Braking and turning in - Trail Braking

[Jersey Tom]

Depends on how you define "planted" but I'd say no, not particularly.

It won't spin out, but you can get some very "lively" rotation and yaw without getting to the limit.

[Richard]

JT - I think you are saying that it is more complex than a linear relationship based on vector x and y. After all, we know tyres slip and slide in relation to the track surface even when there is good grip. We also know that tyre adhesion isn't just based on simple linear rules of friction.

To use the traction circle analogy, rather than a binary grip or no grip at the edge of the circle we have a blurred edge. Cars can venture out onto the areas where the traction is blurred for some advantage in the right circumstances.

It's all about compromise. One car may slide through a bend carrying more velocity, the other may maintain better traction but carry less velocity. One or other might give greater or lesser speed for that section of track. I'm deliberately using "more" and "less" because it is not an absolute science of grip or no grip.

[Jersey Tom]

With regard to the "blurred edge" analogy, that's not quite what I'm getting at. And while the "edge" does move based on downforce, temperature, whatever... I'd say the limit itself isn't that blurred. Exceed the combined limit on a front tire in trailbraking and you will lock it up and push. Exceed the combined limit on a rear tire in trailbraking and you will lock it up and have a good chance of spinning out.

But, when going through a corner... even a big sweeping constant speed corner... you can feather the throttle or put some pressure on the brakes, tip some load from the rear axle to the front, and get some extra yaw attitude or rotation. This is not necessarily limit behavior, and I'd say it's a trait that most anyone who's done any driving (autocross, sim, circuit, whatever) has experienced.

Incidentally, that behavior is the reason I picked the quote in my sig.

Tires and vehicle dynamics really aren't inherently that crazy and mysterious - but people let them be so with a hail of hand waving and vague terminology.

[Richard]

Thanks JT

A friend also sent me this useful info....

Maybe it helps to look at the "traction circle" in terms of forces at the (generated by the) tyre.

Like this:


What you call "the traction cycle" is the "rim of the crater" (global peak). Note, that this illustration is "normalized", which means it reference to 1 (100%) of the max. capability of the tyre. In absolute values, you would get back to your "ellipse or heart shape).

Some (most?) people will do this normalization, when assessing driver performance, as in how much the driver is able to utilize the 100% grip available.

If you go "outside" the circle, you lose force, so technically it is the same, as being a given distance from the "Circle" on the inside (same amount of force from the tyre).

To do the correct normalization, requires good tyre data, and also accounts for the increase in "grip" due to aerodynamic downforce.

But the limit is always 100%, even when the values change in magnitude (G or N)

How much the tyre will "fall off" on the other side of the maximum, will depend on the tyre and the conditions in question.

[raymondu999]

I don't think that's particularly true to be honest. Braking early can be tactical. For example if you were behind another car, which has much worse traction than yours; say for example out of La Source. If you didn't think you had enough of an advantage to get the guy by Eau Rouge; you'd probably brake slightly and focus on getting a better run through Eau Rouge.

[Intego]

Left foot braking is driver-dependant. IIRC Barrichello e. g. is a driver who brakes with his right foot ...
I also thought about left foot braking with the right one still on throttle for exhaust gases. I believe that the rules state that the gas flow into the engine has to be proportional to throttle position (pedal). So blowing could be inefficient mileage-wise ...

From Scarbs:

Now the throttle pedal position must map more closely the actual engine throttle position, thus if the driver is off the throttle pedal, then the engine throttles must be correspondingly closed.

[Coefficient]

Left foot braking is driver-dependant. IIRC Barrichello e. g. is a driver who brakes with his right foot ...
I also thought about left foot braking with the right one still on throttle for exhaust gases. I believe that the rules state that the gas flow into the engine has to be proportional to throttle position (pedal). So blowing could be inefficient mileage-wise ...

From Scarbs:

Now the throttle pedal position must map more closely the actual engine throttle position, thus if the driver is off the throttle pedal, then the engine throttles must be correspondingly closed.

Barrichello was forced to adapt to left foot breaking years ago as monocoque dimension became ever tighter and no longer allowed room to hold the left foot out of the way. This was whilst still at Ferrari I believe. Also, left foot breaking became fundamentally the quicker way to drive a modern car so he had to adapt.

[Gridlock]

Barrichello was forced to adapt to left foot breaking years ago as monocoque dimension became ever tighter and no longer allowed room to hold the left foot out of the way. This was whilst still at Ferrari I believe. Also, left foot breaking became fundamentally the quicker way to drive a modern car so he had to adapt.

Not according to one Rubens Barichello, on The Flying Lap 3 weeks ago. But we digress..

[gold333]

There are times you must exceed the oval and break traction to get the car turned in.

There are certainly things you can do to get the car to pivot on entry - I agree this can be advantageous - but has nothing to do with exceeding traction.

Senna mentions in his own book that his method of advancing the apex (late apexing for higher exit speed) included drifting the F1 car around slow and medium speed corners from 88-91.

Surely a car that is drifting has broken traction past slipping and is sliding. Senna specifically mentions opposite locking and holding the slide while the car pivots (yaws).

[Jersey Tom]

I wouldn't necessarily agree with that assessment, no.

Tires are always operating at some amount of sideslip. Just because the car is a bit "hung out" doesn't mean you've exceeded the limit capacity of the tires. Just means you've done something to make one axle work at a higher sideslip angle than the other.

[Tim.Wright]

I think there are two effects here to consider

1. You cant operate the car at 100% of the tyre force capabiliy because there is no stabilising forces so its too difficult to control. Therefore, eveyone will operate at (for example) 95%. Now you can get 95% of the tyre force capability in two conditions. One slightly below the optimum slip angle, and the other slightly above the optimum slip. When drivers talk of drifing a car, I believe this means that they are at 95% of force capability on both axles but the rear axle is operating over optimum slip angle and the front is operating slightly under.

This means the car must be slightly oversteering to start with and the driver is able to control it after rear has past its optimum but while the front still has grip capability.

Its very rare that a car will be neutral through every corner, so to get the optimum out of it, you generally need to have one axle near or at its saturation point in order to get the full potential from the other axle.

2. Its possible to have higher lateral accelerations in transient conditions than you could acheive in steady state conditions and I think this is how Senna was operating the car. If you look at a Milliken Moment Diagram, you will see that the peak lateral acceleration can occur still with some yaw moment so by definition this is not a steady condition.



Tim

[raymondu999]

I wouldn't necessarily agree with that assessment, no.

Tires are always operating at some amount of sideslip. Just because the car is a bit "hung out" doesn't mean you've exceeded the limit capacity of the tires. Just means you've done something to make one axle work at a higher sideslip angle than the other.

From a conceptual/theoretical point of view; I kind of understand now. But could you give an example where this would happen in practice, JT?

Its very popular in sim racing.

How many sim racing apps/games can properly simulate this though? I'm thinking the tyre modeling would be quite resource intensive - do they even have the physics engines to simulate it at a personal computer level? Or would they just have response curves which the sim would refer to? (i.e. basically it has a database of inputs vs responses - giving out a response by comparing your input to the database of inputs)

[allstaruk08]

i know it was spoke about a little bit up the thread but if you press the brake and the throttle at the same time you run the risk of cutting the engine out. theres a setting in the ECU that has a safety cut off, i remember a couple of seasons ago brawn telling schumacher to calm off the brake when on the throttle in the corners. after the race he explained schumacher used the brake as a kind of T/C and was running close to cutting the engine.

[raymondu999]

Why would you be on the throttle on trail braking entry though?

[timbo]

Why would you be on the throttle on trail braking entry though?

Not on entry usually, but sometimes during the initial braking.
And sometimes a touch of brakes on exit to combat oversteer.
All Schuey's techniques.