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Drafting a truck to make it home

I was coming home from a 220 mile round trip journey and encountered a headwind on the return that pushed my wh/mile up to 380, a level that looked like i would arrive home with 0 miles. the car at 72mph seems to get about 70% of the ideal range meter. I was faced with the decision to slow down to 50mph or so or draft a semi truck. i got behind a car hauler and my wh/mile average went down to 260-270 which at times was better than ideal range. so drafting at 71mph added 30% to my range vs not drafting. I made it to the house with 20 miles still left in the battery.

After only 2 milliseconds of trying to compile all this data, my computer just crashed!!!

Mark E;
Which would you rather be behind: An SUV doing an emergency brake, or a tractor-trailer? A car, or an SUV?

The smaller the vehicle, in general, the faster it will stop and the less reaction time you have. The maximum reaction time is provided, for your convenience and safety, by large trucks.

There is a good summary at http://www.visualexpert.com/Resources/reactiontime.html

@Timo: Agreed, the time taken for the brakes to actually start working is longer a bigger part of the equation than you think. The report above estimates 0.3 seconds. This is part of the reason emergency brake assist was invented. The human action time is a component of the brake action, but not all of it. How fast do you think you can move a lever 3-4 inches? That's the brake pedal, and it isn't an on/off switch, but a linear movement that takes time.

@Brian: There is a trade-off. The truck cannot stop as quickly, but blocks more of the view ahead to anticipate.

Lots of drivers completely overestimate their ability to react in an emergency. The reaction time is dramatically increased if you are not expecting it from if you are sitting, attentive, waiting for an event. If you are drafting another vehicle and look briefly at your dashboard and back to the road ahead that is 0.5 seconds at a minimum, plus the surprise factor and the time taken for you to realise that this isn't just the vehicle ahead slowing down a little, but doing an emergency stop.

In general driving we see brake lights come on all the time, so they don't automatically trigger us to emergency brake - you need to compute that the vehicle ahead is actually slowing down rapidly and you need to brake harder. This takes time, and if you have a buffer of less than a second you just don't have any to spare.

Additionally, as I have mentioned a number of times, when you are that close to a large vehicle you simply can't see the road ahead to get additional cues as to what is happening, and shorten your own reaction time. I have found myself braking *before* the vehicle ahead on a number of occasions as I'm observing what is happening and anticipate.

Further, a string of vehicles drafting on a freeway is a recipe for disaster, as the number of 'draftees' increases so does the risk. If you are unfortunate to be in the middle the accident can be even more severe for you as you get squashed from both sides.

As a distance bicyclist, drafting makes a HUGE difference in how much work you need to do. Like night and day. Having said that, I'm with the majority here that drafting is not something I'd want to do in a car at 70mpg behind a semi. Scary enough if the cyclist in front of you has to go around a pothole and that's at 20mpg.

+100 Mark E, thank you.

So many people feel like they are smart/skilled enough to do what everyone else can't or shouldn't, and if something goes wrong, there's always some unexpected extenuating factor that caused the accident, so it just can't be their fault alone. Normally speeding works fine!

Well duh! Accidents happen because the unexpected happens. And when more than one unexpected thing is going on, then the chance something will go wrong are multiplied.

It's not worth it.

its not worth it...but if you are suffering range anxiety, which I was yesterday coming back from Palm Springs (to south Orange County) with very significant head and cross winds, it really did help. I commented to my daughter that I wish I had a permanent sign in the car so I can notify the truck that I'm going to draft him/her, so they know...

Anecdotally, on a 400 mile roundtrip to Houston, felt like about a 15% efficiency gain in Wh/mi at 2 seconds back an 18 wheeler. Safe and efficient.

You find good drafters. They either don't mind you are there, or don't even notice and maintain a smooth constant speed. I wish I could say I'm drafting, or thank them after a long haul. I find it a unspoken friendship. If I'm in my big truck, I don't mind if someone tags along. I just make sure to give them fair warning when I see traffic slowing up ahead. Usually just slowing without the brakes sends the message. If I get suspicious of a braking event, I'll break the draft, check out what's up ahead. If he's just being jerky, I'll find another partner.
As for road debris, I checked the box next to paint armor... it has held up great so far! Taken a few rocks so far with no paint damage.

Regen will give you a few extra meters because the car will slow down before you touch the brakes.

Did a 228 mi. trip last week that included mountain passes on I5. Drafted part of the way on both out and back legs. I stayed pretty far back but still had a good benefit. Averaged 312 w/m on south leg into a headwind and 291 on the north leg. On the north leg, picked up an empty log truck to draft. This may be the ideal draft. They have a strong wake, have the power to go up the hills at the 65mph and without a load they can see you in their mirrors and you can see past them fairly well. Still stay back enough to be safe and not cause them to be irritated.

I friend of mine says he makes the trailer go out on the dirty shoulder momenterily, kicks up all kinds of shit for tailgaters or bicycles

In case you cannot stop in time, your beautiful car will go underneath the semi and your head may move to a different place. It will be quite a ugly scene.

Being unable to stop as fast as a big truck happens only to daydreamers. The MS has exceptional stopping power; trucks, not so much.

Tailgating a large truck is actually very dangerous. Depending on your definition of 'drafting' it could be extremely dangerous or only moderately dangerous. It's not about the stopping power of the brakes or anything else, if you are less than a second behind the truck you are extremely likely to hit it if for some reason the truck does a quick stop.

It's also illegal and you can be ticketed for doing it.

The truck that can stop in 1 second from highway speeds, or in within 1 second of the MS' stopping time, has yet to be invented or produced.

For those that think drafting close to a truck is a good idea just think about how the trucks higher suspension will pass harmlessly over a fairly large stationary object and how little time you will have from the point you can see it. Good luck.

I absolutely agree with Rob M and have a true story to prove his point.

Little more than a year ago I was following a semi in the early morning darkness, closer than I should have. When the truck cleared the WHOLE carcass of a large deer, I did not have sufficient time to react and run over it in my Infiniti G35 at approximately 75 mph.

I bet the repairs cost you deerly.

The cost was covered by insurance company (not Hartford).

The biggest problem was to get rid of the stench. My dog was perpetually insisting on doing thorough investigation of the underbody.

@Brian H, the truck doesn't need to stop in 1 sec for you to hit it. It just needs to slow down rapidly before you have time to react. If you are too close then you have already hit it. It's basic math and physics. A truck slowing at 0.7g changes speed at 16mph ie you are closing that fast.

Note that I have never said you would hit it at 60mph.

Basing my opinion on the observance about how a lot of people drive: texting, talking on the phone, not paying attention... MOST people should not draft any motor vehicle. Most people seem to be horrible drivers.

A Minority of people have the capability of safely "drafting" a semi IMO.

It must slow down for 1 second longer than you, and decelerate as fast or faster. That's the basic math.

@Brian, my last comment to you on this topic. Apologies for metric units.

- The original topic was about being 1.5 to 2 car lengths behind the truck/SUV/Whatever
- 2 car lengths is 10m in a Model S
- at 70mph you are travelling at 30m/s
- 1g deceleration is 9.8m/s/s
- average human reaction time is just over a second if not expecting something

If the truck runs over something that you didn't see coming you have about 333 milliseconds before you hit it as it is a stationary object that appears 10m ahead of you and you are covering 30m every second. Good luck with that.

If the truck sees the object and does an emergency brake, at say 0.7g you are 3m behind it and closing at 7m/s before you even start to brake. That gives you approximately 300 milliseconds to scrub off *more* than 7m/s as the truck is still decelerating at 7m/s/s. The model S brakes are very good and can pull 1g or so on a good surface. It's not enough, you need to decelerate at 3x 7m/s/s = 21m/s/s or 2g to avoid hitting the truck.

Lets say you do really well and scrub off 5m/s in the time that you have. You still hit the truck at 2-3m/s which is 5-6mph. Best case.

More likely reaction time after drafting a truck for 20-30 minutes will be closer to 2 seconds as your attention is likely to waver. You hit the truck at the full 7m/s (16mph) as you haven't reacted at all yet.

Unless your name is Batman or Superman, or potentially Fangio.

Most drivers overestimate their ability and underestimate the time taken to react.

Some reduction in risk can be atained if Tesla offers intelligent cruise control option with full auto brake function. There would be interesting possibilities on optimizing drafting if some pressure sensors are installed and the car is allowed to pick the best following distanse with a preset minimum of say 100ft.

Of course one would assume that Tesla auto brake feature works better thatn Volvo's...

www.youtube.com/watch?v=aNi17YLnZpg

I doubt that any active cruise control will let you set a gap of 10m at 70 mph.


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