"Rolling" Speed

AntHarmer · Postby **AntHarmer** » Sun Oct 27, 2013 8:53 pm

TTT wrote:Important thing here is that I wasn't behind some crazy old scientist going down the gateway at 55kmh. That would have pissed me right off.

Haha. The scientist part was tongue in cheek. I finish uni at Robina at 8PM so am not usually on the bridge until after 9 on a weeknight meaning I usually have the bridge to myself. I would never slow down to 55 in an 80 with cars behind me.

AntHarmer · Postby **AntHarmer** » Sun Oct 27, 2013 9:02 pm

davekmoore wrote:Just out of interest, how does driving in neutral sit with you in safety terms? And what does the law say?

\You can fail your driving test for clutch coasting which is a similar thing. According to the QLD new drivers handbook it is "failing to maintain prooper control of the vehicle" however I can not see it mentioned in the road rules themselves in any state. Personally, I dont see much of a problem with it. It is far easier to lose speed (brake) than gain speed in my opinion. That said, the problem that I have with it is it requires you to brake when the engine does a good job of braking on it's own without the need for brake pads to be changed every 5 minutes

davekmoore · Postby **davekmoore** » Sun Oct 27, 2013 9:41 pm

Once upon a time coasting was frowned upon partly because engines were prone to cutting out at tickover leaving you with no servo assistance for the brakes (assuming you had a posh car with a servo). Then, and now, there are lots of other reasons not to coast.

22Silver · Postby **22Silver** » Sun Oct 27, 2013 9:45 pm

5th (or 6th for those lucky ones) gear is good enough coast for me.

davekmoore · Postby **davekmoore** » Sun Oct 27, 2013 10:13 pm

As long as 5th or 6th is also a low enough gear to give you the option of accelerating away from a hazard as an alternative to braking and/or steering. C'mon Miles, we need another Advanced Motorist in this thread.

Blade_Hunter · Postby **Blade_Hunter** » Mon Oct 28, 2013 10:23 am

davekmoore wrote:Just out of interest, how does driving in neutral sit with you in safety terms? And what does the law say?

I don't think that there is any law particularly against it but it wastes fuel in fuel injected cars, the ECU will push more fuel / air mix through the car to keep the revs around 1200 for idle than it would in gear with no throttle.

This is how it was explained to me and it makes sense in my head, I could be completely wrong though

22Silver · Postby **22Silver** » Mon Oct 28, 2013 11:33 am

Does everyone here idle at the lights in neutral or clutch in, first gear?
I tend to clutch it for stop signs, and waiting to pull out into a street from a driveway, and thunk into neutral when I get the very beginning of a red light.

JBT · Postby **JBT** » Mon Oct 28, 2013 11:40 am

I only ever pop the car into neutral when stopped at lights etc. I leave it in gear until stopping. I never "ride" the clutch or sit stopped with the clutch pedal depressed wearing out the throw out bearing.

Rocky · Postby **Rocky** » Mon Oct 28, 2013 12:10 pm

As I said earlier, it is a matter of using your 'common sense' in relation to when or whether you use this tactic.
I fail to see how it could be considered dangerous in a lot of situations. I have the clutch and brake covered in any event.
Compared to all the morons out there driving whilst texting, drunk, or off with the fairies, it is not an issue.
I simply don't believe it uses the same amount of fuel as being in gear at twice the revs.

AntHarmer · Postby **AntHarmer** » Mon Oct 28, 2013 12:57 pm

Blade_Hunter wrote:
davekmoore wrote:Just out of interest, how does driving in neutral sit with you in safety terms? And what does the law say?

I don't think that there is any law particularly against it but it wastes fuel in fuel injected cars, the ECU will push more fuel / air mix through the car to keep the revs around 1200 for idle than it would in gear with no throttle.

This is how it was explained to me and it makes sense in my head, I could be completely wrong though

I had something similar explained to me. That in neutral the ECU is delivering fuel to the car to keep it idling, however when you are in gear and rolling, the rolling is turning the engine over, and as long as you are not touching the throttle it uses far less fuel.

AntHarmer · Postby **AntHarmer** » Mon Oct 28, 2013 12:59 pm

Rocky wrote:As I said earlier, it is a matter of using your 'common sense' in relation to when or whether you use this tactic.
I fail to see how it could be considered dangerous in a lot of situations. I have the clutch and brake covered in any event.
Compared to all the morons out there driving whilst texting, drunk, or off with the fairies, it is not an issue.
I simply don't believe it uses the same amount of fuel as being in gear at twice the revs.

I'm with you on the "common sense" issue Rocky. The reason I mentioned the safety factor in the OP is because I wanted people to discuss the speed at which cars roll in neutral not whether it was safe to do so. Didn't work too well did it?

Caffeine · Postby **Caffeine** » Mon Oct 28, 2013 1:56 pm

AntHarmer wrote:I had something similar explained to me. That in neutral the ECU is delivering fuel to the car to keep it idling, however when you are in gear and rolling, the rolling is turning the engine over, and as long as you are not touching the throttle it uses far less fuel.

Indeed, most modern ECU's will cut the fuel delivery entirely when engine braking. If you have a car with an instantaneous fuel usage display, you'll see it drop to 0.0L/100km when engine braking. When in neutral and idling the ECU will keep supplying fuel.

davekmoore · Postby **davekmoore** » Mon Oct 28, 2013 8:35 pm

An MX5 has a dreadful drag coefficient compared to even a decent hatchback, so that will be a factor (MX5 0.38, Hyundai i30 0.27, fully 23% better).

Did you check the brakes for binding?

Rocky wrote:As I said earlier, it is a matter of using your 'common sense' in relation to when or whether you use this tactic.
I fail to see how it could be considered dangerous in a lot of situations. I have the clutch and brake covered in any event.

True. There are a lot of situations in which it is not dangerous. Also true that you fail to see the other situations.

Rocky wrote:Compared to all the morons out there driving whilst texting, drunk, or off with the fairies, it is not an issue.

If you drive an MX5 you're likely to be better than them, but they can still kill you whether or not you're in full control of your car.

Rocky wrote:I simply don't believe it uses the same amount of fuel as being in gear at twice the revs.

No throttle and in gear = no fuel use. Momentum keeps the motor turning. Tickover and not in gear or clutch disengaged uses fuel to keep the motor turning. Your failure to believe this does not make it untrue.

22Silver · Postby **22Silver** » Mon Oct 28, 2013 8:55 pm

The question now is, what has more cost?
The amount of fuel burned when idling, or the amount of fuel you'll need to use to make up for the speed lost due to engine braking.
I guess if you can use engine brake to stay at ideal speed, you're not needing to use the throttle or idle fuel, and the brakes aren't being used either.

Magpie · Postby **Magpie** » Mon Oct 28, 2013 9:08 pm

davekmoore wrote:An MX5 has a dreadful drag coefficient compared to even a decent hatchback, so that will be a factor (MX5 0.38, Hyundai i30 0.27, fully 23% better).

I disagree as the drag coefficient on my NA6 is, with the rear wing at different positions is:
00 deg. 0.29 CDa 0.61
05 deg. 0.31 CDa 0.65
10 deg. 0.33 CDa 0.69

The results were by doing 6 coast downs (from 120 and 3 in each direction) for each wing position.