Background: not important for question, just informational, and why I'm curious

I took the rallyschool thing on the 16th, and they talked a little about LFB on an AWD, but all my instructors gave different explanations. In the classroom stuff, I thought they said on most AWD cars, it'd only help in that it keeps the turbo spooled and give faster control or something in the turns. But then my first instructor said it was just like a FWD car, but this doesn't make sense to me since brake bias is towards the front, so it seems like it would increase understeer. And I didn't understand my second instructor's explanation.

So I talked to a couple fellow students, and heard some other's talking and they seemed to think it wouldn't do anything, so I didn't think aout it a whole lot. But then on sunday, I was playing in the dirt after driving on some dirt raods for a while, and the guys I was with were using LFB to rotate the car, and I'm like "wow, it must actually do SOMETHING". So then I started trying it, and sure enough it allowed me to induce oversteer, and I could do 90 and 180 degree turns just like with the e-brake. So my question...

Question:

why does LFB induce oversteer, how does it work, etc. Someone please try to explain it too me. Thanks


O yea, rallying awesome!!! And so are dirt roads!

 

Yeah, that guy in the classroom didn't give a good answer when I asked what LFB does for RWD too. Or, should I say, contradictory to what the hands-on instructor said.

 

leftfoot braking induces oversteer in the same way as right foot braking does. similar to trail braking, it basically is used to xfer the weight of the car to the the front wheels, thus giving the car more front traction, and lightening the rear end and allowing it to come around. Basically it works based off of weight transfer.

The key to left foot braking is the fact that you can be on the gas and brake, without having to take the time to move your foot over, and also can be on both pedals at the same time.

Espically in a turbo car, like a suby, when flat out, turbo lag can be the differnce to getting on the gas too late, when in fact you got on the pedal at the correct time, just that the power wasnt there due to turbo lag. So when entering a turn upon braking, you can infact roll into the throttle while still pressing the brake, and slowly let up on the brake to control the accerlation out of the turn.

hope that helps.

edit: sounds like the instructor told you what but not why.

 

......while maintaining your forward momentum with the gas pedal....

 

sorry, I was obviously referring to LFB only......as for the RFB....well....errrr.....so you can do this (watch the vid)

https://www.i-club.com/forums/bay-area-15/awesome-mini-vid-75737/

-Ted

EDIT: admins.....sorry for the crosspost...but it had to be done.

 

Ok, maybe I'm wrong, but I thought you could actually accelerate while LFBing, and get oversteer. but I'm not real sure about that. I guess I'm kinda confused also because they talked about the effects on FWD and AWD at the same time (or right after each other) so its hard to seperate them. maybe it was only FWD car that you can accellerate and use LFB to oversteer with.

So LFBing w/ a AWD car does it through weight transfer, but in a FWD car it does it through reduced rear wheel traction, similar to using the e-brake?

The rest makes sense, though, the part about being able to instantly shift the weight back to the front since your foots already on the pedal. As well as the turbo part. I don't know much (obviously) but my cars NA, so would it have any effect on throttle response (aside from that your foots already there so theres no moving of it) for clarification I'll word it differently as well; if your already on the gas in a NA car will it respond faster? <-- hope that made sense, having troubles with wording!

Thanks for taking the time to help me understand

 

my bad, what i meant was that the latter two do it by applying a deceleration force (thus using up some of the rear wheel traction force slowing so that it takes less turning to exceed the limit) while in a AWD car it does it through weight transfer only?

 

I don't understand how turbo lag is reduced. If the RPMs drop too low while braking, don't you lose it anyway even if you're pressing the gas pedal?

 

LFB only works on dirt?

 

OK, this is just the way I understood it when they explained it at the school, but it seems (to me) to disagree with your interpretation, and it makes a lot of sense to me. Allow me to try, to explain it, then please feel free to explain why I'm crazy

So goin around a corner normally, the back and front tires both have a, lets say, .5g sideways force. And since I'm accelerating, the fronts have a .5g force forward (think of these as vectors). This would make the net force in the rears .5g sideways, we'll say the force on the tire is to the right. The front however have .7g force on them (addition of vectors (sqrt(.5^2+.5^2))). Anyway, for this we'll say the tires can hold a total of .6g of force they can hold, so the front is exceeding this and the car is therefore plowing through the turn.

However if I use the same throttle in this situation with the same turning force, but am also applying the brakes, lets say with a .4g force, (assuming the brake bias is 50/50) then the front wheels still have the .5g sideways force, but now have a .1g accelerating force(.5g-.4g) resulting in a total of .51g while the back has a .4g decelerating force as well as the .5g sideways force. The back therefore has a total of .64g force on them (again sqrt(.5^2+.4^2)). So the front has a force under .6gs (the tires max) while the back has over .6gs resulting in a loss of grip in the back, and, therefore, oversteer.

I know I didn't explain it well, and the lack of acceleration circles make it hard to follow, but this was the way I understood the LFB on FWD thing at the school. Thanks again!

 

It should work on every surface, how easy it is changes some though. And maybe its affect does too.