Left Foot Breaking, can someone explain?

[Nenukknak]

All right we all know about left foot braking and how it is particularly handy in steering 4wd and Fwd cars, especially on slippery surfaces as gravel or snow. But why and also a bit how does it work?

[Zico]

At its most basic purpose, left-foot braking is used to decrease the time spent between the right foot moving between the brake and throttle pedals, it is also be used to control load transfer, as Pentti would say.. "Brake to steer". Combined it offers far more car control simply because the brake and throttle inputs and thus the balance of the car can be juggled in real time with zero foot transfer delay.

In WRC cars the active centre diff means that by pressing both pedals at the same time and activating a certain mapping based on throttle and brake percentages ie, the centre diff could send torque to the rear to induce oversteer.

[Zico]

Theres a full article on the subject on Penttis website, see here.. http://www.leftfootbraking.com/hpcnews.htm

[Koz]

How come Pentti isnt around on the forum these days?

[salmo]

At its most basic purpose, left-foot braking is used to decrease the time spent between the right foot moving between the brake and throttle pedals, it is also be used to control load transfer, as Pentti would say.

As someone who uses left-foot braking in rallycross, and to a lesser extent autocross, I would say it's entirely about weight transfer. The time spent moving from one pedal to the other doesn't enter into it.

In a nutshell, for me (and I think for most people), LFB is most useful on slippery surfaces as a means to get the car to rotate. For example, imagine approaching a high-speed turn on snow. You turn the steering wheel to start the turn, and nothing happens. Insufficient grip on the front wheels causes them to have little or no effect, and you understeer straight off the road. LFB allows you to transfer weight to the front wheels, giving them more grip, and allowing you to make the turn. You could do the same thing with right-foot braking, but then you have lost speed because you have taken your foot off the throttle. LFB allows the weight transfer to the front with little loss of speed.

I don't use LFB on the track, because there is so much more grip there, and because that extra grip makes it more difficult to use LFB effectively. However, the effect still holds, and I do know race drivers who use LFB for any corner that does not require downshifting.

[Nenukknak]

Ok Salmo, so you keep your foot on the throttle as well? And then apply the breaks simultaneously? Or do you play around with both, pressing and releasing as necessary? Or is one kept under constant pressure, while the other is being "pumped"?

[salmo]

Ok Salmo, so you keep your foot on the throttle as well? And then apply the breaks simultaneously? Or do you play around with both, pressing and releasing as necessary? Or is one kept under constant pressure, while the other is being "pumped"?

You're pressing both pedals at the same time, but modulating pressure on each of them. Usually as I'm applying more braking pressure I'll lift more on the throttle, but not always. You absolutely don't want to be pumping the brakes -- that should be a slow and steady squeeze on and off. The right foot on the throttle has more latitude in what you can do with it.

It's actually one of the more difficult things I've tried to learn as a driver, and I am still learning it.

[Zico]

LFB works in different ways for FWD, RWD and 4WD in addition to what Ive mentioned above regarding weight transfer... it also goes beyond that.. something I experienced on a previous test when I spun the car. I couldnt understand why and had to study the physics of what was going on, so this is my understanding of it working with a plate type LSD at least..

Most rwd rally cars have a 2 way LSD with 2 different tighten/lock ramp rates for acceleration and braking, ie depending on the amount of torque applied in either direction it tightens/locks. More agressively on acceleration than on the brakes but by applying the accelerator and the brake simultaniously you can lock the LSD and the rear of the car will become even looser than is usually achieved simply by braking and weight transfer.

With a FWD car I believe what you are doing is effectively adjusting the brake balance.. by applying both accelerator and brake simultaniously you are effectively adjusting that brake balance to the rear at that instant, handy when you really need to keep both hands on the wheel on a difficult section.

4Wd Ive already covered..


Its also an interesting subject to me, so if anyone can elaborate further please do..

[Zico]

As someone who uses left-foot braking in rallycross, and to a lesser extent autocross, I would say it's entirely about weight transfer. The time spent moving from one pedal to the other doesn't enter into it.

Not sure I agree.. despite not being directly related to weight transfer, it is a factor and its a good thing, Pentti also talks about it in the article I linked above.

[Zico]

Also found some videos on the subject..

Fairly basic explanation for FR, FF and RR.. http://uk.youtube.com/watch?v=XVDfDTp0Y7g

Colin in the Focus WRC.. http://uk.youtube.com/watch?v=BMEqOG...eature=related He doesnt mention the mapping of the centre diff in the Focus but I think this is only what Subaru had to use to deal with the inherrant problem of poor PMI in the Impreza.