Poor coasting: why?

[RPM]

Hi, one of my cars is an old 80s Ford Fiesta. It was never sold in the US, so here's a picture of one for reference:

This is a small B-segment car with a carbureted 1.1 liter 50 hp pushrod engine. It weighs in at 780 Kg and has a very poor .Cd of 0.40.

One of the things I never really understood completely is why this car coasts so poorly in neutral when compared to other cars I have owned or driven. It makes the car kind of uncomfortable for an hypermiler (i.e. it needs constant shifting)...

For example a Honda Jazz (Fit for you in the US) weighs only 980 Kg and coasts so much better.

The brakes aren't dragging. And the tires are 155/70 R13 Michelin Energy, hence eco-friendly... When I push it by hand it rolls easily.

Hypotheses:
- It's just too light to coast well.
- Ford uses very heavy gearbox oil when compared to Honda?
- Aerodynamics hurt it significantly even at slow speeds.
- All of the above combined?

I would like to hear from Metro owners for instance. They are light, I think. How well do they coast?

Is there any simple and cheap modification that would help the car coast better (it's an historic vehicle, I don't want to butcher it)? If I could make coast better it would become more comfortable and I would then drive it more...

Thanks!

[Daox]

This is one of the first thing I noticed in my Metro. It does NOT coast nearly as far as any other previous car I've owned, and I've had a few small Toyotas (Tercel and Paseo). The lightweight design combined with the less than stellar aerodynamics just makes it slow down a lot faster. I did notice that once I get to very low speeds (~15 mph) it coasts great...

[RPM]

Thanks, it may be that it is just the nature of the beast + greater mechanical optimization from the Japanese companies.

[user removed]

Barring anything increasing the total drag, the issue is called sectional density. Less weight behind each square foot of frontal area, means the same aero total resistance has less inertia to push your car further when coasting.

regards
mech

[MetroMPG]

Agreed: high Cd and low weight is a frustrating combination! The Metro is a poor high speed pulse and glide machine. But 0.40 is particularly high.

There are lots of 'stealth' aero mods that could be done. And yet others that aren't stealth, but could be easily reversible. Good news: those wheel covers look pretty good already.

Unfortunately, I suspect the hatch angle is a big part of the problem. Fixable, but not as a 'stealth' mod.

[fusion210]

I can't believe it's been almost five and a half years, but I posted about alignment affecting fuel economy. I bring it up because nobody has mentioned it yet.
http://ecomodder.com/forum/showthrea...car-12358.html

Almost a 7.5% drop in setting from zero toe to 1/8" toe in just the front.

At 60mph!

Using the calculator here, rolling resistance with a driver on a first gen Insight is approximately 25% of total drag at sixty miles per hour. If I'm doing this right, that's an increase of rolling resistance of 30%.

What I'm getting at is that an older car with specs for toe in, possibly while being misaligned and maybe having worn parts could really wreck your coasting.

I agree with the other posts regarding high drag and low weight as well.

[MetroMPG]

Great point about alignment.

[oldtamiyaphile]

Originally Posted by Old Mechanic

Barring anything increasing the total drag, the issue is called sectional density.

Thanks for the new term, I've been calling it aero density in my head (although I've been factoring in CD as well as frontal area)

Cars with poor 'aero density' are no fun to hypermile.

I actually prefer driving my Renault with the trailer attached. My CD goes from 0.33 to 0.45, but my weight goes up by 500kg. Although high speed glides are reduced, low speed (where I spend all my time) glides are improved. Sadly, it's still less efficient with the trailer.

[RPM]

Originally Posted by MetroMPG

Agreed: high Cd and low weight is a frustrating combination! The Metro is a poor high speed pulse and glide machine. But 0.40 is particularly high.

There are lots of 'stealth' aero mods that could be done. And yet others that aren't stealth, but could be easily reversible. Good news: those wheel covers look pretty good already.

Unfortunately, I suspect the hatch angle is a big part of the problem. Fixable, but not as a 'stealth' mod.

I will look into making a belly pan. The car has absolutely no underbody protection, which I dislike because the engine gets dirty very quickly and I like to keep it detailed. So I could kill two birds with one stone. I am not much of a fabricator, must gather up courage.

The .Cd is high but the frontal area is small: the car is 1.3 meters tall and 1.585 m wide (thank God for small miracles ).

The air flow must detach at the top of the roof for sure. They even made the roof raise slightly at the edge to promote a cleaner separation.

[dirtydave]

sweet ride!
I noticed this too with 4 people in my car vs 1 person