Short Stems and Progressive Geometry Explained
When we talk about ‘progressive’ geometry, it means moving further away from all-road and cyclocross bikes, where the gravel sector started, and learning from mountain bikes, which have unlocked more capability, confidence and (most importantly) fun by exploring slacker geometry.
Let’s clear up one thing from the start – it isn’t about the stem. The goal is to lengthen the front of the bike for added stability over rough and/or steep terrain and that requires a shorter stem to preserve the reach to the handlebar. A shorter stem does create a smaller arc for the bar and your hands as you steer the bike, which would quicken the steering slightly, but this is already offset by the wider handlebars fitted to gravel bikes.
The long reach fit concept first appeared in mountain biking. Gnarlier and/or steeper terrain requires a longer front-centre (BB to front axle distance) so that your weight is further behind the front axle and the bike is better able to roll over rocks, bumps etc. The effect is to reduce the forward tipping caused by hitting a rock or bump.
It’s easy to picture this force when using extreme examples. First, picture a unicycle, on which the rider sits directly above the front axle. Even a very small bump will cause the rider to tip forwards over the wheel. Now imagine a recumbent bike, with the rider laying reclined between the wheels, barely above the front axle – it’s almost impossible to cause the rider and bike to tip forward over the front wheel. A recumbent obviously isn’t an effective solution for riding gravel, so it’s necessary to find a solution that gives enough of this stabilising effect without compromising the bike.
Extending the front centre can be achieved in two ways: 1) a slacker head angle to push the axle away; 2) a longer toptube that moves the whole front of the bike further away.
Each has its trade-offs. Changing the head angle directly and profoundly affects the steering characteristics. Lengthening the toptube changes the fit and also the handling.
Gravity mountain bikes have adopted both, going as far as using vertical seatposts, zero length direct-fit stems, and fork angles so slack that they look as if a headtube weld has failed.
For a gravel bike, no such extremities are required – we’re looking for nuanced changes – and the benefits always have to be balanced against the trade-offs, so we use a longer toptube and slacker steering geometry, but neither one in extremis. We know that it’s possible to go too far with these ideas and we firmly believe that we have found the sweet spot.
The result is that the Assassin is greatly more capable and confidence-inspiring on rough terrain, while still climbing like one of our road bikes and allowing you to achieve your optimum position. Our prototypes proved all of this over and over during testing on every type of terrain imaginable, chasing KOMs, playing in the singletrack, and loaded up for all-day epic rides.
