GTR Cd and frontal area

[steve_D]

Does anyone know the Cd and frontal area for the GTR with wing?

Want the figures to put in a calculator which tells the HP losses at various speeds.

Thanks
Steve

[Stig]

Given the Mira testing, I'd guess that only the factory would have accurate figures Steve?

[adequatespeed]

There is a way to estimate the Cd.

** important if the top speed is limited by revs then none of this is of any use **

You need an accurate power, top speed and rolling resistance figures.

You have the accurate figures form Ultima from their website on top speed and power.

Then you need the help of someone who has had their car dynoed. (There are lots of people on the board who have done this and am sure could help) to find the rolling resistance.

The power loss figure from the rolling road provide rolling resistance & transmission losses (at least for the rear wheels).

This may be at a given speed but don’t worry as there is a fairly linear relationship between this and speed so just work out the loss per m/s multiply by the top speed and it wil be not too far off.

Take this away form the engine power, this will give you the power being used to push the car ageist the drag.


Now the maths bit:

Drag (Newtons) = 1/2p V*V S Cd

where

p = air density in Kg / M3
V is velocity in m/sec
S is frontal area in square meters(you can measure this)
and CD is the coefficient of drag (you don’t know this yet)

Also

Power (watts) = Drag (Newtons) * velocity(m/s)

+ Rolling Restance & Transmission Losses (at that speed) * velocity


Get all this together and you can get a close enough estimate.

You could improve this with some estimate as to front wheel rolling restance figures - probably find a simple guestimate on the web somewhere.

Different power loss figures at different speeds on the Dyno could give a better estimate of the frictional and transmission losses.

Good Luck




>> Edited by adequatespeed on Monday 8th November 16:27

[andygtt]

I've never heard them mentioned so either they were not recorded, or they are a secret.....

how accurate is that kind of calculation Steve?

[davefiddes]

Not sure the calcs would necessarily work. You need to have the car travelling at full speed at max power in steady state. I thought most of our cars were gearbox limited and ran out of revs before we reached Vmax?

If this wasn't the case then the maths would give a pretty good ballpark figure I would have thought.

[steve_D]

I've just bought a Gtech Pro RR and am waiting for it to arrive.
Whilst on their site I found this calculator and thought I would have a play with it.

Steve

>> Edited by steve_D on Tuesday 9th November 23:16

>> Edited by steve_D on Tuesday 9th November 23:18

[Sky]

You can also calculate CD from the power off (clutch in) deceleration rate. This method will include all drag factors including rolling resistance, wheel bearing friction, tire deformation, etc. As adequatespeed mentioned, you may find good guestimates on the web for rolling resistance terms in order to get down to something close to the aerodynamic drag coefficient. The nice thing about this method is that you do not need to have accurate top speed and power values, the data are pretty easy to collect, and it does include (almost) all drag terms which is nice for many analyses. (We use this method to determine aircraft drag during takeoff and landing roll for subsequent takeoff/landing performance calculations.)

After going through a lengthy derivation, I found a web site that does a much better job than I did of explaining the process: www.teknett.com/pwp/drmayf/cdmodel.htm

As for frontal area... if you are comparing the Ultima only to itself (looking at the effects of engine HP for example) the value that you select for frontal area shouldn't matter. This area is a "reference area" only, and in any comparative analyses these values will cancel out as long as you are consistent, and you use the same frontal area for the determination of Cd and the performance calculations based on that Cd. It is only when you try to compare vehicles with different frontal areas that your results will be skewed.

Hope this helps.
-Sky