IPSC

Bilstien Escort Cups with 350 up front and 500 in the rear. This was the set up as it came from Tony G. I also run Weltmiester sways and the balance of this is great for a street / DE car.

IPSC

Ballistic-Chapman

Do coilovers require stiffer springs? I would much rather upgrade to coilovers in the rear if I can delete the torsion bar.

Anyone else running coilovers with torsion bar delete for an aggresive street car? What spring rate???

chris951

Sorry to get off topic, but is the white and orange car in the pics a Stohr, West, or SCCA racer?

shiners780

No problem. It's actually a Gulf Porsche color scheme, see pics below. My camera sucks so it looks white in the pic above. It's a JP1 Jaguar with a Cosworth V8 277hp engine, 1500 pounds, lots of carbon fiber, digital gauges on the steering wheel. The owner just had it shipped over from England. He got it as commission for helping a friend buy $1.5 million worth of cars from Brazil.

I'm still interested in hearing what other people are using for spring rates with TB delete, and how the car handles when really honkin' on the track. So please share 'em if you got 'em.

shiners780

Streetability should be fine depending on the spring rates you choose. Coilovers certainly make height adjusting and corner balancing a lot easier than with torsion bars.

Here's a random video lap of Watkins Glen from Wednesday. I think it's during my second session out on the RA1's.

http://youtube.com/watch?v=sa0ml1UeXkc

RolexNJ

Very nice laps....

Van

I've heard many people say "I indexed my torsion bars to zero (or neutral) with my coil overs" in these types of conversations, and I just don't get it... I've thought about it until smoke comes out of my ears, and the only thing I can figure is that people have some misunderstanding/misconception about how springs rates work.

What matters is the "wheel rate in pounds per inch" -- meaning how many pounds of force does it take to raise a rear wheel one inch.

It seems pretty straight forward that with a coil over rear end, the spring's rate (let's say 500 lbs/in) will have a reduced effect on the wheel rate due to two things -- the distance along the control arm where it is applying its force (think about the leverage of a breaker bar or piece of pipe on a wrench here...) and its angle to the control arm (at an angle, if the spring mounting point moves up 1", the spring will compress at rate relative to the sin of the angle).

I've taken careful measurements and thoroughly researched the formulas required -- by my calculations, on my car, a rear spring rate of 500 lbs/in will result in a wheel rate of 255 lbs/in. This means it will take 255 lbs of force to raise a rear wheel one inch.

Now, with torsion bars, just like with coil springs, the "spring" effect is linear. Due to the geometry of the rear suspension, the torsion bar must twist 3.475 degrees for the wheel to move one inch. Knowing the diameter of the torsion bar, we can calculate the wheel rate for torsion bars. For example's sake, 27mm solid torsion bars, by my calculations, have a wheel rate of about 216 lbs/in.

Because springs force is a linear function, for BOTH the coil overs and the torsion bars, it takes only half the wheel rate to move the wheels only 1/2 inch, and twice the wheel rate to move the wheels two inches. Obviously, it take zero force to *not* move the wheel -- like if the car is parked in the garage.

So, why do I not buy the "neutral torsion bar" theory? Because the torsion bar is a spring, and the moment the wheel starts moving up or down, its force will be acting on the wheel. In a torsion bar only setup, the bar is "at rest" when the car is jacked up (actually, it will be rotated a little in the opposite direction due to gravity pulling the wheel, hub, control arm, etc down). When the car is lowered, and the car's weight rests on the suspension, the torsion bar will start twisting.

If you take a stock torsion bar setup and just add a coil over shock, your wheel rate will be the torsion bar PLUS the coil over. In the example above, that would be about 471 lbs/in. Because you've increased the amount of weight it will take to move the wheel one inch, the ride height will be different (higher) unless you add more weight to the back of the car (like a bag of sand).

Now, let's say you've done all the math of angles and ride height, and spent the time to reindex your torsion bars to this 'supposed' neutral position -- what you're saying is that, at static ride height, the torsion bar is at rest, and the coil over spring is carrying the weight of the car. However, if the wheel moves up (as if you've hit a bump, or stepped on the gas to transfer weight to the back), it will start to twist the torsion bar -- and, thus, the torsion bar will start helping the coil over by adding its linear spring rate constant. You still have a wheel rate of 471 lbs/in – if you want to raise that wheel one extra inch, it will take an additional 471 lbs of force to make that happen.

So, your spring rate is still the addition of the two springs – as long as you have two springs in the system, both must be accounted for. The only advantage to the reindexing is to adjust your ride height so you DON’T need the bag of sand to achieve the original ride height after you’ve ADDED extra spring stiffness.

Now, to answer the OP’s question, if he hasn’t figured it out already, people’s spring rates span the gamut and personal preference and driving style is what counts.

Personally, I’d recommend about equal spring rates front and rear – which will give you a slightly higher wheel rate in the rear – since it’s the wheel rates that matter. As for the wheel rate calculations, that’s a subject for another day (and, if I recall, I’ve commented a bunch on it in the thread Skip mentions).

shiners780

Van, I've thought about the "neutral" torsion bar setup as well and like you, I never fully understood the point of it's usefulness.

Spring rates certainly do, like you say, run the gamut. Basically I wanted to determine where my current setup falls into the mix. I believe my driving style is not extreme in any way, so my setup should not be at either of the extreme ends of the setups being used by others. Hence my "poll" of what others are using.

You've done some great math work relative to our car's suspension geometry! Thanks for sharing that. Interesting stuff.

The one thing you confused me on was this statement: "Personally, I’d recommend about equal spring rates front and rear – which will give you a slightly higher wheel rate in the rear"... If the actual spring rates are equal front and rear, the rear wheel rate would be lower not higher than the front. Maybe I didn't read your statement correctly?

Thanks again for all the info.

Oh, and thanks Rob!

333pg333

Shiners, I knew I'd seen you on Youtube before. I remember the BMW cutting across you on another day. Looks like a good fun track with some nice banked corners that you can go flat out on!
As for 'Neutral' t-bars, possibly what they're referring to in a round about way is some cars that have to run with t-bars in their respective series', 'customize' their t-bars so they have no bearing on the suspension and with the aid of the 'helper' springs can effectively run a coilover rear setup.

IPSC

When Tony G set me my current set up, he had estimated about 275 effective spring rate out of the 500lb rear springs. So he wasn't far off.

IPSC

Van

A little about motion ratios (the ratio between spring rate and wheel rate):

This website has good diagrams: http://www.miracerros.com/mustang/t_wheel_rate.htm
And for a more detailed discription of the formula, Wikipedia has a good entry, too: http://en.wikipedia.org/wiki/Suspens...%29#Wheel_rate

Anyhow, it's all pure math and geometry -- first you measure the distance between the chassis pivot point and where the centerline of the front strut crosses the control arm, and then you measure the distance from the chassis pivot point to the centerline of the contact patch (centerline of the rim).

On my car, that gives me a ratio of about .73 -- that means, if the strut was perfectly vertical, then the wheel rate would be 73% or the spring rate. However, with a strut angle of 72 degrees, it turns out to be a little less -- 50.4% by my calculations. Now, this may change depending on YOUR rims and ride hight (strut angle).

Using the same math, the ratio of spring rate to wheel rate is 51.1% -- just a little higher than the front. That's why I say, with identical spring rates, you'll have a slightly higher rear wheel rate.

*Note* for the picture of the rear geometry, the distance from the shock mount to the pivot line of the rear control arm (the line between the inner mount and the outer mount) is about 12 inches.

Skip Wolfe

The only thing I can't figure out is Porsche Motorsports state the rear effective rate is 42%, so I am worried we are missing something with the rear measurements/calcs.

Van

My car is lowered a bit, which gives the shock a more upright angle, which could account for a few percent, and I'm running rear spacers. And then, if my measurements are off by a little bit, say a quarter or half an inch, that would change the values, too.

If the Porsche values were calculated by actual testing, with a scale and with a dial indicator to measure wheel movement, then the rubber suspension bushings would also be a factor, because they will take some force to overcome their "rest" position.

Someday I plan to use a scale, floor jack and dial indicator to check my measurements -- so until then, take this for what it is -- some stranger's measurements with a tape measure!

Personally, I trust Porsche's data very much, but also don't think my data shows that large a variation.

Oddjob

Setting the rear t-bars to "neutral" is in reference to how they are indexed, not their affect (or lack of) on spring rate.

Pretty common when using helper coilovers to set the rear bars to neutral at ride height. In reality it doesnt actually happen, but you can get it pretty close. Idea is that the car will sit entirely on the coilover spring and the t-bar is not being twisted (with the car stationary). When the suspension is loaded/compressed, then the torsion bar is twisted adding to the combined wheel/spring rate. However, when unloaded (when braking, or the inside wheel in a turn) the torsion bar acts in the opposite direction as the coil spring, reducing the overall combined wheel rate. Some guys like this affect, claiming that it reduces back end climb when braking.

333pg333

I have posted this before but it is in context. I was asking Karl from Racers Edge to figure out my rates which are 616lb/in f and 708 lb/in r (effective) and he was replying to someone else calculations that were not quite correct;

"I have not spoken with Bob, but believe me, my numbers correct (or close to it - see the following explanation). The info on Paragons site is stuff they got from me a few years ago. In any case, the 47% is a number that I have calculated By taking measurements. I am now pulling out one of Porsche's own Motorsport sheets that shows all the rates of thier Turbo and "Cup" cars back when they ran competitively. Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then give the variable rate coilover helper spring rates at 34-65 N/mm which is 194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.

Bob is right about them being inboard but his numbers are off. Actually they are correct I think in that the motion ratio is about 65%. But when calculating wheel rates from spring rates it is the motion ratio squared that is uses. So 0.65 ^2 is , guess what,... 42.25% which is the number that Porsche's own sheet claims as I outlined above. So your torsion bar is 177 at the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in coilover (initially I had 661 lb/in which is attributable to my measurement error - I had 47% and Porsche lists it at 42%).

So it may be a bit stiffer in the rear than I might run but with the ability to tune sway bars etc... you should be Fine. Your setup is actually much stiffer in the front than the Porsche cup setup which ran progressive front springs (200 - 371 lb/in) with the rear setup I described above which is not that far from what you have( yours is 296 at the wheel and theirs was 259 - 334 lbs/in at the wheel).

Hope this make sense, but believe me, what is above is 100% correct. I can fax you the Porsche motorsport sheet if you think it will help.