traction and weight door car

[ulrace]

Ran on the salt for the first time at WOS.  91 Firebird GTA, 4 speed, about 900 horsepower. Had handling problems; springs in rear were too soft at first and corrected that. Went straight as long as i feathered throttle. At mid 190s i could not keep it straight and pulled chute. i had added 600lbs of lead to car, keeping balance at 50.5 front and 49.5 rear. I had been told i needed it to weigh about 5000lb. It was a little under 4000 with me in it. I am thinking that it started spinning the tires at the 190+ speed. My record is 237 (B/GC) What is the thinking on this? Any suggestions appreciated. Still trying to figure out how to add another 1000 lbs, preferably reversible as we also run the car in the standing mile events.
thanks
bob larimore
1123 B/GC

[Interested Observer]

Check, and then zero out the bump steer.

[Rex Schimmer]

Bob,
Doing a little math and assuming that the salt coefficient of friction the maximum amount of power that you could put down at 190 mph and 2000 lbs on the drive wheels is about 400 HP, the salt was pretty good so if you assume that the coefficient of friction was .5 it would be 500 HP. So with 900 HP available it is probably right that you were spinning the tires. If you only added weight to increase traction then you would need to have a calculated rear wheel weight of 3500-4000 lbs. This would be at 190 mph and in reality you want to go 237. So doing a little more basic calculations and making the assumption that to go 190 mph your car needs 400 applied HPs and that aero drag represents 85% of the total drag (the rest being rolling resistance)  at 237 mph you will need 660 HP to over come aero drag and 100 HP to overcome rolling resistance it certainly looks like you could be a record challenger. You really need to do something to increase rear axle weight to provide more forward force. A glance at the Gas Coupe rules looks like any type of aero down force, i.e. rear wing or spolier, that is not stock is not permitted so ballast looks like your only choice.

You hear Bonneville called the " long white dyno" and this a great example, at 190 mph Bob's car starts to spin the tires (we assume) using that speed and the estimate of the salt coefficient of friction we can calculate the horsepower to attain that speed, 400 in Bob's case, and if we had the frontal area we could calculate a pretty good estimate of the coefficient of drag. Obviously all of these numbers rely on "best guesses" like what the salt's coefficient of friction but it does give us at least some idea of what the potential of Bob's car could be if he can get traction. I have not taken into account such things as drive line efficiency or reduced air density because of the altitude, temperature etc.

Rex

[jacksoni]

I ran an '86 in Trans Am configuration, basically the same as the '91 GTA. Frontal area is 21.4. Factory Cd in the .29 range some sites say. That can be improved. Mirrors off. Lower as much as possible and since air to radiator comes from under the front valance is close to /Alt configuration as it sits (no grill). A front air dam is legal. Not sure the GTA, my trans am had the Pontiac wing  which is legal. It does add some down force ( I took off as didn't need that in my case- 4 cyl, 122", 170mph) and causes some drag. Look up the article about Gary Eaker's firebird which ran high 290's in late 1980's. ( he now owns and runs the Aerodyne and A2 windtunnels which are mentioned in the Aerodynamics threads here. Used to work for GM and then Hendricks and knows what works and what doesn't in this situation) If you can't find it I have it at home (not home now) and can look it up.

[Sumner]

.... Went straight as long as i feathered throttle. At mid 190s i could not keep it straight and pulled chute....

Have you looked at your CP/CG situation.  If you spin the tires and the back wants to start swapping ends with the front it is probably off by a bit.  A car that has it's Center of Pressure (CP) behind the Center of Gravity (CG) is going to be less likely to spin around if the tires break loose. The further the CG is ahead of the CP the better in that respect, but we also need traction. In the case of Hooley's car we have gone from about a 3300# car to a 6400# car by adding weight. All of it ahead of the rear axle and to help move the CG further forward some midway in the wheelbase and some in the nose of the car. So we added weight for traction but also weight to try and keep the CP behind the CG.  If you add all the weight in the back and can avoid any wheelspin then the car won't spin, but if you do have wheelspin then it will spin even quicker.  

Here is a fairly simple way to get close on where your CP is ....

http://1fatgmc.com/car/14-Hooley/14%20-%20hooley-construction-2014-2.html

Here is a formula that will help determine how much weight you need on the back of the car to run a certain speed.  Use this one -- Drag Force --HP to run a Speed -- Thrust -- Weight Needed on Drive Wheels ......

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#Drag%20Force%20--HP%20to%20run%20a%20Speed

Sumner

[jacksoni]

What Sumner said! don't put all the weight in the back of the car. if so you get to see the start line, the finish line, the start line, the finish line etc etc

[bearingburner]

Come to Loring and gear for 11/2 miles on pavement. Corvette ran 260+ this fall. Plenty of traction.

[jl222]

  I have a book on the 1000 hp formula I cars [when they had turbos with no boost limits and 1500 hp qualifying engines] most have 65% rear weight, some tried to go to 62% but would spin the tires.
in addition to 65% reat weight they had additional downforce from wings. Oh, wide tires on pavement.

  How much weight is on a TOP Fuel dragster rear tires? anyone ever notice how flattened they are going through the lights?

  What do you think the cp is on Fast Freddy's 384 mph rear engine lakster with wing is?

  Oor 222 Camaro weighs 4860 lbs has 3000 lbs on rear tires has a 10'' spoiler and 2300 hp at the wheels and has gone 294 mph in the 1st mile.

  Oh, with 10'' wide tires, but they don't work just read ''traction at Bonneville' '

  You might see that I don't buy into the center of pressure theory.

                     JL222 

 

[jacksoni]

As long as  there is mechanical traction the car will go straight. A freely spinning tire is different. By the way, I agree with your wide tire/narrow tire thinking. There are other reasons for narrow tires. For the little that is worth.....

[ulrace]

Thanks to everyone. You have given me a lot to consider. We will be back next year. Sumner; great to hear from you. I have used most of your formulas. Hope you are doing well. Say hi to Hooley if you see him.
Bob

[jl222]

You might consider a ladder bar setup or 4 link were you can adjust the instant center for more or less shock or bite to the tire.

What tire are you using and maybe less pressure.

     JL222

[ulrace]

have frontrunners ; 26" (ford 9" 2,47 ratio. 60lbs in tires.
4 link with top bar adjustment.
bob

[SPARKY]

you can get to 2.28 gears with GM axles with c-clip eliminators  pick up nearly 4% more power to tires
TE tractive effort

[Leadfoot]

What's this ? A 12 bolt center section or complete rear ended ? Strange makes a 12 bolt center section for the ford 9 inch.

[SPARKY]

either with the GM 7.5   or the Caddy 10-12 which is a 8 7/8"