Speed Secrets: Getting the Most Out of Driver Feedback
By Ross Bentley
April 11, 2016
One of the best parts of the 3-day Speed Secrets Driver Development Camps (which we put on years ago for young kids transitioning from karting to car racing) was the sessions we did with Jeff Braun. Most of you know that he's a great friend of mine, someone I have the utmost respect for, and he just happens to be one of the best race car engineers in the world. But what makes Jeff special is his ability to explain things in ways that many engineers cannot.
Which is exactly why this week's feature article is so cool. Jeff is describing the terms and phrases that drivers use - or should use, in some cases - to describe how their cars are handling. As Jeff knows better than most, if the drivers describe the problem the right way, the fix is easy (or, at least, easier). -Ross
Most young or new drivers think they have to be good at engineering the car, giving good feedback, or be experts in debriefing. They're concerned with how to explain what the car is doing and worried about how to put that in terms that the race engineer will understand. They worry that the engineer will think they are non-technical. They have all read of or heard about Senna or Prost or Dixon and how good “technically” those drivers were. They think they should be, too.
Most good engineers don’t want a driver to tell them what to change or how to set up the car. They want the driver to tell them how the car feels to them, what it's doing and most importantly, what they want it to do differently so they can drive it faster. Then, the engineer can look at the data and see why the car's doing what the driver describes. By knowing that, and knowing what the driver wants it to do, the engineer can use his knowledge, data, simulations, and other tricks to make the car do what the driver wants. If the car feels the way the driver wants it, he will get the most out of it that he can.
The driver's goal then becomes clear. Don’t try to be an engineer, just explain to your engineer WHAT THE CAR FEELS LIKE. I hear many explanations of what the car feels like when I'm race engineering cars. I thought a discussion of a few debrief terms I've heard over the years would help drivers explain the feeling better, and engineers better understand what the driver is saying. The fact that the driver and engineer are often the same person may give you both a different perspective on some common feelings!
THE CAR IS TOO FREE
Race cars have two basic types of handling. One: the car will not turn into the corner and the driver has to keep adding steering wheel to make it go where he wants. That's Push or Understeer. Two: the car turns too well and wants to spin out. That's called Loose or Oversteer. To be the fastest, you want a car that's right in the middle and does not do either. On ovals, the drivers will really split hairs and get detailed on how loose the car is. The Days of Thunder movie did get one thing right - "loose is fast." But too loose and you crash.
So drivers and engineers want to walk that fine line to get a car that's "free," but not loose. Free is the feeling that the car is not bound up or "tight" from understeer, but at the same time, not so loose that it requires the driver to lift or countersteer to avoid crashing. So drivers refine and focus the engineer on the amount of "loose" or "free" they have. Free is good. TOO FREE is getting to the point where the car starts to be scary and on the verge of forcing the driver to slow down to avoid spinning. In qualifying, a driver wants a car that's TOO FREE for a long run, because it's faster and new tires can help for a few laps. We have many pictures of loose or oversteering cars. The photo below shows you what a FREE car/truck looks like from behind. Note the slight angle of the truck on entry and the direction it's traveling. That's FREE. Only the driver knows if its TOO FREE.
IT BLOWS THROUGH THE TIRE
Drivers will use this expression, usually when talking about the front tire (but sometimes it can be a rear tire). So let's look at the more likely front tire case.
A car can understeer, or push, or be tight in many different ways. Drivers try to be exacting in how they describe a push to the engineers so the engineers can best fix the issue. To just say "it pushes" is not nearly enough. One of the ways a car can push is by "blowing through the tire." The driver feels this when he turns into the corner, the front tires start to grip, and the front end responds with a direction change to the apex. But as the load increases on the outside tire, it very suddenly loses grip and starts to slide. The car stops turning and you have a classic understeer/push, where the driver has to slow down, reduce the load on the tire to gain grip, and make the car turn. The distinguishing thing here is that the tire "tried" to make grip, built up side load, and it all felt good. Then, suddenly, it could not take the last bit of additional load, "BLEW THROUGH THE TIRE" and started to slide.
The engineer will make a much different change in this case than if the driver said the push was caused by "a flat slide" or "it rolls over and then gives up late into the corner."
This one is different from being TOO FREE. A driver will use this term to describe a car that he does not like, usually on the entry to a turn. The feeling usually starts while he's still going straight and applies the brakes. It can continue while he starts to turn into the corner, but usually if it's really nervous on the brakes, he has slowed enough and has to gather it up before he dares turn in. It feels like the rear lacks grip as he brakes and the rear of the car moves slightly side-to-side at very high speeds, sort of a "wiggling" of the rear as it tries to settle down and the driver figures out what the car is going to do.
Drivers hate this because the car's not doing one single thing and they have to wait for the nervousness to calm down before they can turn in. It's not a comfortable feeling and kills confidence right when you need it the most... at high speed, entering a corner.
A nervous car is also very slow because the driver has to slow early, let the rear settle down, then turn in at lower speed; now the whole turn is slow. So a nervous entry is a disaster and something the race engineer will work hard to correct, because almost nothing the car does following the entry matters - the nervous entry dominates everything that happens after that.
Engineers will look at aero balance to help (move the center of pressure rearward, more rear wing, less rake, less front wing, etc). They can use alignment changes like adding more rear toe-in, or standing up the rear tire camber. The diff plays a huge part in entry stability. Increasing the pre-load or reducing the coast diff ramp angle can help. Shock settings are a big part, like increasing the low speed front bump to control the pitch of the car (front moving down/rear moving up), or increasing rear low speed rebound. Packers and bump rubbers can be used in the front third to control pitch of the car. Even simple things like brake bias can impact a nervous entry.
IT FALLS OVER
It's always a balance for the engineer to make the car stiff enough with springs, anti-roll bars, and shocks to give the driver good feel, a stable, crisp platform, and at the same time, not make it so stiff that it hurts ultimate grip. Drivers will usually report a car "falling over" when it's too soft and rolls too much when turning. There are many details to this condition:
The car can fall over if it's too soft and the car starts to turn, makes grip, continues to turn and everything is going along well, but then has rolled over so much that the suspension is now in a range where it's not happy, and the car loses grip and slides. That’s usually not so bad for the engineer; at least it tried to make grip.
The car instantly falls over on turn-in and never takes a set; the driver is chasing it to the apex, trying to figure out what its going to do. This is really bad, as it messes up the whole corner and you're going to be really slow.
It can also fall over on the front. This feels like the outside front makes grip, the car loads that tire, and the chassis rolls, but then it continues to roll and falls over and slides the tire. Some drivers report this as "tripping over the outside front" or "hooking around the front" (see more below). Again, at least it tried to make grip, that’s good.
Most of the falling over issues are solved by going stiffer with spring, shock, or anti-roll bar. Many drivers confuse falling over with grip. The best drivers can accept a bit of falling over as they know the car's making more grip. The less experienced drivers need a super-stiff car that is very directional. But they get that at the cost of less grip. These guys will complain of a "FLAT SLIDE." We will discuss that at another time.
HOOKS AROUND THE FRONT
We talked about "falling over" and this one is often a byproduct of falling over. Most drivers report this feeling when they feel the front of the car has so much grip that it plants hard and the rear comes around due to the excessive front grip. The driver feels the front so powerfully, making so much grip, that it sort of digs in the outside front tire and pivots the car around it. Other ways a driver will report this feeling is "it pins the outside front," or "it's like a stake is driven through the outside front and it goes loose."
What causes this? Many things. If the car falls over from being too soft, it can roll, the driver slows the entry, and then the front tire recovers and gains the grip. Suddenly, the car hooks around that tire, making it super snap loose. It is also possible for the aerodynamic center of pressure (CoP) to be too far forward and pin the front from the excessive front downforce, making it hook from an aero standpoint. The driver can also cause it if he has a understeering car and charges the corner, gets the understeer, lifts to avoid running out of room, and then when the car speed is low enough and the tire gains grip, he has tons of steering angle in the car and BAM! Hooks around the front.
What do we do to prevent this, as engineers? Not have the car too soft so it falls over. Watch the CoP, don't get it too far forward, and hire talented drivers who know how to drive an understeering car... ha ha.
So there can be many reasons for the feeling. The best drivers can point the engineer in the right direction to fix it. By the way, the drifting guys WANT their car to "hook around the front," so it's not always a bad thing.
I could go on and on with these, so that’s a good excuse for a Part Two of Driver Debrief Terms. I really need to explain the term “Wonky.” That came up often while recently engineering a LMPC car at Sebring.