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Thrust and it's relation to weight on the drive wheels:
OK now things are getting interesting as to get "a record" we found out we only needed an additional 132 Rear Wheel HP. Should be easy with a blown motor, at least until we realized it is a lot harder to get a blown gas motor to live on the long course at B'ville vs. an un-blown alcohol motor. So Hooley builds a new blown 400 small block chevy for 2005 that looks to have what it takes on paper. Will it get us a record? Short answer, NO!! We go and run in August 2005 when the course is rougher than for many years and get in 4 runs. Most in the 200-212 mph range and at only 25-30% throttle. Our problem was not one of HP, but of getting the car to go down the rough course straight. On the fourth run the blower decides to blow the front seal and we are done. Probably a good thing considering the shape of the course. Over the next year a NASCAR style flap, funny car type cage addition, more data collection abilities and larger rear skirts were added to the car ( you can see the changes ( HERE )). The previous year larger spill plates were also added to the sides of the spoiler ( Changes for 2005 and you can read about our 2004 to 2006 runs ( HERE )). I'll talk more about the reasoning behind the changes a little later.
About this time I became aware of "weight" and the need for understanding what weight can do for a LSR car. In most forms of auto racing weight is a thing to get rid of, but you often hear that it can't hurt you in land speed racing. Well this is true and not true. The perfect car is going to have the weight needed for traction at it's top speed and no more. This is hard to predict exactly, but we will try. Next you won't ever be happy with your top speed so you will need to be able to add weight or remove weight as needed. So I say build the car as light as possible, without going to extreme measures to make it light, and then be able to add weight to the car in locations that will do you the most good.
Let's look for a moment at the traction deal on the salt. On the drag strip where you have excellent traction and as light a car as you can build the problem is spinning the tires off the line. As the car goes down the course and hooks up it is very hard to overpower the course and spin the tires on the top end unless you are a top fuel dragster. Almost the opposite happens on the salt where the cars are geared so high. Since the salt only offers about a .4 to .6 traction coefficient vs. pavement under ideal conditions it is still pretty easy for a car to spin the tires off the line and in the first gear or so. Then normally the car settles down and traction isn't a big deal until the speed gets high and the aero drag goes up. At speeds of 200 and over you are trying to push through a "wall of air" with the car. It is a lot like putting the nose of a car up against the side of a building and getting in the gas. Not too easy to spin the tires, but put some water under the tires and now they will spin real easy. At high speeds the pressure on the front of the car is just like the side of that building and the rear tires will spin unless they can get traction through more down force on them either by weight or by wings or spoilers. The condition of the salt of course is going to be a major contributing factor.
My feeling is that if you can get traction without a spoiler or wing you are ahead of the game as any down force they give you will also be at the expense of some added aero drag as these devices have a frontal area and a certain Cd and create some turbulence and thus aero drag. and we have seen the effects of aero drag at high speed and how aero drag requires more HP to overcome than added weight to the car in most cases.
So how do we figure if we have enough weight on the drive wheels?? The next formula breaks horsepower into it relationship with thrust (lbs force) and speed (mph).
Thrust (lbs force) = HP X 375 / Speed
By dividing thrust by the friction coefficient of running on the salt we end up with the weight required on the drive wheels, or at least a figure that lets us know if we are close.
Weight needed on Drive Wheels = Thrust / friction coefficient of the surface we are running on.
(There again you can find these formulas on the spread sheet referenced before ( HERE ))
So let's look at the Stude in 2004 when we ran about 220 with our projected 444 Rear Wheel HP. According to the the formula above the thrust works out to 757 and for a friction coefficient of:
.6 we needed 1261 lbs on the drive wheels and for
.5 we needed 1514 lbs on the drive wheels and for
.4 we needed 1892 lbs on the drive wheels
The car with Hooley in it weighed a little over 3000 lbs. then and was about 50/50 in weight distribution, so we had about 1500 lbs. on the back. On the run where he got up to 219 he was only there for a few seconds at the end of the run and he didn't remember the car getting loose, but this was his first 200+ mph run, so there was a lot going on in his mind. The salt was pretty hard and dry in 2004, so I'm guessing the friction coefficient was .5 or better. In 2005 the salt was rough and our suspension was all bound up and not working good so anything over 210 mph resulted in wheel spin and the car trying to spin out.
But 2006 proved, at least in my mind that these formulas can work and can give you an idea if you are going in the right direction in setting up the car. 2006 found the salt a lot smoother than 2005, but more moisture was coming to the surface (during the heat of the day) making the friction coefficient worst than 2004. Our first run down the salt was pretty much a check of the car and the air/fuel at part throttle without any WOT, still it was a 200 mph run. Over the winter besides the other changes to the car mentioned above Hooley had added 250 lbs. of weight right under the front universal joint/transmission output shaft. He had also added pockets to put weight into the car about in the same area except at both sides of the car in front of the rear wheels. Now the car weighed about 3300 lbs. and was balanced about 50/50 front to rear and we could add up to another 700-800 lbs. of weight we took with us in 50 lb. increments.
On the second run we wanted to run more WOT to check the air/fuel there and Hooley spun the tires and then spun the car at 223 mph. Looking at the previous data above it looked like with the amount of weight that we had on the rear tires when we ran 219 two years earlier we were about at the maximum speed we were going to be able to run without spinning the tires. So this year (2006) we had added 250 lbs. to the car, but the salt was wetter and slipper. Instead of wasting runs thinking Hooley could peddle the gas and make it we decided to add weight base on the above information. After that run no more problems spinning the tires and the car was much more stable with the addition of another 600 lbs. to the car. Now it weighed about 3900 lbs. with about a 50/50 weight distribution.
We ended the week with a new B/BGCC record of 236.921. On the record run he averaged 239.740 in the 5th mile with a 241.179 exit speed. Since we hardly picked up anything in the 5th mile (average vs. exit speed) we feel we have reached the potential of the car as it is now, but have we??
Using the spreadsheets I've talked about above it shows he probably needed to be putting down about 576 HP (685 Crank HP) to the rear wheels to run 240, his exit speed. The 240 was at about 7070 rpm and after having this motor on the dyno the 576 HP figure is pretty close to the dyno numbers we have, adjusted for rear wheel HP.
If we look at the "thrust" at 576 HP we see it is probably about 900 and for a friction coefficient of .6 we needed to have about 1500 lbs. on the rear wheels and at .5 we would need 1800 lbs.. I figure we have about 1900-2000 lbs on the rear wheels now. Even though Hooley said it was good at 241 mph I think we might be at about the upper limit speed wise before we again have to add more weight if we want to go faster without spinning the rear tires. Of note is the fact we made the record qualifying run and the backup run early in the morning both days when the salt was better. The heat later in the day can bring the moisture up and make the track slippery. Keep that in mind if you are having traction problems. Maybe moving to the morning if possible will help you if you are right at the point where you have to add weight and you don't have any.