Who's good at sums?

[MikeP]

Just checked my speedo on a measured mile using a certified stopwatch.

Steady 60mph = 64.5 seconds. That explains my variation between speedo and Nav II. (Having re-read my previous post on the variation in GPS info, I've realised I said that the speedo was under-reading, should have said over-reading!)



So who's good enough at sums to tell me what the true speed is please?

Going to repeat the excercise with the Nav II soon.

 

[adamski49]

If I've done it right... 55.8mph assuming 64.5 seconds was to cover 1 mile

Adam

 

[carlhowe]

1 mile is 1760 yard. 1760 yards on 64.5 seconds is 1760/64.5 = 27.28682 yards per second.

So in 1 hour that is 27.28682 x 60 x 60 = 98232.558 yards

divide this by 1760 to get back to mph = 55.8

Before someone tells me, I know there was no need to convert to yards, but lose count when typing non significant zeros.

Carl

 

[MikeP]

Cheers!

Sounds about right to me but then I'm completely innumerate!

 

[carlhowe]

bugger, he replied while I was typing....

is it a case of 'great minds think alike' or 'fools seldom differ'??

Carl

 

[MikeP]

Done a few more runs, same (chained) measured mile and same calibrated stopwatch.

At a steady sixty (not easy trying to keep it bang on with the digital read-out from the NAV II) 60.5 secs. Allowing for the signal delay that's close enough for me

It's comforting to know that the optimistic speedo will keep me on the conservative side regarding Maurice Gatsonides invention

Another question now for those who can do hard sums; the variation quoted in my first post, is this within 10%?

UK legislation (Construction and Use) Regs 1988, states that a speedometer should have no more than a 10% error, Goods Vehicle Tachographs no more than a 2% error (the latter are checked and calibrated). That's why Gatso, Truvelo, Speed Marshall et al, allow 10% +.

This is just out of curiosity. I've noticed that the variation between speedo and NAV increases considerably with speed. At 30 mph on the speedo the NAV is showing around 28 - 29 mph. Con & Use Regs don't specify whether the 10% is an average across the speed range. It would be difficult to complain that it's out by more than 10% at 100 mph but bang on at 20 mph! I feel certain that BMW would say ride at 20!!! Would be nice to get them to pay for calibration though!

 

[Spout]

OK. So how much does a fresh tyre and a fully worn tyre affect readings?

Assuming 3mm wear the tyre is 6mm (1/4 inch) less diameter. So bike will travel 3/4 inch further per revolution on a fresh tyre. Therefore, bike will be actually travelling faster for a given speedo reading. Or am I talking b*ll*cks?

 

[Bikerbear]

Gotter - the weather's too good to be doing sums and measuring miles - ride the bike and enjoy it - 10% for any given speed, will be a tenth - 20 = 2mph, 30=3mph 44 = 4.4mph 67 = 6.7mph 110 = 11mph & so on and so forth!!!. Its quite simple really - bit like me!!