Re-gearing and speedo error - Sportbike Forum: Sportbike Motorcycle Forums
 
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post #1 of 5 (permalink) Old 08-23-2001, 11:28 AM Thread Starter
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Question Re-gearing and speedo error

I re-geared my Falco last week. I went from a 2.56 (16 up front, 41 in back) to a 2.8 (15 in the front, 42 in back) ratio. I was expecting my speedo to be way off. I verified it by being paced by my wifes Volvo at 15, 25, 30, 40, 55, 65, and 75 mph (I know her car is reading the correct speed). At any given speed I was no more than 2 mph off. As far as I can tell, the speed is not read off the front wheel, so I was expecting an error. Does anyone have any insight as to why this did not occur? Was my speedo in error before the change and I just happened to correct it (this seems unlikely). Hmmmm....

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post #2 of 5 (permalink) Old 08-23-2001, 11:07 PM
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speedo error

When I think about it, why would the speedo be off unless you changed the size of the wheels?. The speedo sensor reads off the bolts on your rear brake rotor. You have not changed the brake rotor so bolt revolutions would be the same.

As far as your rear wheel and speedo are concerned it takes X amount of wheel (not sprocket) revolutions to propel the bike at Y speed, why would changing sprockets be a factor?. Regardless of the gearing you use to reach a determined speed, the wheel is still turning at a comparable rate.

If that is confusing, think of it this way. If you switched off engine, removed the sprockets completely and rolled the bike down a hill the speedo would show a reading, even though the engine speed and sprocket gearing are completely out of the picture.

The difference of course would be if a bike's speedo sensor reads off the front sprocket in which case it is computing engine RPM to arrive at vehicle MPH. This means you could remove your chain, start the bike up and run it through the gears and the speedo would show speeds even though the bike is going nowhere

hope this helps,
Gregory
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post #3 of 5 (permalink) Old 08-24-2001, 02:18 AM Thread Starter
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Thanks!

Since it wasn't read off the front wheel, I assumed it was read off the sprocket, which is where the confusion came in. Thanks for clearing that up. (file this under things the owners manual should tell you)

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post #4 of 5 (permalink) Old 08-24-2001, 04:41 AM
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What you will notice with bikes is that your speedo accuracy can vary with tire wear. By putting the sensor in the back Aprilia have just magnified this problem.

There are some things nobody needs in this world, and a bright red, hunchback, warp-speed 900cc café racer is one of them -- but I want one anyway, and on some days I actually believe I need one. That is why they are dangerous.
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post #5 of 5 (permalink) Old 08-24-2001, 05:05 PM
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speedo

Quote:
Originally posted by lucifer
By putting the sensor in the back Aprilia have just magnified this problem.
But in doing so they have solved the problem of knowing just how fast you were going when you clicked into 6th gear while riding a wheelie

Take care,
Gregory
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