Bike geometry 101 - HOW TO TRANSFORM YOUR RIDE

Mountain Bike Rider|Summer 2020

Following on from part one, where we showed how your bike’s geometry influences your body position and centre of gravity, we now look at how you can manipulate components and settings to improve your shape on the bike
Alan Muldoon

My journey into understanding and adjusting bike geometry began in earnest over two decades ago. At that time, I was trying to adapt a full-suspension trail bike into a downhill race machine. The goal was simple: make the bike slacker and lower, and at the same time add some much-needed progression to the coil-sprung suspension.

The first step was to make a scaled drawing of the frame, suspension linkage and the bike’s geometry. I then calculated where the shock needed to be mounted in order to achieve the numbers I so desperately desired. Just as I was about to drill a hole in the frame and reposition the shock, it dawned on me that if my calculations were out, the only thing I would achieve would be to seriously devalue my bike.

So instead of drilling the frame, I sold it and bought an Intense M1 SL – a frame with more holes in than I knew what to do with. Pretty much everything on that Intense was adjustable, much in the same way as brands employ test mules today to trial different geometry concepts without having to build unique prototypes for each new idea.

And while that old Intense taught me a lot about bike geometry, suspension and how adjusting both can impact ride quality, it was really just the beginning of a journey that I’m still on today. Now though, the tools at our disposal for tweaking geometry are much more sophisticated, so there’s really no need to break out the Black & Decker if you want to slacken your head angle or lower your BB height. My thinking on bike geometry has evolved too, so if you missed the Geometry 101 feature in the July issue, I would recommended reading it first as it will help you understand why you might want to change your bike’s geometry before we get to the how part.

In a nutshell, that feature looked at how bike geometry affects the weight distribution of the rider, so if you want to change that, adjusting the geometry is an easy way to go about it. It’s not the only way, however. How we position ourselves on the bike affects weight distribution too, and something as simple as the pedals you run can make a big difference – the straightforward act of dropping your heels to keep your feet firmly planted on flat pedals having a knock-on effect all the way up the kinetic chain.

So before taking a deep dive into all the main options for physically altering your bike, what better place to start our tweeker’s guide to mountain bike geometry than by taking a closer look at how simply changing your riding position impacts geometry and handling.

RIDER POSITION

By far the easiest and most cost-effective way to change the ride dynamics of your bike is to manipulate how you interact with it. Being conscious of your weight distribution when riding, and actively changing it, however, can be difficult. It’s like trying to change how you sit at your desk. Being told not to slouch in the chair is all well and good but it doesn’t do much to actually change your habits. Adjust the set-up of the chair, however, and we can really help cement better posture. It is no different with the riding position on your bike.

Let’s say you want to load the front tyre more to increase grip on flat corners. One way to do it would be to simply bend your arms more to help shift your weight over the front end and actively load the front tyre. But just like the seated posture example given above, that’s going to be a difficult change to instil, especially if you’ve been riding for any length of time. A more effective approach would be to simply lower your stem height by 5mm. And while that sounds like a tiny change, because it alters the position of the single heaviest component on the bike – you, the rider – you’ll be amazed at how big a difference it can make.

Rolling your handlebar forward or back is another simple way to alter weight distribution without having to change the stem length or height. If you find yourself too stretched out on descents and struggling to get off the back, rolling the handlebar towards you by a couple of degrees will automatically shift more of your weight to the rear of the bike and give you the wiggle room you need.

We discussed saddle position and seat angle at length in the first Geometry 101 feature, and while it has very little impact on the overall handling of the bike when it comes to seated climbing, fore-aft saddle position can be game-changing. One of the biggest mistakes we see taller riders make is slamming the saddle all of the way back on the head of the post to create more space in the cockpit. While this will improve the overall fit, it also places more of the rider’s weight over, and even behind, the rear wheel on really steep climbs. Altering your riding position has downstream effects on suspension too. So something as simple as sliding your saddle rearwards increases how much the suspension compresses under the rider’s weight; a compounding effect that generates a less optimum pedalling response. And let’s face it, no one, especially taller, heavier riders, needs to make climbing harder than it already is. So slam that saddle forward on the head of the seapost and start to gain the benefits of a more forward riding position on the climbs.

SUSPENSION SET-UP

When we look at bike geometry, we’re looking specifically at static numbers with the bike un-weighted. Plonk a rider into the mix and complexity increases by a factor of 10. Rider height, rider weight, body proportions, setup and riding style all impact the dynamic geometry. By eliminating all of that noise, static geometry actually becomes a useful way to compare bikes. It’s in the noise, however, that we can begin to manipulate the geometry.

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