Drafting Lorry/Buses

[Deleted User]

Glad to see Im not the only one. I try to keep to the right and bob my head out regulary like Im going to overtake to see what is ahead. Also like somone said I know the routes well, traffic lights etc.

Ben6899

Assuming your brakes are working fine, then minimum stopping distance is governed by the coefficient of friction between your tyres and the road surface. But you should also take into account the distance travelled while perceiving what is happening.

Friction force, F = umg
u = friction coefficient
m = rider and bike mass (kg)
v = velocity (m/s)
g = acceleration due to gravity (m/s/s)
t = thinking-perception time (s)
d = total stopping distance (m)
db = braking distance (m)
dt = thinking distance

Braking
Work-energy principle: the friction force must do enough work to reduce your kinetic energy to zero.
Work due to friction = -Fdb = -madb = -mugdb = -0.5mv2 so db = v2/2ug

Thinking
dt = vt

Total
d = vt + v2/2ug

So you just need to plug in the variables v, t and u. For reference, the Highway Code approximates u to 0.67 or 6.57m/s/s.

veronese68

I used to know a girl that looked like she'd run into the back of a bus.

rick_chasey

Retro...

keyser__soze

I only do it when I know I've got a bail-out route on a wide road, usually to the left and I'm confident I could stop should they slam on the anchors. I probably sit about 10 metres back at pace, and I've been gobsmacked to see how close some other cyclists get, once getting passed by a lorry doing 35+ on the Embankment with a spotty jeans-clad hybrid-riding teenager less than three feet off his bumper. I could see a ped walking up to the zebra crossing by Tite St and thought this guy was going to be toast when the lorry stopped but fortunately for him the it ran the zebra.

DonDaddyD

BigMat wrote:

rjsterry wrote:

Soem data on stopping distances here http://www.ukspeedtraps.co.uk/stopping.htm.
This site - http://www.exploratorium.edu/cycling/brakes2.html - reckons a bike can stop in about 10m from 30mph. Seems a bit optimistic to me, but much less than a truck.

Obviously, things are different on wet roads.

Is that stopping distance with the aid of the now stationary lorry you were just drafting?! I might get in the draft of a motor vehicle, but usually will be on its right wheel so I can pop my head out to see what is ahead and have an escape route if they slam on the brakes. Tend not to get too close either .

This. Always this.

tiny_pens

Used to do this when I lived in a city. JCBs are the best as they drive at about 25mph so its easier to keep up. When I used to draft them, the air brakes used to make a sort of tchh sound before they activated so you got an audible signal that they were slowing down. Wouldn't want to rely on it as the design may well have changed in the last 10 years.

YMMV

sketchley

Keyser__Soze wrote:

I only do it when I know I've got a bail-out route on a wide road, usually to the left and I'm confident I could stop should they slam on the anchors. I probably sit about 10 metres back at pace, and I've been gobsmacked to see how close some other cyclists get, once getting passed by a lorry doing 35+ on the Embankment with a spotty jeans-clad hybrid-riding teenager less than three feet off his bumper. I could see a ped walking up to the zebra crossing by Tite St and thought this guy was going to be toast when the lorry stopped but fortunately for him the it ran the zebra.

I thought you needed to be much closer to the vehicle to get a draft off it....

Off to google.....

CiB

I just assume my luck will hold, hope that nothing will go wrong and get tucked in close where the pull is strongest, right up the towing vehicle's jacksy. It might sound & look a bit crazy but my view is that I've never seen a car or lorry stopped on that stretch of road and the chances of it happening whilst I'm pulling this mad stunt is a bit remote so I'll make the most of it.

Paulie W

EKE_38BPM wrote:

UKspeedtraps says a lorry's stopping distance at 30 mph is 26.8 metres, so a "bicyclist" can stop in a shorter distance than a lorry can, but, if the cyclist is less than 9 metres behind the lorry when the lorry slams on the brakes, the cyclist will ride into the back of the lorry at full speed because they will not have reacted to the lorry braking.
Ouch!

But that assumes that the lorry stops dead when it breaks! At 9m and over you're not getting much of a draft!

Pufftmw

I was draughting a French coach this morning up Loampit Hill but the pedestrian lights went red, so he had to stop

Would have loved to follow him all the way to the top @ 30mph

kurako

I drafted a milk float off Chelsea Bridge a couple of months ago. The feeling of speed was... er.. not particularly exhilerating...

It did provide a nice respite from the headwind and it was quite easy to keep up with.

keyser__soze

Sketchley wrote:

Keyser__Soze wrote:

...10 metres back...

I thought you needed to be much closer to the vehicle to get a draft off it....

Off to google.....

Effect is better the closer you get but it's definitely noticeable even with a fair gap. 10m gap when doing 30mph (less than a second gap in terms of time) looks/feels a lot closer than 10m apart while stopped - plus with the 9m figure being bandied about I didn't want to sound unsafe

redvee

Kurako wrote:

I drafted a milk float off Chelsea Bridge a couple of months ago. The feeling of speed was... er.. not particularly exhilerating...

Like this?

http://www.youtube.com/watch?v=mCno1ezwG30

A slightly quicker draft I did manage to find, it helps if you remember the title you gave the clip.

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

You can hear the wind noise increase as the gap gets bigger. We had just pulled away from a set of TLs which is why I was so close at the beginning of the clip.

fossyant

Just don't draft one of those big tractors pulling a trailer of manure for miles...you'll stink. Don't ask how I know !

mattv

A few years back I heard a lorry about to overtake me, crept upto 28mph on the road bike off the lip of a hump back bridge. It overtook me down the main road, I get right on its bumper. Surrounded by flies! Damned dust cart! Kept with it til I got overpowered by the smell (it was july) then let it drop me.

notsoblue

mattv wrote:

A few years back I heard a lorry about to overtake me, crept upto 28mph on the road bike off the lip of a hump back bridge. It overtook me down the main road, I get right on its bumper. Surrounded by flies! Damned dust cart! Kept with it til I got overpowered by the smell (it was july) then let it drop me.

Defeatist...

You should have scalped the m***** f*****.

mattv

It was doing 40mph. My normal routine was to keep 34mph for over a minute on that stretch near weston super mare. I aint quite that fit!

jimmypippa

The good thing about lorries is that they leave such a massive hole in the wind that you can stay a fair distance back and still get a lot of the benefit.

SteppenHerring

30mph? That's nothing.

I'll occasionally take a tow from a bus keeping on the right corner but it's a bit iffy.

Interestingly on that Wikipedia page about cycling records it doesn't mention Wilko's 24 hour from last year. That was an awesome feat.

EKE_38BPM

Paulie W wrote:

EKE_38BPM wrote:

UKspeedtraps says a lorry's stopping distance at 30 mph is 26.8 metres, so a "bicyclist" can stop in a shorter distance than a lorry can, but, if the cyclist is less than 9 metres behind the lorry when the lorry slams on the brakes, the cyclist will ride into the back of the lorry at full speed because they will not have reacted to the lorry braking.
Ouch!

But that assumes that the lorry stops dead when it breaks! At 9m and over you're not getting much of a draft!

You mean brakes. Or maybe breaks is correct? If the lorry had a catastrophic failure causing it to come to an immediate halt, that would be classed as a break.

Anyway, the lorry would have been braking for 9 metres whilst the cyclist would have been giving it their all. Then they would have realised that that 9 metre gap is rapidly deminishing and would have slammed on the anchors. At this point the cyclist would be hoping that they had an escape route planned and/or that bike brakes braking from 30mph are better than lorry brakes are from braking from ~25mph. That is only a ~5mph closing speed but if the cyclist then has an off (due to bad braking technique, bad road surface, bad luck or whatever), they would probably be collected by whatever was following (assuming that vehicle didn't maintain the correct stopping distance).

You feel free to slipstream at 9 metres or less, I'll take less of a slipstream and feel that little bit safer.

jimmypippa

EKE_38BPM wrote:

You feel free to slipstream at 9 metres or less, I'll take less of a slipstream and feel that little bit safer.

I never use distances but times, "only a fool ignores the two-second rule"

You get no benefit from cars at such a delay, but lorries are still good.

veronese68

redvee wrote:

A slightly quicker draft I did manage to find, it helps if you remember the title you gave the clip.

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

You can hear the wind noise increase as the gap gets bigger. We had just pulled away from a set of TLs which is why I was so close at the beginning of the clip.

Did you mutter something that sounds like banker because you hit a pot hole or because he dabbed the brakes before accelerating? Made me chuckle.

noodles71

I only do buses as the engine in the back allows for you to hear near exactly what the driver is doing with his speed. When they back off I am ready to yak on the brakes. Also as someone else said, know your route and it's potholes.

Another one to watch out is if you are in the main lane you might not have the bus lane as a backout plan as black cabs in London can always be trusted to be roaring up the left in the bus lane to get ahead of the bus as it goes to slow down. Evelyn Street near McDonalds on the ride in from Lewisham/Greenwich can get pretty nasty after drafting down the long straight as most cabs will try and get pole position into the left hander.

redvee

Veronese68 wrote:

Did you mutter something that sounds like banker because you hit a pot hole or because he dabbed the brakes before accelerating? Made me chuckle.

Can't remember what I uttered but the cause was the HGV slowing slightly for a speed camera, then acceletating after.

byke68

JCB's for me! Also the occasional street cleaner.
Once caught up with a moped, spotted it in the distance on a long straight bit of road and caught it just before we got to Braintree. Turned out it was a greasy, spotty work colleague, nearly fell off it when he realized I was along side him! Happy days!

supersonic

You can theoretically be done for breaking the law:

Dangerous cycling.

Source: Road Traffic Act 1988 Section 28

Offence: It is an offence for a person to ride a cycle on a road dangerously.

Cycling without due care and attention.

Source: Road Traffic Act 1988 Section 29

Offence: It is an offence for a person to ride a cycle on a road without due care and attention, or without reasonable consideration for other persons using the road.

I never ride close to other vehicles, I like a clear view of the road and for drivers to know where I am.

petemadoc

I've done it a few times but only very cautiously. It can feel dangerous as fook especially if you can't see the road ahead.

I drafted a tractor for a while the other day, it was doing about 20mph. It was getting too easy so I decided to overtake. Once in front things got a bit more difficult :shock: But I did'nt want to suffer the embarrassment of being caught by the tractor so I had to bury myself until it was out of view. :twisted:

beverick

I'll take a tow where I can and where it's safe but I nearly came to grief last week when I hit a pothole which emerged from under the bus in front of me and I didn't have time to react. Fortunately the only damage was a cracked pannier rack, snapped pannier fittings and a couple of scratches in the case of the work's laptop :oops: !

BTW, buses in areas of North and West Yorkshire (West Riding buses?) carried stickers on the back of them during the 1980's after a number of serious injuries and deaths involving cyclists drafting.

A mate of mine drafted one of these in the dales a couple of years ago:
http://www.farmingmachines.co.uk/machinery/M02976

It was being pulled by a quad bike and had the added incentive of having two or three sheep in it at the time.

Bob

supersonic

I had a word with my brother who is a cop in WY - the force have pulled people over for doing this!