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Br(e)aking the Wheels?

At first glance, it looks like the most elegant way to stop an inline skate by braking the wheels. It works perfectly for a bike, so why shouldn't it work for a skate?

Many skate brake proposals work that way. They press something to the wheels or in between the wheels. Or they have little disk or drum brakes.

Let's look a bit closer. There are a couple of fundamental issues with this approach.

Construction Issues

These mechanisms have to be strong, precise, and light (think of disk or drum brakes). Which typically translates into "expensive, delicate, and maintenance intensive"!

Wheel Damage

If there are four or five wheels, mounted in a rigid frame, it can not be assumed that all wheels carry the same load. For instance, let one wheel be a little bit smaller. When braked, this wheel will frequently lose contact, stop, rub along the road, and get damaged (the well-known "flat spots"). Unfortunately, this damage will not "average out" as normal wear does. If a wheel has a small flat spot, it will frequently stop with exactly this spot down. So it will further rub along the road at this point and damage will get worse.

This is not only theory. Early EZSkates had this type of brakes. After a few minutes of testing, a full set of expensive wheels was broken!

People who propose this type of brakes are very aware of the problem. There are ways to reduce the risk:

  • The heel wheel has the highest risk of getting damaged. So many constructions only brake the front wheels. Others prefer to brake all except the front wheel.
  • You may construct the brakes in a way that brake force is limited. Or recommend to the driver not to brake too hard. Well...
  • WBS use special rubber instead of PU wheels, clearly because of this reason.
  • Tentatively... if you had something... such that the wheels don't stop...? Viscous forces or active electronics come to mind. Certainly possible, but probably heavy and expensive. Smartwheels claim they have solved it in a similar way: when a wheel loses contact, brake force is automatically and immediately reduced.

Heat Dissipation

If you need the brake to stop at a red traffic light, all types of brake will probably work fine. What about more serious applications? Let's assume a skater rolls down a long decline, of 10% slope. Say speed is 10m/s (36km/h), and mass is 80kg. So we have to dissipate some 800W of power. Some fraction of this is certainly going into air friction. But still, we have to burn a few hundred Watts. Which is quite a lot.

If the wheels are braked, all this power has to be dissipated into the air. There is no other medium around. This type of cooling is promoted by high temperatures, high air speed, and large cooling surfaces. None of these factors is easy to have with an inline skate.

The amazing thing about the brake pad is that it gives us access to a more effective cooling medium than air: the road! The pad is in good thermal contact with the road. Because we travel with high speed, the road has no time to heat up while the pad runs along (see below). So the pad can continuously dissipate heat into the cool road ("cool" relative to the melting point of the rubber pad). It's almost like a water cooled system: a contiuous flow of cool medium is carrying the heat away!

For a quantitative discussion, see this memo. In order to calculate the expected pad temperature, it would be necessary to measure the thermal conductance of the pad to road interface. This will depend on road roughness, pressure and other factors and is not easy to measure. Practical experience, however, has never shown any indication of melting so far. Given a thermal conductance and power to transfer, temperature difference is inversely proportional to interface area. So again, it definitely helps to have a large brake pad.

Special Brake Wheels

A sort of compromise are special brake wheels. They are permanently braked (often adjustable) and used as like a brake pad. So, while driving, they are somewhere above the road. When braking, they are pressed to the ground. See example. Some time ago, Ultrawheels had a similar system called DBS.

Brake force can be adjusted to match weight and individual preferences of the skater. However, once set, it seems that it cannot be modulated during braking. If the brake wheel spins, brake force can not be further increased by pressing it to the road. This would limit the ability to do emergency stops. On long declines, overheating can be expected.

The EZBrake Approach

EZSkates use a standard brake pad approach, the EZBrake. We have seen that there are quite fundamental reasons to do so:
  • The construction is robust and simple, replacement pads are cheap and can easily be changed
  • There is no risk of damage to the delicate and expensive wheels
  • Heat dissipation is less of a problem.

Although this is a standard brake pad approach, don't confuse it with the heel stop:

  • The pad is actively pressed down, so all wheels stay on the road
  • The pad is centered under the foot, so you stand on it with full load, even with long frames
  • The pad is much larger (ca. 90 x 25 sqmm): less heating and longer lifetime.
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