edfishjr

Cool. The ND2s all use stiffer bumpstops.

oofie810

This got me reading your blog, and more importantly, enjoying your writing style.

edfishjr

Nice!

edfishjr

Arthur,

This is the roadmap I plan to follow. Might be useful to you and anyone else who wants to go down this path.
1) I will buy one new bump stop, measure its length and its spring rate. Obviously we need to know the exact length because that's our legal limit for whatever we replace it with and seems to me best to start with a new one. I think we also need to know its spring rate so we can figure the stock natural frequency (Fn; stiffness) front and rear while cornering. An Fn that doesn't include the bump stop isn't worth much on a Porsche because there are two springs at each corner, not just one. Of course, the Porsche springs are progressive, which also complicates things.
2) First cut I will input unsprung measured weights from the front strut of a 993 into a spreadsheet I have (and can give to you) to calculate the Fns. I will add the steel spring rate and the bump stop rate together for the spring rate input. This may take a little finessing of the results from the bump stop test as it is probably not particularly linear. I may need more than one number, like the initial rate before bump stop contact, the starting rate including the bump stop, the mid-compression rate and a full-tilt cornering rate. Probably the last is the most important, because I want to protect for and know the stiffness when a tire hits a bump while at the cornering limit. The Motion ratio also goes into this spreadsheet.
3) At this point I can take a look at available bumpstops. If you or anyone else wants a quick, approximate answer at that point I think I'll be able to say something like this: "the stock Fn at the front is about X.XHz. If you want to be conservative and get in the ball park you can add this bump stop, cut it down to this length and bore it out to the shaft diameter (maybe not necessary) and you will get approximately XX% stiffer which will put you at about 2Hz. This should be a good improvement (less roll-faster response) without knocking you into a regime where you need to change your driving style to make it work while retaining similar ride as stock and still reasonably within the capability of PASM or aftermarket passive struts."
4) For the precise, full-tilt answer, I will have to have a new-to-me Cayman that I can partially disassemble to get precise unsprung and sprung weights and measure motion ratios directly. (I have learned not to trust any MR number from any source that I didn't either take myself or watch the measurements being done. Measuring MRs directly is really a very simple process, but it's amazing the different ways people find to screw it up. Maybe it will be .98 front and rear like people say, but I'll know for sure. The 993 MRs we measured were not the same as any numbers we could find published. Some well-known people were spouting numbers that I found were obviously incorrect.) I'll then use my spreadsheet and also the Autocross To Win dynamics calculator to hone in on a precise answer that will take the car to Dennis Grant's 2.2Hz front and 2.5Hz rear, find bumpstops that might work, and then start testing and tuning. This will probably include a shock valving effort. I may not immediately disclose the results of those calculations and the tuning program, keeping the data as a trade secret for a while if I need to retain a competitive advantage. Note: If 2.2/2.5 works well for autocross on a front-engined 944, and it works on a front-mid-engined Corvette (not mine; a friend's CAM-S car), and it works on a rear-engined 993, I think it'll work on a mid-engined Cayman.

arthurc604

Thanks for writing that for us. I think I will need to have Joe Cheng over for dinner to explain to me each and every one of your sentences. I kid, I do have a rough idea of what you mean in each of your points but I certainly have a lot of learning to do. Perhaps in the off season I'll disassemble the car and have a go at measuring MR.

The most (and not very) sophisticated thing I've done so far is to DIY align the car.

edfishjr

read fellow Canadian Dennis Grant’s on-line book Autocross To Win and all will become clear

WillyDaP

Dang, after reading Ed Fish's dissertation even though I started autocrossing in 1981 compared to his brilliant set up tricks I am just a fish outa water! Since I have a bad back, arthritis , lost all my hair, and need bifocals , I may have to drive my Cayman down to Huntsville, when I get it, just to have Ed fix me up - or make sure I don't do anything wrong. Seriously, interesting write up and one which explains to many how serious some folks are to be super competitive. Impressive info and since I understood about as much as ArthurC did ( pretty sure less ) I will just concentrate on having fun and primarily doing Time Trials.

edfishjr

Being a former mechanical engineer I learned this stuff along the way more as an interesting hobby than some need to be super-competitive. Being super competitive in Street requires more money and time than I've been willing to give, always chasing the best car and the best tires and then constantly testing the heck out of everything.

People have been urging me to do time-trials. I might try it at some point.

I've basically decided to sell my present car after Nationals (first week of September) and then get a Cayman base. If the 987.1 does go down to CS then I'll look for one of those. If not, I'll look for a base 987.2 which is in my present class, BS. Competitiveness of the 987.2 base will be purely course-dependent, I expect, i.e. it can rarely win. While the 987.1 Cayman S is also in BS, I have no interest in owning that particular car. Finding either of these choices with the right options may not be easy.

oofie810

Interested in hearing what components you will use if you end up with a 987.1 or even a 987.2 car.

edfishjr

Leading candidate for struts are Bilstein B6 converted to take-apart (I think that's possible) and running the stock PASM springs since it will be a PASM car. The struts would then be revalved to match the actual spring rates that result from the bump-stop mods combined with the stock springs. B6 damping curves are naturally somewhat double-digressive, so that's good. There would be no damping adjustability, but once converted it's relatively easy to modify the internals, or so I gather. Wheels will be special-order 18" with specific offsets that I'll have to figure out to maximize track width and also be legal yet not rub. There will be no legal way to lower the car as far as I know, or make it as narrow as the ND2.

The result should be a car that's race-car stiff, doesn't roll much and thus doesn't lose all it's negative camber in front, transitions very well, won't need a huge FSB, is balanced and has lots of tire for it's weight and thus lots of grip, more than anything else in CS including the ND2. It will still need an adjustable FSB to add to the roll stiffness, keep the rear tires on the ground (no LSD for the 987.1) and to allow front/rear balance adjustment.

BmacIL

I did this with my 997.1 and B6's with Fat Cat Motorsports. I am extremely happy with the results. Target low speed damping ratios were just over 0.7 and it provides an ideal balance of racing response and compliance. Secondary ride (and the resulting grip) is sublime. This, coupled with alternative bumpstops (firmness and height), and packers for tuning has been a great investment.

edfishjr

I've been a follower of Shaik's work since I discovered Fat Cat while doing the bumpstops on the 944. Back then I got the impression they were mostly Miata-centric, but I see they now have a pretty wide range of experience. Most likely I'll go with him.

BmacIL

Yeah the only thing I need to give him some feedback on is that the big FSB trend that often works great on FR cars just hurts on mid/rear engine cars with limited camber, and packers/stops can only do so much. I am down on FSB rate ~60% compared with what I ran at 2022 nats, though still about 30% more than OE front.

edfishjr

I plan to use DG's calculator to fully analyze the roll stiffness balance, including when on the bumpstops, and specifically to figure out what target FSB rate makes sense. I haven't fully watched all the videos, but when he worked on a 987.1 Cayman (base) a few years ago I got the idea that Shaik did not fully analyze the car and create a full spreadsheet the way he has done for Miatas. Did he do that for your car?

BmacIL

At a basic level, but he's more limited in the data he has on the 997. I did some myself in parallel and the problem is it doesn't cover transients well and the compromise needed between steady-state balance and transient balance with a car like mine. As I've adapted to the car and learned to extract more of its performance, I have been able to move to a more aggressive roll stiffness balance in combination with tires that play very nice in fast transients (Yoko A052).