Mountain bikes have come a long way since the dawn of our little sport. Long gone are the days of winged lizards and beaked fish that roamed the earth when the sport was young (yes, the bike on the right has two forks. Circa 1995), but that doesn't mean the evolution of the MTB as stopped by any means. MTBs are a really difficult design problem, they need to be adept at high and low speed, steep climbing and steeper descending; nimble on hardpack, stable on bumpy stuff, grippy in mud, and these days aren't given a second look if they're more than 22 lbs. Thinking about designing one makes me want to sing this (don't ask me how I made that connection, but imagine you're singing it to your third not so good prototype design as you're hauling it out to the trash and it totally works. Go ahead try it, here's the music)
Anyway, I'm in the market for a new bike. I'm convinced the top tube on my '06 Santa Cruz Superlight is too short for my 6' 4" frame, and a quick look at the geometries of the bikes on the market right now revels the fact that bike manufacturers are doing some very different things now than they were when my SL (generation 2 of 3) was the hot new thing. To illustrate what I mean, here are some graphs of the changes in key geometry measurements of 7 bikes from 4 manufacturers over the last 10 years or so (all bikes size XL or equivalent.. I scaled up the old Blur to an XL to compare with the new Blur XC carbon. And yes, I know it's a limited sample, but this is only a blog, k?):
There's has been a clear trend in slackening of head tubes across all the bikes surveyed. In 2009 every single model is at an all-time low in terms of this measurement. Somewhat relatedly, wheelbases have also been increasing across all these bikes.
It's also worth noting how short top tubes were in the early days of MTB, as illustrated by the 1997 HKEK, and how closely most manufacturers have converged on this parameter. We have come along way indeed, thogugh the Yeti seems to be clinging to the old ways of shorter top tubes.
There doesn't seem to be any significant trend in seat angles, some companies seem to be going forward, some back, some nowhere, though a common design decision of late has been to push out the top tube, slack the head tube and steepen the seat angle as seen with the Epic, FSR, and Superlight above.
ST angle increase or no, every single bike above has achieved a net rearward movement of the rider's center of mass with respect to the center of the wheelbase, when combined with a slacker HT angle, that means that all will likely be more confident descenders. My hypothesis about the steepened STs is that it's a compromise to improve climbing efficiency on steep grades and power on the flats at the expense of rearward positiong for downhills. As a die-hard seated climber and endurance racer with big long femurs, my initial reaction to steeper ST is "bleghhhh", but one never knows, this could be some sort of silver bullet...
Another effect of a slackened head tube is increased Mechanical Trail (as defined by David Gordon Wilson's "Bicycle Science"), which increases the amount the bike's center of mass drops and shifts to the inside while turning. In my experience, this tends to make the bike feel livelier under the rider for a fixed sharpness of turn (though you need to turn the bars farther than with a steep HT angle), making it easier to move the bike out of plane with one's body.
In all, I'm in agreement with most of the evolutionary changes seen above, though I'm still skeptical of the more forward [wrt the pedals] riding position created by steeper ST angles. With XC courses becoming more technical every year and light materials allowing for bigger travel and bigger bikes for less weight, shifting XC bikes in a plusher more stable direction makes a lot of sense.
Over the next couple of weeks I'm going to be riding some new bikes, and will be back with more blather on this thread...
[Edit: A note on why I chose the bikes I did. As a Santa Cruz Rider I'm particularly well acquainted with their history. Eric rides a Yeti. Specialized is a recognized standard in MTB, as was GF some years ago (I also used to ride a HKEK). Both these companies also had awesome archives on their websites. I excluded Trek, despite their being a huge company, because IMO they couldn't make a mountain bike that rode well until the advent of ABP and they didn't have geometry in their archive. Then I got tired before adding more.]