THE CRAZY WORLD OF MTB GEOMETRIES (PART 1)
2 months ago
When a new model of mountain bike is released, one of the first topics discussed is the various angles and measurements of the bike. People will make all kinds of predictions and observations about a new bike purely based on these numbers and though many of these numbers are important, they do not paint the complete picture. I have heard arguments about different bikes based only on the geometry figures that manufacturers publish on their websites, with neither party having ever even ridden the bikes in question … crazy! One thing is for sure though, the various measurements and angles of a bike greatly affect the way they ride, their intended purpose and the characteristics of the brand’s design philosophy, and though I do not believe that good geometry numbers mean a good bike, I do believe that more often than not, bad numbers are usually indicative of a bad bike.
So what do all the numbers mean? Before I begin, it is important to understand that different mountain bike genres need different frame angles and measurements. Making a mountain bike a better, faster, more efficient climber (such as a marathon bike), will mean compromising on its stability and ability going down steep, rough descents. The fastest way to define genres is by looking at the amount of suspension travel on a bike: XC/Marathon 90mm – 110mm; Light Trail/All-Rounder @ 120mm – 130mm; Trail @ 140mm – 150mm; Enduro/All-Mountain @ 160mm – 180mm; DH @ 200mm – 220mm travel.
With that in mind, let’s first talk about the two important angles that riders debate the most on bikes. The head-angle of the bike is the angle at which the fork would intersect at level ground. Marathon and XC bikes have less travel and are designed with steeper head-angles (around 69 degrees) than Enduro bikes with more travel and slacker head-angles (around 65 degrees). Over the past couple of years, head-angles have been made slacker across the genres. For example, 71 degrees was the norm for a Marathon bike three years ago, but new models are being released with slacker head-angles around 69 degrees. The same goes for all the other genres where head-angles have been slacked a couple of degrees. What this means is that the bikes are more stable at higher speeds and down steeper hills. But it has negative consequences as well, such as increasing the bikes wheelbase, making the bike climb a bit worse and making the steering a bit lighter and less direct.
And so, with slacker head-angles we have seen steeper seat-angles introduced to help alleviate some of the negatives of slacker head-angles. This is the second very important angle in the geometry figures and it describes the angle at which your seat-post is leaned back in comparison to the bottom bracket (BB) and the ground. It can get complicated though with actual seat-angle vs virtual seat-angle, because most modern MTB frame designs have a bend in their seat tube and so the angle of the seat-post changes as the seat-post is lowered or raised when measured from the BB. As a result, the virtual seat-angle is the measurement in the geometry diagram that we compare and debate across bike models. A steeper seat-angle puts the rider in a more forward position which improves the climbing on the bike. It also puts the riders weight more over the front of the bike which is better for cornering and overall bike control (dropper posts allow for this forward position and stop you from feeling like you are going to go over the handlebars down a steep descent). Steeper seat-angles also have a bonus benefit of allowing bike designers to squeeze in a little more rear wheel travel at the top of the travel arc.
Next up are the important lengths on the geo’ chart. First up is the chainstay length, which is an easy measurement from the BB to the wheel axle. The average chainstay length on a 29er is 440mm. Less would be considered short and more would be considered long, but that is not the whole story. Marathon bikes and Pedal-Assist bikes are built to climb well and so they have longer chain-stays (helping keep the front wheel on the ground during steep climbs), whereas longer travel trail and enduro bikes aim to have shorter chainstays for nimble cornering and keeping the front wheel poppy and manual friendly. The conundrum is that the more travel you want, the longer you need the chainstays to be to facilitate the bigger arc in the wheel travel without raising the BB too high. To get around this and to keep the chainstay numbers low and the travel numbers high, frame designers use clever pivot placement and advanced rear suspension setups such as horst-link (FSR), VPP, DW-link (Maestro) and R3ACT systems, which allow for the chainstay to grow in the beginning of the travel, before the wheel begins to arc back towards the saddle. The placement of pivots is a whole new debate and arguably the most important (and difficult) feature when designing a full suspension frame, yet very little in the geometry numbers describes the full characteristics of the suspension design (Part II of this article delves deeper into this).
The second important length, and the measurement that is today’s number one debate between frame designers, is the reach. It is the vertical distance between the BB and head-tube, and it changes for different frame sizes. It is the most important measurement when comparing frame sizes between different manufacturers, and the trend has been for the reach numbers to grow longer and longer for the same frame size. Frame designers have been employing a greater reach on their new bikes to facilitate the use of shorter stems (and wider handle-bars) and to bring the riders weight further forward (an attacking position when both climbing and descending). Mondraker for example, have extremely long reach numbers on their frames encouraging direct mount 0mm stems. Giant and Santa Cruz have also increased their reach numbers drastically and if you compared a medium size frame from one of the current models, to a large number of their previous models you’d get similar reach numbers. But longer is not always better J and there are definitely ideal reach numbers for different riding styles, the rider’s unique physical dimensions and the amount of suspension travel on the bike.
That’s it for this month, but we are far from finished with this discussion. Look out for next month’s copy of Full Sus where we will talk about the last few important measurements (BB height, Stack, and Seat-Tube Length), but more importantly we will dive into the shortfalls of the traditional geometry numbers described above, and I’ll tell you about the real numbers we should all be asking the frame designers about.