When building a bicycle, there are numerous factors to consider when determining how stable the end product will be. Each contributes, to some degree, to the handling of the bike. The problem here is that you will hear no end of debate as to which of these characteristics is the most important factor to consider and how much each actually impact the ride.
Because stability is highly desirable in some bikes, and less so in others, I’ve come to the conclusion that what bikes need are something like the Quarterback Rating in (American) football. For those that don’t follow the sport, QB Rating is a number that is derived from a formula using five performance indicators as variables. The end result of this formula is a number in the range of 0 to 158.3. The higher the number, the better. What I’d like to do is create a Stability Rating formula that takes into account some of the key factors in a bike’s design so that it becomes possible to have a single raw number that makes one-to-one comparison of bicycle stability.
To that end, I’m going to propose that the following variables be considered in the rating system.
This is one of the areas I’m having trouble — there’s any number of variables here that could be used. Trail could be used, which is a function of wheel diameter, fork rake, and head tube angle. Or we could account for those variables separately in the final formula. Personally, I think it makes sense to just use the value for trail, but as that value a.) changes with tire size, and b.) isn’t easily accessible to Joe Average Rider, it might make more sense to use the individual values. This is an area where I think some discussion is useful. Do we need to discuss front-center values?
Bottom Bracket Drop (or Rise)
Because this ultimately has a value that can best be described as a positive (drop) or negative number (rise), the possibility of the formula generating a negative number is there. As part of the formulation, it might make sense to ascribe a value of zero to anything with a rise.
Lastly, the length of the wheelbase would play into the stability valuation, as well. Longer wheelbase = more stable.
I’d like this blog post to become a discussion point of the factors we should take into account for the purposes of creating the formula. Please leave opinions in the comments below.