Power to weight ratio vs. drag

[lsrjunkie]

So, I've been having a rather lively debate with a friend and fellow car guy. He insists that land speed racing is no different from any other type of racing. The best recipe for a something fast is a light car with a ton of power. I, however disagree with him. My thinking is that a heavier car will give you the benefit of more traction, without the penalty of aerodynamic drag. He says that the aero will give you the additional traction, and the trade off of weight for drag will be negligible. What say you more experienced racers?

[Tman]

For the most part he has been proven wrong by the masses. Pardon the pun

[lsrjunkie]

I tend to agree! When a 7000lb roadster is capable of the speeds I have witnessed with my own eyes, it sure lends credability to my argument!

[Ron Gibson]

People ask me why when I tell them my roadster weighs 4000 lb.  I explain that the tires are 5" wide at the hard rubber tread, have 70 psi required, no aerodynamics allowed, traction of GOOD salt is about .6 of asphalt and trying to put 900+ hp  to the ground on basically sandy asphalt takes weight.
A while back someone here stated if you stand on it in second gear and break the tires loose, you need more weight.

YMMV
Ron

[John Burk]

Adding weight reduces drag and lift proportionately and makes cars easier to drive . It also lowers HP / lb . If you have lots of drag and HP weight is like lowering drag . If wheel spin , lift and handling are not a problem adding weight has the same effect on acceleration as lowering HP .

[Rex Schimmer]

I have been a proponent of "lead down force" for a long time as aero down force can be a bigger component of drag than the car itself at high speeds. Aero down force is not free, every aero device that generates lift. or down force in our case, has some level of drag and both the lift and the amount of drag are a cubic function, i.e. if you double the speed you will have 8 times as much down force and 8 times as much drag. My views on using aero for down force are being modified when it comes to cars that are trying to go 400+ in the 5 mile limitation of the Bonneville race course. For these cars it is as much drag race as it is top speed through the 5 mile traps. Many of the 400+ cars have never been able to use all of their power simply because they will spin the tires at almost any speed. One potential solution is to have a light car with big horse power and a method of developing large amounts of aero down force at fairly low speeds and then be able to maintain that same level of down force, and drag, by manipulating the aero surfaces during the run. There is one car that is presently pursuing this approach and it may be at the salt next month so watch for the NACA 66 car from Rob Freyvogel they might just suprise a lot of people.

Rex

[manta22]

Rex;

Isn't it

Drag goes up as the square of the speed?

Power required goes up as the cube of the speed?

Regards, Neil  Tucson, AZ

[John Burk]

For Rex's suggestion of having down force at low speed , maybe the ultimate method would be a second smaller engine driving an evacuation fan like the Chaparral sucker car .

[Stan Back]

. . . and then there're classes where you're unable, by the rules, to induce a lot of downforce.  That's when weight may be the only answer.

As for second gear -- that's the gear you use to get to third -- and it's the gear you use to really put on the steam.  If you're slipping in fourth, you might need to reapportion the weight.  But the lower gears, you need to drive thru them.  I learned this after 10 years.

(But haven't applied it that well.)

[Jonny Hotnuts]

Down force for traction works great until you get a little out of shape and the aero weight changes instantaneously. This not only means the car is out of shape but now doesnt have the same forces holding it down as it did before it got out of shape.

I remember a spectator telling a friend and fellow racer how he was doing things all wrong and it would be easy to build a car to go faster then he had. I suggest you tell your fellow car guy friend the same thing I told that gentleman, and was something on the lines of 'talk less and more do'. He should put his theories to practice....I will be willing to bet that his steadfast concepts will change dramatically when he is going backwards at 150!

~JH

[Stainless1]

Everything is a compromise...
finding the spot that it all works together is the trick.  Too heavy and you don't accelerate fast enough to get to your max speed in the distance allotted... too light and wheel spin/slip increases and you don't get to your max speed.   Find that sweet spot that allows you to use all of your power with minimum slip....
We use lead and powdered tungsten as required.

[lsrjunkie]

Thanks for all the input guys! I have been lucky enough to find this group and be taught the finer points of playing this game. I'm sure my buddy will learn a thing or two out on the salt with us this year!

[nebulous]

I look at it as speed trials. not getting to the finish line first!