i have just thought of another little thing that will affect it slightly .. tyre size.
now the standard front is 90/90 (is that right) and i have a 110/80,
i once did a formula for a renault 19 forum to prove that a car was actually going faster than the speedo read when you fitted 18 inch tyres ,
this equated to the fact that the speed was measured from the gear box and so it would make a car tyre turn a certain ammount of revolutions giving a set speed for the ammount of turns,regardless of tyre size. so if you had a 13 inch tyre on it would still read the same speed as an 18 as the speedo is measured from the gearbox. (yes i also adjusted for tyre wall death)
using circle measurements i worked out that the difference between the standard 15 inch tyre and the 18 inch low profile was quite significant and it worked out that at 100mph on standard rims you were actually doing about 110 on the non standard..
ok so putting this onto the bike , this is slightly different as the speed is measured from tyre rotations, so you can forget all about engine speed, revs, gearing etc.. if your front tyre is oversize your speed will read slower because its taking longer to rotate..
ok i just spent the last 20 minutes working out that my tyre is oly 2.9% longer on the circumfrence.. big deal. so when my clocks read 100 i am really doing 102.9.. , theres probably more error than that anyway!!
at 60 mph i am really doing 61.74..... i think i will give up now as its not worth trying this one anymore..there obviously isnt as much difference between tyres on bikes, i suppose this is because your only changing the side wall height by a tiny ammount and not the entire rim and side wall..i will put down what i did roughly below .
17 inch =43.18cm wheel diameter
profile =% of width so a 90/90 is 90% of 90 width = 81 or 8.1cm
mine is 110/80 so 80 % of 110 which is 88 or 8.8cm
mutiply the tyre profile by 2 (two sides) and add the rim diameter
8.8 x 2 + 43.18 = 60.78 (110/80)
8.1 x 2 + 43.18 = 59.38 (90/90)
multiply these figures by pi.. 3.1415926536
pi x 60.78 = 190.95cm (120/80 tyre circumerence)
pi x 59.38 = 185.55cm (90/90 tyre circumfernce)
so to work out the % difference we take the following
190.95-185.55 = 5.4
5.4/185.55 x 100 = 2.9% difference
however i can take joy in the fact that the tyre will should last longer....
190.95/100 = 1.9095 (converted to metres)
185.55/100 = 1.8555
1 mile = 1 609.344 meters
so over 1 mile the tyres rotate the following ammount of times
1609.344/1.9095 = 842.81 (120 tyre)
1609.344/1.8555 = 862.34 (90 tyre)
lets say a normal wearing tyre gives 5000 miles then that would be 4311700 rotations on a standard 90 tyre(bloody hell!!!)
5000 x 862.34 = 4311700
so if the 120 tyre does the same ammount of rotations we can work it out as follows
4311700/842.81= 5115.86 miles... so screw you guys i get another 115 miles out of the tyre.. lol.. thats not actually true
i will appear to get an extra 115 miles due to that 2.9% difference.. if a tyre last 5000 miles it lasts 5000 miles so all the above is bullsh*t..
anyway im off out now to do things that are constructive... LATERS