Sunday, 8 March 2020

Beyond the Bicycle: Room For Manoeuvre

As regular readers will know, one of my cycles is a Christiania box tricycle. Acquiring it has taught me quite a bit about how different cycles handle, but of course there's so many configurations out there.

My tricycle is known as a "tadpole" because it has two wheels at the front and one at the back - a fat body and a tail like a tadpole! The braking system consists of a rear coaster brake (aka a back pedal brake) and a pair of disc brakes on the front which are operated by a standard lever. The front brakes can also be held on with the press of a button to act as a handbrake to stop the cycle rolling off.

In terms of steering, there is a handle on the box and the box pivots above the axle connecting the front wheels. To turn left, one pushes the handle on the box to the right which is completely different to a standard cycle.

Handling is completely different to a two-wheeled cycle (and two-wheeled cycles all handle in their own ways). For a start, you are now under the control of the crossfall of the surface you are riding on - this means a slope sideways compared to the line of travel. 

We need crossfalls to make sure a surface sheds water, but as a tricycle rider a steep crossfall will mean you're having to lean into the slope (so if the crossfall sheds water to your left, you'll lean to the right) and when you are in motion, it will tend to pull you to the low side. If a road has a camber, it can mean that you'll stay towards the centre where the slope isn't too steep.

As you can see in the image above, as the crossfall increases, the action of the centre of gravity of shifts to the low side of the cycle and this pulls it towards the low spot - the steeper it is the more pronounced the pull and so at speed this can create a safety risk. There's of course complications with the load in the box, the weight and height of the rider and so on but the point is that once the rider in on three wheels, the ability to stay vertical when the crossfall changes has gone.

The third image (1 in 12) is at the steeper end of the slope for a dropped kerb. It very common for dropped kerbs to be used to transition between the carriageways and cycle tracks (often shared use). On a 2-wheel cycle, the change from the flattish crossfall to a steeper cross fall is generally easy to negotiate; with three wheels, the cycle is tipped and so as a rider, you have to make the transition more slowly and lean into the the change. With the example above, the cycle is coming towards us and so the rider will be leaning to our right.

The image above is pretty common layout and so in order to move from the carriageway to the shared-use cycle track, the rider will push the handle on the box to the right. As the left wheel goes over the kerb, the whole cycle starts to tilt to the right and the rider leans left to compensate for the shift. The right hand wheel goes over the kerb just before the rear wheel and to the whole tricycle is now tilting to the right. Immediately, the tilt reduces as the tricycle goes up the sloping side of the dropped kerb to join the path (below).

It's easier to show you how this works rather than describe or draw it because as one becomes proficient, one anticipates what is happening and one moves in the saddle to compensate for the shifting forces. 

However, this kind of transition cannot easily be performed at speed such is the risk of tipping over because the turn (left in this case) coupled with being tipped to the right both force the rider to the right. If one doesn't compensate for the two, then one is tipped off and into the road.

The photograph below shows how transitions should be done to make it safe for tricycles in that it is a slope along the line of travel rather than across the line of travel (ignoring the horrible cycle lane).

So the principles here are avoiding steep crossfalls and not mixing changes in crossfall with having to turn. For a really interesting experience, some of the bus stop bypasses on CS2 in Whitechapel bring all sorts of movements into play in a very short distance and there is one site in particular which always catches me out;

The red arrows show the crossfall towards some road gullies. As one cycles along, the crossfall is to the left. As one passes the first gully, the cycle track then bends left at the start of the bypass and so one will tend to lean left to counteract the forces pushing out to the right. However, in a very short distance, the crossfall switches to the right as the cycle track bends to the right. The problem is, the tricycle tilts to the right, but unless one corrects shifts the lean from the left to the right quick enough, one risks falling off to the left.

Speed of course is another factor. At very low speeds, the tricycle can be turned very sharply. So in the case of turning through 90°, doing so at a very low speed allows precise positioning, even if the crossfall isn't desirable.

The photo above was taken with the camera level. We were coming out of the retail park onto a shared use path which had dropped kerbs over the site access. The turn was sharp and so with the tilt of the dropped kerb I would have to lean away from the slope (towards the bush) otherwise the tricycle would tip into the road. This is the opposite of what I said above and so shows the difference in handling at low speed and higher speeds (and this makes it really tricky to explain).

The cruising speed for the tricycle is around about 12mph (20km/h). It's slower uphill and faster downhill which brings more dynamics. The tricycle weighs about 35kg, can carry a load of up to 100kg plus the rider. It actually handles a little better with some load. Whether or not it is loaded, the tricycle takes effort to get going and it takes a bit of distance to stop.

I also find it more stable when I'm pedalling or braking (using the pedal brake) because I am braced against the pedals. If I've spun out on the pedals (going downhill) I have to switch to touching the rear brake a bit to stay braced. Under no circumstances can I use the front brake at speed because it forces the weight forward and the steering becomes twitchy - I need to slow with the rear brake and use the front brakes for a stop as the speed drops right off.

Going faster means that I need to keep further away from kerbs (whether an upstand or a drop next to me) to allow a bit of wobble room. Like any other cycle, I'll be anticipating what is going on and making corrections for speed and positioning of the tricycle itself as well as my body position.  I have to try and spot potholes a long way ahead because I'm thinking of missing them with three wheels which are not in line. I'm now proficient to the point where I can go reasonably quickly around gentle corners by leaning into them and if I'm showing off, I can do so by lifting my outside wheel off the ground!

It's taken me quite a lot of words and some images to try and explain how my tricycle handles. It is completely different to my main two-wheel cycle and that in turn is different to my folding cycle. Different riding positions, centres of gravity, wheel sizes and so on. On one of my Netherlands trips, I rode a large bakfiets which has different handling characteristics again;

While it was easier to cope with changes in crossfall, the steering on a backfiets is initially unexpected. A standard cycle has the handlebars over the front wheel and it's pretty intuitive. A bakfiets has the handlebars in the centre of the cycle (or a touch further back) and they're connected by control rods to the front wheel. When turning, you have to start a touch before you would with a standard cycle because the front wheel is much further forward. It doesn't take long to get used to it, but it is different to a standard cycle.

Every configuration is a bit different. We might be towing a trailer which changes how a cycle feels. A tandem has two (or more) people adjusting their positioning. There are side by side tandems, recumbents, tandems with one rider upright and the other recumbent, all sorts of cargo cycles and many others, so how can designers accommodate them all?

One way to check a design is using swept path analysis. In my day job, I use AutoCAD which has swept path analysis for all sorts of vehicles. In short, I can design a layout and drive little cars, lorries and buses around on the screen to check it works. It's very motor vehicle based and the whole dynamic of people leaning can't be replicated, but someone cleverer than me has had a go to replicate various types of cycle which give an idea of the space needed;

The image above shows the approximate space needed to perform a U-turn with different configurations at a relatively slow speed. The Design Cycle is taken from the Design Manual for Roads & Bridges standard CD195 "Designing for Cycle Traffic" which has commentary on the space needed for people cycling in a fair bit of detail. The Design Cycle is an all-encompassing configuration which can be used as a proxy - if it works on a layout, then so will most cycles. 

Swept path analysis will show that as the speed increases, more space is needed to change direction and so a tight layout with tight turns will soon show up as needing people to travel slowly, whereas more space and gentle curves will show that people can cycle at a decent speed (which is the point of transport cycling).

The clever engineers at Sustrans are doing even better things with other software which has included allowances for rider lean and real-world testing of different cycles. Hopefully there will be a commercial release before long and with any luck, other providers will see the value in tools for cycling infrastructure design. What using the Design Cycle does confirm, however, is that staggered barriers are impossible to pass;

As you can see from the photograph above, there's no way I'm going to be able to turn round this barrier and across that crossing.

With the Autotrack model, I have played around with the Christiania model quite a bit and it's pretty accurate, but it can't replicate the effetcs of crossfalls and gradients so experience needs to play a part. My message to engineers is they really need to get some experience of riding some different configurations of cycle. 

I know it's easier said than done and so there are a few of us thinking how we might be able to facilitate the experience as some continuing professional development. Beyond that, there are plenty of people on social media with different types of cycle - ask them how their machines handle and what helps or hinders them.


  1. My partner Jane rides a conventional trike. The biggest problem for her is on split paths with a central white line, often raised, where the sides have been badly maintained. She has to ride with one wheel on the pedestrian side to make this work on one local path, Old Stanborough Lane in Welwyn Garden City, which then makes life fun with central bollards at junctions. Of course she can't take it on the train, despite needing her trike as she has a sight disability which means she cannot balance on a 2 wheel bike.. (Don't get me started on tandems on trains and especially LNER!)

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