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Transmission System

  1. May 28, 2015 #1
    Hey guys,

    I am a bit confused while calculating top speed of the car using gear ratio, Torque, Power and Engine Speed(RPM). Can someone help me out with this?

    Specifications are given as follows:

    Maximum Torque : 19.2 N-m @ 3800 RPM
    Maximum Power : 10 Hp @ 2800 RPM

    Tire Specifications: 25x8 - R12 or 254/65 - R12

    Gear Ratios:
    Initial Input Reduction - 2.869
    First Gear Ratio - 4.6
    Second Gear Ratio - 2.733
    Third Gear Ratio - 1.667
    Fourth Gear Ratio - 1.115
    Final Forward Reduction - 2.38
    Final Backward Reduction - 4.17

    I am looking for top speed at each gear at different RPM, as well as acceleration, Torque and Tractive Effort.

    Your answers will be really helpful.

    Thanks and regards

    Karan Dua!
  2. jcsd
  3. May 28, 2015 #2


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    Homework Helper

    Wheel diameter is 12 inches? Tire width = 254mm = 10 inches. Tread wall height = 65% x 10 inches = 6.5 inches. Tire diameter = 12 + 6.5 + 6.5 = 25 inches. Multiply initial and final gear ratios (6.82822) by each gear ratio: 1st = 31.41, 2nd = 18.66, 3rd = 11.38, 4th = 7.613 . See if you can continue from here.
  4. May 28, 2015 #3


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    Staff: Mentor

    Welcome to the PF.

    Can you give us the context of the question? Is is a homework problem, or a schoolwork project, or a personal project? What kind of car is this that is running on only 10hp? Some kind of a go-kart? :smile:
  5. May 28, 2015 #4

    jack action

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    Gold Member

  6. Jun 1, 2015 #5

    Just a project!
  7. Jun 1, 2015 #6

    Randy Beikmann

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    Karan, my first comment is that your horsepower and torque peak speeds can't be right. The power peak is (necessarily) always at a higher speed than the torque peak. You may have a typo, or got bad data.
    As for top speeds in each gear, it depends on the aerodynamic drag Fd and rolling resistance Fr of the car. At each vehicle speed, calculate the amount of power it takes to propel it: (Fd+Fr)*v. Then pick a gear and calculate the engine speed omega at that vehicle speed. If the engine can make more than (Fd+Fr)*v, then the car can go faster. Keep trying higher vehicle speeds until the engine can't produce enough power to propel it, or until it hits redline.
  8. Jun 5, 2015 #7
    Hey Randy,

    Thanx a lot for your reply.
    I just want to correct myself.

    Power : 10 Hp @ 3800 RPM
    Torque: 19.2 N-m @ 2800 RPM

    How do I find acceleration at each gear @ different RPM of engine.

    For example: How do i find acceleration @ 1st gear with gear ratio of 31.45:1 @ 2800 RPM?
  9. Jun 6, 2015 #8

    Randy Beikmann

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    Karan, I couldn't tell what the tire radius rTire was from your tire spec's above, and didn't see the car's mass.
    You'd need to calculate the tractive force F, which is (19.2 N-m)*(31.45)/rTire. Then it's just a=F/m.
  10. Jun 6, 2015 #9
    The tires which we are using will be 25x8-R12. And the r will be effective rolling radius if i am not wrong? And mass of the car is 400 kgs. And what aabout acceleration on slope? How to calculate that? And do we calculate torque at each gear @ different rpm? If so, how?
  11. Jun 6, 2015 #10

    jack action

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    The acceleration is found with equation 1b on this page. The force Ft in that equation can be found with http://blog.mechguru.com/vehicle-design/tractive-force-calculation-for-a-vehicle/ [Broken] (TE = Ft).
    Last edited by a moderator: May 7, 2017
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