Bontrager computer on Brompton

[PitPony]

I have a Bontrager Gotime computer. I can set the computer for different wheel sizes using the codes provided by Bontrager. The problem is that there isn’t a code for the Brompton wheel size. Has anyone dealt with this problem ?

[backnotes]

According to the Go Time manual, the 4 digit number is simply the "rollout" of the tyre in mm, or in other words, the circumference.

According to this thread viewtopic.php?t=62099 the rollout of a Brompton tyre is 1267mm, but that is calculated rather than measured. It should be right enough, but you could measure it yourself with your particular tyres.

The manual has instructions to do this:

To measure rollout
Tools required: helper, pen, and measuring tape
+ With the valve stem of the wheel directly over the floor, sit on the bike.
+ Have your helper mark the floor at the valve stem.
+ Roll the bike forward one revolution of the wheel, so that the valve stem is
again directly over the floor.
+ Mark the new location of the valve stem.
+ Measure the distance between the marks. If needed, convert to millimeters.
The result is your custom wheel size. Go to the next page to program the
GoTime with your wheel size.

The manual also has instructions for entering a custom wheel size on page 24. That's the manual you get via this link https://manuall.co.uk/bontrager-gotime- ... -computer/

Otherwise, if you put in 1267 for now, and then tell us what make and size tyres you have, that will be right within a few %, and then someone on here may may be able to come up with a more accurate figure.

Good luck!

[Paulatic]

Usually with computers you enter the circumference in millimetres.
I used to put a spot of tippex on the tyre. Sit on bike and roll along some clean black tarmac. Measure the distance between the white spots in mm and that’s your number.

[PitPony]

Thanks backnotes. I had just put in a guesstimate of 1176. I had assumed that as the Brompton wheel is smaller than 1.5 the Bontrager code for 1.5 (1185) should be reduced. If it’s still underestimating distance I’ll try your suggestion of 1267. Thanks again

[simonhill]

After putting in the appropriate figure, I then fine tuned mine using a GPS along a straight road back and forth a few times.

We discussed measured mile roads a while back, but can't remember if there are any in the UK. I did see one in NZ.

Edit: my Brompton is set to 133 (presumably 1330 for you) with Schwalbe Green guard tyres. Can't remember why I set it like that, but it is in accord with my other bikes on a set route.

[rmurphy195]

Usually with computers you enter the circumference in millimetres.
I used to put a spot of tippex on the tyre. Sit on bike and roll along some clean black tarmac. Measure the distance between the white spots in mm and that’s your number.

Chalk does just as well!

[PitPony]

Update : a setting of 1330 suggested by simonhill gives the same distance as my planning tool app, so that’s good enough for me.
Any Brompton riders on Schwalbe Marathon Racer 35.349 might want to try this when calibrating their computers.
Thanks for all the helpful posts.