Scott,

Exactly, and it does include the roll rate from both springs and bars - and should include tire rate and chassis effects, but for practical use these are ignored. I would prefer to use "total roll resistance rate" rather than "wheel rate" for this discussion, to be clearer, as wheel rate in roll can be dfferent than the wheel rate in bump, depending on the suspension design (your bump and roll rates from the springs should be equal with independent A arms). They become very different if sway bars are in the mix.

So the expanded version would be (Front Total Roll Resistance Rate/(Front Total Roll Resistance + Rear Total Roll Resistance)) Roll resistance is usually expressed in different units than wheel rate in roll, being a torque rather than a linear force, but they are proportional and since we are looking for a ratio, the units drop out anyway.

For your setup, and this is a just a guess because of your unknown sway bar rate, but working backward from the result you got - that it's noticeably more neutral to loose, an effective bar rate of 150 would give an initial FRRD of .50 and your changed setup would be at .45 This would predict some shift in balance toward loose, but also better transients and improved camber control.

If you know the dimensions of your sway bar, (inside and outside diameters, effective bar length, effective arm length and motion ratio of the end mounting points - same as the spring value if the attach to the same point) there are formulas to calculate the rate, although the effective rate is usually less than that because of bar and mount deflections.

I should have pointed out that because a given car is neutral at an FRRD of .45, for example, that doesn't mean that the next one will be. I didn't mean to imply that because some Cobras have FRRD % of .6 to .7 that they will push because of that number. It's merely an observation that the ones I've seen in that range do seem tight. The number only tells you approximately how the setup will balance. You really have to test the setup to find what FRRD % will give the best handling compromise, but once you know it, it's real use is revealed as you can use it to calculate how to increase or decrease your total roll rate, or change the amount you get from bars versus springs to adjust to specific track conditions, while maintaining the balance you've found works best.

Sorry for the novel, but it's a favourite subject.

John, thanks for the advise. I drove it again last night and it feels like it wants to oversteer quickly. I am thinking of going down in rear spring rate to around 500 from 700, since all I have are 500's or 700's. Anyway, when I go to the track, I had rather err on the side of the car pushing mid-corner and exit. With the 600's in front, it turns in way faster than before, so I like that. Scott

John is Right
 

John-

You observations reflect what we are seeing as well. A little loose seems to work pretty well for these cars.

-Dan

John.

Good discussion about F/R balance. But, do not forget roll axis and polar moment in your calculations.

I find that these are often overlooked and add confusion to the setup process.

You really need to know the moment arm from the CG to the roll axis, as well as migration of these points in a dynamic environment to make any meaningful spring, roll bar and damper choices.

Of course, this is all complicated due to driver input and lack of data acquisition capabilities. It is all seat of the pants with these cars, and I have found this to be wrong 90% of the time. It is really easy to dial a car out. What feels good is often slower in lap times.


You guys really need to plot your suspensions for camber change, roll centers, instant centers, pitch and yaw effects, suspension freqency and chassis frequency, etc. Then you will have enough data to start making some intelligent choices as to springs, anti-roll bars, and dampers.

I know, it is a bunch of work, but until you plot these things and understand what the effects are, you will chase your tails in the setup.

I Know, I spent a number of years chasing my tail.

Richard,

My problem isn't usually in passing over the details, rather it's in chasing them past the point of declining returns. I'm always tempted to do a full dissertation, but this isn't the place and I think the message can get lost in the numbers. My professors ensured that I acquired the engineer's usual excessive regard for measurement and analysis - I do my best to resist.

I agree with your general point. I think that much of the Cobra's reputation for difficult handling is due to a lack of development. The original car was never fully optimized, and now we face the large variety of sometimes questionable suspension compromises that are considered "Cobra". My live axle FFR is primitive and under-designed compared to one of your creations. Yet with two years of the kind of analysis that you propose behind it, and some opportunistic development of solutions and feed-back testing, it is now a very good race car that competes well. Despite having a less than sparkling pilot, to be honest. Those that start with less tattered sow's ears should be able to do at least as well.

I also agree that for some of us, a reasonable kinematic analysis should be done as a starting point, and that the information gleaned should direct the intial optimization as well as the on-track tuning. I also agree that the seat of the pants part is not usually done well. I wince every time I hear someone pronounce that they've thrown - fill in your favourite piece or adjustment - at the suspension, and now "it handles great". We will dyno test and obsess and spend endlessly on horsepower, but whenever I ask about suspension setup protocol or skidpad testing or data acquistion or even tire data - the glazed regard appears.

Now here comes the resisting part: Having said all that, I think we have to keep perspective on the issue. We are not (mostly) professionals, and most of us, despite calling it racing, never do more than circulate on tracks at comfortable speeds. The majority of us can probably get the improvement we need without the full development package. By doing just the most basic testing of our existing combinations we should be able to cure the most obvious handling ills and learn to adjust for basic track conditions. Those chasing the bowling trophies will have to do more, but perhaps the rest of just need something akin to a Stage 1 suspension setup program.

I had some sophisticated measurement and computational tools available, but that certainly isn't the usual case. So, faced with an existing design that was getting around the Glen in a reasonable but typically Cobra fashion, what would you do first with the tools available to the average enthusiast?

cheers,
John

I appreciate all of the comments and am mostly interested in learning. The cobra is the first and only road race track car I have ever driven. I finally have driven the cobra enough on the track to kind of get a feel for what it is doing and want to improve it. But, my feedback is limited as I am not as experienced as Richard or John behind the wheel. I have enjoyed talking to Richard in the past but am lost on some of the concepts in his last post. Even if I could understand it, I don't know if I could apply it. But, by asking questions, I will keep learning and improving the handleing of the cobra along with the driver. I spoke with finishline motorsports a few days ago about the set up they are running. The shelby spec racer cobra's have the original suspension design as far as spring placement and angles other than a raised rear roll center(for some reason). My hi-tech is also the same as the originals. The spec racer runs a 600 to 650 pound front spring and a 400 to 450 pound rear spring with a sway bar front and back, spring rates depending on track. So, John, your front to rear total wheel rate of .6 to .7 is probably dead on with the spec racer. I just want to have a little more fun. Scott

John,

I agree most heartedly with your statements above.

As to what I would recommend for a basic setup process for the typical Cobra follows: (Please Note: this leaves out camber curves, Spring rate changes, anti-roll bars, etc. We assume that these settings cannot be changed at the test session due to lack of tools, parts, etc.)

1. Find a skid pad or like area where one can drive the car in steady state. At least 200 foot radii if possible.

2. Unhook anti-roll bars and set dampers to the softest settings possible. Set tire pressures to lowest recommended settings. Check static cambers all around. Chalk tires inner and outer edges 1 inch up sidewall and 2 inches into tread. Write everything down!!!!!!!!!

3. Drive car around skid pad at a medium speed (No sliding) in the highest gear possible to establish driving technique. (It is very important not to over drive the chassis) Do 10 or so laps to bring tires up to temp. Check tire pressures. Check tire roll by examining the chalk. (If you have a pyrometer, check temps. This is the best way to establish minimum tire pressures.) If too much/too little tire roll adjust pressures. WRITE EVERY THING DOWN!!!!!!

4. Again drive around the pad, but this time progressively increase speed until the car will no longer maintain the arc. (Important!! Time the laps.) At this point you will discover whether you have understeer or oversteer as a basic characteristic of the chassis. You also know what lap time the car will produce with the current settings. Check hot tire pressures and look at the chalk. (Once again, if you have to much/too little tire roll adjust pressures.) If tire contact looks good, go to next step.

5. Now we are going adjust tire pressures to see if a "Neutral" chassis can be achieved with pressures alone. (Note: 1 psi tire pressure is equal to aprox. 50>75 lbs wheel rate.) Adjust pressures up in 1 pound increments on the end that is sliding first. Drive around pad until car will no longer hold the arc. (TIME THE LAPS.) What change did the tire pressure make in the balance? Keep adjusting and testing until the chassis is neutral. At this point, examine your data. What is the tire pressure differential front to rear. (If it is more than 4 lbs. your spring rates need changing. The tire pressure differentials will tell you approximately what spring rates changes are needed.) Check the times. Are you quicker? If you are not, you are overdriving the car, either with throttle input or steering input.

6. Now hook up your anti-roll bars and do the whole thing again. The car will feel much better due to lack of roll. TIME YOUR LAPS!!! Adjust tire pressures until the car is neutral. If the tire pressures are more then 4 psi differential, your bars need resizing. (Or adjusting if so equipped) Are you quicker?

Now comes the tricky bit. Dampers. You can use the skid pad to start the adjustment process, but setup of dampers is really track and surface dependant, but the pad will get you started.

1. Mark off a entry point onto the skid pad and a exit point. (You will want a run up area that allows you to reach the speed that you were attaining in the previous tests.) Leave the dampers set soft.

2. Treat the skid pad like a 90 degree corner that you do not have to brake for. Accelerate to the turn in point and pick up the radius of the skid pad and follow it to the exit point. (Have someone time you between these points) What did the chassis do? Did it initially understeer? Oversteer? Was the transition too fast or feel abrupt.

This is one adjustment that your are going to have to do by feel. The best thing to do is to change the damper settings in stages. Front bump harder, Test. Rear Bump harder test. Front rebound harder, test. etc. etc. When the chassis feels good to you in its turn in and exit transitions, check your times. Are you quicker? Add in braking on the entrance. Adjust as required.

OK. Now that you have done all of this, you should have a chassis that you can take to the track and at least be able to get around with some confidence.

Now, you can start the real chassis development and spending contest.

Richard,

Well done! A concise method that could be followed on any reasonable piece of parking lot. I'll propagate it with credit if you don't mind.

Perhaps a caveat should be added for the live axle crowd. Because of the roll/bump rate split caused by the side and end view geometry differences, we are unlikely to be able to get a balance as per #5 on spring alone, without resorting to very high rear bump rates. For my initial setup I tried to calculate a bar contribution for the rear that would give initial balance with no front bar and then increased the bar contributions proportionately for subsequent bar-in balancing, once I was happy with the frequencies. Just another advantage for the IRS crowd.

Regards,
John

John,

You are correct. Beam axle stuff gets really difficult due to geometries. Roll center height, toe changes, torque reaction, etc.

I really do not have a lot of experience with beam axle cars as most all of my time has been been spent with formula and sports racing type chassis.

I have found that the setup basics as I described still work, but the data for spring and roll rates is very much different than a IRS.

(Of course there is the old long parallel radius arm high camber change IRS design, which would make a beam axle look easy.)

for some reason the above posted twice. Delete does not work.

It posted three times!!!

So Scott, now that you have had some time to digest all the "Black Art" of chassis set-up from two very knowledgeble sources, and a couple of fools like me, where are you? What did you try and what results did you manage? I need some help here as Cranky has me in a dual to the .001 second with another small block car at the Glen. He's got a lot of current seat time, 50+ Hp, about 200# less, but I'm not afraid...I got a Big Block!!
And a few tricks he doesn't know about. Now where did I put that JATO booster...

McFEz

McFez, You have the downhill (weight advantage) and against the wind(more torque) and the BB fear going. I am running this weekend at PIR so we will see. I am at 600's in front and 500's in back with some extra rear springs both ways if its pushing or oversteering. Corner entry is much better but mid corner and exit ,its pushing a little, with street tires. I had rather have some push first time out and then adjust from there with the slicks on. scott

Well, I ran PIR yesterday. The cobra started out with lots of oversteer. I thought at first, that my back tires were goimg away. I started making air preasure adjustments. I started the day at 22 pounds cold all the way around. I wound up at 16 pounds in back and 19 in front cold. Hot preasures were 24.5 to 25 all the way around. At this point, the car was working great, controlled drifts in several corners. The spring change up front allows the car to turn in quicker and reduces the chassis roll I was feeling before. The tire presure changes really helped. I know that goodyear says 28 to 30 hot will give you the best bite. I had been hearing from some Stock car guys running the goodyear bias ply's that they were running less preasure, starting in the teens cold, mid 20's hot. Well it really helped me. Comments? I think I'm close now as far as set up goes. The driver as usual needs all the help. Good group of guys running too. There was only one car that got by me yesterday. A well modified zo6. Lots of other zo6's, zr1's, 911's and a few mustangs. At PIR you get to run turns 3 and 4 in the cup oval which is around 100 mph for me. Fun but scary with that wall up there. Scott

Scott,

What were your hot tire pressures when you started at 22 psi cold?

Was there a big difference front to rear?

Also, did you get your tire temps after you got your final settings?

Have you changed your damper rates to match the new spring rates?

Those Z06's are pretty quick, aren't they.

Richard, 22 cold, hot temps were 26 front and 28 rear. Yes, final temps were 24.5 to 25 all the way around when starting at 16 rear and 19 front. I have not changed the dampers. I would not know which way to change them. Also, trying to stay with 1 change at a time. Yes, the zo6's are quick. The vette guys all trail brake at most corners in the infield. My cobra does not behave under trail breaking. It behaves much better with early brake, earlyier throttle in the corner. scott

Hi Scott,

What I meant was tire temperature. The reason that I ask this is that your pressure gain is a bit odd.

It indicates that you are working the fronts way harder than the rears. There is nothing wrong with this, but it indicates that you have some gains left in the chassis setup.

I know that damper stuff is difficult, but it is more critical than any other part of the setup at the end.

Richard, explain that to me. I thought that the 6 pound preasure gain up front versus a 9 pound gain in the rear would indicate that the back tires are working harder? The back tires are also wearing much faster than the fronts. when I have taken temps in the past, the backs were 10 to 15 degree's hotter than the fronts. A friend was bringing a tire temp guage and forgot. The pro shocks were set at 5 and 7 out of a 1 to 8 range, whatever that means. Scott

Richard
I'm confused by your statement "working the fronts way harder than the rears". The fronts gained less pressure than the rears both initially, and with the final setup. I thought the harder working tires gained more heat and thus more pressure. I understand how Scott adjusted the pressures (and the corresponding effective spring rates) to gain balance. And a little about checking tire temps to help get full use of the contact patch available. But shocks????

Please explain your thoughts on this for us. And any light you can shed on shock set-up would be most appreciated.

Don

Scott and a427sc,

.Sorry Guys, a brain fart.

I miss-read the post. I had reversed the numbers in my head. (It is a habit for me to always speak of front numbers first and rear numbers second)

However, the numbers indicate a even bigger dynamic inbalance in the chassis as they are. This could really be just Scott's method of driving (BIG RIGHT FOOT), too small a rear tire, wrong compound, or rear bump rates too high.

I do not know the valving on pro-shocks, but from your numbers ("The pro shocks were set at 5 and 7 out of a 1 to 8 range" Note: I am assuming 5 front and 7 rear and equal bump/ rebound adjustment per click) I think that you might have the rear set too high in bump and the front may be too high in rebound.

And your statement "My cobra does not behave under trail breaking. It behaves much better with early brake, earlyier throttle in the corner" indicates too light a bump in front and too light rebound in the rear. That aside, In a high horsepower to weight ratio/short wheelbase car, your method of driving has proven to be the fast way around. (So stop giving yourself a bad time about your talents. You would be real surprised at how few drivers can even tell understeer from oversteer.)

However, it is fairly normal in a high horsepower car to see the numbers that you are seeing. There is only so much that can be done with the dampers to bring temps into balance. (The dampers control the transients remember, and therefore the loads the tire sees over a given amount of time. If the bump rate on the rear is too high, the weight transfer is too slow, therefore allowing the tire to spin due to improper transfer timing.)

The next step if the dampers are right and you continue to have high rear temps is to split compounds. (You must take temps before you look at compound changes) I do not know if Goodyear has a slightly harder rear compound in your rear sizes, but you may need to go a bit harder in the rear to handle the output of your motor.

a427sc. Dampers are a subject that we can use every bit of bandwidth available on this forum to discuss and debate and still not truly understand the effects on a given chassis. that is why all good race teams have a engineer that does nothing but dampers and he/she spends all of their time on a damper dyno in the truck all weekend. (PS: they get paid real well to know how to make them work. Just go to a NASCAR race, you will notice a really big trailer that says PENSKE SHOCKS on the side. this is the busiest trailer in the paddock. The same in all the series, the damper truck gets all the attention from the race engineers.)

Scott, Sorry to tell you this, but you are at the point that you need to go to proper race dampers. Penske etc. I know the numbers are big, but you will not get that thing where you want it until you have proper dampers. (Just ask Boris.)

Sorry for my Sunday stupidity. I need to read better