FINALLY made it to the track this week. Monday night through Friday night I went through a fairly major punch list to get the car ready for the track, as follows:
- Revamp throttle linkage so it fits under the hood
- Install the hood and adjust
- Measure locations of four link holes
- Plot 4 link possibilities in CAD program, calculate possible instant centers
- Adjust for "best guess" four link bar locations
- Install collector tethers (new NHRA rule)
- Install up to date seat belts/harness
- Install hood hold down latch
- Fabricate installation straps for flywheel cover, install cover
- Install dash pad, connect oil pressure warning light and shift light
- Tie up wiring on wiring panel, reinstall panel under the dash
- Wrap the wiring under the hood with protective sheathing
- Install roll cage side bars
- Install parachute and connect parachute cable
- Wash car, vacuum interior
As I'm sure you can see I had a busy week, burning lots of midnight oil to get all this done. I finally got done on Friday night but was too tired out to load the car on the trailer, so I left that for Saturday morning before I headed for the track.
One of the interesting parts of the week was trying to figure out the best location for the four link bars. There's lots of online information on this subject, and so I spent some time in the evenings doing a little research after I had mapped out the hole locations and drawn up a picture of the car in my CAD program. According to the online web sites on this topic, one thing you need to know is where your "anti-squat", or "normal" line is. This is a line connecting the rear tire contact patch to the intersection of a horizontal line running forward from the car's CG to a vertical line running straight up from the center of the front wheel. The instant center is then calculated by extending lines forward from the four link bars until they intersect. The position of the instant center relative to the anti-squat line will tell you something about how the car will behave on launch.
Having the instant center directly on the anti-squat line, and lined up with the CG, is supposed to give you the most efficient launch. Using the CAD program I was able to find an upper and lower bar position that was very close to this point. Here's a picture from my CAD program showing these positions:
In this drawing the instant center is about 4.5" behind the CG, and is right on the anti-squat line (referred to as 100% anti-squat). Another alternative was to have the instant center below the anti-squat line; the web sites said that this less than 100% anti-squat, and would cause the rear of the car to squat when it leaves. This results in a softer "hit" on the tires. Stick cars that drop the clutch or automatic cars that use a trans-brake are supposed to hit the tires pretty hard to begin with, and so they may need less than 100% anti-squat to maintain traction. The drawing below shows one of the positions I mapped that had much less than 100% anti-squat:
Notice also that the drawing above has the instant center far forward of the CG. This will tend to raise the front end on launch. An instant center behind the CG will tend to raise the rear end of the car on launch.
You can also position the instant center above the anti-squat line, for more than 100% anti-squat. This is supposed to hit the tires harder and result in better traction, but at the cost of wasting energy by raising the rear of the car on launch. In the drawing below the instant center is positioned for much more than 100% anti-squat, and also it is behind the CG, causing the rear of the car to rise on launch:
I have a transbrake on my car but was planning to footbrake it on the launch at the track; just holding the RPM to 3000 or so on the line, and then releasing the brake and mashing the accelerator on the last yellow. As a result, based on what I'd been reading, I figured if anything I might need more than 100% anti-squat, and I also wanted to be sure to sneak up on any instant center location forward of the CG, in order to avoid an unexpected wheelie. So, I set up the car to be neutral as shown in the first drawing above.
Saturday morning my friend Kevin came over early and helped me get the car loaded on the trailer and truck all loaded up. We hit the road for the track around 7:30. After a few miles I stopped in a Menards parking lot to check the trailer and the car tie downs; here's a picture:
And yes, that is my brand new F-150
It has the Ecoboost V6, a built in electric brake controller, and other options like trailer sway control. It pulled the trailer beautifully on the 120 mile trip to the track, and got about 13 MPG while doing it, which was much better than my 2005 F-150 with the 5.4L ever got. I am rather amazed at the power and driveability they got out of that little 3.6 liter V-6. The two turbos certainly help...
At the track we were met by my friends Kurt and Steve P., who had come to help out. The weather report said that there was a chance of thunderstorms and it sure looked like it, with the weather being overcast and extremely humid, but we must have lucked out because it turned out to be dry all day. After getting the car unloaded and then through tech, I started making some passes right away. I had no idea what to expect on my first pass, so I just did a pretty good burnout and let 'er rip on the last yellow. The front of the car came up but the wheels spun violently. I made a complete pass but was pretty much all over the track doing it. The time was not too impressive, much slower than I had been hoping for. Track prep was obviously a problem, but so was the car's setup.
After coming back to the pits I talked with the other guys about how the car was working. It was leaving with no rear end movement, indicating that the instant center was indeed on or very close to the anti-squat line. The guys also told me that the track was wet forward of the water box, and that I needed to release the line lock and come out of the box a good ten feet with the tires spinning to get them dried off. After kicking it around a little I decided to change the instant center right away, moving the bars so that I had more than 100% anti-squat. I had brought all my CAD charts with me so I knew where all the different positions of the bars would put the IC, and I picked the one below, to keep the IC close to the CG but just give me a harder hit to the tires:
After making the adjustments I went back out for the next pass. This one was positively embarrassing. After doing a pretty good burnout, which I'd hoped would help, the car left the line spinning even harder than before. The next instant the rear view mirror flew off the windshield and landed on the shifter, and the shift light flashed, and I shifted all the way past second into neutral and hit the rev limiter. I let off and coasted down the track to an 18 second ET
Back in the pits Steve P asked me how it was possible to miss a shift with a two speed transmission. That smartass LOL! After a ten minute break I went back out, determined to do a little better. I had put the rear view mirror back on, but it needed a Torx wrench to tighten it, and I didn't have one, so I did the best I could with an allen wrench. Didn't help, the damned thing flew off again on the launch. Again I had no traction, and got out of the groove after the 1-2 shift, and coasted down to a 12 second ET.
Back in the pits we were thinking that the tire spinning had gotten worse, not better, with the change to the instant center. We decided to do one more thing before going back in to mess with the bars again, and that was to lower the tire pressure. I had been running at about 13.5 psi, so we reduced it to 12. I hadn't really wanted to do that, because going much below 13 makes the car kind of squirrely on the top end, and at the speeds I was running I didn't relish that possibility, but then if I didn't have any traction it was a worse situation, so we brought the tires down to 12 psi. Back out on the track I had given up on the rear view mirror and left it in the pits, so at least I didn't have that distraction. The car still spun the tires but I was able to stay in it this time and get in a full pass. 60 foot time was no better than any of the others, but the car at least seemed more drivable off the line with the lower tire pressure.
It was around noon now and so we took a break for some track food, and then I crawled back under the car to reset the four link bars. I was pretty convinced I'd gone the wrong direction the first time, and that I need to lower the instant center for less than 100% anti-squat. I also thought it would be a good idea to move the instant center forward of the CG, because the car had shown no tendency to pull the front tires on any of the previous runs. The IC position I ended up at is shown in the drawing below:
The next pass was much better; the tires still spun hard but the car was much more controllable on this pass. 60 foot time was still dog slow, but the car just felt better. I was convinced that moving the instant center was an improvement this time. For the next pass, I decided to try some pre-load on the right rear tire, to help even out the traction. You do this by shortening the right upper bar in the four link setup; I shortened it by about a half turn. I also set the front struts a couple clicks softer on the extension setting, so that the front end would rise more easily. Back out on the track the car felt about the same, with a very marginal improvement in 60 foot time and ET. Still nowhere near where I thought it should be, but at least we were moving in the right direction. With the air we had, and the slippery track, I knew there was going to be a limit to how well the car did.
My friend Kurt had been coaching me on the four link stuff, because I'm kind of a newb when it comes to that, and he has set up a bunch of four link cars. He had been putting tie wraps around the rear shock shafts to see how far the shocks had been moving on the launch, and now he crawled under the car with me and showed me that the right rear shock was bottoming out on its travel, while the left rear still had a good half inch of travel left. Kurt suggested we adjust the right rear coil spring on the coilover shocks to make it stiffer, and try to keep that shock from bottoming out. So, I put two turns on the collar to compress the spring on the right rear shock, and went back out for pass #7. This time something funny happened. Before the burnout as I sat in the box, one of the track guys came along and was looking very carefully at the starting line. Kind of like he was looking for a problem. After a minute or so he gave up, and they gave me the signal to do the burnout. But this time when I launched the car darted to the left on the launch, and I had to back out of the pass after 100 feet or so.
Back in the pits we concluded that it must have been the track, because none of us could see how the change to the spring could have caused that big of a change in the car's launch characteristic. We were almost out of time for the test and tune, but I decided to make one more pass. We got the tire pressure right back to 12 psi, and this time I chose the other lane for the run. The result was the best run of the day, more than two tenths quicker than the best previous run. Sixty foot time improved by .05, but it still sucked.
At this point I figured I was track limited, and with the test and tune over we packed up and headed home. Today I got the car back into street trim with mufflers and street tires; this week's project is to get the car tuned up on pump premium and put a few miles on it before next weekend, just to check everything out for street driving. Next Saturday is another test and tune, at a different track which is only 1/8 mile, but has a reputation for better starting line prep. I'll be continuing the chassis tuning next weekend, hopefully with some better 60 foot times. Drag Week starts two weeks from today, and while I'm sure I won't have the car perfectly dialed in, at least it will be in the ballpark before the start of the event. Here's a picture of the car sitting in the pits, and another picture of it doing a burnout at the track on Saturday, courtesy of my friend Kip, who showed up to spectate later in the morning; I'll post another update next weekend.
