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Comment by squigs25 | Hacker News Reader

It's really difficult to emphasize how excessive this is. As a mechanical engineer, I knew just by reading the title that this is really excessive, because the titanium alloy used is definitely a marketing stunt.

Titanium has unbelievable tensile strength for its weight, but there's no good reason to make an "underbody shield" out of titanium except for publicity. It would make way more sense to use steel (and maybe you could make a case for something ultra light weight, like carbon fiber, but probably not).

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What are the characteristics of Titanium regarding corrosion and wear? One thing I have heard from several of the highly technical owners on the forums is that one of the reasons for the design choice of a mostly aluminum body is that there is no structurally weakening corrosion in reasonable use of the vehicle, even in areas that use a lot of salt and such on the roads.

Titanium has good anticorrosion properties. You see it often, for example, in stuff like heat exchangers that run in seawater for long duration. Considerably more stable than aluminum in most environments.

" the shields prevented any damage that could cause a fire or penetrate the existing quarter inch of ballistic grade aluminum armor plate that already protects the battery pack"

Ballistic grade aluminum armor plate... If there is one thing Tesla does well it is publicity. I know there are high strength steels but it seems like they want something that is strong and light. If it sounds like something impenetrable to the average customer that's even better for Tesla.

exactly my point - this is purely a marketing stunt.

Well the chances of actually penetrating the aluminum shell of the battery pack seem pretty low anyway. They are just answering the media shit storm they get for any little problem with an equally over kill replacement that will lead to media shit storm.

There's no such thing as bad press. As long as Tesla has people looking at them they're going to use the opportunity to push their product.

Just for giggles, could it be that adding a heavier material such as steel would have a more significant impact on range due to the additional weight? Part of the upgrade pitch is that users only see a 1% decrease in range. With steel, perhaps you see a 3-5% decrease, and range has been the key argument for Tesla against other competitors.

0.1% decrease. Believe it or not, I know several owners who would not be satisfied with even a 1% decrease, especially if it is adding a safety feature that most of them feel is so unlikely to happen, they would have preferred never to have temporarily disabled the low height mode.

> Ballistic grade aluminum armor plate...

That's from the same marketing phrase-book as 'aircraft-grade aluminium'.

Lightweight armour ( e.g. personal protection ) uses ceramic plates.

Heavy armour ( e.g. tanks ) still uses steel alloys with composite and plastic layers. There were a few AFVs with aluminium armour, such as the M113 and Sheridan, but ballistic protection was restricted to small-arms and light shrapnel. A Sheridan could be ripped apart by a 12.7mm DhSK.

A quick Google search indicates that it's used commonly in armored cars and military vehicles.

eg: http://www.alcoa.com/global/en/products/product.asp?prod_id=...

They were already using a 1/4 inch steel plate as an underbody shield but it wasn't strong enough to prevent piercing and fires in certain situations.

Apparently the people at Tesla calculated that the cost of adding these plates was worth the benefit of improved consumer protection and good publicity.

Weight is not an issue for this part because it actually helps further lower the cars center of gravity.

I wish my car had a titanium underbody, that would simply be badass.

But titanium is less dense than steel, hence preferred when weight is important. For an electric car, weight is paramount, I would think.

Actually, nope!

While it requires more initial force, that extra energy due to added mass is stored in kinetic energy, and contrary to popular belief, it does not affect efficiency. There was actually a myth busters episode about this.

Electric cars would store this added energy in their regenerative breaking, so it shouldn't affect energy consumption.

Also, the added weight of steel over titanium would be negligible, maybe 100 lbs tops.

I didn't see the Myth Busters episode, but isn't there still a loss due to the inherent inefficiency of the motor and regen breaking?

I would think that 100 extra pounds, coupled with the inherent energy loss from inefficiency, would be a significant factor in the car's range. That's 75% of one extra passenger.

This site (http://www.jurassictest.ch/GR/) lets you plot a course for several EV models and set up some parameters such as weight. Adding a passenger to a Tesla knocks 9 Km off the range, according to them.

Regenerative braking does not capture 100% of the force used to accelerate to that speed. Far from it.

According to this article: http://www.teslamotors.com/blog/magic-tesla-roadster-regener...

The Roadster's regen braking is at most 64% efficient. I'm guessing in real driving scenarios, it will be much less.

TO answer everyone's questions below, think about the following example:

You have one car that's 1,000 lbs and one that's 10,000 lbs. When you start each car, it will take a lot more energy to bring the 10k lb car up to the same speed as the 1k lb car, but after that, the effects of drag will weigh more heavily on the lighter car (F=ma, so the rate of deacceleration is a lot higher for a smaller mass), causing constant fluctuations in the motor shutting on and off.

With the the 10k car, the transitions would take much longer. The time in between accelerations would be longer, and the motor would be on for longer spurts. You gain a great deal of efficiency for simply leaving a mechanical system running vs constantly modulating a mechanical system (i.e. shutting it on and off).

This would work in favor of the heavy car being more efficient.

Of course, the heavier your car is the worse your 0-60 split would be, so for a performance car that's a negative.

Also, the heavier car would experience greater bearing resistance + friction. This is really difficult to quantify, and my gut is that it's outweighed by the momentum advantage.

But overall, for a car that ways 2k lbs, a 5% increase in mass is pretty negligible, especially for casual highway and local driving.

> but after that, the effects of drag will weigh more heavily on the lighter car (F=ma, so the rate of deacceleration is a lot higher for a smaller mass), causing constant fluctuations in the motor shutting on and off.

All other things being equal, a heaver car will experience the exact same aerodynamic drag as a lighter car. Only the cross-sectional area and shape of the body come into play when determining how much opposing force air resistance provides. By the same token, the motor must supply a constant amount of torque to the wheels to counteract that force. If the force remains unchanged, so too does the torque required to counteract it. You're right about the rate being different, but as far as I know the motor doesn't cycle on and off like an air conditioner compressor does. It maintains a steady output.

What we're left with is an additional component of drag due to rolling resistance. A heaver car will deform the rubber tires more than a lighter one, and put more normal force into the bearings, causing that rolling resistance to be higher. I'm not sure what fraction of the total drag is due to rolling resistance vs. wind resistance--like you, I bet it's comparatively small, but still significant enough to show up in reduced range.

> Also, the heavier car would experience greater bearing resistance + friction. This is really difficult to quantify, and my gut is that it's outweighed by the momentum advantage.

I may be wrong here, but I don't see having an increased momentum as being an advantage. It takes more energy to build up that increased momentum, and you don't get it all back from regenerative braking

A heavier car may be more efficient, if you calculate efficiency as [Energy used] / [Total mass], but the better way to calculate the real-word efficiency is [Energy used] / [Mass of stupid driver who doesn't deserve this car as much as I do, when do I get to have one?]

Sorry - was away for a while. The drag FORCE will be the same, but the lighter car will deaccel faster as I clearly stated above. F=m*a thus: F/m = a

Think of it like this: it's really easy to push a car that weighs one pound. Much harder to push a car that weighs 1000 pounds.

The momentum advantage was explained in the first point.

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