I guess my question is: How does weight affect how a car handles. Like 2 cars with the same distribution and same suspension but one 200 lbs lighter. How much would this differ? Not a question I need answered, just something I thought up at work today.

Less weight=less inertia.
The car is less likely to want to get away from you when cornering if it weighs less.

hmmm...I am no expert on this subject but I know "it ain't always that easy". Other variables still come into play. But the experts will soon start chiming in, I expect, and we all will be made wiser.

Benno

[This message has been edited by Jdlog (edited 08-04-2003).]

Yes, it really is that easy.
Inertia is a VERY important thing. Inertia is what makes you spin out. Inertia is what lets you spin your tires. Inertia is why it hurts when you hit a wall.

the math is simple (off the top of my head)

friction your tires is able to attain is directly proportional to the weight pushing the tire down

and intertia - 1/2 Mass * Velocity square

so inertia is also directly proportional to Mass (weight)

so it balances out from a handling perspective

but by taking weight off the car you able to accelerate faster.

It can't be that easy, there are some very well handling cars that are heavy.
3000GT is close to 3900 pounds and handles great

a Diablo VT is 3600 pounds

An evo is like 3500

THere are heavy cars that handle well.

Yes, it is that easy. A heavy car can be made to handle well if the suspension is designed with the weight in mind. But if you take a stock Diablo, and then take another one exactly the same, but with 200lbs removed, it will likely handle better, and accelerate quicker.

well, the question was with the 2 cars being basicly equal. I think there was general drag racing rule of thumb, that every 100 pounds is a 0.1 of a second on the 1/4 mile. And another one, that a car that weighs twice as much, needs 4 times the HP to get the same time in a 1/4 mile. dont know the math of this, but I'd expect it applies to sideways inertia also. Of course, most of the sideways inertia is all on how wide the tires are, and of course grip/tread/compound. But back to all things being equal, except the weight, I honestly dont know the math, but 200 pounds is a good amount of difference, and would have a big affect on a skip pad.

no, since inertia and friction are both equally a direct function of weight, when you add or subtract weight from a car you do not change its ability to corner or stop

the only factor to consider is where the weight is located on the car - the lower the CG the flatter the car will stay in the corners

so bolting 500lbs of lead to your roof is not the best thing to do :c)

Sorry, it was early and I did not read the post right. If the weight dist. is kept the same I'll agree. On to the 200 pound weight take off. That is a lot. Unless you like driving a stripped car. Like the Honda guys. Personally I won't do that. I want to go fast and be comfy. Plus I prefer bracket racing. It is all about skill, with a little luck tossed in. As for weight reduction, a few people at the autoXes I have been to have noticed added weight can help. One for the Corvette guys kept a 250 pound guy riding with him because the car "felt" more balanced. I have noticed this too when I have someone riding with me that weighs a little more than I do. I know at least one other guy that thought the same thing in a fiero. There is so much that goes into suspension that as a theoretical question it works, but in practicality it is very difficult.

Handling is a combination of cornering power and feel, here I can only comment on cornering power since feel is very subjective.

Back to the original question:

Decreasing mass decreases the effect of inertia. Decreasing mass reduces friction since friction increases with greater force between the surfaces (more weight). These are not equal however. A tire which has a vertical load of 500lbs may have enough traction to handle 700lbs, the result is a cornering force of 1.4g. The same tire with a 1000lb load will have an increased traction of 1000lbs however the lateral acceleration has now decreased to 1.0g, at 1500lbs load the traction may be 1250lbs and lat accel is now 0.83, and at 2000lbs the traction may be 1500lbs and lat accel is now only 0.75g (Adams, H. (1993), Chassis Engineering: Chassis Design, Building & Tuning for High Performance Handling, HP Books)

The g-forces produced during cornering will increase the tire loading to the point where the tires no longer have grip and will break away, the greater the mass the sooner this will happen even though friction has increased.

Heavy cars that handle have large tires and suspensions tuned to make the best use of those tires, if they were lighter they would handle even better.

BTW small reductions in unsprung weight can have more impact than much larger reductions of sprung weight. Light weight wheels, tires and brake components will allow the tires to follow the road surface more effectively and increase grip, ask the autocross guys who run in the more open classes, they often use wide tires on (very wide) 14" or even 13" wheels, the reason is that the smaller wheels are generally lighter (unsprung weight) and the rim is nearer the centre (lower polar moment). 17" or 18" wheels may provide great street or even road course handling however they may not be optimum for really tight cornering.

So the answer is, 200lb weight reduction (with the same weight distribution as before) and no other changes should increase the cornering speeds of a car, however there are many other factors to consider. If the springs are stiff and you reduce the weight the car may now be too stiff to maintain grip on an irregular surface so you may lose the advantage.

Ther, clear as mud, hope you aren't totally confused.

Ira

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The Iron Duke Resource Site

Sig design by MinnGreenGT

[This message has been edited by Monza76 (edited 08-04-2003).]

I cant follow the resoning of the statements you quoted.

If the tire has a vertical force of 500 lbs and its able to supply a side force of 750lbs, then why wouldnt a vertical force of 1000 lbs provide a sideways force of 1500?

tire traction is non-linear at some point, at which the tire will break loose and skid -the example you quoted sounds like the tire is just ready to slide at 750 lbs of side force.

obviously if you overload the tire, all bets are off. I would want to see some experimental data (graphs) taken with tires loaded at different levels before I would be convinced. The quote you gave is worded as if the data is hypothetical.

[This message has been edited by Ken Wittlief (edited 08-04-2003).]

quote Originally posted by Ken Wittlief: no, since inertia and friction are both equally a direct function of weight, when you add or subtract weight from a car you do not change its ability to corner or stop :c)

wrong just not so at all!!!!
inertia is the restance to change in direction or speed, less weight WILL make any car handel better, corner faster, and stop quicker, shorter. in addtion to a quicker 1/4 mile time.

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Question wonder and be wierd
are you kind?

personally I think the question is a mute point - weight directly reduces acceleration

and in every motor sport I can think of, acelleration is 90% of the game.

The best handling car no matter what it weighs, will be the one with closest front to rear weight balance, providing the tires, shocks, and rest of the suspension is up to handling that weight. Yes the lighter, the better generally. Taking off 200 pnds is not nearly as important as 'where' you take it off. Most racers have movable lead weights to help get the car to that neutral handling. Remember too, that alignment and many other things also affect it to a degree. Corvette got its biggest handling improvements in years by moving the tranny to the rear to gain more rear weight to get closer to 50/50. If you look at specs of the fastest stock sports cars, the ones with 50/50 or close are turning fastest lap times.

quote Originally posted by Ken Wittlief: If the tire has a vertical force of 500 lbs and its able to supply a side force of 750lbs, then why wouldnt a vertical force of 1000 lbs provide a sideways force of 1500? tire traction is non-linear at some point, at which the tire will break loose and skid -the example you quoted sounds like the tire is just ready to slide at 750 lbs of side force.

Tire friction is ALWAYS non-linear. The F=uN friction model you're using doesn't apply to viscoelastically deformable materials like rubber.

A tire's "coefficient of friction" is a function of normal force, and is inversely related thereto.

What Monza and rayb posted is exactly correct; as a tire is loaded more heavily its "G available" (ratio of max lateral force to normal force) decreases.

Lighter cars can grip better, in general. There are a LOT of other variables involved, like tire temperature (race tires used on a car that's too light never get up to temperature and don't perform well).

Heavy cars CAN handle well. Making heavy cars lighter overall will make them handle better.

quote Originally posted by Ken Wittlief: so inertia is also directly proportional to Mass (weight)

Let me say this first.. I am not all that great with science.. i am not singling you out you just put it the best and easiest to quote..

i guess alot of people forget weight and mass are 2 completly different things.. and ive seen alot of people in this post mix the 2 up

mass is the matter that makes up a object..

weight is the gravitational pull on mass..

that is why we weigh less on the moon but still have the same amount of mass

in my opinion without naming laws or theroys is that more weight in a handeling perspective will benfit and hurt the driver in different situations..

lets say you put 200lbs of cement in the back of the fiero.. (it probably wont fit but if it did).. it would keep your tires on the ground better.

IT would be easier to climb rainy hills or keep traction on snow roads..

even on a turn it will help keep your tires planted..

BUT when your tires do loose traction and your ass end does come out it will be harder to overcome the fishtail and keep your car straight..

to stop an object from moving you need a equal or greater opposing force. more weight = more force needed.

thats my point i guess.

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quote Originally posted by warden: even on a turn it will help keep your tires planted..

No, it won't. The extra weight on the tires will reduce the "G available" from the tires which will reduce cornering performance, as well as upsetting front/rear contact patch loading and making the car more likely to spin out.

Yes, the extra weight would provide more traction in snow, but that's another matter. More traction in rain (resistance to hydroplaning) is largely a function of tire pressure.

Nice try. The only situations in which you need to distinguish between weight and mass are during transitions in which weight transfer changes dynamic weight distribution temporarily, and in the case of race cars with heavy duty aerodynaimcs packages that increase force on the tires well beyond vehicle weight without increasing the mass that has to be accelerated.

[This message has been edited by Will (edited 08-04-2003).]

Hmmmmm.....

If you remove 200 pounds from a Fiero, does it handle better?

Obviously, it will accelerate better, and also it will brake better.

But what will happen to the car's handling?

OK, I think I am VERY qualified to answer this question, being as I have removed more weight from a Fiero than almost anyone else so far.

And, the answer is:

Assuming that you have the car very well balanced, with reasonably neutral handling to start with, the handling, (cornering force) will get worse!

What? Worse! Yep, Worse. The G-force number that can be generated on a skid pad will decrease.

Why? Because there is less weight to develop cornering power with the current suspension. In order to get the cornering force back the overall suspension rate at each wheel must be decreased by the same percentage as the decrease in weight at each wheel. Also, the unsprung weight should also be decreased proportionately, (assuming the 200 pounds was removed totaly from sprung weight), to get back to the same handling the car had originally.

Of course, for example, if your rear springs are already too soft, then removing weight off the rear may increase the cars' handling potential. (more than stiffening the rear springs would)

Here is a list of most things that affect traction available at each wheel:
Keep in mind that each has practical limits in it's effectiveness.

tire compound, softer = greater traction

tread surface area, more/wider = greater traction

unsprung weight (tires, wheels, brakes, spindles, roughly half of A-arms, tie rods, shocks, etc.), less = greater traction

spring rate, softer = greater traction

Anti sway bar rate, softer = greater traction

shock rate, softer = greater traction

car sprung weight, more = greater traction

Keep in mind also that we are using the term traction (at each wheel) to define handling for the purpose of this discussion. I feel this is appropriate, becasue not only cornering force, but acceleration force, braking force, ability to change directions rapidly, and any other criteria you want to call "handling" of a car all depend greatly on the available traction at each wheel under varying conditions.

When I dropped my Fiero's weigth from 2500 pounds to 1700 pounds, the handling got terrible, compared to the other 2500 pound Fiero I had autocrossed the year before. I have been reducing spring rates to where the front has 200 pound/inch springs, and the rear has 250 pound/inch springs, and I will be going softer yet, just to get some of the traction back for cornering, braking, etc.

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Gerald Storvik

http://www.8shark.com

Phew I just read all of this and you Fiero guys are very smart. This is good stuff. I like challenging questions.

Ha haaa! I just knew our crowd would not let us down.

Benno

Glad you enjoyed it....

Here is an extra bump in case someone missed it, or has extra input.

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Gerald Storvik

http://www.8shark.com

Rather than try to explain a the complicated relationship between weight, weight transfer and tire theory, and handling I would like to recommend that anyone interested go to your local bookstore and buy "SPEED SECRETS Professional Race Driving Techniques", by Ross Bently. ($15.95) Chapter 1, is about car theory. I learned a lot from it.

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Owner since December 6, 2002, 1st Fiero, 1987 SE, 2.5, Auto.

oh HANDLING!

I thought you meant, like, shipping and handling :c)

shark93726 what did you do to that poor car??
800 lbs is more like starvation then a diet
the first thing that comes to mind is you removed too much metal making the frame weaker and allowing it to flex too much
then you went tooo weak on springs and bars trying to compensate

or you got the front rear or side to side weights tooooo far out of wack
balance is very important in handeling tooo

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Question wonder and be wierd
are you kind?

RogerGarrison was most correct in a practical sense; weight distribution is a key factor in handling. All the mathematical exercises are just that; exercises. If we are talking about Fieros, throw out all the math, we're talking Fieros, not a Porsche or a Ferrari where all the balancing and weight distribution, proportional torque and balancing engine weight and rear end weight are factored in during design. The Fiero was a "what's on the shelf" fabrication. It didn't matter if the parts were absolutely correct, as long as they fit, they were used.
So-what are we dealing with? A car with a low center of gravity (a plus); a car with a wide wheel base (a plus) and until 1988,a car with a really less than par suspension. To compensate for some of the negatives, I have tried and am very pleased with the following: Double up on the front steering stabilizer (the difference is remarkable) and I have ADDED a 35 lb. steel plate from a weight set in the front. I bolted the 35 lb. plate to the inside (hollow side) of the donut spare. Located here, the weight takes up NO room. 50 lbs. would probably be better, but with the weight securely fastened to the spare, God forbid you had a flat and need the spare, with the 35lb. weight and the spare,it already weighs over fifty pounds- not easy to get out.
This arrangement may seem somewhat unusual, but it helps even out the front/rear weight distribution, and the car handles better than it did before.

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I'm going to address a few points here.

Stiffening the suspension too much will make a car handle worse. Why? Because if the roll rate (combination of spring rates and sway bar rates when the car is rolling sideways) is too high then the car transfers too much weight to the outside tires. That loads the outside tire much more than the inside tire.

Because of what Ira and Will pointed out, a car will have greater cornering grip if both inside and outside tires are loaded evenly.

Gerald's problem was that the spring rates were now too stiff for the weight of the car. He was transferring too much weight to the outside tires and his cornering traction was reduced. This got better when he reduced the spring rates accordingly.

Firefighter, the reason adding weight to the front worked for you is not because it evened out the weight bias. By adding weight, you did the opposite of what Gerald did -- you now have less spring rate per pound of front weight. You could have gotten the exact same result by softening the front springs instead of adding the weight.

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Doug Chase
'88 Fiero Formula 5-spd (autocrosser)
'88 Fiero GT 5-spd (daily driver)
'85 Fiero GT 5-spd (rally car)
'87 Fiero SE V6 5-spd (for sale $2200)
'88 Fiero Formula 5-spd (parts car / future rally car #2)
Custom roll cage fabrication available
Custom exhaust fabrication available

Yes-whenI noted above (prior post) that I added weight to the front, I knew about lessening the spring rate per pound. It's based on a mathematical combination which is used for the volume of a conical device or a cylinder. It takes into consideration the tensil strength of the spring, taper (if any) and the spacing between the coils. Goes like this:V=1/3 Pi r(2)h. Actually, I ran out of the magical "spring softening" Pixie Dust I needed, so I just added the weight.

Wider rear tires have the same effect

"throw out the math"

?!

as long as your car exists in our space/time the laws of physics apply.

if you dont get the right answers from the math, you are using the wrong equation

this is the difference between engineers and shade tree mechanics - sure you can make a small change and try it, change it somemore and try it, change it some more... until you stumble over the correct combination of variables

or do the math and get it right the first time. :c)

Ray B: What I did is documented on my web page.

But the short version is that I removed the entire interior, heater, air conditioning, all glass, most of the roof, the doors, engine cradle, gas tank, the ribbing under the hood and rear deck lid, the trunk, and most of the frame behind the motor compartment. The door openings were reinforced and braced with 2" steel tubing, and the back edge of the motor compartment was also reinforced and braced with 1" square steel tubing. The entire rear strut suspension has also been changed to a modified De Dion suspension.

So, actually, the frame should be stiffer than the original configuration. The engine is all aluminum, about 250 pounds, with about 200 hp. A rear sway bar has also been added, to balance the handling.

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Gerald Storvik
http://www.8shark.com

[This message has been edited by shark93726 (edited 08-06-2003).]

I think we have a couple of conflicting issues here. Cornering power is just g-numbers, it is the result of overall grip in a steady state corner. Handling is a far more subjective combination dynamic balance and "feel". Reducing weight by 200lbs without any compensation in the suspension will result in a more responsive car because of reduced inertia, however, as Gerald pointed out, cornering power may suffer because the springs now transfer too much weight to the outside wheels overloading them at lower g-numbers.

Ken, I have the graph showing the relationship between tire vertical load and lateral acceleration, however my scanner just decided to be uncooperative. It is in Herb Adams' book. The relationship is not linear, it is closer to a logarithmic curve, steep at first but flattening out.

Many drivers stiffen up their cars springs and sway bars to improve subjective feel, the result is a more responsive car, but with lower cornering limits. Adding roll stiffness to one end of the car increases grip at the other end, that is why a rear sway bar reduces understeer. These are however steady state improvements, transient response is a far more complex issue since there are far more variables. Steady state cornering is basically a juggling act with two objects, inertia and grip (yes it is oversimplified but give me a break here), transient response adds other objects such as dynamic weight ditribution under acceleration or deceleration, changes in spring rate due to ride height changes (with variable rate springs), alignment changes due to ride height variations, polar moment, tire distortion under acceleration or deceleration, etc...

As you can see you are left with one big decision, do you want ultimate cornering power or do you want a stable fun car to drive. BMW has made its reputuation by building cars that feel right, they are seldom the fastest cornering cars in a group of competitors, but they are predictable and consistent: Confidence building. Little quirks like the drop throttle oversteer (not a steady state issue but a transient one), and bump steer (again a transient issue brought on by road surface irregularity effecting wheel alignment), are not confidence builders, BUT they do little to compromise the cornering power available.

Gerald's "diet" is a race only alternative, if you read his site you will realize that unsprung weight and spring rate are critical issues, especially with a very light car.

The only big plus of the stock (84-87)fiero's brakes are that they are very light. Add the fairly light stock 14X6 wheels and use appropriately sized tires (on the light theme, lets say 185/60 front and 205/60 rear) and you will probably have a slight improvement over the larger tires in response, even though cornering power would probably be lower.

I'm rambling here now, let me hear what you guys have to say about these little observations.

Ira

what about when your tires break loose, and you are in a four wheel drift (power-slide)?

there is more than one aspect to cornering - breaking loose is not the end of the world if you car doenst spin out, right?

and in the real world (driving on unfamiliar roads) that is more likely to happen than doing loops on a track.

quote Originally posted by Monza76: Gerald's "diet" is a race only alternative, if you read his site you will realize that unsprung weight and spring rate are critical issues, especially with a very light car.

On a similar note, the Lotus Elise (1800 lbs wet) is fairly softly sprung with a larger degree of body roll than one would expect, but is still one of the best handling cars around.

Here is where this gets really wierd, take a base modle Fiero with the softest available spring rates, remove the sway bar, mount wide sticky tires on very light wheels, use a tire about 10% to 15% wider at the rear and you will have a car which has lots of body roll but sticks like glue. At this point the shocks and struts are the biggest limiting factor.

I do not recommend this setup because it is a steady state car, in transient response the low roll stiffness will allow too much dynamic weight transfer, it would probably display some awkwardness in a slalom, except of course for those struts and shocks again. If these parts can effectively slow the weight transfer it will handle well.

I had a 1980 VW Rabbit (German built) it had soft springs and no sway bars (except for the rear twist axle) but very good shock control. That car could handily outcorner almost every car in its class.

As for weight distribution, the Porsche 911 is arguably the most successful sports/GT class race car of all time, with all that weight hanging out back. Weight distribution is important, I am not implying it is not, but as Will points out it is just one factor in a very complex equation.

Just consider farther, MacPherson struts are a cheap unsophisticated front suspension for economy cars and Porsche 911s and the simplest independent rear suspension this side of swing axles are semi trailing arms like the older Porsche 911, so you take a car with a rediculously short wheelbase, hang the engine way out back, put econo-car suspension designs on it and what do you have? Pure magic in the hands of the right driver. The real answer is in the details, and that is why, set up right, the Fiero can handle.

Sorry, drifted away from the point for a moment, it all comes down to one simple fact. If you make any changes to a car, you will have to make more changes to accomodate them. If you lose weight you will need softer springs...etc... Same applies to engine performance.

Ira

Ken - Sorry, I didn't mean to ruffle any feathers when I said "throw out all the math".I am not an engineer as you must be and also not a shade tree mechanic. I am however a math aficionado from Quantum Mechanics to the Heisenberg Uncertainty Principle, geometric analysis and the father of a Mensa math scholar. I love mathematics and have a great respect for it and its proponents. But I do have a very pragmatic side when it comes to dealing with the car I love - the Fiero. So, restated- "Throw out half the math". Mathematics is an exact science with few if any issues that are conflicting or unsolvable. However, dealing with a Fiero and its numerous issues is an ART; perhaps half science at best. If math covered all of our issues, the Forum would have half the questions it presently has, since no one would be seeking the opinions of others and learn of their efforts and successes gained through trial and error. Each question asked would have only one answer. The first mathematician to respond would always WIN, no other answers or opinions would be necessary. The Forum would loose all of its "charm" and "character" if we couldn't express our our opinions in this "Art Form". In my former post, I only expressed my opinion as did you previously. Again, I did not mean to contradict anyone.
Oh- Know of any good "shade tree mechanics?"

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when the roof went, so did alot of the stiffness in the frame/unitbody
you now have a flexing car that cannot handel
the tork loads of stiff springs so softer setups will help BUT the loss of a stiff frame is the major problem.
I would try to reinstall the roof metal or build a FULL cage to replace it to get back frame stiffness and stop flexing/twisting that is hurting the handeling

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Question wonder and be wierd
are you kind?