Left: In 2015, 96 million plastic "shade balls" were tossed into the Los Angeles Reservoir to improve water quality and save some water. Image by Eric Garcetti/via YouTube

The team found a range of previously published estimates of water use for these three resources. Because manufacturing practices vary around the world, the researchers collected both local values for water consumption and a global average.

Details vary from car to car: when dismantling, make notes or drawings of how all these parts are arranged and which way round they go.

Work with the car raised on axle stands under frame members. Chock the wheels that are on the ground, and apply the handbrake if it operates on those wheels; otherwise release it. Loosen wheel nuts before raising the wheels.

Lubricate the hub with an approved grease, consulting the car handbook or a dealer for details of the type and quantity.

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The procedure for refitting a driven hub is exactly the same as for a non-driven hub. Clean all parts thoroughly (except plastic shields) with paraffin or white spirit, inspect bearings and renew if damaged.

Haghighi said people need to consider environmental conservation on a global scale, rather than just a local one. They must also consider the entire life cycle of the technology. If the water saving the Los Angeles Reservoir comes at the cost of consuming water in other parts of world, Haghighi said, is it worth it?

There is also an extra grease seal on the outboard side of the pair of bearings. As usual, renew all grease seals whenever you dismantle a hub. There may be additional spacer rings between the seals and bearing races.

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But a new study raises an interesting question: Could saving water in the Los Angeles Reservoir come at the cost of consuming water in other parts of world?

Detach the water shield from the hub carrier. Prise out the inboard oil seal , taking care not to scratch its seating. You may now be able to pull the drive flange out of the hub carrier.

Cars with front-wheel drive have front-wheel bearings that resemble closely those in the rear wheels of rear-wheel-drive cars which have independent suspension (See How car suspension works ).

From there, they calculated how much plastic, oil, natural gas, electricity and water went into making the 96 million shade balls.

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The shade balls could still be useful during a drought if a reservoir accounts for when and where these shade balls are made, said Jay Lund, director for the Center for Watershed Sciences at the University of California Davis. If the shade balls, or other water-sapping technologies, are made during periods when water is abundant, the net effect of manufacturing may be zero.

“Savings in just May through June 2010 [with mandatory water restrictions] is 10 times larger than the 300 million gallons which they’re getting per year for the shade balls,” Bhaskar said.

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In 2015, the world watched as a video of 96 million “shade balls” getting dumped into the Los Angeles Reservoir went viral. The purpose of the balls: to improve water quality and save water.

The water saved from evaporation by shade balls is small compared to other water-saving strategies, said Aditi Bhaskar, a hydrologist at Colorado State University, who wasn’t involved with Haghighi’s study.

“With water, the impacts are much more local, both in time and space,” Lund said “It’s quite plausible to manufacture shading materials at a time and a place where water is relatively abundant and employ them to reduce evaporation at a time in the place when water is really valuable.”

But a team from the Massachusetts Institute of Technology, Imperial College London and University of Twente in the Netherlands reported Monday that it may take more water to make the shade balls than what they save. To offset this loss, the shade balls must sit on the Los Angeles Reservoir between 1 to 2.5 years.

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Remove the hub dust cover and hub-nut locking device in the usual way — on some cars this means raising and removing the wheel, then replacing and lowering it.

The LA Department of Water and Power said they plan to keep shade balls as a permanent solution to water quality issues on the Los Angeles, replacing them every 10 years. LA officials also deployed shade balls at three other reservoirs, between 2008 and 2012, where they stayed temporarily for five to nine years.

At the time, Californians faced a record-setting drought, and conserving water was on everyone’s minds. Mandatory water restrictions led to brown lawns and shorter showers.

The main difference between driven and non-driven hubs is that a drive shaft projects into a driven hub from the inner side.

The inner race of an inboard tapered roller bearing may remain on the drive shaft. If so, tap it off gently with a soft-faced hammer.

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The hub bearings are exactly like those of a non-driven hub, and you dismantle them in the same order. But it is usual for the 'hub' - properly called the drive flange - to turn on bearings inside the hub carrier. So the inner race revolves, while the outer race is stationary.

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The Los Angeles Department of Water and Power said the shade balls could keep contaminants out of the water and reduce the evaporation of the reservoir by 85 to 90 percent during a drought. The officials said the annual savings could amount to up to 300 million gallons of water, enough to supply drinking water to 8,100 people.

Shade balls have lowered bromate levels and stopped algae growth on several reservoirs in the Los Angeles area. They were first used in 2008 on the Ivanhoe Reservoir.

You may now be able to pull the hub carrier free. If not, disconnect the drive-shaft outer joint (See How to check U-Joints ). Support the part of the drive shaft that remains on the car to avoid straining its joints.

To estimate the amount of water it takes to make shade balls, the MIT team conducted what’s known as a “blue” water footprint — an analysis that estimates the amount of water used in the process of manufacturing a product. In this case, the shade balls are made of a plastic called polyethylene.

Extend the handle of a socket wrench with a length of pipe for extra leverage. Check if the hub nut has a right or left-hand thread, then loosen it.

In some driven hubs the arrangement is similar, except that the `stationary axle' - the hub carrier is hollow and the drive shaft runs through it to connect with the hub.

There is likely to be a plastic water shield on the inner side of the hub assembly, either attached to the drive shaft or clipped to the hub carrier.

This means shade balls would need to stay on the Los Angeles Reservoir for at least a year or more to save water. Depending on exactly where the shade balls are made, it could take up to 2.5 years to recover the water used.

Haghighi’s team then examined industry reports from plastic manufacturers to determine how much oil, natural gas, and electricity goes into making polyethylene.

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If the car has a MacPherson strut (See Renewing MacPherson-strut inserts ), remove it from the body and the track control arm (but with unit replacement suspension remove the lower half of the strut only).

The creators of shade balls originally used them to prevent chemical treatments in the reservoir from reacting with sunlight and creating bromate, which is a carcinogen regulated by the EPA. California began deploying shade balls in 2008 as a cost-effective way to reduce bromate levels and also prevent algae growth. The LA Department of Water and Power picked shade balls over other, more expensive covers for the city’s biggest reservoir that would have cost an extra $250 million dollars.

Since shade balls have stayed on the Los Angeles Reservoir since 2015, they are likely now starting to save more water than they cost to produce.

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With tapered roller bearings , if you only want to check and lubricate them, there is no need to drive out the outer races if you can still see them clearly.

Move aside a disc caliper or take off a brake drum (if not integral with the hub) in the usual way (See Renewing drum-brake shoes ).

If not, support the carrier on blocks (if detached from the car) and knock the flange out, using a suitable socket or pipe as a drift to avoid hammering it directly.

Amanda Grennell is PBS NewsHour’s 2018 AAAS Mass Media Science & Engineering Fellow. She recently earned a doctorate in chemistry from the University of Colorado Boulder as a NSF Graduate Research Fellow. Outside of the lab, Grennell acted as senior editor for the graduate student blog Science Buffs, artistic director for the first annual ComSciCon Rocky Mountain West and a writer and social media manager for Chembites.

The team cautioned that water savings from the shade balls will likely be less than 300 million gallons during years when there is no drought. That’s because evaporation of water decreases when the air isn’t as dry. The actual period of time needed to recover water from shade ball manufacturing might even be longer.

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The bearings may remain in the hub carrier, in which case you can remove them in the same way as bearings remaining in a non-driven hub.

Dismantling is much the same as for a non-driven hub (See Removing wheel bearings ). But there is one major difference — the hub nut is usually very tight; if possible, loosen it with the wheel on the ground .

“We know that high-density polyethylene itself is made using oil, natural gas, and electricity,” said Erfan Haghighi, an environmental scientist and mechanical engineer at MIT, and lead author on the paper. “And each of these energy sources are water intensive.”

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Shade balls cover the surface of the 175-acre Los Angeles Reservoir. By blocking sunlight, shade balls decrease levels of bromate, a carcinogen, which forms when sunlight reacts with treated water and naturally occurring bromine. Photo by Patrick T. Fallon/Bloomberg via Getty Images

On a rear-wheel-drive car you may not need to remove the hub carrier: if you can disconnect the drive shaft, you can dismantle the hub with the carrier still in place.

The force needed to loosen the nut is such that you may have to prevent the car from moving — you may even need someone inside applying the brakes .

On cars with no central hole in the wheel, loosen the hub nut with the wheel off: a helper applying the brakes is essential.

A 2014 study showed that of all the water saving strategies used in LA in 2010, mandatory restrictions of water use — essentially stopping people from watering their lawns — saved the most water.

Haghighi’s team estimated that manufacturing 96 million shade balls required between 66 and 766 million gallons of water — the equivalent of 100 to 1,000 Olympic-sized swimming pools. This large range is due to the thickness of the balls, which can vary dramatically.

The type of bearing used also has much in common with non-driven wheel bearings (See Checking the half shafts ). However the bearings of live rear axles are quite different.

The outboard grease seal may also be left on the drive flange. Prise it free, taking care not to scratch its seating, and discard it.