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Disastrous effects of rotational inertia?

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Syndrome View Drop Down
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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 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote buckinghams_pie Quote  Post ReplyReply Direct Link To This Post Posted: Aug/26/2019 at 11:01am
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote sujandinesh Quote  Post ReplyReply Direct Link To This Post Posted: Aug/28/2019 at 6:17am
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?  
RVCE BAJA: 2011-2015
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Syndrome Quote  Post ReplyReply Direct Link To This Post Posted: Aug/28/2019 at 8:29am
@sujandinesh
Yes, all the values are based on our components.
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