How To Brake?

[FTS]

I certainly do agree with Chris, not that my word counts for anything against his , but what is the most important thing is the confidence of the driver and the feel s/he has. Slamming the brakes at the very last possible moment never felt like a solution to me, at least with street cars; the situation may be different with race cars with heavy aero.

[NickW]

So last weekend I tried to shake off the rust and go racing at Mid Ohio.

The driving part was fine. Unfortunately the chassis and engine tuning side of it was a complete disaster. Things started to click inside my head about 24 hours AFTER the weekend...

As the saying goes, there's a race happening, I'm just not in it.

http://contour.com/stories/wka-mid-ohio-tag-2-2011

The race line is still appropriate. If you watch and listen closely in the video of the lap, you will be able to tell the chassis is binding up in the rear on track out from the apex, putting me 30 ft down on the competition every lap in the tighter corners, and that the kart probably has too much Ackerman causing the turn in looseness.

[FTS]

It is pretty amazing to see this level of action this close to the ground. Setting up a kart is really complex I think, although I have NO experience with it, but without any springs/shocks, it is all geometry and probably millimeters of adjustments.

[Skypalace]

Very interesting discussion.

My personal answer is that it depends on what the purpose is, of the session and lap. For all-out qualifying, trying to get every last hundredth (I just had a race weekend where top three of us were within 23 thousanths of a second in qualifying), then going from full throttle to full brakes is quickest, IF 1) you don't end up over-slowing and killing your mid-corner and thus exit speed, as is VERY often the case when using max brakes, and 2) you don't lock up your brakes (on a non-ABS car).

On a race pace, I'm usually (but not always) a little less aggressive in braking zones, I brake a little earlier and a little less hard, to reduce chance of lockup and give more room to increase braking if needed to make the corner. Depends of course on the competitive situation, am I chasing someone or forced to defend, or am I in a 'comfortable' position where keeping up a pace is all that's needed to prevent a position change.

The over-braking is what most people end up doing if they're trying to 'banzai' the braking zone, so that's why it ends up being better not to apply the brakes too hard - it lets them modulate their overall speed easier. At full near-lockup braking, any GT/Cup car has such amazing brakes that the speed comes off at a very prodigous pace, and it's very difficult to stop braking at the right point. Also, if you're aggressively braking, and you brake even a little too early, you need to reduce the braking forces a fair bit later in the braking zone to prevent over-slowing. All of this is much easier to control at lower braking levels, which I believe is why it's often faster (and certainly safer) to brake less. However, if you're getting off the brakes at the right time, and you're not locking up, simple physics says you want to be on the gas as long as possible, then on the brakes as late and hard as possible, to maximize your speed over every foot of the course. Not very easy for us mere mortals to do, and the additional time gained by late/hard braking is usually overwhelmed by the time lost in having too low a mid-corner and thus corner exit speed, killing speed all the way to the next braking zone.

I've learned a lot about braking while going from an ABS car (996 Cup) to a non-ABS car (997 Cup), with manual bias control. I've had to adjust my braking to trail-brake less and get more braking done before turnin, or I was regularly locking up the inside front wheel late into the corner. This actually caused me to change my lines somewhat.

In the wet (just came off a weekend with 5 wet sessions), I'm of course easier on the brake application, as I'm always sensing for traction, including under braking. Downshifting is less aggressive also (lower in the rpm range) or it'll cause rear lockup (in both 996 and 997 I use downshifting/engine braking to augment the car's rear braking).

Many drivers seem to want to maximize their braking, and brake as hard and late as possible. If they have data or video, I often see much more time to be gained in better controlling getting OFF the brake, and rolling through the corner as quickly as possible, and back onto the throttle as early as possible, minimizing time through the corner and all the way down the next straight. There's often a full half-second or more available there from a single corner, versus perhaps a tenth by braking a little harder and later. So yet another reason many instructors/coaches want you to concentrate on areas other than braking as hard as possible.

Final thought - for DE drivers (I instructed for many years but haven't lately), the goal is to get drivers safely around the track, on a good line, slowly increasing their speed as they approach the limits of their cars and their own abilities. It's way to easy to exceed the abilities of driver OR car by nailing the brakes hard, and lots of bad things can happen, as the start of a braking zone after a long straight is the fastest the car is going. I strongly believe (both in DE and in racing) in gently approaching limits from underneath them, and I've too often seen bad things happen in DE from things like mid-corner throttle lift from someone carrying more speed than they're comfortable with, or seeing something ahead that they're not comfortable with, even though their car can handle their current cornering speed.

Next topic: pinching exits, why do I see SO many drivers keep full (or nearly full) steering input well after apex, while applying full throttle, and not unwinding the wheel and going all the way to trackout? Have seen so many incidents... Another topic though :-)

[FTS]

What can I say, an excellent first post; welcome to the community Jim.

I certainly agree that the physics of vehicle dynamics dictate maximum braking for the shortest period of time and being on throttle for the longest periods is the fastest possible way around any circuit. The theory goes out the door when the human starts making the decisions; we are not consistent and most often not rational either.

What I find interesting and something that I would not have thought about is how you had to change your braking to use less trailing with a non-ABS car. It does make sense, I just would not have thought that might be one of the outcomes.

I have been watching F1 since 9-yrs old, that's over 30 yrs btw, and what grabs my attention the most is how seldom those drivers brake on a straight line. Granted they have significant help from their aero, but being on such balance and sensitivity is something to be admired I think.

I like your next topic as well, I hope you do start the thread

[Skypalace]

Thanks! Re F1 drivers, certainly they need to brake in non-straight lines, esp. at say Valencia today where they're almost always in a corner.

Now I think I understand better why they're constantly adjusting brake bias - moving bias rearward will let them trailbrake deeper into a corner before locking up. Too far rear though and esp. earlier in the braking zone you might lock up the rears, that's much worse. So at least for mortals like me, I err on the side of front brake bias, better to have front lockup and understeer through a corner entry, than to have the back step out when under any side loading (esp. entering a high or medium speed corner) and swapping ends (spin at best, into something hard at worst). Though if I'm never getting any lockup on the data (easy to see from logging of the four wheel speeds), then I'm just not braking hard enough, I do want to see a touch of lockup at max braking, just minimal (one wheel slowing a little, never full lock) and fairly even front to rear :-)

I did expect to take some time adjusting to non-ABS, I didn't expect to have to change lines etc. either. I honestly didn't realize (except now in retrospect) how late into corners I'd been trail-braking, and how often the ABS in my 996 must have been keeping the inside front wheel rolling.

So I definitely pay attention to braking, but as with the 996, most of the time I make up when improving lap times is in carrying more speed through the corner (ie. getting off the brake earlier), and getting on the gas earlier.

[NickW]

Originally Posted by FTS

It is pretty amazing to see this level of action this close to the ground. Setting up a kart is really complex I think, although I have experience with it, but without any springs/shocks, it is all geometry and probably millimeters of adjustments.

The setup is magnified by the type of racing- road racing a kart is really a study on not only driving ability but also setup- both are required to be competitive. What's really important in road racing is- you can't win the race in the first corner but you certainly can lose it there. The races are 30-45 minutes long and if the track is bumpy you'll get worn out quick.

If you watch the video closely you'll see that on the back straight I peak over 90 mph (92 to be exact) according to the GPS, and take most turns about 70mph. Some people would say that karts don't have to slow down much for turns, but the inverse is true too- since everyone is trying to slow down the least (because we all want to win) it's a dogfight. To put it into perspective, laptimes in my situp class should be around 1.38.xx at Mid Ohio. That's a kart with a water cooled 125cc two stroke, no gearbox, disc brakes in the front and rear. Sit up Shifters are about 7 seconds faster, laydown shifters even more so.

There's no ABS, no traction control, no power steering. If we have Tillotson carbs we can tune the needles on the fly, if it's a Keihin/Mikuni/d'Ellorto then we have to watch the weather station like a hawk and put the right jets/needles/tubes in. We check chassis alignment three or four times a day, and change alignment/setup throughout the day to keep up with the track conditions.

Brake bias is adjustable on the fly- however, i don't know too many people who use it unless there's something quite a bit wrong. Karts are rear biased- rear locks up before fronts, because the rear tires are bigger (bigger contact patch), weight is rear biased, and the fronts are useless if they lock up first (neither brake nor directional control). Because the bias is rearward, you learn to really carry speed through the turns- threshold braking while turning marginalizes the brake AND turning ability, probably either sending you off spinning or over slowing the turn- basically , if you can threshold brake into the turn you're not going fast enough. At least, that's how it works in karts.

In cars it's a different story because there are a lot of engineered aids- brake bias is computer controlled... ABS, TC and SC intervene with most "optimistic" input... Steering rack ratios are overly slow requiring a lot of input... And engines are so tractable that you are rarely out of the power band (unless you're really goofing up) which covers up a lot of mistakes. Basically, modern cars are much easier to drive fast on tracks than cars from even as recent as 10 years ago.

[FTS]

I think you did catch my big mistake in my sentence; I meant "I have NO experience" in setting up karts

Your explanation on kart setup is precisely what is so difficult in driving karts competitively IMHO. Thankfully, it is a smaller vehicle and getting on a table/jacks is easier to make setup changes, but you really need to understand the geometric options, which are finite of course. At the same time, not having the help of shock/spring settings makes the whole thing even more difficult.

Do you use A/R bars on karts?

And driving... OMG, I cannot imagine working within a window of 20 mph; each 1 mph, or even 0.5 mph, must count. So, if you brake 5 ft too early or too late, you can either loose a spot or race, or worse, find yourself moving the lawn. Amazing.

[NickW]

Originally Posted by FTS

I think you did catch my big mistake in my sentence; I meant "I have NO experience" in setting up karts

No big deal. Everyone has to learn from somewhere. There are lots of people far more experienced than me. I've only been racing karts for about 7 years.

Your explanation on kart setup is precisely what is so difficult in driving karts competitively IMHO. Thankfully, it is a smaller vehicle and getting on a table/jacks is easier to make setup changes, but you really need to understand the geometric options, which are finite of course. At the same time, not having the help of shock/spring settings makes the whole thing even more difficult.

Do you use A/R bars on karts?

Kart setup is not easy.

First and foremost is seat position. That determines pretty much everything (because the biggest lump on the kart is the driver).

Front end setup can be separated into different ways- ride height, stiffness, and alignment. Ride height determines front end jacking, stiffness can be adjusted with a removable bar between the front yokes, and alignment determines camber, castor and toe. There is also track width, hub stiffness, wheel width and steering Ackerman to consider.

Rear end setup is determined by ride height (axle height on most karts are adjustable), axle stiffness, where and which bearings are "locked in", hub stiffness, wheel size and stiffness, and track width. There is also a removable bar in the back that determines chassis twist.

Then there's the kart chassis midsection which determines transient response from front to rear- most chassis' have a removable fourth rail that goes from the midsection to the back, and the front tray can be tightened/loosened according to what the needs are. Not to mention seat stays (adding, removing, loosening, tightening)...

Those are the basics... if you can get a handle on those you're doing better than me!

There are no antiroll bars on karts. Antiroll bars are transient torsion springs that connect the left and right suspension- they are a tuning aid and ideally should be as soft as possible if not disconnected (I know that is a controversial statement), corner spring rate and suspension geometry should determine roll. On my GT2 I have to use them as a bandaid for a conservative setup- the car pushes everywhere. With a more aggressive setup coming (springs, shocks, alignment) I hope to be able to back off the bar settings to get more bump compliance (curb crashing).

For karts you should think of the entire chassis as a giant spring, and compliance for what you want it to do is managed on a micro and macro level. If you get a good handle on how to tune a kart chassis, tuning a car is not much more difficult. Ultimately you need to think about how to work on the end that needs improvement and not take away from the one that works.

And driving... OMG, I cannot imagine working within a window of 20 mph; each 1 mph, or even 0.5 mph, must count. So, if you brake 5 ft too early or too late, you can either loose a spot or race, or worse, find yourself moving the lawn. Amazing.

Oh yeah! That's what makes it fun (or in my case that weekend, a lesson in futility). Losing 30 ft every tight corner means just keeping up is a W in my book. I'm planning on going to Grattan in a few weeks, let's hope it's better. Check out Grattan if you want to see a crazy track!

edit-

Actually, I was thinking about the "20mph window"... it's actually less than that. Top speed is a given- if you can't keep up you're out to lunch. So the relative operating window is actually your cornering speed- entry, apex and exit speeds. That is probably a much smaller window- 2 to 3 mph at most if you're competitive. It's the same in most motorsports- relative speed differences usually determine the outcome.

[FTS]

Well, this has been a great schooling for me, and I truly appreciate it. It is fascinating to me, chassis setup that is. I used to be involved with it in stage rally cars, which I thought was difficult, but karts are much more difficult based on the details you so graciously provided. We basically played with shock setup (travel/bump/rebound), ride height, springs, wheels/tires and roll bars; other than that, we left it alone.

Thank you for the details Nick

[bman]

Originally Posted by Skypalace

Next topic: pinching exits, why do I see SO many drivers keep full (or nearly full) steering input well after apex, while applying full throttle, and not unwinding the wheel and going all the way to trackout? Have seen so many incidents... Another topic though :-)

I am surprised how often I see this and it's scary! I can understand a beginner-intermediate driver doing it but I've seen it too often with intermediate-advanced drivers. Moreover, I have found a resistance to change when you point it out. I've found myself practically begging drivers to "OPEN THE WHEEL", lap after lap. And often the response is "WHY?"....You really get concerned when this happens because you worry the driver has limited feel/sense of what's going on.

I think it is applicable to this braking discussion because often it's a symptom caused by over braking and early release of the brake. The driver isn't rolling into a corner, so they're on the throttle too early.

[NickW]

I also see a lot of exit pinching. What I've noticed as a beginner's symptom is usually apexing too early. Many overly enthusiastic novice drivers begin their turn in too soon. You rarely see this problem with a late apex turn in- those drivers usually have too little speed into the turn and also too much space on track out from the apex.
The other problem I see with many drivers is not getting the wheel straight early enough- or taking too much time turning.
Another symptom is the "threshold to the apex" braker who overestimates his/her traction circle, usually missing the apex completely and thus also flying off the outside of the track exit.
When I instructed (and am about to start for the Chicago region PCA soon) I used to tell students to literally try to hit every cone on the track. I would estimate how much they were off, and encourage them to really aim for the cones on the track, thus demonstrating how much they were pinching their turn in and track outs by not using the entire track.
Teaching students how to read a track is the best lesson for them.

[Skypalace]

Here's how I (used to when I instructed) explain releasing the wheel on exit.

I explain that there is always a limited amount of traction available, and the goal in a corner is to always use all of the traction available. So you brake fully, but have to start releasing the brake as you turn in, to 'release' traction so that you can start using it to turn. When you're at apex at max turnin, you can't be on the brake or throttle as you're using all your traction to turn.

When you want to start accelerating from the apex to track out, you need to 'release' traction to make it available for acceleration, and you have to do this by releasing the wheel as you slowly apply the throttle. By the time you're on full throttle at trackout, you have to have fully released the wheel, or you won't have sufficient traction!

I also do a visualization exercise - think of a string tied between your toe and the steering wheel. When you're on full braking, string is all the way tight from the bottom of the wheel to your toe. As you turn, turning the wheel will lift your toe on the brake. As you press the accelerator after exit, think of it pulling the outside of the steering wheel back down, so that you have to accelerate.

I think that many novice and intermediate drivers are scared of getting too close to the trackout, as they're worried about getting onto (or over) the exit curbing. After all, their instructors have told them numerous times about bad things happening when they drop a wheel off the outside of a curb on exit. So in response they're staying away from track out, but their mechanism for doing so isn't doing a trackout to a lesser point, it's to keep full steering input in while exiting the corner, which at best delays their throttle input until they're pointed straight (killing their corner exit speed), or at worst adding throttle with full input, potentially causing spins or worse (exiting track when overcorrecting after attempting to catch a spin for example).

Jim

[Larry Herman]

Originally Posted by FTS

I certainly agree that the physics of vehicle dynamics dictate maximum braking for the shortest period of time and being on throttle for the longest periods is the fastest possible way around any circuit.

Fatih, why would you think that?

[FTS]

Hmmm, seemingly a simple question, but this one really made me think Larry Am I making an assumption that may not be correct?

Fastest way through a circuit is, or should be, shortest distance while having the longest time periods on the throttle, which means shortest time on brakes, which requires maximum braking needed, no more, for the car to go through the corners. What am I missing?