The dots are at the values for ~70% critical damping given the spring rates (steel + rubber) and masses (sprung and unsprung) at a shaft speed of 3in/s. (I assume that all the energy of compression in the rubber is given back. Though this is mostly true for steel springs, which have very little damping, it's not really very true for elastomers whose damping rates vary a lot.) For bumpy autocross courses this is what I use when I can get the correct (or reasonably correct) spring rates, either with steel or steel + elastomer. For the B6 struts it looks like I can get an Fn of about 2.3Hz in the rear and 2.1Hz in the front given the available aftermarket bumpstops. I'd like to eventually get about 10% higher, but it looks like I'd have to mold my own bumpstops to achieve it. I want the knee in the force curves to be between 2.5 and 3 in/s which is the range of shaft speeds that can be caused by driver inputs. That is the range in which you want significant energy extracted from the spring-mass system to reduce force variation at the tire patches. Beyond the knee I want the forces to climb as little as possible because damping at input frequencies above the square root of 2 times Fn increases transmissibility of high-frequency inputs into the mass of the car while causing higher force variations at the tire contact patch, i.e. reducing grip over surfaces with small, sharp bumps.

One of the things I do with my consulting company, E&F Consulting, is suspension design and analysis.

The problem (and opportunity) presented by these Porsche cars is that the steel spring rate is very low. Lower in terms of Fn than would be the case if they didn't design the car to use the bumpstops as auxiliary springs in the corners. The problem then is that the handling will degrade markedly over the years with deteriorated bumpstops where, in a Corvette for instance, that doesn't happen. The opportunity is that we can increase the spring rate with aftermarket bumpstops. Many other car companies are now doing exactly the same thing, including the ND2 cars I'm competing against.

 

It's too bad that no one takes the time to tune shocks the way the OEMs do. It would be fun to iterate from OEM, stock B6, and your calculated values and see what parts could be made to work the best and where the sweet spot actually is. Calculation and dyno graphs are usually only good for a starting point. There are so many compromises and islands that exist with unexpected dyno curves. If we had to tune without a dyno, or without working configurable prototypes we would tune without a dyno for sure. Most people who like dyno plots would probably be surprised how much you can change the character of a car without making much visible change on the graph. You can also kind of show what you want to if the graph reader isn't savvy. Your shock guy is showing you force versus continuous velocity, and you have roughly drawn a regular F-V curve. Did you ask him for continuous velocity, or is that just his default output?

 

I wondered if anyone would notice that. That's his default output.

What do you think about the large gap in the rebound curves for the fronts?

 

It's a little outside my wheelhouse for the way we use the data. I would typically look at a force displacement curve, because there isn't much beyond pressure balance that I can adjust to change the response relationship when im locked into MP parts. The monotubes I use are also not a traditional compression damping circuit. I would guess that damper would look a lot better on force displacement than it does on the graph you have, which is not to say it's actually better. I always complain that most FV curves are a 50mm sine wave, and not much of what the car does is a 50mm perfect sine so it's not super relevant. Continuous velocity over 1" stopping at 10 inch per second actually does make some sense for motorsport. It made me suspect that your damper person might be sharper than the average.

 

I think he is much sharper than average, part of why he is so successful with clients in almost every form of racing. I'm the oddball autocrosser.

It turns out he's dynoed a LOT of B6's of every stripe. He predicted I wouldn't like what I would see. He meant the large amount of hysteresis indicated by the big gaps in the traces, especially front rebound. He knows I'm used to the tight traces you get with Penske 8300. (But those are 4x the cost.)

 

I'm a bit confused by the hysteresis crossing over at the 6.2in/s compression mark on the rears - where's the energy coming from? Just a factor of dyno acceleration characteristics given the continuous velocity plot?

 

The plot is continuous data sampling as the damper reaches 10in/sec and then slows down. One of the big differences between the sides of the hysteresis is that the side accelerating to 10ips starts as the damper is changing direction. The decel side does not come after a direction change. Lots of things happen when the damper change direction, and that can all result in lag. As a side note, all this data is condensed down into two single data points for the F-V curve that most people show or look at.

 

Well, this is disappointing. I don't think that the B6 struts are legal in CS after all. Both The front strut and rears are is about 1.5" longer than the stock length. The bumpstop length is not the problem. It's the overall length of both struts. the front strut.

"The fully extended length must be within ±1” (±25.4 mm) of the dimension of the standard part."

I've been measuring the heck out of these things to the thousandth. It took me a while to work out how to compare the inverted design to the stock design, but that didn't really matter since they violate the requirement above. It seems that when I first measured them I confused fronts and rears, comparing B6 fronts to stock rears and vice versus. That worked out well! But, it was just random that it looked correct but was not. I can conceive of a way to make them legal, but it would require complete disassembly of the damper and then some machining. I'll work on that some more.

For now I'll go back to the stock struts with one step up in durometer and see how that works.

Edit: only the front strut is too long.

 

If the dampers are able to be disassembled enough to revalve them it shouldn't be too hard to add a spacer to cut down the extended length.

 

Might very well be possible.

 

A Cayman in CS: Events 3 & 4
We had a two day event this weekend. The bad part was that there was rain both days. Not much good for testing. However, both days I got one or two runs with substantially dry sections where I could get a feel for the high lat-G behavior.

The runs were encouraging. If you've been following along, you know that immediately before these events I installed the Tarett 5-hole front bar and a new set of bumpstops one full-step up in durometer from the last event. The car was definitely stiffer in roll and a little more bouncy than last time. The stock struts are really not up to the job with these high spring rates, but the car is still controllable. I calculate the present natural frequencies at 2.10Hz front and 2.45Hz rear. I made a wild guess and set the front bar to the middle hole.

I'm carrying a front spring rate of 505lb/in, which is 155 from the metal spring and 350 from the bumpstop. The rear is 745lb/in, 228 from the metal spring and 517 from the bumpstop. For comparison, the KW Clubsport springs are 400F/685R. Note: when people normally talk about Porsche spring rates they rarely add the stock bumpstops into the mix. When an aftermarket setup eliminates the bumpstop contribution and compares their new spring, say in a coil-over setup, to the stock spring, the result may be a misleading comparison.

I had also maxed out the rear camber and reset the rear toe to zero. I'm sure the thrust angle is wonky, but I'll get that taken care of at an alignment shop soon.

How it Drove
The car was well-balanced in both wet and dry conditions with big grip as before. In spite of the difficult wet/dry/wet conditions I never spun, never hit a cone. I DNF'd once when the car hydroplaned straight through a deep puddle when I needed to be turning. External observers stated that the car seemed to have only very minimal roll in the corners. I think I may have been approaching the speed of the Miatas in the slaloms as I learned to trust the car and ask more of it. I hope to get some ND data to compare to. Transient response seems very good.

Results
Day 1 was mostly wet, so I didn't expect to make much of dent in the swarm of Miatas. I managed 4th of 13 in CS and 8th pax of 110. I was closer to the fast guys in CS in the wet than I expected. My previous experience is that a big weight difference is next to impossible to overcome in the wet, even if the amount of tire is proportional. 600lb is a big weight difference.

Day 2 we got drying conditions for our final 4 runs. I was in the lead of CS all day, both for Day 2 and the two days combined, partly because of others' cone trouble. The last run one of the drivers ahead of me on day one got close enough to take first for the 2-day total by 0.007s. So, I ended with the fastest time in CS on day 2 and taking 2nd for the two days combined. Over the two days I was 6th pax. 6 of the top 10 in pax were CS drivers, with 2016 DS nat champ Dennis Sparks leading the way in Pro class in his 2024 ND in 4th. I'd been pretty close with him both days until he threw down on his last run and jumped ahead by over a second. Dennis trophied in CS at 2022 Nats. He told me he thought it was great that I had developed an alternative to the ND for CS. The top 8 pax positions are shown below.

Plans
Except for the alignment I have no plans to change the car before the two Bristol events. Instead, I'll be working on the driver in two events before Bristol. In the meantime I'll be trying to figure out what to do with the too-long Bilstein B6's. I have a friend that can turn down the standard bumpstops to fit inside the Bilstein lower housings, which means I'll definitely have to test them to determine the real spring rates. But the big thing is to either find a substitute shorter shock shaft or get the shafts shortened and re-threaded.

edit: added comparison to KW Clubsport Coilover spring rates

 

It’s not necessary to shorten the rod to cut droop stroke. Your shock builder has to have something that can work as a top out stop in there

 

This!

 

Or use that space to learn about the magic of rebound springs.

 

Admittedly, I don't know much about these things: Would an internal spacer or stop to limit droop change the hydraulic characteristics by reducing the internal oil volume? Maybe not enough to matter? I thought internal oil volume was a key design characteristic and the basic reason for remote reservoirs.