# DIY - Left Foot Brake LFB Switch



## totheMax (Jun 11, 2008)

Been trying to practice left foot braking.. using this technique to help me get around sharp turns faster next time I go to the track but mostly to help me keep the boost up..... I am pretty bummed that the TDI's drive by wire and ECU program does not to let me accelerate and brake at the same time.

The moment I press on the brake with the left foot, the car just coasts... even if I put the right foot right to the floor .. it just coasts... the moment I lift off the brake, it lags about 1.5 seconds then starts to accelerate normally... The car is programmed to cut off fuel when braking...

Here's a small mod that I did which bypasses this safety feature and allows LFB .... 

*WARNING: THIS IS SUPER DANGEROUS AND NEVER TO BE USED AT THE SAME TIME CRUSE CONTROL OR ON THE ROAD OR OTHER THAN ON THE TRACK...

DO NOT ATTEMPT THIS MODS, NOT EVEN AT YOUR OWN RISKS *

Material below is from: http://www.rallyracingnews.com/lfb.html

Left Foot Braking
In Front Wheel Drive
Theory:

When driven under acceleration with no brakes applied, a front wheel drive car will understeer when the front wheels are turned. To overcome the understeer, a technique called left foot braking is used to change brake bias (balance) to the rear. By using a combination of throttle and brake you can change from understeer to neutralsteer to oversteer. How does it work? Read on!

Weight Transfer:

Think about this for a second. When you stand on the throttle, which way does your body try to go? Toward the rear wheels! Right? Now what happens when you brake? Your body tried to go forward! This boys and girls is weight transfer! By using the basic rules of weight transfer and a combination of throttle and left foot braking, you can be in control when the unexpected happens on a rally (which always happens)

How Does It Work?

Brake bias in normal street cars is toward the front wheels for safety reasons. If you have ever pulled up your hand brake when turning on a wet road and spun out, you know why they build cars this way. What you did was bias the brakes to the rear, and this is not what Grandma wants in her Buick coming home from the supermarket. But as rally drivers we can use a means of biasing the brakes to the rear. Some of the things that can be done include using a brake bias control valve or installing more powerful brakes on the rear wheels. That works wonderful on a track, but on a rally it's hard to change your brake bias in the middle of a turn that suddenly changes from dry dirt with good grip, to wet slime with no grip.

Enter Left Foot Braking!

Once your have mastered LFB, when the road surface changes, you can change the brake bias by either adding power or brake pressure ( this is also called "modulating"). Example: You are in a turn and the rear of the car starts to slide out more then you want. You could release some brake pressure and increase the throttle. This will change the oversteer to understeer. The reverse is also true. If the turn tightened, you can change to oversteer by increasing brake pressure against the throttle. Also by balancing the brake and throttle you can get a neutralsteer.

Why Does It Work?

A tire has 100% of it's possible traction when it is rolling straight with no braking or acceleration. When you change any of these factors the tires affected will have less available traction and break loose earlier.By applying brake against the throttle in a front wheel drive car, two things happen. One: The front wheels keep turning. And two: The rear wheels try to lock up. If you are in a turn, the back of the car will start to slide toward the outside of the turn. To control or stop the slide, apply less brake and more power ( this changes the oversteer toward understeer and stops the rear wheels from sliding toward the outside of the turn ).

Putting Theory Into Practice:

Let's Give It A Try: Now comes the fun part, driving the car. Find a safe place to practice, a gravel parking lot with no trees will do.( If you live up north and it's Winter, try a frozen lake, this is ideal because everything happens at low speed ). Set up some traffic cones to make a Tee intersection. Now get in your car and practice driving the turn without left foot braking, but try doing the normal braking with your left foot.(What did he say??) Yes, try braking with your left foot! You can't learn to LFB until you train your left leg to brake! If you have never tried using your left leg, your will find it's like trying to write with your left hand if you are right handed. This is very important. Until you can drive doing the braking with your left leg, you can't do LFB. Once you can brake with your left leg, now let's try something new. First think about what you want to do, then start your practice turn. *After turning in, apply the brake against the throttle.* If you did it right you should feel the rear of the car start to slide to the outside of the turn. When you are pointing in the direction you want to go, let pressure off the brakes and increase the throttle. If you did it right you will be amazed at how the car seemed to pivot on the front wheels. So wipe the silly grin off you face and try it again, and again, until you can start or stop the slide at will. Congratulations, you now know what LFB is!

Practice Makes Perfect:

The first few times you try the exercise above, you will probably find the car slowing down in the turn but not much else. *Try using less brake*. You will be amazed at how little pressure is needed. *Also remember to turn in before you apply the brake.*I don't mean try to turn before you have slowed down for the turn, I'm talking about after you have braked and selected the proper gear for the turn, you must let some pressure off the brakes before turning in.After you get bored with just making a left or right, try setting up an "S" turn and play with that until you can keep up your speed through both turns. Remember to treat each turn separate, and lift off the brake and turn in each time. When you have mastered Left Foot Braking you will find that you will have control of your car like you never thought possible! It's both faster and more important safer. So don't get discouraged if it seems impossible to master, keep trying and all of a sudden it will seem easy! Ed Brennan 1995

I followed the instructions originally developed by Evan Sussman



_Quote:
Originally Posted by Evan Sussman 

"At the brake pedal there is a switch. It's a two pole switch, (this means it does two things at once.) One switch is normally open (when pedal is not pressed) when you press pedal it closes, and the brake lights come on. The other switch is normally closed (pedal not pressed) when you press the pedal it opens. As far as I know this does two things. 1)disable cruise control and 2)activate anti-trailbraking GRRRR!!!

After some testing with a multi-meter, (they even made it so it doesn't test right without fiddling with it correctly) I figured out the above.

I decided to make a bypass switch. I took two pieces of wire and ran them up to about the radio where I drilled a hole in the little plastic trim. (perfect place for switches) I attached these wires to a single pole switch, and the other ends I tapped into the two wires attached to the normally closed side of the switch.

Making sense? Now when I switch the switch on the dashboard it bypasses the brake switch, even though it opens when you press the brake, the signal still passes it through the dash switch.
..........
Hopefully this gives you guys a starting point. It worked quite well. There were two issues. *Safety First!!! DO Not engage cruise control as braking will not disable it when the dash switch is on.* The second issue is that IIRC the glow plug light would blink after a few minutes of having the switch on. It goes away after switching it off, and I'm pretty sure I never got a MIL light._











and I installed the LFB switch as described in Evan Sussman's instructions above.

I tapped into the No 2 & 3 wires... they are the yellowish and blueish wires in the middle of the switch. I ran a blue and yellow wire to the empty spot above the radio. Drilled out the blank rectangle filler and installed a switch in there like my W/M switch. As it was pointed out, it is key to NEVER bypass this circuit while using cruise control.

Before I put in the switch, I just did a test by closing the circuit with a small jumper and took it for a test drive... Way fun.. Came back and installed the switch.

Note that while I was testing it and LFB (Left Foot Braking) hard while I had lot of throttle, I did get a flashing glow plug CEL ... turned the switch off and it went away.. .turned the switch back on and continued LFB and it did not come back.

In addition to being able to turn the rear of the car fast, this function will be particularly useful on the track because it will allow TDI to keep it's boost up while slowing down. Before, when I took my foot off the throttle to brake before a turn, I lost boost, then there was a lag before the turbo kicked in. With this method, I was able to left foot brake while keeping boost up 15 to 25 psi range. The moment I take the foot off the brake, she takes off and the boost is right there....

Can't wait to try this on the track.
















































The finished product:





















EDIT: Note that this function can also be done through modification of the OEM software. Couple of the top tuners have already modified the OEM software to allow LFB. One of them is currently working on a software mod for TDI's that will do this. I will incorporate this software mod with the 4 bar map tune I will get with my larger turbo. Will keep this thread updated when I do so.


----------



## scenturion (Apr 11, 2009)

Not strictly true (or else why would Schumie do it?), even on tarmac/asphalt/concrete. Additionally, one thing that isnt mentioned here is that with left foot braking, the engine remains loaded. If you have a turbocharger, this means that your turbo never spools down, which should help minimize turbo lag upon acceleration.


----------



## totheMax (Jun 11, 2008)

Woww! what a great analysis of the benefits of LFB.. tnx for posting this. Like you said, I think it's even more beneficial for cars w/ a larger turbo. In hte new year, I will be getting a 2260vk and running 38 to 40 psi boost.. This turbo has a huge wheel and is subject to major turbo lag. I found this even with my 1722. During hairpin turns, I would just RF brake normally and then jam on the throttle near the apex and would feel the turbo lag big time.

This technique will let me slow right down while keeping the boost and power up there. I expect no turbo lag way faster exits using this technique.. 

Time will tell as I have not tried it on the track yet 



scenturion said:


> Not strictly true (or else why would Schumie do it?), even on tarmac/asphalt/concrete. Additionally, one thing that isnt mentioned here is that with left foot braking, the engine remains loaded. If you have a turbocharger, this means that your turbo never spools down, which should help minimize turbo lag upon acceleration.


----------

