Dorkiphus.net - Upgrading 964 Suspension

Quote:

Originally Posted by RunningonMT (Post 514926)

The JIC Cross system that I have been driving is 8 kg/mm (~450 lbf/in) and 12 kg/mm (~670 lbf/in), so moving to 600/800 will be quite a change and I will have some learning to do!

Have ordered 2-way adjustable with remote canisters. I understand the basic advantages of independent adjustment of compression and rebound and that the pressures can be changed in the remote canisters to allow small adjustments in "spring rates". Collin, would you expand on my meager understanding? Can you recommend a "how to" book on suspension adjustments?

Definitely -- glad to hear your interest! And very glad you were able to commit to the remotely-adjustable 2-way'ers.

An excellent, well-rounded book on chassis tuning/development I would recommend is Carroll Smith's Tune to Win. It tends to focus on formula cars and will likely cover a wider breadth of topics than you may care about. But it does discuss suspension in great detail and will provide you with good insight into the "how and why" behind what to tweak. It was last published in 1978 but the principals are still relevant.

A related book, Carroll Smith's Prepare to Win, provides an excellent range of best practices for successfully operating a race car at a high level. It won't tell you whether to add more rebound or spring (this is what Tune to Win is for) but it will give you a good foundation to ensure your suspension doesn't fall off when you take it out on track for the first time. That sort of thing.

I have a number of much more technically-involved books I can also recommend if you're interested. But they are essentially text books so caveat lector.

In regards to your newly-purchased suspension. There are a couple key advantages to having remote reservoirs over a fully-integrated setup:

1) Heat management: shocks get very hot, which can lead to poor damping properties through uneven density/viscosity of the fluid (in this case, nitrogen) and hysteresis + cavitation. Moving the primary reservoir of fluid to a remote location helps decrease the temperature of the fluid and provide more uniform damping characteristics.

2) Total volume of system: by adding remote canisters, the gas volume of the system is increased -- often by 2x or so. The more gas in the system, the higher the likelihood of more uniform gas characteristics. As an aside, we could increase the diameter or length of the shock to obtain a greater volume of fluid (ignoring any packaging issues that will probably limit us anyway). But for various reasons this isn't always desirable since increasing shock diameter increases piston size and increasing length increases piston travel. Adding a remote reservoir helps achieve better damping characteristics without sacrificing the shock dimensions itself.

BTW: a common misconception w.r.t. the remote reservoirs is that you should/could "tune" them similarly to how you might tune your tire pressures. As a rule, don't. The manufacturer states their recommended shock pressures -- abide by these. I've seen far too many problems created by people thinking this is yet another element of your suspension you're allowed to tweak.

Sometimes, it's a necessity and must be used as a bandaid. For example, BMAN's Cup car shocks were old and tired and needed to be rebuilt (I don't think he'll mind me using his car as an example). To compensate for the lost valving, the bandaid was to bump shock pressures well above their spec'd value. It helped provide the damping characteristics they wanted, but just remember that a properly-valved damper operating within spec will perform better.

Your brand-new shocks should already be pressure-balanced from the factory. Run them at what MCS recommends and tune them using the standard bump/rebound knobs.

As for adjustability: You'll now be able to independently adjust bump (AKA "compression") and rebound, which will greatly improve the control you have over your suspension and will be perfectly sufficient for your racing. However, this is just part of the story: whenever we discuss damping, we look at the velocity of the suspension as it navigates its travel. And when talking shock velocities, we prefer to segregate the total range we experience into two regimes:

1) Low-speed regime: Typically defined as anywhere from 5-25 mm/s (note that 0-5 mm/s are not typically controlled by the shock absorbers). Roll, pitch, and heave of the chassis fall here -- in other words, cornering is considered "low-speed".

2) High-speed regime: Typically defined as any velocity over 25 mm/s. Note that we usually only focus on the 25-200 mm/s range, since over 200 mm/s is usually only experienced when we hit curbing. 25-200 mm/s are the bumps in the road surface itself.

As you can see, we tend to think of the low-speed regime as "more important" than high-speed -- since this is what controls our ability to go around turns. So for a two-way shock, we can safely assume that it is biased towards the low-speed regime and will not give us independent adjustability of the high-speed regime.

Three-way adjustables typically segregate the bump adjustment into low- and high-speed, with a single adjustment for rebound.

Four-way gives you separate adjustment over low- and high-speed for both bump and rebound.

Just for fun: So does it go any higher than this? Yes, unfortunately. I've worked with both five- and six-way adjustable shocks in the past (Ohlins and Penske both used to sell a set, IIRC), though with improved shock technology everyone seems to be moving back to a standard four-way. The extra one or two degrees of freedom would vary depending on the manufacturer. For example, the fifth degree would change the inflection point of the graph (i.e., when it transitions from low- to high-speed) and the sixth degree would move the entire curve up or down.

Hope this makes sense. I was attempting to write this during a meeting this morning and I'm terrible at multitasking. :)