how important are the sway bars???

how important are the sway bars? which one will be needed the most being that the car has 65% of its weight in the back? what are the correct sizes? which one has to be stiffer(front or back)? i'm at this point now so i want to get it done.

thanks

luis

They are very important!

Hotshot, The question you ask is not one that can be answered with one sentence. As a matter of fact most books I've read can't answer it completely.
Let's see if I can give an editted answer. There are 3 things that effect the answer. 1) corner weight of the car
2) spring rate
3) shock rates
what a sway bar does is effectively increase the spring rate as the car turns and the chassis starts it's roll. The need for a sway bar or the size depends on the amount of spring rate on that corner of the car. If you use enough spring the there is no longer a need for a sway bar. In most cases that much spring would mean the car would ride like a gokart the rates would be so high.
A lot also depends on the chassis of the car. Some cars like a front bar but not a rear and visa versa. The trick is balancing the spring & shock rates with ride quality and size of sway bar. I do have computer programs that can get very close to the needed combination with minor adjustments being made at the track.
There are several very good chassis books out there, you might want to pick one up.

Determining the correct sway bar from zip is no small task. It all depends on what your application is. The best point of reference to start from is the manufacturer's recomended settings (RF's are pretty good), then go from there.

Most manufacturers will have a setup thats safe and comfortable for the road which tends to slight understeer, but you may have a more sporty requirment.

The first rule of thumb is - the stiffer you make it, the less traction you will get at that end of the car. ie if you want to increase oversteer or decrease understeer, you either stiffen the back or soften the front.

The second rule of thumb is - the softer you make it, the less responsive it is to steering input.

The ideal setting will also depend on your driving style and can differ considerably from driver to driver.

As you can see it can be quite a task, which is why race teams spend lots of time and money on testing at each track before they go racing.

--------------------------
RF40

The interesting thing about the sway bars for me is the torsional stiffness of the chassis and the weight distribution in the accelerating and decelerating processes. An adjustable pair of sway bars is usually used here in Aus with our V8 tintops. This is because of fuel usage and the change of weight. These "blade " swaybars are adjustable from the seating position and drivers make adjustments due to fuel load, type of corners in the next section of the lap and weather conditions.
I hope to have adjustable swaybars in my race car which is being built at the moment.
Best wishes, Robert

Interestingly enough there was a discussion on sway bars (we call them anti roll bars here) some time back within our club. As can be gleaned from the above responses the only real answer is to experiment and find out what is right for you and your style of driving and USE of the car. Road use will allow a softer set up than a race or track car.

Just chucking on stiffer springs will reduce the amount of roll in a car. However that is NOT THE BEST solution as when you come to a bump the whole car car jump and lose you traction. If that is in a corner you are in deep trouble. A compromise is required and adjustable bars are the real answer. If you want a good track day car that is also useable on the road I would suggest a stiff sway bar with a mild upgrade on springs or even variable rate springs. This way bumps get absorbed well but you can load the suspension up for a corner with a correct turn in at a corner.

Most GTD cars use a 20mm solid anti roll bar. Some guys have made their own tube bars which are larger in diameter. Some use the blade system for adjustment and some use a sliding pick up point for the drop link. This latter is often the cheaper and easier to make yourself route. Most are not adjustable and use an off the shelf item.

Just by using nylon (polyurethane?) blocks in the mounting over rubber blocks will stiffen the car without changing the bar rating itself. The amount of pliance in a rubber bush is enourmous and you may get over an inch of roll before the slack is taken up. This is the reason why people change suspension mounts and put in nylon bushes instead of rubber ones or even go the whole hog to heim joints.

The standard GTD springs for an uprated car (!?!) are rated at 325 lbs at the rear and 250 lbs for the front. On GTD's anything softer is too soft in my opiniion for any kind of use. Combine with our 20 mm bar and you will have a good fast road set up. If you want to track use the car then add anything up to another 125 lbs per spring keeping a split between the front and rear of at least 50 lbs. If you go another 125 lbs you will have a solid car. The popular choice on GTD's at present is about a 400 lbs rear spring and a 325 or 350 front. This is combined with a stiffer than 20 mm roll bar, say an equivalent to a 21 or 22 mm bar. By the time you are playing around with spring rates like these often you are also in the territory of variable rate sway bars and possibly making them yourself.

What do I have? I use a 500 lbs rear spring and a 450 lbs front spring and I admit this is too stiff. I only have a 20 mm non adjustable sway bar. This set up does not comply with what I would recommend as above and requires changing. Problems I encounter is that all my stiffness is in the springs and not utilising the sway bar enough. Also I rely on chassis torsional stiffness as a suspension item. I hit the kerbs quite hard during one hill climb and cracked my windscreen. Ideally I wish to soften my car down to 425 rear and 360 front or something close to that. Then I want to change my anti roll bars to adjustable and a bit stiffer than what 20 mm gives me. Variable rate springing is something to consider as you can have a soft initial spring rate say 300 lbs but then after a little bit of roll it progressively rises to 800 lbs. This is in use now on a number of our members cars and gives excellent road ride quality and if you wind up your sway bars at trackside then also gives good perfomrance on track too.

With my existing set up, I was "rushing" down a road and hit a full width of the road road crest. As the spring preload is not correct on my car the rear of the car got airbourne. A scary moment. So make sure that your preloads are ok.

Something to bear in mind too about springs. Unless you pay a good sum for them you may get a high level of inconsistancy in the spring rate as stamped on the spring. I beleive that variances of up to 50 lbs have been discovered here. Test your springs at least to check what rate they are but also to get pairs of springs that are the same or close enough rated. A hydraulic jack, bathroom scales and a fixed level of compression should be enough home found items to allow a comparative test to be done. If you have access to a bearing press or similar then this is one stage better.

Dampers that I use are Koni twin adjustables ie bump and rebound are set independantly of each other. Single adjustable dampers will also work fine with the above ideas.

Oh and you do need sway bars at both ends of the car in case you were wondering.

Of course to really screw with your mind there was this chap Anders Hilderbrand who won the Swedish GT championship in a heavily modified GTD Mk2. He ran no sway bar on the rear and a very stiff one on the front. Spring rates were 900 lbs at the front and 1100 at the rear. The chassis was clearly a suspension member! The car didn't know what ordinary road looked like!

If you have an RF or other make then I would be surprised if the relevant factory can't advise you at least where to start. Remember your school trigonometry? If our dampers and coil over units are at different angles to yours then the springs rates go out of the window. Depending on pick up points, the more vertical the unit the less stiff the spring needs to be to supply the sme level of resistance. Ultima's have highly angled damper/spring units and use 750 lbs plus springs from what I have seen.

Hope this helps. Maybe Andrew could give some feedback on this subject as he has put a lot of thought and effort in getting his car how he likes it.

Malcolm M

Hi, also be aware that damper loads can be greater than the shocks imparted to the chassis by springs...
I have hundreds of references on suspension, and have designed about 10 different cars suspension systems...it is a very complex area, and for a given car is very much dependent on the tyres and the road surfaces expected.
Feel free to email me for any details!


Robert lawrence

what i have is a sabre kit. it uses Hal 10 inch adjustable shocks with 250 lbs in front and 350 in the rear. this car will be driven mostly in the street. race track time will be very little but i would like it to handle very good as a street car. the part number for the shock is:

7001293 proma star 4" w/poly
reference part # DR4855P-1

thanks

luis

[ December 14, 2001: Message edited by: HOTSHOT ]

Sway bars as measured off a real Gt40

front 5/8" dia. 8-1/2" arm (measured from pivot axis to end mount.

rear 3/4" dia. 8-3/4" arm

end mounts are adjustable front and rear for effective arm length. Pivot mounts are aluminum blocks. End mounts and connection at suspension are rod ends.

don't know the exact spring rates on this car but look VERY stiff like the 900 1100 in Malcoms reply

the chassis I'm building for mostly street occasional race use has 300lb front and 350lb rear springs. The sway bars will be as the original car.

Sway bars are used for 2 reasons. One to keep the chassis from rolling too much on turns thus keeping the tire/wheel square to the road and maximize the contact patch. And the other to (if neccesary) equalize the front/rear suspension roll amounts so that one end is not too stiff causing it to lift one tire, again maximizing the contact area.

HOTSHOT these bars are solid

[ December 18, 2001: Message edited by: Kalun D ]

kalun

are these solid or holow?

thanks

luis