Why not neutral when going down a hill?

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
Scott in NM
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Why not neutral when going down a hill?

Post by Scott in NM » Wed Dec 01, 2021 5:49 pm

On p. 30 of TM9-8012 is a CAUTION: "When descending a hill or grade . . never depress the clutch pedal or shift into transmission into "N" (neutral) position." So why not? I've never been told this so it is something new. My great-uncle, a professional truck driver, told me 40+ years ago that 18 wheelers downshift to brake to control speed, but we didn't quite touch on this topic. I've always coasted in neutral on a long hill with all the manual transmission vehicles I have owned with no apparent ill effect. The way it is phrased implies the problem arises when the clutch is re-engaged. Can someone enlighten me on the real reason this CAUTION exists?

Scott


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Re: Why not neutral when going down a hill?

Post by Ron D » Wed Dec 01, 2021 7:23 pm

Hi Scott,
It's generally considered unsafe, mainly due to loss of engine braking, but also the adverse affect of coasting on handling characteristics (going downhill fast, especially on curves or taking evasive action), and probable delay or difficulty re-engaging the transmission in an emergency situation. It's illegal in many states. Google it. And it doesn't save fuel either. Pretty sure you won't find any professional driver who says it's either safe or a good idea. Besides, the Army said not to do it, and that's good enough for me. I've been driving manual transmissions from 3 to 18 gears for more than 50 years and don't do it. There's no good reason to do it. I wish you wouldn't do it either, no matter what you're driving.

Good luck!
Last edited by Ron D on Wed Dec 01, 2021 7:31 pm, edited 1 time in total.
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Re: Why not neutral when going down a hill?

Post by parker007 » Wed Dec 01, 2021 7:29 pm

talk to a truck driver, they say if in neutral it like death sentence. keep in mind usually you cant get back in gear once in neutral when brakes cooked.

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Re: Why not neutral when going down a hill?

Post by GPW1263 » Wed Dec 01, 2021 9:42 pm

Well, there are hills and then there are HILLS. :lol:

Chiefly, relying ONLY on your brakes while going downhill leads to unnecessary brake wear (provided everything goes as planned) and/or brake fade (brakes do not perform well at high temperatures) and circumstances may just be that you are starting your trip downward with worn brakes and then finding out your brake linings have either fully departed their foundations, or extreme temperatures have caused many a commercial truck's brake drum to crack and come apart (and who hasn't seen a large piece of a brake drum from a semi laying alongside the road?) :shock: In a highly regulated industry, one would think this to be a highly unusual occurrence. (It is not.)

Needless to say, there are many ways for things to go sour. This is exactly why you see runaway lanes or arrester beds alongside downgrades on major highways that have long or steep grades. If you live in a part of the country where even finding a significant grade is unusual, then much of this may just sound like a lot of bunk to you. But, there are areas where observing these basic rules is very important.

Saying you have had no ill effects is a pretty bold statement considering you are not able to see your brakes before, or while coasting down hill and if you aren't having to use your brakes, well....you're not on much of a hill.

Now consider that your clutch. When fully engaged, it can get you and your vehicle UP a hill. Thus, it can do just as well getting you DOWN a hill and left engaged, it will happily stay engaged and in many cases, the arresting power of your engine will do much to conserve your brakes.

Of course, changing gears on a downgrade can be problematic (especially if you find you cannot get back INTO gear) but much of that has been worked out with modern, manual transmissions. But if you happen to have worn synchros on that old slush box, well...you might just be setting yourself up for failure.

In all, there is little or no profit in coasting down a long, or a steep hill. Conserve your brakes for when you really need them.

My observations are as a result of handling motor vehicle accidents over the course of 30 years.

Cheers,
GPW1263
Last edited by GPW1263 on Thu Dec 02, 2021 6:54 am, edited 1 time in total.

Scott in NM
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Re: Why not neutral when going down a hill?

Post by Scott in NM » Thu Dec 02, 2021 5:10 am

Excellent answers! Good thing I'm only ignorant, not stupid. No more neutral when going down hills. BTW, I live in the mountains, and we have MOUNTAINS, not hills, so after reading the caution in the 8012 this became quite important. Good to know. If good enough for the pros (and the Army, even though I'm Air Force), good enough for me. Thanks guys.

Scott

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Re: Why not neutral when going down a hill?

Post by Wolfman » Thu Dec 02, 2021 6:01 am

All good points. There is one more aspect to consider.
If you just climbed a steep hill to get to the top, you are most likely in a lower gear and or range.
Now going over the top and going down, you are still in a lower gear.
Reverse ratio from the rear end to the clutch comes into play. And this is especially true if in Low Range.
The same gear ratio from the engine to the rear end, that gave you all that power to climb the mountain, will cause the clutch disc to extremely over speed if freewheeling down the other side, in gear, with the clutch pedal depressed and the jeep is allowed to accelerate.
In this situation, the clutch disc can reach some outrages RPM. The lining can disintegrate from centrifugal force.
How do I know ?
I worked at a tractor dealership. New tractors came in by rail. We went to the rail yard. Took the new tractors off flat cars, on to a dock and rolled them down a ramp. Slowly.
Put a new guy on a new tractor. He was cautiously driving the tractor off the flat car in low range, 1st gear, onto the dock.
When he got to the ramp to roll down, he pushed the clutch in and let the tractor go. When he got to the bottom, the tractor wouldn't move.
We pulled the new tractor to the shop and split it to see what happened.
The roll down created enough clutch disc RPM, it exploded.
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Re: Why not neutral when going down a hill?

Post by Andy Howell » Sat Jan 15, 2022 4:30 pm

I just came across this thread & can help.
The reason not to shift into neutral in ANY manual transmission while coasting is because of a lack of lubrication of the mainshaft gears / bushings / mainshaft.
OK you're rolling along at 40 MPH in high gear, the main shaft & is connected to the input & the rear axle is connected to the mainshaft. 2nd & 1st are rolling around the mainshaft at Different speeds than the mainshaft is rotating, but since the trans is being driven at engine speed Lube is being sent by the rotating mass of the gears & everthing is happy. But when you top that hill & you see a stop sign 2 miles away at the bottom & knock the trans into neutral this happens: The rotaing gear mass slows down to idle speed, but the mainshaft is now being driven by the rear axle & vehicle speed, which very likely is increasing. The mainshaft is spinning Inside the gearset at Warp speed now, but since the gear mass- that the mainshaft & gear bushings depend on for lube- is still at idle speed there is insufficiant lube to properly lube the mainshaft, gear bushings & the pocket roller bearings in the input shaft. It won't fail right away but you can see how using a trans this way will damage it eventually.
I can't count how many 9, 10, 13 & 15 speed Fullers & countless others I've repaired damaged by this driving habit!
On the Other End of things disingaging the clutch & coasting w/ engine at idle speed while in gear spins the input shaft in the pilot bushing at warp speed also, that little bushing only has the lube that was applied when it was installed, so the lower the speed in which it gets used the longer it will last!

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Re: Why not neutral when going down a hill?

Post by YLG80 » Sat Jan 15, 2022 11:52 pm

Hello,
Read this: https://www.lightfoot.co.uk/news/2018/0 ... -downhill/
Also in a physical standpoint, if you simply disconnect the engine from the wheels, loosing the engine brake, when turning the centrifugal force towards the outside will not be compensated and reduced by an another force.
That a simple force composition with the engine braking sent to the ground through the tires.
It give a similar very dangerous effect if you enter in a turn while coasting…
You have good chances to simply quit the road…
Also regarding the law, certain states are simply prohibiting coasting because it’s obviously very dangerousv, for example Iowa.

993B2095-4A3C-492F-8A22-1731CA091372.jpeg

Or as mentioned here:

E3D5F17B-18D1-4D28-904F-D8D58737F337.png

And mentioned in many army vehicles TM’s.

Never try to get down the Alps moutains while coasting and use only the friction brakes. :mrgreen:
It’s gone be a one shot and an « aller simple » or « one way ticket » to paradise.
You will quickly end up with no brakes at all…
Not to mention that with the increased speed, if you don’t think to select a high gear you will never be able to reengage a low gear.

That’s a more fun challenge to try to get down a mountain using the friction brakes at the minimum.
I often do that in the Swiss Alps… and I’m prettry sure that my engine is deligthed to be well lubricated while driving down.
Yves
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Re: Why not neutral when going down a hill?

Post by gerrykan » Thu Mar 17, 2022 4:30 pm

Another consideration with low geared vehicles such as M38 or M38A1, is that if the coasting speed exceeds the safe operating RPM range of the engine, then re-engaging the transmission could over rev the engine causing damage or destruction. This would likely be a connecting rod exiting through the side of the block, or oil pan.
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Re: Why not neutral when going down a hill?

Post by Andy Howell » Tue Mar 22, 2022 12:30 pm

As far as the Transmission Manufacturer is concerned, they could care less about our safety or over speeding the engine.
Coasting the transmission in neutral will sooner or later result in a Non-Warrantable Transmission Failure.

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Re: Why not neutral when going down a hill?

Post by Scott in NM » Thu Mar 24, 2022 8:53 am

Darn, I love a good discussion! Thanks for all the inputs, hope others learned as much as I have.

And as far as a warranty goes, I think mine ran out about 70 years ago.

Scott

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Re: Why not neutral when going down a hill?

Post by Andy Howell » Fri Mar 25, 2022 6:55 pm

Scott in NM wrote:
Thu Mar 24, 2022 8:53 am
And as far as a warranty goes, I think mine ran out about 70 years ago.

Scott
You mean you didn't spring for the "Extended Warranty"? :roll:

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Re: Why not neutral when going down a hill?

Post by YLG80 » Fri Mar 25, 2022 11:10 pm

Scott in NM wrote:
Thu Mar 24, 2022 8:53 am
Darn, I love a good discussion! Thanks for all the inputs, hope others learned as much as I have.

And as far as a warranty goes, I think mine ran out about 70 years ago.

Scott
Hi Scott,
My question : « Why do you want to shift to neutral or depress the clutch when going down a hill »?
What advantage do you expect?
Yves
Ford GPW 1943 - Louisville - DoD 12-7-43
serial 164794

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Re: Why not neutral when going down a hill?

Post by Andy Howell » Tue May 24, 2022 5:07 am

YLG80 wrote:
Fri Mar 25, 2022 11:10 pm
Scott in NM wrote:
Thu Mar 24, 2022 8:53 am
Darn, I love a good discussion! Thanks for all the inputs, hope others learned as much as I have.

And as far as a warranty goes, I think mine ran out about 70 years ago.

Scott
Hi Scott,
My question : « Why do you want to shift to neutral or depress the clutch when going down a hill »?
What advantage do you expect?
Yves
You aint been around Truck Drivers very much, have you? :lol:

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Re: Why not neutral when going down a hill?

Post by YLG80 » Tue May 24, 2022 6:09 am

Andy Howell wrote:
Tue May 24, 2022 5:07 am

You aint been around Truck Drivers very much, have you? :lol:
Exactly, that's the reason why I ask: what advantage do you expect?
Speed, fuel consumption ...

Fuel consumption, certainly not.
Yves
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serial 164794


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