Fork spring upgrade on the cheap; watch out, we're doing math!
So I just had Jamie rebuild my rear shock and respring it, and according to my wishes and waistline, he sprung it with a 900lb rear shock. Shazam that thing is stiff. Being low on funds, I figured I'd figure on figuring the front. So, tldr version: I cut the front springs and added a longer spacer.
My bike came with .90 fork springs, already better than the stockers but not stiff enough for my weight. I pulled the springs out and started mathing at them. The forks have 6.3" of travel. The springs have 33 active coils, with average of .25" space between the coils. Therefore, the spring has 8.5" of compression available before coil bind. Taking the 6.3" of travel, plus the .5" or so of built-in preload, that gives me an 'extra' spring length of 1.7". Grabbing my trusty caliper, I measured the spring wire diameter and coil diameter, and plugged the numbers into a spring rate calc I found on google. Assuming that racetech was honest about the spring rate, I calculated a conversion factor between their claimed .9 and the calculated .81, then started changing the number of coils. Turns out that taking three active coils removes .75" of potential compression, still leaves me with about an inch of leftover compression, and changes the effective rate to just under 1.0. So, I cut three coils off, heated and bent the new end, and ground them flat like stock. I then cleaned the springs up, dropped them in and measured for my spacers. There was 4.45" of space between the tops of the springs and the tops of the fork tubes. The fork caps, fully assembled, are 1.5" long to the spring mount surface, and I wanted .5" of preload built in, so I cut the spacers down to 3.5" and put everything back together. It all works fine, but I haven't had a chance to adjust for sag and check for travel so I'll update this as soon as I get to it. Until then, does anyone have questions or corrections on my math?
Edit: potential downsides to this. The spring could sag earlier than it would due to overcompression. The spring could break due to overcompression. Meteors could hit my bike. Honestly there's not a lot of downside to this I see. Realistically I figure I may have decreased the life of the spring by some amount, but not much. Also if you look at spring rate graphs side by side, you'll see why it's actually a more controlled and better ride to go with a higher spring rate than a softer one with the preload cranked up.
Go soothingly on the grease mud, as there lurks the skid demon. Press
the brake foot as you roll around the corners, and save the collapse and
In order, current to oldest, *denotes still have
DL1000K5*, VFR800X, MT-16*, DL650K4, XR400R, XR650R, XR400R, FZ1, GL1000*, VF750C, CB250, GL1100, GL650, DT50. I'm sure there's others but I forgot them.
Last edited by whodat; 09-04-2013 at 08:36 PM.