advisability of letting out clutch

[Guest DeSotoStan]

Gentlemen,

Does anyone have any opinion re: depressing the clutch pedal on a FluidDrive transmission while descending a hill? I find that my car ('49 DeSoto) goes faster without the engine slowing it down (the momentum from the car's weight is faster than simply coasting with the engine engaged).

SPECIFICALLY, IS THERE ANY DANGER OF DAMAGE DUE TO LETTING OFF THE CLUTCH PEDAL AT THE BOTTOM OF THE HILL, WHEN THE ENGINE MAY HAVE SLOWED DOWN, & THEN SUDDENLY THE TRANSMISSION ENGAGES ?

Does this make sense?

Thank you,

Stan

[Matt Harwood]

Of course it'll go faster in neutral than with the engine drag, but given the marginal brakes on these cars, I always prefer to have some engine braking going downhill. I don't like that runaway truck feeling of being in neutral and coasting, and with the brakes on an older car, you can easily gather more speed than they're capable of safely shedding. I always advise going downhill in gear and not in neutral--the extra speed just isn't worth it.

If you are going to do it, however, I would recommend goosing the throttle to speed the engine up to approximately the transmission's road speed before engaging the clutch. The engine is at idle when you're coasting, and engaging the clutch at, say, 60 MPH at the bottom of a hill, and you instantly accelerate all those parts to road speed, which puts a HUGE strain on the drive line. You wouldn't wind it up to 3500 RPM and dump the clutch from a standstill, would you? It's almost the same thing you're doing here. Again, keeping the clutch engaged while going downhill is probably the gentler thing to do, but if not, wick the throttle before you dump the clutch.

Hope this helps.

[Dave@Moon]

There are dozens of reasons why this is unsafe. Wear to the transmission/engine is almost a minor consideration in fact. I would be more concerned with shock to the u-joints, differential, and wheel bearings ( a MAJOR consideration in a 1949 DeSoto).

However you should know that this is also illegal in most states.

Don't do it.

[Rusty_OToole]

Highly unsafe and not recommended. It is better to leave it in gear. The rule is use the same gear going down as you used going up.

[61polara]

Do you put your modern car in neural to go down the same hill? It will gain speed also. There is no need to do it and it is unsafe. You need engine braking desending all hills to maintain control of the vehicle and prevent run away and loss of control.

[Guest DeSotoStan]

Thank you very much, Matt, Dave, Rusty_OToole, & 61 polara,

This seems pretty unanimous. Glad I didn't go with the advice from the car's former owner (who I always thought knew a lot about old cars), who told me that this practice was OK.

I'm very glad I asked.

Stan

[Rusty_OToole]

It's OK if you don't mind being killed. Amount of danger depends on how long and steep the hill is, and how fast you are going when you hit the bottom.

Old cars with drum brakes are known for brake fade in such situations. It used to be rather common for guys to do what you are talking about and arrive at the bottom of a hill with no brakes at all.

In some Western towns the savvy guys would not park on the main street for this reason. Every once in a while a car would come down the mountain out of control and wipe out 3 or 4 parked cars.

With Chrysler products you were fairly safe as their hand brake is a drum on the back of the transmission completely separate from the service brakes on the wheels. So at least you always had some brakes to fall back on.

[Bleach]

I've always coasted down hills in neutral in my "modern" vehicles, even the ones with automatics. It's just a habit I've had since I first started driving many years ago. My father did it and I got that idea from him. I've always kept it about the speed limit and rarely went over if the cars/trucks I drove managed to do so.

One other thing, I've felt that some automatics actually freewheel when you go down hill if you let off the gas, which feels like you're coasting. I've actually never had an issue with brake fade except one time driving down a steep mountain road for several miles while driving a friend's mid 90s FWD Pontiac Bonneville with an automatic. I had to put in a lower gear for the transmission to help with the braking.

[ply33]

I've always coasted down hills in neutral in my "modern" vehicles, even the ones with automatics. It's just a habit I've had since I first started driving many years ago. My father did it and I got that idea from him. I've always kept it about the speed limit and rarely went over if the cars/trucks I drove managed to do so.

One other thing, I've felt that some automatics actually freewheel when you go down hill if you let off the gas, which feels like you're coasting. I've actually never had an issue with brake fade except one time driving down a steep mountain road for several miles while driving a friend's mid 90s FWD Pontiac Bonneville with an automatic. I had to put in a lower gear for the transmission to help with the braking.

Sounds like you are more experienced with disc brakes than with drum brakes. Drum brakes definitely fade when they get hot and a long mountain grade is brutal on them.

In a newer car (last 30 or so years) I have no issue with coasting and using the brakes to control speed. In a vehicle with drum brakes all around that is fool hardy bordering on suicidal in my opinion. Growing up in a mountainous area in the era of drum brakes, the lack of effective "compression braking" with an automatic transmission is the reason my family only had manual transmission vehicles.

[Bleach]

You're correct about the disc brakes Ply33. Most of my cars and trucks in the last 20 years were equipped with them. The cars and trucks I started driving with were all the drummed variety.

As much as it sounds I'm not advocating coasting. Everyone's driving situations are different and care should be exercised if one isn't 100% certain of their vehicle's capabilities and own driving skill.

[Matt Harwood]

Just as a side note, if you're driving a modern, fuel-injected car and putting it in neutral to coast down hills, you're wasting gas. Every manufacturer since the advent of EFI in the mid- to late-80s shuts off fuel to the injectors when the throttle is closed and the car is coasting in gear. Putting it in neutral re-engages the idle circuit so it is using gas.

So regardless of whether the brakes are better, the fuel systems are definitely more sophisticated and you're wasting gas by coasting in neutral instead of in gear. Yeah, it's negligible, but trying to save a negligible amount of gas is why you're doing it, no?

The more you know...

Edited October 4, 2011 by Matt Harwood (see edit history)

[Bleach]

Just as a side note, if you're driving a modern, fuel-injected car and putting it in neutral to coast down hills, you're wasting gas. Every manufacturer since the advent of EFI in the mid- to late-80s shuts off fuel to the injectors when the throttle is closed and the car is coasting in gear. Putting it in neutral re-engages the idle circuit so it is using gas.

So regardless of whether the brakes are better, the fuel systems are definitely more sophisticated and you're wasting gas by coasting in neutral instead of in gear. Yeah, it's negligible, but trying to save a negligible amount of gas is why you're doing it, no?

The more you know...

Just wondering then, if the fuel is shut off, then why is the motor still running?

Also I've seen the RPM's drop to idle when putting the engine in neutral as opposed to when the vehicle is in "D" in an automatic.

Just wondering.

[Matt Harwood]

The wheels are turning the engine--it's momentum. The engine will turn without fuel, right? You can do it with a wrench, and the starter does it all the time. The momentum of the car drives the engine when you're coasting, and 3000+ pounds in motion carries an awful lot of momentum. Cutting fuel doesn't instantly seize the motor, does it? The engine is just a big air pump, but at this point, the wheels are driving the pump rather than the pump driving the wheels. Engine braking happens because of compression and friction inside the engine, not because of idle speeds governed by the amount of fuel the engine receives, and is why you can coast farther in neutral.

It's for this same reason that the engine speed doesn't return to idle when you coast in gear. The wheels and engine have a fixed relationship when it's in gear, X number of turns of the wheels equals Y number of turns of the engine. So at 60 MPH, on the gas or coasting, the engine should be turning about 2000 RPM (just for example). An automatic car with a lock-up torque converter will change things a little bit, but it won't return to idle speed if you're coasting in gear because the wheels are driving the engine at road speed. Of course, if you coast in gear long enough, eventually you'll coast down to idle speed and it'll start feeding fuel again (actually somewhat before that, see below).

Putting it in neutral, yes, returns it to idle. However, in a fuel injected car, I bet it's not quite idle, probably a few hundred RPM above idle. There's a reason for this, which keeps it from stalling when you abruptly put in the clutch and release the gas. It will only return to "true" idle speed at low road speeds.

And as a note, even when you're coasting, when the engine speed drops to a certain point, it will start feeding fuel again to prevent stalling and bucking. On my '93 Mustang, for instance, it will start feeding fuel at about 1200 RPM when I'm coasting in gear.

Hope this clears things up.

Edited October 4, 2011 by Matt Harwood (see edit history)

[ply33]

The wheels are turning the engine--it's momentum. The engine will turn without fuel, right? You can do it with a wrench,.....

...Hope this clears things up.

Methinks you did not see the smilies icons on his post.

[Bleach]

I had my mind stuck in an automatic when I wrote my last post.

I currently have two manual transmission fuel injected vehicles. One is a diesel Mercedes and the other a '94 F150. Out of habit, I coast in neutral with both going down hills. The diesel will just slow down in top gear if I let off the pedal because of the compression of the engine. I haven't really paid attention what the F150 does down hill since I've only had it about a year and I don't drive it much. I'm sure it does some engine braking as well like you mention.

I drive my wife's Honda with an automatic the same way, I coast it out of habit because it will engine brake plus it'll even downshift by itself on some grades which is really annoying. The car is an appliance and likes to do stuff on it's own if you like it or not.

[ply33]

The car is an appliance and likes to do stuff on it's own if you like it or not.

My new car (2004 model acquired in 2003) turns off the engine on down grades unless you put the shift control lever in "B" (for engine braking). It is, after all, a hybrid. And it definitely "likes to do stuff on it's own".

[Dave@Moon]

My new car (2004 model acquired in 2003) turns off the engine on down grades unless you put the shift control lever in "B" (for engine braking). It is, after all, a hybrid. And it definitely "likes to do stuff on it's own".

Even a hybrid has some engine breaking action, although it comes from the motor/generator generating power. However they can get away from you on a hill, and Prius (among others) has a transmission gear named "Brake" similar to the "Grade Retard" found on old Turboglides for when the hill is too steep. In it the motor/generator generates even more electricity, holding the car back.

I remember that when hybrids were first introduced 10-12 years ago there were several states that had to modify their vehicle code to allow for the car's ability to shut the engine off while moving. If you have a hybrid, it is legal in 50 states for the engine to be off while moving.

If not, you're likely breaking the law. Don't have an accident while you're doing this!:eek:

[Guest DeSotoStan]

Wow! Looks like there's a gray area even here. I usually mind being killed, but there are days . . .

The hill I did this on wasn't steep ( perhaps 30 degree angle), & my top speed was perhaps 45 mph, so there's no danger of losing control. Am I correct in assuming that "brake fade" occurs when you apply the brakes for extended periods? In other words, brake fade will not occur by simply coasting (with trans disengaged), right?

All of you who (at least partially) condone depressing the clutch pedal, isn't there still a concern about the wear to the U-Joints, etc , when the transmission finally engages, at the bottom of the hill when the car is going about 45 mph?

& like you said, RustyOToole, I DO have the handbrake.

Thanks,

ss

[ply33]

Wow! Looks like there's a gray area even here. I usually mind being killed, but there are days . . .

The hill I did this on wasn't steep ( perhaps 30 degree angle), & my top speed was perhaps 45 mph, so there's no danger of losing control. Am I correct in assuming that "brake fade" occurs when you apply the brakes for extended periods? In other words, brake fade will not occur by simply coasting (with trans disengaged), right?

All of you who (at least partially) condone depressing the clutch pedal, isn't there still a concern about the wear to the U-Joints, etc , when the transmission finally engages, at the bottom of the hill when the car is going about 45 mph?

& like you said, RustyOToole, I DO have the handbrake.

Thanks,

ss

Fade on drum brakes occurs when the brakes warm up. If it gets hot enough the linings will get glazed but they will be fading before that. Basically the radius of the shoes is fixed. And if it was a well done brake job in the old days then they ground the linings do the radius was the same as the brake drum. However the brake drum expands when it gets warm. That results in the radius of the drum being different than the curve on the shoes so the contact area between the lining and the drums decreases.

If it does not get hot enough to glaze the linings, then everything will return to normal when drum cools down and shrinks back to its original size.

[Dave@Moon]

Coasting in Neutral or Gear to Save Gas - Coasting and Fuel Economy - Popular Mechanics

Since we're talking about a DeSoto, the fuel injection discussion in this article is mute. The rest is directly applicable.

[c49er]

DeSoto had fuel injection!

[Bleach]

DeSoto had fuel injection!

Yeah about 9 years after the OP's car was made.

[c49er]

Close enough for me. Quite rare too!

[Bleach]

Close enough for me. Quite rare too!

Rare to the point of being unique. Only one working example exists in a '58 Desoto Adventurer convertible. It was sold at a Mecum auction not too long ago but that is another story that may be in another thread.

[c49er]

I prefer carter ball and ball Chrysler and DeSoto's over the Hemi V8 cars. Glad the FI DeSoto sold at one of the "Big Boy" auctions. Must have gone to a good home.

[Rusty_OToole]

Brake fade occurs when you "ride" the brake pedal for a long time. It is caused by heat buildup.

You will have less chance of fade, and less brake wear, if you apply the brakes sharpish and slow down then let it coast again, than if you rest your foot on the pedal and maintain a steady speed.

Once again Chrysler was better than the others, by putting a completely separate brake on the transmission. This meant you had at least partial braking even in the case of complete failure of the service brakes.

Best and safest is to leave the car in gear and allow the engine to slow the car.

[Guest DeSotoStan]

Thanks,RustyOToole,

I NEVER let out the clutch anymore. By "separate brake", you mean the handbrake, right?

ss

[ply33]

Thanks,RustyOToole,

I NEVER let out the clutch anymore. By "separate brake", you mean the handbrake, right?

ss

Mopars of that era had the parking brake on a drum at the back of the transmission. Absolutely no parts in common between the service brakes in the wheels and the hand brake on the transmission.

Well, almost no parts in common: If your drum comes off on the rear axle you'll lose both the service brakes and the handbrake. Don't ask how I know.

[Rusty_OToole]

Yes on Chrysler products of that era you have 2 completely separate brake systems unless the wheels fall off the car.