mx5-kiwi

New Trackspeed Superlight kit arrived and Pads are imminent from Supermiata (thanks top both of these compamies for awesome service as usual) so ive been pondering braking a bit while we wait the weeek or two for shippinmg and customs etc...

There are some clever people here with math capabilities far beyond my own. So im wondering if anyone has the time to do a rough calculation for me...

At a couple of tracks we race at, I find that I tend to coast the last little bit in to a braking point rather than retain full throttle and then hit brake pedal.

This is almost always on the long back straights, for argument sake, travelling at 200km/h and I might coast for approx 20 - 30m (maybe up to 50m at that speed..) before hitting the brake pedal hard.

I presume there is a way to figure out the time lost doing this versus staying foot flat accelerating? My guess is vehicle weight, HP and aero drag all play a part....?

I have always presumed it was minimal and figured keeping the weight transition from accelration to braking smoother and the better accuracy for the brake marker in terms of too early or worse too late, was best.

Anyone have any thoughts on this or example calculations that might prove or disprove my theory?

Madjak

I bring no calcs, but the time savings are minimal. The extra few kph you might gain from staying on the throttle over the last 30m results in gains of only a few hundreths of seconds. Time you might gain by focusing on getting more ideal braking. Also, its common for race cars to sacrifice outright top speed at the fastest straight to gain better gearing especially if it results in a better run out of specific corners.

If I run my c/r 5 speed dog box with 4.78 diff my top speed is 190 kph at 9000 rpm which I hit around 100m before the braking point. I have to lift and coast but that box is still faster than my 6 speed because the gearing keeps me in a better power band on several corners before that straight. I'm basically gaining 5kph down the entire length of the straight and only losing that 5kph for the last 100m.

The only problem is when overtaking cars and you get a draft. When you go to overtake and you hit that top speed cap early and lose a car length or two.

Schroedinger

codrus

If you can eliminate the coasting without upsetting the car (which should be possible) then it will improve your laptimes, but yes, the gains from that are fairly small.

--Ian

Savington

iTrader: (31)

One of the very first thing new drivers are taught is to move their feet faster. There is no logical reason to spend 50m coasting.

This comes down to friction circle basics. You want to spend your time accelerating at max capacity, braking at max capacity, or turning at max capacity, or some combination of those. If you are coasting, you are sitting dead center in the friction circle, which is what you are trying to avoid the entire time you are driving.

Savington

iTrader: (31)

Pro V8 Supercars driver, no delay between throttle and brake:



Max Papis driving some sort of Cup car, zero delay between throttle and brake (usually less than zero delay, he actually preloads the brakes before lifting)

z31maniac

Underrated road course driver.

HarryB

Holy. Talking about braking hard.

mx5-kiwi

Thanks for the input.

I appreciate that reducing or having no coast is a definate improvement and somethign to work on. Was just wondering if my hunch was correct that it was for pretty small gains in the meantime though.

As the season seat time and cicuit familiarity come up im pretty sure I naturally reduce this (particularly when actually racing) but watched one of my in cars on another thread on a, waiting for traffic to clear superlap lap and realised the coast was quite apparent.

Cheers.

Neddy

rrjwilson

I would like to throw in an additional consideration.
Going straight from throttle to brake means you have more momentum which means as the load transfers to the front your front end grip is higher.
This translates to better initial turn in which dependent on the corner could be a real make or break to your line considerations.

Jackie Stewart has written a few things about training the current top drivers because they are not as aware about the changes in chassis dynamics as the old school folk.
I'll try to dig out the one i really liked about braking but it basically said jumping on the brake created huge swings in load which made the vehicle unstable where as if you give the pads a kiss first the load swing isn't as high allowing you to pile on more brake pressure and therefore load.
He likened it to jump out at someone if i remember rightly.

Edit:

Madjak

I don't think anyone is saying that it's not optimal to reduce time between throttle and brake under ideal conditions. However in the case of a pro-am driver with restricted gearing options and limited car setup options, the time savings are minimal and would be overwhelmed by the error within the rest of the lap.

Its best to look at your data logs as they will tell the full story. You'll see how much you lose and how much you can gain of the later braking is giving you less ideal corner speed.

Savington

iTrader: (31)

Stewart is a great driver but not a physicist. The reason you roll into the brakes has nothing to do with chassis dynamics and everything to do with weight transfer and suspension loading.

Under braking, as the car transfers weight onto the front tires, you can increase brake pressure. IOW, the max allowable front line pressure increases as the weight transfers. If you stab the brakes immediately and exceed that initial max line pressure, the tires lock, but if you allow the weight to transfer first, you end up able to brake harder.

The justification here is to roll quickly onto the brake pedal. Once you touch the pedal, the weight starts to transfer, and in a stiffly-sprung race car, it doesn't take long. Ergo, the technique taught in every race school is to quickly roll onto the brakes instead of stabbing them as hard as you possibly can. When I say "quickly", I mean it - "rolling" onto the pedal should take a couple tenths of a second at most in a race car, maybe a bit more in a street car with softer suspension.

MrJon

To throw the most basic of rough estimate math at this, 200km/h translates to about 55m/s, so in your worst case of 50m it's taking about .9 seconds to cover that distance. If you were traveling 220km/h that's about 61 m/s so over the same 50m that's about .82 seconds. For a savings of .08. ish. And that would really be quite a bit bigger savings than reality because you aren't going to instantly accelerate to 220, also I doubt you can gain anywhere near that much speed in ~1s. So a few hundredths seems like a good working estimate of the absolute worst case time cost.

d = 1/2(a*t^2) + v*t , d is distance, a is acceleration, and t is time if you want to take the next step in the math.

codrus

Now do that for 10 corners and you've got a few tenths per lap. It matters more on the shorter straights too.

"lift and coast" is a technique for saving fuel/brakes with minimal lap time cost, but it's still significant compared to a full-speed lap.

--Ian

Savington

iTrader: (31)

I would love it if all of my competitors would chop the throttle 150ft early into every corner. I'd win a lot more races.

mx5-kiwi

Well lets just clarify for the sake of the threads (and my driving) integrity.

Given we race in a field of 20+ cars each time and we are regularly in the first group (despite handicap racing) with class, race and series wins you can rest assured it is more a roll off the throttle than an early "chop".

I may not be the last of the late brakers but im sure as **** not giving up positions by doing that!!!

My reference of coast was more that I'm not flat to the floor accelerating as I line up the braking marker. At 200 - 215 km/h this is probably around 30m and as calcualted above (thank you very much Mrjon, that was the calcualtion I was after) maybe transitioning over a 0.2 - 0.5 of a second.

Probably too long but as pointed out it's only a very small time loss.

Further I tend to only do it on the straights with the long braking zones (high speed) not throughout the course for the reasons said in previous post. Accuracy to braking marker, turn in point and most importantly being on the throttle for the best exit.

I figured that trading a tenth or two for that might be worthwhile BUT is definately an area for improvement. Will have to watch more of my in car to see if it is the case still.

Appreciate the thoughts and input.

jmann

Here is a very good lesson on braking. Tenths per corner add up around the whole track. A few hundredths is a lot when you consider that 20 cars qualifying pos.1 to 20 can be separated by a few tenths
https://driver61.com/uni/braking/

mx5-kiwi

Good link thank you.

Intersting that Phase one (pasted below) is pretty much what I figured in the first place. Will practice this off throttle on to brake speed more on Assetto Corsa. Am also practicing longer 30 minute races in preparation.

The Merc Cosworth 190 against the gt2 and similar cars makes for a fun challenge Anyone one else on there?

Phase 1: The movement from the throttle to brake pedal must be as fast as possible. Any time lost here isn’t huge, but it’s still time lost.

Phases 2: When we are applying the brakes we don’t want to shock the car, which will cause it to break traction. In the same breath, we don’t want to take too long to get to maximum braking capacity. It’s a fine line to get this perfectly and this phase requires a lot of feel.

If your inputs are refined enough, you can begin to feel when the tyre starts to under rotate – something we’re going to go over in the next section of this article.

Phases 3: Next, it’s a case of modulating brake pressure to keep the car at maximum deceleration and around the threshold of grip.

Phase 4: Finally, as you’re approaching turn-in, you’ll begin to smoothly release the brake pressure, so the front of the car rises to a balanced platform (see weight transfer article here). Hopefully, at this point, you’re at the correct speed and on the perfect racing line. If so, you’re almost halfway to taking the perfect corner!

Madjak

I didn't think we were discussing applying this over every corner, I thought we were only referencing the close to terminal speed at the end of a very fast straight. In that case lifting slightly early to get a better prep into a hard braking zone makes next to no difference. The calcs above are assuming the car is still accelerating a lot (20kph in 30m at 200kph... thats faster than an F1), however in my case (N/A) the acceleration in that last 30m is next to nothing so I'd be lucky to pick up 2-3kph if that. So it's more likely a difference in the thousandths than hundreths of a second. For a high powered turbo car you might lose a bit more though, but still it's close to nothing. If you are a driver that feels that extra pause results in a calmer more controlled braking it is absolutely worth doing.

It really comes down to the car setup and driver. Sure it's optimal to be accelerating right up to the brake point, no-one is disagreeing there, but there is absolutely no point doing that if it unsettles the car or driver during braking especially given the savings in time are so minimal. If you are unsure, look at the logs and judge it yourself. Try it both ways and see which is quicker.

FYI, I log both front and rear brake pressure so I can see how the brakes are performing. Since removing my brake booster I can see that my braking is far more consistent and closer to the limit throughout the entire braking zone, plus I've pushed the rear bias up so my rears are doing a lot more work. Prior to removing the booster, there was always a large spike in brake pressure on initial brake application and then fluctuations throughout the braking zone as I blipped the throttle. This means I can be far more aggressive on the initial brake application than before. My car has crazy cams though so this might be different for others, but if you haven't switched to dual masters, balance bar and brake pressure logging then you aren't maximising your braking yet. If you get the braking zone right, there is a lot more time to save than in that last 30m on a straight.