Youll find on endurance bikes, the bottom bracket drop will be lower to add to stability, because youre lowering the centre of gravity, says Allen.Some of them we love and respect, like the fact that a water bottle is a water bottle in all walks of life except when on a bicycle.
Others can be infuriating and the inconsistencies within geometry charts definitely fall into the latter category. As bike fitter at Soigneur in London, Tim Allen puts it, geometry creates a bikes personality. Then, stack and reach arrived and the vast majority manufacturers have incorporated the standard. Reach is the horizontal distance from the centre of the bottom bracket to the middle of the head tube. Comparatively, the endurance ready S-Works Roubaix in the same size has a stack of 585mm and reach of 375mm. When Im looking at whether a frame geometry is going to fit with what I want for a client, theyre going to be the most important measurements. When it comes to the top tube the angle of the head tube and seat tube will have an impact if a bike has a 55cm top tube, and a 70 angle, and another has a 73.5 angle, effectively the reach on the 73.5 bike is going to be longer. A gravel bike or endurance bike thats got disc brakes and more tyre clearance will have a longer fork to cater for that, so the head tube is going to become shorter but it doesnt increase the drop, Allen comments. ![]() Stack is less easy to manipulate without making changes which can negatively affect handling. Trail is the distance between the tyres contact patch with the ground and where the head angle line hits the floor. Head angle is the angle at which a line would travel through from the head tube to the steering axis, to hit the floor. The smaller the fork trail number, the faster the bike is going to respond. The number is the relationship between the head tube angle, and the fork offset so how far the axle of the fork sits out from the crown of the fork. If you were to extend that fork trail, it will slow handling down and make the bike more stable. This is clear within the Specialized Venge charts where a size 49 has a trail of 63mm and a 52 cuts it down to 58mm, down to 52mm on a size 61cm. Its one of those standards thats never really changed, if you look at a small bike it will have quite a steep seat tube angle, 74-74.5 degrees. As the bike gets bigger that seat tube angle tends to get slacker, even down to as much as 72.5 or 72 degrees on the biggest bikes. On smaller bikes I tend to find it a lot better, at 74 degrees you can usually get the saddle very central on the rail. It can be represented in two parts: rear centre (from the centre of the rear wheel axle to the bottom bracket) and front centre (from the centre of the BB to the centre of the front wheel axle), though few brands provide this level of detail. If you stretch the two wheels out itll have a longer turning circle, which will feel more stable at lower speeds. As an example, a size 54 Specialized Roubaix has a wheelbase of 987mm vs 978 for a Venge. This standard has changed over time, with factors such as larger tyres playing a role. Partly to allow for plenty of clearance in corners, though you can manipulate this with shorter cranks.
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