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=BMEqOGejlrw&feature=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.

[salmo]

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.
Good point.

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
Hmm, I'm having trouble seeing how the brake bias could be directed toward the rear with LFB; could you please explain?

I think LFB is especially useful with FWD cars because the front wheels are often overworked as it is, and more likely to let go. This is why most FWD cars are inherently understeery, and why LFB is a good tool for getting more weight transferred over the front wheels.

I forgot to mentioned another great use of LFB, which is for turbocharged cars (like WRC cars). By keeping your right foot on the accelerator, you can keep the turbo spooled during even slow turns, offering a huge advantage when accelerating out of the turn.

[salmo]

Hmm, I'm having trouble seeing how the brake bias could be directed toward the rear with LFB; could you please explain?
Never mind, I just watched that video clip that explains it pretty well. That's a great clip; thanks for the link.

BTW, I can't believe that bit at the end where he demonstrates LFB with heel-and-toe downshifting! He says "not many people can do this." Yeah, no kidding! I've never even heard of such a thing before. It looks pretty wild, and I believe him when he says that it's probably not worth it. I'll continue to right-foot brake with heel-toe when I'm downshifting while entering corner.

[Zico]

Good point.

Hmm, I'm having trouble seeing how the brake bias could be directed toward the rear with LFB; could you please explain?

Only with Fwd.. Think of brake distribution ie F65 R35 as an optimum set-up, If you apply the throttle while braking the power to the front wheels cancels out a percentage of the braking force while the rear remains constant.

[Zico]

I forgot to mentioned another great use of LFB, which is for turbocharged cars (like WRC cars). By keeping your right foot on the accelerator, you can keep the turbo spooled during even slow turns, offering a huge advantage when accelerating out of the turn.

With the modern WRC cars having such effective ALS, this use for LFB now offers limited effectiveness compared with the early Group A cars when it was a far more critical skill to learn.

Most people who try it for the 1st time in their roadcar usually hit their head on the windscreen , and think they will never develop a feeling for it, but practice helps no end and its worth noting that its actually far, far easier to modulate in a rally/race car due to the brakes lack of servo assistance.

[Nenukknak]

Ok I understand somewhat. But what I don't understand is why the same can't be accomplished, with either just the throttle or just the break. Do you use the throttle to push the drivenwheels through the breaks, and to lock/break harder on the non-driven wheels. But if the car doesn't decelerate under braking, because you're still holding down the throttle, how can the weight still transfer?

[Zico]

Weight transfer will occur with either the throttle and the brake, and to a lesser degree simply by letting off. By accelerating while still on the brake you add the adjustment of the brake balance rearwards to the equation, making the rear able to swing round even more easily, reducing the need to use the pendulem effect, ie scandinavian flick.
If the car doesnt de-accelerate while you do this you either have a very powerful engine/weedy brakes combination or you simply are not being aggresive enough. Naturally rallycars usually have decent anchors..

[pentti]

How come Pentti isnt around on the forum these days?I am still here and I read WRC forum.It's just I have been very,very busy to write. But this subject is some thing which I want to help and correct.
In fwd you use brakes and throttle same time to help steering.Harder you brake and 60% throttle more the car turn into corners.95% teason lfb in 4wdr and rwd is just a lot quicker(at least 0.5 seconds)as reaction time.

[pentti]

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=BMEqOGejlrw&feature=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.That first clip is 95% nonsense.Colin was quite good.

[Nenukknak]

I am still here and I read WRC forum.It's just I have been very,very busy to write. But this subject is some thing which I want to help and correct.
In fwd you use brakes and throttle same time to help steering.Harder you brake and 60% throttle more the car turn into corners.95% teason lfb in 4wdr and rwd is just a lot quicker(at least 0.5 seconds)as reaction time.

But you do also use LFB to steer in 4wd (and RWD) as well, not just because reactiontime is quicker?

And how do you use the brake and throttle then? Do you keep the pressure constant on either one or the other, or do you alternate pressure on both brake and throttle as is needed?

[Steve Boyd]

Most people who try it for the 1st time in their roadcar usually hit their head on the windscreen , and think they will never develop a feeling for it, but practice helps no end and its worth noting that its actually far, far easier to modulate in a rally/race car due to the brakes lack of servo assistance.

Another problem with modern road cars is the emergency braking assistance part of the anti lock brake system. This assumes normal right foot braking an measures the time taken between lifting the throttle and applying the brakes. If the time is less than a set value it assumes you're making an emergency stop and applies maximum brakes whatever the pedal pressure. Left foot braking with these cars can make the brake computer think you want to make an emergency stop - hence face on windscreen!

[Zico]

Another problem with modern road cars is the emergency braking assistance part of the anti lock brake system. This assumes normal right foot braking an measures the time taken between lifting the throttle and applying the brakes. If the time is less than a set value it assumes you're making an emergency stop and applies maximum brakes whatever the pedal pressure. Left foot braking with these cars can make the brake computer think you want to make an emergency stop - hence face on windscreen!

Good point.. hadnt thought about that. I've always owned older cars that dont have the emergency braking assist.. doubt Id ever want a car that had that.

Nenukknak, with Rwd (and plate type LSD), left foot braking should only ever be used as a reaction time/foot movement time saver, the pedals should not be applied simultaniously unless you really want to lock the diff and spin as I quickly learned.. ;)

[RichardM]

Modern electronics in cars can be quite irritating. My VW GTi won't let you use both the brake and the accellerator when left foot braking. Once the brake goes on, the computer assumes that you want the accellerator off.

[Xeroid]

NIce bit of video from the past with some good foot action shown in the early part. Just a nice vid anyway from Group B days.
http://www.youtube.com/watch?v=RWqWprRRQoc&feature=related

[pentti]

NIce bit of video from the past with some good foot action shown in the early part. Just a nice vid anyway from Group B days.
http://www.youtube.com/watch?v=RWqWprRRQoc&feature=relatedAnother mis leading video but entertaining.notice how Walter Rohl moves his left foot away from brake pedal back to left.Now he looses the most advantage of lfb.Rreaction time.

[pentti]

But you do also use LFB to steer in 4wd (and RWD) as well, not just because reactiontime is quicker?

And how do you use the brake and throttle then? Do you keep the pressure constant on either one or the other, or do you alternate pressure on both brake and throttle as is needed?Good point. You use brakes in 4wd to move weight transfer.More weight in front, less in back.So yuo can kill understeer.But you can do same with right foot.It is just with lfb your reaction time is much quicker.It is very seldom that you use brakes and throttle same time(unlike in fwd)in 4wd.

[pentti]

Modern electronics in cars can be quite irritating. My VW GTi won't let you use both the brake and the accellerator when left foot braking. Once the brake goes on, the computer assumes that you want the accellerator off.Stay away in any VW group cars(VW,Skoda,Seat and Audi).If you brake and throttle same time computer thinks you have stuck throttle!

[Daniel]

I would have thought not having boring/characterless cars was good enough reason to stay away from VAG group just on it's own.

[jparker]

Good point. You use brakes in 4wd to move weight transfer.More weight in front, less in back.So yuo can kill understeer.But you can do same with right foot.It is just with lfb your reaction time is much quicker.It is very seldom that you use brakes and throttle same time(unlike in fwd)in 4wd.
I read the same thing from Richard Burns Rally School (part of the game).
Now I know who he learned that from :)