Good with a calculator?

[Globalti]

I reset my computer to take account of a new slightly smaller tyre, but instead of setting it to 211 cms circumference I stupidly set it to 16 inches! Went out this morning and soon realised something was wrong when blasting along and the speedo was showing less than 20 mph.

Now the computer has registered 2.52 hours and 27.12 miles, but using my map measuring method I reckon we rode 48 miles in that time, giving a good average of 19.05 mph.

Can you calculate from the 16" wheel setting to confirm if my map measuring method is accurate?

 

[HLaB]

According to Sheldon, depending on tyre width a 16inch tyre had a circumference of 128 or 108cm (quite a difference )
Conservatively taking the larger figure your computer would have read 211/128 = 1.648 times too small so the recorded 27.12miles was actually (1.648 x 27.12) 44.694 miles. On that basis your average speed is quite a bit lower (15.6mph) but you need to know what circumference was actually used.
Taking the 108cm would be 1.954 times too small (52.99 miles)

 

[MacB]

all the computer does is count the tire revolutions and convert it to a distance...so divide the distance you did by the 16 inches to get the revolutions and then multiply that number by the correct tyre size.....easy

 

[Globalti]

Thanks. I've just found the Velo 8 instruction leaflet and it gives a 16" wheel setting as 119 or 120 mm circumference, so... 211/120 = 1.75 times too small, therefore 1.75 x 27.12 miles gives 47.68 miles, meaning that my map measuring method is pretty damned accurate! This means an average speed of 16.75 mph though, am I right? It's more believable as we had a stiff headwind coming home.

Incidentally my map measuring method consists of counting every single time the route crosses a blue grid line, even if it dives across then back again, then halving the number. It never fails.

 

[HLaB]

Is it 2 hours 52 mins or 2.52 hours.

If 2 hours 52 mins; 47.68/ (2+(52/60)) = 16.6 mph

If 2.52 hours; 47.68/2.52 =18.9mph

 

[Globalti]

Looking at the instructions again I see that it's 2 hours and 52 minutes.

 

[Angelfishsolo]

+1

all the computer does is count the tire revolutions and convert it to a distance...so divide the distance you did by the 16 inches to get the revolutions and then multiply that number by the correct tyre size.....easy

 

[ufkacbln]

Don't forget to factor in weather and atmospheric pressure, low atmospheric pressure causes the air in the tube to expand and the tyre to increase in size, equally rain cools the tyre and causes the volume to decrease and reduce tyre size.

 

[Norm]

Indeed, although you need to know the answer before you can calculate the answer.

The rotational forces will also expand the tyre diameter, so you need to know how fast you were going to calculate the amount that the tyre has expanded before you can calculate how fast you were going.

 

[Globalti]

Silly boys!

 

[Norm]

It's true. I couldn't resist and you and MacB seemed to have the answer covered between you a few hours ago.

 

[Garz]

Still it's interesting and keeps the geek within us on our mathematical toes!