Drafting a truck to make it home

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.

cloroxbb | 22 March 2013

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.

Brian H | 22 March 2013

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

Mark E | 22 March 2013

@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.

vgrinshpun | 23 March 2013

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...

Mark E | 23 March 2013

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