frame mounted front bike rack

One of the things I really like about bicycles is that they really push one's understanding of dynamics. For an automobile, putting a heavy load ahead of the front axle, or behind the rear axle, tends to reduce stability. The cure for that would be to distribute the load evenly, front to back. So a heavy load (eg an engine) in the front can be countered with a load in the back, acting like a seesaw.

But that same trick doesn't guarantee success for loading a bicycle, because bikes need front-to-back stability in addition to roll stability. This arises because when turning, bikes have to bank to one side while also turning. To use aircraft terminology, there is a coupling between the roll and yaw axis.

Ships use a keel to independently stabilize the roll axis, but bikes can't just stick a keel into the ground, so must rely on tire resistance, which is coupled to the steering direction. What this means is that for some ways of loading a bicycle, it may genuinely end up being unrideable at any speed, in any direction.

The simplistic rules for land vehicle stability are to keep loads within the wheelbase, and preferring closeness to the axle's height, with the peak height at the exact middle of the wheelbase. For a standard bicycle, the rider is the biggest load and is positioned optimally per these rules; history shows that some really tall bikes work just fine.

Not bad! Thanks

It relates to the headtube angle, and contact patch where the tire meets the ground, aka trail. Bikes that are designed to carry weight on the front have a very different geometry. They typically have a tall and steep headtube without much tilt back towards the rider. They also typically have a longer wheelbase, aka distance between the contact patch of the front and rear tires. The chainstays also tend to be longer. All of these changes make the ride feel sluggish without weight, and are the key factors of handling and steering. In other words, a road race bike versus a road touring bike geometry, but all bicycles have these same factors. Practically speaking, it is the difference between leaning into turns to steer and turning with the handlebars more, which also impacts high speed stability versus low speed stability.

As a former Buyer for a chain of shops. I only need to see a bike's headtube to tell what it was designed for and its size. The only exception to this rule is extremely compact mountain and triathlon bikes where I need a few other visual clues due to a lack of substantial variation in the headtube across sizes.

Let me illustrate the point to hopefully make the concepts more tangible.

I learned the hard way about bicycle weight and handling properties. When I got my first job in a bike shop I rode 33 miles each way every day. I tried racks and stuff on a road bike, and a touring bike with panniers. I was fastest and safest by riding a backpack 2 days a week on a road race bike with strategic clothes caching to keep that trek around 1.5 hours each way.

A seatpost mounted bag rack on a road race bike, weighed down, causes the bike to handle badly. You might think, like I did in my twenties, that the weight will act on the seatpost no different than if wearing a backpack, but that is not the case. Think of it like this, if you need to brake hard and fast you get off the saddle and hang your weight off the back of the bike as far back and low as possible while pulling both brakes. Your feet are still on the pedals, but your center of gravity has shifted rearwards to make you stop faster, so you are already naturally shifting weight to change handling. The center of gravity impacts how the bike self corrects from leaning forces and does not require input to maintain a straight line when moving. If the weight is shifted rearward overall, the bike will feel like your inputs are sluggish and lagging behind like there is a disconnect between input and motion. If the weight is overall forward, leaning will feel very unstable and steering will feel like a very small input causes too much motion; it will feel very unstable.

If front panniers are added to a bike that is designed for them, they are designed to treat the front fork like its own independent geometry system. The goal is to distribute the weight on both sides of the trail and steering center line (like a line projected down the middle of the head tube center to the ground). The bike will have a smaller distance between these two points - trail and steering center, and weight should be distributed evenly between these. A setup like this will feel like a heavier front end, but stable and predictable with both leaning and steering inputs. Any solid obstruction around the front wheel will make crosswind forces impact the steering and this can be very disconcerting at first. It is a very pronounced phenomenon on road race bikes with large front carbon wheels due to the low weight and surface area. Even on a touring bike, front panniers should be a last resort only used when absolutely required, due to the negative effects.

That is the basic gist of geometry and how loading impacts handling dynamics.

I don't like any extra weight on the front axis, too. It makes steering less direct and more strenuous and destabilises the bike by a lot. Once rode a bike with a front basket attached to the frame (bike sharing), and that felt much better as the front wheel handled lightly as usual.

I cant find any sellers online for that one!