projectr13b

I read through some of the DSP thread and have been reading online, but I need some semi-specific advice on running DSP.

At least for this season, my car will still be mostly stock, but I already bumped myself out of STX and STU because I'm running a catless midpipe. Instead of pushing myself backwards into a class I won't really be competitive in, I've decided to propel myself forward into a class I won't really be competitive in. I'll have to run street tires for this season (likely BF Comp 2s since they will last me the season and are cheap and grippy).

I think I'll also be able to grab some coilovers early in the season, but I was curious what spring rates I should run front and back. The car will be close to the stock weight. If anything I'll be able to remove a bunch of the rear and trunk interior, but I don't think it'll drop a lot of weight. I've got to stay street legal for at least this season unfortunately. Hoping to get a tow vehicle and trailer in the future.

I'll also be upgrading the coils at some point, but mine are fairly new.

Doubtful I'll get them this season, but anyone running 10" wheels, what offset are you running? Also, I read 17s are better to run, but I also still see a lot of people running 18s. Any options on wheel size?

Any other advice would be great. I ran one event last season and found myself dropping really low in the RPMs in second gear. I'm used to running an ES Miata where I'm all throttle and very little braking. The RX-8 feels like a very different beast so any tips will help. I don't expect to be coming close to our local DSP times but I'm hoping to keep up with the STX/STU guys this season.

Thanks!

RX8Soldier

iTrader: (24)

There's plenty of info to be found in these forums. Members have been racing since the rx8 came out over 10 years ago, and much of it is documented.

Do your homework and based your decisions on what you learn. After all, most people offer opinions based on personal experience, not science... and experiences vary.

projectr13b

Thanks. I'm still reading, just looking for any insight I can get. Varying experiences help me narrow down what may or may not work for me since I'm still really new to the car.

Tamra

I highly recommend you do some light reading through the street prepared rules section of the SCCA handbook: http://cdn.growassets.net/user_files...pdf?1452294335

Just as a heads up, removing the rear seats and the trunk carpet is illegal in street prepared (would bump you to street mod), so you don't want to do that. Losing weight comes from race seats, lightweight brakes, etc. In street prepared the car has to stay mostly intact.

Seems like it might be easier and you'll be more competitive to just pick up a high flow cat and run STX. DSP assumes you have have full suspension, tune, wide wheels, Hoosiers, aero, and more.

Or the really really easy button (and by far the cheapest) since your car is stock aside from the cat, is to pick up an OEM or equivalent catalytic converter and run CS. Then all you need is tires. The RX8 is a competitive CS car.

Nathan Atkins

The science answer for springs rates for autocross pretty much comes down to how fast you think you can get through a standard 25 pace (roughly 75 feet) slalom since every other transition is slower than that.


you want your springs to be able to react to the body movement you're going to be throwing at it in such a slalom. so the slalom is about 75 feet, so if youre going through it at one second per cone (about 51 mph), you want your natural frequency of your spring to allow a response in half a second. so a natural frequency of 2.0 will stroke in 0.5 seconds.


the rate that corresponds to 2.0Hz for the front is right about 11k


you want your front and rear to finish with their movement about the same time. So since your axles are about 9 feet apart and youre going 51 miles an hour the rear axle lags by about a eighth of a second or so. Knock a eighth of a second off the rear frequency response should be about 2.67Hz, but that would make the car pretty loose so probably best to average that frequency and the front 2.0 to get 2.33 and be on the safe side. a 2.33Hz frequency in the rear comes from a 10k spring.


So get 11k/10k, and since 11k can be difficult to find since its odd and higher than 10, just get a 12k for the front for 12k/10k


You can dial in balance by adjusting swaybars and such, and error on the side of understeer.


So that's a starting point, If you think you can get through that slalom faster than 52 mph then increase the rates, slower than decrease the rates. I'll be using 12k/10k, others use 14k/12k. Mike Khun used his old 10k/7k STX springs when he got started and did alright. The higher you go the more responsiveness youll have at the expense of ultimate grip.


standard sized springs are cheap to buy in online classifieds and tend not to lose value, so theyre worth experimenting with. So there always the trial and error find what works best for you method.

roflcopter

iTrader: (2)

I've been grappling with spring rates for DSP as well, after my last event(first in DSP) I realized that my suspensions needs addressing in a major way to account for the added grip. In the slaloms I was able to 'outrun' my suspension, the car would pitch left from turning right and I would be turning back left before it had even finished pitching left... this resulted in some not fun handling at the limit.

I have been doing TONS of research over the past couple of years as far as suspension goes(although not for the RX8 and not with a decent budget in mind, mostly for our Lemons cars) and so far I kinda have my head wrapped around all of it. The writings of Dennis Grant and a few others have made me realize that you need to think very analytically when working on all this stuff.

My questions for you guys have to do with swaybars though, and a bit about spring rates. My calculations so far with an attempt to get a natural frequency for the front of around 2.1-2.2(stiffer than what you say is needed in autoX but also not necessarily a bad thing) and keeping the rear right around 2.5 have yielded basically the same results as you, 550lb/in(10k) front and 600lb(11k) rear springs. I think this is the avenue I am going to pursue first and it seems like it will give good results.

As far as sway bars go, those spring setups provide a front roll couple of around 47% without any sway bars, which would be a VERY loose setup from what I have been reading, but with the stock sway bars(sport suspension front with stock rear) it would put that at a seemingly more manageable 53% FRC. For those of you with experience, is that a manageable roll couple in an RX8? I know it's a bit lower than the Miatas run for track stuff, but this is also autoX.

The follow up questions is if there is a sway bar available that closely matches the stock bars but adds adjustability(just front or rear, both shouldn't be needed) or should I go with a spring rate that gets the roll couple in the neighborhood while using some other aftermarket adjustable sway bars and design around them since the sway bar options are more limited than the spring selection?

Please show me where I am making bogus assumptions if I am making any, I'm good at that from time to time!

Kennetht638

iTrader: (2)

I think that there's too much reliance on natural frequency calculations in spring rate selection because of Dennis Grant. I believe that his website is a valuable resource, but because it's pretty much the only readily available detailed technical design guide for autocross suspension, it's taken as gospel. It's not a bad idea to shoot for a certain natural frequency to get yourself in the ballpark, but only because it's a good way to get close to the general stiffness that other people are successfully running on autocross cars with the same tires while taking specific masses and rates out of the equation. The desired natural frequency is probably actually a function of what the tire wants, but we don't have enough data to figure out what that is. But I bet if you trawl through a ton of autocross builds on the internet, you'd be able to derive it empirically, and you'd find that the standard deviation is probably pretty big too, because there's a decently wide range that work well enough, where the differences are easily swamped out by driving skill variance.

Nathan, I think your methodology for determining a natural frequency goal makes sense, but only if you consider it for the natural frequency in roll because that is the primary motion in a steady state slalom, so calculating your ride rates for that situation doesn't make a whole lot of physical sense. The cool thing about sway bars is that they allow you to have different stiffness in ride and roll/one-wheel-bump, which also means that you get different damping ratios in both; They're not just a fine adjuster for steady-state balance. For example, using light spring and heavy bar would leave you relatively overdamped in longitudinal motion like braking and accelerating when compared to a setup with the same roll rate with only spring and no bar. However, hitting that balance just right is VERY difficult to calculate, particularly on a production car with lots of camber/toe change in ride/roll, so most people just tune for it.

All that said, I've run a few different setups on my car and have learned my share of lessons through experimentation. In stock trim, I ran the car on street tires and on R-compounds and I thought it was pretty pushy. I calculate ~67.4% front roll couple, though I could be a little off on bar rates, plus this doesn't take into account the fact that the car actually uses its bumpstops as springs. Since I went to STX/DSP, I've tried a few different spring rates and bar configurations, but what I've found is that my car is just on the pushy side of neutral at about 66.4% FRC, and going stiffer on the rear bar to 64.7% makes it pretty loose and softer on the rear bar to 67.7% makes it pretty pushy. To give a sense of how small this window is though, when I ran the original Rivals on the car, I'd run it at 66.4% FRC for my first run and my co-driver's first run, but once the tires were warmed up it would become loose, so I would soften the rear bar to the 67.7% FRC setting and the car would behave correctly again.

So, roflcopter, based on my own experimentation, I think a 53% FRC would be undriveable, and a 47% FRC would be faster driven backwards. While I think that the roll couple numbers that make my RX-8 work seem crazy on paper, I know that the calculations don't account for a lot of things that the car is doing so rather than trying to figure out what's "correct" through math, I copied some people who were fast and then spent a lot of time fine tuning it.

blu3dragon

iTrader: (1)

Agreed. 53% FRC is going to be undrivable.
64 to 66% FRC is about right based on my experience. The way to get there while keeping a higher freq in the rear (desireable) is to use an aftermarket front sway bar with the factory rear, or no rear sway bar.

Using the factory sway bar ratio (which typical afternarket F&R sets will match) you need to keep the front stiffer than the rear. There are plenty who do this successfully. I've not tried it this way so can't say it is better or worse.

roflcopter

iTrader: (2)

Thanks for the input guys! This is why I like this forum and the people in it, you can get honest answers and not a lot of speculation.

When I have the disposable income to pursue suspension parts I will shoot for something in that range for the FRC. I really like the idea of keeping the rear springs as stiff as possible, it just makes sense in my head for some reason and I feel like it's going to come down to deciding which direction I want to take.

Blu3dragon, if I am reading you correctly you run the stiffer rears with little to no rear sway? Do you notice any sort of disadvantage to this? I know it's pretty common practice in other cars, especially FBs, to run with no rear sway.

Kennetht638, thanks for putting the roll vs bump thing in my head, that makes a lot of sense. From my understanding running with less bar would be advantageous in that situation though because during one wheel bump you want the suspension to act as independently as possible so that it maintains forces on the wheel being bumped and doesn't alter anything on the other side causing a momentary loss of grip in excess of what is absolutely unavoidable. As far as using the bump stops a lot in these cars goes, how are we supposed to calculate for that and make our suspension work as a system? That seems to be one of the big points the FCM guy makes and seems to be part of his secret sauce. Just as an exercise of imagination... how far would we have to stiffen springs before the bump stops aren't an issue anymore? Is it attainable or should we just deal with needing to ride the bump stops?

blu3dragon

iTrader: (1)

Correct, I'm running the stock rear sway bar, along with an aftermarket miata front bar (stiffer than the stock rx8 one, but not as crazy stiff as an aftermarket rx8 one). I also recently learned that things are much better in the wet if I disconnect the rear sway bar completely.

I subscribe to the same theory as FCM in that less is more when it comes to sway bars since it allows more independent wheel travel and therefore deals better with bumps.

The only disadvantage in my case is that I am not stiff enough for the track. That is actually due to having compromised spring rates so I can still daily drive the car
I'm on 425F/375R, which I think works out to about 1.9Hz and 2.0Hz respectively. So, despite the theory of less bar is more, it might actually be better for me to run stiffer bars on track since my springs are too soft.

btw, shock tuning is going to be important as well. I'm pretty sure FCM could get you a complete setup that would be very close to optimal in a single shot ;-)

roflcopter

iTrader: (2)

Thanks for the input!

I will definitely be running plenty of spring for the track/autocross as this car is only used in those situations and I am trying to make it as competitive as possible in DSP. As far as your setup goes, do you find any advantage to running natural frequencies that are trending towards more in the rear? It makes sense that you would want the back to be higher so it keeps up with the front but that seems VERY hard to accomplish without some serious sway bar gymnastics on RX8. Even with the larger sport sway bar up front and no rear bar it still doesn't get the FRC into a reasonable range when trying to get the rear 2-3 tenths higher than the front.

As far as FCM goes, I would love to go that route(or talk to a few different shops and see who I feel most comfortable dealing with) but that is not financially viable quite yet sadly.

blu3dragon

iTrader: (1)

I've not tried it the other way so can't really compare. What I can say is that my car feels awesome on track. The sway bar gymnastics are not that big. You just need to look for an aftermarket front to go with the stock rear (there's a thread on here with all the sizing). You can also pick up a smaller rear from an NC, or run with no rear if needed. And you don't need as much as 3 tenths higher in the rear. 5-10% higher than the front should be good.

TeamRX8

iTrader: (25)

bump

I’d suggest that if you’re taking advantage of the aero rules plus the fact we’re on R-compound tires that you’ll want to be up around 3 hz; maybe a bit higher or lower depending on the specifics of your vehicle. Most of the rates suggested above are fine for a street tire class like STX and also still decent enough to drive on the street. IMO that’s not where you’ll want to be for SP.

Whether you’re on the OE 1.0 diff vs 1.5 ratio diff like most aftermarket LSDs will influence the rear rate a lot as will the size/effectiveness of the rear spoiler. Either of these two factors are not going to show up on the spreadsheet percentages indicated above. Plus people are throwing those percentages around without defining the parameters that they’re based on.

Just something to consider.