Excess weight

[Kenneve]

As part of the preparations to take delivery of a new EX CVT, I spent Thursday morning taking all of the loose items out of the old car, ready for the handover in part exchange.

Whilst doing this, I thought it might be interesting to weigh everything, well just for the fun of it really!
However, I was somewhat staggered by the total, which came to 72lbs.
My steel standard spare wheel was 32lbs leaving 40lbs for all the other sundry items, mats, books, first aid kit, in car cameras, sat nav,  you name it!

I suppose it's not surprising that Honda have done away with the spare wheel etc. I did take drive without all the junk and it was possible to tell the difference.

Incidentally, somewhat off topic, has anyone figured out how to mount the in-car camera on a Mk3 screen so that it's within the swept area of the wipers and does not impinge on the passengers view too much?

[guest1372]

72lbs = 32.7 kg

In a respected car opinion site they mentioned a drop of 7mpg between a full and 1/4 tank of fuel, that's a similar weight.  If you never use the backseats, they are quite heavy - had them out a few weeks back when giving the car a deep clean in preparation for sale (old Jazz could be made quite van like if you wished).
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TG

[mikebore]

In a respected car opinion site they mentioned a drop of 7mpg between a full and 1/4 tank of fuel,
 

This doesn't pass my BS test !....can you give a link to read what they actually did.

Full to quarter is about 30 litres or 22.5 kilo.

What did the article say about effect of three passengers (3 x 70 = 210 Kilo) on fuel consumption?

[Skyrider]

In a respected car opinion site they mentioned a drop of 7mpg between a full and 1/4 tank of fuel,
 

This doesn't pass my BS test !

Or mine, how long before someone reccomends adjusting tyre pressures to compensate for tank contents?

[guest5079]

In response to the question about how much difference two back seat passengers make, when doing a 'Google' on air con I came across an article about research done at Millbrook. I hope that's correct but it is the Industry testing grounds. There, a list was made regarding what made a difference to fuel consumption and I was surprised how much  difference that two rear seat passengers made. If my old brain remembers ( probably very little) the car used was a Vauxhall 1.2. more than that I am afraid is in the annals of history.

[guest1372]

This doesn't pass my BS test !....can you give a link to read what they actually did.
Full to quarter is about 30 litres or 22.5 kilo.
What did the article say about effect of three passengers (3 x 70 = 210 Kilo) on fuel consumption?

Probably should have added more context and I tend to agree, so slightly under quoted the figures given, any way here's the piece.

http://www.honestjohn.co.uk/our-cars/skoda-yeti/the-big-question-whats-the-economy-been-like/

Over the course of the past four months, I've been achieving 56mpg overall, with motorway driving coming in at around 60mpg.

Filling up makes a big difference. Putting 55 litres of diesel into the tank adds around 45kg of weight, so all the time you're filled to the brim, you're carrying the equivalent of two very large suitcases around with you. In fact, as one of my previous updates shows, it's actually more weight than the back seats.

It has a serious impact on fuel consumption. Even with the lightest of light right feet, it knocks between 7-10mpg off what I can achieve when the Yeti has between a quarter and half a tank of fuel. So that means instead of achieving 56mpg, I tend to get between 48 and 50mpg, depending on conditions. That means the first half of full tank could actually be costing me between 50 and 60 miles of range. At a stroke, that cuts my theoretical range down from 774 miles to to 714 miles.

Circumstances really are everything, maintaining mass at a constant velocity is very different to accelerating or lifting that same mass in energy terms.  Static mass would have the benefit of lowering the car presumably helping aerodynamic efficiency; at idle mass is irrelevant, at steady 70mph (31m/s) drag will be predominant, it's more down to how rapidly and how often the vehicle will change between these states.

It takes ½J to accelerate 1kg from 0 to 1m/s, it takes 1½J to accelerate 1kg from 1m/s to 2m/s, so triple the amount.
E_x,y=½(my²−mx²)

So comparing 0 to 10 mph and 65 to 75 mph for 30 litres of extra fuel:
E_0,10 = 222J
E_65,75 = 3106J

For three passengers at 210kg:
E_0,10 = 2098J
E_65,75 = 29377J

This indicates it requires 14x more energy to accelerate the mass an equivalent amount at motorway speed than from standstill (of course this is just the mass not the drag).


--
TG

[Skyrider]

I don't get stressed about fuel consumption,  I use a car, it uses fuel. As long as my car is doing about 100 miles per quarter tank all is well. If you worry about fuel consumption to the nearest .005 of a mpg use a bus.

[andruec]

I've never felt that weight could make much difference. I mean the car itself already weighs over 1 tonne. An extra 50kg of weight is only 5%. If the car can achieve 50mpg unladen that would imply an extra 2mpg which is going to be unnoticeable. In any case weight will only impact performance when accelerating. There might be a slight increase in rolling resistance from the tyres being more compressed but it'll be minuscule.

I went up to North Wales at the weekend (190 miles) and on the way up as I left the 50mph roadworks at Stoke the car readout was showing 70.8 mpg! I actually wished they were a couple of miles longer as I'm sure she could've got to 80mpg 

I did the whole journey on cruise control at 60mph (apart from those roadworks and a bit of street driving at either end. When I got back home she was showing 67.9 but after I filled up (same pump as last time) it came to 64mpg. The real shock though was getting back home after driving 380 miles and seeing the fuel gauge still above a quarter. Shocking to think that the car's range is going on for 600 miles when driven like that but only 400 or so when commuting 

[mikebore]

Back to the original statement that the difference between full and a quarter (22.5Kg) causes 7 mpg drop, which is 2% increase in mass of the car.

As you say TG, energy is proportional to the square of the speed but linear for mass. So all else being equal, (driving conditions, number of accelerations at each speed etc, which they probably weren't with Honest John), the extra energy used for a 2% increase in mass should be 2%, not 16% which 7mpg implies.

[mikebore]

Back to the original statement that the difference between full and a quarter (22.5Kg) causes 7 mpg drop, which is 2% increase in mass of the car.

As you say TG, energy is proportional to the square of the speed but linear for mass. So all else being equal, (driving conditions, number of accelerations at each speed etc, which they probably weren't with Honest John), the extra energy used for a 2% increase in mass should be 2%, not 16% which 7mpg implies.

What I wrote above is wrong. The 2% mpg penalty for 2% mass only applies of course while accelerating. Deceleration and steady speed running there would be no penalty for a mass increase (to a first order).

So the overall penalty for extra 22.5 kg in actual driving would be much less than 2%.

I definitely call BS on Honest John 😏

[andruec]

Anyway, I'm not taking the 35kg of golfing equipment out of my car. It's essential. You never know when you might have a few hours spare

[guest1372]

It was just one persons opinion rather than a true test, so a bit misleading really.  I think the energy for a 10mph speed change at higher speeds shows that cruise control is probably very beneficial for fuel consumption, although CC users can be particularly poor drivers sometimes - cruising up to the rear bumper - not letting slip road cars merge etc. but that's a whole other thread.

The chipping wedge and ½ dozen balls should probably never be left out of the car, same rationale for the office putter.
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TG

[guest5589]

although CC users can be particularly poor drivers sometimes - cruising up to the rear bumper - not letting slip road cars merge etc. but that's a whole other thread.
--
TG

Speaking of joining motorway from slip roads.. have a look at this
https://www.youtube.com/watch?v=8sZ8QXaQS2M