Help with a vehicle dynamics problem

In summary, the equation on the right side of the page should be: T-uNr is the net torque producing the rotational accelerationas per your lhsbut on the RHS of the equation you have used T as the net torque producing eqn as you have substituted ALPHA value by T/mr2. instead get t as function of time the get alpha as function of time. use omega= omega at zero time + alpha*t for getting velocity at any time t.
  • #1
kroq
7
0

Homework Statement



I'm building a little model vehicle that will be powered by a twisted rubber band.
Givens:
m_front wheels
m_back wheels
m_body
coeff. of friction (static) = 0.2 assumed
T = Torque applied = 0.86 N-m based on lab tests
Weights (of above parts) by multiplying 9.81
wheel radii

Homework Equations



After using FBD's and Conservation Principles, I have 9 equations (3 from each Free body diagram ... Fx, Fy, and Mz summations equaling either zero or m*a depending on the acceleration happening).

Unknowns, Forces btwn parts (Fx, Fy), and the Normal Forces btwn the wheels and the ground.

I fed this understanding into Excel's solver add-in ... and I'm having trouble identifying the best objective function to use.

My overarching objective is to have the fastest vehicle, but the velocity calculation for anything with a wheel (assuming constant acceleration) is just a function of the input Torque, mass and the radii of the powered wheel.

I want to use solver to optimize the vehicle velocity and solve for all forces.

Am I missing something elementary here?

Thanks for any help.


The Attempt at a Solution

 
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  • #2
My overarching objective is to have the fastest vehicle, but the velocity calculation for anything with a wheel (assuming constant acceleration) is just a function of the input Torque, mass and the radii of the powered wheel.

I want to use solver to optimize the vehicle velocity and solve for all forces.

Am I missing something elementary here?

Thanks for any help.

Without seeing the specific equations, I can't offer much help. Your approach is definitely correct, but when you say "vehicle velocity", I assume you mean the velocity after the elastic band becomes completely unwound? Remember that the elastic band loses torque as it unwinds; I don't know whether you've taken that into consideration in the calculations.
 
  • #3
Hi Ideasrule,

Thanks for your reply.

This car will have a sizable rubber band (looped several times) and will still be unwinding as it crosses the finish (~350 winds and about 12 inches long)

Thanks for the tip. Torque would definitely decrease during unwinding, which leads me to believe that I should set up a numerical analysis of this breaking up the race into several iterative steps ... to track the acceleration, velocity and displacements.

I forgot how to setup solver to solve for simple system of equations to solve for my forces. That's really what I want it to do.
 
  • #4
Hi,

Attached is my free body diagrams and equation set.

Even when trying to solve for my forces by hand, I'm running into inconsistencies ... by default (summation of forces & moments ... I should have 9 equations, right?) ... but I have fewer unknowns which doesn't seem right.

Am I missing something here for this simple problem?

And I'd love to use Excel's Solver to quickly solve for all forces quickly because I will have to use this understanding with much greater complexity and over several iterations.

Any help is welcome and appreciated.
 

Attachments

  • equations.jpg
    equations.jpg
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  • #5
Hi all, I would be greatful for a 2nd pair of eyes on my system of equations. What am I missing?

Thanks much!

K
 
  • #6
Anyone? Thx
 
  • #7
i think there is an error in your torque equation on the right bottom side of the page.
T-uNr is the net torque producing the rotational accelerationas per your lhs
but on the RHS of the equation you have used T as the net torque producing eqn as you have substituted ALPHA value by T/mr2.

instead get t as function of time the get alpha as function of time.
use omega= omega at zero time + alpha*t for getting velocity at any time t
 

1. What is a vehicle dynamics problem?

A vehicle dynamics problem is a situation where the movement or performance of a vehicle is impacted by various factors, such as road conditions, vehicle design, and driver behavior. It involves understanding the forces and movements that act on a vehicle, and how these affect its stability, handling, and overall performance.

2. How can I identify and diagnose a vehicle dynamics problem?

Identifying and diagnosing a vehicle dynamics problem involves analyzing different aspects of the vehicle, such as its suspension, steering, braking, and tire performance. This can be done through various methods, including visual inspection, test driving, and using specialized equipment like sensors and data loggers.

3. What are some common causes of vehicle dynamics problems?

There are several factors that can contribute to vehicle dynamics problems, including worn or damaged suspension components, improper tire pressure, unbalanced wheels, and misaligned steering. Driver behavior, such as aggressive driving or overloading the vehicle, can also lead to issues with vehicle dynamics.

4. How can I fix a vehicle dynamics problem?

The appropriate solution for a vehicle dynamics problem will depend on the specific issue at hand. In some cases, simple adjustments or repairs, such as realigning the wheels or replacing worn suspension components, may be sufficient. More complex issues may require specialized equipment or the expertise of a trained mechanic.

5. How can I prevent vehicle dynamics problems in the future?

Regular maintenance and upkeep of your vehicle can help prevent many common vehicle dynamics problems. This includes checking and adjusting tire pressure, getting regular wheel alignments, and addressing any issues with suspension or steering components as soon as they arise. Additionally, practicing safe and responsible driving habits can also help prevent unnecessary wear and tear on your vehicle.

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