12 Jan 2008

Spent a few hours carefully measuring a front suspension upright to load the dimensions into WinGeo3. It’s time consuming to find the center of rotation of balljoints. Yes, in the interest of reliability, availability, and cost, I’m specifying stock balljoints at the outboard ends. That makes it easy for the builder to get them from any parts store.

Shocks: Those who read the Kimini book recall my angst over shock selection and pricing. Good shocks are very expensive; I recently saw a new astronomical price for one> shock… $2000. I say unless it comes with topless maidens who install it, I have to wonder what the price gets you. Maybe it includes having an F1 champion fly in and drive your car. Anyhow, I’m wondering once again how I’m going to deal with the expense. The shocks, by necessity, must have adjustable ride height and adjustable valving, preferably being double-adjustable.

In the back of my mind has been to use sportbike motorcycle shocks. It’s nothing new; people have been using them in Locosts for a while now, though many builders are clueless about spring rate versus installation rate. They change the springs to make it all work, then wonder why they can’t set the shock valving the way they want. Anyhow, new bike shocks are also expensive, too, BUT, many bike owner remove them when they upgrade the suspension. That’s excellent news for us because they end up very cheap on Ebay. As a comparison, the Koni shocks on Kimini are about $500 each, plus a $50 spring. The price of a used sportbike shock and spring is $40-$50. Yes, about 10 times cheaper. This is a huge reason to consider them even if they aren’t perfect. Heck, the saved money can easily pay for the entire drivetrain! Bike shocks can> work in a car; the secret is to use them as-is, without changing spring rates. That’s because the shock valving is made to work with a specific spring rate; so as long as it remains unchanged, it doesn’t care what it’s installed in.

This brings us to the next topic, whether to mount the shocks in the traditional outboard position, with the outer end on the lower A-arm,
or to use pushrod suspension. The traditional position is simple, easy, light, and easy to adjust. Using pushrods means more joints, a
pushrod, rocker-arm, and potentially burying the shock inside the body where it’s hard to access. Pushrod suspension came from Formula One where they had to get the shock and spring out of the 200mph air blasting over them. It allows putting them just about anywhere, which is important due to underbody airflow management. We have no such problems! However, many people think pushrod suspension is cool (and it is) but is the added complexity worth it? No… and yes. Technically it’s not, and completely silly, adding unnecessary complexity and making adjustment a┬ápotential pain. BUT, what overrides these objections is the $1800 we save. It means we have to compromise our “technical reasoning” on a cost basis.

This reminds me of something Dave Norton (Shrike designer) said about push-rod and rocker-arm suspension, “Just because you CAN design something doesn’t mean you Should design it.” He was referring to the added complexity versus what it does for performance. I have to agree.

Because of the complexity I had no intention of using pushrod suspension until I found how inexpensive motorcycle shocks are. Due solely to pricing, it’s probably the way to go. It does mean adding two rod-ends and a rocker-arm bearing. The one problem with designing in specific motorcycle shocks (and writing a book that uses them) is that over time, that particular unit will become hard to find. To offset this, I’ll choose one from a popular (therefore, common) which will hold off the obsolescence for a long time. Lastly, the ride height adjustment on the shock is unusual, using a ramp-type ride height adjuster instead a threaded collar. This could present a problem, BUT, push-rod suspension lends itself well to adjusting ride height elsewhere, by changing the length of the push-rod. So consider the decision made – probably.