Thread: Lighter flywheels

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  1. #1 Lighter flywheels 
    Young Brit
    Join Date
    Feb 2012
    A while back I ran the numbers for switching from an iron to aluminum
    flywheel for my Triumph TR8. There are a couple of approaches to doing
    the math. The more rigorous approach is to calculate the polar moment
    of inertia for the two different flywheels, adjust for the square of
    the overall gearing (transmission, final drive and tires) and convert
    to an equivalent linear inertia. The second method (the one I chose)
    is to start with a known linear to rotational equivalent and ratio from
    there. The known relationship I used is a solid disk rolling on its
    edge. It has an effective inertia exactly 1.5 times what it would be
    if it wasn't rotating. That means the rotational component is 50% of
    the linear component. Adjust for the square in gearing and you have
    the answer. I wrote a little program to do the calculations. I assumed
    a 12" diameter flywheel which is the Buick/Rover diameter, less the ring
    gear. The circumerence of a circle is the diameter multiplied by pi.
    So if you roll the flywheel along the ground it will move 37.7 linear
    inches per revolution (= pi * 12). A 205/50/15 has a diameter of
    approximately 23.1 inches. My TR8's final drive ratio is 3.45:1 and
    first gear is 3.32:1 so one revolution of the flywheel results in the
    car moving around 6.3 inches. Ratio the squares and take half
    ((37.7/6.3)**2)/2 = 17.9. So each pound removed from the flywheel
    (equally across the face) is the same as about 18 pounds of weight
    removed from the car when in first gear. So if you remove ten pounds
    from the flywheel (equally across the face), the result is equivalent
    to removing 180 pounds of vehicle weight in first gear. The effect
    goes down for each higher gear, of course. If that weight is removed
    from the outside of the flywheel only, the effect is about 2.78 times
    as strong since a solid disk has a radius of gyration of 0.6 times the
    radius (1.0/0.6)**2 is 2.78). 2.78 * 180 is 500 lbs equivalent weight
    reduction. A non-trivial effect.

    I don't recall whether the TR8 flywheel has the big hub like the
    GM 215 flywheels so I ran the numbers both ways, assuming a 3.45:1
    final drive ratio, 205/50/15 tires and LT77 gear ratios of:

    1st 3.32:1
    2nd 2.09:1
    3rd 1.40:1
    4th 1.00:1
    5th 0.83:1

    along with flywheel weights of:

    stock flywheel - 32 lbs
    lightened steel - 22 lbs
    aluminum - 11 lbs

    If the stock flywheel has the big ring, then lightening it is similar
    to removing from the perimeter (from 32 to 22 lbs). In the numbers
    below, I didn't do it that way but a more accurate approach for the
    aluminum flywheel would be to assume a reduction of 22 to 11 lbs equally
    across the face and add that to the difference of the 32 to 22 lbs across
    the perimeter. In any event, a lighter flywheel looks like a good thing
    to do for performance. Here are the numbers:

    32 to 22 lbs (across face assumption):
    1st 177.5 lbs
    2nd 70.3 lbs
    3rd 31.6 lbs
    4th 16.1 lbs
    5th 11.1 lbs

    32 to 22 lbs (perimeter reduction assumption):
    1st 493.4 lbs
    2nd 195.5 lbs
    3rd 87.7 lbs
    4th 44.8 lbs
    5th 30.8 lbs

    32 to 11 lbs (across face assumption):
    1st 372.7 lbs
    2nd 147.7 lbs
    3rd 66.3 lbs
    4th 33.8 lbs
    5th 23.3 lbs

    32 to 11 lbs (perimeter reduction assumption):
    1st 1036.1 lbs
    2nd 410.6 lbs
    3rd 184.2 lbs
    4th 94.0 lbs
    5th 64.8 lbs

    Rotational inertia is mass multiplied by the distance from the
    rotational axis (integrated over the surface). The effect is
    stronger farther away from the hub. The best is from the
    perimeter. Equally across the face is less effective and near
    the hub is the least effective. In my example, dropping 10
    pounds from the perimeter is equivalent to 493 lbs weight
    reduction in first gear while across the face was equivalent
    to 177 lbs. Reducing from the hub is even less effective since
    the effect is a function of the diameter squared.

    Dan Jones
    St. Louis, MO
    1980 TR8 Convertible
    1977 TR7 Coupe
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  2. #2  
    Ole Boy
    Join Date
    Feb 2012
    Mt Pleasant SC
    Dan, I was a 3.33 student in school and I would still make sure I sat next to you!!

    "Stick a Wedge In It"
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  3. #3  
    Great write up and very easy to understand.

    A lighten flywheel definitely makes a noticeable difference in our cars, along with most cars out there. I have driven a few different kinds of cars before and after lightened flywheels and it was one of the most noticeable modifications I made.

    I will move this to along with the cam design topic over to the technical section in a few weeks.

    The Wedge Shop
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  4. #4  
    Young Brit
    Join Date
    Feb 2012
    Another awesome post.
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  5. #5  
    Ole Boy
    Join Date
    Feb 2012
    Mt Pleasant SC
    dan- May i pass that along to other forums?

    "Stick a Wedge In It"
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  6. #6  
    Young Brit
    Join Date
    Feb 2012
    Thanks guys. Feel free to forward the posts. I've got a few more I'll
    post as time permits.

    Dan Jones
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  7. #7  
    Little Brit
    Join Date
    Feb 2012
    With the lighten flywheel and I would assume the resultant loss of inertia, how does this affect the on and off throttle response on the street? I always thought it would result in more abrupt on/off throttle response, good for the track but, annoying on the street. I would also some loss of inertia on launching. Just curious from people who have done this change. Thanks
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  8. #8 flywheel 
    Little Brit
    Join Date
    Feb 2012
    I would like to know how much to reduce thickness and how far to go in towards crank flange and do you cut any on clutch face side. I have access to a large lathe so I could do it myself also any rebalance need done?
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