Re: Simultaneous Application of Gas and Brake Pedals

Now look at a modern car. The brake and accelerator pedal have

> little or no difference in height. Is is entirely possible for > the right foot to press on both pedals at the same time.

The heavy duty floor mat (Mopar brand, which I use in the winter) in my 300M tends to creep forward and up against the center console - which means it gets up and behind (and to the right) of the accelerator pedal. This reduces the amount of foot-space to the immediate right of the accelerator pedal and moves the right foot a little to the left instead of being centered on the accelerator pedal. I've found that in this position I brush against the *back* of the brake pedal when pulling back on the gas.

All in all I'd have to agree that there should be more spacing between the gas and brake pedal.

But I think that it's a manditory design criteria that the brake system of any car is supposed to be able to over-power the engine in all situations. Back in the days when you had a spring that pulled back on the throttle plate, if that spring broke you could have WOT (wide-open-throttle). I can't imagine the braks system of any car not being able to stop the wheels from turing - even in that situation.

Reply to
MoPar Man
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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!

Reply to
James C. Reeves

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.

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.

Reply to
nospam.clare.nce

Isn't more braking power put to the front wheels of a car due to the weight distribution properties during stoping? I don't know the ratios, though.

How do you figure? Antilock does not help with heat tolerance or dissipation.

Yes, but you should still have pressure for at least a couple brake presses stored up in the system - same as if the engine stops while driving.

Reply to
Scott Ehardt

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.

Reply to
Daniel J. Stern

Yup, the ratios are appx 85/15 for FWD and appx 60/40 for RWD

Correct, there is a vacuum check valve in the booster inlet that should prevent the vacuum from dumping out when the engine is under load. It usually takes between 8 and 10 (and sometimes more) pedal pumps to deplete the stored vacuum in a brake booster with the engine not running.

Reply to
aarcuda69062

Well said. I would also submit that this mysterious sudden application of throttle is likely only reported in cars with automatic transmissions, since the resultant crash would probably be avoided if one foot was on the clutch--i.e. the person backing up (in this case) and experiencing a sudden racing of the engine would instinctively stomp the clutch and get the vehicle stopped, perhaps more slowly since some of the pressure is on the throttle as well as of the brake...you get the idea.

Then the driver would look down and realize what was happening, and take their foot off the accelerator, hence there'd be no accident to leave them all dazed, confused, and fully convinced that the "engine just raced unexpectedly" --since after an ordeal like that many people are often somewhat traumatized and don't really have a clear idea what the hell just happened. Just my 2=E7.

BTW I'm new to this forum, very interesting & informative.

Reply to
James Goforth

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.

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

Reply to
Matt Whiting

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).

Reply to
MoPar Man

No, but because antilock brakeswork smoother if they don't lock in the first place, manufacturers tend to install smaller less effective brakes on cars with antilock as standard.

Reply to
nospam.clare.nce

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.

Reply to
nospam.clare.nce

Huh??? If anything brakes have gotten bigger, not smaller. Especially since larger wheels are so popular nowadays. You end up with a lot of room under the rim to put in bigger brakes. Also, the anitlock system does not give the brakes more braking capacity. If the brakes heat up to the point they start to fade, anti lock systems are not going to help.

No there are not. You can easily test the theory out. Mash the gas pedal to the floor. Then mash the brake as hard as you can. I guarantee you the car will eventually stop.

-------------- Alex

Reply to
Alex Rodriguez

Also, of course, there's a check valve in the brake booster, so even if you had no vacuum whatever in the intake manifold you'd have several stops before you lost power assist (not arguing with you, pointing out another reason you're right).

Reply to
Joe Pfeiffer

I've yet to drive a car or truck that couldn't stop from any speed up to

60 MPH with WOT. I've never tried faster speeds, not even in my teenage days. :-)

Matt

Reply to
Matt Whiting

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.

Reply to
Joe

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')

Reply to
Bill Putney

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).

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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.

Reply to
MoPar Man

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.
Reply to
Steve

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???
Reply to
Steve

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.

Reply to
Steve

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