downshifting at high speed

[berdollos]

When I was in high school many years ago, I heard that a kid was driving about 60 mph and tried to downshift (in a manual shift car) to first. He supposedly went through the windshield.

Is this a plausible story? What would happen if you were driving fast and tried to downshift into first? I always wondered but am not willing to try.

[Homer]

It's possible. Your engine would rev waaaay up, but the gearing would slow the vehicle considerably, and very quickly. You could conceivably be thrown through the windshield, but it's not really likely. It's more likely your engine goes "RRRRRRRRRRRRRRRRRRMMMMMMMMMMMMMMMM" *POW* ::puffs of smoke::

--Tim

[sethdallob]

Most modern cars have a chip on your transmission that prevents this from happening. On my dopey little base Firebird, if I'm going more than 20, it will not shift to 1st, only to 2nd.

[Turbo Dog]

I had a friend once who did the similar thing.. wanted to see what would happen if it was going 80 in his truck and dropped it into first gear. He didn't go through the windshield, but his transmission left many pieces on the road.

[brad_d]

I have trouble with the kid-through-the-windshield story because there's no way you could stop any more suddenly than you could at maximum braking. Unless the transmission blew up and part of the shell wedged itself into the pavement - that might stop you really fast....

I seem to recall reading somewhere that 1st gear on modern transmissions typically has no synchronizer - is this correct? If true, I'd think that would make engaging 1st gear at any significant speed pretty tough, which seems to be the case on my truck.

I accidentally downshifted into 2nd rather than 4th at about 60 mph once. The engine raced like hell, the back tires tried to skid, and I never even got the clutch fully engaged before I jammed it back to the floor.


I've heard some complaints from BMW M3 drivers that it's very easy to accidentally do just that on the car. Far more likely than transmission damage, I believe, is engine damage - race that thing up to 11,000 RPM or so, and valves start lagging behind their lifters when closing, causing contact with the pistons. Bad news.

[Sweet Walter]

Down shifting from too high of a speed will stress every part of your drivetrain. Back loading the gears in your differential and transmission, putting more torque through your clutch than it was ever designed for and if you are foolish enough to get into the throttle, your experiment might cost you an engine. Imagine that your momentum is another engine pushing in the same direction as the one in your car. Kablooie!
Worst case would be locking up the rear wheels, this would slow you down, but since they are unloaded they won't be slowing you down that much. Imagine the difference between bicycles with only rear brakes and those that you can stop with the front. Now that our rear wheels have broken loose, any steering input will likely start the rear end to come around to pass the front. Since we are good drivers we steer into it and fish tail into the nearest convenient row of bushes, very out of control.
If your freind went through the windsheild, it was likely from the wreck he caused.

[jon787]

On my 1991 Honda Accord it is also impossible to downshift into 1st gear above about 15 MPH.

Once I accidently shifted into 2nd instead of 4th while accelerating on the freeway. My engine went almmost to the redline when I released the clutch.

[barbitu8]

Originally posted by brad_d
I seem to recall reading somewhere that 1st gear on modern transmissions typically has no synchronizer - is this correct? If true, I'd think that would make engaging 1st gear at any significant speed pretty tough, which seems to be the case on my truck.


No. In fact you should downshift to 1st if you're going slower than 5 mph (at least in my Honda - according to the manual). In the olden days, when they did not have a synchronizer, you weren't supposed to downshift to 1st at any speed. Since the invention of the synchronizer, this is not true.

[Homer]

Originally posted by brad_d


I've heard some complaints from BMW M3 drivers that it's very easy to accidentally do just that on the car. Far more likely than transmission damage, I believe, is engine damage - race that thing up to 11,000 RPM or so, and valves start lagging behind their lifters when closing, causing contact with the pistons. Bad news.




That's very true. Miss a shift while pushing your M3 too hard, and the engine is toast. MANY M3s have lost their engine while at the track. My friend's dad bought him one for his 21st birthday, and just a few weeks later, he let his older brother race it at I-70 Speedway. One missed shift, one month, and $5000 later, the M3 was back on the road.

--Tim

[Lawmill]

I've never heard of a true manual transmission that electronically prevented you from shifting into 1st at high speeds. In my experience, changing the transmission fluid will allow you to downshift at higher rpms, so any limitation is probably synchro related.

Most cars do have rev limiters. They prevent the engine from going too fast. On my MR2, it kicks in at about 8200. I'll bet that a new M3 wouldn't be able to go 3k rpm past redline(8000) if a stock rev limiter were present.

I always thought that reverse was the only gear without a synchronizer. This makes sense, because you only enter reverse when you're motionless.

[Gunslinger]

Lawmill: In a lot of 'em (mostly trucks), 1st isn't synchro-ed.



When he was young and stupid, my uncle had a '56 Buick hotrod. One time somebody pulled up beside him at 55 mph wanting to race...my uncle reached up (three-speed on the column) and dropped it into 2nd...and spun the tires. The other guy decided he didn't want to race. :)

[galt]

Lawmill:
Don't rev limiters work by cutting off the gas? If that's the case, you could still downshift into a gear that pushes you past redline, since it's the car's momentum that's spinning the engine.

[Pessor]

Kids...

The vehicle is equipped with safety belts. Nobody goes through the windshield if these (aside from common sense) are employed. Should you desire going through the windshield, strike a telephone poll without the use of this important safety device at high speed... But please, in the instance of a BMW M3, keep the thing out of first gear before doing so. Homer's brother will give your widow top dollar for the drivetrain.

[berdollos]

This was in the 1950s, with a manual transmission car, and before safety belts.

[Lawmill]

Don't rev limiters work by cutting off the gas? If that's the case, you could still downshift into a gear that pushes you past redline, since it's the car's momentum that's spinning the engine.

No, you're thinking of the speed limiters in many newer cars. A rev limiter just prevents you from going too far past redline. The engine is still providing power.

[galt]

berdollos:
It still sounds highly implausible. Think about this: if the downshifting force were great enough to stop the wheels completely, the car would still not stop fast enough to make you go through the windshield, because it would skid. I'm guessing "go through the windshield" was merely hyperbole on the part of the person telling the story.

[Pessor]

Yeah there's the real answer... not likely, no matter what you do with the drivetrain, to be a quicker way of slowing the car down (sans impact) than the car's brakes. Maybe split your lip open on the steering wheel if not using safety belts... (smiley)

Okay... here's the question. Where are the smilies???

[wolfseyn]

How about Reverse @ 60mph!
YEAH!

[berdollos]

not hyperbole, cause the kid died.

What I do not know is whether this was how he died,
it was what I was told.

[The Ryan]

Did his whole body go through the windshield? I could see someone's head going through the windshield, but not the hole body.

Pessor: smilies (http://boards.straightdope.com/sdmb/index.php?action=showsmilies)

[Pessor]

Thanks 'The Ryan' for the smiles link...

As for the friend that got killed (now I'm under the impression that he died in the 1950's or so...) doing this? There's a story that you've yet to hear in the category. The kid died doing something sinister but he didn't self-inflict, especially in the form of an inappropriate downshift.

Here I'll say it... Was he intoxicated?

Also, If the story originated from back in those days, who told it and continues to tell it?

[Badtz Maru]

I don't think there is any way of stopping a car quick enough to throw someone through the windshield, seat-belt or not, short of running into something. As pointed out above, even if it caused all wheels to stop spinning completely, you would just skid. I've locked my brakes at high speeds before (70+) and though it was scary as hell, I did not experience that kind of deceleration.

[galt]

Maybe he threw it into first, screeched to a halt, and was shot by the guy behind him. :)

Lawmill: Ok, then, so how do the rev limiters work? I've hit mine while going well under the speed cap on my car, and it just seems as though the gas is cut off.

[berdollos]

He was a boarding student at my prep school and in my class.
He died in a traffic accident on a highway around Baltimore.
The story was that he downshifted at high speed. But I never knew if that really happened.

No one has talked about it recently. It is just an event and a question that surfaced in my mind.

[Lawmill]

Lawmill: Ok, then, so how do the rev limiters work? I've hit mine while going well under the speed cap on my car, and it just seems as though the gas is cut off.

That's weird. I autocross and it's common practice to let the engine bounce off the rev limiter for a few seconds before a turn when it would be normal to upshift. The engine still seems to be providing power.

I don't know how it works, probably part of the ECU. I know some at-home and professional modifiers who remove them when performance upgrades would benefit from higher possible rpms.