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Welcome back to the Bikerumor Ask A Stupid Question series. On the menu today is the fork crown steerer unit, and how it copes with head angle adjustments. Joining us are two fork brands specializing in the production of Upside Down, or inverted, forks:
Slackening the head angle off seems to be popular at the moment with the availability of angle adjusting headsets. I always wonder, though, is it possible to ever run a head angle that is too slack, in terms of what forces the fork’s crown steerer unit is manufactured to tolerate? The combination of a mullet setup with offset bushings and an angle adjust headset could alter the head tube angle of a bike by several degrees. Is it safe to do this?
Intend: The forks are tested with a specific angle of 65° in my case. This means the normal head angle is already mentioned in the fatigue and overload tests. If we want to go lower with the angles, it is a question of mathematics.
For example, at an axle-to-crown measurement of 555mm the sinus of 65° compared to 60° is 503mm to 481mm. This means a small difference in the forces on the fork, but it is also not so huge that the forks will explode immediately.
Of course, in a statistical manner, the problems with creaking, stiction and so on will increase, but if we are talking about a head angle of 62° instead of 64° I do not foresee bigger problems for the forks.
Bright Racing Shocks: My fork design approach is not affected by the mounting of variable headsets because the two contact areas between the frame and fork remain unchanged.
In the design of the steerer tube/fork crown assembly, I take into account three fundamental directions in the application of force. These three directions have been refined over the years using both telemetry data and dynamic test bench experimentation.
I trust these three directions (which you can see in the diagram) both for XC use and for enduro and e-enduro use. Of course, the force levels applied to the three directions (vectors) you see in the diagram are not always the same and vary according to the use for which the fork is designed.
The 68 degrees you see as the frame’s steering angle is arbitrary and can vary without causing major effects in f.e.m. analysis. This is because in MTB (unlike on the road) the variation in the direction of the stresses is continuously variable.
Here is a diagram in which you can see how the forces coming from the wheel axle are discharged into the fork structure. In this last diagram you can see how it all affects the crown.
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