Introduction: Building a Tricycle or Quad
I’ve been looking into the design of a trike with two wheels at the front, it has been quite a learning experience. Here I would like to save you some time, frustration and bruises by giving you a heads up about steering a three or four wheeled cycle. I’ll describe the problems then describe simple designs to overcome them.
Step 1: Using Stub Axles
Many start by mounting stub axles on the sides of tubes that acts as a steering axis. It works like this:
Step 2: What's Wrong
Now that’s fine for a garden cart but it is intrinsically unsafe for transport. Let me show you why, here is one of the wheels seen from above being put under force.
Imagine you hit a hole or bump. The resulting force tries to push your wheel back relative to the cycle. Your wheel then wants to turn around the steering axle and sends a force back through the system to your handle bars and the other wheel. It can rip the steering from your hands. The same happens when braking, if one side exerts more force than the other, this is felt through the steering. If you are lucky you may end up in a ditch rather than facing oncoming traffic.
The shorter the hub, the less force you feel. Good, but the problem still persists.
Step 3: Learn From Existing Answers
Examining a modern car tyre gives us a hint at the answer. It is a simple concept but it took me a while to grasp it.
Here’s a cross-section of a car wheel. I’d always wondered why they were shaped like a bowl. It turns out it is not just to give space to put the brakes, it’s to allow center steering.
The wheel pivots around the point (A) in line with the area where the tyre touches the ground so it does not suffer the problem described above.
There is another trick cars use. Take a look at a parked car if its wheels are left at full deflection. They lean over a little. And there’s the answer for bicycle wheels.
Step 4: Center Steering
When laying out your frame put your steering pivots at an angle. This diagram show one wheel shown from the front. Some people choose to tilt the wheel but standard wheels are not made to resist side forces, their spokes fail comparatively quickly on bumpy or winding roads.
Note how a line drawn from the axis touches the ground in line with the area where the tyre touches the ground. Any bump or braking force now acts on the frame.
Step 5: Helping You Steer
The good thing is this turning force can be used to our advantage.
Simply aim the axis slightly in front of the point where the tyre touches the ground. This creates the “Caster Effect” which provides your steering with a tendency to want to return to straight. The distance between the steering line (S) and the place the tyre contacts the ground rail is called Trail.
When people discuss steering you are likely to hear the words Rake, Caster and Trail, now you know what these are.
Step 6: Turning Circles
So now you can steer safely but there are some other considerations to reduce stress on the wheels and make your journey more efficient. Take a look at this vehicle taking a tight bend. It is important to note that cars turn relative to the center of their rear axle.
I used an SVG from the Wiki article about Akerman Geometry as the car in this diagram.
The important points are:
- The circumference of the outside track is larger than that of the inside.
- The wheel on the inside has to turn a slightly tighter track so it’s angle to the car body is larger.
- Wheels on the outside have to travel farther and hence spin faster.
Step 7: Akerman Geometry
The answers have been around a long time. Remember I mentioned that a car seems to rotate about the center of its rear axle...
A German carriage builder called Georg Lankensperger cracked the problem, his English Agent patented it for him so it is now known as Akerman Geometry.
At its simplest build your steering arms point to the center of the rear axle. It is easy and elegant. Don’t worry too much about the arm lengths, it keeps working.
Step 8: The Last Turn
The last thing to think about is illustrated in the previous diagram about the curve. If you have two driven wheels then they need to be able to turn at different speeds. A way to envisage this is keep one wheel in the same place (X) and rotate the whole axle. The moving wheel needs to turn while the other doesn’t.
I have seen the result of people welding both wheels to an axle. At a corner the wheels need to turn at different rates, they either snap their linkage or skid, at best they cause a breaking effect because one tyre is dragging.
An answer is to only have one driven wheel and the other freely rotating. Sadly it brings all those asymmetric dangers. Another answer is to put a slip clutch between the two but it is inefficient due to the same breaking effect.
Best of all is to get a differential gear (D). They are available for tricycles and feature internal gears or use dry bearings. You could even make one yourself, but that’s another Instructable entirely.