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Flywheel question for a truck

  1. Aug 9, 2011 #1
    Hello, I have a question on a flywheel. I have been racking my brain over this for months, and still no good answer.I i have a 4 wheel drive truck, that is 4,500.lb. 100hp at 4,000.rpm, and 200lb. ft at 2,000.rpm, at the flywheel. The vehical drives at 50mph at 2,500.rpm. with a 1 to 1 final drive ratio. with a loss of 40% thrue the driveline. 31x10.5 tires. What would be a better flywheel? a 10lb or an 30lb. flywheel? on flat ground i could see that the heavier wheel would keep up inerta, but would the lighter wheel be better for going up hills of a constant grade, sutch as a 2mi. hill of 15% grade. if i needed to accelerate up this hill, there would be more power at the wheels/tires, using the lighter flywheel and therefore use less engine power to do the same amount of work, or would there be enough energy in the 30lb. flywheel to help get up the hill. Just an old guy trying to get better performance from an old truck, and trying to figure out some science.:) any help would be wonderfull!!!
  2. jcsd
  3. Aug 10, 2011 #2


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    lots of big, scary numbers and performance specificities, you might have more luck in the engineering forum.
  4. Aug 10, 2011 #3


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    The main purposes of a flywheel is to allow the engine to idle at a reasonably low rpm and to allow for an easier launch from a standing start if using a manual clutch. If the clutch is too light, then idle rpm will need to be set higher and launching will require more throttle input and more precise application of the clutch. If the clutch is too heavy, you're wasting power when the clutch is accelerated. Considering small to medium size cars use 15 to 20 lb clutches, it would seem that a 10 lb clutch would be too light for a full size truck.
  5. Aug 10, 2011 #4
    Something about this makes me think it's a homework problem.

    Assuming you're being truthful, I'm puzzled by some things:
    Where did you get those numbers? If you're just guessing, that's fine, but they didn't look realistic, so I calculated the missing values:
    200 ft/lbs @ 2000 = 76 HP
    100 HP @ 4000 = 131 ft/lbs
    That just looks wrong to me.

    A vehicle that heavy, with so little power will likely work better with a heavier flywheel. What engine are you using that has such a wide range of flywheel weights available?

    Uh, no, that might be the transmission ratio, but "final drive" is considered to be the drive axle's ring & pinion ratio, or the ratio between engine RPM and axle RPM.

    Flywheel weight will not affect steady-state speed. A lighter flywheel will let the engine change RPM more quickly, but in the real world, with a heavy truck and a low-power engine, you aren't going to be accelerating very rapidly up any grades! :smile:
  6. Aug 10, 2011 #5
    Pantaz, Lets just use your hp and torque specs, i was just rounding #s. The truck is a 1981 toyota 4wd, with a 22r carb. motor. your specs are just about right on the mark. I understand that taking off would be more difficult, and even harder to start if hauling something. but if i have the right gearing. And yes i am sorry for the mix up on the "1 to 1 ratio." Would it not accelerate faster, or at least keep more engine torque to the ground by using the lighter flywheel? if this is not the case please can someone show me why in a formula. Yes i also know that a more powerfull engine may be the better route to go, but that relly is not the point. I am trying to understand the "why", between the two flywheels, I can order a 9 1/2lb.,12lb,16lb,21lb,31lb,32lb,and a 38lb. flywheel for this engine. again, any help would be great. Thanks Tom
  7. Aug 11, 2011 #6
    I searched for specs on the OEM flywheel (about 25 pounds) and I found some helpful information. The general recommendation for a 22R in a 4x4 is the 30lb flywheel. If you are running taller than stock tires, lower gearing (ring & pinion) is also recommended.

    I recommend you look over some of the 4x4 forums, such as http://www.pirate4x4.com/forum/forumdisplay.php?f=12". Many helpful people with real-world experience in modifying their rides.

    The flywheel weight influences how quickly the engine can accelerate, but it also works in the opposite. External forces at the wheels (such as gravity when climbing hills) will be able to slow the engine more quickly.

    As to supplying a formula for comparing flywheels, good luck with that. :smile: At its essence it's about rotational inertia. BUT... there are a lot of variables, including how the vehicle is driven and over what type of terrain.

    FYI: Engine http://www.4x4wire.com/toyota/faq/parts/" for 1981-87 22R: 96 HP @ 4800 rpm, 129 lb/ft @ 2800 rpm.
    Last edited by a moderator: Apr 26, 2017
  8. Aug 11, 2011 #7
    Thanks for all of the help, I believe that i understand now. Thanks again. Tom
  9. Aug 12, 2011 #8
    If the hill takes longer than a second or two to climb, I doubt a heavier flywheel will do anything for you in that regard. I don't think they store nearly enough energy to make a difference.

    Heavier flywheels generally make it easier to start moving (especially uphill), as they make it harder to stall the engine. However, if you're looking for performance (at the sacrifice of "comfort") then generally the lighter the flywheel the better.

    I have heard claims that a heavier flywheel will help you launch a car under specific circumstances in drag races, but that's another story and I can't speak too much about that.
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