60-0 in 136 feet! Meaningless!
While browsing through a few old car magazines this morning, I was reminded of an old gripe I have with nearly every automotive publication out there. The effectiveness of brake upgrades (and brakes in general) is almost always misrepresented in testing — everyone seems to quote 60-0 and 100-0 stopping distances or times, despite the fact that brake upgrades are virtually incapable of improving those statistics.
- Fact: The tires are the limiting factor in single-stop deceleration performance
Almost any vehicle is equipped from the factory with brakes powerful enough to stop the car at the threshold of wheel lockup from 60 or even 100 mph. What this means is that a car with factory brakes will perform as well in single-stop tests as the same car with a $6000 brake upgrade. So what determines braking performance? The single most important factor in single-stop braking performance is the tires – magazine braking tests are mostly measuring the tires and the surface they’re rolling on.
So why do people spend thousands of dollars on high-performance brakes? The function of performance brakes is actually heat management, and it takes pretty aggressive driving to expose the weaknesses of factory equipment in most cars — much more aggressive than a single stop from sane speeds will reveal. Well-engineered brake upgrades are necessary to dissipate the heat generated in repeated stops. Large, wide rotors have an advantage in thermal mass and heat transfer from airflow through the central radial vent. Pad compound upgrades to otherwise stock brakes are effective at allowing the car to stop repeatedly in aggressive driving for two very good reasons:
- A single aggressive stop from very high speeds is less likely to run the pad surface up to a temperature where its coefficient of friction drops.
- Operating at higher temperatures dramatically increases the efficiency of radiant and convective heat transfer; the rate at which energy is dissipated skyrockets.
Why do some cars have a reputation for great stopping, even with modest tires? Static and dynamic weight distribution are a very large factor (secondary to tires) in stopping performance. Because a tire’s coefficient of friction decreases as it is loaded, it is most efficient to avoid excessive weight transfer. Porsche “Hand of God” brakes have little to do with the actual stopping parts themselves and more to do with having a very low, rearward-biased center of mass. When I hear a writer gush about an MR2 with stock brakes out-stopping a field of cars with expensive upgrades, leading the reader to believe that brakes are better left untouched, I can only wonder why they insist on wasting time and magazine space on “brake testing”.
Next time you’re reading your automotive publication of choice, especially one featuring upgraded vehicles, and the writer runs on about stopping distances and “big binders” (or whatever their cute phrase of the month is) you’ll know to roll your eyes dismissively and ignore statistics for statistics’ sake.
Technorati : brake tests, brakes, stopping distance, upgrades
on January 23, 2007 at 7:36 am Dan wrote:
Good points. But you fail to mention whether the magazines you’re reading are comparing cars (as in the buff books) or doing before-and-after tech testing (as in the enthusiast mags). If the latter, your points are well taken. If the former, then it’s a legitimate test pitting the stock tires, the stock brakes and the stock weight distribution of various competing cars.
on January 23, 2007 at 5:05 pm Chris wrote:
Good points yourself. I meant to refer to those vehicles which have been modified for performance, however slightly or extravagantly, and reviewed in comparison to other cars — sometimes the other cars are also modified, but often new cars are used as the benchmarks. Yes, in short, enthusiast publications.
on February 13, 2007 at 1:42 pm Wider tyres - Will they increase braking efficiency-Modifications & Accessories-Team-BHP wrote:
[...] First, take a look at this informative link, which I found pretty enlightening: The Pansy Patrol - Info for the 3000GT / Stealth Community To summarize, brake upgrades (i.e. bigger discs, grooves, drills, etc.) are mostly for heat dissipation to reduce brake fade when brakes are being used repeatedly. It is the tyres’ contact patch area (and compound I guess) that are crucial in the actual slow-down of a vehicle once the wheels lock (or modulate at that level without locking, in the case of ABS). Rear discs may help some, but on most Indian cars, esp. with front-wheel drive (viva?), this won’t make a big difference. And it’s probably not worth the hassle and price. This option makes sense if you’re adding vast amounts of power to your car, i.e. with a turbo/supercharger or constant NOS. Which I don’t gather is the case here. The one option that IS worth trying though (in addition to bigger tyres), is upgrading the front-discs system and any parts thereof. Hope that helps! __________________ "We’re all riders on the Storm!" - Raoul Duke [...]
on February 13, 2007 at 6:17 pm Andrew wrote:
I’m sure you taken this into consideration, but a lot of the time, brake upgrade kits are being reviewed for older cars. 2nd gen RX7’s , base model 240’s, etc , which really didn’t have spectacular brakes, both due to aging and old design. New cars are, well, better by design, in my experience.
on February 13, 2007 at 7:52 pm Chris wrote:
While not all cars are equipped with adequate brakes, it seems to be the case more often than not. Obviously I haven’t driven everything with four wheels, but in my experience from sportscars to econoboxes, everything made in the last 20 years usually stops well once. Thanks for the reply Andrew!
on September 27, 2007 at 3:41 pm Fraser Elliott wrote:
While it is true that tires are what stops the car, which as you’ve pointed out is revelation enough in itself to many people, braking modulation is key. The better modulation afforded by larger-diamter rotors allows a greater force to be imparted by the tires before lockup than would be possible with smaller-diamter rotors, and this alone is why bigger brakes, properly balanced, CAN in fact reduce stopping distances. In plain english, nearly any car, even one with undersized brakes, can lock its tires. A car with properly sized brakes can actually get more work out of the tires before forcing them to lock, thereby resulting in shorter stops.
I don’t drive a VR4 but you’ve done a good job with the tech I see around here.
on September 27, 2007 at 7:01 pm Chris wrote:
Thanks very much for this addition Fraser; there is plenty of truth there. It’s excellent that you point out properly balanced as well, because many brake upgrades change a car’s braking bias for the worse and can hurt braking efficiency. Few people will look at brake balance very critically, and almost nobody (’cept the serious road racers) would go to the trouble of fine-tuning bias with a dual tandem master or proportioning valve.
I would love to do a real before-after brake upgrade comparison between several cars to get some real numbers to play with. Whatever the findings, it would be really interesting to see.
on October 8, 2007 at 2:05 pm Fraser Elliott wrote:
Here’s one pertaining to Grand Prix’s.
The Grand Prix shares platform and most of its suspension with the Monte Carlo and the Impala. But inexplicably, the highest-performance version of the Grand Prix, the GTP, is saddled with the puny 10.9 inch front brakes of its less powerful brethren, whereas the Monte Carlo and the Impala got 11.9 inch, much heavier front rotors. Everything else about the cars’ brakes (with the possible exception of ABS calibration) is exactly the same, including the calipers and pads. The only difference is the size of the rotor and the caliper bracket which spaces it further from the hub.
This makes the junkyard parts-bin 12-inch upgrade a very popular and cost-effective upgrade for GTP’s..for $100 and 30 minutes work you can really make a substantial improvement to the car. They’ll take substantially more abuse before overheating, thanks to the much higher capacity heat sink provided by the heavy rotors, and an instrumented test bore out an actual stopping distance improvement as well. The results can safely be pinpointed to pretty much the size of the rotors only, because everything else about the brakes including the very pads themselves remained constant: http://www.thrashercharged.com/tech_htm/brake_test.shtm
Real testing of this stuff is hard to come by, but this is one I could put my hands on.
A car magazine (might have been Hot Rod) did instrumented testing of several high-performance and track brake pad compounds on a Mustang a few years ago and showed a huge improvement was possible not only in hot durability and high-speed braking distance, but also in lower-speed, lower-temperature situations where coefficient of friction was the prevailing factor. The tires remained the same throughout the test, which again shows that there may be more power to be had through a given tire than a given braking setup is letting it produce.
on October 10, 2007 at 3:42 am Chris wrote:
You certainly do your research. I’d have trouble refuting any of that as the facts certainly seem quite clearly laid out. The one exception I’d like to raise is that these are ABS-controlled stops. In the same way that we established brake modulation is improved through increased brake leverage, ABS performance might also improve.
Because threshold braking only involves the initial ramp-up to maximum, and very little modulation thereafter, transients are a minimum factor. ABS is all about transients. ABS is oscillating between full lock (if you applied enough pedal to trigger ABS, you have enough hydraulic pressure to lock the brakes), and full release. A more “modulatable” braking system will achieve lock faster and release quicker also. Without any elaborate simulation, it strikes me that this could improve braking distances and that the effect would be somewhat magnified compared to a stop by threshold braking.
The Mustang article sounds of interest if you can find it on the web somewhere. While I believe most cars can brake at the threshold of lockup on normal rubber in a single-stop situation, exceptions could and probably do exist. Is a Mustang underbraked from the factory? Perhaps. I think a first-generation 3000GT VR4 violates this assumption also — at least my old Stealth was resistent to locking the brakes when that was my intent.
I’ll finish with a counterpoint: for each brake upgrade that stands to potentially improve single-stop performance, I bet there are five more that change bias enough to worsen performance slightly.
on May 24, 2008 at 12:01 pm how brakes work wrote:
[...] reality of how brakes work. This is a short and succinct description of the reality of car brakes.http://www.pansypatrol.com/brakes/How Brakes Work? : How Car Stuff WorksFeb 22, 2006 … The modern automotive brake system has been [...]