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Thread: Reducing the rotational inertia to maximize efficiency

  1. #1

    Reducing the rotational inertia to maximize efficiency

    I just read an article that has inspired me to think once more about rotational inertia.

    Gordon Murray Frames a New Future for Automaking | Autopia | Wired.com

    The other thing that kills you is rotational inertia. If you’ve got a bigger car, you need bigger wheels and tires, and you need a bigger engine, so the crank and the flywheel and the gears are all bigger. If you build a small car with smaller, lighter wheels and a smaller engine, smaller transmission, the inertia drops as well.
    I've mentioned in other threads that I am keen on getting semi-solid forged wheels for my Tesla Roadster. I want lighter and stronger than the standard Tesla Motors forged wheels. I am trying to find a way to get SSR Type-C SSF wheels in a 5/110 bolt pattern. They do not seem to have a 16x6 wheel, but they do have 16x7 and 17x7.5

    SSR Wheels
    $450 16x7.0J +42 offset 12.1 lb (normally 5/114.3)
    $510 17x7.5J +49 offset 14.9 lb (normally 5/100)

    A set of these for the Roadster might only be $1,920.00 and would total 54 lbs. That's a savings of 14 lbs versus the $2,900 forged Tesla wheels and a savings of 36 lbs versus the stock cast Tesla wheels. The catch is that SSR Wheels would need to make about 40 of these and do crash tests just to change the bolt pattern from what they're already selling in these sizes. I fully expect that the crash testing will raise the price from the $1,920 for similar models quite a bit. Even if they cost more than the Tesla Motors upgrade, I still think it would be worth it.

    If anyone else is interested, please say so. It's about time that I called SSR Wheels again to see whether they can quote the cost of altering their existing product with the Tesla Roadster bolt pattern. It would be nice to know that someone else might want lighter wheels without sacrificing strength and performance.
    1990 Honda CRX Si - Barbados Yellow
    1998 AM General HMCO - Competition Yellow
    2000 Honda S-2000 - Berlina Black
    2011 Tesla Roadster 2.5 - Very Orange

  2. #2
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    Maybe you could have material machined out of the forged wheels just for a racing set? With the forged wheels being seriously over-engineered, they might tolerate having some material removed.

  3. #3
    Quote Originally Posted by zack View Post
    Maybe you could have material machined out of the forged wheels just for a racing set? With the forged wheels being seriously over-engineered, they might tolerate having some material removed.
    Well, maybe I'm just a sucker for advanced engineering, but the SSF technology is supposedly denser than normal forging. I just don't think there's any way to drill 3.5 lbs of aluminum from each of the standard forged wheels and have anything left. Besides, the latter would require a significant amount of risk by trusting a one-off modification that hasn't been proven.

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    Understood. Yet, I discover something about my Roadster that isn't quite "proven" on a regular basis. 8^D

  5. #5
    EU Model S P-37 VolkerP's Avatar
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    Lighter wheels are better, no question. Lighter wheels improve surface contact as the wheel returns faster to the ground after being lifted by a bump.
    WRT efficiency:
    wheel weight is added to "unsprung" vehicle weight, which increases rolling resistance on bumpy roads. It's total weight that counts, regardless of distribution.

    rotational inertia is responsible for how much energy your drive train must deliver to make the wheels turn, even without moving the vehicle. Imagine your car jacked up, with the rear axle in the air. BUT the energy contained in the turning wheels is recovered when slowing down (e.g. regen braking, or slowing down when going uphill). So where is the point of reducing rotational inertia, besides of the benefit of lighter wheels?

    If you go for that, check that there are significant mass reductions near the wheel rim. A fly wheel concentrates mass at its outside. Go look for the opposite. Benchmark your expensive new wheels before purchase by letting them roll down a slope. The wheel with it's mass concentrated near the axle will outaccelerate the standard wheel.

  6. #6
    Thanks for the details! Good stuff.

    Quote Originally Posted by VolkerP View Post
    So where is the point of reducing rotational inertia, besides of the benefit of lighter wheels?
    If you favor quick acceleration significantly more than regeneration, like me, then reducing rotational inertia is really effective.

    If you go for that, check that there are significant mass reductions near the wheel rim. A fly wheel concentrates mass at its outside. Go look for the opposite. Benchmark your expensive new wheels before purchase by letting them roll down a slope. The wheel with it's mass concentrated near the axle will outaccelerate the standard wheel.
    That's a good test. I won't be able to benchmark before purchase, though, since I'll probably have to buy 40 of these to get one set. (on that note, does the Model S also have this bizarre 5x110mm bolt pattern?)

    The disadvantage with the Roadster is the "larger" diameter. I realize that 19" rims are all the rage, but serious racers are actually down-sizing to 13" wheels. My existing SSR Wheels are 15" and weigh only 8 lbs each. That's significantly less mass than most wheels available, even the stock 14" wheels for my old Honda (except the 13" wheels mentioned above).
    Last edited by S-2000 Roadster; 2012-01-17 at 09:58 PM.

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    Member driver_EV's Avatar
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    Is there any consideration regarding having matching custom wheel size on front and rear? say, 16x7 all around? I think this may allow a more balanced handling (less under-steer).

    Also difference in weight between example pair of 16" and 17" SSR wheels being ~ 5lbs:

    ref:
    SSR Wheels
    $450 16x7.0J +42 offset 12.1 lb (normally 5/114.3)
    $510 17x7.5J +49 offset 14.9 lb (normally 5/100)

  8. #8
    Senior Member dhrivnak's Avatar
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    I would consider lighter wheels but not for a year as I need to pay off the Roadster first. .

  9. #9
    Electron Pilot Todd Burch's Avatar
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    Quote Originally Posted by VolkerP View Post
    So where is the point of reducing rotational inertia, besides of the benefit of lighter wheels?
    In addition to S-2000 (related to Andre 3000?)'s acceleration argument, with regen you only recover 80-85% of the original energy...and that's if the battery's not near a full SOC. So you're still losing at least 15% of the additional energy required to rotate heavier wheels.

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