25/09/2010
The Yamaha WR250R is a highly capable dual-sport motorcycle, renowned for its versatility. However, one of the most effective ways to tailor its performance to your specific riding needs, whether it's tackling gnarly off-road trails or cruising on the motorway, is by adjusting its gearing. This article will guide you through the process, explaining the components involved, the impact of different ratios, and the steps to perform the change yourself.
Understanding the Role of Gearing
Your motorcycle's gearing system consists of two sprockets: the countershaft sprocket (front) and the rear sprocket. These sprockets are connected by the drive chain, transmitting power from the engine to the rear wheel. The ratio between the number of teeth on these sprockets determines how many times the engine crankshaft rotates for each rotation of the rear wheel. A lower gear ratio (e.g., larger rear sprocket or smaller front sprocket) means the engine revs higher for a given road speed, providing more acceleration and torque but a lower top speed. Conversely, a higher gear ratio (smaller rear sprocket or larger front sprocket) results in lower engine revs for a given speed, improving fuel economy and top speed but sacrificing acceleration.
Common Gearing Changes for the WR250R
The stock gearing on the WR250R is typically a 13-tooth front sprocket and a 43-tooth rear sprocket, giving a ratio of 3.31:1. This is a good all-around setup, but many riders opt to modify it.
For enhanced off-road performance, riders often choose to increase the size of the rear sprocket or decrease the size of the front sprocket. Common changes include a 14-tooth front sprocket or a 45, 46, or even 47-tooth rear sprocket. This lowers the gearing, providing more low-end grunt for climbing hills, navigating technical terrain, and accelerating out of corners. However, this will result in higher engine RPMs at highway speeds, potentially increasing fuel consumption and engine noise.
For riders who spend more time on the road or in higher-speed situations, a common modification is to increase the gearing slightly. This might involve a 13-tooth front sprocket with a 42-tooth rear sprocket or a 14-tooth front sprocket with a 45-tooth rear sprocket. This higher gearing will reduce engine RPMs at cruising speeds, leading to a more relaxed ride, better fuel economy, and less engine wear.
Table of Common Gearing Ratios and Their Effects:
| Front Sprocket (Teeth) | Rear Sprocket (Teeth) | Gear Ratio | Primary Effect |
|---|---|---|---|
| 13 | 43 | 3.31:1 (Stock) | Balanced performance for mixed use |
| 13 | 45 | 3.46:1 | Improved acceleration and low-end torque, slightly reduced top speed |
| 13 | 47 | 3.62:1 | Significant increase in low-end torque, noticeable reduction in top speed |
| 14 | 43 | 3.07:1 | Slightly improved fuel economy and reduced highway RPMs, less aggressive acceleration |
| 14 | 45 | 3.21:1 | Good compromise for increased highway comfort and still decent off-road capability |
| 14 | 47 | 3.36:1 | Closer to stock but with a slightly more road-biased feel |
Tools and Parts You'll Need:
* New front and rear sprockets (ensure they are compatible with your WR250R)
* New drive chain (often recommended when changing sprockets, especially if the old one is worn)
* Chain breaker tool (if you need to shorten or lengthen the chain)
* Socket set (metric sizes, typically 10mm, 12mm, 14mm, 17mm, 19mm)
* Torque wrench
* Allen keys/hex wrenches
* Pliers
* Grease
* Chain lube
* Motorcycle stand (rear paddock stand is highly recommended)
* Gloves and eye protection
The Changing Process: A Step-by-Step Guide
1. Preparation:
- Park your WR250R on a stable, level surface. A rear paddock stand will make this job significantly easier by lifting the rear wheel off the ground.
- Ensure the engine is cool.
- Gather all your tools and new parts.
2. Accessing the Front Sprocket:
- Locate the front sprocket cover, usually a small metal or plastic cover on the right side of the engine, near the footpeg.
- Remove the bolts securing the cover using the appropriate socket.
- You may need to remove the plastic chain guard as well.
3. Removing the Old Front Sprocket:
- The front sprocket is held in place by a washer and a nut.
- To prevent the countershaft from turning, you can either:
a) Place the motorcycle in gear (first gear is usually best) and apply the rear brake.
b) Use a chain whip or a strap wrench to hold the sprocket.
- Loosen and remove the sprocket nut. This can be tight, so a breaker bar might be helpful.
- Remove the retaining washer and then slide the old front sprocket off the countershaft.
4. Installing the New Front Sprocket:
- Clean the countershaft splines with a rag.
- Apply a thin layer of grease to the splines.
- Slide the new front sprocket onto the countershaft, ensuring it's oriented correctly.
- Reinstall the retaining washer (often a new one is supplied with the sprocket) and tighten the nut.
- Torque the sprocket nut to the manufacturer's specifications (refer to your WR250R owner's manual or a service manual). Torque specifications are crucial for safety and proper function.
5. Accessing and Adjusting the Rear Sprocket:
- The rear sprocket is mounted to the rear wheel hub via several bolts.
- You'll also need to loosen the chain adjusters on the swingarm to allow for chain slack.
- Remove the rear sprocket nuts and bolts. You might need to hold the rear wheel or sprocket to prevent it from turning.
- Carefully remove the old rear sprocket.
6. Installing the New Rear Sprocket:
- Clean the hub and the mounting bolts.
- Place the new rear sprocket onto the hub, ensuring the correct orientation.
- Reinstall the sprocket bolts and nuts. It's good practice to use new ones if supplied.
- Tighten the bolts by hand initially.
7. Installing and Adjusting the New Chain (if applicable):
- If you're using a new chain, you'll need to determine the correct length. This usually involves comparing it to your old chain or using a chain breaker tool to cut it to size.
- Loop the new chain over the front and rear sprockets.
- Connect the chain using the master link or rivet link provided.
- Proper chain alignment is critical. Ensure the sprockets are perfectly in line.
- Adjust the chain tension using the adjusters on the swingarm. The general rule for chain slack on a WR250R is typically around 20-30mm (3/4" to 1 1/4") of up-and-down movement midway between the sprockets. Consult your owner's manual for the exact specification.
- Tighten the rear axle nut to the manufacturer's specified torque.
8. Final Checks and Reassembly:
- Reinstall the front sprocket cover and any other guards you removed.
- Double-check that all nuts and bolts are tightened to the correct torque.
- Spin the rear wheel to ensure everything moves freely and there are no binding issues.
- Apply a quality chain lube to the new chain.
Important Considerations:
* Chain Wear: When changing sprockets, it's highly recommended to replace the drive chain as well, especially if the old chain is stretched or worn. Mismatched wear between sprockets and chain can lead to premature failure of all components.
* Speedometer Accuracy: Changing the final drive ratio can affect the accuracy of your speedometer and odometer, as they are often calibrated to the stock gearing. You may need a speedometer healer or recalibration device.
* Rider Skill and Terrain: The best gearing for you will depend heavily on your riding style, the typical terrain you encounter, and your personal preferences. Experimentation might be necessary.
* Legalities: Be aware that significant gearing changes might affect your motorcycle's compliance with local noise or emissions regulations, although this is less common with minor adjustments.
Frequently Asked Questions:
Q: How often should I change my sprockets and chain?
A: This depends on riding conditions, maintenance, and the quality of the components. A general guideline is to inspect them regularly. If the chain has stretched significantly, or the sprocket teeth are hooked or worn down, it's time for replacement.
Q: Can I just change the rear sprocket and not the front?
A: Yes, you can. Changing just one sprocket will alter the ratio. However, for optimal performance and longevity, it's best to replace both sprockets and the chain as a set.
Q: Will changing my gearing affect my fuel economy?
A: Yes. Lowering the gearing (more acceleration) will generally decrease fuel economy, especially at higher speeds. Raising the gearing (more highway comfort) will generally improve fuel economy.
Q: How do I know if my chain is too tight or too loose?
A: Check the slack midway between the sprockets. Too little slack will cause excessive wear and stress on the drivetrain. Too much slack can cause the chain to slap, jump off, or damage other components. Always refer to your owner's manual for the correct slack measurement.
By understanding the principles of motorcycle gearing and carefully following these steps, you can effectively modify your Yamaha WR250R to better suit your riding adventures. Remember to prioritize safety by using the correct tools, torquing fasteners appropriately, and performing thorough checks after the installation.
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