The number of gear ratios in a constant mesh gearbox depends on how many forward gears and reverse gears are designed into the gearbox.
1️⃣ Forward Gears
- A typical manual constant mesh gearbox has 4 to 6 forward gears in most passenger cars.
- Number of forward gear ratios = number of forward gears.
- Example:
- 4-speed gearbox → 4 forward ratios
- 5-speed gearbox → 5 forward ratios
2️⃣ Reverse Gear
- Usually 1 reverse gear is provided.
- Reverse gear uses an idler gear to reverse rotation.
3️⃣ Total Gear Ratios
Total ratios=Number of forward gears+ reverse gear
- Example:
- 4-speed manual gearbox → 4 forward + 1 reverse = 5 ratios
- 5-speed manual gearbox → 5 forward + 1 reverse = 6 ratios
4️⃣ Key Points
- Forward gear ratios vary for torque multiplication and vehicle speed control:
- 1st gear → high torque, low speed
- Top gear → low torque, high speed (cruising)
- Reverse gear typically has a ratio similar to 1st gear to provide adequate torque for backing.
Summary:
A constant mesh gearbox usually has 4–6 forward gear ratios plus 1 reverse gear, so the total is 5–7 gear ratios depending on the design.
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