How to Choose a Fixed-Gear Ratio: Why 48×17 Is the Standard, with a Skid Patch Chart

With one gear, balance starts, cruising and hills

A bike with gears lets you pick one for each situation, but a track bike or single-speed handles everything with one gear. On a fixed gear, the pedals also keep turning as long as the rear wheel does, so your cadence rises on its own on descents and at high speed. Set the ratio by balancing these three things.

• Starting from signals: the heavier the gear, the harder it is to start from a standstill
• Cruising speed: whether you can reach your usual speed at 80–90 rpm
• Hills and descents: whether you can push up climbs, and whether your legs spin too fast going down (a fixed gear won't let you stop pedaling)

Gear weight is usually compared by gear ratio (front ÷ rear) or by development, the distance per pedal revolution (gear ratio × tire circumference). Overseas references often use gear inches, and among track bike riders around 70–80 inches is seen as the standard for city riding.

Why 48×17 is called the standard

Complete track bikes often come with 48T at the front and 17T at the rear. With a 700×28C tire, the numbers are as follows.

At an easy 60 rpm that's 22 km/h, and at 90 rpm it's 32 km/h, just the right weight for keeping up with city traffic. And since 48 and 17 share no common factor, the skid patches reach their maximum of 17, so the tire wears evenly. Not too heavy, not too light, and long tire life: that's why it became the standard.

Recommended ratios and speeds by use

These are speeds calculated for 700×28C, with skid patch counts for one-footed / two-footed skids. If unsure, start from 48×17 and move the front or rear teeth by 1–2T depending on whether you want it heavier or lighter.

Even for gears of the same weight, the skid patch count varies greatly with the tooth combination. For example, 48×16 and 51×17 both have a ratio of 3.00, but 48×16 has just 1 skid patch (2 even two-footed), and 51×17 likewise has 1. If you want a ratio around 3.00, choose a combination that doesn't divide evenly, such as 47×16 (2.94, 16/32) or 49×16 (3.06, 16/32).

What are skid patches? How to calculate them

A skid means stopping the pedals on a fixed gear to lock and slide the rear wheel, slowing you down. When skidding, you nearly always stop at a set pedal position, such as with the crank of your dominant foot level. The number of separate spots on the tire that scrape the ground is the skid patch count. Too few, and the same spots wear down repeatedly, creating flat areas that quickly ruin the tire.

1. 1

   Find the greatest common divisor of the front and rear teeth

   For 48 and 17 it's 1, for 48 and 16 it's 16, and for 48 and 18 it's 6.

2. 2

   Reduce the teeth by dividing by it

   48/17 can't be reduced, so it stays 48/17. 48/16 becomes 3/1, and 48/18 becomes 8/3.

3. 3

   The reduced rear number is the one-footed skid patch count

   48×17 has 17, 48×16 has 1, and 48×18 has 3.

4. 4

   For two-footed skids, double it when the reduced front number is odd

   A rider who can stop with either foot also stops half a crank revolution apart, so the patches double if the reduced front number is odd. 48×16 (3/1) has 2 two-footed, and 49×17 (49/17) has 34.

Generally, 7 or more one-footed patches spread tire wear enough, and 3 or fewer are combinations to avoid. Enter the front and rear teeth in the "Track bike skid patches" tab of the Bike Gear Ratio Calculator to see one-footed and two-footed skid patches, gear ratio and development right away.

Chart of gear ratios and skid patches (44–52T × 14–18T)

Each cell gives the gear ratio followed by the skid patch count as one-footed/two-footed. Combinations with 3 or fewer tend to wear the tire unevenly, so take care with them.

The chart shows that a 17T cog gives 17 skid patches with every chainring except 51T. That's because 17 is prime and shares no factor with anything but 51 (= 17 × 3). This ease of use is one reason 17T is the standard rear choice. Conversely, 48T paired with 16T, 18T or 15T gives few patches, so take care when changing the weight while keeping 48T.

How many chain links after changing gears?

Track bikes and single-speeds have no derailleur to take up chain tension, so you set it with the rear wheel position (where it's fixed in the dropout) and the number of chain links. Link counts can only be even, so after changing gears you need to check that the rear wheel position stays within the dropout's adjustment range.

For 48×17 at 400 mm between axles, for example, either use 96 links and move the rear axle forward to 398.3 mm, or use 98 links and pull it back to 411.2 mm. Changing the rear cog by 1T changes the chain needed by about half a link (about 6 mm), which moves the rear wheel by half that, around 3 mm. On frames with short dropouts, a half-link chain is another option.

What to watch for on public roads

On public roads in Japan, the Road Traffic Act and prefectural rules require a bicycle to have a brake on both the front and rear wheels. Fixed-gear skids and stopping with your feet don't count as brakes, so riding a brakeless racing track bike on public roads is a violation. Treat skidding only as a supplementary technique, and ride with brakes fitted front and rear. Check the rules that apply where you live as well.

• For a first fixed gear, starting light (around 44×17 to 46×17) makes starts and descents less scary
• Cadence rises on its own on descents: 40 km/h on 48×17 is about 111 rpm
• On rainy days, skids bite too easily and the rear wheel tends to slide. Slow down early with the brakes
• If uneven tire wear bothers you, change the gear or rotate the tire's mounting position to shift it

Comparisons with geared road bikes and the relationship between cadence and speed are covered in Cadence and Speed Chart, and the tire circumference for an accurate speed display in Tire Circumference for Bike Computers.

FAQ

What gear ratio do you recommend for a track bike?

For city riding, 48×17 (ratio 2.82) is the standard. On 700×28C at 90 rpm it gives about 32.5 km/h, with 17 skid patches. For hilly areas, 44×17 to 46×17 is easy to handle, and for speed on the flat, try around 49×17 to 50×17.

How do I calculate skid patches?

Divide the front and rear teeth by their greatest common divisor, and the reduced rear number is the one-footed skid patch count. For two-footed skids, it doubles if the reduced front number is odd. 48×17 has 17, and 48×16 has 1 (2 two-footed).

Why is 48×16 considered a poor choice?

48 divides evenly by 16, so there's just 1 skid patch (2 even two-footed), and the same part of the tire keeps wearing down. For a similar weight, 47×16 (16/32) or 49×16 (16/32) are candidates.

How fast does a gear ratio of 2.8 go?

On 700×28C (circumference 2130 mm), development is about 6.0 m, giving about 28.6 km/h at 80 rpm, about 32.2 km/h at 90 rpm, and about 35.8 km/h at 100 rpm. Speed = gear ratio × tire circumference (m) × cadence × 60 ÷ 1000.

How many chain links change when I change the cog by 1T?

Changing the rear cog by 1T changes the chain length needed by about 0.47 links. Links can only change in steps of 2, so a 1T change can often be handled by adjusting the rear wheel position, and a change of 2–4T calls for revisiting the link count.