Tutorial 1.3: Friction and Traction¶
Time: ~15 minutes Prerequisites: Tutorial 1.2: Gears and Torque
The Ice vs. Grass Analogy¶
Imagine running on two different surfaces:
Running on Ice Running on Grass
🏃 ~~~~slip~~~~ 🏃 →→→→
≈≈≈≈≈≈≈≈≈≈≈≈≈≈≈≈≈ ~~~~~~~~~~~~
Hard to push off! Easy to push off!
You slip and slide You grip and go
LOW FRICTION HIGH FRICTION
Your robot wheels work the same way! Without friction between the wheels and floor, the wheels just spin without moving the robot.
Types of Friction¶
Static Friction¶
The force that prevents sliding when objects aren't moving yet.
Robot at rest on floor
+--------+
| | ← Trying to push
+--[O][O]+
═══●●●═══ ← Static friction holds!
The robot doesn't slide because
static friction resists the push.
Kinetic (Moving) Friction¶
The force that resists sliding when objects ARE moving.
Robot sliding sideways
+--------+ ←←← Sliding direction
| |
+--[O][O]+
═══→→→═══ ← Kinetic friction pushes back
Less friction than static!
Once you start sliding, it's easier to keep sliding.
Key Insight: Static friction is STRONGER than kinetic friction. This is why: - Your wheels grip better when not spinning (accelerating gradually) - If wheels spin too fast, they break loose and "burn out"
Why Wheels Need Friction¶
Without friction, wheels can't push the robot forward:
Good Friction Bad Friction (Ice)
+--------+ →→→ +--------+
| | | |
+--[O][O]+ +--[O][O]+
⟳ ⟳ Wheels grip ≈≈≈⟳≈≈⟳≈≈ Wheels spin freely
═════════ ≈≈≈≈≈≈≈≈≈≈
Robot moves forward! Robot stays still!
Wheels push, floor Wheels just spin,
pushes back no grip = no movement
VEX Wheel Types¶
VEX offers several wheel types, each with different friction properties:
Traction Wheels (High Friction)¶
+===========+
| ▓▓▓▓▓▓ | ← Rubber tread pattern
| ▓▓▓▓▓▓ |
+===========+
Best for: Pushing matches, climbing
Trade-off: Can't slide sideways
Omni Wheels (Medium Friction Forward, Low Sideways)¶
+------+
/ ⟋ \ ← Small rollers around edge
| ⟋ O ⟋ |
\ ⟋ /
+------+
Best for: Maneuverability, turning
Trade-off: Can be pushed sideways
SECRET: Rollers let wheel slide sideways!
Mecanum Wheels (Omni-Directional)¶
+------+
/ ╲ ╲ \ ← Angled rollers
| ╲ O ╲ |
\ ╲ ╲/
+------+
Best for: Moving in any direction
Trade-off: Complex programming, less pushing power
Why Your Robot Uses Omni Wheels¶
Your robot is configured with 4" omni wheels. Here's why:
Tank Drive Turn with Traction Wheels
+--------+ +--------+
| | | |
+--[█][█]+ +--[█][█]+
↑ ↓ Opposite
direction SCRUB! Tires fight each other
Wear, power loss, hard to turn
Tank Drive Turn with Omni Wheels
+--------+ +--------+
| | | |
+--[O][O]+ +--[O][O]+
↑ ↓ Opposite Rollers let wheels
direction slide sideways!
Smooth, easy turning!
Omni wheels let the robot turn smoothly because the side rollers slide instead of scrubbing.
Center of Gravity¶
Your robot's center of gravity (CG) is the balance point:
Balanced Robot Tipping Robot!
+--------+ +--------+╲
| CG | | CG ╲
+--[O][O]+ +--[O][O]+ ╲
═════════ ═════════ ╲
FALLING!
Tips for Stable Robots:¶
- Keep CG low - Heavy components (battery) near the bottom
- Keep CG centered - Don't put all weight on one side
- Wide base - Wheels spread apart resist tipping
Side View:
LOW CG (Stable) HIGH CG (Tippy)
+--+ +==+ ← Heavy top
| | | |
+---+--+---+ | |
| | +---+--+---+
| ████ | ← Battery | |
+--[O]--[O]+ +--[O]--[O]+
Traction in Push Back¶
In the Push Back competition, traction matters a lot:
Pushing Match Scenario:
Your Robot Opponent Robot
(Omni Wheels) (Traction Wheels)
+--------+ +========+
| 5kg | →→→→ | 8kg |
+--[O][O]+ +--[█][█]+
You push... But opponent has:
- More mass (harder to move)
- Traction wheels (more friction)
Result: You get pushed back!
Strategies for Better Traction¶
-
Mixed Wheels: Traction in front, omni in back
-
Weight Distribution: More weight over drive wheels
- Lower Gear Ratio: RED cartridges for more torque
- Driver Skill: Don't spin wheels - gradual acceleration
Code Connection: Wheel Measurements¶
Look at src/robot_config.py:
# 4" omni wheel circumference = 4 * pi * 25.4 = 319.19 mm
WHEEL_TRAVEL_MM = 319.19
TRACK_WIDTH_MM = 295 # Distance between left and right wheels
WHEEL_BASE_MM = 200 # Distance between front and back axles
Why do these measurements matter?
Track Width¶
Top View:
+--------+
| |
[O]--295--[O] ← TRACK_WIDTH_MM
| |
[O] [O]
+--------+
Wider = More stable, but slower to turn
Narrower = Less stable, but faster turns
Wheel Base¶
Side View:
[O]--200--[O] ← WHEEL_BASE_MM
Longer = More stable forward/back
Shorter = Can tip forward easier
The drivetrain uses these values for accurate autonomous movements!
Summary¶
| Concept | What It Means | For Push Back |
|---|---|---|
| Static Friction | Resistance before sliding | Wheels grip when accelerating |
| Kinetic Friction | Resistance while sliding | Less grip when wheels spin out |
| Traction Wheels | High friction, rubber | Best for pushing matches |
| Omni Wheels | Side rollers for sliding | Easy turning, but can be pushed sideways |
| Center of Gravity | Balance point | Keep low for stability |
Exercise: Wheel Selection Challenge¶
Design Challenge: You're building a Push Back robot. Consider these options:
Option A: 4 omni wheels - Pros: Easy turning, smooth driving - Cons: Can be pushed sideways
Option B: 4 traction wheels - Pros: Maximum grip, hard to push - Cons: Hard to turn, scrubs tires
Option C: 2 traction (front) + 2 omni (back) - Pros: Good grip + smooth turning - Cons: Front can be lifted
Questions: 1. Which would you choose for a scoring-focused robot? 2. Which would you choose for a defensive robot? 3. Can you think of other combinations?
Answers¶
-
Scoring-focused: Option A (omni) or Option C (mixed) - you need maneuverability to quickly collect and score blocks
-
Defensive robot: Option B (traction) - maximum grip to resist being pushed and to push opponents
-
Other combinations:
- 2 omni (front) + 2 traction (back) - good for pushing while maintaining turning
- 6-wheel drive with dropped center wheel - always 4 wheels touching for stability
- Mecanum for sideways movement (complex programming required)
Ready to test your knowledge? Check out the Physics Q&A Review!