mark kibort

HP to drag ratio is much more of a factor at the higher speeds. also, a given amount of weigth will effect the Hp/weight ratios if a car starts out at a lighter weight. obviously, its always better to have a better aero car if HP to weight is even. On the same token, its better to have a lighter car, even though HP to weight is the same (all other things being equal like tire size, chassis width, etc). Since most of our cars have near 20sq-ft of frontal area and speeds are near 100mph on most tracks, for most of the time, the total aero drag is not that significant. (50hp range.) improvements of say 10%, which is a lot on aero would only have a 5hp advantage).

(edit: added descriptions below)

3000lbs/300hp 10:1
3100lbs/300hp 10.3:1
3000lbs/305hp 9.8:1
3050lbs/305hp 10:1 (at 100mph, 5hp aero advantage is worth 50lbs)
3100lbs/310hp 10:1 ( at 130mph, 10hp aero adatage is worth 100lbs because power required goes up with the CUBE of speed. speed went up by 30% and 30% cubed is 2.1X, so power goes up by 10hp vs 5hp at 100mph.)

What this shows is that if someone has a 10% aero advantage, its probably like having a 50lb advantage at near 100mph racing, but near 100lb advantage (near double) when the speeds get near 130mph as the power required goes up by the cube of speed. (and so does any differences) (strictly talking acceleration)

FredC

I did that too, but trimmed down in time for limerock. Believe it or not, when i visited the scale (running on 1/4 tank) the car and I weighed in at2,703#........ I thought the operator was ****ing with me.... All that winter meat, cheese, wine, and some more... 1 pound over the minimum weight after sweating a couple of pounds with jogging.... I carry 35 pounds of ballast (I mean the car does).

Gary R.

I'll see if we can't get you more legit at the Seneca Lodge bar....
Figures my car is so light that even with my fat *** in it I need the spare tire and a cool suit cooler full of water to be safe!

APKhaos

Weight makes a difference.
My car car run as PCA Stock F at 3145# or SP3 at 3250#. That extra 100# adds at least a 1 second laptime difference at my home track. In this case, my fat a$$ is normalized out of the equation winter nothwithstanding.

Gary R.

The real question is the effect this has on... STREAK!!!!!

hacker-pschorr

+1

Weight has no effect on top speed, it just extends the time & distance it will take. At Bonneville you have all the time in the world, the extra weight keeps the car on the ground (just like downforce) without the added drag.

Streak

None. None whatsoever. I am far too talented for weight to be a factor.

DrJupeman

I believe it is a squared equation, which means that to double your speed you need 4x the power. No cube:

Aerodynamic drag (force) = 1/2 D x A x Cd x V^2

where D = air density, A = frontal area, V = velocity relative to the air, Cd = coefficient of drag

Veloce Raptor

Correct.







Professional Racing and Driving Coach

d15b7

hi guys.

my thoughts are that given a hp/wt rule, and given that you might have a heavy ballasted car with lots of hp running against a light car with much less hp -- the result at the end of the race may be fairly dependent on the track that you are racing on.... for instance, i'd always pick the heavy/high hp car if the track had lots of high speed turns and high speed long straights (like WGI and VIR). i think that the higher top speed potential of the heavy/high hp car would give it an advantage. now, if the track being raced on were very twisty, with many slow turns, and short straights (say Shenandoah for instance; i've RACED on that one, and man it was tight!) i'd pick the light/low hp car. in that case, the nimbler handling and better slow speed abilities would really come into play; that, plus the tires would hold up better/longer (and possibly the brakes, too).

so, it might be a 'horses for courses' kind of thing.... there is no rule in GTS that states that you must run the same car at every race, is there? the thing to do would be to build two cars; a heavy ballasted/high hp car for the long fast tracks, and a light/nimble/low hp car for the tight ones; then bring the right car for the right track, every time!

and streak (and good hands) -- my Probe is 2660 lbs min (running Toyo RA-1s. i have to weigh 2735 lbs if i choose to run Hoosiers -- so far, i always run Toyos) at the end of the race, and my hp is 160 whp (i'm allowed up to 174 whp, but my motor won't make that!). that would put me in GTS-2, if my car were german, right?

todd
ReidSpeed

Larry Herman

I wonder about this sometimes. Last time I was at Summit Point (Nov 2007) with the RSA I ran consecutive 1:21.9s hauling Chris Cervelli's 250 lb butt around in the passenger's seat. I am not sure that without him I could have gone sub 1:20. Maybe it was his encourgement that overcame the ballast!

APKhaos

Larry,
My best time at SP was duing a six lap knife fight with a very close competitor in last year's club race. There's nothing like external stimulus to get your game sharpened up. Once I finally shook him off it was really tough to get back done to those times.

One of the things I really like about the Racepak IQ3 is that it shows the progressive delta between current lap and best lap, which means you see if you are falling behind your best lap for the session. Great motivation!

Streak

Bring it on Reid!

You would only flog me by 1.5 seconds instead of 3! That would be refreshing.

There must be a German part on it somewhere. Wiper blades? touch up paint? Anything?

mark kibort

Are you sure about that? Better double check you math below. You are right, aero drag goes up with the square, but then power HAS to go up as the cube. So, "yes" cube.

Let's say you had 200bs of drag at 100mph. (and had 50hp)

How much drag would you have at 200mph?

What would be the power required to reach 200mph if the drag went up by a factor of 4x. (square of velocity).

You might want to repeat the answer to VR.

mk

mark kibort

Can you explain why you think he is correct?

Ill give you a hint.

drag goes up with the square of speed

Power= Force x Velocity

This is in chaper one of your Junior Highschool Physics books.

Still think he is correct?

Ill give you a little hint: power goes up 8x (Cubed) for doubling speed. Dont agree? Why dont you explain why. Im all ears.