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post #1 of 3 (permalink) Old 1-15-2020, 4:05 PM Thread Starter
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Fork Spring Preload Starting Point

Question for all of the suspension gurus out there…


I am replacing the fork springs on my ’09 and am trying to calculate the correct length, as a starting point, for preload. When disassembling the stock springs, I measured 29 mm of compression; measured from the top of the uncompressed spring assembly as it sat in the fork tube, including the black puck, to the spot where it sits when compressed under the circle clip. Since all of the references indicate that the stock spring is a .585 kg/mm rate, this seems to me that the 29 mm compression would yield 17 kg of compression preload, rounded.


I went with the RaceTech .85 kg/mm fork springs, a rate indicated by both their website and that of Sonic Springs for a 165 pound rider. If I understand the way I should be doing my math, the same ~17 kg of preload would be achieved with only 20 mm of compression on the new spring due to the increased rate.


On the workbench I measured the stock spring, washer and spacer at 53.8 cm (I omitted the black puck since it was a constant). So it seems to me that a starting point for setting the preload on the new springs should be a total spring, washer and spacer length of 51.8 cm, correct?


All of this assumes that the ’09 stock springs are .585 kg/mm and that, despite being woefully under sprung the factory preload recommendation is somewhat correct…
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post #2 of 3 (permalink) Old 1-16-2020, 2:07 PM
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Your logic is sound. I never checked the sag with the stock springs, but I'd bet it's not optimum. You want about 30mm sag, with you on the bike. Did the Race Tech kit come with a PVC spacer? If they didn't include that in the kit, then the stock spacer should work. My guess is your Race Tech springs are the same length as the stockers and using the stock spacers will reduce sag to about where it should be.

You don't know for sure until you put it together and check the sag. I cut the springs on my ex to yield about the same spring rate as your new springs and I'm not sure but I think the springs were right about the top of the fork tubes with the forks extended.

You can find out how to measure sag on the Race Tech website or in the Suspension section of our Wiki.
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post #3 of 3 (permalink) Old 1-16-2020, 11:11 PM Thread Starter
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The Race Tech kit did come with a length of PVC to be cut for the spacers and several washers, presumably for fine tuning, as I think there were six or eight of them. In their included instructions, RT gave a pretty good lesson on generic suspension setup and actually went as far as stating that a standard preload should be about 20mm. Their springs were actually shorter than the factory spring, though for some reason I didn't write down the difference; it was a fair amount, maybe 30 mm or so. At any rate, I will try to have to remember what I did so that when I can actually test ride it in a few months I can adjust accordingly, if needed.



And good heads up on the wiki; I have been scouring it for about the last month before pulling the trigger on the springs. It also guided me to install a 400 lb/in spring on the rear shock and I threw some FOG bones on for good measure. I went with an AFCO spring that I ordered through Speedway Motors. Though shorter than stock, there was enough threading on the stock shock to get a beginning preload on that (using the same math as above) without having to resort to a bunch of spacers/washers.
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