i think there is a difference from using your ebrake vs LFBing

ebrake looses rear wheel traction and doesnt affect weight transfer much it basicly pulls the car back

LFBing affects weight transfer alot to the front causing better traction up front and loose of traction in the rear

with the ebrake you dont get the traction enhancments up front just the loose of it in the rear

edgar,
i could be wrong

 

nope your car will still hold boost if your on the trottle

havent you seen any racing? from speed touring car to rally keeping the revs up right befor the green flag comes out and dumping the clutch on high revs is the key to jumping out in front with a turbo car...

edgar,

 

It's all about weight transfer...that's all it is. Same as e-brake, except the real brakes work much better. The reason you would want to use your left foot instead of just RFtrail braking is to keep the revs high to keep the turbo juiced. If you are accelerating while braking, the weight bias is still to the back of the car so it won't rotate the car. The only way to change the weight bias of the car is to apply a force. The only way to do that is to apply an acceleration. Three ways to accelerate: brake, gas, or turn. If you want the weight forward (so the rear will get light and slide), you have to brake. If you want to be able to resume at max boost right after you slide the rear out, you need to keep the engine in the rev range where it has full boost. You CAN do this with heel-toe technique (just don't hit the clutch or shift when you're heel-toeing)... but sometimes it's easier to just use your left foot to brake instead (if you don't need to shift or if you have an automatic). It is phyically impossible for LFB to be different than heel-toe (heel-toe without shifting that is) if you apply the same pressure to the brake and gas pedals in both techniques.
I hope that's clear...dunno though, I'm not great at written explanations like this.

 

The "weight transfer" theory is true, but only applies if the car is deccelerating. If you are speeding up or maintaining speed then there is no "weight transfer".

 

What's really going on in LFB is an effect of brake bias. Brakes on most cars are set up to exert about twice as much torque on the fronts as on the rears.

So let's say you have AWD with an open diff (50/50 torque split). You hit the gas and get (for example) 2 units of torque on each wheel.

You decide to LFB. You send one unit of torque to the rears and (due to brake bias), you send two units to the fronts.

The brake torque on the fronts (2 units) cancels out the driving torque (2 units).

On the rears, you subtract the braking torque (1 unit) from the driving torque (2 units) which leaves you one net unit of driving torque.

Front Net Driving Torque: 0 Unit
Rear Net Driving Torque: 1 Unit

You have now taken a 50/50 torque split AWD system and turned it into RWD. Which allows you to induce power-on oversteer, which you may know by experience or by working out the traction circles.

In WRC they actually use a different diff map for LFB, which sends even more torque to the rears, to make the effect stronger. For those guys, LFB is like an "Oversteer Button".

The other benefit of LFB (in turbosupercharged cars) is keeping the engine loaded during situtations where the driver does not wish to accelerate (steady state cornering). Which obviously keeps boost up, allowing more rapid acceleration when exiting the corner.

The downside of LFB is brake heat and brake wear, and fuel consumption, because the engine and brakes are fighting each other.

I learned LFB in karting, driving single-speed karts with a centrifugal wet clutch and a narrow powerband. You can use LFB to keep the revs up in low speed corners because the clutch will slip. When you release the brake, the revs are already high, and the kart takes off nicely. We never had problems with brakes or the clutch, so it was basically free horsepower.

 

Thats exactly what I wanted to know, thanks for the great explanation! It all makes sense to me now!!!

So it isn't really weight transfer thats doing it since you CAN accelerate while inducing oversteer through LFB. much thanks!

 

That's where I think the problem with the class comes in too. The presentator just acted as if the brake forces are the same for all 4 wheels.

 

It's pretty unlikely that you would get the same brake torque on the rears as on the fronts. If you set a car up like that, it will want to spin under braking, unless you have huge rear tires or a strong rearward weight bias. I think even F1 cars have something like 60/40 front/rear brake bias.

 

This is the coolest thread EVAR on i-club.

Someone asking a question and people stepping up to lend a hand with real friggin' info. Props to all of ya.

 

Kravda, I should be able to answer your question about weight transfer, more specificallY WHY weight transfers.



No. weight transfer applies when you have more than 1 wheel.

Think about a unicycle. You could accelerate and be pushed back, but WEIGHT would be push back.

It's because in a vehicle contacting the ground in front and rear points can't just
have weight get "pushed back" into thin air. In a car, the center of gravity will be "pushed back"


So in acceleration, we can draw a force vector pointing backwards from the center of gravity.

Looking at a side view of the car, you can translate the force vector into a bunch of vectors pointing at a tangent, clockwise.

Now let's conduct analysis on what happening at the front and rear of the car. You can now see that a these "clockwise" vectors will be pointing UP at the front wheels, and DOWN at the rear wheels.

Thus, force is pushing down at the rear wheels and up on the front.


This is the reason why weight transfers during accelerating and braking





Kravda, if you're in college, just go ask a mechanical engineering professor. Every college should have an ME professor whose specialty is dynamic systems, and he should pretty much know a lot about cars.

 

Because the error that a lot of people make with researching stuff about cars is that everything is an ABSTRACTION. For example we can approximate power being generated by an engine as the amount of joules created when a volume of gasoline burns, and then multiple that by the number of cylinders and rpm's. But doing so would net you a linear torque and horsepower curve, which is obviously incorrect. You need to be careful in what principles you can UNIVERSALLY apply (physical theories), and abstractions created to aid in the layman's understanding of something.


So by all means, if you're still interested, there's a lot of cool stuff and textbooks you can pick up about things like suspension and braking components. But beware, you will encounter quite a lot of math, because if you take any physical object and start analyzing it, sooner or later you'll find yourself modelling its physical aspects with linear algebra, differential equations, etc. If you take waveforms and start analyzing them...eh whatever I'll shut up now.

 

Is this for AWD? Wouldn't your rear tires have some accelerating force also? And can a tire be accelerating and decelerating at the same time?- (the .5g-.4g= .1g thing) Wouldn't the .5g acceleration go away once the brake is applied? This explaination kinda makes sense, but it's hard to understand and very possibly flawed in the physics behind it. I'm not sure how much physics the instructors really know- they do know how to drive, and have real-life experience- that's whats important.

From my knowledge of physics (BTW it's my major in college - And go ahead, call me a nerd ), the first thing that comes to mind is this:

Since the front brakes are doing most of the braking, it causes the front tires to 'stick' moreso than the rears, and under lateral acceleration (cornering), the rear tires are therefore more apt to deviate from the circular path and 'swing out,' following more or less the tangent to the circle.

The friction of your tires is the force that causes centripital acceleration (vector pointed toward center of circle). This force keeps you traveling in a circular path... so, since your front tires are experiencing more of this frictional force under braking, they can still follow the curve. Since the rears don't have enough of that frictional force, they lose traction and the rear of the car slides out.

This all follows the 'ball on a string' model, where if one swings a ball attached to a string in a circular path (think lasso), when released, the ball continues in a straight path, tangent to the curve. In this case, the tension in the string is the force causing the centripetal acceleration, keeping the ball from deviating from the circular path.

And since frictional forces are dependent on mass, weight transfer must also be a factor, as others have already noted. But...We'll save that lesson for another day.

Physics ain't so hard to understand now, is it?

 

Nah, physics rocks dude! A person with a solid background in physics and biology is going to be a baller in the years to come.

lol. I entered college planning to be a physics major until I almost killed myself wearing metal in the wrong place at the wrong time working at the SLAC accelerator summer after my freshman year. That and the depressing mood of the physicists there made me promptly switch majors. I admire you for sticking it to the man!


That's why I switched to electrical engineering, where the danger of my arm being torn apart walking through some huge magnetic field wearing a watch is now nonexistant, and the probably of me being killed by electric shock is almost certain.