Stupid sigma computer question

26*25.4*3.15= 2074 and 2074/1.61=1287 for mph, both of which is pretty close to the 2089. The difference is in the width of the tire (the 1.9). A wide tire will have a larger diameter than a narrow tire on the same wheel. You have the right equation, but you're diameter is slightly off.

M

mike_stay said:

26*25.4*3.15= 2074 and 2074/1.61=1287 for mph, both of which is pretty close to the 2089. The difference is in the width of the tire (the 1.9). A wide tire will have a larger diameter than a narrow tire on the same wheel. You have the right equation, but you're diameter is slightly off.

M

ok - many thanks for the post - Steve

Steve Wells said:

I have question about a sigma cycle computer. I have to key the
wheel size.

Quoted message said:

My tyre says 26X1.95 and the manual for my Sigma BC1200 RDS
says 26X1.9 = 1298 (mph) or 2089 (km/h).

One mile is 1.61 kilometers and therefore 1298 x 1.61 = 2089. This is
the distance the bicycle travels in one wheel rotation in millimeters.
The instrument counts rotations and computes distance and speed from
that.

Quoted message said:

Can someone explain how they come up with this?

Quoted message said:

I thought I should use:

Quoted message said:

tyre size (inches) X 3.14 X 25.4 / 1.61, but I can't get this to
come up with the figures in the manual.

Do that differently if you want to get it accurately. The rolling
circumference of a tire varies with load, and this is dependent on
inflation pressure and load. For this reason, measuring that distance
directly under normal tire inflation is appropriate. To do this,
position the wheel accurately with the valve stem directly over a
visible mark or piece of tape on the floor, sit on the bicycle in
normal operating posture and roll forward one revolution until the
stem is again directly between wheel center and floor. Measure that
distance in millimeters and enter that number in the metric mode of
the speedometer. If you switch to English, the instrument should
convert it, but if it doesn't, enter that value divided by 1.61.
Working from nominal tire dimensions is often in error greater than
that which would occur in measuring the roll-out circumference.

Jobst Brandt
[email hidden]

quote=''Steve Wells said:

I have question about a sigma cycle computer. I have to key the
wheel size.

Quoted message said:

My tyre says 26X1.95 and the manual for my Sigma BC1200 RDS
says 26X1.9 = 1298 (mph) or 2089 (km/h).

One mile is 1.61 kilometers and therefore 1298 x 1.61 = 2089. This is
the distance the bicycle travels in one wheel rotation in millimeters.
The instrument counts rotations and computes distance and speed from
that.

Quoted message said:

Can someone explain how they come up with this?

Quoted message said:

I thought I should use:

Quoted message said:

tyre size (inches) X 3.14 X 25.4 / 1.61, but I can't get this to
come up with the figures in the manual.

Do that differently if you want to get it accurately. The rolling
circumference of a tire varies with load, and this is dependent on
inflation pressure and load. For this reason, measuring that distance
directly under normal tire inflation is appropriate. To do this,
position the wheel accurately with the valve stem directly over a
visible mark or piece of tape on the floor, sit on the bicycle in
normal operating posture and roll forward one revolution until the
stem is again directly between wheel center and floor. Measure that
distance in millimeters and enter that number in the metric mode of
the speedometer. If you switch to English, the instrument should
convert it, but if it doesn't, enter that value divided by 1.61.
Working from nominal tire dimensions is often in error greater than
that which would occur in measuring the roll-out circumference.

Jobst Brandt
[email hidden][/QUOTE]
Mnay thanks for this post. I will try the roll-out and see what I get.
Thanks for all the explanations... Steve