If you've ever built a one-off bike or transplanted an engine into another bike, you will have had to deal with chassis geometry. This is a critical but all too often neglected area of building that dream bike. If not done properly, you'll likely end up with a bike that has some undesirable handling traits, or worse, is dangerous to ride. Getting the bike's static attitude, engine location in the frame, front trail, and rear suspension characteristics correct is more important than getting the last horsepower out of that tuned engine.
Ever wondered what kind of rear suspension characteristics a championship winning MotoGP exhibits? Well, so did we! Fortunately, in 2007 Yamaha released some details of the previous year's bike (2006) during a press briefing, and these drawings allowed us to model the rear suspension of the M1.
And here is the result in terms of rear wheel rate and the so called motion ratio. As you can see, the wheel rate has about 20% progression over its wheel travel and the motion ratio a 10% increase. This is fairly typical of modern race bikes although many show even less progression, often near zero, and a "flat" motion ration.
A Recent Example
Here is a brief example of a recent "special" that a customer brought to us to sort out a rear suspension problem. He reported that the suspension feels quite soft initially, but then "goes really hard" after a couple of inches of travel. He's come to us after the original builder suggested that he needed a harder spring.
After taking some measurements and running it through the computer modelling, it quickly became apparent that the revised linkage mounting points introduced a huge non-linearity, or progressiveness. This was so severe that the suspension essentially locked after about 2-3 inches of rear wheel travel: The spring becomes more than twice as stiff. Somehow, we doubt that a stiffer spring would have solved the problem.
And here are the plots, first the before, and then after with revised linkage and attachment points:
The remedy was a revised linkage and relocating the mounting point. So, a bit of fabrication and welding. The result was like night and day, another happy customer!
As a result of the considerable number of "specials" we've built (or rebuilt) over the years, we have a proven process to ensure that we get the best from such projects.
Typically, this consists of:
- If an existing bike, we carefully measuring the existing geometry including head angle, fork offset, swinging arm pivot, front sprocket location, rear suspension linkage and mounting points, etc.
- Create a computer model of the bike that allows us to optimise such parameters as front trail, rear suspension linearity and motion ratio, pro/anti squat behaviour; and analyse forces encountered at critical structural points
- Propose alteration to an existing chassis to improve any shortcomings
If we are building a chassis from scratch, such as the frame kits we are currently offering, we use the same process but have more scope for an optimal layout. For existing chassis that customers bring to us, there are invariably some compromises imposed by the existing hardware. It has happened, though, that these compromises were too great and a new frame was the only "fix". One such case is illustrated in our YZR500 replica blog.
While it is difficult to provide a guideline on cost of this service, we never had a customer complain about the cost, especially given the change it make to the bike and the pleasure of riding it. Please get in touch to discuss your requirement and we'll gladly give you an estimate of the costs.