JT Sprockets Review

[xpertsnowcarver]

Please post your experience with JT Sprockets in this thread instead of creating separate threads.

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Overall Rating (From 1-10): 2 (Not Recommended)

Product: JT Sprocket 38 Tooth Rear Sprocket
Year: 1988-2008
Make: Yamaha
Model: XV250
Part number: JTR857.38

Description on packaging:
"Material: High Carbon Steel SAE1045/49/SCM415 Japan
This product complies with all OEM manufacturing standards for motorcycle sprockets.
JT Sprockets: Motorcycle & ATV Sprockets
Manufactured by Jomthai Asahi Co., LTD.
Made in Thailand"

Place of purchase: Old Bike Barn
Item Code: 87-249909
Price: $29.95
Link: http://www.oldbikebarn.com/Yamaha-Re...ategory=259701

First Impression:
Made a phenomenal improvement in gas mileage. I went from 65MPG to 85MPG. Max MPG I've reached was 97MPG (in combination with increasing the front sprocket from 15 Teeth to 17). Since this motorcycle does not have a tachometer, I can't say what RPM it was running at before and after.

Final Impression:
Things just started getting worse and worse and worse. Within 3500 miles, I had to significantly tighten the chain THREE TIMES! This was absurd, and I could not figure out why. They chain was always lubed every two to three weeks and showed no signs of stretching or excessive wear...

Well, I decide to remove the engine to replace a few gaskets. When I went to remove the chain over the rear sprocket, I was shocked to find this.



In the pictures, you can see a clear comparison of the used 38 tooth sprocket with 3500 miles on the motorcycle and a brand new 38 tooth sprocket I purchased last week. Let's keep in mind that this motorcycle is only a 250cc...

So why would it wear out so bad??? Well...The problem is rather obvious. It is simply a combination of two things.

Reason 1) The motorcycle was originally engineered to operate with a 45 Tooth sprocket. With this sprocket, it clearly has a greater diameter compared to a 38 tooth sprocket.

By decreasing the diameter of the sprocket, the engine will operate with a greater load (greater resistance to acceleration per say). Think of it like this. When you remove a wheel lug nut, which is easier to loosen the lug nut with? A 7" long ratchet? Or a 15" long ratchet?? Of course the 15" long ratchet requires less force to turn the lug nut. So, the 7" long ratchet will require more force to accomplish the same result. That is what is happening to the sprocket. Since we are decreasing the diameter of the sprocket, then engine ends up applying more force on the teeth, causing a significant amount of wear. Also, because there is less teeth on the sprocket, there is less surface area for the chain to push against.

Reason 2) JT Sprockets did not account for reason 1 and did not increase the hardness of the sprocket to be able to withstand the increase in force against each tooth. They did use a much harder material for the front sprocket. (See the following post for a review of the front sprocket.)

Final Notes:
Made a mild attempt to communicate with JT Sprockets about this via email. I asked if there is some kind of warranty. I received no word from them. If you would like to decrease the size of the sprocket, be sure to decrease it only slightly. I went with the extreme and decreased it 7 teeth. Instead, decrease the rear sprocket size by no more than 3 teeth. But!! If you can figure out a way to convert to a wider chain and sprocket, then you may be able to further decrease the diameter of the rear sprocket. (The wider the chain and sprocket, the more surface area the force can distribute over and decrease wear on the teeth.)

[xpertsnowcarver]

Overall Rating (From 1-10): 8

Product: JT Sprocket 17 Tooth Front Sprocket
Year: 1988-2008
Make: Yamaha
Model: XV250
Part number: JTF569.17T

Description on packaging:
Not Available.

Place of purchase: Old Bike Barn
Item Code: 87-249177
Price: $15.95
Link: Yamaha Front 17 Teeth Sprocket

First Impression:
Made a phenomenal improvement in gas mileage. I went from 65MPG to 85MPG. Max MPG I've reached was 97MPG (with the combination of decreasing the rear sprocket from 45 Teeth to 38). Since this motorcycle does not have a tachometer, I can't say what RPM it was running at before and after.

Final Impression:
3500 miles and this sprocket looks to be in decent shape. I do recommend increasing the size of front sprocket in order to save fuel, but remember that there is a trade off. If you want to save fuel, you must sacrifice some torque.

Final Notes:
Material is made of a harder steel than the rear sprocket - Chromoly SCM420 steel alloy. They should have made the rear sprocket out of the same material, but in order to provide an exceptional price, they need to cheap out on the material. Going the extreme of getting the biggest front sprocket is okay to do.