Rear swaybar tech needed

[shmoov69]

Ok, after seeing my car in pics at the MMCC last summer it looks like it has quite a bit of roll in the rear.
First, how do you figure out how much bar you need without going too much?
Second, can you have the bar on the car and the links on the rear?
Third, does the bar have to be parallel with the rear with the ends parallel to the ground?

The suspension is a home fabbed TQ Arm setup with coilovers.
Pics are on my Fquick I think. WWW.fquick.com/shmoov69
Thanks!

[exwestracer]

You can mount the bar either to the chassis or axle housing (as stock). It's mostly a matter of convenience. Yes, the bar should be parallel to the axle, the bar arms (ends) should be parallel to the ground, and the end links should be 90deg to the arms in both planes.

Every bar has a spring rate, just like a coil. That's why racers prefer the straight, splined bar with separate arms. That way you are only changing the active bar rate when you change sizes. You will need to know the up and down travel of the rear springs in roll to figure a bar rate that will reduce travel the amount you want. It's VERY easy to go too stiff on the rear bar. The car will be "slidey" and lots of fun to drift around, but probably not as fast.

[Flash68]

One good start would be to talk to Dave @ Hellwig or Marcus @ SC&C.

[Dave@Hellwig]

We offer an adjustable 3/4" rear bar for leaf spring applications. We can do other sizes on a custom basis. To get the sway bar size in the right ballpark you need to know your roll center location vs stock and how much spring rate you have.

[shmoov69]

Thanks guys. It is coilover with 200# springs. It sits approx 3 1/2" lower than stock in the rear and with the springs, panhard and all the brackets, there is not much space!
How the heck do I figure the roll center??
Thanks!

[ArtosDracon]

Looks to me like you're going to proabably need a splined bar mounted to the front of the axle. Roll center on a solid axle rear suspension is going to be at the average height of the panhard bar. Given the height of your panhard and your 3.5" rear drop you likely have a very short fulcrum between the CG and the RC so you shouldn't need too stiff of a bar back there. Given that your coil overs appear to be mounted vertically your sign rates for roll are pretty easy to calculate. Without having the car corner scaled or at minimum front and rear scaled it's technically impossible to calculate the exact right bar for you. First and foremost we need to know what length of arms you can fit, that will dictate the size of the bar you can use. We also need to know the springrate on the front as well as what the roll stiffness is on your front bar. That way it's possible to ensure you don't over-do the bar width and get the car too stiff on the rear.

Theoretically, to create a completley neutral roll steer, we would need to calculate total roll stiffness for the front, taking in to account your roll center, CG, total weight, corner weights, springrate and your roll bar. Then calculate the exact same rate for the rear without the roll bar, and supplement your rear with a roll bar so that the rear roll resistance is proportional with the front based on weigth bias.

[Ron.in.SoCal]

Robert - you're hired! That's some good stuff I'll reference when it comes time to tune my suspension...

[shmoov69]

Holy cow!! That is some advanced tech support there!! LOL!
Well, that is alot for me to take in and try to figure out with my pea sized brain. I was hoping there was an easy button or something! LOL!

That gives me something to shoot for and I'll try to get that info sometime in the near future. I'll prolly have to bug you some more with more questions.
Thanks for the detailed answer!

[ArtosDracon]

My pleasure, and it's certainly not "wrong" to pick somewhere to start based on an educated guess and tune from there. Take Penny as a perfect example. It's got a budget most of us can't touch and some of the best minds and manufacturers on the planet all throwing everything they've got at it, and it takes some tuning. There is always going to be one HUGE X factor in suspension design. The driver. I tend to reference an "ideal" when talking about suspension design, because there is this supposed magic spot where the planets align and your car will be perfect, until you add the nut between the steering wheel and the seat, lol.

Personally I like a balanced roll resistance with slight negative ackerman and 2-5* of roll oversteer. To me that makes a car feel tight and responsive without risk of understeer. Some guys will like to do more oversteer modulation with the right pedal, and while that's fun, it's probably not going to be the fastest.

[ArtosDracon]

Robert - you're hired! That's some good stuff I'll reference when it comes time to tune my suspension...

I'd love to actually hear that some time soon. Being an undervalued accountant gives me nothing but spare time.

[shmoov69]

Thanks Robert. I drove the car hard in the turns only that weekend. I've farted around on the street before, but never really "hard". Lots of hard in a straight line, but not turning. So, needless to say my skills leave alot to be desired! I don't know what a "perfect" feel is to be honest. The car wants to push coming out of a hard turn, but that could also be the locker rear too. The pics had the car twisting in the rear a bit more than I expected to see. Do the coilovers being totaly vertical have anything to do with that?
Thanks!

[ArtosDracon]

The coil-overs being vertical will only affect roll rates a very small ammount, and would make the maths a lot harder if done by hand. Personally I've become obsessed with adjustable rate rear bars lately, and given your fabrication ability that may be a good idea for you, that will let you dial it in for your preference and the track. You could go all the way up to the bars that are in-cab adjustable, that rotate the swing-arm so that it is allowed to deflect and absorb some of the force before it's handled by the actual torsion bar, but that gets really expensive, really fast. A frame mounted splined bar with custom arms with multiple holes would be a very easy way to get a bar on the car for about $200 and let you tweak it. And once you have all the hardware, a replacement bar to change the base rate, in case you don't have enough adjustment in the arms, will only set you back about $80, instead of another $200 bent bar.

[shmoov69]

Cool, thanks Robert! Where is the best place to get the ends and bar?

[U_SAY_GO]

Do you have a rear bar on the car now?

How much does the car weigh?

What is your front to rear weight %?

Without knowing the weight of the car I would say 200lbs. on the rear is too soft.

Just my .02, but I would start by doing the old tie-wrap on the shock trick to see how much travel you are using. A rear bar is going to help your turn-in and responsiveness, but "stick" axle cars can't handle much rear bar at all because it will "lift" the inside tire and upset your contact patch.

Remember it's all about front to rear balance. If the car is well balanced and you add a rear bar you will need to stiffen the front to match. If the front is too stiff relative to the rear the car will be really "loose". I really wouldn't put too much value on how it looks in a picture unless you have other data to back up what the picture shows.

Hope that helps...

[ArtosDracon]

Cool, thanks Robert! Where is the best place to get the ends and bar?

I like speedway motors, but I don't have much experience with many circle-track parts suppliers, and there are a lot of them.

[shmoov69]

Remember it's all about front to rear balance. If the car is well balanced and you add a rear bar you will need to stiffen the front to match. If the front is too stiff relative to the rear the car will be really "loose". I really wouldn't put too much value on how it looks in a picture unless you have other data to back up what the picture shows.

Hope that helps...
so, if it needs a bar, it will be loose in ther rear correct? The "body roll' look in the rear really doesn't mean anything? I honestly don't know much about all this! It is all a learning curve for me, I'm used to going straight! LOL!
Thanks!

[ArtosDracon]

The rear look of roll does mean something. The yaw characteristics are what really matter. If the front is stiffer than the rear it will plow if you can get enough traction, if the rear is too stiff, it will over-yaw or feel like it's trying to come around on you.

[shmoov69]

The pushing is kind of hard to tell honestly. The rear has a locker and it seems that makes it want to plow, but maybe it could be something else? It is definately not loose in the ass unless the boost comes on hard and it just looses traction.......which it did on the auto-x quite a bit!! LOL!!