The abbreviated guide to bicycle dynamics
Head tube angle/aka Rake
But the forks are only part of the equation when it comes to creating trail and ultimately stability. Head angle is used to create more or less trail on a bike. The steeper the head angle, the smaller the trail and ultimately, more twitchy the bike. The slacker the head angle, the greater the trail and more stable. Modern long travel forks tend to have more offset than their shorter travel cross country forks, thus when combined with slacker head angles, create quite slow steering but stable bikes, ideal for heading in the direction of down, fast. Given the same set of forks, you can change the steering and handling characteristics of a bike by altering the rake. This is why we are staring to see adjustable head angles on mountain bikes now, as riders want to be able to tune the way the bike handles based on personal preference or trail conditions. Into all of the above, factors like changing your position on the bike, playing with geometry, tyre sizes etc. etc. can all affect the way the bike steers and stabillizes. Regardless, the basic physics forces still apply and are common to all types and sizes of bikes.
Wheelbase contributes directly to both directional stability and cornering agility. The longer the wheelbase, the more stable a bike is, while the shorter it is, the quicker it will turn into corners but less stable. Cornering reaction slows for a longer wheelbase as the front wheel needs to be turned further to initiate the turn, hence require more rider input. This is due to the fact that a longer wheelbase creates a longer tangent to the arc of the corner. For this same reason, a deflection, such as a bump, has less effect on the straight line stability as, in simple terms, reaction time is slowed. Another attribute of wheelbase is stability under weight transfer forces such as braking and climbing. A longer wheelbase distributes the center of gravity between the front and rear wheels more effectively (ie. evenly), as for the given height of the center of gravity, the less the weight transfer from front to rear. Of course, on a bicycle, the wheelbase equation is a little more complex and is split between two sub measurements, the chainstay to bottom bracket and bottom bracket to front wheel…
Chainstay length is perhaps the most important component of the wheelbase length. Unlike a motorcycle which has a large lump of metal… and ‘stuff’ called the engine, which weighs a good deal, as well as a rider, a bicycle has a single mass – the rider who is also the engine. Correct biomechanics tell us where the rider should generally be positioned for good transfer of the power to the crank and that happens to be behind the bottom bracket (when measured vertically). As such, the center of gravity (COG) on a bicycle automatically has a rearward bias which pushes it towards the chainstays. The direct result of this is that chainstay length is a critical measurement in achieving a good blend of stability, agility and climbing ability. It is also the one geometry measurement that remains constant between different sizes. With the rearward bias of the COG, if the chainstays are too short, the bike will ‘pitch’ to the rear, or lift, on steep climbs as the weight transfer is ‘pushed’ to the rear* on an incline, pitch heavily forward under braking but be very nimble through corners (this is the common trait of the original San Andreas, which has relatively short stays for the travel the frame can allow). If on the other hand the stays are too long, then the bike will climb and brake ‘flat’ yet be sluggish through corners. Too long or too short will also contribute to straight line stability accordingly as previously mentioned. *We are not accounting for additional affects such as chain climbing which is an effect of shorter chainstays and pulls the front of the bike up under power ie. is prone to wheelies.
Bottom Bracket to Front wheel
Bicycles come in different sizes to accommodate different sized riders and the two measurements that change accordingly are the seat tube, which raises or lowers the rider and the top tube, which allows for a comfortable reach for the rider to the bars. The bottom bracket (BB) to front wheel (FW) distance is the result of the combined lengths of both the seat and top tubes. As the BB-FW distance varies with the different sizes, it is an inherently compromised measurement when it comes to creating a bike’s geometry, as for each different size the COG shifts slightly and each size will behave a little differently. In general, if the measurement is too short, it will will place more of the rider’s weight to the rear of the bike, pushing the COG towards the stays, enhancing front wheel lift in climbs; while if it’s too long, the rider might be uncomfortable and feel incapable of of riding the bike effectively, even though it will create a better climbing and more stable bike.