Ford Fusion Lightweight Concept

[CodeRedR51]

Build a Fusion with aluminum, high strength steel, carbon fiber, EcoBoost 3-cylinder and a few other light weight tricks (check out the skinny wheels) and you can shed nearly 800 pounds.


http://www.autoblog.com/2014/06/04/ford-builds-lightweight-concept-fusion-video/

 

[YSSMAN]

This is one of the coolest things Ford has done recently, and its a shame we're 10+ years away from a lot of this tech being applicable on regular family sedans. Considering the performance penalty would take us back to ~1998 in terms of acceleration, the fuel economy savings would be great. I'd have to wonder how different it would be running regular sized, low-rolling-resistance wheels and tires.

 

[CodeRedR51]

Most aftermarket wheels are lighter than what OEM's provide now, so not sure why they didn't just go that route.

 

[Exorcet]

I really hope this catches on. Out of all the strategies to increase fuel economy this is my favorite. At the very least high end auto makers should try to introduce light weight materials more. The Sesto Elemento was a wonderful supercar concept. I prefer it heavily to the hybrid cars showing up now, but for everyone seems to be going for those hybrids instead.

I don't really care if a regular road car is hybridized so much as they aren't strictly about performance, but I think major weight reduction would be competitive even then. Having a bit more acceleration doesn't really hurt when merging on highways and whatnot. CoG is going to shift a lot more with luggage and passengers though.

 

[homeforsummer]

Most aftermarket wheels are lighter than what OEM's provide now, so not sure why they didn't just go that route.

It's not weight so much as rolling and aerodynamic resistance, I'd think. Most cars like the Fusion now seem to run on tires wider than bona-fide sports cars of the 1990s (at a guess, I'd be surprised if it's less than a 225-section), mainly to ensure they still grip well and support the ever increasing weight of cars in that sector. Which is fine, but it does harm rolling resistance and there's quite a bit of aero drag from four steamrollers spinning around like that too.

Make the car way lighter and you don't need as big a footprint. It's a virtuous circle, since the wheels and tires get lighter (good for performance) and thinner (good for economy). Same as the 1.0 Ecoboost in it - in a full-fat Fusion, it'd be pretty slow. But in a Fusion that barely weighs more than a Fiesta it probably isn't too bad. You make the car light, you can fit a smaller engine... which weighs even less and makes the car even lighter, so even less of an engine is needed.

Of course, light weight means when you do bung a bigger engine and fatter tires on there it should be pretty brisk, too.

 

[CodeRedR51]

 

[Slash]

I literally almost just posted this.


Although, the last time they tried saving weight they ended up with this:

Needless to say, that didn't last long...

 

[RocZX]

I have a few problems with it....

1.The tire look like space saver spare tires, seems like they might be dangerous high speeds. Especially maneuver at high speeds doesn't seem like the holdup. Is there any law that regulates tire size ?

2. Seeing how the windows are blacked out, does it ever have an interior or any federal mandated safety equipment? I would how much that would add to the weight of it?

3.I wonder how durable the materials that they use are and how long they last?

 

[Exorcet]

I have a few problems with it....

1.The tire look like space saver spare tires, seems like they might be dangerous high speeds. Especially maneuver at high speeds doesn't seem like the holdup. Is there any law that regulates tire size ?

It might be just as stable as the regular car. With less weight you can move a car just as much with less grip. One thing to consider though is that most road cars lift. The lower the weight, the more noticeable the lift (or downforce) will be.

2. Seeing how the windows are blacked out, does it ever have an interior or any federal mandated safety equipment? I would how much that would add to the weight of it?

Good question. R1600's diagram suggests an interior, but it may be stripped down compared to the normal car.

3.I wonder how durable the materials that they use are and how long they last?

This shouldn't be an issue. The reason lightweight material isn't use in cars is basically down to cost. The aerospace industry uses materials far better than you would find in even the best road cars and they have much tighter safety and reliability concerns to worry about. Low weight doesn't imply fragility at all.

 

[Danoff]

I wouldn't compromise on the tire width. Stopping distance is a key safety feature. With less mass you don't need as much grip to stop in the same distance, but you still have the added factor of reduced frictional force from the weight of the car. So it's not just width, but reduced grip on the surface that you do have. And you know people are gonna put 70,000 mile tires on that bastard.

Even if somehow those wheels allow the car to stop in the same distance as the stock fusion, I say make it better. Stick with the wider tires and stop even shorter. That's more important than a few tenths of a mpg.

 

[homeforsummer]

To be honest, Ford would probably put fatter tires on it for customer perception reasons alone. Even if the tires were equally safe, given the weight of the car (and if it's a car with the same mass as a Fiesta, then it doesn't need tires any wider than those on a Fiesta), a lot of customers would look at it and assume it's not going to stop as quickly.

 

[Danoff]

To be honest, Ford would probably put fatter tires on it for customer perception reasons alone. Even if the tires were equally safe, given the weight of the car (and if it's a car with the same mass as a Fiesta, then it doesn't need tires any wider than those on a Fiesta), a lot of customers would look at it and assume it's not going to stop as quickly.

I just don't get that compromise. If you're trying to win a fuel economy competition, fine. But for something that's supposed to represent potential fuel savings for consumer cars, I don't see that as a compromise... or I didn't until I watched the video. The rims are carbon fiber... that about answers it.

 

[Keef]

This is one of the coolest things Ford has done recently, and its a shame we're 10+ years away from a lot of this tech being applicable on regular family sedans. Considering the performance penalty would take us back to ~1998 in terms of acceleration, the fuel economy savings would be great. I'd have to wonder how different it would be running regular sized, low-rolling-resistance wheels and tires.

The OEM tires on a Fusion Titanium with 18 inch wheels weigh 25 pounds each. As for low-resistance tires, that doesn't have anything to do with weight. The only ways to effectively save weight via tires is to use poor quality tires that are simply manufactured with less material or make the tires smaller. I can almost guarantee the stock wheels weigh at least 25 pounds each. It is easy to make wheels lighter through more precise engineering. Original equipment wheels are typically well overbuilt for long-term reliability purposes.

Sadly, Ford doesn't give us any specifics on the size of the tires on this concept but I can tell you that the BMW i3's similarly skinny tires, sized 155/70-19 in the front, weigh 16 pounds each. The BMW's are a ridiculously huge diameter of course, bigger than this Ford concept, but generally it seems like when you chop 1/3 the width off your tires you save 1/3 the weight.

Most aftermarket wheels are lighter than what OEM's provide now, so not sure why they didn't just go that route.

Not only are they trying to prove in-house technologies, but it's probably tricky to find aftermarket carbon wheels that skinny. Above I showed how the skinnier tires could save a whole 40 pounds so if you used normal-sized aftermarket wheels you would only save weight in one place, not both places.

EDIT: Keep in mind guys that this concept isn't about gas mileage, it's about saving weight. And speaking of skinny tires, look at what the BMW i3 offers. They're skinny as hell. But the car still has acceptable stopping distances because of the shape of the contact patch. The tires are tall (19s, optional 20s on a tiny car is ridiculously tall), not wide, so they still have good longitudinal grip because the contact patch is oriented in that direction. If you wanted lateral grip you'd arrange the contact patch the other direction by making the tire smaller and wider. Obviously a big, wide tire will give you the best of both worlds. But that's also considerably heavier.

 

[Eunos_Cosmo]

I like the thought, but I hope they can do it with more standard materials. Carbon is expensive, time consuming in the production process, and produces toxic fumes when it burns. It's pretty amazing what Mazda has done (now and in the past) with plain 'ol steel.

 

[RocZX]

Sadly, Ford doesn't give us any specifics on the size of the tires on this concept but I can tell you that the BMW i3's similarly skinny tires, sized 155/70-19 in the front, weigh 16 pounds each. The BMW's are a ridiculously huge diameter of course, bigger than this Ford concept, but generally it seems like when you chop 1/3 the width off your tires you save 1/3 the weight.
.

From MotorTrend.

Chassis highlights include tall, thin 155/70R19 tires (3.6 kg, 31 percent each) riding on 5.0 x 19-inch carbon-fiber wheels (4.5 kg, 42 percent each)

Seems like they are the same size as the i3.

 

[Keef]

I like the thought, but I hope they can do it with more standard materials. Carbon is expensive, time consuming in the production process, and produces toxic fumes when it burns. It's pretty amazing what Mazda has done (now and in the past) with plain 'ol steel.

I agree. I think there's a lot of work to be done with steel structures for most manufacturers. The metal makes a ton of sense on most levels and Mazda has managed to cleverly design the complex structure to use as little material as possible while actually increasing strength and safety. That's not easy to do. The main reason I admire Mazda is because they're often glad to tackle problems that aren't easy and most other manufacturers eschew in favor of higher profits. If you don't risk it you'll never truly innovate.

From MotorTrend.

Seems like they are the same size as the i3.

Interesting. Makes me wonder if they're the same tire. According to Tire Rack, these sizes/models of Bridgestone Ecopia tires were developed specifically for the i3 and i8 and are the only tires that come in that size.

 

[homeforsummer]

I just don't get that compromise.

It's not a compromise if it offers the same performance.

The physical size of the car doesn't make much difference here - a Fusion with the mass of a Fiesta will stop at the same rate as a Fiesta with the mass of a Fiesta. In theory, possibly a little better, as the wider track and longer wheelbase will give it greater stability than the narrower, shorter Fiesta.

Putting wheels and tires like this on a regular Fusion would be a recipe for longer stopping distances and poor lateral grip. Doing so on a Fusion that's been on an 800 lb diet should be no less effective than existing heavier, fatter wheels and tires on a Fusion that still carries that 800 lbs around with it.

Also, and this bit is pure speculation so feel free to disregard, but I suspect it's easier to brake a wheel with much lower rotational mass than it is a regular wheel and tire too. As long as the tire itself has enough friction on the road surface you've got the benefits of both a lighter sprung mass to slow down, and less rotating mass to try and stop rotating.

If any actual physicists have any input to this I'd be quite interested to hear it, so I'm not just saying things that might be a load of tosh...

 

[Kent]

It's not a compromise if it offers the same performance.

The physical size of the car doesn't make much difference here - a Fusion with the mass of a Fiesta will stop at the same rate as a Fiesta with the mass of a Fiesta. In theory, possibly a little better, as the wider track and longer wheelbase will give it greater stability than the narrower, shorter Fiesta.

Putting wheels and tires like this on a regular Fusion would be a recipe for longer stopping distances and poor lateral grip. Doing so on a Fusion that's been on an 800 lb diet should be no less effective than existing heavier, fatter wheels and tires on a Fusion that still carries that 800 lbs around with it.

Also, and this bit is pure speculation so feel free to disregard, but I suspect it's easier to brake a wheel with much lower rotational mass than it is a regular wheel and tire too. As long as the tire itself has enough friction on the road surface you've got the benefits of both a lighter sprung mass to slow down, and less rotating mass to try and stop rotating.

If any actual physicists have any input to this I'd be quite interested to hear it, so I'm not just saying things that might be a load of tosh...

I agree on pretty much all of this except the final bit... Only because the rotating mass is so small compared to the actual braking load.
It definitely does make a difference but if that difference is great enough to be shown in the traditional feet or meter measurement of braking distance is unknown to me (though I doubt it would show up).

As a whole I see this sort of project as a great experiment but of no real value in the immediate future. Should someone or some company figure out how to use composites, and carbon fiber in a economical way I'm all for it, in the mean time I'll leave CF wheels to Konnneeggsseeeggggzzzr@!#$u8085gh (or whoever)... Though I have no doubt Ford's full might would do it better.

 

[niky]

Taller, narrower tires aren't really a handicap in terms of braking. Or shouldn't be, in any way that really matters. While you sometimes get better braking with wider tires... this is because of fundamental differences in tire compounds and types between wide fitment tires and narrow fitment tires. Hell, I've seen a Corolla on ballonish 185mm economy tires brake within two meters of cars equipped with big brakes and ultra-wide sports tires on a dry track, so YMMV.

Narrow tires built for performance should minimize any dry braking issues while still enjoying the hydroplaining advantage inherent with the narrower patch.

In my experience, it's the tire stiffness (and, unfortunately, new tires are designed to run pretty stiff for lower rolling resistance, which means it's a common handicap) and the rubber compound that make the big difference, in particular, the low-rolling resistance compound used on these tires.

 

[Eunos_Cosmo]

Also, and this bit is pure speculation so feel free to disregard, but I suspect it's easier to brake a wheel with much lower rotational mass than it is a regular wheel and tire too. As long as the tire itself has enough friction on the road surface you've got the benefits of both a lighter sprung mass to slow down, and less rotating mass to try and stop rotating.

Braking systems are massively overpowered. A normal Fusion probably has enough braking power to lock up all four wheels at 80mph if it weren't for ABS. If this were the days of unassisted, manual-servo brakes with no ABS, I might agree with you. A typical family car has something like 400-500bhp of braking power.

 

[Danoff]

It's not a compromise if it offers the same performance.

The physical size of the car doesn't make much difference here - a Fusion with the mass of a Fiesta will stop at the same rate as a Fiesta with the mass of a Fiesta. In theory, possibly a little better, as the wider track and longer wheelbase will give it greater stability than the narrower, shorter Fiesta.

Putting wheels and tires like this on a regular Fusion would be a recipe for longer stopping distances and poor lateral grip. Doing so on a Fusion that's been on an 800 lb diet should be no less effective than existing heavier, fatter wheels and tires on a Fusion that still carries that 800 lbs around with it.

It's still a compromise. Why not put it on a 700 lb diet and keep the wider tires and out brake the fiesta. Whys is the fiesta good enough?

Taller, narrower tires aren't really a handicap in terms of braking. Or shouldn't be, in any way that really matters. While you sometimes get better braking with wider tires... this is because of fundamental differences in tire compounds and types between wide fitment tires and narrow fitment tires. Hell, I've seen a Corolla on ballonish 185mm economy tires brake within two meters of cars equipped with big brakes and ultra-wide sports tires on a dry track, so YMMV.

That's not really a scientific comparison. The size of the brakes, as @Eunos_Cosmo points out, is not really that much of a factor because every car can lock up the wheels on braking. The size of the contact patch is directly related to friction so it has to be a factor.

Narrow tires built for performance should minimize any dry braking issues while still enjoying the hydroplaining advantage inherent with the narrower patch.

Yea you might get some standing water benefits with a narrower tire, but I'm primarily concerned with stopping distance on dry pavement.

In my experience, it's the tire stiffness (and, unfortunately, new tires are designed to run pretty stiff for lower rolling resistance, which means it's a common handicap) and the rubber compound that make the big difference, in particular, the low-rolling resistance compound used on these tires.

I'm 100% certain that you are correct that the compound makes a massive difference. Let's assume it's the same compound used on both the original and narrower wheels so that that's not a factor in this discussion (because there's no reason that it should change when going from one to the other). More contact patch = More braking in normal conditions. More braking = more safety. Why would we compromise safety in a significant way to save a few tenths of a mpg.

The auto industry is so ridiculously focused on mpg these days that I'm concerned that they (and buyers) will lose sight of what's actually most important - keeping your face out of your windshield.


Here's an excerpt from Continental explaining some of their economy tire technology:

The Conti Premium Contact 2 has a unique 3D tread groove which not only improves the braking performance but it also gives greater protection against aquaplaning. The use of cat's paw technology not only gives improved fuel efficiency but also shorter stopping distances. The tyre has a narrow contact patch while in motion for improved fuel efficiency yet when you apply the brakes the contact patch widens and spreads like a cat's paw giving more rubber contact on the road for shorter stopping distances.

That paragraph effectively talks about just about everything we've discussed.

 

[homeforsummer]

It's still a compromise. Why not put it on a 700 lb diet and keep the wider tires and out brake the fiesta. Whys is the fiesta good enough?

All cars are essentially "good enough" right now.

UK magazine Autocar tests braking capabilities whenever they run a full road test on a car. Since I can remember - say, the last 20 years or so - the vast majority of cars will stop from 60 mph in about three seconds. I've just picked four mags completely at random from a nearby shelf:

2013 Kia Sorento - 3.04 seconds
2014 Mercedes CLA - 2.9 seconds
2009 Tesla Roadster - 3.35 seconds (damp track)
2013 Kia Picanto - 3.19 seconds

Occasionally, you'll get something with fairly serious brakes - two I've picked intentionally:

2004 Renaultsport Megane - 2.7 seconds
2014 Porsche 911 Turbo S - 2.6 seconds

Incidentally, most of those stop from 70 mph in around 50m, according to the data. Regardless of whether they're 2 tonnes of SUV or less than a tonne of city car, and regardless of tyre width.

Essentially, beyond @niky's comment about compound (a hard, eco-biased one will ultimately have less grip than a sticky summer tire), you're flagging up an issue that doesn't exist. The way of making a car stop better is to spend much more and stick some carbon-ceramics on it, like the Porsche 911 Turbo has. Not to just make the tires wider.

And even then, the benefit is limited - the Porsche stopped from 70 mph on a dry track a whole 13 meters earlier than the Picanto (and you'd kind of hope so, given its brakes are about the diameter of the Picanto's entire wheel and tire). But on a wet track, the difference was about 2 meters. All the tire width in the world doesn't help if it's not getting to the road surface.

Incidentally, the safety issue you flag up also sounds like a problem that doesn't really exist. Modern cars brake ridiculously quickly with very little fuss. Provided those "few tenths of an mpg" don't affect that performance, I don't really see what the problem is. I'd be much more keen on arguing the case for something that matters, like improving visibility. Enormous A-pillars are much more of a problem for me than a margin-of-error difference in braking.

 

[Danoff]

All cars are essentially "good enough" right now.

UK magazine Autocar tests braking capabilities whenever they run a full road test on a car. Since I can remember - say, the last 20 years or so - the vast majority of cars will stop from 60 mph in about three seconds. I've just picked four mags completely at random from a nearby shelf:

2013 Kia Sorento - 3.04 seconds
2014 Mercedes CLA - 2.9 seconds
2009 Tesla Roadster - 3.35 seconds (damp track)
2013 Kia Picanto - 3.19 seconds

Occasionally, you'll get something with fairly serious brakes - two I've picked intentionally:

2004 Renaultsport Megane - 2.7 seconds
2014 Porsche 911 Turbo S - 2.6 seconds

Incidentally, most of those stop from 70 mph in around 50m, according to the data. Regardless of whether they're 2 tonnes of SUV or less than a tonne of city car, and regardless of tyre width.

It's a pain to look up the widths on all of those, but I'll suggest that they're similar compared to this fusion concept.

Essentially, beyond @niky's comment about compound (a hard, eco-biased one will ultimately have less grip than a sticky summer tire), you're flagging up an issue that doesn't exist. The way of making a car stop better is to spend much more and stick some carbon-ceramics on it, like the Porsche 911 Turbo has. Not to just make the tires wider.

Carbon-ceramics won't help you avoid an accident on city streets. If you can lock up the tires you have on your car, you've got enough brake for the tire. Carbon ceramics are good for track use, but for single use, cold performance (which is what keeps you from hitting the other guy) it's no better.

There are three main components of stopping. Downforce, coefficient of friction, and area. I see no reason to talk about coeficient of friction (tire compound) because it can be put on any size tire.

And even then, the benefit is limited - the Porsche stopped from 70 mph on a dry track a whole 13 meters earlier than the Picanto (and you'd kind of hope so, given its brakes are about the diameter of the Picanto's entire wheel and tire). But on a wet track, the difference was about 2 meters. All the tire width in the world doesn't help if it's not getting to the road surface.

I have acknowledged this point (twice).

Incidentally, the safety issue you flag up also sounds like a problem that doesn't really exist. Modern cars brake ridiculously quickly with very little fuss. Provided those "few tenths of an mpg" don't affect that performance, I don't really see what the problem is. I'd be much more keen on arguing the case for something that matters, like improving visibility. Enormous A-pillars are much more of a problem for me than a margin-of-error difference in braking.

Ok, let's stop talking theory and actually look at the numbers. What's the stopping distance on this fusion lightweight (with the light weight tires they use)?

 

[niky]

The size of the contact patch is directly related to friction so it has to be a factor.

Yea you might get some standing water benefits with a narrower tire, but I'm primarily concerned with stopping distance on dry pavement.

I'm 100% certain that you are correct that the compound makes a massive difference. Let's assume it's the same compound used on both the original and narrower wheels so that that's not a factor in this discussion (because there's no reason that it should change when going from one to the other). More contact patch = More braking in normal conditions. More braking = more safety. Why would we compromise safety in a significant way to save a few tenths of a mpg.

A wider contact patch is not necessarily a bigger one. Just a wider one. A long contact patch should, theoretically, make for better braking... that's the whole concept behind wrinkle-wall slicks on dragsters... elongating the contact patch at the moment of launch, rather than widening it. (Same theory as the Continentals you cited, I believe)

A bigger contact patch is better in dry weather braking, up to a point, but if an extra few meters saved under braking were all that mattered, we'd all be driving around on well underinflated tires... tire wear, heat build-up, squidgy handling and possible carcass damage be damned!

And bigger isn't always better. Especially when it's wet and you can't rely on the stickiness of the rubber itself to provide grip, as on wet roads.

We already compromise braking by having ultra-wide tires (that hydroplane) with stiff low-profile construction that doesn't help traction under braking as much as softer construction does. We make up for this by using very soft, very pliable rubber that literally flows into the cracks in the road... providing more surface area without needing tire deformation.

If I recall correctly, Bridgestone selected the bigger 19" size for these new LRR tires so they could elongate the contact patch without causing too much distortion in the carcass. So the tires can provide the necessary grip under braking without generating too much heat from tire distortion in regular use.

Theoretically. Personally, I haven't met a low-rolling resistance tire that I've been satisfied with in terms of braking, yet.

 

[Danoff]

A bigger contact patch is better in dry weather braking, up to a point, but if an extra few meters saved under braking were all that mattered, we'd all be driving around on well underinflated tires... tire wear, heat build-up, squidgy handling and possible carcass damage be damned!

It's not that a few extra meters is all that matters when it comes to driving. But this is a trade - a few extra lbs for a few extra meters. I'm saying (without knowing the numbers) that I don't think I like that particular trade.

 

[Eunos_Cosmo]

It should be noted that upgrading brake system components is almost always done to increase braking endurance, not performance. As has been said before, most stock brake systems have more than enough power to lock up all four wheels from reasonable speeds. If you get bigger rotors with more aggressive pad material or even upgrade to carbon rotors, you won't necessarily stop more quickly, but you will be able to stop quickly all day long.

 

[niky]

It's not that a few extra meters is all that matters when it comes to driving. But this is a trade - a few extra lbs for a few extra meters. I'm saying (without knowing the numbers) that I don't think I like that particular trade.

Well, it's not something we can really judge until they release numbers, but I don't expect the difference to be all that drastic, if there is any difference at all. And I expect most of the difference will simply be down to the tire compounds.

Interestingly, here' s C&D test... same tire model, different sizes... a loss of 40mm tread width equals an increase in braking distance of just four feet.... which is actually short enough to be within test-to-test variation... noise... and can be tweaked simply by lowering the pressure of the smaller, narrower wheels for more bite under braking (again, if the tall and narrow tire were built to do this, same effect).

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

 

[Danoff]

Well, it's not something we can really judge until they release numbers, but I don't expect the difference to be all that drastic, if there is any difference at all. And I expect most of the difference will simply be down to the tire compounds.

Interestingly, here' s C&D test... same tire model, different sizes... a loss of 40mm tread width equals an increase in braking distance of just four feet.... which is actually short enough to be within test-to-test variation... noise... and can be tweaked simply by lowering the pressure of the smaller, narrower wheels for more bite under braking (again, if the tall and narrow tire were built to do this, same effect).

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

I think I see where you got the number from, and you're comparing a much heavier 19" rim to a 15" rim. So in addition to the weight increase from being wider, it also has a massive weight increase for diameter.

 

[homeforsummer]

It's a pain to look up the widths on all of those, but I'll suggest that they're similar compared to this fusion concept.

Sorento: 235
Picanto: 165
CLA: 225
Roadster: 175/225 (F/R)
911 Turbo: 245/305 (F/R)
Megane: 225

Fusion: 155 (if the article above is correct)

So... no, they're more or less all different.

Interesting: There's a 70mm difference in section between the Picanto and Sorento. The Picanto is on eco tires, too. The Sorento weighs 1.9 tons, the Picanto about one metric ton less. There's about 15 hundredths difference in their braking times.

If you don't think the fact the Picanto stops in about the same time as the Sorento is down to weight, what exactly is it down to? Since there's quite a difference in tire width and diameter? I'd not particularly like to drive a Sorento on 165-section tires but do you really think it would take longer to stop if it weighed a ton less than it does?

I'm really, genuinely struggling to see the problem here. Tire width clearly doesn't make much difference to braking distance, because it's almost always applied in correspondence with vehicle weight. A lighter vehicle simply doesn't need massive tires in order to stop in the same distance as a heavy vehicle with massive steamroller tires.

Ok, let's stop talking theory and actually look at the numbers.

Come on Danoff, you're better than that - so far I'm the only one who's actually used numbers, rather than theory and suspicion.

Or are real-world, repeatable braking tests in controlled conditions, from one of the world's most reputable car magazines, not suitable for a discussion about braking figures?

What's the stopping distance on this fusion lightweight (with the light weight tires they use)?

Absolutely no idea, since it's a prototype and its existence has only just been made public. But since all road cars brake at pretty much the same rate and the Fusion weighs about as much as a Fiesta, I'd say about the same rate as a Fiesta. At the very least, the same rate as a Picanto, with 10mm extra width but brake rotors like you'd find on a bicycle.

Checked through my collection: Autocar has tested the Fiesta too, 15 October 2008. 2.7 seconds from 60 mph.

Admittedly, that's on 195-section tires. There may well be a tiniest percentage difference between that and a Fusion on 155s. Though the Fusion is also on 19" rims (if the article is correct), whereas the Fiesta is on 15"s. I'd be surprised then if the contact patch isn't similar. Edit: And @niky's post while I was typing this suggests there's very little difference, given the extra diameter.

But even then, that's missing the point. I've already provided figures showing that tire width makes very little, if any difference across a broad range of cars. Weight, on the other hand, appears to - since a tiny Kia on rock-hard eco tires apparently stops as quickly as an SUV on tires a third wider. And a Ford Fiesta on 195s stops as quickly as one of the best hot hatchbacks on sale at the time, on performance-biased 225s.

 

[Exorcet]

Let's assume it's the same compound used on both the original and narrower wheels so that that's not a factor in this discussion (because there's no reason that it should change when going from one to the other).

I'm not an expert here at all, but when you dive into real tire work, it gets non linear. I remember the FSAE tire guys spending months looking at load vs grip and none of the graphs were as neat as you'd expect. With an 800 lb weight difference and this being an experimental car and all, I think a different compound might come into play.

More contact patch = More braking in normal conditions. More braking = more safety. Why would we compromise safety in a significant way to save a few tenths of a mpg.

The stopping distance is a part of safety, but not everything. Someone brought up hydroplaning before. The tire width might be primarily for weight savings, but might also have some secondary benefits.

Going on a bit of a stretch, light cars are more sensitive to lift like I said before. They obviously haven't done anything on top of the car to address that. The underside of a car like this generally just gets covered with panels and blocked by an airdam. Maintaining smooth flow under the car is a pain mostly because of the wheels. Smaller wheels would make it easier to control the underbody air flow for both lift and drag. If you do end up with a better lift coefficient you might make back what your tires lose from lack of width. Now Ford didn't say much about aero, but aero and MPG's go hand in hand, especially at highway speed.