# Attempting to model a "roller coaster" with curved rails and friction-based pad braking system

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Kenneth MacLean on 15 Feb 2013
Answered: Steve Miller on 24 Nov 2021
Hello,
I feel like I may be getting in over my head with this being my first attempt at using SimMechanics R2012b 2nd Generation. I am attempting to model what is essentially a simulation of a roller coaster cart mounted on curved rails. The cart is intended to begin vertically, slide down the rails under the force of gravity, and then be guided into horizontal motion by the track curvature. This presents my first question:
1. I do not know how to model the joint between the cart and rails, due to the curvature. Were the rails not curved, I believe that a simple prismatic joint would do. Does it matter how I choose to define the rail solid body?
Once the cart has leveled out, I intend to have it interact with ground-mounted brake pads, which will have an externally-actuated braking (normal) force. I am much more lost with this, though it seems like there might be some relevant examples. Should I be handling the brake pad and rotor (plate) interaction as contact forces similar to the mini-golf example?
2. Any clues or examples as to how to model the brake pad/rotor interaction, taking into account the fact that they would not initially be in contact?
It seems like what I initially thought of as a fairly simple machine might be a very complicated modeling problem in SimMechanics. Have I chosen the wrong software, or am I just thinking about it wrong? My eventual intention is to design and simulate a servo-hydraulic control system which regulates the cart's deceleration. Sorry for the long multi-part question, and thanks for any help.

David on 29 May 2014
Hi Kenneth,
I'm looking for a joint/constraint that enables motion along a curve as well. Since we know that the prismatic joint is suitable for straight motion, we could go for this "creative" solution:
1. Add a prismatic joint between the body of interest and a dummy body.
2. Add a planar joint between the dummy body and the world frame (assuming motion in a plane).
3. Compute and add an input motion for the dummy body in such a way, that the prismatic joint moves during the simulation.
I have yet to try this strategy myself, but this could be a way to constrain the body of interest along non-straight paths. For a 90 degree bend for example, the prismatic joint should rotate around the center of the circle, by means of a revolute joint at the center and a prismatic joint translated by the radius.
In case I come up with some meaningful results I'll post them.
- David
Kenneth MacLean on 4 Jun 2014
Hi David,
I eventually just elected to use MSC Adams for my project in the interest of time, but I'd be interested to know if you manage to come up with anything.

Steve Miller on 24 Nov 2021
This can be done using a Point-on-Curve Constraint which is available in Simscape Multibody (formerly named SimMechanics).
--Steve

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