Finally tested the coil-over shocks with a proper press and load cell. As suspected, there’s a sizable gas preload pressure before the shock starts to compress. It was a lot of fun, actually, as I rarely have the proper tools to do this. I’m always having to guess or approximate what I’m doing due to not having the right tools, always making me feel bad about doing a half-assed design job. However, when I’ve got the proper tools it’s a really good feeling knowing that the data I’m collecting isn’t “roughly” this, or “about” that – it’s exact. With this data I can confidently design the push-rod suspension, and know it’ll work first time. That’s a great feeling to have even before it’s built.
After 25 years of resisting, I’ve finally given into probably having two sets of wheels and tires, one for the street and one for the track. Given that, I’ve backed off on buying the lightest street wheels possible, since they’ll have to deal with potholes (and bonehead tire installation shops…) Street wheels are far from light but then again they cost less than 1/3 that of a really light racing wheel. So it was a bit surprising to come to terms with purposely buying something that isn’t the best. Yup, today, street wheels were ordered. Theoretically they aren’t needed but they’re invaluable for suspension mock ups and checking clearances. Having CAD is pointless when detailed dimensions aren’t available, which is a lot of the time. Wheels are especially bad in this respect, and it’s a rare manufacturer that supplies accurate cross-sectional drawings. The best anyone gets is bolt pattern and back-spacing.
The ultralight track wheels that cost crazy money will come later after the car’s built and ready for the track.
And so it begins. Having decided on the front tire diameter now the real suspension design begins. Using Mitchell’s excellent WinGeo3 software, I’m working through many iterations, trying to get static FAPs (Force Application Points.) What’s frustratingly familiar is how I can get static FAPs, and roll centers for that matter, but I’m not happy with the camber gain curves yet. I’m not complaining and quite enjoy the iterative process. It’s a good feeling knowing that once this step is done, the car will have a stable, predictable nature to it, just like Kimini has. And like the Mini, it took weeks to gradually settle on those elusive points in space about which the suspension will pivot.
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.