[Gibs]

The directions for the cheapo speedometer I have doesn't give any direction about where on the spoke I should place the magnet. is there a distance from the center hub that it should be at? Also based on the directions it told me to use the diameter in mm * 3.1416 to find my calculation for the speedo. I have a 2013 Specialized Allez with DT Axis 1.0 wheels and 700 23C tires. I used a calculation of 2097 for the input in the speedo. Am I way off base here? Sorry for the stupid question. I am new to this stuff. TIA



Gibs

 

[RoadrunnerLXXI]

It doesn't matter where on the spoke you place the magnet. What does matter is the distance gap between the magnet and the sensor which read it. I think most computer requires 1 to 2 cm gap give or take. The instruction should tell you what is the recommended gap. As to the circumference to enter, the instruction should have a table guide. Just enter the number for the 700cc x 23 tire. For example, for my cateye, I would type 2096 for a 700cc x 23 tire.

 

[Gibs]

Thank you. I had read that but wasnt sure about the magnet. Mine is 5mm's and has no issue picking up the signal. I think I will change it to the 2096 that you posted. The reason I started to question was that the speed I had on it and my GPS were different and the only variable would be the number. Thank you for the inputs.


Gib

 

[Hooben]

Not only that, but another thing to think about is the MPH. Most computers come calibrated to kilometers per hour. Make sure and change that to miles if that's what you want.

 

[JCavilia]

Your number of 2097 (that's the ourside circumference of the tire in mm) is certainly in the ballpark, and close enough for almost any purpose. Tires do vary a bit, and if you want to be really fussy about it, you can do a rollout measurement. Basically, roll the wheel a full revolution on a smooth surface, with it bearing approximately the weght it carries when riding, and measure how far it rolls. A nifty way to be sure you've rolled one full revolution is to put a dab of water-based paint on the tire so it leaves a mark on the road, and measure the distance between successive marks.

But you're close already, probably well within half a percent (probably much closer), and in truth nobody needs to be more accurate than that with this stuff. Changing from 2097 to 2096 will make a difference of less than half a mile in 1000 miles. More than close enough.

BTW, it's different than the GPS because the GPS is less accurate for this type of measurement.

 

[Kerry Irons]

Thank you. I had read that but wasnt sure about the magnet. Mine is 5mm's and has no issue picking up the signal. I think I will change it to the 2096 that you posted. The reason I started to question was that the speed I had on it and my GPS were different and the only variable would be the number. Thank you for the inputs.

Your calibration number is probably close, but you can calibrate the computer. Find a road with mile markers (they are put in place by people with accurate surveying equipment) and ride at least 10 miles (20 is better). Compare the distance your computer reads with the actual distance (note that some computers keep calculating after you stop so take the numbers on the fly). If you get 20.1 miles for 20 miles actual distance then ratio your calibration number times 20/20.1. You'll be far more accurate than a GPS unless you are riding a straight line the whole time. The GPS unit cuts corners in a random fashion.

 

[Gibs]

Thanks for all the inputs, I happened to ride by a speed sign on the base today, the type that tell you how fast you are going and have barney fife on the other side of them. Anyway, the speedo was right on what it said. so I think it is close enough. Again thanks everyone.

 

[Lelandjt]

I have to setup a lot of computers for customers with all different size tires. I always roll the tire on the floor 1 revolution and measure that. It only take a couple seconds and you know it's accurate.

 

[Yukikaze]

Thanks for this information! I have been using the wrong tire circumference for my Polar RCX5. I'm glad I looked through this thread.

 

[Kerry Irons]

Thanks for all the inputs, I happened to ride by a speed sign on the base today, the type that tell you how fast you are going and have barney fife on the other side of them. Anyway, the speedo was right on what it said. so I think it is close enough. Again thanks everyone.

Since the speed signs are only to the nearest mph and could easily be off by 1-2 mph, you're looking at a 5-10% error. You might be able to make the argument at 70 mph but not at 15-20 mph. Calibrate your computer - it's not that hard.