global west

[Lowend]

You are "allowed to clearance the firewall for exhaust clearance"

[Skip Fix]

At one time any mod for GT-1 was allowed in CP, they didn't use to bump you into mod unless it was a big change. But been a few years since I cut corners with SCCA.

[David Pozzi]

I saw a Corvette driver get protested because his steering wheel was 1/2" smaller diameter than stock. Also wheel offset has bitten a lot of people at nationals.

[Mean 69]

Sorry for the highjack on this, but what is the current hot C-Prep car/setup? Seems like there is usually one type of car that has the stronghold on a particular class, and the owner/drivers are especially passionate about anyone else coming in with something that can, or is percieved by them, to have any advantage at all. Case in point? NASA American Iron, "dominated" by late model (modified) Mustangs. Every year when the rules are looked at and changes to increase the competitive field are suggested, the incumbent Mustang owners SCREAM foul thinking that thier cars won't be competitive anymore. Result? Significantly dwindling numbers of cars in the field, and little (negative, actually) motivation for suppliers to bring in new technology or applications to come and play. I'd guess it's far less of an issue (i.e. number of cars) in SCCA, but...

David/Norm/Tom, do you have an approximation for the roll resistance for a leaf sprung car that is based on the torsional resistance of a particular leaf spring (i.e. the number Dave quoted)? The whole mess gets convolved when you put the springs in a car, they are coupled to the rigid rear housing, etc, it's not so straight forward as I see it? I know the argument for the Cat 5 forward bushing is that it reduces the friction associated with roll, there was a great article from one of the circle track suppliers I read a while back but can't find it now discussing this point.

I'm actually surprised that the "a leaf spring, is a leaf spring, bla-bla-bla" folks haven't interjected on this thread, which is refreshing actually. They aren't all created equal by any means. I can't speak to the Cat 5 stuff in my own personal experience as I have never driven a car with them, but a good friend of mine has. In fact, he won an AIX race out here in his rookie season two years ago in a 70 Camaro, which should be one heck of an endorsement for them, right? Er, except that he was the only car in AIX that finished that race, which is not to take ANYTHING away from the win at all, but rather to point out that there is often more to the story than may meet the eye (I can comfortably assure you that the other link suspension/torque arm cars aren't running out to swap over to leafs as a result of his win). I'll see if I can get some opinions from him, he doesn't post on the internet forums. One thing that I am certain he will tell me, make sure you put locking hardware/locktite on the bolts that hold the forward spring bucket onto the frame, it's kind of an issue when that piece comes loose...

Mark

[Norm Peterson]

Sorry for the highjack on this, but what is the current hot C-Prep car/setup? Seems like there is usually one type of car that has the stronghold on a particular class, and the owner/drivers are especially passionate about anyone else coming in with something that can, or is percieved by them, to have any advantage at all. Case in point? NASA American Iron, "dominated" by late model (modified) Mustangs. Every year when the rules are looked at and changes to increase the competitive field are suggested, the incumbent Mustang owners SCREAM foul thinking that thier cars won't be competitive anymore. Result? Significantly dwindling numbers of cars in the field, and little (negative, actually) motivation for suppliers to bring in new technology or applications to come and play. I'd guess it's far less of an issue (i.e. number of cars) in SCCA, but...

At least for the Mustang entries,

I know Padberg was running struts and Meier used to run shortened struts. I don't know what Meier ran this year but Padberg whooped all of us. Most of the fast cars are running a three link setup except Padberg who is using a torque arm. Not too many foxes anymore but the rules allow the same mods for everyone as far as suspension type. I like the idea of SLA on a fox but it was proven that it wasn't necessary last year.

I also know of one Satchell link car, but not whether it went to Nationals.

David/Norm/Tom, do you have an approximation for the roll resistance for a leaf sprung car that is based on the torsional resistance of a particular leaf spring (i.e. the number Dave quoted)? The whole mess gets convolved when you put the springs in a car, they are coupled to the rigid rear housing, etc, it's not so straight forward as I see it? I know the argument for the Cat 5 forward bushing is that it reduces the friction associated with roll, there was a great article from one of the circle track suppliers I read a while back but can't find it now discussing this point.

You actually have three components of roll resistance.

One is the roll resistance that's due to the vertical spring rate and the lateral separation between the springs (just like with coils; it's a K*t² thing).

Torsion along the leaves is a T*L/(G*R) computation, where the value of torsional resistance R can vary (either step-wise as in multi-leaf designs or gradually if either the width or depth varies gradually - read the latter as requiring an integration of R over L). Most of this comes from the front portion, but the rear also contributes.

The third is due to the fixity of the leaves at the axle, and the general approach that you'd use to write your own formula for sta-bar stiffness would work (i.e. keeping the properties of the arms in terms of the leaf dimensions and the torsional section in terms of axle tube dimensions instead of lumping them both in terms of bar OD and ID). Once again, you have to consider the front and rear portions of the spring as well as leaf property variation over their length.

Note that bushing and shackle stiffnesses will reduce the roll resistance actually seen by the sprung mass, and that the bushing effects are nonlinear (progressive). It's a "springs-in-series" deal, where 1/Ktotal = 1/K1 + 1/K2 + . . . + 1/Kn.

The presence of sliders instead of shackles changes the boundary conditions such that there would be rather little torsional resistance developed by the spring end so supported (as soon as the maximum + loading of the + to - unit loading due to the torsion exceeds the - unit loading due to vertical load, one side of the spring will "lift" and no more torsion can be developed . . . over maybe a 2.5" leaf width that won't take much and the rate becomes zero at this point, so you can use zero and ignore Item #2 for a slider ended leaf portion).


Dave - those sort of weenie protests have to be taken from Stock Category classes. That's also one of the reasons I'll probably never run Stock unless I'm temporarily at a jobsite, found an event, and entered with the rental car.


Norm

[chicane67]

Chicane
Just curious what you consider to be a better leafspring? The L2 has an extra leaf on the front side and is made from much thicker gauge steel than any other leaf I have seen for our cars.

One thing I can't abide by is GW's reccomendation for Koni shocks, they just don't have the right rates. You really need a tune Bilsten (at least) to make things behave properly.

Uhm, my FIA road race spring for one. It can be tuned from a 125lb build, up to whatever. The highest rate I have built is a 390lb. Spring steel type and where its from, spring steel thickness and how you stack it... individual leaf length, the type of spring clamps and where they are loacted... are all part just of the equation. There is WAY more to a leaf spring design that just adding a forward leaf to the spring pack. I dont discount Doug's spring... it has its place. But, there is much more to leafs than most know.

I dont like the off the shelf KONI's for various reasons, but Bilsteins are a piece of cake. You give me corner weights, spring rates and its intended use... and I can come up with valving numbers. I might just have a little experience with Bilstein and changing rear rates...

[Lowend]

I'm assuming the FIA spring is your creation? Is this a setup where you are swapping leafs in an out and moving clamps around to tune? Sounds like something I may want in the future.

[chicane67]

Actaully the original design was from Guldstrand, for Penske, and I implimented all the late model technology and thought... not to mention the time invested into multiple sets of springs and doing the swaps finding the balance for my rates.