bagger

I'm looking for advice and justification for an ideal average brake application rate on the track. Specifically, I'm looking for the time from just starting to apply force on the brake pedal to full force on the brake pedal. In the picture that I attached <brake force time history.jpg>, this is the "t = ?" time. To keep the discussion focused, I am NOT interested in the times of the follow on events; either holding at full force or reducing force back to zero.

A side note is that I suspect that the time to full brake force is not dependent on how much speed you want to loose. In other words, the force time history curves for mild braking are the same as the time history for serious braking up to the time of getting full brake pedal force. You just hold that force longer if you want to loose more speed. In the picture, the solid line is for a mild speed reduction and the dashed line is for a serious speed reduction. Feel free to comment on this subject too.

I know that a single rate will not be ideal for every situation due to track conditions, traffic, tires, and the turn itself. But an average rate is still a good place to start. My gut tells me that this time is about about 1 second for my stock 911 3.2 Carrera. But I want to know if other people agree or disagree and, more importantly, what their justification is for the rate that they use.

The ubiquitous but frustratingly non quantitative advice (see book list below) is some version of "not to fast but not to slow" or "brake hard but don't jab" along with "you want to get the car suspension settled for the turn but you don't want to unbalance it". I have not yet read (or heard anyone give) an absolute time or rate based on objective performance characteristics of the car. In other words, nobody says that it should be 1.5 seconds based on the stiffness characteristics of the car.

I'm sure a lot of people just have an intuitive sense for what the ideal time is. And they can prove it to themselves because the car just feels optimally set through the turn. BUT what I'm hoping for is an objective technical justification. I'm a structural dynamicist so I have a good shot at understanding the physics. I suspect that the ideal initial brake time needs to be at about the natural frequency of the car rocking about its lateral axis. But I'm hoping somebody else has worked out the details.

Sources that do not have the answer that I'm looking for:
Beckman's Physics of Racing
Bondurant's High Performance Driving
Smith's Drive to Win
Bentley's Speed Secrets

Larry Herman

The answer that you are looking for is not a fixed number. First, the amount of time that you would want to ramp up to maximum braking is dependent mostly on the stiffness of your suspension and to a lesser extent on the grip of your tires. With sticky tires and very stiff springs, you can ramp up pretty fast; definitely under a second. With a stock car, the ramp up must be slower; could be almost 2 seconds.

Second, your example can only apply to situations where you want to get to maximum braking and hold it for more than a few seconds. You would never use maximum braking to drop 15 or 20 mph as it would unsettle the car too much and you would lose a tremendous amount of speed.

ervtx

The application of brake force will change a freely rotating tire into one that is slipping across the surface of the track by tranforming vehicle kinetic energy into frictional force between tire and road surface. A sticky tire will have a wide range of frictional force that it can absorb as it transitions from freely rotating (zero friction), to tire slip (optimal friction for braking), to lockup (maximum friction). In contrast, a tire with zero grip translates kinetic engery within a very small range of frictional force between rotating and lockup.

Therefore, the application of brake force for tires with zero grip provides very little margin for error in terms of reaching lockup, and sticky tires provide a very large margin for error in terms of reaching lockup. From the perspective of avoiding lockup (achieving sustainable threshold braking), tire grip should be the primary variable in determining the optimal rate of brake application.

I can think of a lot of things that influence tire grip - weight of the vehicle, size of the contact patch, tire compound, tire pressure (or how much the size of the contact patch increases under braking), tire temps, track surface temperature, track surface material, track surface conditions (e.g. wet vs. dry), brake bias and how uniformly the chassis distributes its kinetic energy to each of the 4 tires (mechanical grip of the vehicle as a whole), etc.

Given the number of variables involved, could you actually do something with the millions of possible "objective technical" answers to your question. Perhaps the human brain can learn the "good enough" subjective answers for wide enough permutation of factors such that informed intuition becomes the best approach.

Veloce Raptor

I am not as verbose as these other guys, so I'll juust say that it depends on the situation and the corner...









Professional Racing and Driving Coach

SundayDriver

I worked with a data coach some time back and we discussed this exact item. To give it perspective, he had been working (paid - only job) with various pro race teams from Indy cars to sports cars.

From the point of throttle lift to full brake pressure was 0.15 seconds for the pros. I was right at 0.20 seconds. This is as I recall and it has been a few years - I distinctly remember the '15' number, so I am assuming it must be 15 hundredths.

Not sure that the natural frequency of the car matters that much - if the dampers are doing their job, the car is not going to oscillate, rather it will settle because the damping is greater than critical.

ervtx

You could always post a picture. Yours are usually worth more than the proverbial thousand words!

Veloce Raptor

..

Sean F

Jerry Austin posted this in another thread and he knows what he's talking about

http://www.optimumg.com/OptimumGWebS...e_tech_tip.pdf

bobt993

Since your starting new I would think in terms of where you want to stop braking relative to the apex in each turn on the track. Not how hard but where the car has obtained the best entry speed and is somewhat stable in the corner.

SundayDriver

Great article - I took their class and even added the two math channels discussed. But I had never seen that article before. (I guess I remembered correctly about the times in my previous post.)

ervtx

OMG, you are RIGHT! I see it clearly now. Thanks!

"Notice that this driver takes 0.22s to build maximum pressure. There is still some room for improvement
as there are many drivers who are able to build maximum pressure in 0.20s or less."

Improving my time to peak brake pressure from 2.2 to 2.0 seems doable. Improving from 0.22 to 0.20? Ha! HAHAHAHAHA!!!! Yeah. Right!

Good info. Thanks.

bagger

Thanks for the posts everybody!

Sean F, many thanks for the link; it is the most explicit and technical answer that I've seen. It seems to say that the smaller the time the better. I can't tell if that advice is for a street car or F-1 though. Plus, it doesn't give justification. But I e-mailed OptimumG with my questions, so hopefully they will answer.

bagger

This is the answer from Chris Meissen at OptimumG:

> > Yes typically you want to build brake pressure as quick as possible.
> > The reason for this is that initial braking is typically done in a
> > straight line with little lateral acceleration. Therefore to achieve
> > the best performance you always want use your tires at their maximum
> > capability.
> >
> > So initially when you brake in a straight line you want to immediately
> > be using the brakes and tires at their peak performance. When you
> > start entering the corner you will want to ease off the brakes as the
> > lateral acceleration of the vehicle increases. As the lateral grip
> > provided from the tires increases the longitudinal grip capability
> > will decrease.
> >
> > So even though you don't have a extremely stiffly sprung race car, you
> > will still get the maximum speed from initially braking as hard as
> > possible, as long as you don't lock the brakes up or lose the
> > stability of the car.

By losing the stability of the car, I mean that some cars under hard
> > braking may have a tendency to spin because the real wheels can become
> > almost completely unloaded. You are correct that with a properly setup
> > 911 this shouldn't happen, since a 911 has the majority of its weight
> > on the rear wheels.

Potomac-Greg

At my age, to achieve 0.22 (let alone 0.20) would require me to start braking a good 1/2 second before I even think about it. Which may be impossible.

RickBetterley

Jeez, that is a damned impressive writeup. Thanks for the link.