Disastrous effects of rotational inertia? |
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Syndrome
Bolt Sorter Joined: Sep/01/2018 Location: India Status: Offline Points: 13 |
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Posted: Aug/25/2019 at 8:58am |
Hello
I am working on the longitudnal vehicle dynamics for our vehicle, to select a gear ratio. So far I wasn't consider any rotating mass, and assumed a vehicle mass of 225 kg (including the driver). Then I came across this concept of "Equivalent Rotating Mass" where in, basically, we take into account the rotational effects of rotating mass (as it'd require a higher torque to give that mass an angular acceleration and linear acceleration; more on that here : http://www.thecartech.com/subjects/auto_eng/car_performance_formulas.htm ). When I calculated the equivalent mass for our vehicle it rose up straight by 115 kg!! (therefore, total car weight 340 kg). Basically this has flipped over all of my calculations. I was aiming for a speed of 60 kmph and now, at the ratio for dynamic balance at 30° incline, my vehicle can't pass 40 kmph. What the hell happened? How credible is this? And do other teams even actually take this into account? Because I haven't found a single mention of 'this problem' (massive mass increase) on the forum anywhere. And on the thread by Akron (http://forums.bajasae.net/forum/absolute-top-speed-of-competition-ready-car_topic1037.html) , people have taken the mass of their vehicle as ≈200 kgs. I don't see any way that could be the case with ROTATIONAL inertia included. Please help me out. Edited by Syndrome - Aug/25/2019 at 9:01am |
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buckinghams_pie
Bolt Sorter Joined: Jun/09/2019 Location: us Status: Offline Points: 21 |
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Your vehicle mass seems reasonable, your 60kph top speed seems slightly high but not unreasonable, maybe your rotational mass is too high
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sujandinesh
Baja Godfather Joined: Dec/27/2013 Location: Enschede Status: Offline Points: 494 |
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I am not an expert on the subject, but if I look at the equation used to calculate the equivalent mass of rotating parts, I see they have used certain values. I hope that when you made the calculation for your car, you changed those values based on your components (polar moment of inertias, efficiencies etc.).
Is that what you did?
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RVCE BAJA: 2011-2015
General Motors: 2015-2016 Tyre Testing - University Racing Eindhoven: 2016-2017 Tyre Dynamics - TASS International: 2017-2018 Tyre Engineer - Apollo Vredestein The Netherlands |
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Syndrome
Bolt Sorter Joined: Sep/01/2018 Location: India Status: Offline Points: 13 |
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@sujandinesh
Yes, all the values are based on our components. |
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