The Pod and other venues that are set up for 1/4 mile events use a series of beams at specified distances along the track. When you break the start beam the clock starts, and when you break the beams up the track the timing gear gives you a time. You ALWAYS cover a distance of 1/4 mile at these venues every time you visit as the timing lights are always in the same place. To measure your speed there are 2 timing lights a fixed distance apart. Using the equation:
Velocity = distance/time
your average speed over this known distance can be calculated. This is where your terminal speeds may be a little out from what you expect at the Pod as the second terminal speed beam is after the finish line, so you need to keep on the gas until you pass it.
Bruntingthorpe did indeed use GPS, but it was not the same standard as used in your TOM TOM, it was far far more accurate (so much so that I can see the lines people took around the bends). Using many samples per second the timing system uses the same equation as above ie:
Velocity = distance/time
As it knows with very high accuracy the exact position of the GPS device the velocity at any given moment can be calculated. Because it works in 3 dimensions it would even have taken account for anyone that decided to use the width of the runway in there timed sprints. Because it knows your velocity at any given moment it can also therefore determine when certain thresholds are passed, like for example 60 mph. It then looks at the time, looks back at the time the first movement was noticed, takes one from the other and voila a 0-60 time. For the 1/4 mile time, well it measures distance accurately so it knows when you have travelled exactly 1/4 mile from your inital point of rest.
The RSM uses a different set of inputs and equations. It has a speed input from the gearbox so it knows how fast you are going at any given time (as long as you set it up correctly) It also has an acceleration input. To measure the 0-60 times it uses both of these, primarily it uses the speed sensor though. When it senses a change in speed from 0 to anything more than about 0.5mph it starts the clock. As it has a speed input it can easily work out how long it takes to get to 60. It is quite clever though as it will also use the acceleration input to compensate for wheelspin......how? Well using a couple of equations of course. At any given time it knows your speed and the time elapsed so it can calculate what the acceleration should be using the equation:
Final Velocity=Initial Velocity + Acceleration X Time
or
Acceleration = Final Velocity - Initial Velocity / Time
Now as it also has an acceleration input it can determine whether or not the calculated acceleration and the actual acceleration are the same......it does this many times per second. If the calculated acceleration is significantly more that the measured acceleration then you have wheelspin. Easy! If it is less then something is wrong!
To measure 1/4 mile times it uses the speed signal input with the following equation:
Distance = Velocity / Time
Again taking measurements many times per second, so at any given moment it knows how far you have travelled since the previous given moment. Now again it will use the acceleration input to ensure everything tallys up:
Distance = 1/2 x Acceleration X Time x Time
And correct as approprite.
Now on the the Gtech. It has only one input and that is acceleration. Unfortunately it is the acceleration of the unit rather than that of the car (unless you have it rigidly mounted) This is where the problems start. Mount your Gtech on your windscreen and turn it in to the mode to measure Gs and give it a little tap......you will see the Gs change drastically. Now drive along with it in this mode and see what happens to the Gs as you drive along Britains fantastic roads. Now the screen update is quite slow, in the background the processor is getting loads more samples per second. So what? Well this as you will remember is the only source of measurement that the gtech has. To figure out your speed it uses the equation:
Final Velocity = initial Velocity + Acceleration x Time
If the Acceleration measurement is disturbed this can have a profound effect on the calculation. If you dont believe me I will perhaps put together a spreadsheet for you.
Similarly it uses the equation:
Distance=1/2 x Acceleration x Time x Time
To work out how far you have travelled, and shaking or erroneous readings will cause errors to be introduced. The reason that the results from testing at the Pod may have led you to believe that the units were giving reasonable results is probably more to do with the track surface than anything else.
Then there is the issue of gradients. If you start off on a level bit of road and then find during your run the gradient changes this will also affect your time. As you know from setting the unit horizontal before a run the acceleration sensor is quite accurate! Contrary to Richards post you will probably find that you get better results going uphill than down as the sensor will read a higher G than you are actually pulling that will affect the equations. Go downhill and the readings will be lower so it will take you longer to reach the target. It may be marginal with the difference in actual acceleration up and down a hill though. Now if you start to think about the effects of bumps and suspension settings along with the secure method of fastening the unit to the windscreen you may start to see the point!
So in summary, all of the methods of measuring times and speeds use input data and equations to work things out. Where the other methods always use a known physical input (speed or distance) the Gtech suffers from using data that is at best questionable pervesely due to the accuracy of its measuring device. On a similar bit of road you will always be able to get a good comparison, but comparing times from your test track and my test track and anyone elses test track, let alone different peoples car can only ever be a rough guide.
