Cycle Computer

[securespark]

Help...

I have a Sigma BC400 cycle computer which I'm trying to set up for wheel size. My wheel size is 26 x 1.75.

Trouble is, despite the instructions, I cannot work out what to do!

Anybody got one?

Cheers!

 

[andy]

Help...

I have a Sigma BC400 cycle computer which I'm trying to set up for wheel size. My wheel size is 26 x 1.75.

Trouble is, despite the instructions, I cannot work out what to do!

Anybody got one?

Cheers!

you need to program the circumference of the wheel into it. there is usually a guide with it what has the wheel diameters etc

and if you want to make it look like you do more miles than you actually do, add a 2nd magnet (or even 3RD!) to the wheel

 

[david and julie]

When I did mine (not the same make) I put a pencil mark on the edge of the tread and a corresponding one on the ground. Rolled the wheel 1 full revolution and marked the ground again. The distance between the 2 lines (in centimetres) was the figure which I used for set up.

How do your destructions tell you to do it?

 

[securespark]

The instructions say you can use one of two methods to calculate the figure:

Use their wheel size chart, or use the formula wheel height in cms x 3.14.

The things I don't understand are:

1. The wheel size is 66cm x 3.14= 207.24

2. When I look on the chart the figure is 2070. But the machine only has space for 3 digits.

3. The preset figure is 215. Nowhere on the chart can I find a figure like that.

4. There are different values for tyres of same size as mine, but different thicknesses. However, if you use the cm x pi formula, there is no thickness taken into account.


The r/h column in black is wheel size, and the r/h column in red is headed "WS kmh"

 

[pipme]

Tyre section will have effect upon 'rolling radius' due to 'aspect ratio' or profile .. It seems feasible for a bicycle, that the wider the tyre for a given diam. the greater the aspect ratio and hence the rolling radius, mud plugging etc.

eg. Your size 26" x 1.75" (I take it) or 26" x 2.0"

26" = 66.04 Cm base radius = 33.02 Cm

Circumference of 2070 mm(given?) = 207.0 Cm ÷ Pi = 65.89 Cm ÷ 2 = 32.95 rolling radius, very little different to base radius.

But

26" x 1" has a rolling circumference of 1973 mm or 197.3 Cm, working back this gives a rolling rad' of (197.3 ÷ Pi ÷ 2) = 31.4 Cm.

BTW Just a thought 4 digits in mm saves one digit width over Cm to one place. 2070 against 207.0 pits 4 against 5 positions, a print column saver, perhaps??

The narrower tyre has a lower profile.

The other 'red charactered' column I have no suggestion for at all.

All of the above is pure supposition, quite obviously I know naff all about it .. but I have had a 'pop' ... This will force the hand of the 'idle' knowledgeable to come out and jump all over me, but give you the info you need ..

D&J's method of finding the rolling circumference is practical and true, given flat surface, correct tyre pressure and rider on board !!

 

[securespark]

Thanks everybody.

Went to a bike shop, and he did the calc 66 (height from tarmac to top of tyre) x pi = 207.blah

So he entered 207. They have 4 digits so the more accurate computers can be set up with a more accurate measurement.

It works! See other post...

 

[saxton]

Help...

I have a Sigma BC400 cycle computer which I'm trying to set up for wheel size. My wheel size is 26 x 1.75.

Trouble is, despite the instructions, I cannot work out what to do!

Anybody got one?

Cheers!

and if you want to make it look like you do more miles than you actually do, add a 2nd magnet (or even 3RD!) to the wheel

I like it!