Uneven wheel circumferences

Hi guys
I had a question
If you have 2 different size tires in your bike..17inch in the rear and say 24 or 26inch up front..how would you calculate the accurate speed to match your Cycle Analyst display?
The tire circumference should be input into the CA.
Thx

The wheel that you pick up the speed signal is the one that matters, of course. You need to calculate the length of a complete rotation in riding conditions, when you are riding the bike and the tire is inflated at your usual riding PSI. Put a mark on the ground and tire, ask someone to mark the completed rotation on the ground. This will give you a precise speed reading.

ebike11 said:

Hi guys
I had a question
If you have 2 different size tires in your bike..17inch in the rear and say 24 or 26inch up front..how would you calculate the accurate speed to match your Cycle Analyst display?
The tire circumference should be input into the CA.
Thx

Click to expand...

If you are using a CA that picks up the speed signal from the controller (via hall wire), and you have a hub motor, then it's based on the size of the hub motor wheel. If you have a CA that uses and external sensor, then it's based on the wheel where the sensor is located.

MadRhino said:

Put a mark on the ground and tire, ask someone to mark the completed rotation on the ground.

Click to expand...

I have put a dab of paint on the tire tread, rode the bike forward thru a complete rotation, and measured the distance between the paint marks left on the concrete.

99t4 said:

MadRhino said:

Put a mark on the ground and tire, ask someone to mark the completed rotation on the ground.

Click to expand...

I have put a dab of paint on the tire tread, rode the bike forward thru a complete rotation, and measured the distance between the paint marks left on the concrete.

Click to expand...

I measure wheel diameter with a tape, enter that number, then compare the display reading against a GPS speedometer on my phone, and make small corrections as necessary.

I used to use the "dab of paint" method to set cyclocomputers, back when a phone was something that attached to the wall. I used Liquid Paper. Remember that stuff?

Precise diameter is with rider on the bike. That is because the wheel has a larger OD when the tire is not compressed by the riding weight. Of course, if one does ride high PSI the difference is not much. The error gets more important with lower PSI. But, the speedometer precision is not important for everyone.

GPS speed extrapolation has a delay, enough to require constant speed for a while in order to give numbers that are close to reality. GPS speed is precise only as a compilation over a considerable distance.

MadRhino said:

Precise diameter is with rider on the bike. That is because the wheel has a larger OD when the tire is not compressed by the riding weight. [...]
GPS speed extrapolation has a delay, enough to require constant speed for a while in order to give numbers that are close to reality.

Click to expand...

Sure, even the bike's display readout has some latency that must be stabilized before its accuracy is evaluated.

I'll go for a direct measurement of speed over an extrapolated measurement of diameter every time, though. Not that it matters that much, as you point out.

Set it to the ballpark correct setting, like 26 inch wheel.

Then you can use another device, like a car odometer known to be accurate, GPS, or mile markers you have confirmed to be accurate, to fine tune your bike speedo or CA, to be accurate for a one mile distance. It might take a few runs to get it perfect. Run one, you measure short, change it one click, still too short, change it two clicks, oops, too long. and so on.

For me this mattered only when you wanted to know to the quarter mile, the distance traveled in a long ride. You ride the mile, or more if you like, and then tune the device to the greatest possible accuracy.

And yeah, that does account for deformation of the tire as you do it.