Tarett sway bar initial settings

[seebobgo]

Installed on a 87 911 3.2 with Bilstein sports, 22/29 TB. I went half way front, full soft rear. Than moved to 1/4 to the hard in the rear. Any input on where you set them?

[wildcat077]

You will know soon enough if the front sway is too stiff when people following you on the track come to tell you that your car is turning on three wheels.You're better off starting softer in the rear than the front ... last thing you want is oversteer with a 911 !

You have it about right,you'll just need fine tuning to comply with your driving style and tire choice ... streets versus "R" compounds

Cheers
Phil

[seebobgo]

Thanks for the strokes. Over steer at 30 mph is fun but at 90, well I'm getting a little to old for that.

[winders]

Originally Posted by wildcat077

You will know soon enough if the front sway is too stiff when people following you on the track come to tell you that your car is turning on three wheels.You're better off starting softer in the rear than the front ... last thing you want is oversteer with a 911 !

This is kind of funny. How much the front inside tire lifts in the air in a corner is directly related to the relative roll stiffness front to rear and not how stiff the front bar is set.

With the 22mm/29mm torsion bar setup, you already have a 911 that is a bit too roll stiff in the front compared to the rear. But, you don't tell us the intended use of the car. If this is for the track, you want the sway bars to provide as much roll resistance as you can get from them. I would put them both in the middle to start. If you find the car understeers, stiffen the rear. If car oversteers, stiffen the front. The goal would be to have one of the bars near its stiffest setting and have the understeer/oversteer characteristics where you want it.

[seebobgo]

Originally Posted by winders

This is kind of funny. How much the front inside tire lifts in the air in a corner is directly related to the relative roll stiffness front to rear and not how stiff the front bar is set.

With the 22mm/29mm torsion bar setup, you already have a 911 that is a bit too roll stiff in the front compared to the rear. But, you don't tell us the intended use of the car. If this is for the track, you want the sway bars to provide as much roll resistance as you can get from them. I would put them both in the middle to start. If you find the car understeers, stiffen the rear. If car oversteers, stiffen the front. The goal would be to have one of the bars near its stiffest setting and have the understeer/oversteer characteristics where you want it.

Got it, pretty fundamental. Just bring wrenches. Thanks_Bob

[Flieger]

How much the front inside tire lifts, once it looses contact with the ground, is really just a function of the rear stiffness (roll and ride, since accelerating or braking will change weight distribution too). Once the tire lifts off it has transferred all the weight it can. Relative stiffnesses control how soon you lift off.

[juanbenae]

how will the car be used moving forward? you leave me hanging at the "so on" in the signature line.... I read the post 2x and pretty sure you don't say.

[winders]

Max,

No, it is still a function of relative stiffness.

By the way, you shouldn't use "weight". It is "load".

What happens when the inside front tire is in the air is that the outside front tire is bearing the entire load at the front. Just because the inside front tire is in the air does not mean the front suspension is no longer resisting roll. That load can increase or decrease at the front as well. In other words, roll can increase or decrease and it will continue to be resisted by the front suspension.

Scott

[Flieger]

With only three wheels on the ground the rear stiffness is controlling the roll angle, which is determining how high the inside front is off the ground (ignoring longitudinal load transfer). The force exerted by the road on the outside front tire does create a moment about the mass center (which we will assume is on the plane of lateral symmetry of the vehicle), but that moment does not change beyond small sine/cosine effects due to roll angle. The reason is that the weight on that tire does not change (and the track is assumed to be constant). Ignoring mass concentrations,aerodynamic forces, and maneuvers resulting in z axis acceleration the front to rear weight distribution must remain the same for the car to be in pitch equilibrium. If you have only 1 front wheel then there is only one load that can be on that corner. If that load is different then you are accelerating in pitch.

What happens is that as you continue to increase lateral acceleration on three wheels the weight transfers off the inside rear to the outside rear, causing the outside rear to compress and lift the inside front even further due to a combination of roll and pitch. Then you end up on 2 wheels and the dynamics of the problem get a lot more complicated.

[Flieger]

And lighten up. Arguing about spring terminology, weight vs. load. One term might not be technically correct but we are getting the point across.

Sway bar and roll bar and shock all kind of irk me but I have made my peace. I've seen those terms in Racecar Engineering even. I like to say anti-roll bar but have gotten lazy and say sway bar. Still call them dampers, though.

[michael lang]

Bob, in reading your original post, it can be construed as a couple of things. 1. You just installed new sway bars and your trying to figure out the best initial setting or 2. You are trying to correct a problem that you are having with the car.
For the answer to #1, I don't believe there is a correct answer. I think it really depends on what you like/want out of your car. My car is set up fairly stiff and to be honest is really uncomfortable to drive on the street. I absolutely hate driving into Bethesda because the roads around Washington DC are in such bad shape, get it out to a track and it transform into a whole different beast. But those are the trade offs that I decided to make.
For #2, if you are trying to fix something and you installed the sway bars to address a problem, well you never said what the problem was. You started into oversteer but you never committed to that as being the actual issue. Are you getting lift in one of the tires like it was suggested? Or is there something else that is going on? I used to have the r/f tire come real high as I would come around T6 at SPR and I always felt as though it was slowing me up. A few years ago when I went through my suspension refresh, I put in Bilstein sports just you have but went a lot stiffer in the torsions bars. As soon as I stiffened up the rear, the tire lift I was having coming around T6 went away. The benefit was that I was then breaking 1:30's SPR. I don't think moving the adjustment on the rear sway bar to 1/2 or 1/4 stiff is going to make that much of a difference in oversteer, so give it a try and see what you think.

[winders]

Originally Posted by Flieger

With only three wheels on the ground the rear stiffness is controlling the roll angle

Would you have us believe that all of us sudden all the roll control switches to the rear the instant the front inside tire comes off the ground?

The outside front tire still has its suspension, does it not? Unless the car is rotating around a point above the center line of the front outside tire, which it isn't, that suspension is still resisting roll.

The "weight" (mass x gravity) of a car does not move around. Also, if you use weight or mass instead of load, it makes it hard to understand that the loads the tires are handling may exceed the weight of the car. For example, when a 2000 lbs car is braking at 1.2G of deceleration, the tires are dealing with 2400 lbs of load.

[Flieger]

Originally Posted by winders

Would you have us believe that all of us sudden all the roll control switches to the rear the instant the front inside tire comes off the ground?

The outside front tire still has its suspension, does it not? Unless the car is rotating around a point above the center line of the front outside tire, which it isn't, that suspension is still resisting roll.

The "weight" (mass x gravity) of a car does not move around. Also, if you use weight or mass instead of load, it makes it hard to understand that the loads the tires are handling may exceed the weight of the car. For example, when a 2000 lbs car is braking at 1.2G of deceleration, the tires are dealing with 2400 lbs of load.

Wow, did you flunk statics?

If a car is braking at 1.2G in a gravity field of 1G and there is no upward or downward acceleration due to longitudinal curvature of the road, then the car still weighs 2400 pounds. What happens is that the LOAD on the rear tires decreases and the LOAD on the front tires increases in proportion to the cg height times longitudinal acceleration divided by track width divided by 2 since there are two tires per axle.

You say "roll control" but what do you mean by that? The outside front tire now becomes one point of the fulcrum since a 9 inch wide pneumatic tire is closer to a single point than a 60" track width. That tire cannot resist roll to any significant degree. Yes, the normal force from the road is not passing through the cg so it does create a moment, partly a roll moment and partly a pitch moment.

But once the inside front tire lifts your roll stiffness hits an inflection poiint such that the slope of the curve changes to be softer since only the rear is contributing. But again the car is now going to be squatting as well unless you have some really crazy instant center behavior.

[winders]

Excluding aero, and assuming no elevation changes, the sum of vertical loads are always mass x gravity. However, the lateral loads are different and using weight instead of load contributes to confusion in the long run.

When a 911 lifts the inside front tire in the air, the outside front suspension is still resisting roll. If you go faster the car will roll more and the outside front suspension will compress more. The roll center does not all of sudden magically move over the outside front tire when the inside front tire comes off the ground.

My race cars lifts the the inside front tire off the ground just a bit going through turn 3a at Sears point. The only reason I know that is that I have seen photos showing it. The handling of the car feels no different at that point. The cars does not change all of sudden when the tire is in the air. If it did, I would spin off the road as I am near the lateral limits of the car at that point. It does not have an increase in roll so the front outside suspension is still resisting all the roll it was resisting before the front inside tire went in the air.....

[seebobgo]

Originally Posted by michael lang

Bob, in reading your original post, it can be construed as a couple of things. 1. You just installed new sway bars and your trying to figure out the best initial setting or 2. You are trying to correct a problem that you are having with the car.
For the answer to #1, I don't believe there is a correct answer. I think it really depends on what you like/want out of your car. My car is set up fairly stiff and to be honest is really uncomfortable to drive on the street. I absolutely hate driving into Bethesda because the roads around Washington DC are in such bad shape, get it out to a track and it transform into a whole different beast. But those are the trade offs that I decided to make.
For #2, if you are trying to fix something and you installed the sway bars to address a problem, well you never said what the problem was. You started into oversteer but you never committed to that as being the actual issue. Are you getting lift in one of the tires like it was suggested? Or is there something else that is going on? I used to have the r/f tire come real high as I would come around T6 at SPR and I always felt as though it was slowing me up. A few years ago when I went through my suspension refresh, I put in Bilstein sports just you have but went a lot stiffer in the torsions bars. As soon as I stiffened up the rear, the tire lift I was having coming around T6 went away. The benefit was that I was then breaking 1:30's SPR. I don't think moving the adjustment on the rear sway bar to 1/2 or 1/4 stiff is going to make that much of a difference in oversteer, so give it a try and see what you think.

Looks like I started some conversation here.

It's # 1 Mike. I'm good. I'll just bring a small tool box with me. It's tough you find the right setting on back roads. End of May, Summit point. First time out in years. Maybe I'll see you there.
_Bob

[Flieger]

Originally Posted by winders

Excluding aero, and assuming no elevation changes, the sum of vertical loads are always mass x gravity. However, the lateral loads are different and using weight instead of load contributes to confusion in the long run.

Well then, why do you insist on calling vertical loads simply loads? That is rather ambiguous, no? Better to call them weights vs. loads. Or else make it clear by saying vertical loads or lateral loads. Or be like the hip cats and say ||Fz|| and Fy

When a 911 lifts the inside front tire in the air, the outside front suspension is still resisting roll. Yes. If you go faster the car will roll more and the outside front suspension will compress more. No, the weight is the same on the front if we are looking at a pure cornering situation. The weight on the outside front can only amount to the static LF+RF corner weights at a maximum unless there are aerodynamic, longitudinal acceleration, or road curvature forces The roll center does not all of sudden magically move over the outside front tire when the inside front tire comes off the ground. No, the roll center gets closer and closer to the instant center of the outside tire as a greater and greater amount of the total front cornering force is generated by the outside tire. Geometric roll center only applies when the forces are equal, so with no lateral load transfer, or when the two IC's are in the same spot. All this means that your roll center can easily be outside the track.

My race cars lifts the the inside front tire off the ground just a bit going through turn 3a at Sears point. The only reason I know that is that I have seen photos showing it. The handling of the car feels no different at that point. The cars does not change all of sudden when the tire is in the air. Of course not, it is the Fz on each tire that controls the cornering behavior, not the roll stiffness. The corner weights do not change suddenly in a discontinuity, but the roll stiffness does make a sudden shift, meaning the load vs. lateral acceleration or vs. roll angle has a cusp in the curve. The slop changes (reduces). If it did, I would spin off the road as I am near the lateral limits of the car at that point. It does not have an increase in roll so the front outside suspension is still resisting all the roll it was resisting before the front inside tire went in the air.....You aren't understanding. The front provides a roll moment and a pitch moment, but that moment is constant. So yes, it still resists roll but it only resists as much as it did when the Fz on the front inside went to 0. Past that it has a roll stiffness of 0- increasing the front roll will not increase the Fz on the front outside.

You quibble about terminology but you fail to grasp the concepts you are referring to so precisely.

[Flieger]

By the way, I just put an order in a couple days ago for front and rear Tarett 22mm effective rate hollow RSR style sway bars. My Weltie 22's have served me well but the bushings are shot and I want the sways to match the quality of bushings I have elsewhere. I've got 22/29 torsion bars and when my car comes back from the shop (MFI being restored) I will take some measurements of the torsion bar to ball joint (front) and torsion bar to hub (rear) distance, along with the torsion bar active length. I will assume 2200 pounds for my car's weight (including driver) and a 66% rear distribution. I will plug into my spreadsheet and let you all know what I settle on for lever lengths on the sway bars. I don't have raised spindles so my front roll center is a little lower than some cars so my setup will be a little more front-stiff on a raised-spindle car.

[juanbenae]

see what im getting at?









man, i just wanna go racing.. 3 weeks

[michael lang]

car311, did you mean to put this reply in another thread? and after this winter, i think we're all ready to hit the track.

[juanbenae]

Originally Posted by michael lang

car311, did you mean to put this reply in another thread?

nah, I posted a muffler pic cause I lost my smiley face eating popcorn image..