14 replies to this topic

[kober]

This weekends performance of Bentleys in Sebring brought this issue to my mind once again:
There are two main concepts in prototype racing - open cars (LMP) and closed cars (GTP). I suppose that main aero differences are: lift (in favour of LMP), drag coefficient (GTP), frontal area (LMP). But what are the proportions? Is one of these concepts better? And how much does added weight of GTP matter?

[CFD Dude]

I'm not sure what the quantitative differences between the drag and lift is between the LMP 900 and GTP, but there isn't a weight penalty for the GTP. Both cars are required to weight at least 900kg. The only other major difference is in the tyres, GTP cars are required to run on narrower tyres than LMP 900.

Between the aero and tyres, the GTP are supposed to have a higher top speed (less drag) but they aren't able to corner as well as LMP 900 since the cockpit impedes the flow to the rear wing, and the LMP 900 have wider tyres.

What I can tell you is that I was at Sebring last Saturday and there was very little difference between the performance of the Audi and Bentley cars. If the Bentleys had started at the front they could have very well won the race. I'll be surprised if they don't win at LeMans.

Which is better? I would say LMP 900, but not by much under the current rules, but that's more for tyres than aero. Under the rules starting next year both race under LMP1 which allows for the same tyres for both and requires the open top cars to have the same roll over and crash protection for the imaginary passenger as for the driver. Under those rules I can't see any advantage for running an open top car.

[schuy]

Originally posted by CFD Dude
The only other major difference is in the tyres, GTP cars are required to run on narrower tyres than LMP 900.

Does that mean that in rain conditions the Bentley will be nimbler?
(Due to the narrower tyres).

Or do the regs state that upon wet conditions everyone use the same tyre size?

[Pong]

Kober,

Racetech magazine had a long article on the subject approx. a year ago concering the construction of the Panoz LMP.

LMP:
- better aerodynamics


GTP:
- better structural stiffness
- noise problems
- heat problems

regards,
Pong

[CFD Dude]

Does that mean that in rain conditions the Bentley will be nimbler?
(Due to the narrower tyres).
Does that mean that in rain conditions the Bentley will be nimbler?

Good question. The rules don't say anything about rain tyres, but the wider LMP 900 tyres are a maximum not a minimum so the LMP 900 cars could use narrower rain tyres if they thought it would be an advantage.

Interestingly, there is a minimum weight for the wheels and tyres with LMP 900 wheels required to be 8.0kg front / 9.0kg rear, and the GTP and LMP 675 requried to weigh 7.5kg front / 8.5kg rear. I guess if you run smaller rims and tyres you have to ballast the rims.

[Ross Stonefeld]

Didnt the Panoz GTS car win a race 97/98 at Sebring in the as part of an FIA combined open top/GTS race specifically because the narrower tires helped it in the wet?


As far as LMP vs GTP one of the Audi Sport directors recently commented that there's also practical design considerations. For instance its a lot easier to get in and out of an R8 than a Bentley. Its also more comfortable for the drivers, and visibility is better. The Bentley was practically blind after 6 hours at Sebring. They had an onboard camera mounted on the dash and you could hardly see. Meanwhile the R8 drivers had tearoffs and if things got really bad could change visors.

[kober]

OK, thank you so far. I didn't know about the tyre issue, and hadn't thought about the others...

[Scoots]

Assuming all of the rules are the same a closed cockpit car should be faster just because of better control of airflow ... what I don't understand is why so few top-fuel dragsters use canopies? Is it safety? Is it that 7000+ hp makes aero mean alot less? Is it that in 1/4 mile the gains are insignificant? I also wonder why the engines are not covered.

[wingsbgone]

Originally posted by Scoots
Assuming all of the rules are the same a closed cockpit car should be faster just because of better control of airflow ... what I don't understand is why so few top-fuel dragsters use canopies? Is it safety? Is it that 7000+ hp makes aero mean alot less? Is it that in 1/4 mile the gains are insignificant? I also wonder why the engines are not covered.

Agreed!! Can you imagine the effect of F1 (or even CART) aero work on a T/F monster.

Not to hi-jack the thread, nor beat a dead horse, but I still boggle at the idea of an unlimited nitro dragster (free nitro % and gearing), with full aero, over, say a 1/2 mile run. I imagine 450mph would be possible. Anyone know where to get an accel profile for a T/F or Funny Car dragster? Curious to see how the accel decreases over the run (and thereby the effect of aero on the top speed). I believe the simplistic AVERAGE accel is something like 4g for a run.

Ciao.

[Scoots]

Originally posted by wingsbgone


Agreed!! Can you imagine the effect of F1 (or even CART) aero work on a T/F monster.

Not to hi-jack the thread, nor beat a dead horse, but I still boggle at the idea of an unlimited nitro dragster (free nitro % and gearing), with full aero, over, say a 1/2 mile run. I imagine 450mph would be possible. Anyone know where to get an accel profile for a T/F or Funny Car dragster? Curious to see how the accel decreases over the run (and thereby the effect of aero on the top speed). I believe the simplistic AVERAGE accel is something like 4g for a run.

Ciao.

To continue the hijacking of the thread ...

With ground effects they could probably make enough downforce that they could get rid of the huge rear wing, and if you got rid of the length rules the cars could be longer to put even more torque to the wheels, then of course there is the possibility of unlimited tires. In a truly unlimited class I think 400 wouldn't be incredible in a 1/4 mile ... 500 or 600 in a 1/2 mile.

[DEVO]

point of clarification...

Were the Bentley's used at Sebring not LMP900 classified cars? I thought they were but just closed cockpit.

as for all the car classifications that ran at Sebring could somebody explain differences:

LMP900 (min weight 900 kilos, max displacement???, tire sizes???)
LMP625? same
GT?
GTP?

I thought there was 4 classifications and if my list is incorrect could you please explain the different classifications.

Thanks

[Pioneer]

LMP900 (the Audi's)
GTP900 closed cockpit variant... considered in the same class as LMP but slightly different rules
LMP675 lightweight prototype class... more engine resitrictions than the 900 classes
GTS
GT

Someone else will have to explain the difference in the GT classes... I believe the GTS class cars are based on a stock model but are basically custom built racing cars whereas the GT cars are purchased as is from a factory provider.

[CFD Dude]

Someone else will have to explain the difference in the GT classes... I believe the GTS class cars are based on a stock model but are basically custom built racing cars whereas the GT cars are purchased as is from a factory provider.

That's pretty much right. GTS cars can have some pretty serious body modification (including a rear wing, and front and rear diffusers), and the engine is only required to be a mass produced unit, it doesn't have to be available with the car. For example, you can't buy a C5 Corvette with a 7L engine, or a 550 Ferrari with a 6L one. I'm not sure, but I think starting this year the GTS cars could have a carbon-fiber chassis, which neither class could in the past. That was the reason the Zonda was able to race at Sebring this year.

GT cars are required to keep most of the original body work, cannot have diffusers or body tunnels (GT 360 Ferraris have an undertray that covers up the sculpted underbody) and use a race-modified version of the production engine which come with the car. That's where BMW got in trouble with the M3 GTR, the 4.0L V8 wasn't available in an M3, let alone anything else commercially available from BMW.

[Chickenman]

Assuming all of the rules are the same a closed cockpit car should be faster just because of better control of airflow ... what I don't understand is why so few top-fuel dragsters use canopies? Is it safety? Is it that 7000+ hp makes aero mean a lot less? Is it that in 1/4 mile the gains are insignificant? I also wonder why the engines are not covered.

It's been done before. Big Daddy Don Garliuts ran a full aero car with Canopy back in the 80's. Swamp Rat Thirty.

http://www.nhra.com/50th/top50/

http://www.garlits.c...at_30___31.html

Why are Canopies and full aero not used today in TF? Simple...it's been tried and discarded.
Too heavy and they didn't offer all that much of a benefit. By the 90's TF had gone past the Full Aero stage and gone back to more traditional chassis and Aero designs.

TF drag racing is an animal all to itself. The rear wings have a huge, huge amount of downforce due to the cantilever effect. They are also very efficient in design. Much more so than casual observers give them credit for.

Top Speed in TF is not all that important. ET is. Drag is not important. Downforce is. The huge Multi Element, Cantilevered rear wing on Modern TF cars produce much more downforce than could be attained using any form of tunnel. A huge, huge amount of downforce is required. TF cars can spin their tires thru the lights at over 300mph if every thing isn't just right. Amazing what 6,000+ HP can do.


The chassis's are unique . Very long... and very flexible chassis's are the way to go in TF. A chassis with the torsional and longitudenal rigidity of a licorice whip does work well with ground effects. You also want the chassis to be as skinny as possible, shaped like a missile. As Big Daddy found out, in his famous " Blow Over ", a wider chassis can also get more air under it .... a huge consideration when you pull the front wheels and are doing 100mph in about 1.5 seconds!!!!!

Engine covers are of no consequence in drag reduction. The rear tires are second only to the rear wing in drag. The rear wings are also high enough that any aero gains by using engine covers would be insignificant.

One big reason for not using engine covers is that they get in the way. Ever seen a TF team thrashing on an engine between rounds? The whole engine comes apart and literally everything is checked and replaced as necessary in a matter of minutes. And they don't have any extra time. A few seconds can literally make the differenace between making the next run or not.

[Scoots]

I remember DG's car, but just because he didn't stay with it does not mean it is a bad idea. Are there any rules preventing shrouds over the engines and tires?

It may be that it's simply not worth the expense since the primary challenge is in setting up the clutch and in the driver's RT.