Steering drag in high speed corners

[Belatti]

Some food for thought here:

A month ago I saw data acquisition from a steel frame / 120HP / 550Kg-driver included openwheeler. It called my attention that the car entered a 200m radius corner, pedal to the metal at 210Km/h. Full throttle. The exit speed? 185 Km/h...

The driver barely turned the steering wheel. The tyres are a little bit over sized for that car... from my humble point of view.

Why that much steering drag? I dont know the steering geometry in detail, but the first thing that came to my head was and odd ackerman.

[raymondu999]

The front tyres scrubbed too much I guess?

[Caito]

Side wind? It could affect but not all that much, I think.


210 was top speed?

If the car went from 210 to 185 there was a substantial power loss. Do you have more data to calculate how much power was lost? Given speed we could calculate the total force that was stopping the car. Then maybe JT would tell us if the value is reasonable to be accounted to tire scrub.

PS: I know power is not a quantity to be lost, but you know what I mean. Too lazy to write that again correctly.

[mep]

How about the tires creating more friction when they get loaded sideways/get deformed?
Does not need to be scrub which is as I understand it is track wide chance during suspension movement.

[Ciro Pabón]

Well, any curve reduces your speed, specially when all the power of your car is devoted to air drag.

First, because the car "feels" the wind sideways. Second, you're changing the speed vector. Third, any Ackerman drags you. Fourth, even without Ackerman, the slip angle means friction. With 110 HP, you have no excess power to overcome air drag plus everything else.

[Belatti]

OK, I know the reasons why the car slows down. What I cant quantify is how much does every cause adds to the effect. A good way to know it would be to work with the Ackerman and check results.

There is little or nothing to do with yaw aero, "natural" (aka not Ackerman) slip angle drag and tire deformation.

Although Caito mentioned "power loss" and made me think about the fact that the "standard" 1.6L 4 banger engine has no dry sump and 1.6 Gs is not what it was designed for...

[Ciro Pabón]

Well, then check engine temperature. If friction increased by oil slush, this could be seen in the instruments.

As for quantifying the effects, I'll make this assumption:

The lateral slip is the tangent of the angle of the wheel and the speed vector.

The side force is proportional to this angle, at least for low values of it (what's called lateral stiffness).

Max values for the slip (tangent of angle) are around 0.10

Let's say the driver actually moved the steering wheel very little. So, let's say the tangent of slip angle would be around a quarter of max, that is 0.025.

So, for a car to take a corner at 200 meters radius (no apex cut), you need a lateral force of:

a = v^2/r
a = (210 / 3.6 m/s)^2 / 200 m
a = 17 m/s2

So, IF the relationship between "phantom" braking force (caused by the slip angle) and lateral force is equal to slip angle tangent, then

braking force = 17 * 0.025 = 0.42 m/s2

Thus, to obtain a reduction of 25 kph (from 210 to 185 kph), equal to 6.9 m/s, you need a time of 16 seconds of curve (that is, 6.9/0.42).

Too long a curve, I presume. If the curve is half a radius in length (28 degrees of deflection, as a radian in length is 57 degrees), it measures only 100 meters, which at 58 m/s gives you barely under 2 seconds to get this deceleration.

So, either the braking force is higher than 0.025 times the lateral force or the other factors took a significant share of the deceleration. For a 2 seconds curve, you need a deceleration of 0.2 times the lateral acceleration to explain the reduction in speed, which is a lot (over 1.6*0.2 = 0.3 g). You wonder why this car needs brakes: you can substitute them with the steering wheel alone. THAT could be an advantage. You only need to take late apex curves and forget about braking.

On the other hand, you could try to compare the telemetry you have with the one of cars with regular tyres or regular Ackerman (you wouldn't need everything, just speed). As aero yaw and other effects, including oil slush, should be similar for similar cars, then you'll be able to evaluate what's different in this one (tyre width, unusual Ackerman).

If data from other teams is hard to get by, talk with Mike Coughlan or Nigel Stepney. I mean, they could give you figures for braking force vs lateral force.

[Jersey Tom]

Ciro's got it.

Bear in mind, as speed goes up the driver will have to turn the wheel less and less to generate the same slip angles on the front tires... so yeah in a high speed corner you're only going to use a small steer input. Might have to use double or triple the steering input in slower corners to get the same slip.

[Ciro Pabón]

How could I get it, Thomas? Actually, I was expecting you to explain, I was guessing. I confess I didn't read enough before posting.

Some pointer to data on this braking force vs slip angle would be great for Belatti who, I can attest to the Gods of racing, is working his neck to win actual races. If I were rude, I would ask for a discrete PM, but you know me, I would never do that.

Now, Belatti, how many seconds does that curve takes? Please, I'm truly intrigued, thank you.

I'd go with your first explanation, that is, tyres too wide. You know, I never thought wide tyres had this kind of "phantom braking" problem (if Tom confirms that's the case and I'm not misinterpreting his previous post).

After all, ché, if you know the Ackerman characteristics, you know the angles of the wheels, so, if you get to know how much that angle means in terms of braking, well. Then maybe you can show us the results, for mortals to get an idea of how large is that effect, whose understanding, in some situations, could be important to drive properly.

Sideways braking. They say Ayrton was lucky... I have always had the sensation that F1 cars brake harder when moving sideways than when braking as usual, specially when locked, but I must be wrong, am I?



The reason why I'm intrigued is because I guess that that the size of safety areas in circuits is related to your question, at least for the extreme case of "steer braking", if that's how this kind of braking could be called. It would be nice to get an idea of how this works, you know, for class teaching. After all, FIA has invested in a lot of work in safety area size, but for regular engineers little data is available, that I know.

The effect of braking caused by curves is very noticeable in large vehicles on steep slopes with high bankings (or so I think I've felt!), when you tow a large load and you can see the wheels twisting sideways noticeably on every curve. I wonder, if I'm not being carried away, how much does it means on terms of gas, when you consider the economics of a road design. Now that I think about it, well, yes, it IS taken in account in some models...

Are there any rules of thumb, Tom?

[Jersey Tom]

Steer-braking.. slip angle drag.. whatever you wanna call it...

Fx = Fy_Tire * sin(slipAngle)

The more slip angle you put into a tire to generate lateral force... the more that lateral force is aiming "backward" rather than into the cornering. Just that simple.

[Caito]

Given this, is there any better way to take the curve?

Probably like a V-shape. Don't steer, steer hard and go out. I mean for this type of full throttle corners.



I forgot to mention something really important. Does the car has a LSD?( hope the car was not high).

When you steer the speed differential of the rear wheels(plus positive torque for the full throttle) may be causing lock up (or partial lock up) hence the high drag.

I didn't invent this, I was told of it. That some racers on the straight made sharp adjustments instead of a soft movement along not so straight straights.


Hope it helps.

Bye bye,

Caito.-

[Ciro Pabón]

Thanks, Tom, you're very kind. So I was wrong. I assumed Fx=Fy_tire*tan(slip angle). Anyway, the results are almost the same because for so small angle (about 5 degrees), sin and tan are almost equal. For large angles, sin gives you 1, while tan gives you infinity...

I think I now see the point. So, at high speed curves the slip angles are hard. This means the car is at the limit, as you expect in a race.

Caito, I have no idea if taking this curve as a hard curve will give you a better exit, given that the change in speed is at mid road between "large" and "small", that is you lose 25 kph in 210 kph AND you have a relatively underpowered car. As Caito explains, some racers use "late apex" curves, I don't know if Caito agrees with what I said elsewhere, about proposing that late apex tactic is better:

- The smaller the radius of the curve
- The more powerful your car is
- The lower the grip of your car
- The longest the following straight is

This is a midsized radius curve, with a midpowered car, with oversized tyres (high grip?) so who knows. Better for one is to go and ask the driver what he thinks about oversized tyres requiring a gentler or harsher steering.

I'd say gentler, or try narrower tyres to see what happens with harder, late apex steering. However, with 110 Hp is hard to gain a LOT of speed on the exit. You know, if the car is at top speed, regaining those last 10 kph from 200 to 210 kph is agonizing... notice how in smaller categories a lot of drivers try to carry as much momentum as they can on relatively small radius curves, because the bodies aren't what you call the best in the world and air drag is high, so a small vehicle that carries momentum through the curve, in the last curve before the straight, usually takes away curve number one.

[strad]

The front tyres scrubbed too much I guess?

[Jersey Tom]

"Scrubbed" is a little vague

[strad]

Think so? Pretty universally used throughout racing.