Are lightening the 911 and torsen bar size/rigidity related?

[jluetjen]

The answer is yes, weight and spring stiffness are related. It is known as the suspension frequency. If you really want to get into it, you'll need to calculate the natural frequency of your suspension. This is 187.8 * sqrt(Wheel rate (lb/inch)/sprung weight (lbs)). The result will be in cycles per minute. Note that the wheel rate is not the spring rate but the rate measured at the wheel in the normal range of travel. You can see how as the cars weight goes down, the denominator will get smaller.

In general road cars are in the 60-80 range, 80-100 for sporting cars and 100-125 for non-ground-effects race cars. BTW, full ground-effect F1 cars run a rate of about 200-500. In order to avoid pitch the front end should be 10-20 CPM lower then the rear.

I haven't gotten around to calculating the numbers for my 911. But I for one am a believer in a car where the suspension moves and "suspends" the car. If you have ever driven a FF, you'd be surprised how "softly" sprung they are. It's not like they have wooden blocks in place of springs. The springs actually move when on track.

[KTL]

I'm glad John chimed in. He always provides in-depth answers to suspension questions. Seems like I learn something every time he posts!

by dd74:

Two thoughts I have for my car this year is to firstly shed additional weight, then secondly do suspension work which could entail larger torsion bars. I only wonder if the larger torsion bars are necessary provided that the car will be lighter. Hence, can the lighter weight of the car's unsprung weight "act" as if it has heavier torsion bars.

I apologize if I'm not being clear, or that I'm not "catching on." Maybe this is just a bit beyond my ability to communicate it effectively...


Please bare with me.

Dave,

Like Ryan said, a significant weight loss program will mimic the installation of bigger t-bars. But the added benefit of weight loss would be the lesser amount of work/stress to which the tires and suspension components (not only the t-bars but the control arms, bushings, ball joints, etc.) are subjected. Brakes become more effective with weight loss as well!

How far you want to go with the weight loss is up to you........... Kinda sounds like something from The Matrix. "I'm trying to free your mind, Neo. But I can only show you the door. You're the one that has to walk through it."

And we'll bear with you on this subject. But I for one WILL NOT bare with you. Might want to seek out a different kind of BBS for that. Not that there's anything wrong with that........... (obligatory Seinfeld reference).

Just messing with you! Sorry about the spelling correction- damn those homonyms!!

[dd74]

Thanks, KTL. It was a rough day yesterday. Trying to think about weight loss, torsion bars and (outside of this thread) the animated history of Porky Pig, created many disproportionate thoughts and thus, spelling errors on my behalf. Your patience and understanding with this has been much appreciated.

[kjb]

Originally posted by jluetjen
This is 187.8 * sqrt(Wheel rate (lb/inch)/sprung weight (lbs)).

Given this equation, a 20% decrease in weight (2500 lbs -> 2000 lbs) would give a 12% increase in suspension frequency. How much of a torsion bar size increase would it take to have the same effect?

I suspect there's a simple formula relating torsion bar size to wheel rate, which would answer the original question.

/ Johan

[Randy Webb]

The suspension frequency arise from the model for the suspension, which is noting more than a damped harmonic osciallator (as I am sure everyone recalls from 1st year physics). It sets the time period of the excursions (compression and extension). But there are 4 wheels so that is where the model breaks down a bit. The shocks and flex in the tires determine the dampening rate of those osciallations (hence the Brit. term for shocks, and it is more accurate). Probably the main incorrect assumption in such models is that there is no friction in the tires or shocks that affects the amplitude and time period of the excursions.

If there is anything else I can do to obfuscate the original question, let me know.

[KTL]

Originally posted by Randy Webb
If there is anything else I can do to obfuscate the original question, let me know.

Keep it up. You're doing great. Had to reread your post a few times to keep up.

And since I had to look it up, today's Word of the Day is..........

obfuscate (ob-fuh-skate):

verb- 1. To make indistinct or dark. 2. To confuse or becloud

Obfuscate away! That's how we learn stuff.

[dd74]

"Damped harmonic oscillator..."

"obfuscate..."

My mind is slipping...I can barely hold onto this thread...and I created it! Arghhhhhhh!

[Randy Webb]

dd -- you don't need any of this stuff (tho it might provide some aesthetic pleasure, just as a fine equation does to mathematicians).

Just follow your plan. Take off the wt. Drive the car. Change the bars (etc.). See how you like it. Lather/rinse/repeat until satisfied or out of money. I posted that to enhance understanding for those who are fairly deeply into it. Most car people don't realy understand what they are doing, tho Pooschey drivers are better than most.

You need to understand this the way a HiFi afficianado needs to understand psychoacoustics and circuit design. They only need to listen (under the controlled conditions) and compare (in a double blind test). Same for you. Good Luck.

[DG624]

I should have asked before I cahnged my bars('86 911). I just went to 21/26mm and I think I should have done a 21/27 or 22/28. I was worried I would have too much bounce. I have adjustable Shocks which should reduce a rough ride. I have not experienced much oversteer with the current setup. I am afraid to push it too much on the track and in Auto X it is not really going to get lose. How much will a mm make on handling?

[Wil Ferch]

Oh boy...there's a lot more here.
First..I posted an equivalency chart on lb/in spring rates vs torsion bar size on the Rennlist Tech board. I think it was repeated here in one of the posts.
Second...adjustable dampers usually have only rebound control..not jounce. Even so, the total "force" vs shock piston speed graph...as seen on a shock dyno...can tell you a lot if the chosen shock will be harsh or not. If you envision force on the "Y" axis...and travel on the "X" axis...the general shape of the curve is like a camel hump. The top-most point is the total lbs of damping force, and first thoughts are that the higher the force, the rougher the shock will be. Not necessarily so...the "ramp-up" of the curve ( how steep the curve is) as you move toward the right on the "X" axis...is a more accurate determiner of how harsh the shocks are.
Revelation: ....a gentle ramp-up with "high" ultimate force numbers is actually a "softer-feeling" shock than one that has lower peak values...but the ramp is very steep.

Got that ????.....

---Wil Ferch

[jluetjen]

Wil;
I had a copy of the spreadsheet that you described, but I can't find it any more. Didn't it calculate the spring rate of the T-bars as opposed to the wheel rate?

I guess if you combine it with the ride height spreadsheet it can be worked out.

[Wil Ferch]

Correct...it didn't go as far as detrermining wheel rate. For me, that's going way too far anyway. You've got the "tire" component at what?....about 1000lb/in spring rate with some sort of damping model applied...that is now in-series with the suspension ( spring) rate and its dampers??...

I'm an engineer by training at even that makes my head spin...

---Wil Ferch

[jluetjen]

You engineers are all the same!!! If you can't believe that it will be nuts-on to the 4th decimal, you won't even bother going there. With that approach we'll all just sit around repeating conventional wisdoms and urban myths to each other.

Personally I take the approach (at least in regard understanding my 911) that some understanding of the wheel rate and suspension frequency (at the wheel as opposed to at the tire) is better then nothing at all. At least you've reduced the set of complete unknowns and most likely bounded the problem some. There's also a good chance that you can possibly identify and eliminate (after some experience is gained with the data) some gross problems.

Oh well, I guess I'll put it on the list of things that I'll try to figure out on my own when I get some time.

[speeder]

As much as I am fascinated by the technical explanations given here, (and I really mean this), , I do not believe that one needs to know any complicated math formulas to set up a street/track car, anymore than I need to understand the inner workings of my television or computer in order to reap the benefits of it.

Stiffer springs, (or torsion bars)= less suspension travel. Simple as that. If you try to raise or jack up one wheel of a race car, at least 2 wheels will leave the ground simultaneously. Is this what you are aiming for in your daily driver that sees ~10 hours of track time a year? I know that it isn't, and you just want a great handling car, (like me and most of us here), my advice would be to get more seat time in DE events w/ experienced 911 pilots like Tyson giving some input. My car has stock 19/24.1 T-bars, Carrera sway bars and Bilstein sports, All I really need is slightly more aggressive camber settings to lose most understeer, and maybe 26 T-bars in rear. (Which I have in living room). Eventually I will want R-compound tires, but then that will raise the stakes slightly on suspension forces.

For now let's just try to get track time, once I can drive my car the way it is as fast as Tyson/Jack/Otto/Dave B./Marshall can drive it, I will start blaming the car for my lap times. Hopefully this will happen before my reflexes go due to age.

[Randy Webb]

"You engineers are all the same!!!"
- Yo mon -- who you dissin'??

I are sceintist -- not engineer!

Remember: "The first time it's science -- after that, it's just engineering."
(Not that there's anything wrong with that -- to bend a Seinfeld quote a bit...).

BTW, the 4th decimal place thing you describe John, has a name -- it's called the error of misplaced precision. I've seen it at work and the result is not pretty - lost NIH grants, etc.

And, Wil -- I got it.

[dd74]

Very valid point, Denis, which Randy echoed to some degree.

As far as I can follow within the context of this thread is that aside from the math and engineering equations, the thread's subject was diverted somewhat from my initial question (as poorly worded as it might have been), which was can lighter body weight (i.e. f/g bumpers, hood, doors, etc.) compensate a need for stiffer torsion bars.

In my thought the answer to my question is "yes, lighter weight can supplant the need for stiffer torsion bars." But I know better than to rely on what is more or less guesswork on my part, so I posed the question. In return, I received science - which is all very exciting to read - but to understand is beyond me.

My car might undergo another weight loss program soon, so time (track, Mulholland and otherwise) will tell. All I am doing now is the cause-and-effect research.

[dd74]

Originally posted by Randy Webb
...the error of misplaced precision...

You forgot the copyright.

[jluetjen]

...which was can lighter body weight ... compensate a need for stiffer torsion bars?

The answer is Yes , but it is not as simple as...

Stiffer springs, (or torsion bars)= less suspension travel.

There is a lot more to the relationship of spring rate to handling then that conclusion. My apologies for appearing to hyjack the thread and confuse the issue, but I figure that the right answer is a lot more valuable then a pithy one.

Anyhow, at the end of the day I agree that in general seat time is far more valuable then any mechanical tweak to the car.

[dd74]

John - I'm definitely not into pithy answers, and I agree with you about the statement you outlined about stiffer torsions + less suspension travel = better handling. That's why it'll be a step-by-step process with weight loss first, then possibly more suspension work.

[Wil Ferch]

jluetjen:
You really mis-read my comments...

I was simply saying guys like you should be admired for taking it to the nth degree...for me at this point, it makes my head spin!

Never said it wasn't valid or that it can't gain some understanding ... remember...I'm the inventor of the Ferch Method (tm) for finding ride heights with bigger torsion bars ... . That wasn't exactly 5 minutes work , either...

And Randy Webb...amen....precision vs. accuracy. I work in the multi-national corporate world...all precision...lousy accuracy

---Wil Ferch