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Comment by alufers | Hacker News Reader

I wonder if all the tech in those cars requires modifications to the vehicle itself, like a larger alternator to satisfy the power requirements or reinforced pillars to support the weight of the hardware. It'd be cool to hear from somebody who knows how they equip those cars.

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Those modifications seem unlikely, You can't easily reinforce pillars in a car and they are designed to support the full weight of the car in a roll-over situation so a few 10s of kgs of equipment on the roof make little difference. Similarly, a typical alternator is rated to 2kW so far above the required power of a few LIDAR, camera and computing devices. Having said that, early vehicles did have heavy duty alternators installed (although they may have confused these with heavy duty inverters which are needed) and modified suspension components [1]

[1] https://pub-tools-public-publication-data.storage.googleapis...

The linked history of street view does mention that the first instance had an alternator “from a fire truck”.

Fwiw, direct fit high-amp alternators are available for most cars. And they are in the same ballpark of ~200 amps, and high output at idle as a "fire truck alternator".

So it's a fairly cheap modification just to swap out the stock alternator. ~$500 or less.

Car choice/ÉCU programming may come into play here.

I thought cars were “intelligent” enough to not charge while idle or warming up because that’s when emissions and inefficiency are highest.

I doubt that was the case for the depicted 1992 Chevrolet Astro Van.

I think it should "just work". The added load would drive voltage down, so the inputs the ECU gets continue to send the right message. So long as the alternator has enough capacity to meet the demand...

Yeah if a manufacturer wanted to hold off on drawing a higher load to recharge the battery until after the car was warmed up they would just do that by lowering the setpoint. If you try and draw higher current from it it would still meet that lower setpoint.

A typical alternator for a small car is rated for something like 60-120 amps, which at a typical 14v output, is 0.8 to 1.7 kW.

2kW falls into more of a light truck or high-output category.

Cars are typically specced to handle 75kg (~165lb) of baggage on roof rack without breaking a sweat. Think carrying skies + personal gear on a trip across Europe. And if you are concerned about momentum, please note it's common to carry two bicycles on a roof rack; with speeds of 100km/h+ they generate substantial forces due to drag and in case of cornering.

I doubt the whole camera+lidar roof package is anywhere near close; best guess around 10kg (~22lb)

I design mobile mapping vehicles for a different company, so while I can't answer specifically about the Street View system, I might be able to provide some insight.

Our smallest portable system weighs around 25lbs and can mount to just about any vehicle using strong magnets or standard roof rack. It pulls less than 10 amps so it can be powered through a standard cigarette lighter outlet or wired directly to the vehicle's battery.

Our largest system weighs many hundreds of pounds and mounts on one specific vehicle in as many reinforced locations as possible. We utilize factory bolt holes on the roof, make custom brackets to support cantilevered weight off the back, and make modifications to the roof up front to support even more weight. The system typically pulls 40-50 amps however, the vehicle comes with a high output alternator and dual battery system from the factory so we don't need to upgrade them.

Based on the components of the current Street View system, a roof rack is sufficient to support it's weight. I'm guessing it pulls around 20 amps so it likely has a direct connection to the vehicles battery, but wouldn't require an alternator upgrade.

Thanks for the valuable insight, 50 amps look like the sort of current you would see while a car's starter motor is running. Out of curiosity, do you shut down the equipment automatically when the engine is killed? I imagine drawing so much current may interfere with the starter motor.

There are a lot of companies that do this type of vehicle modification for you. It's common for a lot of professional-use vehicles also in construction / utility network maintenance etc.

As others said, the roof is fine. The suspension is quite easy to adjust a bit for the extra weight and that helps also in stability due to the weight being high-up. For vehicles commonly used in these kinds of things there are higher rated alternators and batteries that are drop-in replacements for the standard one.

> that helps also in stability due to the weight being high-up

I don't understand that. Isn't weight high-up shifting the center of mass upwards, making the vehicle less stable?

EDIT: jstanley, olex: that makes sense, thanks!

I think it means the suspension adjustments help in stability, which is helpful because the weight is high up.

I think that OP wanted to express that stiffer suspension is needed for extra weight, and the same stiffness also helps keep the vehicle stable despite a higher center of mass.

Thanks, yes that's what I meant. You want less body roll if your center of gravity is higher, so that requires stiffer suspension and stiffer anti-roll bars to keep the vehicle more level (at the expense of some comfort).

3 times the weight of the car, 1.5x for large vehicles, is a US requirement to how much the roof can support, this is part of protecting occupants during a roll over; this was from 2010 and may have new requirements[0][1]

[0]https://www.martindale.com/antitrust-trade-regulation-law/ar...

[1]https://www.consumerreports.org/cro/news/2010/04/nhtsa-decli...

That's a static load; the actual amount of luggage a car is rated for carrying on the roof is much less (usually somewhere in the mid 100s for a passenger vehicle).

I don't know about the street view cars, but the waymo vehicles have cooling and power run through the pillars to the camera, because the camera has to be cooled.

This information was disseminated to fire departments so that if rescuers have to cut somebody out of one of waymo's cars, they don't accidentally cut through a pressurized coolant line.

A modern car will run into suspension wear/longevity problems long before the pillars care about how much weight you put on them. This hardware is tens, maybe a couple hundred pounds. Roofs can support thousands without deformation. The vehicle will suffer increased tire/suspension wear and "bad to the point of it being hard to justify not using a bigger vehicle" handling long before the structure is in any way affected. They stack cars on cars at many junkyards and the lower cars are just fine save some scratches and dents to the roof skin.

Usually any vehicle mods are done to meet bare minimum standards.

A lot of the mapping cars I see are hybrids, they probably get some lower benefit there.

I would assume they run these things for a fixed period and toss them to avoid opex. I’ve had exposure to lots of commercial customized vehicles, and unless it’s required by law or the use case, not an extra dime is spent.

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