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Advantages of mid-motor vs. rear-motor in a RWD buggy?

Advantages of mid-motor vs. rear-motor in a RWD buggy?

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Old 01-21-2016, 07:40 AM
  #31  
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When I went to the track last year I took my B5 rear motor and my B5M mid motor and would run them back to back to see which car was better on that day for the track conditions. I bet 8 out of 10 times I chose the B5M. That being said. I bought a losi 22 3.0 not too long ago. And now every time I go to the track the 3.0 is better than my B5M. I would say if you want another 2wd buggy. Even though you are considering a rear motor car. I would look into the 22 3.0. and if you really really want a rear motor car. Find a used 22 2.0 rear, or a B5.
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Old 01-21-2016, 10:33 AM
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Exactly where the mass is distributed doesn't matter. You use the distributed resultant if you want to find the moment. The center of mass of a rear motor car is still well within the wheelbase.

Originally Posted by Davidka View Post
I'm no engineer (obviously)
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Old 01-21-2016, 11:55 AM
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Originally Posted by nikg View Post
Exactly where the mass is distributed doesn't matter. You use the distributed resultant if you want to find the moment. The center of mass of a rear motor car is still well within the wheelbase.
It does when considering the moment of inertia, and how quickly/easily the car will rotate, and stop rotating. Like during cornering.
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Old 01-21-2016, 12:00 PM
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Try taking a 1m (or 3ft) pipe and put a 1kg weight on each end, hold it in the middle and try to turn it back and forth. Now out the same weight in the middle and do the same. You will find with the weight in the middle it will be much easier to control.

That's the advantage of mid motor.
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Old 01-21-2016, 12:39 PM
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Originally Posted by nikg View Post
Exactly where the mass is distributed doesn't matter. You use the distributed resultant if you want to find the moment. The center of mass of a rear motor car is still well within the wheelbase.
Sounds great but it doesn't reconcile with what everyone who's driven both have observed.

To achieve the same center of mass with an RM car as an MM car requires that more weigh be distributed forward to the front wheels. This doesn't address that leverage that the mass of the motor has over the rear tire's lateral grip. If the CG is the pivot point, think of the distance the mass resides from it as the length of the lever.
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Old 01-21-2016, 04:15 PM
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The center of gravity is not the axis of rotation. When the car starts to slide, it rotates about a point outside the car.
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Old 01-21-2016, 05:06 PM
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Originally Posted by nikg View Post
The center of gravity is not the axis of rotation. When the car starts to slide, it rotates about a point outside the car.
Semantics... I think you understand the point being made.
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Old 01-21-2016, 05:29 PM
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Originally Posted by Davidka View Post
To achieve the same center of mass... This doesn't address that leverage that the mass of the motor has
Yes it does. That's what center of mass is.
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Old 01-21-2016, 05:43 PM
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Originally Posted by nikg View Post
Yes it does. That's what center of mass is.
You're arguing just to argue. Not interested..
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Old 01-21-2016, 07:00 PM
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Originally Posted by nikg View Post
Yes it does. That's what center of mass is.
No. Center of mass is not the same as moment of inertia. Hold two milk jugs, one in each hand, and spin around with your arms outstretched, and then do it again with your arms held tightly against your sides. In both scenarios, your mass is the same, and your center of mass is the same, but your moment of inertia is much higher with 16lbs of milk held at arm's-length from your center of mass. You'd have to twist your feet against the floor much harder to start and stop spinning with the milk jugs held at arm's-length. Same concept with putting mass near the ends of a vehicle (especially near the front) vs. near the middle.
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Old 01-21-2016, 09:12 PM
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When a car starts to slide, it rotates about a point outside the car. Not in the middle of the car. For two cars with the same center of mass and total mass:

you resolve the mass into a single point at the center of mass,
the radius is the same,
the moment is... the same for both cars
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Old 01-22-2016, 12:54 AM
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No. The center of rotation is always the center of mass. The vehicle may also *orbit* a point outside the space it occupies, which is what happens any time the vehicle is cornering in any way, including sliding. (kind of like how the Earth rotates around its center of mass but orbits a point approximately 93,000,000 miles away, generally referred to as "the Sun".) The center of rotation and the center of orbit combined describe the path the vehicle will take, assuming a perfectly circular corner of course -- if it's ovoid at all, which it always is, you need two or more centers of orbit, but that's getting too complicated to discuss here. Resolving the vehicle's entire mass into a single point is not sufficient to get an accurate result, and you can see that in the real world because two vehicles with the same mass and the same tires will slide differently if most of the mass is at the front vs. at the back. The distribution of mass not only affects the moment of inertia but it also affects the distribution of traction, and if you don't see how that changes the slide behavior of an RC car then there isn't much point in continuing to talk to you.

The problem with academic physics is that it only works with spherical objects at 0C in a frictionless vacuum. The real world is significantly more complicated, which makes teaching the basic concepts too difficult, that's why physics textbooks below the graduate-school level leave out most of the complicated stuff.

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Old 01-22-2016, 07:44 AM
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nikg....use your original account.
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Old 01-22-2016, 07:45 AM
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I think somebody needs a 22 3.0
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