Binski99`

So I bought one of those 12 function bike computers made by schwinn. My tires are 26x2.3. So what should my wheel factor be. It says to multiply the wheel diameter in milimeters by 3.1416. I gave the chart below but I want it as accurate as possible.

Thanks in advance!!!



Schwinn Bike Computer Wheel Size numbers
Wheel Diameter Number
20" 1596
22" 1759
24" 1916
26" (650A) 2073
26.5" (Tubular) 2117
26.6" (700x25C) 2124
26.8" (700x28C) 2136
27" (700x32C) 2155
28" (700B) 2237

w/ tire
ATB 24X1.75 1888
ATB 26x1.4 1995
ATB 26x1.5 2030
ATB 26x1.75 2045
ATB 26x2 (650B) 2099
27x1 2136
27x1 1/4 2155

Big_e

I have a newer schwinn computer and an older one that lists a size of Wheel diameter w/tire: 26x2 (650B)- Wheel factor: 2099. That looks to be the closest, hope that helps.
Ernest

PS: Wonder if your list is a misprint since it says that factor for a 23x2 is 2099 which wouldn't really make sense. Now that I read both of my lists closely, they both say the same thing with the 26x2 wheel. Yours must be a misprint. 26x2 is the closest to your wheel with tire size.

mechBgon

If you want it as accurate as possible, then inflate your front tire to your normal riding pressure, sit on the bike, and measure how far it rolls in one wheel rotation while you're sitting on it.

The usual method is to make a mark on the tire, align the mark to a starting mark on the floor, then roll forward until the mark on the tire is pointing at the floor again, jump off, and mark the floor with your ending point. Now measure the distance between the starting and ending marks.

bottlecape30

that is so weird i bought the same unit the other day.
the thing i can't figure out is where on the spoke to put the magnet?
in my mind where this is placed should make a big difference.

mechBgon

It doesn't matter where you put the magnet on the spoke. The magnet will still trigger the sensor at the same frequency when the wheel's turning, no matter where on the spoke you've placed the magnet. So don't sweat it

Binski99`

I measured one full rotation and that was a distance of 2100 mm so that is what I set it as. Oh yeah that was a typo I made it is 26x2

Thanks for all the help

Big_e

With all due respect, are you sure about that? I would think that different placements could give false readings on the speed. I placed mine right above the spot where the spokes cross as they near the axle. That's kinda how the small illustration has it placed.
Ernest

mechBgon

Absolutely certain, yeah It's a question I have to answer a lot (LBS mechanic since 1989).

If the computer were trying to detect the linear velocity at which the magnet is travelling, then it would matter, because the further out the magnet is located, the faster its linear velocity. But that's not how the computer computes speed. It looks at the frequency of sensor impulses, which is independent of the magnet's location on the spoke. For a given ground speed, any point on the spoke will pass by the sensor at the same frequency.

And that's a good thing, or else it would be very difficult to set up bike computers accurately!

On a separate note, I like to put the magnet at the crossing point of the spokes too, for wired computers (for wireless, I put the sensor as close to the bars as possible). Many spoke magnets can engage two spokes at the crossing point, keeping the magnet from getting bumped out of position as time goes by.

bicycleflyer

All you are doing is multiplying diameter by 3.14 (aka "Pie") to get circumference. Basic math. Some computers want centimeters, some want millimeters, others want some goofy formula. Your's just wants millimeters which is not uncommon. A roll out test will give you the most accurate results.

But for future reference, Sheldon Brown has a master-chart of most all wheel sizes.

https://sheldonbrown.com/cyclecomputer-calibration.html

In your case...use column "F"

Don't get too anal about this. Even if you only use the chart instead of a roll out, you still have better accuracy than the speedometer/odometer on most autos. FWIW .... I just use charts myself.

bottlecape30

Hey thanks,
that makes sence. I remember hooking one of these up to my brothers bike years ago and they had you messure a point on the spoke. Good to know that is no longer nessary.
Cheers

dalesd

https://sheldonbrown.com/cyclecomputer-installation.html
[emphasis added]

mechBgon

On my commuting mountain bike, I have the sensor located halfway up the fork blade, and it has no problem reading speeds approaching 60mph. Having installed thousands of computers in my career, I'd probably know by now if this were actually an issue Put the magnet where you prefer (unless it's a wireless sensor, in which case I suggest placing it as close to the handlebars as practical, for obvious reasons).

robert1234

All the thing does is count the times the wheel goes around so it doesn't matter at all where the magnet is located.

bktourer1

To make sure I had the right #'s in the computer, I found a road marker that started in 0 and rode till I hit 1 mile.
I was a fraction off, not enough to make a difference