Why do racing drivers tap the brake before a corner?

[2kwik4u]

I think that in order to practice proper left-foot braking, you need to rest your heel on the carpet, instead of having it floating in mid-air. That way, I believe that my left is just as sensitive, not more or less sensitive than my right.

[gridgway]

we're getting OT now, but how does left foot braking work? I have tried doing it and can use my left foot to brake, but have not found how it can be an advantage (on track of course).

Graham

[Zod]

Mr Whippy said:

So you won't see many BMW "M" drivers doing it then, but I guess you'll find people who've "upgraded" them suffering pad knock alot more often You see the drivers in Best Motoring DVD's tapping their brakes alot, but I guess it's also to check for bite and pedal travel after heavy lapping and on a long straight. Last thing you want is a tiny pause or too much travel before they bite when your aiming for a turn in point at 100mph!

That's why you will see BMW M drivers doing it. The alternative is to find that the pedal goes a long way to the floor without slowing you down very much on a corner where you really need to lose a lot more speed.

[Jungles]

gridgway said:

we're getting OT now, but how does left foot braking work? I have tried doing it and can use my left foot to brake, but have not found how it can be an advantage (on track of course). Graham

There are various different applications of left foot braking (LFB) in performance driving. Different drivers also use LFB for different reasons. I'll try to lay these out in a historical, and then technical, format.

The first wide-spread use of LFB was in rallying, during the early 80s, when 4WD cars began appearing in the World Rally Championship. At the time, the cars used in the WRC were extremely powerful and had crude 4WD systems with locked differentials and torque split of 50/50 between front and rear axles. Hence, these cars exhibited extreme understeer and were difficult to turn.

Prior to that era, rally cars - indeed almost all cars - were RWD. Driving a RWD car at maximum pace almost always involved applying excess throttle, to cause the rear of the car to slide. Sliding into corners with ridiculous amount of throttle was the norm.

But with the high power and tram-like dynamics of the dominant Audi Quattro in the early 80s, Stig Blomqvist introduced LFB when he drove for Audi, and later taught it to Walter Rohrl when they became team mates. Since then, the technique became wide-spread in rallying.

The whole idea behind LFB in turbo-charged 4WD, was to enable a driver to transfer the weight of the car to the front, without backing off the throttle. Backing off the throttle caused turbo lag (a big problem at the time), limiting the power available for exiting a turn. The effect of weight transfer, combined with appropriate brake bias, also allowed the rear wheels to spin faster than the front. All these allowed the driver to:
1. Create more downforce on the front wheels.
2. Induce loss of traction on the rear wheels.
3. Allow braking without decrease in throttle, maintaining turbo boost.

LFB has also transferred to front-wheel drive (FWD) cars as well. By using LFB in a FWD car, it is possible to almost lock the rear wheels while keeping the front wheels spinning under power, if the technique is applied with too much enthusiasm. Appropriate balancing of the brake and throttle can induce very mild oversteer during turn-in, eliminating the problem of understeer for FWD cars.

Now, onto using LFB... (for FWD)

LFB is a highly specialised technique, and there is a time and place for them. It belongs between two techniques utilising the same concept: trailing-throttle steering, and trail-braking. They all rely on weight-transfer under power or braking, and therefore rely on the car having the correct momentum, and proper application of throttle or braking.

Trailing-throttle is a high-speed cornering technique, where engine braking is used to slow the car for a high-speed corner at 4th gear or above, and neutralise understeer during turn-in.

Trail-braking is slow to medium-speed corning technique, where pure braking (with right foot and no throttle) is used to negate understeer during turn-in. (There are other benefits to trail-braking, but those would be irrelevant here.)

LFB sits in between those two techniques, and generally should be utilised for corners taken at 2nd or 3rd gear. Also, there should be no need to change gears while negotiating the corner. Good driving vision and forward planning is essential.

1)
When approaching a corner, brake (right foot) to slow down, and down-change to appropriate gear, as per normal driving practice.

2)
Place the car into neutral throttle, and maintain that throttle as you initiate the turn-in using the steering wheel.

3)
Very gently apply the brake with the left foot, so that the front-end firms up, and rear-end begin to float.

4)
Modulate the braking, and if necessary, the throttle, to get the car pointing toward the apex.

5)
At or before the apex, release the brake completely and accelerate through the apex and exit.

At step 4, the amount of yaw depends on the ideal driving line through the corner, the type of car being driven, and the surface. Adjust accordingly, but usually you should be pointing slightly more toward the apex than normal. At step 5, if you need to LFB past the apex, the technique is wrong. When passing the apex, you should be able to steer the purely with the steering wheel and the throttle.

Do not mash the accelerator. Try to modify your braking pressure first. Use throttle when braking is not having the desired effect. This is somewhat controversial, and some drivers will say to modulate braking while maintaining throttle, while others will say to modulate throttle while maintaining braking. I personally prefer to maintain throttle, and modulate using the brake, with throttle modulation only as a last resort. Therefore, my "neutral-throttle" at turn-in would normally be higher than if I was to negotiate the corner without LFB.

In RWD cars, LFB is only advantageous at the point of turn-in, to shift the weight to the front without potentially destablising the car when letting go of the accelerator to brake with the right foot. It's a time-saver and helps to slow down more smoothly. After the initial understeer is eliminated, get off the brake. For RWD cars, the advantages of LFB are questionable, and highly depend on personal choice. I personally don't think there is a significant advantage, unless you're driving a car with excessive understeer during initial turn-in (some open-wheel formula cars, such as Champ Cars, are like this), or a very powerful car that is extremely unstable when lifting off the throttle. For turbo-charged 4WD and FWD cars, LFB is quite beneficial, especially in loose-surface rallying.

I hope that covers it adequately.

>> Edited by Jungles on Thursday 18th May 14:56

[roop]

Sometimes it's just to seat the foot on the brake correctly, sometimes it's a confidence test...

[GreenV8S]

Jungles I think you're missing the handling changes that occur when the engine and brakes are applied simultaneously. It isn't a matter of weight transfer; the brakes apply different amounts of braking to the front and rear wheels and this has a significant effect on the car's handling.

In a 4wd car, applying the brakes while accelerating puts a lot of braking on the front and relatively little on the rear, giving the car a net rear power bias and giving it rwd-like power oversteer characteristics. This can be used to overcome the severe understeer inherent in a 50:50 4wd car.

In a rwd car, applying the brakes while accelerating gives you full braking on the front wheels while the brakes and acceleration tend to cancel out on the rear wheels. This can be used to induce understeer to recover from unwanted oversteer (dab the brakes, the front washes out bringing the car back in line, off the brakes and keep going without ever having had to lift off the power).

[Jungles]

GreenV8S said:

Jungles I think you're missing the handling changes that occur when the engine and brakes are applied simultaneously. It isn't a matter of weight transfer; the brakes apply different amounts of braking to the front and rear wheels and this has a significant effect on the car's handling. In a 4wd car, applying the brakes while accelerating puts a lot of braking on the front and relatively little on the rear, giving the car a net rear power bias and giving it rwd-like power oversteer characteristics. This can be used to overcome the severe understeer inherent in a 50:50 4wd car.

That's true, but I missed it.

GreenV8S said:

In a rwd car, applying the brakes while accelerating gives you full braking on the front wheels while the brakes and acceleration tend to cancel out on the rear wheels. This can be used to induce understeer to recover from unwanted oversteer (dab the brakes, the front washes out bringing the car back in line, off the brakes and keep going without ever having had to lift off the power).

This, I didn't know.

I have to admit that my post was very FWD-biased.

Edited as per your information.

>> Edited by Jungles on Thursday 18th May 14:54

[Mr Whippy]

Zod said:

Mr Whippy said: So you won't see many BMW "M" drivers doing it then, but I guess you'll find people who've "upgraded" them suffering pad knock alot more often You see the drivers in Best Motoring DVD's tapping their brakes alot, but I guess it's also to check for bite and pedal travel after heavy lapping and on a long straight. Last thing you want is a tiny pause or too much travel before they bite when your aiming for a turn in point at 100mph! That's why you will see BMW M drivers doing it. The alternative is to find that the pedal goes a long way to the floor without slowing you down very much on a corner where you really need to lose a lot more speed.

Odd, because they weren't tapping the brakes on the CSL in the Enduro Challenge, but those cars with monoblock multi-piston brakes were.

What would tapping the brakes do on a single piston slider? If you were suffering fade a tap wouldn't do bugger all anyway, so why do it? It seems Stop Tech provide the reason, pad knockback, something a single piston slider won't suffer from.

I'm interested in the explanation why you think a BMW driver would *lightly* left-foot tap their brakes while still on the throttle coming upto a braking zone.

Dave

[TheExcession]

On the foot tap theme, I'd heard it was just to make sure there was some pedal there.

On the left foot braking theme, I've been doing it for about 3 years now in my regular road driving, admittedly a lot of this is in an automatic car. I believe it's taken me this long to to get really good at it, and by that I mean it's now completely natural and most of the time I don't actually realise that I'm doing it.

One test little I like to perform with automatics is to brake with the right foot, then whilst still under braking apply the left foot to the brake pedal (it's nice and big and can take both feet), take over braking with the left and remove the right foot back to the accelerator. I do this whilst keeping an eye on the passenger head to see if I can discern the point of foot changeover by them nodding forward of backwards, and most of the time it's toatlls seamless.

One thing that I find thoroughly fascinating about LF braking is the way your body learns to respond to changes in acceleration. As has been said some people can never get it, but modulating the balance and acceleration of a vehicle using both of your feet in responsse to what your inner ear and bum are telling you is a tremendously difficult task to learn. Once we get locked into the habbit of right foot controlling stop and go some people do seem to find it very difficult to get the left foot to do the same.

Just recently I knackered a ligament in my right knee that makes driving a little uncomfortable, Having already mastered braking with my left foot I am now learning to control the accelrator with my left foot too. It's tricky and still requires a lot of concentration, where as LF braking is now totally second nature.

Good stuff,
Ex

>> Edited by TheExcession on Friday 26th May 12:22

[GreenV8S]

I was commuting in a mush box for quite a while, and the idle was a bit hit and miss so I had to learn to left foot brake. It didn't take long to learn to modulate the brakes, but it took ages and ages to teach myself not to 'dip the clutch' while coming to a halt. Fione while I was concentrating, but if I stopped thinking about it, it only needed something to re-awaken manual gearbox habbits, just putting my hand on the gear lever as if I was going to take it out of gear would be enough, and my left foot turned itself back into a clutch control.

[TheExcession]

Yup, these are the really hard habbits to break, but still an extremely rewarding technique to learn when it all goes according to plan

I was thinking a little more about this and was about to post that for people that want to get a taste of left foot breaking without giving the windscreen a Glasgow kiss, then the two feet on the pedal technique might help.

Essentially continue to let the right foot do the braking but get the left foot resting on the pedal too and used following the pressure that gets used. Gradually let the left foot do a little bit of the braking and like you say, if automotion takes over, or it all goes tits up at least everything is in the correct place

[thunderbelmont]

If it's anything like my animal, I tap the pedal to check for "pedal", and then hit it! Effectively a quick pump to bring the pedal up so I can feel it better, also it helps to shake the person up behind me. "What, brakelights, Oh f**k...."

Damn, let the cat out of the bag....

Mind you, it depends on the system on your car, and the weight of your car.
My little beastie requires lots of brake pressure (hence the quick pump for security), as I'm trying to stop 1200Kg of tin-top from 170-180+ (mph, not kph!) on some circuits. After an average race, about 1-2days later, my legs feel like I've just run the London marathon twice!, so you'll get the drift for wanting to have a better feel to the pedal.