Slipstreaming road vehicles.

blackpoolkev

When I am keeping pace with a flow of traffic,I like to take advantage of the slipstream offered by the vehicle in front.
Basically it feels a bit unsafe at times and I got to wondering,what is the braking distance of a bike versus a car? Does anyone have some reliable numbers on this?

desweller

Vastly longer!

Cars are friction limited, but bikes are balance limited, i.e. you'll go over the bars waaaay before you can lock the front wheel up. So keep your distance; cars can stop a lot more quickly...

elmsallmissile

If you stick to the two second rule you'll not go far wrong, you'll not slipstream either though,
just like any other road vehicle you need thinking & braking time, if you are following inches behind and anything happens suddenly in front you WILL crash!

neeb

Yup, you've got to keep alert and anticipate anything that might cause the vehicle to brake. On the other hand, you don't need to be mega close to get a decent drafting effect because of the size of cars compared to bikes. Buses are particularly good.

sungod

a car can stop faster, but it's not about the braking distance, it's the reaction time

at 50kph (c. 31mph) you are going at nearly 14 metres/second, so if you are drafting a metre or two away from the vehicle and the driver drops the anchor, you'll close the gap before you have a chance

i.e. this... http://www.passmytheory.co.uk/learningc ... tance.aspx ...says...
thinking distance at 30mph - 9m
braking distance - another 14m

so assuming you fit the average, you'll go 9m *before* you start braking, i.e. right into the back of the vehicle you are drafting

elmsallmissile

neeb wrote:

Yup, you've got to keep alert and anticipate anything that might cause the vehicle to brake. On the other hand, you don't need to be mega close to get a decent drafting effect because of the size of cars compared to bikes. Buses are particularly good.

If you are drafting a bus how do you keep alert for anything happening in front of said bus, you cannot see anything, at 30 MPH you will have travelled 15 feet before you have even THOUGHT about braking, hence the number of multi vehicle pileups, they all drive too close & THINK they can stop.

cougie

You have to ask what would stop better a bike or a car ?
Just look at how much more rubber they have on the ground.

It will end in tears. Google for Davis Phinney and crash. Yuk.

blackpoolkev

Okay,I understand that the thinking time plays a major part in any situation,so let's assume that the cyclist has a view of the road in front, and both (s)he and the driver react at the same time to a pedestrian stepping into the road.What's the difference in stopping distances?

Nairnster

No official figures, but the car will stop before you.

As said above, in order for you to put enough force to stop as quickly, you will be over the bars.

neeb

elmsallmissile wrote:

If you are drafting a bus how do you keep alert for anything happening in front of said bus, you cannot see anything, at 30 MPH you will have travelled 15 feet before you have even THOUGHT about braking, hence the number of multi vehicle pileups, they all drive too close & THINK they can stop.

Well, they are bigger so you don't need to be very close to get some drafting effect. I only really feel safe doing it on slight uphills where the speeds are a bit lower. With cars you can sometimes see through the windows. But yes, it's risky. Impossible to give any rules of thumb about when it's relatively safe and when it's not, too many circumstantial factors involved.

Speedmenace

drafting is a risky business, if you can see through the car and the driver is aware you are there then its ok to draft for a short while, stay to the left or right of the vehicle. buses generally don't brake hard, and you don't need to be that close to it to get a good draft

just keep your fingers over the brakes and be very aware of your surroundings

cougie

http://m229.photobucket.com/image/davis ... ml?src=www

mouth

Problem if you get close to a larger vehicle is yo can't see enough in front to anticipate its movements.In a car when you can see over it, look through rear window etc thats ok but a bus/van/lorry? Could get real nasty. You may well be the next road safety advert.

Besides if you're training, take the wind.

blackpoolkev

cougie wrote:

You have to ask what would stop better a bike or a car ?
Just look at how much more rubber they have on the ground.

I didn't ask "which stops better?" I asked for the difference between the two.Your "rubber on the ground" insight wasn't really what I was looking for.

redvee

I have done this but only where I knew the stops of the buys I was going to follow, when they broke the 30mph mark my legs surrendered. I did follow an HGV off some lights this week and had to stop as the slipstream was kicking up leaves from the gutter and some were on the large size.

merak

It is incredibly risky, but also incredibly fun and I can remember individually with great joy some of the brilliant drafts I've had from buses, tractors, tractors with trailers and vans. This is in no way a recommendation to do it though.

A much less risky draft is behind mopeds - it's a bit like riding behind a Derny on the road and is safer than following a big vehicle because you can see the road in front - not much different to riding in the wheels in a group (except the moped rider isn't going to give you a warning of hazards like a cyclist would).

optimisticbiker

sungod wrote:

a car can stop faster, but it's not about the braking distance, it's the reaction time

at 50kph (c. 31mph) you are going at nearly 14 metres/second, so if you are drafting a metre or two away from the vehicle and the driver drops the anchor, you'll close the gap before you have a chance

i.e. this... http://www.passmytheory.co.uk/learningc ... tance.aspx ...says...
thinking distance at 30mph - 9m
braking distance - another 14m

so assuming you fit the average, you'll go 9m *before* you start braking, i.e. right into the back of the vehicle you are drafting

Stopping distance for a bikeon a dry surface at 30mph is 10.4m, so less than a car... however that is theory and assumes perfect ability to stop the front brake locking up while keeping the rear tyre on the road... the reality could be 20 - 50% more than that. So assuming you can see why the guy in front is stopping you might not hit them, if you're good enough. Most of us will take some pain....

Big Hairy Wallapers

If you get the chance then try it.
It's great fun but much like several things that are fun it's pretty dangerous.
I do it whenever I get a chance, 3 miles at 30-35mph behind a tractor last week!!
It may end in tears but until then il'll keep on doing it.
Stopping distances are impossible to tell, it's more to do with reaction time.
If I'm behind a car and can tell the driver is uncomfortable with it I'll drop off or go past depending on speed of traffic!
Give it a go and good luck x

Secteur

As has been outlined above, only a fool would do this.

Pickled Pig

Secteur wrote:

As has been outlined above, only a fool would do this.

Why does everyone who wants to take a risk have to be a fool? Was Neil Armstrong a fool? It is foolish to take risks that you don't understand. The OP is trying to understand the risk and is not a fool. By all means be risk averse but don't insult other people who enjoy excitement.

estampida

so I have been doing this since the 1994, mainly with buses

keep a small gap and use sight to maintain the gap

listen to the engine note you will hear the revs dropping off or brakes engaging as they slow down and can overtake

the few emergancy stops normally end in a stoppie, in the extreme cases I have used my left hand on teh back of the bus to push the rear wheel down
just kep a mental picture of the road ahead

we all know where the lights & roundabout issues are on the commute

ride_whenever

As a counter to the funless killjoys above (excluding the immediately previous post), I do it fairly regularly. Particularly behind buses as they have predictable routes and you can hear the engine. For reaction times and stopping distances, on the road bike it can be a touch hairy when they slam the brakes on, so i tend to give myself 3m, but on the mtb it's a lot less and you can react and stop faster.

I have a colleague who makes me look like a sissy though, he will literally rub his front tyre on the back of the buses. Apparently, when the bus brakes hard it tends to slow you down in a fairly controlled manner...

Secteur

And I repeat, with consideration, only a fool would do this.

Fun it may be to some, but foolish it remains to all and you should not encourage people who may not have the awareness / bike handling skills / lack of sense to do this.

merak

Actually quite a few people in the thread have said that they do it while recognising the risks. If you do it recognising the risks and you come a cropper you have yourself to blame - so while not recommending it to others I will continue to do it when the opportunity presents as it is awesomely good fun.

Big Hairy Wallapers

As I have previously said I do it and I love it.
If that makes me a fool in certain peoples eyes I can live with that.
Someone eating junk food and watching Eastenders is a fool in my eyes but that ain't gonna stop 14million people doing it.
You make your choices n I'll make mine.
Mega impressed with the tyre / bus brushing thing - I am a pussy compared to that!!

Was gonna try n crack 50mph inches behind a mates car but David Harmon said Don't try this at home so I didn't!!

Good luck x

sheffsimon

Secteur wrote:

As has been outlined above, only a fool would do this.

Dont draft cars, but certainly happy to draft a bus on my way home from work. Two long roads where quite often I can get behind a bus. Never had an issue with stopping distances/faceplanting the arse end of a bus.

MaxwellBygraves

I sometimes do this, mainly only with farm vehicles that are towing huge trailers.

They're so big that you can comfortably be a way back and still benefit from a draft.

It's dangerous, but so much fun! I would probably think twice about it before drafting on roads I didn't know.

blackpoolkev

Stopping distance for a bikeon a dry surface at 30mph is 10.4m, so less than a car... [/quote]

I must admit,this surprised me.Thanks for the well intentioned warnings as well as the hair raising anecdotes.

I actually feel safer using a slipstream to keep up with traffic rather than allowing a long stream of traffic to squeeze me to the margins of the road,although I accept it's not for everyone.

sungod

OptimisticBiker wrote:

Stopping distance for a bikeon a dry surface at 30mph is 10.4m, so less than a car... however that is theory and assumes perfect ability to stop the front brake locking up while keeping the rear tyre on the road... the reality could be 20 - 50% more than that. So assuming you can see why the guy in front is stopping you might not hit them, if you're good enough. Most of us will take some pain....

it's only 10.4m if the parameters in the model match those of your tyres and the road

CiB

I do it; stuff the risk. My favourite place is the 50mph limit ring road round the edge of town here where there's a clear mile to the next roundabout and no junctions. In all the 5 years I've been driving & cycling this way to work I've not once seen a car stopped on this stretch of road, or seen anyone come to a sudden stop along that stretch. I'm sure someone has or will, but life is about taking a punt, having a bit of a gamble occasionally. And from time to time there'll be a tractor with one of those huge square trailers on the back, or a horsebox lorry, or a cement wagon: the sort of vehicles that accelerate much more slowly than usual traffic and therefore give you the opportunity to draft up to some silly speeds. 47mph is my PB along there so far before the elastic went ((c) Phil).

Zone in on engine note & noise, listen for brakes being applied, have the occasional peek round the side of it, and cover the brakes all the way so that if you need to stop, you're giving yourself maximum chance to do so. On a scale of 10 though the exhilaration factor is up there at 10½ and the risk factor is on the low side. Enjoy.

bompington

OptimisticBiker wrote:

Stopping distance for a bikeon a dry surface at 30mph is 10.4m, so less than a car

Although admittedly I haven't ever measured it, I don't believe this figure for a minute. Try it by approaching a junction at 30mph and hitting the brakes 10m away, and see what happens.