Generally the BB-FW distance is a by product of determining the optimum top tube for the given use for the bike. A jump bike will obviously have a very short top tube (TT), due to the style of riding and need to lift the front wheel easily, while an XC bike will have a longer TT to create an effective riding position and overall stability under a number of applications. Ideally, for good all round performance the top tube will be sized to provide comfort in the ‘cockpit’ for the rider, while placing just enough weight over the front wheel to allow for stability while climbing. One can see the complexity of achieving the ideal balance, especially when you consider that a mountain bike might be ridden through a range of very different terrain that all demand something different in terms of handling characteristics. What’s more, the variation in the BB-FW measurement illustrates the need for the chainstays to be ‘dialled’ in order to create a predictable ride characteristic amongst a range of sizes.
Effects of braking
Reference the following for more details: Motorcycle Chassis Design: Reference pages 101-104 How and why: Motorcycle Design and Terminology: Chapter 7 Pg 88-92
When it comes to the world of mountain bikes, the effect of braking is one of the most deliberately confused dynamics, often used to justify suspension designs. There are some simple basics though that can not changed, denied or overcome. The braking equation (way simplified).
Front wheel forces
The division of braking power is generally attributed as a 70/30 split between front and rear. This means under braking conditions the majority of braking forces will go through the front wheel and this means that the weight shift of a braking bike is heavily forward bias.
This weight transference causes several reactions:
1. The fork dives as the momentum of the weight transfer is absorbed through the compression of the forks, causing the bike to pitch forward. Interestingly, this compression of the forks also steepens the head angle and reduces trail, making the bike more unstable under hard braking – just what you want coming into a hard, rough corner.
2. As the weight shifts forward, the rear suspension ‘unloads’ and automatically extends through the loss of weight derived compression. This also has a side effect of aiding the pitching forward of the bike, further enhancing the weight transference, which ultimately unloads the rear wheel and causing a loss of traction.
3. Through the lifting of the rear and the compression of the forks, the bike actually rotates and squats, lowering the center of gravity.
These three actions are the basic physics of braking and can not be changed without re-writting the laws of physics. Despite claims to the contrary, or the importance placed on rear suspension systems, the single most important factor to the overall performance of a bike under braking come down to the tuning of the fork as the fork’s performance is paramount to the countering and balancing the various transfers that occur while braking.
A well tuned fork will aid the bike in remaining more level while still smoothing the trail conditions to allow for an effective cornering action.