18 Jan 2008

Regarding the bike shock, the spring rate is around 500lbs/inch. The problem is that it’s preloaded, and the high-pressure nitrogen adds to the overall rate. This gives a discontinuity, where nothing happens for several hundred pounds until it finally starts to compress. Until I plot points at higher force I won’t know what I have. This weekend I’ll make brackets to properly support it and run a force vs. compression test on Monday. Oh, and another impressive feature of the shock is the mount bushings. I pulled a bushing out and there are needle roller bearings inside – very cool! My expensive Konis don’t have that!

While a great value, I can see a potential problem using these shocks, which is a shock travel versus spring rate issue. That is, I can trade one for the other, which is fine if one’s not important. Unfortunately there’s a squared term in the installation ratio which makes things more interesting. Instead of getting all wound up over this I’ll remain calm until I get it all into the suspension design software to figure it out. Worst case I have to use different units at the back, or swap springs, which isn’t so bad. That’s part of the beauty of buying off Ebay. If you buy used stuff and end up not needing it, you can sell it for virtually the same as what you paid.

Several Locost builders I know have used these shocks so I know they can work. Of course most are using sportbike engines so the very low weight translates to lots of suspension travel. I’m going with a heavier but more “polite” street engine which will eat into the little shock travel that’s there. Guess I’ll be the first to know if it works or not.

On the tire front, I’m going to try to decide tire diameters this weekend.