Hacker News Reader

Comment by blunte | Hacker News Reader

Hah. I don't care how good your software and individual wheel control is - if even in the perfectly optimized situation you exceed the friction limits of all four wheels, you will slide. And if your software will not allow wheels to slide, then you will have reduced braking. Either way, you won't stop nearly as quickly as you want.

What people fail to realize, particularly in slippery conditions, is that it's often much easier to gain speed than to shed it. Modern traction control systems make it more (too) easy to gain speed without equally making it easy to shed speed.

View on HN

> What people fail to realize, particularly in slippery conditions, is that it's often much easier to gain speed than to shed it. Modern traction control systems make it more (too) easy to gain speed without equally making it easy to shed speed.

Which raises an interesting question: if a traction control system can detect poor traction conditions and compensate, couldn't it just as easily warn the driver to slow down, just like the backup radar on recent cars warns about pedestrians and cross traffic?

I used to own a VW which would turn on a light indicating a high chance of black ice when the temperature conditions lent themselves to that (black ice can form at several degrees above freezing temperature).

I suspect that a car could warn of low traction and recommend slowing down, but no car offers it because consumers won't buy a car that nags them to slow down, Even if it's for their own safety.

I have experienced that VW ice warning. It was awful.

First of all, the possibility of ice is obvious. No warning is needed. That is just silly.

Second of all, it wasn't just a light. It was a loud noise.

Third of all, the warning was delayed. It wouldn't occur right when starting the car. It often jolted me for attention right as I was pulling out into very busy traffic. The warning was a hazardous distraction.

> First of all, the possibility of ice is obvious. No warning is needed. That is just silly.

Partly correct. Yes, the possibility may be in part obvious by conditions such as temperature. But I think most people who are used to driving on icy conditions can tell you that the ice may still come as a surprise. A system that could _reliably_ warn about slippery conditions would be good.

my car does the exact same thing and it's easily dumbest feature a modern car has ever had. It's Canada in January, of course ice is possible. The alert goes off at a random time up to 15 minutes into a journey. Since it's audible you think something really serious has happened and now focus is not on the road but trying to figure out which tire has punctured or if a connecting rod is protruding from the engine block. Then you remember the stupid ice warning and are reminded how much you hate the alert, meanwhile in your distracted state you have driven into the ditch.

Somewhere where it snows once every 3 years it might be helpful. When you live in a place with real winter it actually makes you less safe since your brain has to spend time processing the alert for a condition that's obvious since the entire horizon is covered in a blanket of snow.

It’s not just braking. Porsche says:

> An uncontrolled vehicle would quickly understeer in this situation. In other words, the driver initiates the turn, but the vehicle slides in a straight line without slowing down. The control software in the e-SUV immediately puts an end to understeering. In a left-hand turn, it would brake the rear left wheel and accelerate the right one until a neutral driving situation was restored.

Braking the left rear wheel won’t do much if there isn’t enough traction to correct the understeer.

Even if you imagine the car has a magic gyroscope that can arbitrarily control yaw, if you try to make an 0.2g turn when none of your tires have 0.2g of traction, it’s not happening. You won’t oversteer, and you won’t understeer. You will, however, steer less than you wanted, and you’ll go off the road.

What I want is something like a tail-hook that I can deploy from underneath the car in an emergency situation that will just drag along the ground grabbing on to whatever it can get a grip on, whether that's snow, dirt, or asphalt. If I change my main point of contact with the supporting surface from rolling rubber to a metal spike (or something like that) I'm gonna slow down a hell of a lot faster than I would otherwise.

I’m not seeing how this could work on most driving surfaces. On asphalt you would have to penetrate the surface, which would require extreme force, or somehow apply additional downforce to a friction device, which would reduce braking friction to the other wheels.

> you would have to penetrate the surface

No, you just have to push down hard enough to take some of the pressure off the tires. (In an extreme case you would lift the tires entirely off the ground. A nice side effect is that you would no longer need a tire jack. Hm, I am beginning to understand why car manufacturers don't implement this :-)

Huh? Taking pressure off the tires reduces traction.

Not if that pressure is moved to something pointy that digs in better than tires, or has more friction.

Sounds like a good way to destroy our roads.

I remember my dad ranting in our 88 oldsmobile about how stupid all of those 4WD SUVs were, running around so fast in the snow. He would always say that 4WD could help you accelerate, but not help you brake. But whenever he bought tires, he would always get all-seasons. He found out the hard way (by rear ending an SUV) that a lot of those 4WD vehicles that drive so confidently in the snow are doing so not because they rely on their 4WD, but because they can rely on their tires.

Tires matter. A lot. For the past 25 years, snow tires have gotten continuously better. I recently tried out a set of hakkapeliittas, and the difference in driving confidence was dramatic. I could actually count on my car to reasonably stop in a way that I've never felt before, even with chains.

I had a friend who used to like to say "4WD can help you accelerate but it can't help you stop". I took him out in my '80 Subaru in the snow one day. It had a lever to activate 4WD, and I demonstrated how it would slow significantly faster with 4WD engaged.

The reason is that there was no ABS, and the brake proportioning valve would send most of the stopping power to the front wheels, since most of your stopping power comes from them on the dry roads. But in snow, the fronts lock up before the rears are slowing you down much. With 4WD active, the front brakes were connected to the back as well as the front and would use their stopping power as well.

Today is a different world though, with ABS.

You're right, it's all about the tires. I used to drive an Audi S4 which I would put Blizzaks on in the winter. I would take people out in the snow and blast around, it was amazingly predictable and controllable.

In your case the engine braking made up for, what sounds to be, a poorly calibrated braking system. Yes, front brakes take the majority of braking load and they are designed that way from a braking distribution standpoint. But you have zero advantage with engine braking in a 4WD system vs properly calibrated brakes because on snow it doesn't matter - it simply comes down to the point where you lose tire tire friction greater than the inertia of the vehicle. ABS works to prevent a lockup of the wheel which reduces the opportunity to use friction even friction has been lost.

You are right that some 4WD systems (generally not AWD systems) can have an advantage of engine braking in wet or snowy conditions by not having to use brakes as much. Most drivers overcompensate on braking application in snow or a loss of control situation, which is why ABS works so well against the natural human response. But if you have a driver that is aware of the vehicle mass and the road conditions a proper braking system wouldn't lose out to control of a vehicle being stopped to engine braking in a 4WD system. Most people don't have the driving skill to know when engine braking would even be advantageous over wheel braking. And engine braking done too aggressively is hard to quickly disengage comparative to letting up on too much wheel brake application which can lead to longer periods of loss of control.

Ultimately what Porsche lays claim to is outlandish in extreme situations. They can't beat the laws of physics. And unless their car has active tire studding it wouldn't stand a chance against a quarter mile stretch of wet ice at 4 degree decline. Doesn't matter what vehicle, tires or software you've got. That vehicle is sliding to the bottom with no control.

I well remember the first winter with my new company 4WD pickup. Sure I could go much faster in poor conditions. But once I lost control there was no correcting out of it. You were powerless and just had to wait until you slid to a stop or hit something!

Re: Hakkapeliittas: They are amazing in the snow and ice, agreed, but terrible on the dry at interstate speeds. Coming off the highway on an off-ramp at 70 on Hakkas is one of the most butt-puckering experiences I've had.

After a couple pairs of Hakkas, I switched to Blizzaks, which are not quite as good in snow and ice, but WAY better in the dry.

Front Range CO gets some good snow, but not enough days for the Hakkas to be my preference. YMMV depending on snow days in your location.

Exactly.

I remember a few years ago I came across an article on a major car website that ranked four wheel drive as more important in winter conditions than proper winter tires. I personally don’t care if accelerating takes a little longer as long as I’m able to steer and stop, but I guess everyone has their own preference.

I think it's because you can add better tires to car with 4WD if you determine you need it. You can't add 4WD/AWD to a car with winter tires.

Tires are the dominant factor in my experience. (In deep snow ground clearance is also a significant factor.) Many videos [1], [2] on youtube corrobrate this.

For the non-professional drivers on public roads, AWD is not a safety feature. It's arguably dangerous as it gives false confidence as you exceed the limits of your equipment.

It does provide a convenience at low speeds (e.g. parking lots and driveways) as it makes it more difficult to get stuck but even there tires and ground clearance are the dominant factors.

[1]: https://www.youtube.com/watch?v=EC1E2eDt1JY [2]: https://www.youtube.com/watch?v=iXjzYbpt9Ow

This is why Colorado now requires chains in severe conditions for passenger vehicles. AWD gets you up to speed, it does not get you back down to 0.

Chains in CO are NOT required for passenger vehicles. CO only requires chains for commercial vehicles.

There is a new "traction law" for passenger vehicles, which, when in effect, requires M+S tires OR AWD, and at least 3/16" tread. If you don't satisfy those requirements, you can put on chains.

https://www.codot.gov/travel/winter-driving/tractionlaw

“During severe winter storms, CDOT will implement a Passenger Vehicle Chain Law (also known as a Code 16). This is the final safety measure before the highway is closed.

When the Passenger Vehicle Chain Law is in effect, every vehicle on the roadway must have chains or an alternative traction device (like AutoSock).“ (emphasis mine)

Sounds like a good move. California has done this for years, but it’s much more necessary there because the drivers aren’t used to snow.

You're right, I was misreading your original post, and thought you were referring to the new traction laws. I sit corrected. :-)

Reply on news.ycombinator.com