My shaky math comes out to:

Inertia I=W*r²

Acceleration = T/I (In Radians/sec)

Therefore Acceleration = (T/I)*r (In Meters/second²)

Right?

1 - 12 of 12 Posts

My shaky math comes out to:

Inertia I=W*r²

Acceleration = T/I (In Radians/sec)

Therefore Acceleration = (T/I)*r (In Meters/second²)

Right?

Joined

·
4,334 Posts

My shaky math comes out to:

Inertia I=W*r²

Acceleration = T/I (In Radians/sec)

Therefore Acceleration = (T/I)*r (In Meters/second²)

Right?

there's lots more involved.... real wheels are not solid discs which means that the 'r' for moment of inertia is not the rim. and then you are assuming that there is nothing attached to the wheel? you'd have to figure ALL the rotating weight you are accelerating AND the F=ma for accelerating the non-rotating weight thru space.

Could I use the size of the wheel as a ersatz "Gear", and if so, how would I calculate it's ratio?

Joined

·
4,334 Posts

this all handles the non-rotating parts.... I was just trying to point out that to be accurate you would need to find moments of inertia for all the rotating parts too to predict the acceleration. Not insignificant with the wheels plus brake rotors, all the gears/clutch/flywheel in the drivetrain, and the guts of the motor itself.

Okay, let's throw some numbers in here.

Example car: 2010 Chevy Malibu, stock weight, with a Warp9 in direct-drive through a 4:1 differential.

Vehicle weight: 1556KG

Wheel radius: 0.3345m

Motor torque: 321N-m

So, Final torque at-the-wheel is 1284N-m (4x mechanical advantage from the differential, 1/4 the speed. Right?)

acceleration = 1284/(0.3345 * 1556) = 2.4669. If that's in Radians/sec, then acceleration = 2.4669...*0.3345 = 0.82519 m/s^2

Either way, that's not much acceleration at all. 0.25G? Definitely not going to be winning any drag races with that. So where's my math wrong?

Joined

·
4,334 Posts

Do let me know if you spot any problems. I'm sorry about the Javascript being all on one line, but apparently Blogger loves to insert br tags everywhere. :/

nice little thing : )

Do let me know if you spot any problems. I'm sorry about the Javascript being all on one line, but apparently Blogger loves to insert br tags everywhere. :/

just add fields filling exaples, note or comment in rpm field: torque up to given rpm - constant, also may be a title - calculated theoretical constant acceleration; constant torque model as opposite to contant power (ideal CVT model), where torque and acceleration decreases wile speed increases

may be specify (to be clear) - accel calc. w/o taking into account resistance (wind, rolling)

Dear Sir,

but isnt this equation bit deceptive. It depicts that if one increases the wheel torque to a very large number by using high motor torque and gear ratio, the acceleration would be enormous. Practically such a high acc would cause tyres to skid. Is there way to calculate max allowable acceleration considering the point after which the tyre skids?

I think people are a little more conscious of the limits of tire traction these days, hopefully. It is damn nice to have limits when you are designing a system too!

1 - 12 of 12 Posts

- This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.

DIY Electric Car Forums

A forum community dedicated to DIY electric car owners and enthusiasts. Come join the discussion about electric vehicle conversions, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!

Full Forum Listing
Recommended Communities

Join now to ask and comment!