can we talk sway bars?

[48ford]

My 46 ford(I beam axle) has a 3/4" front sway bar.
I have a bar for the rear(chassis engineering dual leaf springs) that is 7/8",now every car I ever seen has a much bigger bar in the front ,smaller in the rear.
Am I going to have trouble?
What if I put rubber bushings in the rear,and poly in the front.
I know it's not a 'Vet,but i'm looking for a stable car.
any ideas?
Thanks Russ&Irene

[Fat Cat]

My 46 ford(I beam axle) has a 3/4" front sway bar.
I have a bar for the rear(chassis engineering dual leaf springs) that is 7/8",now every car I ever seen has a much bigger bar in the front ,smaller in the rear.
Am I going to have trouble?
What if I put rubber bushings in the rear,and poly in the front.
I know it's not a 'Vet,but i'm looking for a stable car.
any ideas?
Thanks Russ&Irene

There are some good details about rear bars on this thread http://www.roddingroundtable.com/forums/viewtopic.php?t=323

[Bob Paulin]

My 46 ford(I beam axle) has a 3/4" front sway bar.
I have a bar for the rear(chassis engineering dual leaf springs) that is 7/8",now every car I ever seen has a much bigger bar in the front ,smaller in the rear.
Am I going to have trouble?
What if I put rubber bushings in the rear,and poly in the front.
I know it's not a 'Vet,but i'm looking for a stable car.
any ideas?
Thanks Russ&Irene

Russ:

You're not gonna' believe this......

I had a fairly lengthy reply all written up, but when I went to post it, I apparently was posting to the duplicate thread right around the same time Fat Cat was doing some housework.

The message I received was that I could only respond to existing messages...the duplicate, to which I was responding, apparently was no longer in existence, thanks to the compulsively-neat Fat Cat....LOL!!!!!.

My reply was, basically, there are exceptions to the rule, and in your case, the small diameter difference may not be an issue - especially if the rear bar has a longer motion ratio (lever arm).

I'm on the phone with Canada right now, but I'll try to get back - maybe tomorrow - to go into further detail for you.

Bob Paulin

[Bob Paulin]

My 46 ford(I beam axle) has a 3/4" front sway bar.
I have a bar for the rear(chassis engineering dual leaf springs) that is 7/8",now every car I ever seen has a much bigger bar in the front ,smaller in the rear.
Am I going to have trouble?
What if I put rubber bushings in the rear,and poly in the front.
I know it's not a 'Vet,but i'm looking for a stable car.
any ideas?
Thanks Russ&Irene

Russ:

I haven't forgotten you.....just been a bit busy on the telephone.

While you are correct in having observed that in *most* cases the front bar is quite a bit larger diameter than the rear bar, you've got to understand that you have a somewhat unique mix of old (front) and new (rear) suspension geometry and technology going here.

It is also entirely possible for the effective rate of the 3/4" bar to be a bit higher than the effective rate of the 7/8" bar due to motion ratios (lever arms in high school physics speak).

A shorter lever arm on the 3/4" bar could easily cause the 3/4" bar to develop more roll resistance than the 7/8" bar - if the 7/8" bar has a longer lever arm working on it.

There are, as you probably know, exceptions to every rule, and even though I stated in the other post that the "rule of thumb" is to have a rear bar that's approximately half the front bar's diameter and rate, I would at least *try* the particular setup you have mentioned.

I am going to assume that the bar you are talking about installing on the rear comes from the outfit that designed the parallel spring setup, so you have to hope that they have come up with this setup either through some brilliant engineering, or, at least, by trial and error and a lot of customer feedback.

It would be impossible for me to figure out what your front and rear motion centers are, what your COG is, what your roll axis looks like, etc. without physically having the car here to measure and plot the geometries, then plug them into the chassis setup program I have, so it is equally impossible for me to even begin to suggest what might work in your case..

You mention that you would like a "...stable car."

If you REALLY want to find out if the car is "balanced" _ the term "balanced" in this case relates to the front and rear suspension systems working together to handle their own correct percentage of body roll in order to produce a car that offers "neutral" handling characteristics - here's what you can do.

Place a piece of tape or some other marker at 12 o'clock on the steering wheel when the car is pointed straight ahead.

Take the car to a large, smooth, clean (sand-free) parking lot and start doing a circle (maybe 50-to-75 feet in diameter) at low speed.

Take note of where the steering wheel marker is in order to maintain this circle. Let's say it's at ten o'clock to maintain the constant circle.

Now, without touching the brake, increase the speed. Keep increasing and maintaining the speed gradually - using even throttle pressure, not on and off - until you *must* adjust the steering wheel in order to scribe the same circle.

If the car is "balanced", you will not have to change the position of the steering wheel in order to maintain the same circle as the speed increases, but I'm betting you will have to adjust the steering wheel position when the tires begin to squeal a bit.

If you find that you have to move the wheel more towards nine o'clock to maintain the circle, the vehicle is "understeering" and you should work on the front suspension to fix that.

If the wheel must be moved closer to 11 o'clock in order to follow the same circle, then the car is "oversteering" and you should work on the rear suspension to fix that.

If you are close, you might even balance the car with tire pressures.

This method should point to what your chassis needs long before you actually spin the car out or go sliding off straight ahead with the steering wheel at full lock.

This is really an oversimplification of the way we balance oval-track cars when we have the luxury of a long test day, and, of course, there are exceptions to the rules used to "balance" the car, such as factoring in brake balance.

But what I have described above will give you a chassis tuning "tool" that will help you diagnose what your car needs to make it more pleasant to drive and more responsive - thus safer.

My business telephone number is in my member profile - if you have any more technical questions that might require some give-and-take.

I'll probably be around some times during the weekend.

Bob Paulin

[58 Yeoman]

Thanks for the tips, Paul. I'm going to take the 58 out to a parking lot this spring and try that. It has the stock suspension setup and doesn't take corners well. No rear sway bar.

[48ford]

Thank you Bob,
Now I have a starting point,and can figure out why the car is doing this or that.
You were very helpful
Thanks Russ

[Bob Paulin]

Thanks for the tips, Paul. I'm going to take the 58 out to a parking lot this spring and try that. It has the stock suspension setup and doesn't take corners well. No rear sway bar.

Yo! man.....

With the late-'50s GM "cream of wheat" suspension and that notorious "twist-o-flex" X-frame, you'll probably want to determine overall front/rear balance by watching to see if the sparks are coming from the front of the rocker panel or the rear....LOL!!!!!

Just kidding....

While I have stated that I would not want to run a car without a front A/R bar, I really don't feel that *rear* A/R bars are that critical for street use. but I've got no problem with someone wanting to use, and fine-tune their suspension with one.

I've only worked with them on a handful of occasions in oval-track racing. They are more of a road-racing item.

The problem often comes when someone simply sticks a rear A/R bar on and makes no other changes whatsoever.

This can substantially change the rear roll resistance, and can easily upset a nearly-balanced car - that maybe only needed a minor tweak - by forcing the rear suspension to resist more roll than it is designed to do - or should do according to the amount of weight transfer that is occuring at the rear.

Bob Paulin

[OldSub]

If you find that you have to move the wheel more towards nine o'clock to maintain the circle, the vehicle is "understeering" and you should work on the front suspension to fix that.

If the wheel must be moved closer to 11 o'clock in order to follow the same circle, then the car is "oversteering" and you should work on the rear suspension to fix that.

...

Bob Paulin

This is the kind of great practical information I love to find here!  Thanks Bob for taking the time to answer the question so we can all learn a little more.

[Phat]

Hi Bob!   I have to agree with Bob on rear sway bars on street stuff.  I most times have a customer that just has to have one so i build it in with one little trick(i am sure Bob has done this on his oval track cars as i have)
I make one side of the rear bar so i can just pull a pin to disconnect it and let the owner decide.  8 times out of 10 we end up removing the rear sway bar.  Look at some of the vettes...the rear bar is so tiny it rather funny thats it a full blown sports car.  But with the way the COG is its really not doing much.  I also find you dont need them at all in old pickup trucks in the rear.