Skid Patch Calculator
An essential tool for every fixed-gear rider to maximize tire life.
Chart: Skid Patch Comparison
What is a Skid Patch Calculator?
If you ride a fixed gear bicycle (a “fixie”), you often slow down or stop by locking your legs, which in turn locks the rear wheel and causes the tire to skid along the pavement. A skid patch calculator is a specialized tool that helps you understand how this skidding affects your rear tire’s lifespan. When you skid, you tend to do so with your pedals in the same position, which means the same spot on the tire—the “skid patch”—repeatedly makes contact with the ground.
If your gear ratio is not chosen carefully, you might only have a few, or even just one, skid patch. This concentrates all the wear on a tiny area, destroying a tire in days. This calculator helps you determine the number of unique skid patches your bike’s setup provides, allowing you to choose a gear ratio that distributes wear evenly and saves you money.
The Skid Patch Formula and Explanation
The number of skid patches is determined by a simple mathematical relationship between the number of teeth on your front chainring and your rear cog. The calculation involves reducing the gear ratio fraction to its simplest form. The denominator of that simplified fraction is your number of skid patches.
The core formula is:
Skid Patches = Cog Teeth / GCD(Chainring Teeth, Cog Teeth)
Where “GCD” stands for the Greatest Common Divisor—the largest number that can divide both the chainring and cog teeth counts without leaving a remainder. For example, a 48/14 ratio simplifies to 24/7, which gives you 7 skid patches.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Chainring Teeth | The number of teeth on the large front gear. | Teeth (count) | 42 – 55 |
| Cog Teeth | The number of teeth on the small rear gear. | Teeth (count) | 13 – 22 |
| GCD | Greatest Common Divisor of the two tooth counts. | Unitless | 1+ |
| Skid Patches | The number of unique wear points on the rear tire. | Patches (count) | 1 (bad) to 22+ (good) |
Practical Examples
Example 1: A “Good” Ratio
- Inputs: Chainring = 47 teeth, Cog = 17 teeth.
- Units: The numbers represent the count of teeth.
- Results: The GCD of 47 (a prime number) and 17 (a prime number) is 1. The number of skid patches is 17 / 1 = 17. This is an excellent number, as it spreads wear across 17 different points.
Example 2: A “Bad” Ratio
- Inputs: Chainring = 48 teeth, Cog = 16 teeth.
- Units: The numbers represent the count of teeth.
- Results: The GCD of 48 and 16 is 16. The number of skid patches is 16 / 16 = 1. This is a “suicide ratio” because every skid will land on the exact same point, destroying the tire extremely quickly.
How to Use This Skid Patch Calculator
- Enter Chainring Teeth: Count the teeth on your front chainring and enter the number into the first field.
- Enter Cog Teeth: Count the teeth on your rear cog and enter it into the second field.
- Check for Ambidextrous Skidding: If you are skilled enough to skid with either your left or right foot forward, check the box. This can potentially double your skid patches if your gear ratio allows it.
- Interpret the Results:
- Total Skid Patches: This is the main result. A higher number is better. Aim for at least 8.
- Gear Ratio: This shows you how “heavy” your gearing is. Higher means faster but harder to pedal from a stop.
- GCD: A GCD of 1 is ideal for maximizing skid patches.
- Chart: The chart visually compares your single-foot patches to the potential patches if you are an ambidextrous skidder.
Key Factors That Affect Skid Patches
- Chainring Size: Using a chainring with a prime number of teeth (like 43, 47, or 53) guarantees you’ll get the maximum number of patches for any given cog.
- Cog Size: Just like the chainring, a prime-numbered cog (like 13, 17, 19) ensures the ratio can’t be simplified, giving you a high patch count.
- Relative Primality: The most important factor is that the chainring and cog counts don’t share common divisors. For example, 49 and 16 are not prime, but they are relatively prime (their GCD is 1), resulting in an optimal 16 patches.
- Ambipedal Skidding: The ability to skid with both feet forward can double your skid patches, but only if the cog size is odd. If the cog is even, it provides no benefit.
- Even Ratios: Ratios that simplify to a whole number (e.g., 48/16 = 3/1, 44/11 = 4/1) are the absolute worst, resulting in only one skid patch.
- Tire Rotation: While not part of the calculation, physically rotating your tire on the rim can help further distribute wear, but it’s no substitute for a good gear ratio.
Frequently Asked Questions (FAQ)
- What is a good number of skid patches?
- You should aim for at least 8 patches. Anything less than 5 will wear out your tires very quickly. Many riders consider 17 or more to be the gold standard.
- Does tire size (e.g., 700c x 25) affect skid patches?
- No. The number of skid patches is purely a result of the gear ratio. Tire size and width do not change the mathematical outcome.
- What is the “suicide ratio”?
- This refers to any gear combination that results in only one skid patch, such as 48×16. It’s called this because it “kills” your tire very quickly.
- Why are prime numbers so important?
- A prime number can only be divided by 1 and itself. Using a prime-numbered chainring (like 47t) or cog (like 17t) makes it highly likely that the gear ratio fraction cannot be simplified, thus maximizing your skid patches.
- What is “ambidextrous” or “ambipedal” skidding?
- This means you are comfortable and skilled at skidding with either your right foot forward or your left foot forward. Most riders have a dominant foot. Being able to use both can double your skid patches in many gear combinations.
- Should I choose my gear ratio based only on skid patches?
- No. While important for tire life, the gear ratio also determines how the bike feels to ride. You must find a balance between a good number of skid patches and a gear ratio that suits your strength and riding style. For city riding, a ratio between 2.7 and 3.1 is common.
- How do I find out the number of teeth on my gears?
- The tooth count is usually stamped directly onto the surface of the chainring and the cog. If not, you will have to count them manually.
- Can I have too many skid patches?
- No, there is no such thing as too many skid patches. More patches always means more even tire wear, which is always better.
Related Tools and Internal Resources
If you found this tool helpful, check out our other cycling resources:
- Gear Ratio Calculator: Calculate your gear inches and development.
- The Ultimate Guide to Bike Fit: Ensure your bike is adjusted perfectly for your body.
- Essential Bicycle Maintenance Checklist: Keep your ride in top condition.
- Fixed Gear 101: A Beginner’s Introduction: Learn the basics of riding fixed.
- How to Choose the Right Cog for City Riding: A deep dive into rear sprockets.
- Tire Pressure Guide for Urban Cyclists: Optimize your ride comfort and efficiency.