More on The 2 Foot Magic Save Hax
Three days ago, I posted a story on a physics exploit in iRacing that allows saving spins by applying full pressure on the both the throttle and the brakes.
Called the “2 Foot Magic Save Hax”, the issue already raised some ruckus in the iRacing forums before I decided to post a story on it. As usual, VirtualR took a neutral stance on the issue as the idea was to let you, the reader, decide on how serious the issue is using a video provided that shows the problem in effect.
Instead, the story caused on the biggest shitstorms in the history of VirtualR as almost 300 comments have piled up in the initial story, ranging from rational discussion to wild accusations of all kind.
To put this issue to rest, here’s some follow up material on the issue as physic experts Niels Heusinkveld of Reiza Studios and Kunos Simulazioni’s Stefano Casillo have weighed in on the issue.
Click Here to read Niels’ statement
While I don’t think iRacing is nearly as realistic as it should be, this behavior isn’t that strange or magic. To some extend, the engine power cancels out the braking that is done at the rear. Say this is achieved perfectly, then you have a 100% front brake bias, with free rolling rear tires.
The front tires are locked, so regardless of steering, they produce contact patch force perfectly in the opposite direction the car is traveling. Even at 25 degrees car yaw angle, the front tire force is mainly longitudinal (braking). Rear tires purely rolling produce virtually only lateral (cornering) force.
Which way will the car turn? Well its kinda simplified but calculate the torque each tire grip puts around the car center of gravity. Both the rear tires produce lateral force, which both create great rotation potential. The front tires create less yaw torque around the car center of gravity because they are braking more than turning. Result? The car will straighten out.
The results will vary a lot because in each gear, you change the driving torque at the rear wheels yet the braking torque is the same maximum in each gear. I guess in 6th gear, you can’t keep the rear wheels from locking up and the car will rotate more and possibly spin out. In 1st gear at low speed, perhaps the driving torque is much greater than the braking torque, so you still get big wheelspin, which acts kinda the same as a locked tire (no lateral grip), so you may spin out too.
It is likely to work in quite a few cars in quite a few sims. The only “bug” is us, self proclaimed simracers approaching it like a game, doing things you would never do in real life… I needed a gamepad to try this out as I can’t get my feet to do this, even in a simulator!
Click Here to read Stefano’s statement
Some thoughts about the issure;
I hope you all realize that the car will align itself to have the side with less grip “forward”, ie, in the direction of travel. 2 simple way to prove it: get a pen a throw it on the floor, it’ll rotate in a pretty random fashion. Now stick a rubber to one side of the pen and repeate; the pen will always stop rotating straight away, align to the direction of travel with the rubber end BEHIND. The hard way to test this is to get in a parking lot, get some speed, hands off the steering wheel, pull the hand brake and woooosh, ur rear end will end up where your nose was pointing. Simple reason is that the rear tyres locked up loosing much of their directional grip. I hope we all agree about this.
So let’s apply the hand brake thing in reverse, to our front tyres, if they lock up first what is it going to happen? Racing school will teach you that the car “won’t turn with locked front tyres”.. that is right, but it is only part of the story, actually, locked front tyres will align the car into the direction of travel; that is a more complete description.
So if you manage to lock your front without locking your rears you will save pretty much every slide, even a 180 degress slide (remember the car reaction with the hand brake? it swapped ends).
On to the sims now… how come it is so easy to get into this situation of locked fronts and unlocked rears in the sims? Let’s see some of the reasons:
- Brake pedal resolution. In real life, brakes work with pressure and the limit is strength you have in ur foot. In a sim, the brake pressure is determined by a pedal position, with a definite minimum and maximum. The maximum is often set to maximise the control and usage of the brake pedal in normal situations. If a real car has a potential braking torque of X but this (as it often is the case) overpower the tyres potential grip the sim is often set to have the maximum a little above the tyre limit. This avoids having that terrible feeling of locking tyres as soon you touch the brake. It is often the case that this value is not enough to lock the rear tyres (more rotational inertia, more grip and all that stuff) so you get that behaviour you condemn. In real life this would be probably a “PANIC” manouver, just hit the brake as hard as you get.. but there is no “maximum” in real life, so it would be impossible to reproduce it in a constant way as in the sims. In RL you’ll have to find the right pressure needed to lock the fronts and not the rears.. and consistently repeat this in a PANIC situation. Still asking yourself why nobody really bothers teaching this stuff? In RL you fix it the right way ™: you avoid the spin in the first place ;)
- Setup. If we get a 100% front brake bias, we’ll be able to save pretty much every slide. Is it cheating? No, it’s just stupid! it means we’re giving up optimal brake distribution (thus, lap times) to save our asses when we make a mistake. No driver nor engineer in with a sane brain would even think about this.
- Finally, potential software bugs. It happens, I’ve been there all my life :D . It’s important to make sure a rear tyre at high slip angles will lock instantly even with very low braking torque. But heavy front biased braking and gas on will still save the slide if the brake pedal maximum is set too low.
Want to prove all these things right? Get in your favourite sim and set the brake gain as high as you can ( you should be able to lock tyres with 20% brake pressure) then slide, brake hard and watch the car completing the spin slowly coming to a stop ;), just as we see on TV.
Furthermore, peichp2k has put together a video, trying to show the same effect in various simulations. As with the original story, it’s your turn to make up your mind on the issue based on the material provided, hopefully this issue can now be debated without turning into a “simulation a vs. simulation b” issue once again.