I posted in another thread about "polar weight" Its an important agility factor to great handling of any vehicle- I have never heard it discussed and figured since I mentioned it, AND I"M BORED RIGHT NOW, I figured I'd explain its importance to anyone that is interested.

1st off, polar weight is weight relation to the center of axis or gravity in rotation. What this means is the further from center of gravity something is, the more force that is required to move it/ or stop it.

Example- (as we all know) Hold your hands close to your body and twist your hips left and right and left... You can stop one direction and reverse into the other fairly quickly.
Now hold your arms extended out sideways and twist, it now takes more effort to stop and reverse directions, Or just to start the twist movement for that matter. Hold weights in your hands and it gets worst.

How does this effect handling? In a big way. When a car turns into a corner, the front and rear bumpers and/or the side doors are your hands held extended outward. That weight rotates on the car's CG (center of Gravity) when it turns left or turns right. The more weight and /or the further outward the weight is placed, the harder it is to turn the car (and to stop the car from turning after initiated)

Example: Lets take the battery. We can all move a battery around anywhere in the car. Most relocate it to the rear when dragracing to get more weight over the backend. If that battery weights 30 lbs and is sitting in the car right ontop of the CG, then the polar movement weight is 0. Move it up to the stock location (aprox 5 ft estimate based on most cars{note: every car has a different CG- My V6's CG is further back in the wheel base than you V8 guys}) and the polar movement weight is now aprox 750ft/lbs. at 2 ft from CG it is 120ft/lbs and so on.

Moral of the story- Any critical parts to the car that can be mounted more towards the center of the vehicle rther than the outer extremities (like the battery, and MSD box, big frickin speaker boxes, anything....) can help inprove the slalom speed of the vehicle.

 

I have my battery relocated to the right rear passenger seat area
where there is a cavity in the floorboard where the seat cushion use to be. toward the center and as low as I can get it. The car is being setup for local autoX.

 

next time, im going to auto cross with the doors open and weights in them.


just cause.







btw, i donno if you're into motorcycles, but look into what eric buell has done with the new firebolt.....hes big into mass centralization too..

 

good post dean.

that is something that i am pretty sure has never actualy been brought up on the boards before.

 

btw doesnt the same apply for the height of the weight?

for drag racing, having the weight higher up might be a good thing since it would raise the center of gravity and "fling" the car on the back tires more during launch.... right?



while on a car that turns, you want the weight low, so that it isnt tossed back and forth so easily... correct?

 

This is refreshing, actual technical conversation. Since I am bore too, I'll throw in my two cents. This is a good time to bring up the difference between transient handling, and steady state handling. Polar weight, contributes largely to transient handling, but almost none to steady state. At the same time weight balance greatly affects steady state handling, but not much transient handling. Also, the smaller your polar mass, the more difference you make to the F/R ratio with small weight movements.

Transient handling involves any change of direction and/or speed. Or in scientific terms delta in velocity. The components associated with this are shocks, and springs to a lesser extent.

The force applied by the shock is its coefficent times the travel of the shock per a period of time. The spring applies a force equal to the spring rates times its deflection. This means the shock applies a much greater force when entering or leaving a corner, or breaking/accelerating than the springs.

Steady state handling means there is no change to velocity. Since shocks only apply force during a change in velocity the only pieces to act on the car are the springs and sway bars since they only rely on deflection to apply force.

To touch on weight balance that I mentioned earlier: The car will roll around the, get this, roll axis, and pivot about the, pivot axis. These two lines intersect at an imaginary point known as the instant center of the car. These lines are based entirely on the geometry of the suspension. This is where weight balance starts to make a difference. A car with the center of gravity in front of the instant center will tend to push, while a car with the weight in the rear will be loose. This is because the center of gravity applies a force, in a corner, that points in the opposite direction of the center of the curve.

This is counter intuitive to most people who think a front heavy car is always loose, while a rear heavy car is always pushy. This is true, but it goes back to transient handling and polar weight. If you have a stick with a ten pound weight on one end and you try to spin it by holding it in the middle, it will want to spin around the weight. Now, hold the stick in the middle without spinning it, and it will fall towards the weight. The first scenario shows a front heavy car is loose going into and coming off of a corner(transient) and the second shows it will be pushy going through the center.

That is why it is so important to select the proper shocks, springs, sway bars, and tires when the factory didn't do a good enough job of setting up the car. It should also tell you why sometimes more is not always better.

 

Another important thing to consider is the angle of the rear axle to the center of gravity. This changes the dynamic forces on a vehicle as it travels down the road. A few degrees change can literally send your car flying in the air (mostly for drag racing as in most street cars can't pop wheelies). One must also understand compressable flow for gas shocks to truly undertand the forces acting upon them. You don't necessarily want all of the vehicles weight over the CG but instead wan't an equal distribution. You want to spread the vehicles weight over the largest area possible. If you have too much weight at the CG that point begins to pivot and you lose stability and your tractive force is decreased from the parts of the car you want it on. The ideal 50/50 weight ratio also applies to the left and right of a vehicle.

Capairing the forces on your car to holding your hands out is close but you do not have any other forces (beside aerodynamic) on your arms as you swing them. Even though your bumpers are furthest from your CG you now have normal forces acting from the road against your tires and are redirecting parts of the car in the opposite direction that the CG wants to travel. Therefore they are aiding in turning the vehicle. They contribute to the dynamic forces on the car. Just in the opposite direction.

 

Is there a good set of books that would expound on this more? This is meat for my brain - I know there's a lot of physics involved on getting a 3500+ lbs car down the road.... Aerodynamics, inertia, mass, velocity, the stuff you mentioned, all of it is pretty important, but I guess the average mentality is "that's rocket science, I just want my car to look cool/go fast/sound good/blah blah blah..."

 

How to Make Your Car Handle -- Fred Puhn

Chassis Engineering/Chassis Design, Building and Tuning for High Performance Handling -- Herb Adams

Tune to Win -- Carroll Smith
Engineer to Win -- Carroll Smith

Race Car Engineering and Mechanics -- Paul Van Valkenburg

Performance Handling/How to Make Your Car Handle: Techniques for the 1990's -- Don Alexander

Fundamentals of Vehicle Dynamics (R114) -- Thomas D. Gillespie
Race Car Vehicle Dynamics (R146) -- William & Douglas Milliken

the last two are VERY technical books, lots of info on all sorts of suspension dynamics and diagrams for almost any suspension you can imaging. any of these or all of these are really good books on suspensions, and tuning suspension. maybe head to the local book store, see if they have some of them there, and thumb through them. not every book is fore everyone. by seeing it before you buy it you can find the one that should be close to your level. the Puhn and Herb Adams books are a couple of good ones for people without a ton of starting knowledge. hopefully other will chimb in there are some very good books out there, and i would really like to hear what ones other people like.

 

If there's a college book store close to you, you can try to get a vehicle dynamics book there. If it really interests you, I'm sure there are classes at JuCo that you could take.

There is a lot of information out there. You might want to just try searching for vehicle dynamics at the library. Find an author who has at least one good degree. Either a BS in ME or a Masters in Kinematics or Dynamics.

 

content removed since the other post were also removed. to keep the thread going in a tech fact based direction only

 

You really have to drive in a fully suspensioned and tuned chassis thirdgen to understand how scary these cars can become. They are low, wide, have massive amounts of rubber, plus if you have a small engine like AGood2.8 you can really open some eyes around the twisites. Dynamics and suspension geometry are taken in account for every car on the road.

 

Oh I get it... Bigger tire footprint = more grip = better handling performance i.e.: higher turning speeds, improved lateral G's, possibly quicker launches, ect... Gotcha.

 

Let me kinda retract this statement- An all aluminum engine (LS1, etc..) may just about be good bais for the cars natural roll axis when lowered 1" to 2". I have been tinkering with the Idea of raisng my front roll center slightly (or lowering my rear roll center- but that would involve changing to a higher rear spring rate) because I am more on the loose side when it comes to adjustment ranges.

By inserting adjustable lower ball joints, I can retain ride height, be able to readjust my Front A-arms more parallel to the ground, and thus, also raise my front roll center slightly- I don't know if it would help any (this car seems pretty much dialed) but it would be fun to play with and further reduce rear weight transfer in hard braking turns. Power is obviously not a problem factor for me coming out of the turn.

 

get ahold of the local SCCA, running auto-x will be an amazing learning experiance. you will learn more about you as a driver, and your car at your first event, than you could even imagine. if you get one of the 'good' drivers to ride along, and help you for your first few runs, you will really learn alot.

 

We have one in island, but my schedule is up to my eyeballs. I'm forced to settle for zipping through turns and stuff on the street - learned a lot so far, but I have a long way to go. Like: Rain makes roads wet. Aquatreads only prevent hydroplaning. Taking 90* turns at 30+ is NOT good for turning. skidskidskid.... Lucky I didn't hit the 3000-lbs torpedo dummy sitting by the road...

 

Brian Beckman's Physics of Racing

This is a great series of articles, covering all the math you use in how a car handles. Starts with easy stuff, moves on to some pretty cool computer-based modeling. Brian Beckman is a PhD physicist and Corvette racer, so he knows big, fast cars. Check it out.

 

A lot of that is way over my head, but my wife should be able to decipher most of it. Good stuff, anyway, at least he wrote it for stupid people like me, so we could understand it. From what little I could wrap my head around, I got big lightbulbs going on in my brain as to what to do next to the suspension.

 

Ths is right where I want to be!!!

So is this discussion about handling in general or will it be specific to f-body cars?

How much of a factor is it the anticipate turning radiuses? I have heard that the setup for autocross event is different from road course events. Which one is closer to street applications?

 

dean, in his attempt to rejoin the convo, already added to this.

but check out this link

http://phors.locost7.info/phors15.htm

that is from the above physics of racing link, it is a good bit on speeds and springrates ect, and how a bump effects a car at diffrent speeds.

 

To answer you question, everything mentioned applies to most cars most of the time. There are some comments unique to front engine rear drive cars, and some things about solid axle three link suspensions, but for the most part you can take any of this and apply it to any car.

 

It would make sense to make the car handle better (if you could do this, mind you) than it could possibly go fast, right? Just so you don't out-run your handling envelope, right? Just a silly lil' Q.

 

Yes, just like those go-karts at the fair.

 

So it is probably safe to say that for third gen f-bodies driven on the street it woulde be wise to have a system flexible enough to handle the four arenas (Drag Strip, Road Course, Autocross, and Streets).

What are the flexible pieces? What are the compromise pieces?

Jason

 

jrg77 --

that is much easier said than done. it is possible to make an all purpose car, but it will never be as good at anything, as a car built specificly for one purpose. actualy my friend has a GTA, that turns corners on rails, cuts a sub 2sec 60ft (on kuhmo street tires), and is just an all around great performer. it is all in the setup, alot of the auto-x, road-racers and drag racers, all run similar parts, but the geometry they use is much diffrent. the things like shocks/struts, and springs are where the big diffrences come in between the cars.

 

drag strip -- you generally want the car to throw all the weight in the rear... front end lifts easily... makes it turn like ****. you want all the power, NOW.

road course - generally big tires, stiff shocks and springs.. harsh on the streets, and you will get crap for 60' times... go look at that cool vid of the road race vette going down the strip and you'll see what i mean. also, you want the powerband to be smooth with nice throttle transistions...

autoX - depends on how hardcore you go. you could race a dead stock geo metro and still have fun there... but the more hardcore of a car you build.......

streets- kind of a broad catagory.. on one hand, you have people that daily drive basicly all out drag cars. on the other, people drive all out roadcourse/autoX cars.... both a lil extreme but thoes people love it.... most "performace" people on the street are after the image of performance... so a mostly stock car that sounds/looks good is good for them.. and they would find either car extreme.....


basicly it comes down to what do you want and what do you want to do.... because you're not going to impress everyone, you arnt going to be the best at anything, so just make your car fun.

if you daily drive your car, a stripped and gutted pesudo race car is not going to be as fun as a interiored car with a radio... that extra 1/10th more in the quarter is worth it then..... just think about stuff before you mod and you'll be fine.

 

Most of the suspension mods you do for straight line traction will tighten up the car through the corners. However, a big sway bar in the rear will actuallly loosen the car up a little bit going into a corner, but it will still push a little coming off unless you drive it a little loose all the way through. Big sway bars in the rear also minimize body roll during hard drag launches. Extra toe in will make the car respond quickly in the corners, but only adds drag when you're going down the track. Lots of chamber also helps in the corners, but again hurts straight line MPH. Tire pressure plays a very big roll in how a car handles and launches. Generally 5-10 psi more in the front will help handling and drag launches. I run a 30/25 split most of the time. Hard shocks and springs with a short suspension stroke helps both pretty well. I should qualify that last statement with "as long as the tires hook a decent amount on launch" shocks with a lot of jounce and rebound resistance help a drag launch, and naturally eliminate a lot of body roll through a corner. If you have old nasty hard tires, a hard suspension is just going to make them spin like crazy. In that case you want very soft springs and shocks. Setting up a car like this will make it do most stuff ok, but not much really well. The biggest thing that helps a car carve corners is a driver that is smoothe on the wheel and pedals.

 

Basically, if you really really wanted to, you could have a different suspension "module" - a package of springs, shocks, and struts (and whatever else I'm forgetting) you could toss on the car in order to tune it for whatever event you were planning to attend? In other words, keep the body pretty much the same, and just switch out phrehiperal suspension components and tires in order to tune it up to whatever you're running...? Of course, you'd need space in your garage for all of this extra stuff, not to mention the tools and hardware for changing it all out on a frequent basis, and the desire to bust your car down to jack-stands EVERY time... But, I know of a few people who are crazy enough to do stuff like that... Don't think I'd do it, tho...

 

That module setup would be really cool, but the other two modules wouldn't fit in the hatch, and very few have room in thier trailer for that many front ends. Bolt-on subframe modules that connect at the firwall and front engine brace... hmmm...

Is it ride height that gets adjusted or rebound and compression? What other pieces would have to be adjustable?