Wednesday, June 4, 2008

Track Day #6 -- 2008/06/02

So, yet another track day with new stuff. In addition to the rear brake upgrade I mentioned in the previous post; I also bought some new wheels and tires for dry track days. I ended up with fairly inexpensive wheels (SportEdition D5) from TireRack. I chose them because they were the lightest wheels I could get at a reasonable price.

For tires, I wanted something with more lateral bite and sharper roll-off on the traction vs slip angle curve. With street tires, the transition from grip into sliding is really vague and it can be hard to tell when you're pushing too hard. I wanted something that would be easy to tell when I went up to the edge of traction. For this purpose, I chose the Nitto NT-01 DOT Competition tire. I didn't go crazy with the size; just a tame 225/45-17 tire size. It fits easily without rubbing; but it sure does grip well.

The gameplan is the same as always, get used to the new tires and make sure that I'm not over cooking the brakes. Try to work (again) on T7 to try to avoid the understeer problems and maybe try out trail braking now that I have a bit more rear bias in the brakes.

All in all this was my best track day yet! The weather was cool and dry starting out overcast, but ending up with the sun occasionally obscured by cloud in afternoon. After the first session to check things over, I was able to start getting to work. Turn 7 seems so much easier now with the new tires; I can really drive it down into the apex with the throttle.

I also had an opportunity to hustle a bit more through T10 - T12; this is a place I generally have been pretty tame. I've seen lots of videos of folks spinning out at PIR, and Turn 10 seems to be the favourite spot. People will go too hot into there, and not be able to make the transition into turn 11 and end up sliding into the weeds. I didn't want to be one of those people; so I've been pretty calm through there. The new tires really gave me the confidence to start pushing a bit in Turn 10; carrying more speed than I normally do.

Here's an example of something similar to what I'm talking about. It's a little bit different in that the Porsche lost control after T11 entering T12 rather than leaving T10; but you get the idea...

My day was a bit better. I didn't have any scary moments at all (other than avoiding the Porsche, which wasn't really that scary). I didn't even run the last run session of the day. I figured I had such a good day I didn't want to jinx it with something going wrong in the last run group which was a shared run group from all levels (novices and intermediates together). Not that anything would go wrong, but I just didn't have a good feeling about it so started packing my gear; and as luck would have it, just as I hit I-5 raindrops started falling on my windscreen. I would not have liked to be out on a wet track with R-compound tires; so I think it was the right decision.

When I got home, I took a look at the data from the PerformanceBox and saw that I ran a new PB of 1:35.4; nearly 4 seconds faster than my previous best time! I figure that 3 of those seconds are purely in the NT-01s, but maybe 1 second is there because of the increased confidence I got from the setup.

Here's a plot comparing my previous best time (blue) against my new best time (red):

And here's the fast lap on video:

On the brake side, everything looked great when I got home. The pads did NOT have the taper any more and were only 20% used; so they'll be good for another couple of track days maybe three. Seems like I finally got everything clicking. Now I can leave things alone and just concentrate on my driving without worrying about stuff.

UPDATE -- 2006/06/04

I've been having a pulsing sensation in my brake pedal ever since I put the street pads back on the car since the last track day. I figured it was just some pad accumulation on the rotor that I'd have to deal with (e.g., sand it off, or similar). This can happen sometimes when you switch from one pad to another (like I've been doing). Well, unfortunately when I got home from work today, I took a look at the rotor and found a crack in one of the slots that went all the way to the edge of the rotor. Maybe I shouldn't have been so smug as to think I have everything figured out, eh? Also -- note to self: when checking the brakes after a track day check the rotors too!

I haven't taken the rotors off the car, but from what I can tell I have only the one crack. Hopefully it was an isolated deal. For now, I've got to call up TCE and order some new rotors. I really can't run on a cracked rotor for long -- even on the street.

UPDATE -- 2008/06/05

I talked with Todd from TCE today and ordered the new rotors. I got the 12.2" rotor, so it's a tiny bit larger than before (was running 12"). I don't think that the extra 0.2" will help prevent cracking by itself, but it won't hurt either. After talking with Todd for a while, I did walk away with a few things. Chiefly, damage to rotors is usually not done by the braking event itself; rather, what causes the damage is usually caused by not cooling off the brakes well enough. If you just come into the pits without cooling the brakes, the pads will heat soak the rotor and cause thermal expansion issues between the hot part of the rotor (where the pads are) and the cooler part of the rotor (away from the pads). The thermal stress can cause the cracks. There was a black flag in the first run group and I went straight from the track to my pits (where I measured the rotor temps to be about 750 F), and in the first track day with these rotors I got meatballed twice for what turned out to be tire noise. Again, I just went to the pit spot after the meatball. So, I think my takeaway lesson is that if the run session is stopped due to a black flag or I get meatballed, I should run for a little bit in the pits (or maybe even out around the block) to cool down the brakes. I really am not looking to upgrade my brakes again at this point. :-)

UPDATE -- 2008/06/14

The rotors came really quickly, about a day earlier than expected, so everything was set for me to dig into the brakes and get the rotors replaced.

Here's a picture of the cracked rotor:

And a closeup of the crack:

The new rotors are 0.2" larger, so maybe, just maybe, I won't crack these as quickly. We'll see. Here's a comparison shot with the new (very slightly larger) rotor nice and shiny (at least until I bed them in):

I sent the rotor pictures to Todd at TCE and he thinks that I'm really pushing these brakes pretty hard. The purple colour of the brake hat is one of the clues. I guess I'll be looking at a BBK for next year -- probably go with the 13" Wilwood kit. This will require 17" wheels, which means swapping out my street wheels for 17" ($$$) -- hence why it will be next year. :-)

Depending on how my rotors pan out, I figure I'll run the July 29 event and maybe one more August or September and call it a season. I was hoping to run 6-8 Track days this year, and five isn't too bad.

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