One day a guy told me if you turn the steering wheel a bit, to both sides, many times, very quickly while braking the brake distance is shorter. He also said that technique saved him from hitting another car in the back.
Is that actually true? I don't think so... Can anyone explain if it is or not and why?
Thanks!
might be true...sounds logical. the tire scrub might work the stopping distances, because instead of one direction your tires working in, they are working in 2 at a time.
It might help the wheels from locking up (if turning just slightly) because of the friction it adds. But other than that it shouldn't have an effect but to actually add more distance.
My physics head has overloaded
Unless the change of direction is very small.. Don't blame me if I'm wrong on this one pls!
It's not quite that simple though, a slightly(!) rotated tyre might deform in a way that provides a larger contact patch.
I wouldn't recommend this because you're unlikely to do it exactly right in a situation where it counts; emergency braking. I'm not convinced of its effectiveness either.
I'm not a physicist and all of this could be utterly incorrect.
That is an incredibly dangerous thing to do in an emergency situation. If you need to avoid something you should make one steering input, around the object.
Swinging the wheel from side to side will only have one effect, it will unbalance the car and cause a spin. No matter how little you do it, you will add momentum in either direction each time, and that momentum will add up and spin you around.
The safest ways to stop are by pressing the pedal as hard as you can (In an ABS equipped car) and steering around the hazard. Or in a car without ABS, there are two alternatives.
Lock-steer-release, in which you do exactly that. You lock up, turn the wheel between a quarter and a half turn depending on steering ratio, then release the brakes. This will effectively "shove" the front end over at mid-low speeds, and allow you to reapply the brakes in a safe direction. I wouldn't recommend doing it at high speeds because it can cause a spin.
The second is Threshold braking, which is pretty simple in text, but requires practice, practice, and more practice. One thing to remember is that if you have to steer while threshold braking, you need to lift off the brake pedal gradually as you turn the wheel, the more you turn it, the more you lift off the brakes.
I don't mention Cadence braking because it's not effective both in my own experience and in the tests I've seen.
I would go back to your buddy and tell him to learn something about how his car handles, he is going to get himself and possibly others hurt if he keeps thinking that is going to work. It's guaranteed to make the car spin at some point.
Dragon Commando is right.
Best way to brake in a roadcar is to slam it till abs fires, then gradually reduce pressure until abs stops firing. This should get you the correct amount of pressure for threshold braking -the most effecient way to brake.
HOWEVER, as you slow down you can(and should) apply more brake pressure, as the tires can deal with it. So in practice just slam the brakes and hold on. Keep the pressure constant, abs will work for you until you stop.
OR - and this is the real kicker - learn how much pressure you have to apply (at different speeds, and at different grip levels), to brake as hard as you can without engaging the ABS. (ABS trades a little bit of braking distance of the ability to steer, and does this while keeping the braking distance way under the usual all-wheels-locked emergency braking.).
Some ABS' are more eager than others, though - my Skoda Octavia's abs is waaay too eager, while my old Xsara Picasso's ABS were a tad not eager enough. Result: With the xsara I could easily threshold brake, enabling me to brake so hard it felt your eyeballs were gonna fall off, and in my Octavia the abs engages BEFORE I could apply full brakes without locking up (or as I feel I could, from the "butt feeling" in the car - could be the tires it comes with have less grip than Im used to).
TL: DR: Dragon Command is right, your friend is DEAD WRONG (Perhaps I should put "Soon-to-be-dead" wrong), Full brake force with ABS, and learn to brake as hard as you can without locking up.
Whilst cadence braking isn't as good as threshold braking, or even relying on a computer to sort you out, it's a good skill to have.
Assuming your car doesn't have ABS - mine doesn't at the moment, although I suspect my next one well (), and assuming you suddenly come across a hazard ahead, then threshold braking is very hard to do in that circumstance. Braking too little is a common problem, but those nancy-boy genes get removed from the gene-pool quickly! Braking too hard is common - you hit the anchors as hard as possible - in which case recovering from locked brakes is a good idea. This is cadence braking - lift off and reapply the brakes. Repeatedly.
Cadence is very inefficient for braking distances. But it will enable you to steer a bit, unlike locked brakes, and it is marginally better than purely locked brakes in terms of stopping distance if done quickly enough (i.e. the more time you spend OFF the brakes then the longer it'll take to stop).
I've used it in my race car too - go in too hot, the natural reaction is to brake too hard and lock up a bit, especially in the wet. Cadence braking is the difference between making the corner (albeit slowly) and having a high speed frontal collision with something very solid.
a tv show i was watching about go karts showed a kart stopping within 10 meters because he did this weird spin, but i don't think i would want to try that on a normal road, especially in a car, i'd be worried about rolling it.
What slows quicker, a tire on the very limit of grip cadence braking, or a tyre with all 4 wheels locked?
A locked tyre with the car going forwards will have similar dynamics to one with locked wheels going sideways, but with probably less overall grip due to the design of the tyre.
Karts are a bit of an exception with no front brakes so there may well be tricks you can use to use the front wheels to slow down. After all the front brakes do most of the braking on a road car.
If you look at the traction circle, travelling forwards in a dead straight line at maximum braking effectiveness will give you the best braking ability. If you want to keep on the limit of grip and steer a little your going to have to release the brake a little too. Perhaps swaying side to side will alter the contact patch, but I doubt enough to counteract the grip problem.
Traction circle:
Imagine the ring around the edge is the limit of traction.
Lets say your tyres have a max force of 1G. They can handle 1G of acceleration, 1G of braking *or* 1G of cornering. They can't do 1G of braking *and* cornering, so to sway at 0.05G you'll only be getting 0.95G of actual braking force.
</oversimplified example>
If you have ABS, the best thing to do is stand on the brake. The quicker you do this, the quicker you will start to slow. If you only have 0.05 seconds to react, it's better to of slowed a little than of decided you'd like to cadence without actually slowing at all... Once you've started to slow and you have time *then* you can think about cadence braking *if* you ABS system is letting the wheels lock too much.
A modern ABS system, however, should be able to out-brake you as it can do each wheel individually (which, frankly, you can't)
If you don't have ABS, you should still do the same, but instead of standing on the brake, apply firmly until you start to feel the wheels lock, then release a bit and keep going. You'll slow faster to do it half-locked and half at the limit, than half at the limit and then half with your foot off the pedal...
The lateral acceleration squared plus the longitudinal accelerated squared, square rooted, gives your total (i.e. the Pythagorean theorem). So in your nice example of a tyre with a perfect traction circle (although this also applies correctly to an ellipse, but sadly real tyres are more complicated than an ellipse too), adding or 5% of lateral force capability decreases maximum longitudinal force by only 0.13%.
Likewise if a tyre were creating 0.95g braking and 0.05g cornering, the vector length (i.e. the combined total) would not be 1.0g, but 0.9513g. In fact you'd need to be cornering at 0.31g to reduce braking down to 0.95g.
Get some grippier tyres
Weight reduce the car
Get lighter rims (expensive option)
Drive slower, everywhere
Replace any broken suspension or braking components (worn shock absorbers will increase your braking distance and ability to control a car in an emergency situation)
Thanks a lot for the replies guys. When I talked to that guy I was thinking about the circle Jakg showed. Even in the numbers in his example are not right, seems like the logical argument if I want to explain to him that he's a troll
I agree with everything DragonCommando said too, but I think spinning the car with such a little steering is not that easy...
BTW this guy drives a Mercedes SLK which, obviously, has ABS.
Yes, but still... I mean, if you see from outside a dude doing that "technique" with a car you can not even apreciate body roll or anything, neither the car turning, you see it going straight, that's what makes me think is hard to spin it
