I had my car weighed on all fours in order to get a better idea of the cross balance between the front left/right rear and vice versa. I had about a 1/3 of a tank of gas and suprisingly the car only weighed about 100 pounds light in the rear (obtaining a perfect 50/50 balance). The balance was 51.7% front and the cross balance was within 50lbs difference (another suprise). Having said all that, am I assuming correct that when I drive track events, I'm better off with a full tank of gas?

 

Not necessarily as that this the static weight distibution. However, when you're accelerating, more weight is transferred to the rear of the car. That being said, you can still have fuel starvation on the track if you sustain high g-force loads, as I experienced that at VIR on street tires with 6-7 gallons of fuel in the tank. So at the track, I'd recommend running a minimum of 8 gallons to be safe as you don't want to be mid-corner and experiencing fuel starvation.

 

Glad top read your #'s on the weights!

Since I go through about 15 gallons driving solo at an average HPDE event, I fill up at a station close to the track. Usually need to refill when leaving the track. Optimally, a half tank before each session would help........but, lets get real.....the weight difference won't be noticable unless you're an extremely advanced driver. I don't need to spend $5.00 per gallon for fuel at the track. I never noticed a handling change with different fuel loads.

Lets see.......10 gallons at about 8lbs/gal=80lbs. 3300lbs/80lbs= 2.42%....(my math is VERY rusty.....)....Have you ever been passed by an instructor with a student passenger? I have.

Also, the tank is in front of the rear wheels....GOOD!

 

Personally, I fill up before I go to the track and just run it through the day (usually fill up again before I go home). I've not noticed any changes in the behavior of the car with respect to fuel level.

BTW: gas is close to 6lbs/gallon...water is 8lbs/gallon.

As John said, during acceleration (or deceleration) your 50/50 split goes out the window. According to Nissan, the 350Z is normally a 53/47 split: under acceleration it is supposed to wind up being a 50/50. Under braking, I would guess it's somewhere around 60/40.

I wouldn't get too hung up on the 50/50 thing...just keep in mind that a bit more weight up front will increase the stability of the car. Cars tend to lead with the heavy end (much like the heavy side of an unbalanced tire will point down). During braking, you want as much weight as possible far ahead of the front tires in order to help the car maintain a straight path.

For example, rear-engined Porsche 911s are not really a stable configuration for braking. The weight in the tail will cause the car to spin easily unless the driver maintains a reasonably straight line. You'll notice that most agricultural excursions for the 911s occur in the braking zone...and they usually leave the track rear-end first. The flip side is that on the other side of the turn, they can accelerate like hell.

 

Thanks for the input. I was just wondering.

 

This is true in theory, although most 911 spins occur from TTO (trailing throttle oversteer) which is similar to what you are talking about but without the use of brakes. The current generationg 911 (996) is extremely stable relative to older generations. I think it would be very hard to spin a 996 under the brakes (unless you're lifting and braking mid-corner) since they are quite stable and ABS minimizes the risk of loss of control.

Incidentally, the rear-biased weight distribution of the 911 allows it to have some of the best braking of all street cars out there. This is because as the car reaches full braking, the weight shifts forward and it effectively gets close to 50/50 weight distribution allowing the front and rear tires to be used more effectively (with close to even brake bias). By contrast a 350Z at threshold braking probably has a weight distribution closer to 65/35 (f/r) and a fwd car with a poor static weight dist and high CG may go as bad as something like 80/20. The more unbalanced the weight dist at threshold braking, the more front bias is required and the more the braking distances lengthen.

This is why, no matter what kind of stickies you're running and how big and bad your brakes might be, there's always a chance a car with better bias might be able to outbrake you for the turn.

Jason

 

The latest Grassroot Motorsports (For those with NASA you get it for "free") has a great article on brake bias and weight transfer during decelleration.

Check it out for those who are interested in knowing the basics of brake bias and why you need it.

 

Hmmm... how long ago did you receive that issue? I think mine has disappeared into the black hole that is the USPS.

 

Hey Jason, good post there! Yeah, my experience has been solely with the older generations of 911s (70-ish) that don't have ABS or ESP. You are completely correct about the advantage of having more weight on the rear tires during braking, however this does compromise braking stability. That pretty much means that a "good" driver in a nose-heavy 350Z will be able to turn in consistent lap times (due to the additional stability), but a "great" driver in a 996 or 911 can turn consistent AND faster lap times. A 911 just requires more out of the driver to be fast.

My viewpoint is that if a car is more stable, you will have a better chance of success...not to say that 996s or 911s have not been raced with great success. For us "weekend" guys, it's probably best to stick with more mild configurations and focus on just finishing consistently. Screwing up in a demanding car can mean disaster (and not finishing at all). I just think that these cars are not for "beginners."

BTW: it's possible to spin a 996...seen it done once. You just have to mess up bad on trail-braking. But, to be fair, the 350Z will spin like a top under said conditon too! Come to think of it, I've seen a Honda Civic spin that way also.

 

I was at the Virginia City Hillclimb cou[ple years back and talked with a guy that had just come from a Mustang Cobra to a 911, completly differant cars handling wise he said. Another Porsche guy was there told how to enter turns, etc and he improved a lot. He said the tail wanted to come around all the time, after his quick lesson he felt much better, we saw a great demonstration of the tail problem, guy braked wrong in another 911 and put it into the cliff side, luckily vs going off the cliff.

 

We are in agreement, braking stability is theoretically worse in a car with a lot of rear bias (physics is physics after all).

However, braking in a straight line (or even in a curve carefully) doesn't upset the 996 much. They've done amazing things to the suspension in these cars and ABS + PSM (porsche stability management) can keep most drivers out of the weeds even if they do boneheaded things.

I actually don't have much seat time in older 911s (some time behind the wheel of 993s and little in a 964) but I've done a lot of mileage in 996s (see my picture at left) including at the track. They're not that hard to drive. Although, I suppose a novice in a GT2 could be a bit risky

The turbo that you see at left is actually very easy to drive, the only risk is that the speeds you can get to are really out there...

Jason

 

Hey Jason, well I'd certainly love to give the 996 a spin someday...but I don't think that's going to happen as long as I'm an engineer! Besides, for that kind of scratch I'd rather own an open wheel Formula Ford. (I'm deparately looking into racing open wheel...it's become an obsession).

Last time I was out at Gingerman there was a guy with a late-model 911 turbo: that thing was fast! He had Hoosiers and a full cage. He was turning lap times only about 10 seconds behind the track record (rumored to have been Arie Luyendyk). I can totally see what you mean about the speed thing...

 

Someday when I move out of Manhattan I'll have to buy an open-wheeler. I'm leaning towards Formula Continental - I want to play with aero (that's the only thing I haven't done).

But first I have to move out of NYC, get a dedicated production track car (to bring to HPDEs), get a tow vehicle and finally I'll get my open-wheeler. I'm guessing it won't be for a while. For now I'll stick to tracking the Z and 996TT.

Btw, there's an instructor in the BMWCCA with a modded 996 GT2 (650hp, full cage, lost 500 lbs, adjustable Penske suspension, sick-wide goodyear slicks) who runs 55 second laps at Lime Rock! For reference, I run low 1:02s in my stock Z with yoko A032Rs (and I'm one of the faster instructors).

Jason

 

I can honestly say that I feel your pain, Jason! I've found a couple of nice mid-90's Van Diemens with lots of spares (including a fresh 2.0L Duratec) for less than $30k. If I sold the Z, I could buy one...but then I'd need the tow rig, trailer and shop to work on it!! Life can really torture a person, you know? Ah well, that's why I bought the Z: it will serve as my "race car" until the real one shows up.

Yeah, I find it mesmerizing to watch guys who are that fast...it just seems like the earth stops rotating when they exit that corner with the "hyper jets" on.

BTW: How do you like the Yokos? I was going to try race rubber this year on the Z, but my funds have already been allocated to race schools and vehicle dynamics courses. I was thinking of going with the Hoosier R3S04s since they're available in sizes that will fit the 18s on the Z. However, I've heard that Hoosier's quality control is a bit lacking...

 

Yeah, I too would have nowhere to park a trailer since I live in the city.

Unless you're actually competing and/or have a lot of money to blow, I'd steer clear of the Hoosiers. They're only really good for ~10 heat cycles, and then it's time to toss them in the dumpster. So, if you want to pay over $1000 every other track weekend, I'd stick with another R-compound. Unfortunately, there aren't too many size options out for the Z yet (or at least ones sold in the U.S.).

Archman, you going to sign up for the Mid-Ohio NASA HPDE in April? There's going to be a large Z turnout this year.

 

John, I just PM'd ya.

 

The Yokes were not bad - I drove them on the street and on track for a full season. They did a pretty good job in their dual role. The downsides is that they are extremely loud on the street and they were messed up during the whole second half of the season. All those heat cycles and all that mileage meant the grip started decreasing pretty badly. In the last couple of events that I did, they were no more grippy than a decent street tire and the last time I used them, they were downright scary (and the tread flew off one of them). So the to make a long story short, I liked them but they aren't perfect (then again, I put them through a lot).

This year I'm using Hoosier R3S03 - I don't think the R3SO4 comes in the size I want (245/40-18). I'll mount them on my track-package Rays wheels and only drive them at the track. With proper camber, a set of hoosiers should last about 3-4 track weekends - which I can live with. In terms of QC, I have only heard that on the internet (a medium of which I am skeptical) - although their construction is substantially different from that of any street tire or most DOT rated R-Compounds (yielding to a slightly higher level of inconsistency).

We'll see how it plays out. I'm still trying to figure out how I'll get the wheels/tires to the track.

Jason