are racing as that affects rotational inertia). Thickness and material will also be an issue, too thin and it'll shatter when put under load.

    How heavy is the load you are pulling (in this case car + usual occupents)?
    What is the engine configuration?
    That is it's typical torque output?

    What are you desiging the flywheel for? Normal driving or racing?

    I'll think some more and come back.

    Tbh Ranger Mike is better at than me at this sort of stuff. I've never designed or messed with flywheels before (apart from seeing them machined down) so I don't know all the factors to consider.
    Last edited: Nov 3, 2009
  4. Nov 3, 2009 #3
    Re: rotating objects?

    thanks for your reply mate.
    i have only started to have a look into moments of inertia.
    the standard flywheel is steel and i want to make one out of alloy so it is lighter but i should have more metal to play with to retain the strength.
  5. Nov 3, 2009 #4
    Re: rotating objects?

    The key is you want to have enough rotational inertia to overcome the time when the engine isn't making power.

    The inertia will then define the mass and size needed.

    I'm just trying to dig out my design book to find some information on this and how to determine minimum inertia. Bare with me :)
  6. Nov 3, 2009 #5


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    Gold Member

    Re: rotating objects?

    My initial thought is that you should use an aluminum alloy or composite material for the flywheel itself, and a good steel ring gear for the perimeter. In any event, your mass should be concentrated in the outer section.
  7. Nov 3, 2009 #6
    Re: rotating objects?

    Hmm I know there are ally flywheels out there, but it strikes me as a bad idea longterm. The fact that Al had no fatigue limit, especially in something that has constant changing load. A short term lightweight racing flywheel that you needed bulk I can see the point of though.

    I'd say a chromoly steel would be most suitable for a general purpose flywheel.
  8. Nov 4, 2009 #7
    Re: rotating objects?

    so today i have been trying to understand moment of inertia. now i basically understand that for my flywheel i need to reduce the MOI.but i would like to make it out af a different material and need to know how to calculate the existing steel flywheels MOI and what my new one will need to be
  9. Nov 4, 2009 #8
    Re: rotating objects?

    To accurately work out the MOI you need to do integration of the:
    I = int(r^2 * dm)
    The further the mass is away from the centre, the bigger the effect of MOI.

    If you aren't great at maths, then you can create a spreadsheet that works this out. Or use an assumed simplification.

    You would need to use this to find the MOI if you are using a non cylindrical shape. So if you wanted to make the outside edge thicker to increase moi.

    If you are using a uniform cylindrical shaped flywheel, the above can be simplified to:

    I = 0.5m*r^2.
  10. Nov 4, 2009 #9


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    Staff: Mentor

    Re: rotating objects?

    No one asked the obvious questions: doesn't your car already have a flywheel? For what reason are you trying to redesign it? What are you trying to gain?
  11. Nov 4, 2009 #10
    Re: rotating objects?

    Lighter flywheel as he is switching from steel to al alloy. So i'm assuming he wants more response.
  12. Nov 7, 2009 #11
    Re: rotating objects?

    well the standard flywheel that im using is very heavy.i have had in the past material removed from these steel flywheels and of course had them balanced but i have never understood the whole theory behind it.so now i would like to design my own using a lighter material which will hopefully allow me to retain the original strength but also be much lighter.i just need to calculate the lowest weight that i can go achieve and go from there.

    what about this:

    if i want to design a flywheel for an engine that has never had a flywheel designed for it how do you work out the weight that the flywheel needs to be and how do i know where the metal needs to be on the flywheel?

    obviously i must take into account the weight of the starter ring gear and the flywheel - crank bolts?

    the only info i have is what the diameter of the flywheel needs to be
  13. Nov 9, 2009 #12
    Re: rotating objects?

  14. Nov 9, 2009 #13


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    Staff: Mentor

    Re: rotating objects?

    It's a very complicated question. You may have more luck in the mechanical engineering forum. Moving the thread...

    [edit: I also renamed to to be a little more descriptive.]
  15. Nov 9, 2009 #14
    Well I can tell you what you need to find out. Just not how, i'm trying to read up on exaclt what you have to to, it's why its taking so long.

    You need to find the difference in energies, between what is given to the flywheel under the power strok, and what is taken away when it's not on a power stoke, and an acceptable rpm fluctuation.

    You then use this to calcualte a suitable I value. Which knowing the diameter will give you the minimum weight and thickness.

    Just exactly HOW you find the energy difference, is what i'm currently stumped on. I suspect you'd need torque and crank velocity vs crank angle.

    I know this stuff is in Mech Eng Design by Shigley, but I can't find my copy at the moment.
  16. Nov 10, 2009 #15
    thank you mate ill have to get a copy.

    and there was me thinking that my question was so easy id be laughed at ha ha
  17. Nov 10, 2009 #16
    there are some other considerations. is the engine internally balanced, or externally. if external, there will be a minimum weight it will need to be (offset weight + flywheel) what is the max rpm. wear surface has a bunch of considerations. is it to have a different face, (aluminum w/ steel wear surface) max and min diameter OF WEAR SURFACE, not ring gear. minimum outer diameter will be a function of the output vs input. Offset from the crank surface to the wear surface can be critical, too

    aftermarket manufacturers make flywheels in many different configs. aluminum, nodular, stamped steel. but, I have to ask:

    if this flywheel shatters at full rpm, what contains it, and is that containment strong enough to ensure you, and occupents, and all bystanders are safe.
    I have seen flywheels/clutch/pressure plate assys grenade. in a rear wheel drive car, they can RIP YOUR LEGS APART. they will come thru an aluminum bellhousing, often exploding it, too. heres a link to speedways flywheel section, you can see there are lots of possible configurations, from ram couplers to multidiscs.

    http://www.speedwaymotors.com/ProductSummary.aspx?free_text|11/10/2009%207:04:51%20AM=flywheels&deptId=0 [Broken]

    good luck

    Last edited by a moderator: May 4, 2017
  18. Sep 10, 2010 #17
    hello again everyone

    totally forgot about this as was busy with uni work anyway i have decided to lighten my existing iron flywheel now.

    so i know the moment of inertia of the standard flywheel to be 0.1483 kgm^2 (does this sound right?)
    so if i calculated the kinetic energy as
    K.E=1/2 I w^2
    using an angular velocity idle speed of 83.77 rad/s then
    gives me 520.34j

    (am i right so far?)

    so if this is the K.E of the flywheel at idle then how can i find out what K.E is needed by the engine to allow it to get between power strokes?

    really sorry if im incorrect on my workings but im still a bit new to this.

    also i know the radius of gyration of the standard flywheel 1s 116mm so in theory this is the place to remove material?

    i greatly appreciate all help
  19. Sep 11, 2010 #18

    Ranger Mike

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    Gold Member

    hi all..how did i miss this post?
    ifin it were me , i would not want to try to re-invent the (fly) wheel!..sorry, bad pun..
    i would lighten the existing flywheel or buy an after market aluminum flywheel.
    any time you can reduce rotating weight, you will increase responsiveness of the engine. Every pound of rotating weight removed makes the engine think the rotating mass has been reduced by 10 static pounds. thats why the race engine guys build engines with minimum weight crank shafts, connecting rods, pistons, piston pins, cam shaft drive belts vs chain. same with the flywheel..i would also look at reducing diameter of the clutch and pressure plate and the flywheel.
    caution- you can make a real nice grenade by weakening the flywheel by removing material..i have seen racers at the drag strip blow flywheels apart and you have a nice opportunity to lose a few toes!!
    go with small lightweight aftermarket products
  20. Sep 11, 2010 #19
    hi it is extremely common to lighten existing flywheels and this is what i am looking at doing.i will not be removing an unsafe amount of material
  21. Sep 13, 2010 #20
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