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#11
January 24th 05, 04:50 AM
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On Sun, 23 Jan 2005 08:15:23 -0500, Matt Whiting
> wrote: wrote: > >> On Sat, 22 Jan 2005 22:38:54 -0500, "James C. Reeves" >> > wrote: >> >> >>>Couldn't in my 1967 GTO. The engine torque far overpowered any brake >>>pressure I could place on the brake pedal. Now the car wouldn't actually >>>move (the front brakes kept it in place)...but it would sure billow plenty >>>of smoke from the spinning rear tires! >>> >> >> And if it had been front wheel drive? >> That's the rub with many of todays high powered vehicles. You have >> antilock brakes that are made as small as they can get away with to >> keep the weight down (and since they have antilock, it is hard to >> overwork them anyway) and now we have cars with more horsepower than >> the old muscle cars. The power brakes are engine vacuum operated, and >> the vacuum goes for a dump when the engine is under load. > >Horsepower doesn't matter much in this case, it is torque that matters >and only a few cars today have torque ratings above the muscle cars of >the 60s. > > >> So, yes, there are MANY cars on the road today that would have a hard >> time restraining the engine with the brakes even well below full >> throttle. > >I guess it depends on how you define many. I don't think any four >cylinders and probably precious few V-6s can do this. Sure, the large >V-8s probably can generate enough torque to overcome the brakes on the >drive wheels, but I'd have to try it to be sure. > >The logic that suggests that few cars can do this is simple. Look at >how long it takes (in time, not distance) to accelerate a car to 60 MPH. > That tells you how fast energy is being put into the motion of the >car. Most cars take 6 or more seconds. Now look at how long it takes >to stop the same car from 60 MPH. It will often be half this time or >less. This tells you that you can remove that same amount of energy >with the braks about twice as fast (or more in most cars) as you can put >it in with the engine. This gives you a rough suggestion that the >brakes are substantially more powerful than the engine. > >Now, of course, you have to factor in that the engine is working on >typically only two wheels and thus may be wheel spin limited initially, >but that only applies to cars that are fairly high performance. The >brakes are working on all four wheels, however, mostly on the front due >to weight transfer. Even so, I'll bet that only a few vehicles have >engines with sufficient torque to overcome the brakes on even two >wheels, and certainly won't overcome all four as the Audi proponents >originally claimed. > >Keep in mind that most torque convertors stall at less than 2,000 RPM so > you can't consider the engines peak torque, but must look at the >torque available at whatever the stall RPM is for that car's TC. This >will typically be much less than the peak torque. > > > >Matt And the torque at the wheels is typically in the range of 9 to 15 times crankshaft torque with a standard transmission, and higher with a torque converter equipped car. From a dead stop not many cars can overcome the brakes, but when attempting to stop at speed, even relatively low speed, and hitting the accelerator at the same time, it is a bit different story. 
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#12
January 24th 05, 08:51 PM
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#14
January 24th 05, 11:30 PM
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#15
January 28th 05, 05:03 AM
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"MoPar Man" > wrote in message ... > "Daniel J. Stern" wrote: > >> Well, sure, but that was when Deet-riot was still selling cars with >> 300 horsepower and 9-1/2" drum brakes at all four corners. > > 300 HP not necessarily at the wheels. Even 300HP was exaggerated. > > Drum brakes are more efficient (hydraulically speaking) at braking > than disk brakes. Way more surface area too. But more prone to fade > (which does not come into play in the current context). Both true. I've always found manual drum brakes quite capable. Detroit used to make some cars with 300 gross hp, but not very many. Not like now! I think cars today, on average, have more power than they ever had before, on average. If you look at the big performers, of course it's far more obvious. Several on the market with 400 hp, NET. Find a muscle car with that.
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#16
January 28th 05, 11:37 AM
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Joe wrote:
> "MoPar Man" > wrote in message > ... > >>"Daniel J. Stern" wrote: >> >> >>>Well, sure, but that was when Deet-riot was still selling cars with >>>300 horsepower and 9-1/2" drum brakes at all four corners. >> >>300 HP not necessarily at the wheels. Even 300HP was exaggerated. >> >>Drum brakes are more efficient (hydraulically speaking) at braking >>than disk brakes. Way more surface area too. But more prone to fade >>(which does not come into play in the current context). > > > Both true. I've always found manual drum brakes quite capable... That's because of the "self-energizing" property of drum brakes. The geometry of the pivot point is designed such that the small amount of friction applied due to the pedal pressure gets amplified by rotating the shoe into the drum harder (a multiplication effect, a mechanical "power brake"). The downside of power brakes, which is a necessity with disk brakes because they do not have the designed-in mechanical amplification, is that after one or two pumps of the pedal while under throttle, your vacuum reserve is depleted, and you essentially have no (or extremely weak) brakes - this could become critical if a sudden acceleration situation arises (due to driver error, floor mat jam, or vehicle controls failure). The mechanics of the drum brake is trotally immune from that loss of amplification. Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')
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#17
January 28th 05, 02:34 PM
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Bill Putney wrote:
> That's because of the "self-energizing" property of drum brakes. > The geometry of the pivot point is designed such that the small > amount of friction applied due to the pedal pressure gets amplified > by rotating the shoe into the drum harder (a multiplication effect, > a mechanical "power brake"). I thought it was because the 100% of the pressure in the brake line is transfered to pushing both brake shoes into the drum (because the wheel cylinder is pushing 2 pistons outward towards the drum out of both sides of the cylinder) vs disk brakes (where half of the pressure is wasted by trying to force open the calipers and the other half is used to push the pads into the rotor). > The downside of power brakes, which is a necessity with disk brakes > because they do not have the designed-in mechanical amplification, > is that after one or two pumps of the pedal while under throttle, -------------------------------------------------------- A condition I can't imagine happening in the field, unless it's one of these hypothetical mysterious run-away full-throttle-while-standing-still cases. Even in that case, you're not going to be pumping the brakes several times (and depleating the vacuum reservior) - you're going to plant your foot on the brake pedal and keep it there. In that case, you're not going to depleat the vacuum.
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#18
January 28th 05, 05:29 PM
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Joe wrote:
> > Both true. I've always found manual drum brakes quite capable. Detroit used > to make some cars with 300 gross hp, but not very many. Not like now! I > think cars today, on average, have more power than they ever had before, on > average. If you look at the big performers, of course it's far more obvious. > Several on the market with 400 hp, NET. Find a muscle car with that. > > The standardized methods by which horsepower is measured have changed MANY times over the years, not just the one big change (from SAE gross to SAE net) in 1972. The actual definition of SAE Net has been revised many times in the intervening years, as had SAE gross before that. Its meaningless to compare a 1969 "375 horsepower" 440 to a 2005 "350 HP" 5.7 Hemi, except to run the two cars on the same dynomometer (or weigh them and run them down the dragstrip and calculate based on ET or trap speed- a surprisingly consistent and accurate method). Its quite popular now to dismiss the 60s ratings as over-optimistic, but having driven both old and new and having seen both old and new run on the same dyno is a real eye-opener. A lot of the 60s engines were actually deliberately under-rated because the insurance companies were trying to avoid covering high-horsepower cars.
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#19
January 28th 05, 05:34 PM
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Bill Putney wrote:
> The downside of power brakes, which is a necessity with disk brakes > because they do not have the designed-in mechanical amplification, Why do people keep saying this? Disk brakes DO NOT "require" power assist at all. I much prefer the feel of manual disk brakes to any other braking system out there. My '69 Dodge currently has stock Kelsey-Hayes front disks and stock rear-drums, activated by a MANUAL disk brake master cylinder and a MANUAL pedal linkage. The feel is just wonderful, and really only slightly higher pedal effort than when it had a power booster, MC, and pedal setup. There is much more pedal *travel* which allows finer control over braking with the manual setup. The car stops on a dime. And if that streetable example weren't enough, how do you explain the fact that no NASCAR race cars have a power booster, but they all have 4-wheel DISK brakes???
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#20
January 28th 05, 05:37 PM
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MoPar Man wrote:
> I thought it was because the 100% of the pressure in the brake line is > transfered to pushing both brake shoes into the drum (because the > wheel cylinder is pushing 2 pistons outward towards the drum out of > both sides of the cylinder) vs disk brakes (where half of the pressure > is wasted by trying to force open the calipers and the other half is > used to push the pads into the rotor). You might want to stop and think about that... there is no "wasted" force in a disk brake caliper, either the single-piston "floating" type or the 4-piston "fixed" type.
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