installing a sway bar in the rear?

[Lumpyguy]

So I have been wondering if anyone has concidered installing a sway bar in the rear of the 818? I was looking and it looks like I will have a bit of fab work to the chassis and lower links but it looks like it will work.

does anyone know how much room is under the trans to the chassis?

any idea from the track guys how this will affect the handling?

370333703437035

[xxguitarist]

Not having driven the car yet, I'm still doubtful that loosening up the rear any extra will be necessary..

[Lumpyguy]

cool, This is the type of feedback I need. I love to fab and I can go crazy making stuff but dont want to make things worse. I am not much of a track guy but would like to run mine a few times on a track. keep her coming.

[Hindsight]

That swaybar is gigantic! On an 818, it would probably be nearly like bolting the spindles to the frame lol.

I agree with xxguitarist. Maybe a front bar but not rear.

[xxguitarist]

We'll most likely be running a front sway, design is a bit TBD though, as I'm eyeing making a simpler design that mounts out in front of the main frame area. Would be much easier to swap, and easy to make a wide variety of stiffnesses.

[Lumpyguy]

It was for fitment to see if anything would fit and was not sure what I would have used this is what came on the front of my donor car. the rear donor was a whiteline 24mm bar which I am trading for a stock rear bar.

So the concenous is just a stock rear bar in the front.

thanks

[Wayne Presley]

I've driven the cars on street and track in both R & S versions and a rear bar is not necessary.

[Lumpyguy]

Thanks guys

[bbjones121]

So I have been wondering if anyone has concidered installing a sway bar in the rear of the 818? I was looking and it looks like I will have a bit of fab work to the chassis and lower links but it looks like it will work.

does anyone know how much room is under the trans to the chassis?

any idea from the track guys how this will affect the handling?

370333703437035

This causes some undesirable handling characteristics. It has been discussed in one of the very first builds on here.

[Scargo]

I am going to play Devil's Advocate here and ask why some amount of roll bar couldn't/wouldn't be helpful in certain instances, like a race car?
A track car with wide, low profile tires could not benefit from reduced roll? Certainly you would not want the whole rear-end stiff and a roll bar might be pretty light.
Once you have done all you can do with springs, it is hard for me to see how you can do the rest with only tire pressures if you have nine to ten inches of rubber on the ground.

[Santiago]

Once you have done all you can do with springs, it is hard for me to see how you can do the rest with only tire pressures if you have nine to ten inches of rubber on the ground.

Glyn, that first part of your hypothesis has to be taken pretty seriously. Someone in a design-stage who is contemplating a sway bar (at all) has to nail down some goals and uses before really weighing in on the virtues of a sway bar. On its own, there's very little to be gained in having a roll bar "once you have done all you can do with springs." Think about it, a roll bar is a spring, so what more are you looking to do with it "once you have done all you can do with springs" in any given condition on track? You can limit body roll with coil-over springs just as readily as you can with a roll bar; there's no magic in a roll bar. Also, you can alter roll by altering roll-center as well; my Watt's link on the Mustang made wonders of controlling roll by adjusting the main prop position (and not messing with the roll bar at all).

Of course, limiting body roll with the primary coil-over springs may lead to an overly stiff ride. So a roll bar solves some problems for some specific situation (really handy on a street car in which you may want a softer ride overall). But here we're talking about a design goal focused on driver comfort. You might really like it in straight-line acceleration, since a roll bar might leave you free to work with softer rear primary springs.

Here's something I think roll-bars may genuinely excel at offering; an in-car adjustable bar lets you change balance on the fly. In some racing circumstances that might be an asset, particularly if fuel consumption has a large impact on weight distribution over the course of a race. With the placement of our fuel tank, I'm not seeing this as a major consideration in the 818 application, but I can see it in others. I know guys building enduro cars with very large fuel cells placed aft of the rear axle; they're chomping at the opportunity to adjust balance to suit the changing conditions. But these are really "over-time/changing conditions" benefits of the device in question. Apart from that, it's just another spring - one that communicates laterally which I don't like at all for most situations.

Last, don't diss air. =) Once you really have done all that you can with the springs, air pressure is a major fine-tuning tool that should be taken seriously. I'd strongly suggest reading Paul Haney's The Racing & High Performance Tire. It's expensive but easily worth twice the price.

Best,
-j

[Scargo]

I would agree with most of this, but I argue that a roll bar exerts force to the opposite side based on what force it sees that is trying to roll the body, so I don't see it as just "another spring". It is transferring pressure directly to the suspension on the other side.
The normal (usually coil) springs at each independent wheel are not acting directly on the other wheel's suspension. They act independently on the body and through the body is the way that one spring influences the other spring. If you hit a bump or a rise evenly the sway bar does nothing.
I believe most racing orgs don't let you adjust the sway bar once you are on the track. It is a neat trick and I have looked at the rotatable blades.
Aero won't do you any good on a 40 MPH turn, AFAIK. I believe it doesn't do much till you are around 80+. My average speed at Lime Rock has been about 90.

[Santiago]

No disagreement it seems (yet). =)

You're right, of course, as that lateral transfer of the sway bar's force is what I meant (and now corrected in my orig. post) by it "communicating" across the car. To my mind this is generally a bad thing, as it is unloading the inside tire. Sure, since that side doesn't want to fully unload it resists and you get less body roll, but that benefit seems largely tied to avoiding unwanted geometric relationships we can deal with in the basic set up (once we "do all we can with the springs"). I'd rather let the two sides be genuinely independent, particularly since many a corner is not taken on the level-ish part of the track but is run over berms & exit strips (esp. in w2w situations).

Best,
-j

[Scargo]

And the reason "everyone" does one on the front? I love the pictures of some cars with the inside front tire four or more inches off the ground. Has to be some sway bar influence going on there.
I like the idea that, in the rear, (without a bar) you can clobber curbs and not upset the suspension (much) on the other side. Might help a lot in the bus-stop at WGI. Take it NASCAR style...
Hey folks. You can jump in any-time. The water's fine.;)

[Tamra]

My knowledge is limited on the subject... but I would say the main benefit of an adjustable sway bar is that they offer a way to tune the suspension's front-rear stiffness balance more quickly and easily than can be done by changing spring rates. Basically, adding a front bar causes more understeer, and a rear bar causes more oversteer. Saying it another way, a front bar gives more traction to the rear tires, and a rear bar gives more traction to the front tires. The bigger the bar/ the stiffer you adjust the bar, the more prevalent these become.

That's why FWD and AWD cars tend to add big rear bars, and RWD cars tend to add big front bars, especially in autocross due to its transitional nature. It seems like autox Miatas tend to run giant front sways, yet track Miatas tend to run a rear bar (or maybe both front and rear? I know very little about track set-up though, but that's just what I've seen). I think it depends on your application and your car set up for what is "best," but sway bars do allow one additional point of adjustment.

With our Miata, it has an open diff and we were getting a lot of wheelspin on mid corner acceleration. Adding a big front bar helped transfer the weight to the inside rear mid corner, and gave us the ability to accelerate out of corners better.

In addition, stiffer springs make for worse ride comfort. Sway bars allow for control of body roll without compromising the ride quality as much, which is more relevant for cars that will be both street driven and raced.

[Bob_n_Cincy]

Our experience auto crossing so far.
This was the situation we used for testing:
Stock 818S springs and shocks.
Go kart mode 1560 lbs
-2.5* camber front and rear.
2 280# occupants.
4 225/45-16 R888 (old slicks)
24 psi all around
open diff
NO SWAY BAR.
Stock turbo and boost. (195 whp)

Results: Very LOOSE applying gas while turning.

Need to improve drifting skills.
Watching video of front suspension, we are seeing about 2" difference in ball joint height during max turn.

Going to try harder front springs before I think about a front sway bar.
Bob

[CHOTIS BILL]

There is a lot more to suspension tuning than deciding if you are going to add a sway bar or not. Personally I would prefer not to run a rear bar because they tend to lift the inside rear wheel and even if you have a limited slip diff why over work it. It would be interesting to setup an 818R using the procedure I used on my DSR. Even though my car was about half the weight of an 818R they both have about the same weight distribution, have bigger tires in the rear, and have aero downforce. I would start with the weight of the front wheels and then set the wheel rate equal to a percentage of the weight on each front wheel. I don’t know the actual numbers with of an 818R with a driver in it but let’s say the car and driver equal 2000# with 53% of the weight on the REAR then there is 530# on each REAR wheel which is the wheel rate I use for the FRONT wheels . A percentage of this is a number that can be tested for best results. I would guess starting a 100% would be a good starting point even though I have gone up to 400% but you need a lot of downforce to pull that off. With that I would sit the front wheel rates to about 530#. Taking a guess the motion ratio is around 65% then I would use a front spring of 800#. In the REAR I would do the same calculation but I would use the FRONT weight and 47% and assuming a 1:1 ratio I would pick a rear spring around 470#. This may or may not be too stiff and changing front and rear rates by the same percentage keeps it balanced. The finial item to calculate is the sway bar. I found out though a lot of testing and comparing notes with other racers that a front to rear roll couple of 61% front and 39% rear makes of close to neutral handling and for that a 285# front sway bar is required. This may be close or may not even be in the ball park but would be interesting to try.

[Scargo]

I'd really appreciate estimates made based on the data I got from Jim at FFR. Perhaps this will help with it being less of a WAG.

The FFR blue car typically had a 3-1/4 inch ride height.
Roll centers: 1.8 inches front, 2.95 inches rear.
Motion ratios: .877 rear pure motion, but also with a .985 angle correction factor. Front: .795 with a .906 angle correction factor.
Both FFR cars ranthe 300/500 combo. The higher down-force setup on the red car didn’t see enough track time for them to start experimenting with higher rates. (300/500 is what they found best on the medium down-force blue car)
Wheel rates: front: 171 lbs/in
Rear: 380 lbs/in

[CHOTIS BILL]

There are several other items that factor into it. Tire diameter, downforce front and rear, corner weights, unstrung weights, track, and wheel base. The spring rates I suggest my seem high but you don’t know until you try. Softer spring help compliance but higher spring rates give you quick response and unbelievable turn in. I made a large increase in spring rates before a race at Willow Springs. I had race there for over 40 years and knew it very well but as I was leaving the pits for the first time with the stiffer springs the steering response was so quick I almost drove off the track. The only way to tell if stiffer springs are better is in lap times. You can go to stiff and lose tire compliance or stiffer than the chassis can handle. If you would like a copy of the spreadsheet e-mail me at: blomenick at deltatau dot com.

Bill Lomenick