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Good read for newbs and old dogs


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#1 Calypsoblur

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Posted 17 January 2010 - 08:39 PM

http://www.iwsti.com/forums/driving-tips-t...understeer.html

It is on a STI website, but it also applies to all other cars. I disagree with a couple of statements, but the concepts are all correct. He says that this does not easily apply to autocross, but you should still try to use it. The problem with autocross is that cars usually understeer at low speed, have a neutral attitude around 40 mph and start to oversteer after that. However, as he says, you can get the car to do what you want, you just have to learn how.

As for left foot braking, it is extremely usefull in autocross. Ideally, use first gear to get to the first turn, short shift to second and leave the shifter alone. Once you have shifted to second, move your left foot to the brake. I find it much easier to trail brake/brake smoothly with left foot breaking and then roll on to the gas all without any pause in the action. I try not to brake in the middle of the turn, but find that I occasionally do it; I do not recommend it for making corrections. If you are having to use the brakes in the middle of the turn, you are probably going too fast.

Slow in, fast out.
Craig Robson
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#2 Scoob

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Posted 18 January 2010 - 07:28 AM

That was a pretty good write-up. I used to drive an older GC8 automagic for autox and I started left-foot braking in that. I do it in everything, all the time now and it can make a huge difference in autox, as Craig says. But you must practice, practice, practice to get your brain used to it and to train your left foot to be smooth. Of course now, on my kart, I'm still always left-foot braking... :) There is a lot of time to be made up by "keeping your foot in it" in autox situations, whether a tight, technical course, or a more wide open, sweeper-type course.
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#3 Monkeywrench

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Posted 18 January 2010 - 12:59 PM

It has a lot to do with personal preference. I have no problem with braking with my left foot, but I only do it where I want to set the nose of the car prior to turn it (ie: fast sweeper). Usually I'll keep my foot flat to the floor, and jab at the brake with left foot. Quicker than a lift.

Left foot braking works to "balance" the car through transitions. Watch some of the fast go through a slalom and watch their brake lights (though some like to remove the bulbs or tape their lights so competitors can't observe this).

Fast in, fast out
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#4 evremonde

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Posted 20 January 2010 - 11:40 AM

This was a good read. I think i was having these very same struggles in the evo last event. Tried all day to get even a bit of oversteer, with no luck :)

Craig, what statements from the article do you disagree with?

#5 bronxbomber252

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Posted 22 January 2010 - 12:08 PM

Funny that all of you struggle so much with understeer... My car never understeers unless I do something completely idiotic... Im usually fighting to keep my tail in line... Even with skinny 225 fronts and 255 rear tires I get mostly oversteer.... Then again... I do have a car with 50/50 weight distribution, a rear roll stiffness bias, and RWD... :)
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#6 Calypsoblur

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Posted 25 January 2010 - 12:58 AM

I guess it is more like he should have clarified a few statements.

1: I _THINK_ most people who are not experienced and complain about understeer ARE driving too fast/asking for too much from the car.

2: Snap spin is caused from a car that is sliding (with corrective steering added), suddenly gains traction and then spins in the opposite direction because of the weight transfer and steering input. Understeer will never cause snap spin, but correcting understeer can cause oversteer which can cause snap spin like he describes. This is the tactic most used during drift competitions to suddenly change directions in a slide; middle of oversteer slide with corrective steering, lift throttle, rear end gains traction, weight transfers while front turns (because it always had traction) in same direction, presto, the car rotates around the inside rear tire (momentum/inertia) and no weight on the outside so you oversteer in the opposite direction before you realize it.

3: The reason you have to let up a little after hard initial braking is because the springs will push the front end of the car up transfering weight to the back, not because you are going slower. Although, if you have a race car with super stiff shocks and springs and running downforce (F1 cars?), then yes, your brakes will be less effective at 20 mph vs. 120 mph. But for a production vehicle...

4: He says to add air to the rear tires to increase the oversteer. Two reasons, tires have a spring effect and tires roll over. Increasing air pressures increases the "spring rate" and makes the tire more more immune to rolling over. However, the change in traction from the spring rate effect is (generally, when running near ideal pressures) far greater than the change in traction from sidewall flex. Changing the spring rate changes the rate of weight transfer and also the total weight transfer. If you increase the rear spring rate, keeping the front the same, then less weight moves to the outside tires, thus decreasing the amount of traction on the outside causing the rear tires to share the load more, giving more total rear traction. Same happens up front. Now, if we consider different rates front and rear and throw braking into the mix, it gets even more complicated when discussing understeer. In dynamic situations such as trailbraking, stiffer spring rates can amplify the desired effects due to weight transfer NOT happening.

Finally, 5: I feel that he doesn't discuss enough about how the chassis can effect the steering characteristics of the car. Fatter tires are usually put on the rear tires to increase accelerative traction not correcting rear end traction issues. (Porsche actually increases the rear wheel width in the Carrera 4, figure that one out...) How many FWD cars you see with wider rear tires? Under powered RWD cars typically run the same size tires front and rear, even the old cars with horrible suspensions. This makes a great segway to FWD vs. RWD. Tires only want to do one thing at a time, lateral or longitudinal accelerate. When we ask them to do both, we have to decrease how much of each so that it doesn't exceed basically the max. We could have another article on just friction circles. FWD and RWD cars can oversteer during the first "half" of the turn, but only RWD cars can oversteer on the way out. RWD cars CAN understeer in the second half, which leads me to the next topic on chassis's. A car's suspension can go a loooong way to determining a car's turning characteristics. Adding caster and camber can really help traction through the whole turn. Toe-out makes the car easier to cahnge directions.... Suspension designs can effect the traction by allowing changes to the alignment. Beetles are required to run suspension limiters due to the massive amounts of camber change.

Well, I have said too much now.
Craig Robson
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#7 Andre

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Posted 01 February 2010 - 01:52 PM

Really good post Craig, thanks.

I've been left foot braking for the longest time - the comment about heating up the brakes clearly had me in mind (with new pads - I'm good for some flames when I come off the course). Even beyond that, one bad tendency I think that comes with left-footing is to over use the brakes, either cause I/you fail to fully get the foot off and/or tring to 'settle' the car a little, instead of keeping off the brakes and steering through, in slaloms and such. The fact that your foot is near the brake pedal makes it too easy to use them.

Thanks agan, you ALWAYS need to think about your AX driving if you want to go faster - its almost always about the driver.

Andre





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