{"id":203,"date":"2022-01-29T23:19:26","date_gmt":"2022-01-29T23:19:26","guid":{"rendered":"https:\/\/gerardweatherby.org\/camping\/?p=203"},"modified":"2023-01-13T18:26:19","modified_gmt":"2023-01-13T18:26:19","slug":"80-rule-fact-or-fiction","status":"publish","type":"post","link":"https:\/\/gerardweatherby.org\/camping\/80-rule-fact-or-fiction\/","title":{"rendered":"80% rule: faction or fiction"},"content":{"rendered":"\n

Disclaimer: This is my opinion. Heed at your own risk.<\/a><\/em><\/p>\n\n\n\n

Some people on the interwebs like to say you shouldn’t leave a car battery on a concrete floor. This was<\/em> true — back when batteries had wooden cases\u2014something to do with moisture and swelling<\/a>. Since batteries haven’t had wooden cases since before I was born, and I’m old, it’s pretty silly now.<\/p>\n\n\n\n

Some people on the web like to claim you should only use 80% of a tow vehicle’s capacity. I wonder about these people. Do they only drink 9.6 ounces of a beer or drive 52 in a 65 mph zone?<\/p>\n\n\n\n

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What can be asserted without evidence can also be dismissed without evidence.<\/p>\nChristopher Hitchens<\/a><\/cite><\/blockquote>\n\n\n\n

Since I trust Toyota’s engineers more than self-proclaimed experts, I’ve mostly ignored the so-called rule. One reason is what I call proof-by-lawyer<\/em>: if driving at rated capacity was as dangerous as fear-mongers portray, why aren’t there any class action suits against the manufacturers for bad ratings? <\/p>\n\n\n\n

More recently, out of curiosity, I decided to look into it. Where do tow vehicles ratings come from?<\/p>\n\n\n\n

It turns out, in the past, the manufacturers just kind of made them up, so naturally, they would spin a little bit to say they would tow more than the next guy, but not so much as to damage their brand reputation. <\/p>\n\n\n\n

About a decade ago, the manufacturers realized it wasn’t a good situation and got together to standardize the rating. The result is SAE J2807<\/a>, which, for a mere $87, you can buy a copy of. Fortunately, we don’t have to because it’s been reported on in the motoring press. <\/p>\n\n\n\n

It’s not a cakewalk. The highlight is the Davis Dam test, where the vehicle has to ascend 3,500 ft in 11 miles, starting at 100o<\/sup> F, with the air-conditioning on full blast, and not slow down too much. In relatively cool and flat New England, someone like me is unlikely to stress my tow vehicle that much.<\/p>\n\n\n\n

The standards include specifying specific tires, trailer configurations, etc. SRW means a single rear wheel on each side, and DRW <\/em>is dually. The tests must complete without any warning lights.
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\"\"\/<\/figure>\n\n\n\n

A “dually” with four rear tires.<\/p>\n\n\n\n

Test<\/th>SRW<\/th>DRW<\/th><\/tr>
Davis Dam<\/td>>= 40 mph<\/td>>= 35 mph<\/td><\/tr>
Go 5 meters from stop on 12% grade, 5 times in a row<\/td><= 5 minutes<\/td><= 5 minutes<\/td><\/tr>
Hold full rig with just parking brake<\/td>12% grade <\/td>12% grade<\/td><\/tr>
0-30 mph (level ground)<\/td><= 12 sec<\/td><= 14 sec<\/td><\/tr>
0-60 mph (level ground)<\/td><= 30 sec<\/td><= 35 sec<\/td><\/tr>
40-60 mph (level ground): passing test<\/em><\/td><= 18 sec<\/td><= 21 sec<\/td><\/tr>
20-0 with trailer brake, staying in 11.5 feet lane, trailer lighter than tow vehicle: braking test<\/em><\/td><= 35 ft<\/td><= 35 ft<\/td><\/tr>
20-0 with trailer brake, staying in 11.5 feet lane, trailer heavier than tow vehicle:: braking test<\/em><\/td><= 45 ft<\/td><= 45 ft<\/td><\/tr>
20-0 without trailer brake, staying in 11.5 feet lane: trailer brake failure<\/em> test<\/em><\/td><= 80 ft<\/td><= 80 ft<\/td><\/tr>
understeer test: increases from 0.1 to 0.3 g on 300 ft circle<\/td><\/td><\/td><\/tr>
sway test, 62 mph<\/td>>= 1.0 damping ratio<\/td>>= 1.0 damping ratio<\/td><\/tr><\/tbody><\/table>
Selected SAE J2807 tests<\/figcaption><\/figure>\n\n\n\n

I don’t fully grok the understeer and sway test metrics, but they break down to the tow vehicle stays in front of the trailer<\/em> and the trailer gets back in line quickly.<\/em><\/p>\n\n\n\n

Here are some videos I found helpful:<\/p>\n\n\n\n