kurt M

I gave up on corect terms. I was talking about an anti-roll bar once and the person said it did not pertain to him as he did not have a cage yet.

It gets real thick real quick don't it? . The only given is there are no absolutes.

BostonDMD

Thanks guys......it is going to take some time for me to digest all this.......

FredC

I would ask Gary R because he has an identical suspension to yours and has experimented with it (made adjustments, etc...)

Gary R.

Yeah, but i'm still slow...

Paolo - My guess is Dan has yours set on the second to last hole in the rear (one notch above full soft) and 1-1.5 tic marks from full soft in the front. It comes down to YOU and how the car feels to you. I think you are still too new to the "new" car to adjust anything, maybe (with Dan's input) at Thunderbolt after a few runs you can verbalize what it's doing that you would like to change (if anything). Fred and Sean are pretty damn quick and they never mess with their setups, which IIRC are pretty much identical to yours. On the other hand my car felt like I was ripping the front tires off it in T1 at the Glen (bad explanation but WTH). I stiffened up the front bar (per Dan's input) and it felt 100% better. My way of thinking is I stiffen the front until I feel a loss of front grip in turn-in to mid corner, then back off. I run stiffer in front than both Sean and Fred, think it helps me in the uphill/downhill at Limerock and T1 and T7 at the Glen the most, but that's ME, MY car, and MY driving style (or lack of)...

Most importantly if you adjust them make them even and with the SAME free play on both sides. Ask Sean how just the free play can mess you up, he had a horrible time at Summit until we fixed it..

Bryan Watts

Has to do with a few things...this is at least how I have come to understand it.

Camber gain. The camber gain on a front strut generally sucks. And the "sweet" spot of the camber gain (per amount of suspension compression as the car rolls) is generally at stock ride height as designed from the factory. Thus, the lower you go on ride height, the less camber gain you get as the suspension compresses. Not to be confused with gaining positive camber...the rate at which you gain negative camber (relative to the chassis) simply slows and can't keep up with the roll of the car. In order to fight this, it means dialing in a lot more static camber and/or adding a lot more spring rate and bar rate in order to limit the amount of roll on the front suspension and make sure that your tire is operating at it's optimum when the car rolls over in a corner.

Roll Center. The problem is furthered even more due roll center geometry that causes the roll center to drop faster than the CG as you lower the front suspension. So, even though you are lowering the CG when you lower the front suspension, you may actually be introducing MORE tendency towards roll because you are increasing the lever arm between the CG and the roll center around which is rotates. More tendency towards roll = need for more spring rate and bar rate in order to prevent the car from rolling and the suspension from having to go further and further into the poor portion of the camber curve.

Thus, Mac strut front suspensions respond well to lots of camber and lots of spring/bar. You can add grip by using spring/bar to limit roll and help keep your tire happy with it's relationship to the surface. In classes that allow it, moving suspension mounting points can helps a lot towards restoring better roll center and camber gain geometry at the lower ride heights that we run on our race cars.

In the end, no matter what sort of suspension we are talking about, you can't always say that more front bar = more understeer. If that were the case, we'd be adding understeer whenever we increase the bar or spring rate from factory (and that's often the first modification we make for a track car). I don't think anyone puts SOFTER springs on their car from the factory setup in order to gain grip on the track. The book definition of what to do assumes you are operating from a somewhat "optimum" setup. Unfortunately, due to rules constraints, budget constraints, multi-use (street/track) constraints, parts availability, driving ability, etc, etc, very few of us are operating from an optimum setup. Sometimes, adding spring rate and bar rate adds grip and sometimes it takes grip away. If 700 pounds is the hypothetical sweet spot for a certain setup, going from a 650 pound spring to a 700 pound spring may add grip, while going to a 750 pound spring from a 700 pound spring may take grip away.

Of course, you can't consider your front suspension in a vacuum either. Changes you make there are very important to the rear of the car as well.

It's all pretty flippin' complicated, which is the reason why there are lots of ways to get it done. Most BMWs are setup with a stiff front and relatively soft rear (many run no rear bar), but I've seen one very fast BMW in the past that left off the front bar all together and put a huge bar on the rear along with a welded rear diff and he ended up with a very fast car as well. It handled more like a Porsche...it was tough to get into the corner but a rocket on the way out.

2BWise

Nicely put Brian.

Two things I would like to elaborate on and give some more things to think of.
The notion of adding bar at the front to add understeer or adding rear bar for oversteer as states is not an absoulte. One other thing must be mentioned is that the adding of bar at one end versus the other only matters because of the relative rates of the front and rear. While adding bar at the front adds understeer, so does removing bar at the rear. Its the relative adjustment that matters for the balance of the vehicle.

The other thing and one of the reasons often overlooked in the big bar vs. small bar debate is that the arb is an undamped spring. The smaller arb will respond slower, but won't overshoot. Theoretically, you can run into an issue with a large bar that your roll displacement actually increases because it is undamped. I've never seen this but I've never run a vehicle with a large amount of roll stifness from arb's.

M758

What anti-roll bars (proper term) do is actually very interesting from a physics perspective.

To understand what the do you really need to look at change in load on each tire. So lets say you have nice 50/50 car. 2600lbs total 1300lbs front and therefore 650lbs each on each tire. That is static no cornering. Now clear no car is that perfect, but we can make that simplification.


So when you corner in steady state you can at most put 1300lbs on each corner of the car with inside being unloaded. When you look at the tire grip curve 1300lbs of force down per tire with 1300lbs side load gives a max corneirng force of some value. Now on way to adjust that is play around with the static load so that you don't start at 650lbs per corner, but maybe 1000lbs and 300lbs. This what you do when play around with weight jacking. Now most road cars don't screw with weight jacking on purpose since that staic left/right imbalance will improve cornering in one direction, but hurt it in the other. Roundy rounders only turn left so this a handy trick.

Now where do anti-roll bars come in? Well they create a similar effect of weight jacking, but do so evenly from side to side. So you create some weight jack effect from the bars. This can enhance grip or reduce grip. This really gets important when you are looking at front-rear balance issues. Now what is interesting is that no where do the springs come into play yet. That is because vertical force on each tire is a result of cornering load, static weight distributiion, track width and anti-roll bar due the ability to transfer vertical load from one side of the car to the other. Springs merely hold up the body. (This does assume you can the prober camber to ensure optumum tire contact and assume a steady state cornerng)


So this all very theroretical, but the bottom line is that sway bars (as they called by most) have an impact on handling and should be used on road race car to tune balance front to rear. They don't work like springs do so you cannot use sway bars to compensate for soft springs. You can to a point, but they will introduce odd behavior. In practice you want to use the sway bars to fine tune the balance of the car. They work best in steady state corning, but also can be used in transition of turn in and turn out based on which bar is being adjusted.

There are two basic ideas for tuning a sway bars. One is to set them in the middle and adjust forward or aft depending on the desire. The other is to start soft on both the slowly stiffen.

If you really want to use the sway bars to greatest effect you need to take the time to understand what they should do. However most just move them a little bit and then just "see what happens". I have been playing around with mine for a while and know that a few millimeters can make significant difference in handling. In general the stiffer the front the more understeer and the stiffer the rear the more oversteer. However alot depends on where you are in camber curve of the tires and on what you consider is oversteer vs understeer. Sometime you suffer from understeer, but the right move is more front bar to support the car more or so you can adjust driving style to creat understeer in 2-3 corners, but improve stablility elsewhere and the root cause for understeer is a odd line not the chassis.

So play around with them as that is the only way to really determine how they wil impact your car.

Bryan Watts

Larry Herman

Thanks Bryan, that is pretty much what I figured too, but I have never run into that situation (yet) and so have not thought about it much.

Another point about anti-roll bars that I have found true is that the front bar will have greater effect on turn entry due to the increased weight on the front of the car, and the rear bar will have greater effect on corner exit on the back of the car for the same reasons.

Bryan Watts

Good point. Another thing that many forget as well when reading "the book" and determining that adding front bar results in more understeer, is that the corner needs to be thought of in 3 (if not more in some cases) very distinct sections. Entry, steady cornering, and exit. Where your handling problem occurs very much determines what changes you need to make and how those changes will affect the car.

Things are further complicated when it comes to anti-roll bars for auto-xers who, due to rules (Stock class only allows you up upgrade the front bar) or due to needed tradeoffs (optimizing for transition speed and quickness rather than ultimate steady state grip), may run a much stiffer front bar than what "the book" would tell you is appropriate.

kurt M

Doh good reminder, you are not alone in finding this out. forgot to add to tune based on the corner. Tune the entry action with the front of the car and exit action with the rear of the car. This is using a 911 as example. I see more misuse of sway bars than almost any other susp sub system. They are just too billet purty and easy to bolt on while the car is jacked up in the driveway. Too large or too tight and they can make an otherwise mushy car feel like it is on rails at low G but handle poorly when loaded up.

In a way sways are dampened a bit. Their action is resisted by the dampers indirectly. They don't load at all unless the struts/dampers move in relation to each other side to side via body roll or wheel movment. One side up and the other down with roll and one side only in the case of a single side bump or dip. Driving over gators as example. Only the inside tire gets kicked up but the sway transmits energy to the outside tire. In this case it trys to lift the outside tire which is resisted by the compression damping and sprung and unsprung mass.

the90

Bars utilize the load sensitivity of the coefficient of friction of tires; an unevenly-loaded tire pair has less grip then an evenly loaded pair. As was mentioned above, add 500lbs of normal load to a tire and you do not get 500lbs worth of added grip....
The reasons this occurs are not clear, though I've read that tire heating is the suspected cause.

Trucho-951

If one looks at a tire's coefficient of friction (CF) vs. load graph, one can see that it is not a linear relationship. That is to say that a tire’s efficiency at converting load into grip declines as the loads get heavier. The CF curve looks like it peter’s out (is that a technical term ). The heavier loads cause greater slip angles and greater tire deformation.

So a heavily loaded tire produces high levels of grip, but not as much had it’s CF not petered out.

FredC

Come to the race at NJMP, do all practices, qual and races and then try to convince yourself that you are slow... not sure you will be able to

Gary R.

Well, it IS a relative term... Still doesn't look good for TBolt but it isn't over till the fat lady sings..