Is engine braking really necessary?

[xxChrisxx]

You can lock the rears and have the gearbox hitting the limiter without stalling the car.

That statment as written doesn't make mechanical sense. Could you please provide clarification as to your meaning?

Another view on this is the handbrake. You don´t stall the engine just by locking the rears for a second while performing a nice slide.

You don't stall becuase.

You just have to push the clutch in before you stop the car.

You don´t stall the engine just by locking the rears for a second

You do if it's rear wheel drive.


It's not a case of believing me, i'm not 'having you on'.
You can go out and test this for yourself, if you are so inclined.
Next time it gets icy (so it's really easy to lock up), or at high speed if you are a daredevil.

Drive.
Jump on the Brakes and lock up.
This will happen (I hate yahoo answers but this lists what will happen nicely)
http://answers.yahoo.com/question/index ... 307AAf58IX

You'll still be moving, the wheels wont be rotating so the engine won't either (ie stalled).
Release the brake with the clutch still out, you will bump start the car.

Disclaimer: do it with no cars near you as you will lose brakes and power steering.
Any 'moments' during the course of the experiment
are the responsibility for the driver.

[autogyro]

If the clutch is disengaged the engine is no longer connected to the road wheels.

Obviously you engage the clutch again after putting the gearbox in second gear.

If you lock the driving wheels, and don't slip or disengage something along the transmission. You will stall. F1 have anti stall, which pulls in the clutch when the rears lock. In your car your brain tells you to push the clutch.

Nope because the forward momentum of the car will eventually slow down enough for the rear wheels to start rolling again.
You can lock the rears and have the gearbox hitting the limiter without stalling the car.

Another view on this is the handbrake. You don´t stall the engine just by locking the rears for a second while performing a nice slide.
You just have to push the clutch in before you stop the car.


My brain only tells me to push the clutch in because of fear of destroying the gearbox, but it doesn´t change the fact that you can lock the rear wheels while moving and not stall the engine.

Sorry Nando but most of this is wrong.

If the handbrake locks the rear wheels without clutch slip the engine will stall.

If the rear wheels lock at any time whatsoever without a gap in the torque path from the engine the engine will stall.

[xxChrisxx]

I'm posting this as a seperate post to stop it being lost in the clutter.

It is more difficult to stall a road car as the systems are designed to keep the wheels turning pretty much at all costs. Systems designed to prevent disaster should not being confused with the sources of disaster being absent.

[Nando]

You don´t stall the engine just by locking the rears for a second

You do if it's rear wheel drive.

No you don´t because then every drifter in the world would have to restart their car after they have used the handbrake.

If you sit in the car and are doing 100km/h in 5th gear.

If you push in the clutch and put it in 2nd gear then release the clutch, you will hear your engine screaming at maximum rpm´s but the rear wheels will be locked because they are turning at a much higher rate then what the gearbox in that gear can do.

[Nando]

I'm posting this as a seperate post to stop it being lost in the clutter.

It is more difficult to stall a road car as the systems are designed to keep the wheels turning pretty much at all costs. Systems designed to prevent disaster should not being confused with the sources of disaster being absent.

Not sure why you talk to me like i was 3 years old.

Will the next post guide me through which pedal does what as well?

Best one was on the former page, telling me to stay away from other cars when sliding.

Pretty sure that would be filed under common sense in most peoples brain.

[xxChrisxx]

No you don´t because then every drifter in the world would have to restart their car after they have used the handbrake.

They have slipping diffs. Infact they have differentials specifically designed for the task. If you look back, I said that something has to slip (basically allow the engine to rotate relatively to the wheels).

Quote

I also don't wish to derail the thread any more with "how drivelines and transmissions work". If you are inclined we can discuss it elsewhere.

But one thing I do have to ask.

but the rear wheels will be locked because they are turning at a much higher rate then what the gearbox in that gear can do.

If they are turning, how can they be locked?


Also I'm sorry if what I said come across as patronising. It really was not my intent, the disclaimer thing was just a little joke.

[Nando]

No you don´t because then every drifter in the world would have to restart their car after they have used the handbrake.

They have slipping diffs. Infact they have differentials specifically designed for the task. If you look back, I said that something has to slip (basically allow the engine to rotate relatively to the wheels).

You do know that you can drift ordinary street cars?
pulling the handbrake is one of many ways to get the rear end rotating.

And you are also not facing the fact that on the Drift car and a street car the rear wheels will be locked fully. So by your logic the engine would stall on both of them.

What diff is on the car should not matter because the rear wheels are not moving for that half a second or more.

But one thing I do have to ask.

but the rear wheels will be locked because they are turning at a much higher rate then what the gearbox in that gear can do.

If they are turning, how can they be locked?

let me re-phrase that one.

80km/h - tires are spinning at X amount of revolutions per minute.
4th gear at 80km/h have X amount of rpm´s.
Both of them "work together" gearbox can handle that speed because it´s in 4th gear.

Now if you slap in 2nd gear you will lock the rear wheels because the gearbox can´t run at that speed in that gear.

You will then get your gearbox to dance on the limiter screaming and asking you to slow down the car because that gear was only ment for let´s say 50km/h (random numbers)

So because the gearbox at that gear can´t spin as fast as the rear wheels want to do because they are travelling at 80km/h you will end up with locked rear wheels.

However, the locked rear wheels won´t stall the engine because the engine is dancing on the limiter.
The wheels will stop locking when you slow the car down to 50km/h.

You have one part that wants to do 80km/h the other wants to do 50km/h.

Also I'm sorry if what I said come across as patronising. It really was not my intent, the disclaimer thing was just a little joke.

Ok fair enough i did not see that as a joke but that could be on my part.

[Nando]

Maybe i´m missing a part here but could you explain to me how you would stall the engine in the 80km/h example?

For the engine to die you need to kill the rpm´s.

So if we take your thoughts that locking the rears will stall the engine it would mean the rpm has to drop.

So if we have a gearbox at maximum rpm´s and a pair of rear tires wanting to turn at 80km/h.

When those rpm´s drop you would all of a sudden allow the rear wheels to start turning again which would stop the rpm´s dropping and possibly even shoot back up to the limit once again as the car probably is still outside the 2nd gear working "area".

[xxChrisxx]

Before we carry on, I have no knowledge of drifting.

But it sounds like you are describing a rather extreme driveline shunt, forcing freewheeling at high speed due to overrevving. Which surely would just break traction and start the driven wheels spinning.

So I don't know if this is a curiosity of language, ie 'lock-up' to mean 'break traction'. Or if the wheels acutally do stop rotating.

I can't think of a reason why the wheels should stop rotating. But even if they do stop, something along the transmission must be slipping.

As a schematic a transmission can be imagined as a rod. You can't rotate one end faster than the other without:
a: Having something inbetween that allows relative rotation, clutch or similar.
b: Twisting the rod.

As transmissions don't twist very far before breaking, slip is usually the method of allowing the relative movement.

I'll read up shift lock drifting business.

[xxChrisxx]

Maybe i´m missing a part here but could you explain to me how you would stall the engine in the 80km/h example?

For the engine to die you need to kill the rpm´s.

You can still travel forward without the wheels rotating. The wheels are sliding relative to the ground rather than rotating (ie locking up).

This typically happens when you have very low grip. Ice and gravel and maybe in the wet. But can happen on tarmac at higher speeds/forces.

[Nando]

Before we carry on, I have no knowledge of drifting.

But it sounds like you are describing a rather extreme driveline shunt, forcing freewheeling at high speed due to overrevving. Which surely would just break traction and start the driven wheels spinning.

So I don't know if this is a curiosity of language, ie 'lock-up' to mean 'break traction'. Or if the wheels acutally do stop rotating.

I can't think of a reason why the wheels should stop rotating. But even if they do stop, something along the transmission must be slipping.

As a schematic a transmission can be imagined as a rod. You can't rotate one end faster than the other without:
a: Having something inbetween that allows relative rotation, clutch or similar.
b: Twisting the rod.

As transmissions don't twist very far before breaking, slip is usually the method of allowing

I'll read up shift lock drifting business.

Well we have a few scenarios up now, which one are you talking about?

handbrake (actually locking the real wheels for a second) and sliding.

or the 80km/h straight line "test"?

[Nando]

You can still travel forward without the wheels rotating.

Because the front tires are freely moving and you have a considerable amount of weight up in speed, rear tire lock won´t stop the car effectively at all.

The wheels are sliding relative to the ground rather than rotating (ie locking up).

So you mean that the rear tires will spin at 50km/h but the car will travel at 80km/h?

Edit: I will be going to work in 10 minutes so i won´t reply until after a few hours.
You might see me answering other topics during that time from my iphone but discussions like this is just too much to write on an Iphone
Just so you don´t think i will ignore you.

[xxChrisxx]

I'll get the mods to clear this discussion (as we've hijacked the op), and move it to an appropriate area.

It'll also give me time to get a comprehensive post put together.

[olefud]

As another quick point on the graph, I used to race stock cars on a half mile dirt track. If the track got too snotty the rear wheels tended to lock when letting up before a corner. Flywheel weight greatly influenced the tendency to do so.

[xxChrisxx]

I'm currently doing a workthrough of traction from first principles, for posting in a seperate thread.

Could you elaborate a bit on that point.

What do you mean by snotty? When the track loses grip?
Did the wheel stop rotating and remain sliding or just break traction?
Was this under braking, thottle, lift off/overrun at high revs/low revs etc.
Was it on a straight or a corner?
Was it a specific wheel, ie the loaded or unloaded, or was it both?

Thanks