Things you always wanted to know the answer to [Vol. 2]

[Crusoe]

monthefish said:

Driving in the wet.

Does it decrease fuel consumption (less friction between tyre and road) or increase fuel consumption (in the same way it's harder to walk through a swimming pool than dry land)?

Steady state motorway at 70mph type thing you would get a slight benefit from lower surface friction and maybe even a slight upward force of water spray from the tires. In a powerful car you also tend to be a bit smoother with the controls and less likely to drive as fast so probably save more on wet days compared to dry summer days. Cool wet air is also good for the engine performance. To get the road wet you would generally offset all those advantagea against the worse MPG you would get from the rain impacting the car and the wind that usually goes with it though.

[rohrl]

Crusoe said:

monthefish said:

Driving in the wet.

Does it decrease fuel consumption (less friction between tyre and road) or increase fuel consumption (in the same way it's harder to walk through a swimming pool than dry land)?

Steady state motorway at 70mph type thing you would get a slight benefit from lower surface friction and maybe even a slight upward force of water spray from the tires. In a powerful car you also tend to be a bit smoother with the controls and less likely to drive as fast so probably save more on wet days compared to dry summer days. Cool wet air is also good for the engine performance. To get the road wet you would generally offset all those advantagea against the worse MPG you would get from the rain impacting the car and the wind that usually goes with it though.

I doubt this. I'd have thought you'd increase fuel consumption due to having to shift a load of water out of the way of the tyres.

[DervVW]

rohrl said:

Crusoe said:

monthefish said:

Driving in the wet.

Does it decrease fuel consumption (less friction between tyre and road) or increase fuel consumption (in the same way it's harder to walk through a swimming pool than dry land)?

Steady state motorway at 70mph type thing you would get a slight benefit from lower surface friction and maybe even a slight upward force of water spray from the tires. In a powerful car you also tend to be a bit smoother with the controls and less likely to drive as fast so probably save more on wet days compared to dry summer days. Cool wet air is also good for the engine performance. To get the road wet you would generally offset all those advantagea against the worse MPG you would get from the rain impacting the car and the wind that usually goes with it though.

I doubt this. I'd have thought you'd increase fuel consumption due to having to shift a load of water out of the way of the tyres.

I'm with you, Ive driven the same route to work in driving rain and the mpg per the computer (a lying VAG unit) did go down, by a margin

[P-Jay]

DervVW said:

rohrl said:

Crusoe said:

monthefish said:

Driving in the wet.

Does it decrease fuel consumption (less friction between tyre and road) or increase fuel consumption (in the same way it's harder to walk through a swimming pool than dry land)?

Steady state motorway at 70mph type thing you would get a slight benefit from lower surface friction and maybe even a slight upward force of water spray from the tires. In a powerful car you also tend to be a bit smoother with the controls and less likely to drive as fast so probably save more on wet days compared to dry summer days. Cool wet air is also good for the engine performance. To get the road wet you would generally offset all those advantagea against the worse MPG you would get from the rain impacting the car and the wind that usually goes with it though.

I doubt this. I'd have thought you'd increase fuel consumption due to having to shift a load of water out of the way of the tyres.

I'm with you, Ive driven the same route to work in driving rain and the mpg per the computer (a lying VAG unit) did go down, by a margin

Worth considering in the wet that cornering speeds will be lower, meaning accelerating needed between roundabouts and such.

[Crusoe]

A 5% reduction in rolling resistance equates to a 1% reduction in fuel consumption. All other variables being the same a wet road has lower friction and a colder tyre also tends to have lower rolling resistance. Obviously different if you were wading through a river compared to a damp road. Rare that you get a damp road with no other mpg impacting conditions (cold decreasing tyre pressure adds resistance as would the usual wind and rain)

If you have the time or inclination http://www.edccorp.com/library/TechRefPdfs/EDC-103...

[Ayahuasca]

A given volume of wet / humid air will have less burnable oxygen than dry air, maybe that accounts for some of the power reduction?

[deeen]

Ayahuasca said:

A given volume of wet / humid air will have less burnable oxygen than dry air, maybe that accounts for some of the power reduction?

I thought the rapid expansion of the water gave more power, not less?

[Ayahuasca]

Boiling water in a kettle. Sea level, non-pressurised. Just making an ordinary cup of tea.

If you leave the water to boil, does its temperature continue to rise, if so is the rise slower than pre-boil?

[ATTAK Z]

Ayahuasca said:

Boiling water in a kettle. Sea level, non-pressurised. Just making an ordinary cup of tea.

If you leave the water to boil, does its temperature continue to rise, if so is the rise slower than pre-boil?

Nope ... excess energy is given off as steam (latent heat)

[walm]

No and hence no.
At 100degC it turns to steam and buggers off.
You need >1atm pressure to get above 100.

[menousername]

forgive my mechanical ignorance

but why / how does a steering wheel return to centre position when you let go of it mid-turn?

how do the wheels return to "straight" when you let go during a turn (and do that manly thing where you let the steering wheel feed itself through your hands and instinctively know when to grab it) I would have thought that the pushing momentum acting on wheels that are approx half-locked would cause it to either continue its turn or increase the angle of turn...

[Ayahuasca]

So the actual water in a boiling kettle will always be less than 100 degrees C?

[marshalla]

menousername said:

forgive my mechanical ignorance

but why / how does a steering wheel return to centre position when you let go of it mid-turn?

how do the wheels return to "straight" when you let go during a turn (and do that manly thing where you let the steering wheel feed itself through your hands and instinctively know when to grab it) I would have thought that the pushing momentum acting on wheels that are approx half-locked would cause it to either continue its turn or increase the angle of turn...

Caster angle causes self-centring if it's done correctly.

[walm]

Ayahuasca said:

So the actual water in a boiling kettle will always be less than 100 degrees C?

Yes.
Why? What did you think happened?

[boyse7en]

Ayahuasca said:

So the actual water in a boiling kettle will always be less than 100 degrees C?

The water in a kettle will reach 100 deg C and at that point the water will begin to turn into steam and evaporate off. Therefore the water cannot achieve a temperature of greater than 100deg (at atmospheric pressure).

The only reason a kettle would not reach 100 deg would be if the thermostatic switch was set too low.

[monthefish]

boyse7en said:

Ayahuasca said:

So the actual water in a boiling kettle will always be less than 100 degrees C?

The water in a kettle will reach 100 deg C and at that point the water will begin to turn into steam and evaporate off.

...although the white stuff you see coming out of the kettle when that happens isn't actually steam.

[Nimby]

The water molecules in direct contact with the heating element / plate will be hotter than 100C. And it takes time for steam to form at nucleation sites so you'll get actual liquid water > 100C. If you heat water in a container with no scratches or imperfections it can superheat, though that's not likely in a kettle.

[Dr Jekyll]

[quote=boyse7en]


The water in a kettle will reach 100 deg C and at that point the water will begin to turn into steam and evaporate off./quote]

But any body of water at any temperature above freezing always has some evaporating off, and condensing back into visible steam if the surrounding air is cold enough.

I thought the point about boiling is that water is then hot enough to turn to vapour under the surface and bubble to the top without the pressure squeezing it back to liquid.

[Ayahuasca]

There was a poster a while ago, maybe on this very thread, who thought that the bubbles in boiling water were bubbles of oxygen and hydrogen gas separating out...

[Dyl]

Two from me that I've found myself asking recently.

On cars with manual ventilation controls, why does it suggest that front window demisting should be at the 3rd highest setting? Surely the highest will get the most air passing across the window. (Stock photo below)



I recently bought a hard drive case, included with this was a USB connector. Why does the connector have a secondary cable coming from it as in the picture below?