shorter braking distances

[andrew7]

Last night I stumbled upon a competition between an electric FSAE car and a gas racing kart. I personally could care less about "green" cars but what surprised me the most was the braking test, at 70kph, the electric stopped over 90 feet. The little kart with one disk on the axle did a little better. There was discussion about karts scrubbing speed using its tires by sliding all 4 through the corners. It proceeded to run the test again but fishtail the kart back and forth through the test, shortening its distance even more. Then on its last run past the braking cones, the little kart power slides to a stop in only 34 ft. The electric car team not sure what will happen stop their car in 35 feet doing the same stunt. Which begs the question, if in a panic stop situation would sliding your car sideways be worth the risk if your distances were 1/2 or 1/3 of a dead straight stop? I probably wouldn't have believe it had I not seen it. I don't know if the slicks on the track also made a difference vs. a normal car on an average road. Did anyone see this? I think a little extra space and accident avoidance is very important with our cars. I wonder if anyone actually tested a straight stop verses a slide? I'm curious of other Sevener's thoughts.

 

 

http://planetgreen.discovery.com/tv/mean-green-machines/mgm-smart-kart-smackdown.html

[yellowss7]

In the old days, before Antilock brakes, I believe that it was established that Locking all 4 wheels in the dry stopped you faster than Threshold brakeing. Assuming you don't mind flatspotting your tires.

 

Tom

[xflow7]

Keep in mind that the kart gleans *much* more benefit from the lateral slide method of braking than any "real" car would because in doing so it is able to use all 4 contact patches to provide braking force rather than just the rear two.

 

Honestly, I'm surprised that the FSAE car would see much benefit from that technique. That they improved so much leads me to believe that either their brake balance was way out, or they were not making best use of the brakes. How did they stop the car in the first test? Lock 'em up, or threshold braking? True threshold braking is difficult, and particularly from such a low speed on dry pavement where the time duration of the event is relatively short.

 

Secondly, in doing the "power slide" stop, did the drivers truly not make a control input until the beginning of the 35 feet ? I find that hard to believe, because from 70kph=64 ft/sec, even a 0.2s duration to react and get the car into the slide and decelerating would eat up 13 of the 35 ft, which leaves 22 ft to stop from 64 ft/sec ~ 3g which I don't really believe.

 

What I'm saying is that in a true panic stop situation, where there's no advance notice of the impending stop, you'll burn a lot less distance stomping on the brake pedal than attempting to get your car into an appropriate slide to scrub the speed.

 

With respect to ABS, in spite of how it tends to be marketed, the point of ABS is to maintain steering and vehicle stability through the braking event, not necessarily get the shortest stopping distance.

 

Edit: What I have observed in my Seven is that *smooth* application of the brake pedal makes a big difference in getting the car to decelerate well. Indiscriminate stomping on the pedal causes the front wheels to lock up prior to building enough weight transfer to maximize their usefulness.

 

Dave

[Ian7]

I'm going to take issue with the "4 wheel lock up is shortest" comment, but on a theoretical vs practical basis.

 

The coefficient of sliding (locked-up) friction is much less than the coefficient achieved through a properly functioning contact patch, and as such, if it were possible to maintain absolutely perfect on-the-limit threshold breaking, that would produce shorter stops.

 

The fact that most drivers can't produce anywhere near that level is the reason that "locked up" is percieved to be better still exists.

And as stated above, ABS is a stability system, not a minimum-stopping-distance system, as it actually cycles the brake pressure, it doesn't attempt to hold a threshold condition.

 

And as for the thought of a bunch of untrained drivers consciously throwing their cars into uncontrolled broadslides... sheesh.

[TheDingo8MyBaby]

xflow7 is right. You're truly scrubbing off speed when steering in a kart under braking. When you have brakes on all 4 corners of an automobile, you're utilizing all of the braking force possible (assuming the bias is correct).

 

Just to point this out - the reason that most karts only have a rear brake is they don't have nearly as much weight transfer as cars (they don't have any suspension). Having front wheel brakes on karts can also cause unwanted understeer. Karts can pull over 3 lateral G's around corners...which is more than 3x what normal passenger cars will do. Furthermore, they have consideriably less weight.

 

I do, however believe that they are stopping in that distance of 35 ft. No, I don't believe they're pulling 3G's, but I believe the energy is just being directed in a different direction. What they don't mention is how much distance they cover perpendicular to the stopping distance.

 

In the end, I dont think anything revolutionary has been discovered. You still want to threshold brake in a straight line unless you have stupid amounts of room to the left or to the right of you..and even if that's the case it would probably be best just to steer around the obstacle.

Edited July 23, 2009 by TheDingo8MyBaby

[Ian7]

Just to point this out - the reason that most karts only have a rear brake is they don't have nearly as much weight transfer as cars (they don't have any suspension).

 

Weight transfer takes place regardless of the type of suspension, or lack of. Since the center of gravity of the kart+driver is not down in the same plane as the tire contact patches, ANY acceleration forces (fore, aft, or lateral) will cause weight transfer in the relevant direction. Springs, antiroll bars, and dampers merely control the rate and distribution of the weight transferred and subsequent dynamic behavior.

[Ian7]

Karts have suspension, just not "traditional"... :-)

[TheDingo8MyBaby]

Weight transfer takes place regardless of the type of suspension, or lack of. Since the center of gravity of the kart+driver is not down in the same plane as the tire contact patches, ANY acceleration forces (fore, aft, or lateral) will cause weight transfer in the relevant direction. Springs, antiroll bars, and dampers merely control the rate and distribution of the weight transferred and subsequent dynamic behavior.

 

Right- I never said they didn't transfer weight. I said they did not transfer nearly as much. Take that for what you will...

[Ian7]

Sorry, I misread your note as suggesting weight transfer only occurred due to suspension.

 

Moral - read twice, reply once :-)

[TheDingo8MyBaby]

and I forgot to mention center of gravity... so good point there.

[stevet]

I did a bunch of work on ABS in the early days - specifically for Ford's advertising (they were the first co to fit ABS as standard on a car range, the Scorpio in Europe). Every word of copy was scrutinized by the lawyers - we could not claim shorter stopping distances, only better control under braking. ABS will stop you shorter in some driving conditions, but not others (in gravel or snow for example having the wheels lock up actually helps...). We took various journalists and motoring groups to Ford's test center in Belgium and had big fun a for a couple of days testing ABS on lots of different surfaces. Happy days...