theox05

Took the C5Z out for first track day and wanted to get some forum feedback on my car feedback. I am in the midst of converting it to a dedicated track car and have done the following items:

1. Autopower roll bar installed
2. Seats replaced with Sparco
3. Most of interior stripped
4. Katech Belt Tensioner
5. Spark Plugs(TR55) + Wires (Taylor 10.4mm)
6. CCA Pedals
7. MGW shifter + 1 washer Anti-Venom
8. All fluids except coolant

Tires are RE760s with good tread left (tires were on when I bought it) and car has a mild track alignment. Bilstein shocks are on it, but stock sway bars. Used Competition mode for TC. Stock HP.
Front 265/40/17-----Rear 295/35/R18

Ok, feedback time. For reference, here is a one lap vid:
So I drove fairly conservatively, this being first time on track with the Vette. However, I noticed a very distinct tendency to get loose in the rear on high speed corners, particularly ones with a downhill gradient, to the point where my entry speeds had to be lower than I wanted. A couple of times pushing it a bit I nearly drifted through the corner. Couldn't get on the power as early as I would have liked as well, as the rear end really wanted to kick out. You can really see this on the long straight as I cannot carry enough speed from the approach corner to hit more than 120 mph or so. Also, stock brakes suck, but thats easily resolved. I was pretty conservative in braking zones.

For reference, all my laps previous to this are in a 2018 BMW M4. With that car (on RE71s) I can carry more speed, hit 140 on the straight, and the backend stays firmly planted to the ground. That lap is a 2:19. With some better brakes I could probably get that down into the lower teens at my skill level. However, with the M4, I can run under 2:10.

Primary question is what do I need to do to get that backend to stick? Bigger, better rubber? I am planning on going 18 square soon with NT01 315s, but want to make sure I'm not missing something. I had thought even with these moderate tires I would be able to put more power down at my skill level (I am Intermediate), but that was def not the case.

Thoughts?

IBAUCLAPlaya

What is your alignment set to?

theox05

Unfortunately, I do not know. I know from when it was up on the lift that it is a mild track alignment, but I do not have any numbers or one of those high dollar tools. I suppose I could take it in to figure it out unless there is another way. Have to get an alignment anyway when I get new tires/wheels, will do it then at the latest.

IBAUCLAPlaya

My bet is camber and/or toe out of whack in rear.
RE760 isn't a great tire for track as they will overheat quickly and fall off when you're pushing it, but if you're not experiencing issues with that happening in the front I'm guessing you're not at that point yet.

theox05

Good deal, thanks for the input. I'll have that checked out and see whats up. Its entirely possible that the previous owner was doing drag stuff rather than road racing, bad assumption on my part.

tommie

Have you checked to see if anything feels loose in the suspension? control arm bushings, ball joints, tie rods?

TrackAire

How many miles on the car?

Have you really checked the hub bearings, a-arm bushings and other suspension parts for wear and or play? A self steering rear end is a nightmare when you don't expect it.

You really need to have a track alignment done by somebody that knows what they are doing. A fancy machine is useless with an inexperience operator...you can get a professional alignment with strings and tape measure if the guy doing it knows his stuff. I would think a track alignment is going to cost at least $200.00 or more if you get the car corner weighted, etc. We'll worth it in my opinion because you can't diagnose anything if the alignment is out of whack.

What were your cold vs hot tire air pressures?

Did you have the stability control in the "off" position for sure? You do not want the car in "Active Handling Mode".

Your tires might look "fresh" but be dried out....what is the date of manufacturer stamped on the tire and how many heat cycles do they have?

Welcome to the best drug in the world!! Let us know what you find out.

theox05

Taking a deep breath....whew....here we go!
1) 26k miles on the car, of which I have put on 500 (I got a good one )
2) Will do a suspension check, but I think things are good. Been under there messing around a bit.
3) Cold 31, Hot 36
4) Def in Competition Mode, had to dial out the Las Vegas style COMPETITION MODE screen on the DIC with reset button
5) I'm well within the date code on the tires, no idea on heat cycles, they came with the car.

Oh4GTO

I think there are too many unknowns to try to tackle the problems. With C5 and C6 have always been known for a snappy rear. The car is so light and if you unload it out back, it can go very quickly. Wider tires and moving the battery to the back can help.

argonaut

IMO step one is get the car properly aligned. Unlike a BMW, the Corvette is MUCH more difficult to align and it should not be done by anyone other than someone/shop experienced in track prepping vettes. Don't even think about taking it to your local tire shop (been there, done that. As have numerous others. And regretted it). You need to find a race shop with Vette experience. In the northeast places like Phoenix or County Corvette will charge you upwards of $500 for a full setup. Sounds nuts but that's the reality of dealing with a SLA suspension - very few know how to align it and they charge a premium for doing so.

TrackAire

Once you get an alignment done by a track expert, I would then soften the pressures a bit. With your rear 295/35 tires, you have quite a bit of sidewall to work with....I would probably start at 26 psi to 27 psi cold and try to keep the rear pressure at 32-ish hot and see how much more stable the rear is. The other thing to look at after a track session is how much the tire is rolling over onto the sidewall. IIRC, your tires have little "triangles" stamped into the side of the sidewall just below where the tread line meets the sidewall. You want to make sure the tire pressure is soft enough so there is tire wear to the "tip" of the triangle. Looking at a picture of the Bridgestone RE760 Sports...they look like they have a lot of tread block area that can get hot quickly and become greasy due to the tread flexing a lot under load. Just something to keep in mind, especially on hotter track days.

Mark@AMT Motorsport

Sent you an email Matt - but I absolutely agree with everyone suggesting the alignment is off. These cars suck to drive if the alignment is off. Get it sorted and you'll be much happier and faster.

NSFW

What tires did you have on the BMW for the 2:10 laps that you're comparing to?

RE760s are 340 treadwear, which IMO is a nice balance of grip and longevity for a street tire... but not much fun on track.

theox05

Thank Mark, appreciate the feedback!

RE71Rs on the BMW. I think what I'll do is get a track alignment, do some testing, and then go tires.

vettehardt

Don't even bother with competition mode in the TC. It's an early system that does more harm than good. Just turn it off. All it does is apply brakes to keep the car from rotating. Wears down brakes faster and can affect handling. Most all racers with the C5Z06 just turn it off.

Bill Dearborn

I agree the alignment could be the issue along with the tires on the car. I started track driving C5s back in 1997 as soon as my C5 had 1500 miles on it. Previously, I had been driving an 86 C4 Corvette. As soon as I got the car warmed up and started pushing it I noticed a huge difference between it and the C4. The biggest was a lot more trailing throttle oversteer. The C4 was an understeer car so when I went into some turns like the bus stop at Watkins Glen I did what I used to do with the C4 to negotiate the turn, it felt like the back end wanted to pass the front end. What I found is as soon as I got the car rotating into the turn I had to at least get maintenance throttle on to keep the rear where I wanted it. Adding more throttle tended to make the car squirt through the turn, although if I added too much I would push the front tires into understeering. Once I switched to 315/35/17 tires all the way around the car was a cornering monster (the way the SCCA T1 racers used to run back in the early 2000s).

Bill

NSFW

That alone could explain the difference in lap times. You'll be able to brake harder going into corners, carry more speed through them, and use more throttle coming out.

Plus an alignment... I ran 2.5 degrees of camber up front last season and had that changed to 3.0 for this season. 1.5 degrees rear camber, no change for this year. Just a hint of toe-in.

I could see you shaving ten seconds per lap after a few sessions to get used to how much more capable the car is.

And +1 for shutting off active handling and traction control. Every time I forgot to, I was reminded when the ABS kicked in to keep the car straight while I was trail braking and trying to turn in. It's not helpful at all.

Supercharged111

I ran the same RE760 tires at HPR on my C5Z back in 2012/2013 and if anything, the car was a little pushy. No idea the times it ran with those tires, but I cracked off a 2:06 on 295/35/18 square NT05s in an otherwise bone stock car on the old surface. It's clear from the video you're being conservative, but since you have prior experience I'd expect you to start warming up to the car after 3 or 4 sessions unless something is just plain wrong, like a bad alignment as has been suggested. On a cool day, our 260whp 4th gen Camaros can crack off 2:03s on the new surface. With good square tires and the car properly responding to driver input, I'd expect that a C5 would be under the 2:00 mark. I'm not putting this out there to thump my chest, I just want to give you a decent measuring stick. Below are my recommendations on your line.

T2: don't enter full track left, try 2/3 left. There's no room on the exit to make up the ground lost on the entrance.
T3: not sure this'll work on a car with 100whp more than mine, but I don't brake between 2-3 I just float the gas. Back when I tracked the C5, I braked going into T3.
T4: brake later, from 118 I can brake after the 3 marker. I also hold 4th around this corner so I don't upset it with a downshift. I try to maintain that straightaway speed as long as possible, then really yank the car back over to set up for T5
T5: this corner should be a little bit of an early apex corner as the exit is wide.
T6: I'd downshift to 2nd here.
T7: More throttle sooner, I track full out here then yank the car back over to get the car squared up for the entry to T8. T7 is fun as hell on the ragged edge, the car should try getting predictably loose on you near the top of the hill.
T8: Back to 2nd gear here
T9a/b/10: Your hands aren't very busy here, you need to carry more speed overall.
T11: I'd grab 2nd here. If you apex too soon, there's dip waiting for you on the outside of the track that will loosen the car up.
T13/14/15: more speed needed here, car should be hairy coming out of 15.

I know you didn't ask for driving advice, but there are some big chunks sitting on the table. I understand your lack of familiarity and comfort with the car is probably the largest driving factor for lack of raw pace where I pointed it out above. Who do you plan to run with?

theox05

Definitely appreciate the advice, I'm only about 10 track days in at this point, all of those in the M4 until last weekend. As for the vid, yes this was the point where I was basically said "Hey, something may be up with the car, lets just focus on the line and smoothness and sort the rest out later." I ordered the AMT Motorsports Camber Kit, gets here Tuesday! I'll install and then take it in to the Vette shop in Denver for a track alignment. Then I have a dilemma. I do not want to keep paying for HPR + insurance while I'm working the bugs out. So I was thinking of getting some cones and finding a big empty parking lot to try and simulate T3 and T10, where I noticed the drifting tendency the most. Have to do some research on suitable places.

At the end, I'm looking to get into TT then move into W2W in ST with NASA. Would love to pick your brain sometime at HPR and maybe have you drive the car and see what you think. Once I get this sorted I'm planning on being out there at least once a month.

Supercharged111

Our group is kicking around running HPR in a couple of weeks. T3 and T10 are loose corners for me too. I usually try holding my mouth just right.