Choosing A Bike
Handling and Ride Characteristics
We touched on several aspects of handling and ride characteristics in Part II of this series. Specifically, what makes a gravel bike a gravel bike: tire clearance, longer wheelbase, slackened steering, increased trail compared to a road bike, lower gearing, and often higher stack and less reach. All of these attributes exist on a spectrum. Most people reading this will already have at least one bike. Think about how that bike handles. Is there anything you like or don't like about it? For example does the bike feel unstable on uneven ground? Do you experience too much "wheel flop" heading into high-speed turns on the road? If you know the geometry of your current bike you can compare it to the sizing charts of new bikes you are considering and make predictions on how it will handle. Back in the day, frame size was mostly done by eye-balling the seat tube length - and many bikes still have their size listed in sizing charts by this convention. However bikes are no longer consistently built with seat tube and (horizontal) top-tube of the same length, so a frame's "size" doesn't quite mean as much as it used to. So these days I first look at standover height, stack and reach; then I can determine which numbered frame size I probably need (50, 52, etc.… or small, medium, large, etc…). Stack is the vertical distance from center of the bottom bracket (BB) shell to the centertop of the headtube; reach is the horizontal distance between the same two points (see Figure below). Using the bottom bracket as the datum for frame size allows the customer to choose their fit across different brands, since it doesn't matter if the top tube is horizontal or sloped, whereas measured seat tube length can vary depending on the top tube slope. The stack and reach tells me what kind of position I can achieve depending on the stem, saddle, and handlebar options I chose. Stack/reach will also determine if the bike will offer a more relaxed or a more aggressive type of fit. A few manufacturers will also list the stack & rack measured to the center of the handlebar clamp - then you can really compare the position vs your regular bike. So from these numbers I'll know which size of a particular frame will most likely fit. Then I look at the rest of the geometry chart for that size frame. I will mentally compare the wheelbase, seat angle, head tube angle, and amount of trail to bikes I already have and make predictions on how that bike might handle. Looking at two of my own bikes, as an example, my Moots RSL Disc road bike has a trail of 52mm and a wheelbase of 979mm while my Otso Fenrir adventure bike has a wheelbase of 1120 mm and a trail of 88mm. Even though both bikes fit me well, these numbers would seem to indicate that the handling and steering characteristics are wildly different…and that is indeed the case. Just knowing the trail and wheelbase measurements, I did not have to test-ride either of these to know how they would handle.
Above: Stack and Reach are a better indicator of how the bike will fit than the listed size, which used to be the length of the seat tube. Many modern bikes have a down-sloping top tube and so the length of the seat tube is less relevant.