Commuting - How close for drafting?

Drafting always seems like a cool idea and would definitely be a possibility on my commute, but I'm always nervous to get too close to a tall vehicle like a truck or a bus. I wouldn't be able to see traffic or signals in front of me, so a short stop could potentially be disastrous for me if I draft.

So, my question: do I have to get right behind a vehicle to draft? How far back can I be to still get the benefits of little-to-no wind resistance?

Depends on your speed and the direction of the wind.

But DON'T DO IT. In addition to the dangers you mentioned, you won't be able to see potholes or cracks or seams or debris in the road, and something unexpected like that can take your front wheel out and down ya go.

If it's a really large, square vehicle (city bus, dump truck, some semis) the draft produced is quite extensive and you get some benefit even like 30+ feet back. However it depends on the speed, at 25 mph the draft is probably not as strong as at 55 (which you aren't going to be maintaining on your bike no matter how good a draft you've got).

Do it if you want, but I'll relate a story that may make you think twice.

I'm riding to work one day, and there's a light rain falling. I'm following a big postal truck, so the draft was fantastic. I'm about 6 feet behind him travelling at around 40 km/h. The brakes on postal trucks are apparently very good, since the vehicle was able to go from 40-0 in a very short distance. I locked up both wheels initially, but when I was about 2 feet away from the truck, my front tire grabbed and I tilted up and smacked the top of my head into the back of the van. I dropped back down onto two tires, but was so stunned that I forgot to clip out and promptly fell over. By this time, the driver was coming around the back of the truck to load/unload. He asked me if I was OK, to which I said "Yeah, no problem". I struggled to my feet and rode the rest of the way to work. I had to get a new helmet, since there was now a big flat spot on the top where I had hit the truck. Other than that, I got away without a scratch.

I draft minivans and SUV's all the time, but I keep back about 10' and don't do it when it's wet out. I also avoid drafting delivery vehicles of any type, especially postal trucks.

10 to 15 foot.

I do not draft much, since there is plenty room to pass me due the low volume traffic on my commuting hours. But there is a particular section of road where the speed limit is 20 miles an hour. Traffic goes 30, and any good old minivan makes my day ... but I keep my distance +-30 feet.

Now I know why dogs chase cars .... is so much fun!

I usually draft on the right hand side. This still blocks the wind, but allows a quick swerve to the right around the vehicle if you need it.

All properly maintainted motor vehicles can stop faster than an experienced cyclist.

Ones doesn't need to be behind the vehicle to benefit, a great deal of help comes from a stream of vehicles passing on your right. A bus passing can give a boost over 100yrds.

Al

All properly maintainted motor vehicles can stop faster than an experienced cyclist.
I'll have to test your theory. A car with disc brakes can stop faster than me from 40 km/h, but a bus/loaded truck? I dunno... <runs off to steal bus and measure panic stopping distance>

I'm done drafting. I was chasing an SUV down a steep hill and they stopped short to turn into a driveway. I was pretty far behind them and not going very fast, but there was no way I could get to the brakes and stop in time. I was bruised fine, the car was fine, but my dearly beloved Bridgestone was totaled. :cry:

I'm done drafting. I was chasing an SUV down a steep hill and they stopped short to turn into a driveway. I was pretty far behind them and not going very fast, but there was no way I could get to the brakes and stop in time. I was bruised fine, the car was fine, but my dearly beloved Bridgestone was totaled. :cry:
Um - did I mention I only draft on the flats? I'll go down a hill in a full tuck to match a vehicle's speed, but I definitely don't want to draft off of it. That's scary!

Not drafting makes you stronger!
At least that's my excuse. The real reason is that I'm a chicken.

Um - did I mention I only draft on the flats? I'll go down a hill in a full tuck to match a vehicle's speed, but I definitely don't want to draft off of it. That's scary!

Yea, I wouldn't say I was drafting - as in hugging the rear bumper to gain a speed advantage. I came over the top of the hill and caught up with the car faster than I expected. My point is that I mis-judged my distance from the car and their intention to brake quickly. From now on I'm leaving more room between me and every car, in addition to not drafting.

I'll have to test your theory. A car with disc brakes can stop faster than me from 40 km/h, but a bus/loaded truck? I dunno... <runs off to steal bus and measure panic stopping distance>
@40 miles an hour with ABS a bus takes <including reaction time> 120 feet

Drafting on downhills?? What would be the point of that? :)

The sweet spot for drafting depends on the size of the vehicle. I once drafted a Prevost bus/rv down a 20 mile lakeside stretch with a speed limit of 45. It's just like riding in the back of a big group - you are pedaling pretty hard, but there is no way you could manage that speed on your own. During that whole ride I was between 10 and 45 feet away. Also, garbage trucks create a huge draft, but they can smell like poo poo.

I've never gotten a draft worth anything off of a car/suv/truck. With stops, a city bus does a slower average speed than me but I can draft them between stops sometimes. Delivery trucks and such are almost always too fast for me, unless they're caught in traffic-- then i often pass them.

I can be pretty reckless about some things on my bike, but I'm overly-cautious about this: is the benefit worth the risk? Much as pacelining requires confidence in the bike-handling skills of your co-riders, aren't you making a big assumption about the driving abilities of the operator of whatever motor vehicle you're drafting? A sudden stop, a vehicle from the side that couldn't or didn't see you, or a road hazard you couldn't see, and it's RIP.