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Firstly the speed data and distance data don't seem to match. There is up to a 10% discrepency on the data I have! For example:-

On a run of 11.6s to do a 0-60 my average speed is 14.57m/s

Thus, in 11.6s I have covered approx. 169m

According to the distance data I have covered 181m and so my average speed is 15.65m/s.

Secondly, why does the distance data not use the altitude in its calculation? I have been looking at the data to understand how it's calculated and it correllates with pythagoras of x and y position. Is the altitude data not accurate enough or is it assumed that you find some flat road?

Finally, to calculate power I require acceleration, kindly provided in the .csv download. However, this requires a lot of massaging to remove some big values and get it to a stage where I am confident that it's realistic. My car in 2nd can pull 0.5g (on a GTech) so I've set this as a maximum figure. What sort of averaging should I use on the acceleration data to get the best results? I've started to search but as ever it takes a while to sift through the pages and get to a result that is relevant.

## Comments

The acceleration values are there mainly for video use, and are estimates calculated from difference in speed and bearing. In my opinion, to calculate power it would be better to have actual G force data from real accelerometers (not pseudo data calculated from GPS data). Unfortunately RaceChrono does not support accelerometers yet.

I have had real accelerometer support in the works really long, but it seems I just have too many things to do :(

In terms of power caluclation it's more of an exercise than for any proper purpose. I would have hoped that the data was accurate enough to enable calculation of average acceleration every second for example and then use an interpolation of that data for the guestimate of power. The actual acceleration data in the .csv has such a large amount of noise at 5Hz that it's not suitable for the task at the moment.

Fundamentally accelration is the change in speed with time so calculating it is not an issue, the error of the GPS position is the issue so that the transition of the speed data is not as smooth as you'd like to calculate acceleration at the same rate. Over an ageraged period of time you should be able to get data that has a low enough error to give some results that can be used.

I'll have to do a few more runs and see how I get on...

From these runs I was then able to calculate power and air resistance to give approx. power at the wheels. To improve accuracy I averaged the power over a second for each point. A better method will be to manually smooth the acceleration data to remove noise, although on these runs it was a lot better to begin with.

The final stage was to take the calculated power at the wheels and apply a losses equation taken from rolling road data for a quattro car. The data points I had were to convert power ATW to fly power, 1.15 at 3k and 1.35 at 7k, there. This yielded great results and a power curve that was a good approximation of what had been measured on the dyno. I think my weight was a little low for the car but the figures compared to a recent dyno like this:

Power at the wheels, GPS 252bhp, Dyno 257bhp.

Max power, GPS 310bhp, Dyno 318bhp.

Max torque GPS 330ftlbs, Dyno 350ftlbs.

The possible weight discrepancy would explain the majority of the error IMO. Will check the weight again online today.

So overall a good day, got to do the same for my Audi S2 and BMW now...

As for estimating power, I suspect there's too many possible sources of error to get flywheel power from this method. Power at the wheels will be more accurate. As you've determined, weight is an important factor and sadly, you can't get it from the Internet. It's rare that a car will weigh close to the stated weight. Manufacturers don't vary the stated weight of a car based on the options it comes with. Normally, just the different engine sizes are catered for. Factory fitted options can weigh quite a lot, especially things like air conditioning. You need to get the car on some scales or a weighbridge with the driver onboard. Head down to your local recycling centre as they all have a weighbridge and ask the guys nicely!

Cheers,

Andy

The avearge speed was the average of the speed data points (every 0.2s) over the total time so there's no problems there. The distance data was the finish distance - start distance. The avearage speed was calculated from total distance / total time.

The differences are an area of concern although the speeds measured tie in with my expected results and some reecnt 0-60 tests gave results around the 5.5s mark and this ties in with GTech data.

I will look at some of the improved data and see what the distance vs. speed data errors are (using the methods described above).

For power the calculations were:-

(1) P=FV

(2) F=ma

Substiture (2) into (1) for F

(3) P=mav

The power at the wheel calculation is:-

(4) Air Restance = 1/2.rho.Cd.A.v^3

Where:-

P = Power (W)

m = Mass (kg)

a = Acceleration (m/s^2)

v = Velocity (m/s)

Air resistance (W)

rho = Air ensity (kg/m^3)

Cd = Drag coefficient

A = frontal area (m^2)

Conversion kW to bhp = 1.341 kW to bhp.

The data for both cars is widely available and I also have RR plots and run-down losses to have a look at, the power curve was just for fun but was the right shape and within about !0% from the data provided.

The acceleration data provided is very noisy and requires some massaging to get smooth results, of course this can dilute some of the data also and so the error on the power at the fly is large.

As for power at the wheels, that's pretty simple to work into the RaceChrono package with the input of car weight and making the following assumptions. Most modern cars have a Cd around the 0.3 mark and A of around 2m^2 and an assumption of rho = 1.2 isn't far out, this is a small amount in second gear compared to the 220+bhp ATW calculated and measured on the RR. The acceleration data would have to be improved using some data checking to smooth the data using previous results.

I will happily send you some data. I'll make sure it's all OK ;-) and get a copy off to you.

Can I post pictures on here? Have got some nice Ecel charts of the data described.

http://s348.photobucket.com/albums/q326/rusty_mcrusty/RaceChrono/

pw is race_chrono

I've just looked at the .csv file and the discrepency Inoticed between avearge speed x time vs. distance quoted has all but gone. I'll check this on future runs but it seems that the original runs I did had poor GPS. Live and learn! Still, for consistency I'll use the speed data if I need to calculate distance. Great!

Got some number crunching to do on my S2 data and see if I can get some 35 to 65mph runs (turbo lag in 2nd means that 30mph is too low). I'll also see what sort of power curve I can create.

The only thing outstanding at the moment is the acceleration data noise, if I get a satisfctory result I'll post up, likewise if anyone has any ideas...

You need to analyse the graph of the speed data to ensure that it is smooth, I have had runs where the GPS has suddenly shifted or a satelite has dropped out and this means that a section of hte data cannot be used.

The no.1 rule is to apply common sense and if something looks wrong or different to the majority of other runs then it more than likely is.

There seems to be a cyclic error on the acceleration data of about a 1s period. The magnitude of this is reduced with the averaging but a manual adjustment is still required to remove the really silly points.

For losses I have found that the best method is to assume a straight line distribution for the Patw to Pfly figure. I have taken the value for max RPM of 1.35 at 7000rpm and assume that losses at 0RPM are 0!

This gives:-

P(atw to fly) = 1+(RPM/7000*0.35)

TO calculate RPM I have maxed the car in that gear on the limiter (7400RPM) and then traken the peak speed from that run. Another approximation would be to hold say 3000RPM and use that. From this measurement I can then get the RPM/mph data (or other speed measurement/ power measurement) and then calculate RPM from the speed output from RC.

To finish, I have compared the caluclated power at the flywheel and RR data for my S2 and it's getting close, it's certainly the right shape, it's just inertia effects and the change in Cd with speed that give errors now...

As it's a turbo car there is also turbo lag and ECU load to take into account when comparing gears. This is most visible with the sub 5k differences in the power curve.

I think the best method of comparison for my car is a 35-65 test in second (3500 to 7000rpm), the power curve generation is for fun more than anything else but consistent tests should show differences due to modifications.

I've also done 5 0-60 tests and got some interesting results. The launch is the hardest part to replicate, with the gearchange a close second. The results vary by up to 1s but comparing the 40 to 60 times they were all within 0.2s of each other...