TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Reducing the rotational inertia to maximize efficiency

Discussion in 'Roadster: Performance' started by S-2000 Roadster, Sep 20, 2011.

  1. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    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

    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.
     
  2. zack

    zack Member

    Joined:
    Nov 15, 2010
    Messages:
    946
    Location:
    Minneapolis, MN
    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. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    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.
     
  4. zack

    zack Member

    Joined:
    Nov 15, 2010
    Messages:
    946
    Location:
    Minneapolis, MN
    Understood. Yet, I discover something about my Roadster that isn't quite "proven" on a regular basis. 8^D
     
  5. VolkerP

    VolkerP EU Model S P-37

    Joined:
    Jul 6, 2011
    Messages:
    2,451
    Location:
    Germany
    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. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    #6 S-2000 Roadster, Sep 25, 2011
    Last edited: Jan 17, 2012
    Thanks for the details! Good stuff.

    If you favor quick acceleration significantly more than regeneration, like me, then reducing rotational inertia is really effective.

    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).
     
  7. driver_EV

    driver_EV Member

    Joined:
    Dec 7, 2011
    Messages:
    147
    Location:
    Charleston, South Carolina
    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. dhrivnak

    dhrivnak Active Member

    Joined:
    Jan 8, 2011
    Messages:
    2,091
    Location:
    NE Tennessee
    I would consider lighter wheels but not for a year as I need to pay off the Roadster first. ;).
     
  9. Todd Burch

    Todd Burch Electron Pilot

    Joined:
    Nov 3, 2009
    Messages:
    4,577
    Location:
    Smithfield, VA
    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.
     

Share This Page