June 4, 2013

Reading Time: 18 minutes

The abbreviated guide to bicycle dynamics

Rear wheel forces

Reference the following for more details: Motorcycle Chassis Design: Reference pages 99-100 How and why: Motorcycle Design and Terminology: Chapter 5 Pg 79-82

Rear wheel braking applies a different force to the bike, not though to the front but rear suspension.

As disc brakes on a mountain bike are connected to the ‘rear triangle’, when they are applied the effect will be the same across the board regardless of the type of rear suspension employed – ie. when applied, the rear brake force causes a degree of suspension compression. This compression occurs because the braking force is opposite to the force of the applied traction. The amount of the force applied is proportional to the angle created between the pivot point (actual or virtual) and the point of traction.

As with the front forces, this effect is a direct cause of physics and can not be sufficiently overcome without sacrificing performance in some other area, namely the performance of the rear suspension.

Contrary to some claims, the application of the rear brake does not lock out the suspension, as this is physically impossible. Possibly this myth is derived from the physics of rear wheel braking itself. As discussed, the application of the rear brake can cause some compression of the rear suspension. It is plausible that if the rear shock has not been tuned to the rider and the bike, it could be undersprung, meaning that braking compression uses more of the shock’s stroke than would be normally deemed acceptable, hence making the rear suspension far less pliant over rough ground while under braking and ultimately giving the feeling that the rear suspension has been ‘locked out’. Rear brake suspension compression, when used by an experienced rider can actually aid in taking a bike through a corner in a more level manner. With the bike pitching forward under the front wheel braking forces, the rider can use an amount of rear brake to compress the rear suspension and help level the bike through the turn. So… As you can see, a bike is by no means a simple beast and there are many different equations that come into play to create the ideal handling bike. What though can not be overcome or denied, no matter how smart people might think they are (or the marketeers think they might be), are the core physics of a suspended bike in motion. No matter the size, weight, simplicity or complexity of a frame or a suspension system, physics dictate how a bike will react for a given circumstance. Attempting to overcome these physics will create a compromise, so ultimately any mountain bike that claims to over come this, or that, in this situation or that scenario, will be compromised in another. So the next time you hear some mountain bike ‘folklore’ or claim about how something is supposed to work, or not, think about the physics, because physics is a fact Jack and probably the only fact there is. Next: Construction

After 20 odd years working in and around the cycling industry, for myself and others, I decided riding bikes was more fun than working with them. Over that time though, I wrote (and some wrote for me) a lot of stuff about bikes, on blogs and the like. Some was good, some, well... not so much. Rather than loose it all when I shut everything down once and for all, I have kept some of my favourite, and more popular pieces here for... prosperity?

I also am working on new pieces as well...


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