Calculating bellcrank motion ratios
Help, please! I'm trying to redo the motion ratios on my front shocks. They use pushrods and bellcranks. I've been prototyping these in 1/8 flat steel. I've now done five that won't work and it's getting to be a pain to cut, drill, file, install, measure, curse, measure again, curse more, and toss the prototypes.
Can anyone help me with the measurements and formula to compute bellcrank motion ratio? Good sources for this info?
I want a 2.1 motion ratio - one inch of wheel travel is 48/100, or 31/64 inch of shock travel.
Most recently, Prototype3 made a 1.90 motion ratio and Prototype4 made 2.35. I carefully and accurately split the difference in the holes for Prototype5, hoping for the average of 2.12 (close enough). However, it produced a 2.40 motion ratio!
The picture shows the whole left-side bellcrank. Pushrod attaches to the black steel piece, which is what I'm redrilling.
Or, am I doing this the hard way? Is it smarter to make a set that's close and tune with springs? (I'd like to avoid having to buy many sets of springs.)
Thank you for your time and help.
A couple of tips for doing this manually...
A few tips that should help.
1. Try to get a constant motion ratio over the planned range of travel. This leads to more predictable handling than either a decreasing or increasing ratio. This is usually easily achieved by making sure the damper shaft is perpendicular to its bellcrank radius AT THE SAME TIME THAT the pushrod is perpendicular to its bellcrank radius.
(NOTE: a side benefit is that radius changes meant to change the motion ratio have more predictable results)
2. Measure the damper and hub motions for each 0.5" motion of the hub with respect to the chassis. Use a range of hub travel of at least 1" in both directions from the in-use position.
Re: Calculating bellcrank motion ratios
Geremy, if I recall my high school trig correctly and understand your question correctly, the direct answer is that the motion ratio is determined by the ratio between the input arm (A) and the output arm (B), times the sine of the angle between the two arms.
(A/B)* sin(a) = MR.
If the angle is 90 degrees, sine is 1.0 and the ratio is just A/B.
Hope that helps.
Stan