Turning Circle

[atlas_shrugged]

It has always puzzled me why cycle paths are often designed with turns at right angles or even worse. As a result I have kept a keen lookout for any UK design guide which specifies the minimum turning circle for a cycle. This is obviously information kept on a need-to-know basis. So imagine my surprise when I recently came across this very figure in LTN 2/08:

Conventional bicycle
1800mm Overall length
1650mm Minimum turning circle Outer radius*
850mm Minimum turning circle Inner radius**

* The outer radius governs the distance between walls required to execute a full turn.
** The inner radius indicates the size of an imaginary circular obstruction which the cyclist moves around.

So I tried this on my conventional bike. The tightest turning circle *diameter* I could manage was 3m (approx.). If I got off the bike then I could steer the bike within approx 1.7m diameter. I am fairly certain that the Outer 'radius' as described is the outside turning diameter. When LTN refers to a radius the measurement they describe is actually a diameter.

Does this explain the impossible turning circles of UK cycle paths?

[GarethF]

So that's based on dismounting, picking up the bike, turning the handlebars right across and turning round with it?

A few years ago my brother in law was given the proposed layout for the overnight bus parking at the 2012 olympics. The spaces were beautifully herringboned into the space and the whole thing looked lovely. The first detail he raised was that overnight, buses are packed in nose to tail like sardines, since unlike sainsburys there is no need for ready access to each individual vehicle and they all roll out at the same time in the morning. The second point was the scale of the plan. Something didn't seem to add up. It turned out that the guy couldn't fit enough spaces on the plan to house all the buses, so he'd scaled down the spaces so they would fit in the space available. Geniuses!

[Postboxer]

Doesn't the required radius of a cycle path turn also depend on the degrees the turn is, it would be easier to do a 90 degree turn within those radii than it would a 180 degree turn. Can't really explain why, it's because you can cut the corner and escape quicker on the shorter turn, the 180 degree turn your front wheel has to keep towards the outside of the turn a lot more.

My sister knew someone working at the Olympics, they had seen the plan for a drop off area for the busses, possibly designed by the same person, the first point they raised was to ask if it wouldn't be better if the busses pulled up to the kerb on the left, rather than the right, as that is where the doors tend to be in the UK, it had been designed by an American I think.

[pwa]

I've seen a lot of cycle facilities with ridiculously tight turns. Some of them not very old.

[pjclinch]

It has always puzzled me why cycle paths are often designed with turns at right angles or even worse.

Well, there's your mistake...

Conventional bicycle
1800mm Overall length
1650mm Minimum turning circle Outer radius*
850mm Minimum turning circle Inner radius**

* The outer radius governs the distance between walls required to execute a full turn.
** The inner radius indicates the size of an imaginary circular obstruction which the cyclist moves around.

So I tried this on my conventional bike. The tightest turning circle *diameter* I could manage was 3m (approx.). If I got off the bike then I could steer the bike within approx 1.7m diameter. I am fairly certain that the Outer 'radius' as described is the outside turning diameter. When LTN refers to a radius the measurement they describe is actually a diameter.

Does this explain the impossible turning circles of UK cycle paths?

Maybe, but more to the point, I suspect, is that people putting them in (either on paper or on the ground) don't cycle and/or don't test prototypes.

With my cycle trainer hat on, I also train trainers and one of the "learn it by getting it wrong" aspects is always test your bike-handling drills before letting other people loose on them. Even experienced cyclists can easily underestimate how much room is needed for a manoeuvre. Extend that to people who are primarily ticking boxes and don't care or know what's involved and you have what you've seen for yourself.

Pete.

[amediasatex]

Conventional bicycle

And what bout unconventional ones?

Cargo bikes
Trikes
Recumbents
Tandems
Bikes with Trailers
etc.

Any design has to have wiggle room for the unconventional and edge cases, I can understand not routinely designing for a triplet with a trailer, but you need to at least consider cases outside your statistical 'conventional bike length'.

[pjclinch]

Any design has to have wiggle room for the unconventional and edge cases, I can understand not routinely designing for a triplet with a trailer, but you need to at least consider cases outside your statistical 'conventional bike length'.

And while not a triplet with a trailer, I don't think you should rule out a triplet (and not just because I used to ride one).

I think a good baseline set for "if you can't do it easily on these it's back to the drawing board" would be a hand-powered trike, a loaded tandem (and no making it too easy with a Helios or Pino!) and a cargo trike. All should get through without dismount being required.

One notable point about that lot is they're all more cumbersome for fine manoeuvrability than a motor scooter. So it stands to reason that any "access barrier" designed to make life difficult for motor scooters has no place on a cycling network unless you have a specific design goal of light, compact bicycles ridden by fully able bodies riders only (and I'd be interested to see anyone justifying that if they did).

Pete.

[thirdcrank]

It has always puzzled me why cycle paths are often designed with turns at right angles or even worse. As a result I have kept a keen lookout for any UK design guide which specifies the minimum turning circle for a cycle. This is obviously information kept on a need-to-know basis. So imagine my surprise when I recently came across this very figure in LTN 2/08:

Conventional bicycle
1800mm Overall length
1650mm Minimum turning circle Outer radius*
850mm Minimum turning circle Inner radius**

* The outer radius governs the distance between walls required to execute a full turn.
** The inner radius indicates the size of an imaginary circular obstruction which the cyclist moves around.

So I tried this on my conventional bike. The tightest turning circle *diameter* I could manage was 3m (approx.). If I got off the bike then I could steer the bike within approx 1.7m diameter. I am fairly certain that the Outer 'radius' as described is the outside turning diameter. When LTN refers to a radius the measurement they describe is actually a diameter.

Does this explain the impossible turning circles of UK cycle paths?
(My emphasis.)

Bearing in mind your own results, I think that the bit I've highlighted is wrong: you don't need to be Pythagoras, Archimedes or anybody else of that stature to ride a bike round in circles and measure the inner and outer radii. Since you were trying your best to keep your turns tight, it seems likely that anything based on more carefree riding would be more generous.

It's probably fair to say that the major deciding factor is the space available to create the farcility. Izambard Kingdon Brunel couldn't create a 3.3m diameter turning circle in a 2.5m space.

In short, guidelines not followed. Unless anybody believes that never happens.

[[XAP]Bob]

IKB would, there would be a lot of engineering involved to shift whatever mountain was causing the restriction...