First of all: a bicycle is not a vehicle. You don’t drive it, you ride it. Please keep this in mind throughout the article. With CNC machining it would be easy to make size-dependent chainstays but after we have done the research and crunched the numbers there is no reason why. Before we dive into the numbers I would like you to ask this question to yourself first: Are you going to buy two sizes of the same bike? We know that some companies have started to offer size-dependent chainstays as a feature. We also have size-dependent chainstay in our hardtails but we have done our calculations and tests and we think that size-dependent chainstays are not necessary. Especially not on full suspension bikes.
It’s a fact that the size depending chainstay length is not a feature. The rider positioning is changed by reach with every size in order to move the rider mass rearwards as the rider’s physical limits are different from each other. Shorter riders have a lower center of gravity compared to tall riders because shorter people have shorter legs and torso. The human’s center of gravity at riding position is on bellybutton. When on riding position, the hands of the shorter riders are relatively higher than taller riders because the fork AC does not change between sizes. These reasons altogether compensate for the need for the size-dependent chainstay lengths. Therefore short people can manual as easy as tall riders with their sized bike. What comes for weight distribution it’s not a good thing to shorten the full-suspension bike’s rear center. Shorter riders have a shorter reach and more weight is distributed to the front. What comes to cornering, the shorter riders have less mass on the front because their center of gravity is relatively lower and their hands are relatively higher. Therefore, shortening the front center balances the weight distribution because more weight on shorter people is distributed on the bb. If we would shorten the rear center, we would put more weight to the rear and lose more traction on the front. Also, we would at the same time change the leverage ratio to the less short (light) rider-friendly direction. There have been a lot of bikes in the past that have had an adjustable chainstay length but yet still nobody seems to use them very often. The reason why people don’t use the feature is that the bike’s kinematics are also dependent on the chainstay lengths and the feeling that they get from the change can and most likely will come from the suspension.
In raw numbers without taking into count the height of the rider and without changing the rear stay length, the rear axle load difference on Pole bikes is rough 0,63kg when moving from on M to S with a 70kg rider. The weight difference from XL to S would be 1,6kg. This weight difference is easily controlled with handlebar height, braking, and the rider stance. For lighter riders and heavier riders, we need to note that they are using the same leverage curve on shocks. If we would make the rear center shorten there would be less leverage for the shorter rider but more weight. To make it more complicated I need to say that less leverage ratio will make the shaft speed higher and the damping harder if we don’t change the shock tune as well for the lighter rider. Also, there would be a difference in the rebound.
If you are thinking of how are you going to compensate for the weight balance, there is a very easy method of how big a difference you can make by just bending your knees. I encourage everyone to get a scale and put it next to a table. Lean on the table like you would be riding a mountain bike. Bend your knees and watch how the scale changes. That’s how you control your grip on a mountain bike because a mountain bike is not a vehicle. You are the mass that needs to be floated around the corners. You are not a pilot or a driver. You are a RIDER.
I know size-dependent chainstay lengths sound like common sense but it’s just one of those things that is not. Testing this feature would be impossible because the test group could not ride all the sizes because the reach is the biggest thing that they would notice. Let’s just say that if we should change the chainstay length by size, should we change the wheel size at the same time? I say that changing chainstays with size with full suspension is as reasonable as changing wheel size by size. And don’t get me started with the mullets 😉