Length of a vehicle's slipstream?

thelawnet

I did about 10 miles today, just general errands, no long rides, my top speed was 27mph (GPS measured, second-highest speed was 22mph), which I reached on a 30mph road (flat or very slight incline). I pulled onto the road at about 10mph in a line of traffic, there was a 'tipper truck' ahead of me, I tried to keep up (speed limit was 30mph) couldn't quite catch up, but it's a fairly long vehicle, and I'm not sure if it was just me giving it beans or the vehicle's slipstream as I never normally get that fast except on a sizeable downhill. I was in 36/11 gear and I was probably three car lengths behind.

Am I still getting assistance at that distance, or was I just trying hard?

EKE_38BPM

Three car lengths at that sort of speed sounds a bit too close for safety. The Highway Code says the minimum safe distance for a car following another vehicle at 20mph is 3 car lengths and bike brakes ain't as good as cars.

A tipper truck will punch a massive hole in the air so I think you were probably still in the slipstream, but I can't back this up with any proof as to the length it would be at that speed.

sfichele

Probably, here's a quote from wikipedia

On the show Mythbusters, drafting behind an 18-wheeler truck was tested and results showed that traveling 100 feet (30 m) behind the truck increased overall mpg efficiency by 11%. Traveling 10 feet (3.0 m) behind the truck produced a 39% gain in efficienc

The distance is going to depend on the speed of the truck.

But I'd never try to slipstream cars/vehicles closely at speed because if they brake hard - you're ufckde

thelawnet

sfichele wrote:

But I'd never try to slipstream cars/vehicles closely at speed because if they brake hard - you're ufckde

My brakes are pretty good tbh, at 30mph cars sit only a few feet from each other, there was enough space for a car to overtake me and sit in the space for example - it seemed like quite a lot of space.

iPete

Don't worry about them, I follow plenty of vehicles at 30 at about a cars length and sit on the brakes.

I've only gone into the back of 1 lorry to date but was going slowly, typical

EKE_38BPM

Slipstreaming (or tailgating in a car) is fine until the vehicle in front stops suddenly, then it hurts (or gets expensive).

Pufftmw

I would estimate that the stopping distance of a car or lorry @ 30 would be further than a bike's, mainly due to weiight/momentum but interested to find out. Its the thinking distance you need to be aware of.

thelawnet

Pufftmw wrote:

Its the thinking distance you need to be aware of.

surely much less in a bike. Car drivers are usually on the phone, chatting to a passenger. or changing a CD, they haven't got a clue what's going on.

jds_1981

When I'm close behind traffic I tend to sit just to one side. Allows one to see ahead & means that rear-ending should be avoidable via a tweak of the handlebars.

redvee

This is me chasing a tipper truck down the A38 back in March. I guess I was 2 car lengths behind the trailer and when the brakes were dabbed I was on mine straight away, I was riding on the brakes and looking at the back of the trailer. You can hear the wind noise on the camera as the gap increased.

http://www.youtube.com/watch?v=uumVJ5f9638

desweller

A bike's braking is limited by its high centre of gravity. By definition, you are stopping as hard as you can when the rear wheel lifts off the ground. You are therefore not limited by the friction limit of the front tyre, and are therefore not fully exploiting its capabilities.

A car's braking is not limited by the height of it's CoG, so it is able to run the tyres at their maximum friction and stop much faster than a bicycle.

CiB

I had a tow from an artic the other week, hitting 44 at one point.

There is a risk, but it can be reduced:

The road I was on is about a 3/4 mile stretch of clear straight clearway
In 4 years I've never seen a car stopped on that short stretch
The 50 limit is well observed
There are no junctions on it
You ride with fullest attention on the engine note, covering the brakes all the way. Slightest drop in tone means abandon the tow
Know that a 32 ton truck won't stop dead, or in less time than I can.

Enjoy the buzz.

EKE_38BPM

This page has a bicycle stopping distance calculator. Dunno how good it is though. You can change things like surface, adhesion co-efficient and rolling co-efficient, but not things like what type of brakes you have or width of tyres (I haven't put any thought into how much of an effect this will have though).

According to that site, you will be well within the stopping distance and I don't think that is including reaction distance and thinking distance.

Slip streaming is fun, but not really advisable.

snailracer

The problem with drafting trucks is that their drivers sometimes forget they might not be carrying a load when they brake, and truck air brakes aren't terribly progressive in any case.

shouldbeinbed

thelawnet wrote:

Pufftmw wrote:

Its the thinking distance you need to be aware of.

surely much less in a bike. Car drivers are usually on the phone, chatting to a passenger. or changing a CD, they haven't got a clue what's going on.

quite a sweeping generalisation there & if true you'd be seeing dozens of crashes every ride

We cyclists never have headphones in or thinking ahead to our next general errand or putting the world to rights in our heads or deciding what we want for tea, or wondering what that new squeak is or annoyed at the last close pass etc do we.

thinking distance is the innate speed you need to process the actions on the vehicle in front, make a decision as to what you're going to do about it and then physically do it, distraction or not that takes time whilst the other vehicle is already into its stopping phase and then you're stopping with a tiny fraction of braking area a vehicle has without servo assistance & putting the stopping power through a tiny fraction of a vehicles friction point onto the road.

UndercoverElephant

I think that the ideal drafting distance would be dependent on the speed and shape of the vehicle, as well as the weather.

I find that the ideal distance is a triangle of roughly 60 degrees from any of the vertexes of the vehicle, which puts you pretty close. If you go a little further back than that, you end up in turbulent air (dependent on the shape of the draftee), which can drop you pretty quick, a little further back still, and you get some positive effect, but probably not enough to keep up. The distances involved elongate slightly in a headwind, and shorten in a tailwind.

Like others have said, it's not big, it's not clever, but if you don't much like your face anyway, it's great fun. If I'm doing it, I usually stay near the outside corner, so that right-turning cars coming the other way have a chance of seeing me. I have had times where they just see a gap between the draftee and the car behind which they think they can make. It also means you have a bail-out should the draftee brake suddenly.

Of course, I wouldn't recommend it.

Headhuunter

I do draft buses and lorries sometimes quite closely. I tend to hang about a car or half a cars length behind them at about 25-30mph, to the right of the rear, so if it does stop suddenly, I may be able to shoot round the right, especially when crossing London Br which has multiple lanes in each direction. Also I can see round the side of the vehicle to a certain extent.

I'm sure that you could be 10-20m behind something like a double decker bus and still benfit from its draft. Something that size really does push masses of air out of the way.

I'm not convinced that a bike's stopping distance is shorter than a bus or lorry's at any given speed though, they have very powerful air brakes which can bring them to a standstill very quickly and of course you have to factor reaction time into the equation as well....

Confusedboy

When learning to sail many years ago, I was taught that the length of the 'wind shadow' of an object shielding the boat from the full force of the wind, usually a tree on our learning reservoir, was 6 times it's height. So, a tree on the bank 20 metres high would affect the wind driving the boat up to 120 metres downwind, or in proper sailing, a 150m high cliff would affect the boat the best part of a kilometre downwind. Probably a similar ratio applies to road vehicles moving through the air (I mean driving through the atmosphere, not flying...).

vorsprung

My Orbea has new 105 brakes with new blocks on it and currently "stops on a dime"

So I am happier about drafting motor vehicles in a fairly cautious way

If your brakes are less than 100% I wouldn't risk it

The problem is if your reactions are too slow and the lorry stops quicker than your thought then you are in deep shit. In a car if you have a bump it's just a bit expensive. On a bike it can be more...painful

kamiokande

Also wise to avoid drafting on roads you're not familiar with. Get too close and a metal grate, pothole or other obstruction can pop out right under your front wheel.

cee

DesWeller wrote:

A bike's braking is limited by its high centre of gravity. By definition, you are stopping as hard as you can when the rear wheel lifts off the ground. You are therefore not limited by the friction limit of the front tyre, and are therefore not fully exploiting its capabilities.

A car's braking is not limited by the height of it's CoG, so it is able to run the tyres at their maximum friction and stop much faster than a bicycle.

this is true...however you can shift your weight around on a bike, meaning you can choose where the centre of gravity is....and make a very dramatic difference to the braking characteristics because of the ratio of weight split between veehickle and pilot.

I have said previously....my hobby is mountain biking...and i commute to work by bike....

when braking off road....the key to getting rid of speed fast...is to shift your weight back and down as you pull the brakes.....

This not only prevents the back wheel from lifting...but also stops it from breaking traction (i.e. skidding) and shoves all of the braking force down harder than it would if you were simply a passenger on the saddle...

It is important to do this offroad, particularly on steep loose descents...and the technique comes in handy on the road too.

o'course...big old disc brakes help too, but the thing I am most wary of are people riding behind me, not able to stop in time....

MichaelW

I seem to recall that the slipstream extends about 50% of the vehicles length then starts to tail off then gets turbulent.
My best drafting was on a very foggy day, behind a milkfloat.