So I help two chicks change a tire......(Potential Problem)

[exdeath]

The parking pawl only works on the go wheels... just an FYI.

That doesn't even matter with an open diff anyway; the axles and spider gears are free to pivot even with the output shaft all the way through the carrier locked by the pawl. They can't turn together in uniform direction in park, as the carrier will bind, but lift one drive wheel off the ground so that's it's free to reverse, and the other wheel will easily roll in park.

I'd wager that 3000+ lbs on a single drive wheel on an incline would be enough to overcome the clutches in a LSD and still roll also. Wheel chocks are your best bet if you're paranoid and feel you need to jack one side of the car 10 feet into the air for half an hour to change a tire.

Strange thing is that people have been jacking up and changing one rear tire in rear wheel drive open diff cars with rear only parking brakes for years without incident...

 

[ViviTheMage]

You're fine, happened in my Grand Cherokee, it just ... tipped off the stand, we rejacked, no harm.

 

[Koing]

I thought you were going to say they started to pull down your pants...

Koing

 

[AMCRambler]

Seriously? Your car doesn't have a differential? Man you really got hosed...

Before you hurt yourself or someone else, please understand that locking the transmission is not the same as applying the brakes. You might need to try this yourself to believe it. Park your car somewhere level, set the parking brake, and raise the front of the car on jack stands, with the drive wheels off the ground, the parking brake engaged and the transmission in park you'll still be able to spin the drive wheels, you may need to get a little leverage on the wheel by using a tire iron on one of the lug nuts, but once you overcome the friction of the brakes dragging the wheel will turn. What may be more disturbing is that the wheel on the other side of the car will turn the opposite direction of the wheel you are turning. That's just how an open differential works and all modern vehicles (for this discussion) have one. But wait, the parking brake is engaged isn't it? Yes it is, but the parking brake only applies the brakes to the rear wheels, the front wheels are unaffected and will freewheel when both wheels are off the ground.

Now we have to do some inductive reasoning, hold on here things are going to get even more difficult to understand. With one front wheel (assume FWD) jacked up, off the ground, and the transmission in park and the parking brake set you have 2 rear tires that are pretty well locked keeping the car from moving, if you release the parking brake however, the two rear wheels aren't going to stop the car from moving if there is an incline or if someone leans against the vehicle, That one drive wheel that is off the ground obviously can't do anything because it's in the air and the other drive wheel that is on the ground is free to turn as well because the open differential will let the wheel that is off the ground spin opposite the wheel that is on the ground should the car start to roll. The car will roll off the jack then once both drive wheels are on the ground the transmission in park will stop the car.

You're wrong. With an automatic the wheels don't spin with the transmission in park. The car has to be in gear. If it's in gear and you jack both drive wheels off the ground and spin one side, the other side will turn in the opposite direction. With the car in park, you'll be able to turn each wheel maybe an inch or two in either direction because of the slop in the gears, but that's it.

All I can figure is that when he jacked up the car the other side must have lifted off the ground enough to allow the drive wheel still touching the ground to slide. With the stiffer suspension on cars today this can happen.

 

[exdeath]

You're wrong. With an automatic the wheels don't spin with the transmission in park. The car has to be in gear. If it's in gear and you jack both drive wheels off the ground and spin one side, the other side will turn in the opposite direction. With the car in park, you'll be able to turn each wheel maybe an inch or two in either direction because of the slop in the gears, but that's it.

All I can figure is that when he jacked up the car the other side must have lifted off the ground enough to allow the drive wheel still touching the ground to slide. With the stiffer suspension on cars today this can happen.

http://www.factory-servicemanual.com/wp-content/uploads/2009/06/car-differential.jpg

Look at that pic and see what happens when you hold the ring gear/carrier from moving (locked by the driveshaft/transmission). Note the blue spiders still spin freely and allow the axles to move unimpeded, provided the wheels are allowed to turn in opposing directions (eg: one off the ground).

With an LSD it's different, you'd need enough force to break the clutches loose.

 

[kalrith]

The OP, and several people commenting in this thread, should never "help" anyone with car troubles.

TBH when I saw the thread title I figured you knocked one up and she ended up getting kemo before she knew she was pregnant.

+1 million...

If you don't know what you're doing then don't try and "help" people with their stuff. Their $75 flat just turned into a $100+ rear brake job.

ITA. I have no problem calling someone for a stranded person without a cell phone, but it's (a) a liability to help people and (b) hard to tell whether people are legitimately stranded or waiting to rob you, steal your car, etc. (this one is much more important to me now that I have a 1yo son).

There are just too many people out there who would either try to sue you if you botch the repair job or try to rob you from the get-go. I'm not going to risk my son's and my life trying to help people who'll likely be just fine on their own.

IMO with driving comes the responsibility of maintaining and repairing the vehicle. If they can't change the flat on their own, then the onus is on them to call someone qualified who can. That person certainly isn't me.

 

[kornphlake]

You're wrong. With an automatic the wheels don't spin with the transmission in park. The car has to be in gear. If it's in gear and you jack both drive wheels off the ground and spin one side, the other side will turn in the opposite direction. With the car in park, you'll be able to turn each wheel maybe an inch or two in either direction because of the slop in the gears, but that's it.

All I can figure is that when he jacked up the car the other side must have lifted off the ground enough to allow the drive wheel still touching the ground to slide. With the stiffer suspension on cars today this can happen.

You did like I suggest and jacked your drive wheels off the ground to try it yourself? Go try it then come back and offer an apology. If you can't spin the wheels you either have a limited slip or locking differential (which is uncommon in modern passenger vehicles.)

 

[jellyrole]

I'm pretty sure the brake rotor is fine, no harm done.

I'm assuming they drove a manual transmission then? My car has a hard time rolling forwards or backwards with it just in park... Let alone parking brake too.

This thread has lots in common with the parking brake + automatic thread.

This would be a person not to listen to.

We have multiple celebs in this topic it seems.

 

[MrPickins]

Also, why weren't either of the loose wheels, either the spare or the one you took off, placed under the car at one of the jacking points (eg rocker panel pinch weld)?

I was taught to always do this when working with a jack.

 

[Throckmorton]

OP, let this be a lesson to you. Don't help strangers unless you're sure you're not going to create more problems. Changing a tire or working on their car in any way always has a chance of you breaking something, which you're then morally responsible for. You broke their rotor and let them drive away, now it's on you if the brake or hub fails and they crash. Even aside from that, you owe them for the damage to their rotor. Them not knowing who you are doesn't change your moral responsibility. Them not pulling the parking brake doesn't change your moral responsibility, because that became your problem when you swooped in and took over the job.


The only things I'll help with aside from an emergency is a jump start or a vehicle recovery, and only if they have proper recovery points.

You're wrong. With an automatic the wheels don't spin with the transmission in park. The car has to be in gear. If it's in gear and you jack both drive wheels off the ground and spin one side, the other side will turn in the opposite direction. With the car in park, you'll be able to turn each wheel maybe an inch or two in either direction because of the slop in the gears, but that's it.

All I can figure is that when he jacked up the car the other side must have lifted off the ground enough to allow the drive wheel still touching the ground to slide. With the stiffer suspension on cars today this can happen.

The transmission parking pawl is BEFORE the differential. In P, the diff still functions and will let the one wheel on the ground roll.

 

[Pacfanweb]

What difference does it make that there's a baby in the car for changing a tire?

So it fell off the jack. So what? That's not going to hurt anyone. I'd MUCH rather have my baby inside the car than on the side of the road.

 

[alfa147x]

We have multiple celebs in this topic it seems.

haha so true!

 

[Pacfanweb]

http://www.factory-servicemanual.com/wp-content/uploads/2009/06/car-differential.jpg

Look at that pic and see what happens when you hold the ring gear/carrier from moving (locked by the driveshaft/transmission). Note the blue spiders still spin freely and allow the axles to move unimpeded, provided the wheels are allowed to turn in opposing directions (eg: one off the ground).

With an LSD it's different, you'd need enough force to break the clutches loose.

No.

If only one tire is off the ground, and the drive shaft is locked (e.g. in park) the tire off the ground is NOT going to spin.

You have 3 things that move in the diff, and 2 of them have to be free for any movement to happen.

Either: Pinion is locked, then the spider gears will spin and both axles will spin. (opposite directions)

Or: Pinion unlocked, (neutral) one tire off ground, other on ground, turn tire, pinion WILL move, because the spider gears will both spin AND rotate as the carrier itself moves. And the carrier can't move unless the pinion is free.

But if 2 of the 3 are locked, the other one is not going to move. So you can leave it in Park, raise one tire, and it will not move.

Edit: In summary, one rear tire off the ground, trans in Park, tire off ground will NOT turn, at all. It is locked. Unless you are macho enough to make the other tire turn with all the weight of the car on it using a lug wrench. (unlikely) But the diff will not freely spin unless 2 of the three can move.

I just went and tried it on my Jeep, which happens to have one side in the air and no drive shaft. Stuck the axle in it. Put a pry bar between two studs. Had the wife pull on the bar while I crawled under the Jeep and held the pinion.....as long as I held the pinion, the axle in the air would not move. The other side has the tire on it and is on the ground.
The differential is an ARB air locker, which is obviously not locked at the moment, so it functions as an open diff.

 

[StageLeft]

They can't get you; they no information about you either way.

 

[Pacfanweb]

The transmission parking pawl is BEFORE the differential. In P, the diff still functions and will let the one wheel on the ground roll.

Nope, incorrect, see explanation above.

edit: Unless you're just strong enough to make the other tire turn the other way by turning a lug nut. But if it doesn't move, your axle won't spin freely without the other tire being in the air, also.
Easy way to keep this from happening is the e-brake.

 

[Pacfanweb]

You did like I suggest and jacked your drive wheels off the ground to try it yourself? Go try it then come back and offer an apology. If you can't spin the wheels you either have a limited slip or locking differential (which is uncommon in modern passenger vehicles.)

You owe him an apology.

 

[MrPickins]

Edit: In summary, one rear tire off the ground, trans in Park, tire off ground will NOT turn, at all. It is locked. Unless you are macho enough to make the other tire turn with all the weight of the car on it using a lug wrench. (unlikely) But the diff will not freely spin unless 2 of the three can move.

But, the car could still roll if it was on an incline, right?
(assuming the parking brake was disengaged)

 

[Throckmorton]

Nope, incorrect, see explanation above.

edit: Unless you're just strong enough to make the other tire turn the other way by turning a lug nut. But if it doesn't move, your axle won't spin freely without the other tire being in the air, also.
Easy way to keep this from happening is the e-brake.

I didn't notice anyone arguing that turning the lug nut is what makes the car roll.

I'm assuming that the car gets tipped off the jack, and the tire simply allows the car to roll because it's not braked.

BUT, you can definitely apply enough force to the lug nut to turn the wheel, and therefore rotate the other wheel the opposite way. Lug nuts are supposed to be tightened to to about 80ft/lb. With a 1ft long wrench, you'd be applying 80lb. If you think 80lb of force isn't enough to move a car, then you've never pushed a car.

 

[exdeath]

Either: Pinion is locked, then the spider gears will spin and both axles will spin. (opposite directions)

Uhm.. thats what I said? Park on an auto locks the output shaft -> drive shaft -> pinon -> ring -> carrier. With one wheel in the air, the other can freely roll with the other able to reverse instead of trying to roll in the same direction and binding in the carrier. Maybe it takes some muscle to pry on the hub to get it rolling on flat ground, but if it wanted to roll on it's own, there is nothing stopping it if the opposing wheel is in the air (disregarding 4x4, special diffs, etc).

 

[Pacfanweb]

I didn't notice anyone arguing that turning the lug nut is what makes the car roll.

I'm assuming that the car gets tipped off the jack, and the tire simply allows the car to roll because it's not braked.

BUT, you can definitely apply enough force to the lug nut to turn the wheel, and therefore rotate the other wheel the opposite way. Lug nuts are supposed to be tightened to to about 80ft/lb. With a 1ft long wrench, you'd be applying 80lb. If you think 80lb of force isn't enough to move a car, then you've never pushed a car.

80lbs is NOT going to overcome all the pressure and weight of the car being on the one tire on the ground.
You simply don't have enough leverage with a lug wrench to make those spider gears turn and turn the other tire. All the mechanical advantage is against you.

I'll put it this way: If you have one of your drive wheels in the air, the other on the ground, and the trans in Park......if you can impart enough torque on a lug nut of the wheel in the air to actually turn the other tire, then you have FAR more problems than a flat.

Now if you're talking BOTH drive wheels in the air....I can go with that. You can definitely make the other one turn. But not if one's on the ground. Unless you have a cross-threaded lug nut and the strength of Hercules, you aren't going to make the tire on the ground roll.

Looking at the OP's first post, it looks like the car simply fell off the jack. It happens. Stock jacks aren't exactly 2.5 ton shop jacks. They're flimsy.

 

[Pacfanweb]

Uhm.. thats what I said? Park on an auto locks the output shaft -> drive shaft -> pinon -> ring -> carrier. With one wheel in the air, the other can freely roll with the other able to reverse instead of trying to roll with the car and binding the in the carrier.

But that's not going to happen. The mechanical advantage is with the wheel that remains on the ground, not you with the lug wrench turning the other wheel.

And it cannot "freely" roll....there is some internal weight/friction to overcome.

Put it this way: Put the car on a lift. Leave it in Park.

Now, I stand in front of one tire. The other tire is off, and you can put a lug wrench and a nut on a stud. Now you try to turn it, while I hold the tire on the other side still.

I will EASILY defeat your efforts. You will not be able to make that tire turn while I have a hold of it.

So...what do you think is holding that tire 'harder'....My arms, or the weight of the car?

Sure, in THEORY, the tire on the ground CAN turn.....but it's not going to happen. It takes EFFORT to make those spider gears turn. They DO NOT turn easily.

I just tried this in my garage tonight.....one tire on the ground. Wife with a big ass pry bar trying to turn the axle in the air. And me, with one hand, holding the pinion yoke still.
She could not turn it. You couldn't, either. Nor could I. Come on Ex....you know your stuff. Think about it. Try it.

 

[Throckmorton]

80lbs is NOT going to overcome all the pressure and weight of the car being on the one tire on the ground.
You simply don't have enough leverage with a lug wrench to make those spider gears turn and turn the other tire. All the mechanical advantage is against you.

I'll put it this way: If you have one of your drive wheels in the air, the other on the ground, and the trans in Park......if you can impart enough torque on a lug nut of the wheel in the air to actually turn the other tire, then you have FAR more problems than a flat.

Now if you're talking BOTH drive wheels in the air....I can go with that. You can definitely make the other one turn. But not if one's on the ground. Unless you have a cross-threaded lug nut and the strength of Hercules, you aren't going to make the tire on the ground roll.

Looking at the OP's first post, it looks like the car simply fell off the jack. It happens. Stock jacks aren't exactly 2.5 ton shop jacks. They're flimsy.

There's no mechanical disadvantage. The torque you apply to the wheel is the same as what comes out on the other side, just opposite direction.

The wrench is probably the same length as the radius of the tire. So you apply 80lb at that 1ft distance, and you're applying 80lb to the car. It's like if you stand behind the car and push with 80lb. That's enough to make a car roll, much less fall off a jack

 

[EightySix Four]

Pacfanweb is arguing that a person can't create enough force to make it happen, but that's not what everyone else is arguing. They are all saying that the wheels (by some force such as gravity) can still turn (in opposite directions) with the car in park.

Everyone is right, you are just arguing different points.

 

[exdeath]

But that's not going to happen. The mechanical advantage is with the wheel that remains on the ground, not you with the lug wrench turning the other wheel.

And it cannot "freely" roll....there is some internal weight/friction to overcome.

Put it this way: Put the car on a lift. Leave it in Park.

Now, I stand in front of one tire. The other tire is off, and you can put a lug wrench and a nut on a stud. Now you try to turn it, while I hold the tire on the other side still.

I will EASILY defeat your efforts. You will not be able to make that tire turn while I have a hold of it.

So...what do you think is holding that tire 'harder'....My arms, or the weight of the car?

Sure, in THEORY, the tire on the ground CAN turn.....but it's not going to happen. It takes EFFORT to make those spider gears turn. They DO NOT turn easily.

I just tried this in my garage tonight.....one tire on the ground. Wife with a big ass pry bar trying to turn the axle in the air. And me, with one hand, holding the pinion yoke still.
She could not turn it. You couldn't, either. Nor could I. Come on Ex....you know your stuff. Think about it. Try it.

We aren't talking about on a lift on even concrete with a someone holding the free wheel or trying to motivate the vehicle through leverage on the axle. We are talking about 4000 lbs lifted up on a spare jack on a slope with only one of the drive wheels on the ground, and the rake of the car rolling foward on that one wheel as it slips off the jack with nothing locking that wheel but the rotational inertia of the opposing wheel in the air. There is nobody holding the other wheel with a prybar then.

As soon as it fell off the jack and both wheels hit the ground it would stop and ultimately rest on the parking pawl, so it's not going to roll away, but certainly worth being aware that an auto being in park alone is not enough to keep a vehicle from rolling off a jack.

Still this thread is amusing. All this fuss and detailed scenarios and technicalities but people have been changing tires on the side of the road for 100 years without much drama 🙂

 

[Pacfanweb]

There's no mechanical disadvantage. The torque you apply to the wheel is the same as what comes out on the other side, just opposite direction.

The wrench is probably the same length as the radius of the tire. So you apply 80lb at that 1ft distance, and you're applying 80lb to the car. It's like if you stand behind the car and push with 80lb. That's enough to make a car roll, much less fall off a jack

Nope, because you have to first overcome the friction it takes to make the spider gears move, which isn't a little, and then you have to overcome the weight of the car to make the other wheel actually roll.

Just not going to happen. I've changed too many tires this way, seen it done too many times, and even proved it out tonight in my own garage just to satisfy that small bit of self-doubt I had.
Again, my wife was tugging on a 24" prybar, and I was holding the pinion yoke from spinning with ONE HAND. Now, I did have the mechanical advantage of the gearing on my side.....but the tire on the other side did not. And it did not budge.

There absolutely IS a mechanical disadvantage. You are using a lug wrench that MIGHT be a foot long if it's lucky. A car with a 16" rim, that's 8" of radius just for the rim, much less the tire...so even a small tire has as much leverage as a lug wrench.

I've broken sockets and studs by standing on a pull bar this way, and never even thought about moving the car. It takes a LOT more than any 80 ft/lbs to make that thing move.......even if you had a weak engine, by the time the power gets through all the gearing and to the wheels, there's a lot more torque turning them than the engine has at the flywheel. It really takes an awesome amount of power to move a car, if you get right down at the axle and try to spin it.