Our Skid Patch Calculator is a tool used primarily by fixed-gear cyclists to extend the life of their tires.
Advanced Skid Patch Calculator
Optimize your gear ratio for tire life and speed.
Nearby Ratios (Comparison)
| Cog | Patches | Ratio | Difference |
|---|
It calculates how many specific spots (“patches”) on your rear tire will wear down when you skid to stop.
Cycling: Skid Patch Calculator (Fixed Gear)
On a fixed-gear bike (fixie), you often brake by locking your legs and skidding the rear wheel. Because your feet are always in the same position when you lock your legs (e.g., right foot forward, left foot back), the rear wheel tends to stop at the same specific points relative to the ground every time.
If you have a “bad” gear ratio, you might only have 1 skid patch. This means every time you skid, you are burning rubber off the exact same spot on the tire, destroying it very quickly. A “good” ratio will distribute that wear across 10, 17, or 30 different spots.
Understanding gear ratios is essential for any fixed-gear cyclist looking to balance performance with maintenance costs. Below are the most frequent inquiries regarding skid patch mathematics and tire longevity.
What is a skid patch in fixed-gear cycling?
A skid patch refers to the specific point(s) on a rear tire that make contact with the ground when the rider locks the pedals to initiate a skid. On a fixed-gear bicycle, because the drivetrain is direct, certain gear ratios will result in the tire locking in the exact same position(s) every time the pedals are horizontal.
How does a skid patch calculator determine the number of patches?
The calculation is based on the mathematical relationship between the number of teeth on the chainring (front) and the cog (rear). To find the number of skid patches manually:
- Create a fraction using your gear ratio: Chainring / Cog.
- Simplify that fraction to its lowest terms.
- The denominator of the simplified fraction represents your total number of skid patches.
For example, a 48/16 ratio simplifies to 3/1. The denominator is 1, meaning you have only one skid patch, which will lead to rapid tire wear.
Why is having more skid patches beneficial?
Having a higher number of skid patches significantly extends the lifespan of your rear tire. If you have only one or two patches, those specific spots will wear down to the tire casing (the “threads”) very quickly. By choosing a ratio with 15, 17, or more patches, the friction is distributed across more surface area, ensuring the tire wears down evenly.
What are “ambidextrous” skid patches?
Ambidextrous skid patches apply to riders who can skid with either foot forward. If your simplified gear ratio has an odd numerator, the number of skid patches doubles when you switch your lead foot. If the numerator is even, your patches remain the same regardless of which foot is forward.
Professional Tip: Using a prime number for your chainring (like 47t) or your cog (like 17t) is the most efficient way to maximize these patches.
What is the ideal gear ratio for maximum tire longevity?
The “Golden Ratio” for many urban fixed-gear riders is 47/17. Because 47 is a prime number, it ensures the maximum possible number of skid patches for almost any cog size. This specific setup provides 17 patches for single-foot skidders and 34 patches for ambidextrous skidders, offering a perfect balance between speed and maintenance economy.
Does the size of the wheel affect the number of skid patches?
No. While wheel size (e.g., 700c vs. 650b) affects your “gear inches” (how far the bike travels with one pedal stroke), it does not affect the number of skid patches. The patches are determined strictly by the mechanical link between the chainring and the cog.
The Formula of Our Skid Patch Calculator
The calculator uses your gear ratio to find the number of patches.
$$Skid Patches = \frac{\text{Rear Cog Teeth}}{GCD(\text{Chainring Teeth}, \text{Rear Cog Teeth})}$$
(Where GCD is the Greatest Common Divisor)
Simplified Rule of Thumb:
Take your gear ratio fraction (Chainring / Cog) and simplify it as much as possible. The denominator of the simplified fraction is your number of skid patches.
Examples:
- 48×16 (48 tooth chainring, 16 tooth cog):
- Ratio: 48/16 simplifies to 3/1
- Result: 1 Skid Patch. (Terrible for tires; you will skid on the same spot every time).
- 48×17:
- Ratio: 48/17 cannot be simplified.
- Result: 17 Skid Patches. (Excellent for tires; the wheel stops at 17 different points).
- 44×16:
- Ratio: 44/16 simplifies to 11/4.
- Result: 4 Skid Patches.
Note on “Ambidextrous” Skidding: If you can skid with either foot forward (left-forward or right-forward), you double your skid patches (unless the numerator is even).
What is a bicycle skid patch?
In technical terms, a skid patch is a discrete longitudinal area of the rear tire circumference that undergoes abrasive wear when the drive train is locked (skidded) on a fixed-gear bicycle.
Because a fixed-gear bike has no freewheel, the mechanical position of the pedals is physically indexed to the rotation of the rear wheel. When a rider initiates a skid, they typically lock their legs in a specific ergonomic position (e.g., cranks horizontal). This fixed relationship means the tire will contact the pavement at the same relative angular positions every time.
The Mechanics
The number of unique skid patches is determined by the gear ratio of the drivetrain. If Nc is the number of teeth on the front chainring and Ns is the number of teeth on the rear sprocket (cog), the relationship is defined by the simplified fraction:
$$\text{Simplified Ratio} = \frac{N_c}{N_s} = \frac{a}{b}$$
- Standard Skidding: If you always skid with the same foot forward, you have b skid patches.
- Ambidextrous Skidding: If you can skid with either foot forward, and the numerator a is odd, you have 2b skid patches. If a is even, you still only have b patches.
Scientific Implications
- Abrasive Wear Distribution: A higher number of skid patches (achieved by using a prime number of teeth on the rear cog, like 17 or 19) distributes the frictional energy across more of the tire’s surface area, preventing localized failure (burning through the casing).
- Resonance & Integer Ratios: A 1:1 or 2:1 ratio (e.g., 48/16) is the least efficient for tire longevity because it results in only one skid patch, causing the tire to fail at a single point very quickly.