The MINI appears to be in the same ballpark. All of the tests I've found put the MINI's 60-0 MPH stopping distance at about 125 feet (38.1 meters). Of course, 60 MPH is a little slower than 100 km/hr, so you'd have to lengthen that distance by a couple of meters to compare it to the cars in that chart.

 

Ah - that's it! Not necessarily only downforce, but the aerodynamic drag of the car moving through the air is also going to provide a significant rearward force in addition to the braking force from the tires.

 

If weight doesn't make much of a difference, how come it takes 50,000lb 18-wheelers such a long distance to stop? They've got a lot of rubber to the ground, and some pretty sweet brakes (at least on new ones). I didn't have time to read the link above, because I am at work.,,

 

If you look up the braking distances for an 80k-pound tractor-trailer, they're not wildly longer than car braking distances, even though the tractor-trailer weighs 20 times more than a sedan. Maybe a factor of two in most cases.

I think there are a couple of reasons for the longer distances:

Truck tires are built for longevity, not performance, and probably have a significantly lower friction coefficient than passenger car tires.

You can't brake a semi-trailer nearly as aggressively as a car for fear of jack-knifing the trailer. Unless the trailer stays perfectly aligned with the cab, the braking isn't going to be optimum. I think this is probably the biggest factor.

Panic-braking a 50 to 80k-pound semi-trailer involves bleeding off a LOT of kinetic energy in the form of heat at the brakes. As good as the brakes are, they may simply not be up to the task of panic-stopping a loaded rig in the theoretical minimum distance without overheating and losing effectiveness.

 

I just checked out a "Truck Factoral" website... it has the specs for a newer model Peterbilt Tractor with a stopping distance of 390ft 55mph-0. It is quite the difference between the MINI Coopers 125ft 60mph-0. Can tires alone account for that extra 265ft? Or is braking distance on paper a bit different than in the real world?

I would accept that on paper, the distances should be the same, but in reality there are a heck of a lot more variables than tire friction when it comes to braking. I'm a bit confused...

 

Could you post a link to the site? Because 390 feet from 55 MPH -> 0 doesn't even meet federal requirements for loaded tractor-trailers. The maximum allowed braking distance from *sixty* MPH is only 355 feet. (FMVSS section 105/121).

I suspect that the figure is either wrong or they're including driver reaction time in that 390 feet. Or possibly that distance is for an unbraked trailer instead of a braked trailer.

Also, see my previous post about not being able to aggressively "threshold brake" a tractor-trailer like you can with a car, and that the tractor-trailer drum brakes might not be up to the task of stopping a rig in the theoretical minimum stopping distance without overheating.

 

I think they need the Stig to take them both around the Top Gear course.

 

I don't think tires have anything to do with braking distances unless you're skidding to a halt. Cornering yes, braking no.

 

That's why you get better than "idea" numbers. It's the "gummy-ness"..

this is also why road race cars use wider tires to increase ultimate lateral grip despite higher rolling resistance.

Really, the adhesion dynamics of rubber is really, really complicated.

Matt

 

Well, the coefficient of static friction between the tire and the road will determine the maximum braking force you can get out of the tires, in much the same way is it determines maximum cornering force, but I don't don't how widely the coefficient varies among street tires. Most people seem to use 0.8 as a "typical" number, but that could be an experimentally-derived number from the 1950s for all I know.

 

True, although I still think that having aerodynamic drag helping to slow the car down has a significant effect as well. Even with a slippery car, having 3-5 square meters of frontal area moving 100 km/hr is going to going to give a heck of a rearward push. (Too bad we can't do braking tests in a vacuum, eh?)

I could calculate the extra braking assistance from aero drag, although I think I might have to solve it numerically rather than analytically, since the drag force will decrease as the car slows.


EDIT - never mind about the aerodynamic braking. I ran some quick numbers using "ballpark" values for drag coefficient and frontal area (0.3 and 4 sq. meters), and even if the aero drag stayed constant at the initial maximum value (when the car was going 100 km/h) throughout the entire braking process, it would only shorten the stopping distance by about 2 meters. So I guess the problem is that using a fixed value for the tire's friction coefficient isn't an accurate-enough model.

Still, I think my initial point was valid. Although extra weight requires that the brakes bleed off more kinetic energy to bring the vehicle to a stop, the extra weight also *helps* braking by increasing the frictional force at the tires. If weight has an effect on overall braking distances, it's a weak relationship at best.

(And a look at typical braking distances for everything from motorcycles to cars to loaded semi-trailers would support this - the braking distances only vary by a factor of two or three, even though the weight range among the vehicles spans a factor of 100 or more.)

 

I'd like to take this time to apologize to everyone for bringing up the concept of vehicle weight differences affecting stopping distance back in post #4.

I'm sorry.

 

I'll put my 09 MCSa JCW up against that stock Shirrock any day of the week.

 

Well, to try to justify your diversion, wouldn't the heavier car generate more heat when stopping? I'd always understood that lighter cars are usually easier on their tires/brakes, making them have an advantage in endurance racing (including usually better fuel consumption).

I don't know physics well enough to model any of that, though. So I have nothing to back it up.

 

So many different variables... but I think the point of this Fifth Gear episode was to show that they both performed equal in class. Since we are all viewing this from a MINI forum, the real question that should be asked is... (drum roll please) was the MINI sports button "ON" when the MINI was driven?

 

the best "zeroth" order approximation is one that combines a classic friction surface (co-efficient of friction times area times normal force) with a surface that has term that is independant of normal force (or is just proprtional to area). this is a mix of classical friction surface and scotch tape. No clue on the numbers though...

And scabpicker, yes heavier cars generate more heat. The kinetic energy of the car is (1/2)mV^2 + (rotational effects) and is dominated by the mass of the car.

And Scottruiqui, you're right. Drag isn't big at number much below 100 MPH, goes up fast though. (I think at 70 mph we ***** about 18 HP into drag.) The Veyron starts to brake from high speed by angling the wing to increase aero drag more than anything else... Gets some more downforce too, but at the 200 mph range, aero braking is where it's at. That's why dragsters use parachutes!

and to get back on topic, the comparo illustrates two things: The cars are pretty equal on that track, and that the lack of understanding of significant figures trancends national boundaries!

Matt

 

Ah, yes... they included a 120ft driver reaction distance. It is still far above the MINI. I just don't believe an 18 wheeler could stop as quickly as, let's say a Porsche. I'll post the link later... I'm at work.

 

In case there is any question regarding Jason Plato's driving skills, I just came across this tidy example of his front-driving abilities:

 

So I guess the Scirocco isn't coming to the US:
http://www.autoblog.com/2009/04/06/i...irocco-to-u-s/
I was seriously considering the Scirocco over the Mini but I don't regret my decision at all.

 

I think the R20 (R32 replacement) will be faster, or at least should be faster, and is coming stateside. A fwd car with anything more than 300hp and no LSD (which DSG doesn't have) is a waste, I mean having owned an A3 with DSG that was up around 280 hp, I can tell you I had a load of wheel spin, and it was close to its limits.

I still think they should bring the Scirroco over but they have to find a way to slide it between the GTI and the R20, with regards to price.