# Bike computer wheel size setup

I've gotten my self my first mountain bike, along with a Bike computer. This needs to be setup, but im really not sure how i do this. The wheel size of the bike is a 27,5 inches. The bike computer is a Bontrager Trip 100, that comes with the following wheel settings:

700:23, 700:25, 700:28, 700:32, 700:35, 700:38, 29:2.1, 29:2.2, 26:1.5, 26:1.9, 26:2.0, 26:2.1, 26:2.2, 26:2.3

One thing is what setting to enter on the computer, but im also unsure about where i should place the sensor and receiver on the wheel. I believe it must matter if i place the sensor and receiver near the center of the wheel, or if i place it need the edge.

## 6 Answers

Look at page 23 and 24 of the manual and follow the directions for measuring tire rollout. Measure your front tire and select the wheel size on the chart that has the closest measurement to your tire. There can be a big difference in the circumference of different brand/model tires even if the stated size is the same. Measurement of the actual tire circumference will give you the most accurate readings. As far as placement it makes no difference. As long as the magnet on the spoke triggers the pick-up it will count revolutions. The computer does the calculations to convert the data to mph, distance etc. My preference is to mount it as close to the axle as possible. On a mountain bike this keeps it out of the mud and away from branches.

• Yep, measure how far the bike travels in one revolution of the wheel. Even if there is no table in the manual, it's a simple math calculation to convert wheel circumference into wheel diameter, and you'll be a lot more accurate than feeding in a wheel size. (And some computers let you feed in circumference directly.) – Daniel R Hicks Dec 29 '14 at 3:50
• And it makes no difference how far out along the spoke you place the magnet and pickup, so long as it's reliably mounted. The computer measures rotations. – Daniel R Hicks Dec 29 '14 at 3:52
• The problem with the simple math is that it doesn't catch the change in effective rolling diameter, which is impacted by rider weight, air pressure run, and manufacturing tolerances. Those add up to more than you think (per my own experiments), so I now advocate the rolling/measuring as in Batman's post). – Brian Knoblauch Dec 30 '14 at 20:32
• One of the pro tips for this is to use toothpaste. Put a line across the pavement, ride over it, and then measure between the first line and where the tire touches down again. – Eric Gunnerson Jan 1 '15 at 0:55

A 27.5" wheel is a 650b wheel, which is not listed. Your bike computer should have a way to do calibrations the old fashioned way -- by rolling the bike a known distance (typically something like ~10 meters) after mounting the magnet+sensor and from this, it will calculate the necessary parameters for the wheel size. If you don't have that ability, you should pick the closest wheel circumference you can -- note that the sensor will only pickup the wheel per revolution regardless of where it is placed.

As for placing the sensor, the manual(*) for the trip 100 says that the magnet needs to come within 3-5 mm of the sensor. This is typically done closer to the brakes than closer to the hub on a road bike, but you may need to go further down in order to get the magnet to get within that range on a mountain bike (most 650b bikes sold these days, esp. ones marketed as 27.5" are mountain bikes, as 650b road bikes are often marketed as 650b, so I'm assuming you have a mountain bike). If you're within this range, the sensor mount is good. If it isn't, move it to that range. See the manual for pictures.

(*) I think this is only the quick-setup, there should be a more useful manual somewhere, like in the box.

On this page, the following information is given.

Use a roll out method ie measure circumference with a mark on the tyre and roll forward one exact revolution measure with a tape measure the distance covered preferably sat on bike, then put dimension into the custom setting for wheel size, youtube shows a good example.

According to the manual linked to by @Batman, it looks like you can enter a custom wheel size after you have clicked through all the predefined wheel sizes. Based on the fact that it's 4 digits, the custom wheel size is most likely in mm.

Another way of calculating the circumference, is to take wheel diameter (total distance across, and multiply by pi (3.14). If you don't have any measuring device and want something reasonably close, take the bead seat diameter, for 650B, this is 584mm, add in 2 times the tire size. Assuming you have 2 inch tires, that's 50.8 mm. So we end up with 584 + 2 * 50.8 = 685.6 mm diameter, and when multiplying by pi gives us a circumference of 2152.8 mm.

• The only problem with calculating this based on the sizing specified by the manufacturer is that tire sizing isn't exactly reliable -- you can buy two 2 700x28 tires but have significantly different circumferences (and they may not clear in the same frame!). – Batman Dec 29 '14 at 22:53
• Yeah, like I said. This is only if you don't have a measuring device, otherwise the rollout method is the most accurate. – Kibbee Dec 29 '14 at 23:59

The most accurate way I have found to measure the circumference of a blown up cycle tyre is to lay the wheel on a flat surface with a line drawn across it. Place the wheel with the centre of the hub on the line supported with a couple of wood blocks just high enough so that the spindle is clear of the surface.Place a set square or a 10 cm + board upright on it's edge on the line and against the tyre. Measure the distance from the set square/board to the centre of the wheel hub. Multiply this dimension x2 to give the diameter of the tyre then multiply the diameter by Pi (3.14).

To setup bicycle computer you need to find out circumstance of your wheel in mm. Most reliable way I know is the following.

• enter some approximate value and call it c0 (circumstance initial)
• drive some track of well known length, athletic course, rowing track, 100km ride, anything that has some length and you know long it is. let's call this value dA (distance actual). Make sure that your tires are inflated properly.
• read distance measured by your computer with initial value, and call it dM (distance measured)

now you can calculate actual circumstance cA as follows:

cA = (dA * c0) / dM

Great ideas above re calculating actual circumference via wheel travel (toothpaste!)...I just used a marker placed based on valve stem, rolled out one revolution (valve stem returns to 6 o'clock) and got 2209mm (it is a new single speed 650B/27.5" MTB). Unfortunately my CPU did not afford a manual entry so I used this table:

http://www.bikecalc.com/wheel_size_math

to locate the nearest circumference. I chose to go down in value (assuming that my weight will reduce the overall circumference (as will treadwear, reductions in PSI, etc.) and used the nearest correlated value to find one of the 15 options that were pre-loaded into my CPU. Figure it is not an exact science, but who cares? The relative values (ride to ride, month-to-month) of actual use are more predictive of what the step counter (or CPU in this case) measure.

• Welcome to Bicycles @beardgod. I have removed the negative remarks; please check out our help center, especially the be nice part. Again, welcome. – andy256 Aug 30 '15 at 0:29