I am kinda surprized that no one has posted that decelerating does not actually transfer weight to the front. It is the action of the brakes on the wheels and the suspension giving way (brake dive). Its the torsional force on the tire that lifts the chasis at the front wheels. If you were able to somehow rotate the wheels in the oposite direction and go foward, when you brake the "weight" would be transfered to the rear...

Every action has an oposite reaction. The tortional force is negated by the pull of gravity on the rear of the chasis. Therefore, your total braking capacity given infinite friction tires is the force needed to lift the car past the forward-backward center of gravity at the front wheels.

Just something to think about.

------------------
'84 3800SC, XP cam, 3.2 pully, HPTuners, N* TB, 4sp, SPEC 3+ clutch, Aluminium flywheel, pacesetter headers, F355 body kit, weight reduction

^ ?
really?

so, if I had a bathtub full of water on top of my car, and zero give suspension, the water would not shift to the front of the tub (and spill out) when I hit the brakes?

it is true that there is no physical movement of weight. and, yes, it does also load up as a torsional force - the overall affect is known as weight shift. if you were able to actually measure the weight of all four wheels during a braking event, you would in fact see more weight on the front wheels, and less on the rear.

quote Originally posted by Pyrthian: ^ ? really? so, if I had a bathtub full of water on top of my car, and zero give suspension, the water would not shift to the front of the tub (and spill out) when I hit the brakes? it is true that there is no physical movement of weight. and, yes, it does also load up as a torsional force - the overall affect is known as weight shift. if you were able to actually measure the weight of all four wheels during a braking event, you would in fact see more weight on the front wheels, and less on the rear.

If you were to secure the water in place i.e. freeze it, no the water would not shift (aside from compression, but thats kinda extreme). If you were to apply the stoping force along the center line running front to back, it would have nearly the same weight distribution as it did when it was moving.

How does a plane or bullet slow down without falling end over end? Thats cause they don't use tortional forces to slow down. It uses lateral friction (drag).
Or better yet a shuttle in space? It should go into an extreme spin when it tried to connect to the space station, but it doesn't cause no weight is transfered.

Weight transfer cannot be avoided. It is the simple consequence of accelerating something (not thru the CG) with mass.

The front suspension's compression during braking is simply a result of the weight transfer.

You could have a front suspension with 100% anti-dive. It would not compress at all during braking.

However, the same amount of weight transfer to the front is still there, regardless of the anti-dive percentage.

[This message has been edited by pmbrunelle (edited 10-05-2009).]

quote Originally posted by Dracor: If you were to secure the water in place i.e. freeze it, no the water would not shift (aside from compression, but thats kinda extreme). If you were to apply the stoping force along the center line running front to back, it would have nearly the same weight distribution as it did when it was moving. How does a plane or bullet slow down without falling end over end? Thats cause they don't use tortional forces to slow down. It uses lateral friction (drag). Or better yet a shuttle in space? It should go into an extreme spin when it tried to connect to the space station, but it doesn't cause no weight is transfered.

You sure you want to stick with that no weight transfer theory?

quote Originally posted by Formula88: You sure you want to stick with that no weight transfer theory?

Yes, if you were to put a jet engine in that bike for reverse thrust it would slow down with out doing a wheel stand. Again explain the shuttle and airplanes....they don't do wheelies when slowing down.....

An airplane's engine's thrust can be directed to push more or less directly on the centre of gravity and centre of aerodynamic pressure.

A wheeled vehicle has its contact patches on the road, wayyy below the CG.

[This message has been edited by pmbrunelle (edited 10-05-2009).]

quote Originally posted by Dracor: Yes, if you were to put a jet engine in that bike for reverse thrust it would slow down with out doing a wheel stand. Again explain the shuttle and airplanes....they don't do wheelies when slowing down.....

it is inertia
in an airplane and in the shuttle, you do feel the the braking force pulling you forward
another fine airplane example is the "vomit comet". doing weight shifts to the point of interior weightlessness. then the opposite, doing weight shifts to make feel like you weigh more than you actually do. it is the same force at work: inertia
noone is claiming the car/airplane/vehicle changes actual weights. it is the forces of inertia creating additional loads which do not exist when not in motion.

Learn the difference between mass and weight.

So lowering your car not only improves aerodynamics, and handling, but also braking? (Since there will be less front end diving?)

quote Originally posted by RotrexFiero: So lowering your car not only improves aerodynamics, and handling, but also braking? (Since there will be less front end diving?)

If the braking system is tuned properly, then yes. Tires are more sticky when lightly loaded. One way to reduce the load on the front tires when braking is to lower the car.

It will degrade the theoretical stopping distance of a Fiero with unmodified brakes.

However, as the car pitches less, the pedal becomes easier to modulate. It's a bit of a wash-out IMO.

Also, there seems to be confusion regarding weight transfer. It is not the same thing as nosedive!

[This message has been edited by pmbrunelle (edited 10-05-2009).]

quote Originally posted by Pyrthian: it is inertia in an airplane and in the shuttle, you do feel the the braking force pulling you forward another fine airplane example is the "vomit comet". doing weight shifts to the point of interior weightlessness. then the opposite, doing weight shifts to make feel like you weigh more than you actually do. it is the same force at work: inertia noone is claiming the car/airplane/vehicle changes actual weights. it is the forces of inertia creating additional loads which do not exist when not in motion.

You're getting close here. Yes inertia is the force, but when you slow down you feel a "pull" forward, not down. You might feel a slight lift in the seat due to the car rotating on its front wheels.

I wish so badly that I could post pics at work. Look back at your college/high school physics. Rotational forces are in a straight line, not in a circle. The counter force for a wheel going forward on a car is UP behind the wheel (down in front of it). Even if you rotate the caliper on the disk, its still going to be the same counter force. The caliper is lifting up on its mounts which pulls the back end up, the rotational force on the chasis pull the back wheels from the ground allowing for less friction.

Here is an interesting thought, why are disks always to the inside of the car? Thats cause it moves the pivot fulcrum of the chasis closer to the center relative to the mounting points of the caliper.

Draw up a vector diagram of the wheel to the chasis, and see where the forces go.

Remember that weight is the resulting force pushing down so when we speak of weight transfer, it doesn’t mean that anything moved (as in water in a bath tub). Weight transfer means that you had some percentage of the car’s weight on the front and rear axles and then it changed do to acceleration or deceleration.

Also, I think we for this discussion we should assume the mechanism causing acceleration and deceleration involves the tires only, no rocket motors, motorized fans or aerodynamic speed brakes.

Imagine that you built a car to run on a track such that part of the car could hang below the height of the tire contact patch.
If the center of mass were made to be at the same height as the contact patch there would be no weight transfer when accelerating or decelerating.
If the center of mass were below the height of the contact patch, weight (the downward force) would shift to the back when decelerating and forward when accelerating. That’s backward from the real world.
Motorcycles and regular bicycles are subject to these effects more because their center of mass is higher.
It is the same with cornering. Because the center of mass is above the road, weight is transferred to the outside wheel. That is why a car leans and a bike rider leans the other way to compensate. That is also why Indy cars are nonsymmetrical. They only turn in one direction.

[This message has been edited by gem1138 (edited 10-05-2009).]

quote Originally posted by Dracor: You're getting close here. Yes inertia is the force, but when you slow down you feel a "pull" forward, not down. You might feel a slight lift in the seat due to the car rotating on its front wheels. I wish so badly that I could post pics at work. Look back at your college/high school physics. Rotational forces are in a straight line, not in a circle. The counter force for a wheel going forward on a car is UP behind the wheel (down in front of it). Even if you rotate the caliper on the disk, its still going to be the same counter force. The caliper is lifting up on its mounts which pulls the back end up, the rotational force on the chasis pull the back wheels from the ground allowing for less friction. Here is an interesting thought, why are disks always to the inside of the car? Thats cause it moves the pivot fulcrum of the chasis closer to the center relative to the mounting points of the caliper. Draw up a vector diagram of the wheel to the chasis, and see where the forces go.

Caliper placement is irrelevant for the purposes of our discussion...mainly the result of packaging issues...
I think you need to review your own textbooks.

quote Originally posted by Dracor: Yes, if you were to put a jet engine in that bike for reverse thrust it would slow down with out doing a wheel stand. Again explain the shuttle and airplanes....they don't do wheelies when slowing down.....

You probably think the airplane wouldn't fly off the conveyor belt either.

The shuttle and airplanes don't slow down via their wheels (well, the shuttle does have landing gear brakes, but doesn't use them in orbit ).

Ok, this will be my last attempt to explain this.

I never said the weight doesn't shift on a car ...I said IT DOESN'T COME FROM acceleration/deceleration.

Here is a bad picture to illustrate the forces. If my colors are off, thats cause im colorblind.


The red are the forces at work. The yellow is the pivot point. The blue is direction of rotation. When torque is applied to the chassis via the brakes (the box) it causes the rear to lift. Just like when a dragster takes off, the back has the torque so it raises the front end. Its how motocross riders control their pitch in midair by revving or braking the rear wheel (an effective weightless environment).

Yes, I understand the difference between weight and mass. Yes, the end result is more weight is on the front wheels, but the Op asked WHY. This is WHY.

Sorry "Magic" is not a good enough answer for me. I want to know why the front end is diving down on a car and not other vehicles.

If you understand component forces and simple machines, this is obvious, but I don't have time to explain that.

quote Originally posted by av8fiero: Certainly if you reduce said front bias too much you will negatively affect braking performance and car control, but as long as you don't go too far you will increase braking effectiveness. Reducing said front bias wouldn't really affect braking in a light braking instance anyway so this comment is really irrelevant to this discussion.

It's the emergency situation that I'm worried about. I couldn't care less about normal braking. It's maximum braking, where I'd be the most worried about spinning out. When braking slowly there is room for error. (or at least time to get off the brakes and let the car settle again)

Factory braking systems have front bias well in excess of what would be required for optimum braking, therefore reducing this excess bias improves your braking. That is all i'm talking about. Unless a vehicle has abs the fronts are almost certainly going to lock well before the rears wet or dry in a factory braking system. If you're designing a braking system from scratch you're mostly correct in your statements. I'm talking about modifing a factory braking system to work better than as it was originally designed and delivered by the factory. A factory braking system has too much front bias. The fronts WILL lock first wet or dry. Signifigant front bias is absolutely necessary in any braking system for ultimate braking performance, I'm simply stating you can improve a FACTORY braking system by reducing some of the front bias. Have you done ANY roadracing or autocrossing? I'm sure if you have you know a factory system is not optimized for braking efficiency, it's optimized for average drivers to have as much car control as possible with a WIDE saftey margin built in as well. ALL I'm saying is that you can reduce that saftey margin and improve your braking performance

------------------
88blackchopv8

[This message has been edited by av8fiero (edited 10-05-2009).]

Not to labor this but to understand the daunting task that average brakes must perform (not to mention do this again and again without failure) check out this website.

http://www.1728.com/energy.htm

You can calculate how much energy is developed by a car (Fiero) in motion.

No need for an online calculator:

E = 1/2 m v^2

FWIW... I have archived somewhere that the typical car will experience the effect of approximately 17% weight transfer under hard braking. So, assuming for the sake of this discussion that 17% is accurate, a FWD car that has 63% of the weight on the front wheels would effectively have 80% of the weight on the front wheels during hard braking. Rear brakes are not very effective, and prone to lock up (even with stock proportioning valves).

A rear engine car with a 60-40 rear (static) weight would effectively shift to 57% front.

Under hard (forward) acceleration, the weight shift is towards the rear. Drag racers figured this out decades ago. And part of the reason FWD cars (at least in stock form) have more difficulty getting off the line.

Suspension changes can affect the percentage shift, but I do not know (and doubt) that the same changes would have the same effect on all cars.

While the foregoing might lead to the conclusion that a rear wheel drive car is ideal, it also assumes straight line braking. Adding turns creates another set of dynamics that come into play. In the end, a mid (rear) engine car offers the best compromise.

------------------
FierOmar

quote Originally posted by Austrian Import: Everybody always says the caliper bore size on the rear is slightly smaller than the front. Could someone be specific? By HOW MUCH?

49mm Front
45mm Rear

Assuming 4:1 pedal and 100 pounds of force from your foot, that would yield 2977.6 pounds of clamping force for the front, and 2511.2 pounds for the rear. This is not including the brake booster, or any bias produced from the prop valve.

Borrowed from Ogres cave: http://home.comcast.net/~fierocave/

Read it all, and then comment.

quote Originally posted by Freshj: 49mm Front 45mm Rear Assuming 4:1 pedal and 100 pounds of force from your foot, that would yield 2977.6 pounds of clamping force for the front, and 2511.2 pounds for the rear. This is not including the brake booster, or any bias produced from the prop valve. Borrowed from Ogres cave: http://home.comcast.net/~fierocave/ Read it all, and then comment.

closer to 48mm rear if my memory serves me right. At least for '84-'87.

Really, this brake discussion is getting ridiculous. Deceleration is the same as acceleration only applied in the opposite direction of movement. Anyway, as the CoG is higher than the contact patch you have a moment. Therefore, the load on the front wheels will increase and the load on the rear wheels will decrease. The opposite will happen when accelerating. This is regardless of whether or not the front end dives. Just pick up a pen and a paper and start calculating, it's really very simple. Compare a static vehicle with one accelerating at 1G and one decelerating at 1G. After you've done that, consider not continuing the discussion. I know I will. consider not continuing this discussion that is.

Yeah, I agree with you. I basically give up on butting into brake-related threads. It's apparently impossible to teach a lot of people simple concepts.

[This message has been edited by pmbrunelle (edited 10-08-2009).]