NWARC Lapping Day

Pacific Raceways (Kent, WA)

August 5th, 2004



Contents



Summary




Pictures and Video Highlights


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Small Video (360x240) (946KB)
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This was almost at the end of the 2nd session. You can hear that my car was not being especially cooperative when I was turning right as it was squealing a lot. The driver of the SHO said his tires were getting greasy at that point, and that his instructor had told him to hit the brakes in the middle of 3A. He hit them a little harder than necessary, and that brought the car around... right in front of me. Fortunately, this happened at the slowest point of the track with decent runoff room. It was still a little unnerving coming around a corner and seeing the car in front of you sideways. I had my head up and was already looking at the entrance to 3B. The SHO (being yellow!) was pretty easy to spot, so I just slowed down cautiously and steered to the outside of the turn.
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Small Video (360x240) (973KB)
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Ok, so I can't always show it when I'm driving well or when others make mistakes, can I? I guess I gotta take my licks, too.

One of the new things I tried this time was short-shifting into 4th gear before turning in for Turn 9. I didn't really need to do this at the speeds I was going as I had no problem getting to the track-out of 9 before running out of revs. However, some of the Group A guys were suggesting hitting 4th before 9 (to people in general, not specifically me), so I figured I'd try it. I found that it did let me concentrate on the corner more rather than watching my tach and thinking about shifting. However, as you can see from this video, there's a downside to this: if you shift as you're still turning slightly right before the turn-in, you might get 2nd instead of 4th. I guess my 2nd gear synchro is in good shape :-} Fortunately, I was only going somewhere a little over 70mph at this time, so I didn't over-rev the engine. It surprised me, but I had time to recover without unsettling the car for the corner. Yet another reason to get that roll bar so I can install my 4-point harnesses.






Track Map

Pacific Raceways Track Map
©2004 Pacific Raceways, Inc.

Track Description

The Pacific Raceways Road Course is an SCCA and NASCAR certified, 10-turn (9 turns with a "kink" in the main straightaway), 2.25-mile course. There is a total elevation change of 125 feet, of which 100 feet occurs in 1/2 mile. The course winds through natural wooded terrain, making it one of the most unique and challenging courses in the United States.
©2004 Pacific Raceways, Inc.



My Car Setup





Equipment Report





Numbers and Data




Front Tire Failure

I had to go home early from the track because I broke a belt in my right front tire. Since the course is run CCW, that would be the outside front tire on 9 of the 12 turns. As best as I can tell, the tire started to fail (maybe belt(s) started slipping?) and made the car track strangely after the last event. I first noticed this as I was driving home from the track after the previous event. The car's primary symptom was that it pulled to the right. I mistook this odd tracking for having picked up marbles from the cooldown lap my previous session. It didn't go away in subsequent street driving or in the first session of the day this time. It didn't get worse, either. In the second session of the day, there was a small pop from the right front of my car during a LH sweeper. It felt like I'd run over a small bump, perhaps like a 1" high speedbump on that tire. As I continued cautiously onto the front straight, then there was pretty severe vibration whenever I was over about 70mph. Below that speed, it felt fine, with the exception of the aforementioned "pulling to the right" stuff. I took the rest of the lap very carefully (and slowly) and went into the pits. I was unable to find anything wrong via visual inspection and shaking the wheels to check the wheel bearings. I went back out for my cooldown lap since the session was basically over anyway. After I returned to the pits, I turned the tires all the way to the right to get a good look at the tread. The tire in question had a (1/4" ?) outward bulge along the inboard edge of the tread for pretty much the whole circumference of the tire. At one point on the circumference of the tire, the tread had a part (outboard of the centerline) that looked "caved-in".

I took the front tires to the Firestone shop to ask them what happened to my tire. They said that it's definitely a broken belt. They noted some "odd wear patterns" and asked me if I'd been doing burnouts. I told them the car was AWD and there's no way I was doing burnouts on those tires.

They are Firestone Firehawk SZ50EP 245/45/ZR17 tires. The front ones have 3/32" to 4/32" of tread left, and the rears have 2/32" to 3/32". There are no cords/belts/badness showing in any of the other three tires. All tires show reasonably normal wear patterns (given their use). With good tires, the car tracks straight and has no strange tire wear, so I believe my alignment is good. I've definitely never hit anything (pothole, curb, construction debris, etc.) while driving on these tires. They have 2.5 track days, 2+ seasons of autocross, 4-5 autocross schools, and 5000mi (tops) of street driving on them.

Now I gotta spend money for new tires sooner than planned. Grumble grumble. But I get to get new tires :-)

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Before Event

Here are the front tires after the previous event, and before this one. Both of them looked ok upon external visual inspection. The tire on the left is the one in which the belt broke in this event.

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Before Event

Here are the rear tires after the previous event, and before this one. Both of them looked ok upon external visual inspection. The gray stuff you sees stuck to the tread is race tire debris picked up by the tires at the track. It wears off after a little street driving.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from front of car.


Note that just to the left (in the picture) of the center of the tread, the tread is "caved-in." Just to the right of the center of the tread, the tread bulges out slightly.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from front of car.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from right side of car.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from front of car.


Note the odd wear and tread build-up on the tread on the left side of the picture.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from right side of car.

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Immediately after pulling into the pits and parking.
Right (passenger's side) front tire. View from rear of car.


Here you can see the depression and the bulge more clearly. The tread looked like this no matter how the tire was rotated. i.e. it wasn't one spot on the tire; it was like this all the way around the tire.

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Immediately after pulling into the pits and parking.
Left (driver's side) front tire. View from rear of car.


Note that the tread is flat and there are no weird wear patterns or tread build-up.

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Immediately after pulling into the pits and parking.
Left (driver's side) front tire. View from rear of car.

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Immediately after pulling into the pits and parking.
Left (driver's side) front tire. View from front of car.

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Immediately after pulling into the pits and parking.
Left (driver's side) front tire. View from left side of car.

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After driving 30mi home at 50-60mph.
Right (passenger's side) front tire. View from right side of car.

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After driving 30mi home at 50-60mph.
Right (passenger's side) front tire. View from rear of car.


You can see the bulge on the left in the picture, but the depression on the right side is no longer there.

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After driving 30mi home at 50-60mph.
Right (passenger's side) front tire. View from right side of car.

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DSC00043.JPG (78KB)

After driving 30mi home at 50-60mph.
Left (driver's side) front tire. View from rear of car.

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DSC00044.JPG (52KB)

After driving 30mi home at 50-60mph.
Left (driver's side) front tire. View from left side of car.

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After driving 30mi home at 50-60mph.
Left (driver's side) front tire. View from left side of car.

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After Event

The tire on the right was the right front tire, and as you can see, it will not even stand up straight without my holding it. In its position in the picture, the tread touching the floor was more-or-less making full contact with the floor. The (still good) left front tire is on the left for comparison. The right tire sure looked a lot worse once it was off of the car.

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After Event

Closeup of the tread of the right front tire.








Analysis of What Little Significant Knock There Was

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Here you can see the only significant knock (above 3) of the first session. It happened within the first minute I was on the track, right around turn 10. As you can see from the Throttle data, I was giving it a constant throttle input around 30% and the ECU was in closed-loop fueling mode (as evidenced by the oscillating O2 sensor voltage) before the knock occurred. The knock spike was coincident with a small throttle increase from 31% to 34%. It also appears that the ECU went into open-loop operation at this time, since the O2 voltage went above 0.80 and stayed there for a little while. I don't know why the O2 voltage dips down to 70% as the knock goes away, unless that's evidence of a misfire, where lots of oxygen would be present in the exhaust gas. Also note that the ECU did not determine that the knock was severe enough to start retarding ignition timing, as the timing only dropped from 36 degrees BTDC to 33 degrees BTDC, which I believe is in response to the throttle increase and not the knock event. Timing returns to 36 degrees BTDC as soon as the throttle goes back to 31%. I suppose load data would have been useful at this point to infer what was happening with manifold pressures.
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This time, the knock occurred around Turn 9, and again it was right out of the gate for that session. This makes me wonder if perhaps there's a cooling issue here since the car was idling for 10 minutes or so in pre-grid before entering the track. I don't seem to get this type of knock once I've gone around the track a few times. That, coupled with the fact that I'm generally "taking it easy" on the first lap, makes me wonder about heat soak issues.

In any case, the knock again occurs at partial throttle, around or below 50%. In these cases, though, the oxygen sensors were reporting very rich conditions (above 0.95V), so I don't think fueling was the problem, unless it's that fuel distribution to the individual cylinders is not uniform.

For the first knock event, the knock begins while the ECU is in close-loop fueling mode and rises in magnitude as the throttle is increased and the ECU goes into open-loop mode. Timing gets pulled, but it never goes below 22 degrees BTDC. This event was between 3000 and 4000RPM, at which RPM the turbos are capable of making full boost, so it's likely that manifold pressures were high at this time. It's possible that this knocking would have continued or gotten worse with time, but it stopped when I shifted to 4th gear early.

For the second event, I had just increased the throttle from 30% to 53%, and the oxygen sensors were again reporting very rich conditions. Timing was pulled, but again, I think this was mainly due to acceleration and load rather than in response to the knock.



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Last Modified Tue Aug 17 2004 21:35:19 Pacific Daylight Time