johnc Posted April 25, 2005 Share Posted April 25, 2005 Let's say you have a car that has a 55/45 weight distribution and you can get it to 50/50 by hanging the battery, fuel tank, etc. out behind the rear axle, thereby increasing its polar moment (or yaw inertia). Is that change a good idea? Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted April 25, 2005 Share Posted April 25, 2005 Is this one of those questions you already know the answer to? Quote Link to comment Share on other sites More sharing options...
blueovalz Posted April 25, 2005 Share Posted April 25, 2005 Somebody hi-jacked Johnc's user name! Quote Link to comment Share on other sites More sharing options...
johnc Posted April 25, 2005 Author Share Posted April 25, 2005 Is this one of those questions you already know the answer to? You betcha! Its a test. Somebody hi-jacked Johnc's user name! Doubtful. They would get my reputation along with the user name and nobody wants that! Quote Link to comment Share on other sites More sharing options...
jt1 Posted April 25, 2005 Share Posted April 25, 2005 According to a self proclaimed Idaho Zcar expert, 55/45 IS the ideal weight distribution!! John Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted April 25, 2005 Share Posted April 25, 2005 http://www.miata.net/sport/Physics/index.html you may want to read thru this Quote Link to comment Share on other sites More sharing options...
johnc Posted April 25, 2005 Author Share Posted April 25, 2005 People are already digressing... Here's a hint: Which is more important: balance or transitional response? The answer may depend on the intended use... Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted April 25, 2005 Share Posted April 25, 2005 http://www.miata.net/sport/Physics/index.html you may want to read thru this,first and keep in mind that the tires used and the suspension set-up will have a great deal to do with the results BTW you generally want to keep the( center of gravity/ballance point) as low and centralized as you can in a road race style car 55/45 weigth the center of mass between the wheels would be prefered over 50/50 with a good deal of mass at the extreme ends of the cars length, in fact most of the weight SHOULD BE between the axles and as close to the ground as you can get it! Quote Link to comment Share on other sites More sharing options...
Mikelly Posted April 25, 2005 Share Posted April 25, 2005 Grumpy has the answer if my teaching is correct. I'll assume a road race intended use since that applies to my intended use! The complaint with the Porsche 911 world class handling has always been the wonderful handling it is, right up until you get to the knifes edge of it...and it cuts you deep! Then the pendullem effect (That Jim M. Experienced a few weeks ago in his nice shiny new GT3 at supersonic speeds on a road course!)kicks in and sends one spinning off into the grass... Having the weight low is nice... having it low and balanced within the two axles would be even better. The biggest complaint I've had about fuel cells in Zs is that you are stuck hanging the thing out past the rear wheels... right where the previous tank was, and is still not optimal... This is also the reason I moved my battery from the rear deck and into the area behind the passenger seat. That weight will offset a significant amount of PMI while aiding in weight ditribution from front to rear, and moving it lower still! And John, Lets not give Kim Blough any credit here, my friend... a 55/45 balance is not optimal for a V8 Zcar, especially when sitting the motor so high in the engine bay and forward over the steering rack... Bad is bad, no matter the color, and a 55/45 ratio on a road race setup is NOT gonna win TTD no matter what Mr. Blough's memory will have him believe. Mike Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 26, 2005 Share Posted April 26, 2005 My favorite story regarding this subject is Dennis Hale's 215 aluminum V8 510 race car. It is a true front mid engine with the firewall moved back A LOT so that probably 1/2 the engine is in the passenger compartment. He built that car specifically with PMOI in mind and it has 50/50 with almost no weight outside the axles, and he has described it to me on a couple occasions as "nearly undriveable". My friends who have seen it in person say it looks like one hell of a race car, but apparently it loves to spin... just like the extremely low PMOI would suggest. Weight distribution is not the be all end all of handling and I would disagree that 55/45 cannot handle. I don't know the weight distribution of the British touring cars, but I bet they would love to see 55/45, and yet they handle pretty damn good. I've seen some pretty fast front drivers at autox too. I can think of a particular Fiesta that people would actually LAUGH at... right up until they saw that it beat their Vette, Mustang, Camaro [insert your supposedly fast car here] and realized that it REGULARLY took TTD at our old Santa Maria autox. EDIT--All that said I like my Z with 50/50, and I can tune it to do what I want, either plow or oversteer. Just like you should be able to do with 55/45. Quote Link to comment Share on other sites More sharing options...
Mikelly Posted April 26, 2005 Share Posted April 26, 2005 First car I ever raced in was a Ford Fiesta... Most fun I had in a car until I found my First Datsun Zcar... Mike Quote Link to comment Share on other sites More sharing options...
johnc Posted April 26, 2005 Author Share Posted April 26, 2005 For a road race vehicle, weight distribution is more important then yaw inertia. In fact, as Dennis learned, a little yaw intertia is a good thing. It makes a car more forgiving and stable because the same inertia that causes bad things once the car starts rotating also makes the car harder to rotate to begin with. For an autocross vehicle, low yaw intertia is more important then weight distribution because autocross is all about transient response. During a run at an autocross the vehcile is rarely in any kind of steady state cornering. Quick transient response will give faster course times then good balance, if the driver can stay ahead of the car. From one of Mark Oritz's newsletters: From a vehicle dynamics standpoint, I would opt for a more balanced setup rather than less yaw inertia, especially in a car that is nose heavy now. Within limits, yaw inertia can be coped with by driving technique. In some situations, it can even make the car faster. Overall, though, less yaw inertia is better, particularly when the course demands high yaw accelerations, as when negotiating chicanes or street-circuit turns that come in quick succession. But steady-state handling and ability to put power down are more important. A rear-drive car with only 45% rear weight and a powerful engine is seriously traction-challenged, especially when exiting turns. Any further reduction would not be good. With more front percentage, you will actually have to stiffen the rear suspension, at least in roll, with respect to the front. Otherwise, you will be adding understeer. The inside rear will then be extremely light when cornering. It probably is now. One situation where yaw inertia can make a car faster is where the car may unexpectedly encounter a slippery spot in the middle of a turn. If the slippery patch is short enough so the front and rear of the car hit it separately, the car will experience understeer and then oversteer in very quick succession: it will do a wiggle. If it has little yaw inertia, it will do a big wiggle. If it is close to the limit, it may spin. If the driver wants to allow a margin of safety to increase the chances of catching the wiggle before it becomes a spin, for a given level of risk the driver must stay further from the limit in a car with little yaw inertia. For this reason, in the days of high-speed open-road racing, many engineers regarded yaw inertia as desirable, and this was thought to be one of the advantages of a front-engined car with a transaxle, and a problem for the rear-mid-engine layout. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted April 26, 2005 Share Posted April 26, 2005 Mark Ortiz is the guru of all things chassis... Got any harder questions John? Quote Link to comment Share on other sites More sharing options...
BillZ260 Posted April 26, 2005 Share Posted April 26, 2005 The biggest complaint I've had about fuel cells in Zs is that you are stuck hanging the thing out past the rear wheels... right where the previous tank was, and is still not optimal... What's the best solution for this? I am going to re-do my fuel cell and would like to optimize it's location...I know it's a fairly general question but I think it adds to this topic since mentioned as a sore point in Z design. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted April 26, 2005 Share Posted April 26, 2005 What's the best solution for this? I am going to re-do my fuel cell and would like to optimize it's location...I know it's a fairly general question but I think it adds to this topic since mentioned as a sore point in Z design. Smaller tank would help. Quote Link to comment Share on other sites More sharing options...
johnc Posted April 26, 2005 Author Share Posted April 26, 2005 Move the tank as far forward as you can while still leaving enough room to get a wrench on the nuts holding the diff to the moustach bar. Quote Link to comment Share on other sites More sharing options...
BillZ260 Posted April 26, 2005 Share Posted April 26, 2005 More questions on the tank, I'll start another thd... Quote Link to comment Share on other sites More sharing options...
ZHeadV8 Posted April 26, 2005 Share Posted April 26, 2005 VERY interesting thread, thanks for starting it John.... My car is in build, I have been advised by may (including yourself John) to keep to 400BHP to save overpowering the car and to get the weight distribution right. I would like to point out that I have in fact for once in my life listened I am not 100% sure I understand yaw inertia from this thread but my engineer has decided the tank will be BETWEEN the rear suspension turrets, a custom made fuel cell to fit behind the seats. This was to allow space for twin exhausts and to provide a clear exit from under the car to aid with aerodynamics (having an under belly pan and maybe some venturi a la ferrari 430. Also to show off the tubular control arms (OK I know I am a poseur) That will centralise the weight distribution but will it also lighten the back end and cause handling problems ???? He is a rally engineer who builds awesome world class cars (has worked for Nissan / Toyota etc). For a very fast road car we can cornerweight the car any way we like by moving and relocating stuff. What in your opinion is the best way to set this up ? Quote Link to comment Share on other sites More sharing options...
zlalomz Posted April 26, 2005 Share Posted April 26, 2005 Is this too centralized? I hope he has a good fire suit. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 26, 2005 Share Posted April 26, 2005 I think that's how they did all of the CP cars back in the day IIRC. That's a little too close for comfort IMO. Quote Link to comment Share on other sites More sharing options...
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