Saturday, October 18, 2014

Proof that you do use math after high school

I'll do my best to explain this, as while I have a BA in English, I don't have one in math.  However, some help from a teacher colleague next door who knows his numeracy guided me through this process.

When I first fit the zx9r swingarm, it appeared that the combination of linkages, dogbones, and swingarms from 2 different bikes resulted in a swingarm with incorrect angle... it pivoted down too far, so the bike would be stood on its nose in this configuration.  I needed to do some measuring to be sure, as my eyes might have been playing tricks.

First step was to reinstall the stock swingarm and take some measurements.  I liked how the bike handled in this guise, so it was a good place to start.  The straight edge ensured my measurements (using the floor as a datum line) were more accurate.  After measuring the swingarm axle distance from the floor, and the swingarm pivot from the floor, as well as the swingarm pivot length, I could determine the swingarm pivot angle in this configuration.

With a diagram and formula provided, I got all scientific...  The unknown angle, or the angle of the 7R swingarm, turned out to be 11.1 degrees.

I now wanted to replicate that same angle with the 9R swingarm.

I installed the 9R swingarm, using the 7R linkage and dogbones.  By plugging in the numbers from the slightly longer 9R swingarm, it turns out my suspicion was right.  The swingarm did sit at a steeper angle, adding 22.5mm more ride height.  This would equate to 40mm over the totally stock 7r configuration... I had added 20mm of ride height with the WP shock

Too much?  I didn't know -- but my goal was to reset things back to how I parked it a few weeks ago.  I needed to ideally get the swingarm "up" a couple of centimeters to mimic the same attitude.

I had been warned off creating/buying longer dogbones to lower the rear end, as I was told it might screw up the suspension travel or rate.  When I looked at the upper shock mount, I saw that there was some room to move...  I ended up removing the nut that fit on top of the upper shock mount.  It was about 3/8 of an inch thick, so perhaps removing that would lower the rear end of the bike.  Considering people add spacers or jack it up with no ill effects, perhaps the opposite will be true as well.

I took some measurements and... I am now much closer.  Right now the rear ride height is only 7mm higher than where I had it set.  A good place to begin.  I also allows me the full range of ride height adjustment built into the WP shock, so should I want to quicken up the steering even more, I have room to do so.

So where I am sees the 9R swingarm, the 7R knuckle, dogbones, and shock, and the spacer removed from the shock.  Another 7mm of ride height shouldn't adversely affect the handling!

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