Next in line are two 29er trail machines who happen to be good friends and just happen to be about the same size as myself. Both are similar in setup, and both riders have a similar style to myself. The second of the two is a long time friend going way back who lives in North Carolina. Similar riding terrain as ours here in the great state of New Hampshire but has it’s differences of which I won’t go into here. If you’ve ridden in New England and in North Carolina, there are distinct differences and likenesses in the terrain. The second rider, Yuri from North Carolina, (We met as undergrads sharing an affinity and passion for all things two wheels at PSU) is pretty excited about his bike and has started two threads on MTBR. One on the build, of which you can find here, and a second on some tough parts decisions found here.
In the discussion of the bikes setup in the first thread, the question of “Trail” came up. I believe it was in reference to the specific fork choice (Fox 32 FLOAT 29 / 100 Fit Terralogic – I know, that’s a long one… I think I have the right order) and HT angle (70.5*). All of Fox’s 2012 forks to my knowledge have a rake of 44mm in accordance with the most recent 2011 specs (keep in mind that on the Fox 2012 34 29″ Fork Specification the Rake Spec states 51mm +/-2.5mm , which I believe is G2 – It does not appear that they have the 100mm 29 Spec’s for 2012 up yet which I cannot confirm to be indeed 44mm. 2011 Fox F29″ 100mm forks were 44mm of rake clearly indicated on their specification sheets. Rock Shox clearly indicates 46mm of rake for their 2012 Reba 29″ fork family here). Couple that with the HT angle, Wheel size and tire choice gives you a specific Trail number. For more on the background and discussion of trail, here is a great list of trail explained:
1. Bicycle Steering Trail via Mechanical Hacks
2. Trail and its Effects via Spectrum Cycles
3. Geometry of Bike Handling via Calfee Design
4. And my personal favorite, as the builders personality really shines through: Trail, Wheelbase and Considerations via Rivendell
You should note that all of these articles focus on trail largely as it relates to road bikes / rando setups and IMO where most of the knowledge and theory about trail traditionally originates in the cycling world. I believe there is a lot of fresh input coming from the motorcycle world that crosses over from suspension guru’s. But the reader must note that Mountain bikes are very different than road bikes. On the road, the cyclist is set in his/her position and their center of gravity (CG) is for all intense purposes locked with the exception of say leaning through a turn or getting out of the saddle either uphill, sprinting, etc (I’m generalizing a bit).
However, in the case of a mountain bike, the rider is very dynamic and his/her CG really is all over the place. Sitting, standing, climbing, descending, turning, balancing, sometimes hopping ‘trialsy’ moves, bunny hopping, slow speed tech, burly dh runs, etc. And to completely confound the equation, let us add the layer of a front suspension fork (I’m not even going to get into full suspension). As the fork cycles through its travel, it effects head tube angle, bb height, Center of gravity, wheelbase and yes as a result since HT angle is ever changing as the fork goes through it’s travel: Trail.
I’m going to state something very delicately, taking great care and thoughtful consideration. I cringe as I’m typing these words right now… My own opinion on Trail: it gets a lot of emphasis, maybe too much emphasis on a mountain bike, especially when it’s a moving target and there is no purported ideal number. I believe there are set ranges for a specific type of bike, but to positively state that there is one number that rules them all is just not true. As you will read, in my own opinion, there are several factors that determine how a bike rides or rather “feels” and the reader can interpret this as bicycle handling. And when I state my own opinion, it’s as of today, March 24, 2012. This may change with added input, experience, rider input and developing styles of riding. Until then, let’s roll up our sleeves and get started.
My own opinion on geometry is a bit more like this: First and foremost is rider positioning relative to the bike and the ground. Since the ground or rather terrain is so demanding and dynamic as it relates to a mountain bike, I believe there are several factors that must be considered and thus must be balanced to deliver a proper handling mountain bike. This also goes for region: East Coast vs West Coast. I’ve found that it is important to question those who state “it must be so”. So what do I consider in the equation of handling and geometry? Rider physicality, rider weight, position, weight distribution and center of gravity are what begin the recipe for me.
What dictates this is:
0. Design Criteria: Wheel Size, Fork choice, and bike setup to name a few in addition to those I just stated above.
1. Bottom bracket height – this places the riders CG higher or lower and begins to establish the “feel” of the bike, which to me equates to On the Bike vs IN the Bike.
2. Head tube angle – This is tied to BB Height which effects the riders CG which effects the quickness of the bikes steering. Lowering CG quickens steering.
3. Fork Rake – this is also tied to HT angle and BB Height IMO – BUT 9 times out of 10 is self determined by the fork manufacturer so unless I’m building a fork, it’s kind of a standard that is plugged in.
4. ST Angle – this positions rider weight distribution between bb and rear dropout but also is a determinate of rear tire clearance relative to wheel size which is a consideration depending on the riders setup.
5. HT Length – I think this one is an important one because it helps to position the cyclist and their weight distribution. Too stretched out you’re putting too much weight on the bars, too upright and you’re commuting down the trail.
6. Rider Cockpit: Personally, I get this from several measurements which are in no particular order: reach, inseam, height, weight, BB center to center of saddle/seat post interface, height of saddle in cross section, a measurement from center of saddle/seat post interface to the center of the stem/fork steerer, stem length, bar height/reach, crank length.
7. Rider cockpit determines TT length. Rider Cockpit is determined by the cyclists physicality, measurements taken from existing setups, complaints, aches, pains, the old “This hurts after X time in the saddle”, etc. and thoughtful consideration of how the rider is placed on the bike now vs how I want them positioned “IN” the bike so they experience the ride how I intend it. I’m building you a bike I want you to ride and take away an intended experience which I believe to be my own personal signature. Call it style for lack of a better term. You’re coming to me because you’ve connected with my bikes and relate to my ride philosophy.
8. Standover – as much as possible without causing a detriment to the integrity of the frames strength.
9. Wheelbase is a result of all of the above and IMO is not a determining factor if you’ve got your HT angle, BB height, Chainstay Length and rider cockpit pinned. It is what it is relative to that specific cyclists physicality with relation to the bike fit IMO. Or should I state ATMO in this case?
10. Head tube angle and fork rake determine mechanical trail & ground trail (two different numbers because they’re determined in different manners).
BUT BB height effects steering quickness and is also determined by a set of compromises and balancing clearance for your given terrain. I think on a 29er, the ideal east coast BB height lies somewhere between 12″ and 12.375″ – but the reader should note that is a moving target. I personally think that 12.5″ on the east coast is a bit too high and you can train yourself with thoughtful pedal cadence to adapt to a lower bottom bracket height between the two numbers I state above and minimize pedal strikes. The benefits of a lower rider CG helps to quicken steering, and lends to an “IN The Bike” Feel. Having the rider positioned up and a bit back also helps with this “feel”. For example, an early proto I built had a 11.75″ BB Height, and although pedal strike was troublesome at times and I really had to think about cadence during the rides, that bike literally devoured turns when you leaned the bike hard. It became very evident from the ensuing rides that BB Height was something that is of importance in addition to other considerations stated above and below.
With regards to HT angle, a steeper angle makes a mountain bike kind of touchy, (and decreases the trail number) here on the east coast and too slack of a HT angle (increases the trail number) can make it feel sluggish. I think a combination of HT angle and BB height really, in my own opinion if rake is a set number by a particular fork, determine that bike “feel” when you’re positioning the riders CG. I don’t think there is one determining factor to how a bike feels but rather several and it all starts with the rider physicality, their weight, the terrain they ride and their chosen wheelsize. The study of “Morphology” comes to mind with this and like a good recipe, you need just the right ingredients and balancing relationships between many factors.
Then again I am a custom builder, and so I must consider a list of determining factors because each rider is very different.
Let the fur fly…