Yes, that's what I meant.

Yes a small change to the f / r ratio and it 'feels' like it's more prone to turn or rotate, but that's just on the street so far. Track will be the judge.

Haven't done full alignment measurements yet. Maybe today. I think it must have lowered by a small amount too, which will give more neg with the trailing arm suspension.

Hmm, not sure about where the T bars were sitting in terms of indexing, but they had been re indexed so my guess is neutral. I'll check w Sean.

I agree Elliot, Jim knows his stuff.

 

Actually I had the wrong front wheel rates. It should be 661 lb/in (= 140N*5.7*.91*.91)

So the new ratio is 661/386 = 1.71 pretty close to the old ratio 519/296 = 1.75

 

Couple thoughts I have based on this thread and the discussion in the spring rate thread that was going on the 944 board:

Keep in mind that the effective rate calcs are a mathmatical model, and may not be a perfect match to the real world suspension system.

But even if we assume the effective rate calcs to be absolutely accurate, the next point of contention is what effective rate balance (front to rear) works the best. This is actually the primary discussion. How to balance the car's handling, which is not the same as balancing the theoretical effective spring rates.

You can use the math formulas to attempt to estimate a desired spring rate, but in reality, it comes down to how does the car feel and handle with the selected spring rates.

Seems like its pretty common for a lot of guys to believe that the car should have equal effective spring rates front to rear, 1:1. This is likely based off the idea that the car being approx 50/50 weight distribution should have 50/50 spring rates. And/or an attempt to match the factory stock spring rates, which are close to 125 lb/in front stock springs and 125 lb/in rear (23.5mm t-bars). But a stock street 944's dynamics are quite a bit different than a dedicated track car running 4-5 times higher spring rates, increased damping rates, 2-3" wider wheels and tires, lowered 1.5", larger swaybars, different LSD lockup rates, altered alignments, etc.

No matter how you do the math, many dedicated track/race 944s are not setup with anything close to a 1:1 spring rate balance. They have more bias to the front, some a lot, some not as much. Pat's 1.7-1.75 ratio effective rates are somewhat similar to setups that I have used and feel comfortable with (my cars have been in the 1.5-1.7 range). But I know a lot of guys and have driven cars that have a much lower ratio (1.3 or so), and they are fast too. And I am currently looking at running a little more rear spring (lower balance ratio) on my track car.

A lot of the handling balance is affected by other variables I listed above (damping rates, swaybars, etc). And track layout and driver style/preference have a big impact on desired balance. Big fast tracks with high speed sweeping corners demand different setups than short low speed tracks with tight corners. The faster the turns the more understeer you want, the slower the turns the more you need the car to rotate with the throttle.

My point is, I guess it does not surprize me that the car feels slightly different when swapping spring rates, especially if removing t-bars, even if the theoretical effective spring rate balance is the same as the previous setup. In the end, it will come down to track testing to fine tune the car's setup for your own style and track layout. And for those of us that like to tinker, that is the fun and rewarding part.

 

See, here's what happens. Jim starts talking tech and you start seeing stars in your head, like you just got hit with a sledge hammer. But then he starts talking about how the car feels and driver style and springs with and then without T-bars and faster, based on one style, though just as fast with another and then you go.....



What The Hell Was This ABOUT AGAIN?

 

Actually to further your point Jim, it also seems as if our chassis' can handle quite a variance in springs and possibly other components. As you state, there are guys running in the 1.3 range and maybe even closer to the 1.1 range (although I think this would be a rarity?). I imagine that the really competitive guys have a bunch of gears that they swap in and out too which would help not only the powerband, but also some handling moments. E.g. If you did want to run with a 1:3 setup on a faster track, but there was a hairpin or two that the car just push understeered like a rutting pig, then perhaps a different ratio might help in such instances. More often it's just a bit of patience and taking your medicine at times like these. My guess is that if our car's have an archilles' heel, it is in slow spreed corners. Often displaying that push understeer spoken of by many.

Well it's a fun learning curve if anything.

 

Sorry - it made sense in my own mind (when I was talking to myself about it)....

 

I agree. And that is basically the point I wanted to get across. Nothing specific to this thread, maybe more so in the 450/275 spring thread on the 944 board, but there are guys that state emphatically that certain springs will or will not work (often based off of something they read, not first hand racing/track experience). But especially when you start playing with adjustable dampers, adjustable swaybars, etc., there are a lot of spring rate combinations that can all be made to work relatively well.



I have always raced in stock classes, so I am limited to stock curb weights and gearing and power and DOT tires. The guys that run the modified classes with significantly reduced weight, more power, custom gears, full slicks, etc are going to have even more variables that make the car differ dramatically from a stock 944 setup. See some tender and main spring setups on those cars, making progressive rates using big 1000-1500+ lb/in springs.

 

Jim, in your estimation if you were setting up a car for a faster, momentum type track, yet it had a couple of slow speed hairpins or similar, how would you go about reducing the push understeer in those slowspeed corners. Notwithstanding, there is also a school of thought that you drive around perceived car setup issues and just deal with it. Slow down a bit earlier and get a 'set' and just take one's medicine.
Others would add a bit of toe out, but then that has it's consequences under braking. I've used both, and they work, but are ultimately unsatisfactory. Winding up more rear bar, but then this often can lead to snap oversteer on corner exit.

Perhaps it's always a matter of compromise.

 

Ok, time to debunk another old myth…

Just because you have ‘equal’ 50/50 weight distribution you do not need equal spring rates. In fact you do not need to have a set ratio between front and rear. If you want to set it up right you need to set it up right! That means tuning it for the chassis, track and driver preference. You actually tune each end separately. You can find the rear spring rate that makes the rear perform the way you want and then find the spring rate that makes the front perform the way you want it to. Front and rear do different things, you want good braking while turning in characteristics from the front and good traction while applying power in a corner form the rear. Neither end can be so stiff that it is upset by rough track conditions. You do want to keep the overall vehicle dynamics from going to crap with a really weird set up but you will find that the ratios can vary quite a lot without really affecting the general handling.
Next myth – the ‘feel’ of stiffness (harsh or compliant ride) is much more about shocks than spring rates. The stock rear effective rate is right around 175 in/lbs (as mentioned earlier). If undamped that will move up and down a huge amount and bottom the suspension travel all the time. (3” of compression would be caused by only 525 lbs of force in each rear corner). When you hit a curb on the track you are imparting way over twice that force….Hard braking and starting a turn in will also give you that kind on loading in the front. It’s the shock/struts that are absorbing that kind of energy. They really control the ‘ride feel’ of the vehicle.

Matching the spring rates and the shock settings takes a lot of work to get ‘prefect’ – and ‘perfect’ is different for each track and each driver.

 

Whoops - I just saw Jim's post....and he debunked the myth first!

 

Bad, bad, bad habit that is very hard to brake once you start doing that (I know – I am a master in driving around a problem!).

Sometimes you have to do this at a given track day but if you start to do it a lot you get to the ‘close enough for me’ set up and then you wonder how the other guy just bested you by 2 seconds. If you are just driving your street car on the track you can ‘just live with it’ but if you are tinkering with the set up then keep working to get it right.

My favorite example is when I took my wifes Audi S4 to the track – I got a brand new set of Victoracers and I was going to drive it on instructors day and then turn it over for the 2 day DE for her to drive. I found that due to the weight of the car (4,000 lbs with me and a full tank) it required a fairly harsh move to get it to rotate well (chop throttle to induce a little over steer and then full throttle to get a little slip angle). After the second session the tires were hopelessly chunked and the caliper temps were over 400f. I guess my technique for ‘driving around’ the handling issue were not appreciated by the car….or the wife!

 

Chris - thanks for adding your insight, good to see some actual consensus on the internet once in awhile.

Certainly there is some compromise to everything, but you do try to maximize the car's handling in all conditions. Obviously easier said than done.

What range of speeds are you seeing between the fast corners and slow hairpins? Are you talking autocross slow, and Daytona fast all on the same track?

Are your KW's double adjustable?

 

Chris, well yes I agree with you in sentiment. The 'drive around the car' was really just something I picked up from a Ross Bentley book. His take was that it isn't always the car that is not performing up to scratch so to speak.


There's a couple of pretty slow corners that I can get some scrub-push understeer and a few quite quick ones, the fastest being T1 at perhaps 110mph on trackout.
I am just starting up with using a datalogger so don't have hard figures to hand.

Yes KW double adjustables.

The best way to show you is a film clip. T2 is a strange turn with a few ways of dealing with it. I use the double apex method for now. It's off camber too. T9 is the hardest braking corner and one where I have just backed off and taken a wide straighter entry so as to scrub of most of the speed in a straight line, then late apex. I'd rather trail a bit more and have a mid point entry/exit. The last turn T12 before main straight is setup by the preceding corners and I get it wrong here, but I have a better idea of what to do now.
Anyway, any ideas are welcome. Oh, and see if you can notice our newly patented secret weight distribution contraption. It's very subtle but a studied eye like yours should just be able to make it out. hehehe.


http://www.youtube.com/watch?v=GNHjD3Da5RU

 

The passenger seat is a nice feature. What are the two red nuclear missile launching switches on the center console, ABS reset and/or?

You are fighting the steering wheel a little to get the car turned in, but not bad. I have seen worse. I don't hear the front tires squealing. What tires are you using?

Pretty technical track, and a lot of tight/slow turns.


A couple thoughts:

Track is quite wide and allows some choice in line thru several of the turns and series of turns, which does give you some options and ability to adjust your driving style and line to the car's handling. Would be helpful to do a lot of comparison with data (now that you have it).

Is your car at or near full stock weight? The heavier it is, the more inherent push it will have in tight turns.

Looking at the video (which is difficult to assess), I certainly would consider setting the car up to rotate well (more rear spring/bar to be able to induce and control some mild oversteer). And also play with the adjustable dampers. The jounce and rebound adjustments allow some ability to play with turn-in characteristics independent to track-out. In theory, you would like to get the tail to rotate on turn-in but then tuck back in on track-out when you get back on the power.

You really want the front to turn in well, and I can see why you are concerned about push; too much understeer would hurt the car on this track.

Likely some option to do some deep trail braking in some to many of the corners. If the car gets too tail happy for the high speed corners, you may have to use a later apex those turns.

A lot of shifting required. Almost seems like a regular 951 3.375 R/P would be better suited for this track?

 

I was just at Watkins Glen with Ross last week….! Interesting guy, I like some of his driver ideas…but some are a little too far out there for me.

So – on to the video –
1) The non Porsche guy is quite the ‘tosser’ (non driving technical term)
2) Nice ‘secret’ contraption!
3) Do most Aussie track go counter clockwise? Almost all US tracks are clockwise. Seems weird since it both favor the nonstandard (for the country) driver position!
4) One of my favorite driver quotes may help – “it is amazing how many drivers, even at the Formula One level, think that the brakes are for slowing the car down”
5) Once you get to the real upper levels for track driving you do most of the balancing in the corners with your right foot. Your set up will change a great deal as you master this.
There are some turns at Watkins Glen where the fast way through the corner is a abrupt turn in followed by lots of power to evoke a some oversteer and then manage the throttle, if you don’t you will understeer. The set up of the car for power though the corner is totally different than the ‘gentle’ method.
Of course the set up for racing vs time trial is different too….