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#1
04-27-2012, 07:06 PM
 coulombKid NEAA Member Join Date: Jan 2009 Location: Mesa Posts: 236
Force, mass, & acceleration (sanity check)

vehicle mass: 1,500 kg
motor torque: 140 nm
low gear: 1.86:1
ring gear: 3.66:1

Neglecting all losses I calculate a rear axle torque (in low) of 953 nm.
With a 0.33 meter wheel radius I calculate a reaction force of 2,888 newtons.

Newtons law indicates that this contraption (neglecting losses & assuming 100% tire adhesion) would be capable of accelerating at 1.92 meters per second per second.

Assuming that 1 g is about 9.81 m/s/s that would indicate a maximum acceleration of about 0.196 gees. I rarely do this type of calculation and usually not in the metric system. How would this hypothetical vehicle drive? lead sled? rocket? Am I even close?
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#2
04-27-2012, 08:05 PM
 Duncan Senior Member Join Date: Dec 2008 Location: Southland New Zealand Posts: 1,701
Re: Force, mass, & acceleration (sanity check)

Hi Kid

60Mph is about 30m/sec so 2m/s/s will give 0 - 60 in 15 seconds

Your "low gear" is 1.86 : 1 ? - sounds like quite a high low gear
#3
04-27-2012, 08:11 PM
 Joey Senior Member Join Date: Oct 2007 Location: Oregon, USA Posts: 271
Re: Force, mass, & acceleration (sanity check)

1.92 meters/s^2 = 6.93 km/hr/sec

Under constant acceleration you could reach 95 km/hr (0-60 MPH) in 14 seconds. However without changing gears you would be at 5280 RPM. I know my motor will not have constant torque out past 2500 RPM.
#4
04-27-2012, 09:01 PM
 coulombKid NEAA Member Join Date: Jan 2009 Location: Mesa Posts: 236
Re: Force, mass, & acceleration (sanity check)

Quote:
 Originally Posted by Joey 1.92 meters/s^2 = 6.93 km/hr/sec Under constant acceleration you could reach 95 km/hr (0-60 MPH) in 14 seconds. However without changing gears you would be at 5280 RPM. I know my motor will not have constant torque out past 2500 RPM.
If I used a Powerglide the torque converter, if used, would help. The maximum rpm recommended for the Kostov 13 is 4450 RPM. That would indicate that the governor should be adjusted to shift at about 50 MPH. Neglecting converter slip that would put the car at 94 MPH in high when the motor is at it's maximum recommended speed. The 140 nm figure came from their print. I've never seen a Kostov 13 used by any of our members so it's all bench racing at this point. I've got all the drag race parts for the TH700R4. Sounds like it would be a much better tranny choice once I weed out the mechanical fuse parts we're familiar with for the more modern automatic.
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Last edited by coulombKid; 04-27-2012 at 09:24 PM.
#5
04-27-2012, 10:42 PM
 nulluser Junior Member Join Date: Mar 2012 Location: SE Michigan Posts: 15
Re: Force, mass, & acceleration (sanity check)

I will do it in English units for reference.

Weight = 3307 lbs
Mass = 102.7 slugs
Torque = 103.3 ft-lbs
Final Ratio = 6.8076:1

Force = Torque / Radius * Final Ratio
Force = 103.3 ft-lbs / 1.083 ft * 6.8076
Force = 649 lbs

Force = Mass * Acceleration
Acceleration = Force / Mass
Acceleration = 649 lbs / 102.7 Slugs
Acceleration = 6.32 ft/sec^2
Acceleration = 0.196 g

Velocity = Acceleration * time
Time = Velocity / Acceleration
Time = 88 ft-sec / 6.32
Time = 13.9 Seconds

This is all assuming constant torque of course.
#6
04-28-2012, 02:51 AM
 MalcolmB Senior Member Join Date: Jun 2008 Location: Newcastle upon Tyne, UK Posts: 521
Re: Force, mass, & acceleration (sanity check)

Quote:
 Originally Posted by nulluser I will do it in English units for reference.
Not wanting to be pedantic, but we switched to the metric system decades ago. I thought slugs were those slimy creatures that eat my lettuce

Maybe you meant the Imperial system?

On topic: 0.2 g sounds a little undergeared to me. I've been aiming for 0.3 g minimum to cover odd situations such as steep hill starts and climbing ramps.
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#7
04-28-2012, 04:36 AM
 Woodsmith Spam Busting Admin Join Date: Jun 2008 Location: Manchester UK Posts: 4,370
Re: Force, mass, & acceleration (sanity check)

I think we call it imperial, the Americans call it English, because we started it.

Metric is much easier to work with given SI units, which I think maybe French.
Imperial is much more real world, at least for us older Brits.
#8
04-28-2012, 07:31 AM
 bliksem Member Join Date: Aug 2009 Posts: 30
Re: Force, mass, & acceleration (sanity check)

It is called the imperial system because it is a royal pain in the butt to use.
#9
04-28-2012, 10:41 AM
 Salty9 Senior Member Join Date: Jul 2009 Location: 42.596N 122.688W Posts: 741
Re: Force, mass, & acceleration (sanity check)

Quote:
 Originally Posted by bliksem It is called the imperial system because it is a royal pain in the butt to use.
Too true and the reason I switched from an engineering major.
#10
04-28-2012, 10:45 AM
 Ryan800 Member Join Date: Apr 2010 Location: Seattle, WA, USA Posts: 76
Re: Force, mass, & acceleration (sanity check)

My car puts somewhere around 810 Nm to the wheels and weighs about 1125kg (with one person). My tires have about a .31m raduis, so this comes out to a theoretical low speed acceleration of 2.3 m/s2.

My car is very slow, and i'm going to put a larger motor in soon. However, it does start and drive up all of the hills in the area, of which there are many. It definitely does not go 0-60 in 15s because torque starts falling off around 25mph.

If you can get more torque or lower gearing, definitely do it. If not and you live in a flat area you are probably fine as long as you can accept very minimal performance. If you have hills, you will probably get tired of the slow starts pretty quickly, I know I did.

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