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I can imagine. The older the chassis design the more stiffening has an effect (and a huge effect with BMWs designed prior to the E39+ era). I'm not trying to imply otherwise above but just making the point that the huge benefits of the past as I mentioned are becoming harder to come by with modern body structure designs. However, the marketers of strut bars (speaking of BMWs here) continue to purport massive benefits to the masses of people spending money on them for modern car. Few, if any, of those folks (imo) will see the benefit of that bar versus the hundreds of dollars they spend.

Many of my older cars would flex as you described when jacked. If you ever opened a door (if you could even do so) on one while jacked, you sure learned quick not to try to close it. On my E39 and others, if you jack one end the whole side easily goes up, and you can open/close doors all day long with no issues. It's just an illustration, however crude, of how stiff modern BMW body structures have become. One look at those natural frequency charts of the torsional and bending rigidity tells the story another way. They're at the point now where additional gains are becoming very hard to achieve.

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This is a pain since most front-wheel and all-wheel drive cars have understeer, and you want to add a stiffer *rear* sway bar. Why in the world do the rules spec a front sway bar for stock and not a rear?

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Yeah, heck add RWD cars to that too since most all modern cars are heavily tuned to understeer no matter what.

In the end, it simply penalizes various cars versus others and yields classes that are often heavily populated with just 1 or a handful of cars that can yield the best result.

Imagine a stock class where the following are allowed: camber plates, front and rear sway bars, and shocks. That's it. This one "minor" change would allow a whole host of cars to be competitive with each other. Additionally, it would dramatically lower the costs of competing since the number one on-going expense is tires, and many stock class cars eat the edges of expensive high end R-comps alive (hence allow camber plates). It would be easy to go through an autocross season and spend over $2000 on tires alone every year as it is now. In any event, this will never happen, so it's just a dream anyway.

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Chuck so you're saying that the reason folks increase the front sway bar is because the BMW looses too much negative camber at the limits of the suspension travel? Has the suspension geometry of newer models solved that?

I'd have thought double wish bone front suspension would go a long way towards eliminating the problem. I've not crawled under my M3 enough to recall what setup it has for suspension in front. Its not the track car so mostly I don't care.....

Ron

Chuck to answer your one question about the Talon. It really isn't quite as "bizarre" as you might think. I added the strut bar "knowing" it would solve part of the problem. After all I had just spent $$ on it, so it must work!

I must have modified my approaches to turns after adding the strut bar (I had significant time on track before doing so) without really paying attention. Of course this change in my driving was to help me manage the added (unknown to me) understeer. Once I took the bar off I didn't modify my approach which lead to me inducing more oversteer then I had in the past.

So the car was fine it was me that messed it up by trying to make it better. A lesson I try to get across to all the students I work with in our track program. I think many a well handling car becomes worse by throwing money at the wrong things, at the wrong time, without a good understanding of what the final goal is and how the parts added will get you there. That's why I like these kind of discussions. Always something to learn.

Re M3...all BMWs have strut front suspensions (well, at least most all, I'm not sure what the old M1 had, etc).

Re swaybar...it's not just BMWs. Just about every car in stock class autocross uses a huge front swaybar as it's the only thing that can be changed (other than shocks). Just like most other modern cars, all modern BMWs are a compromise with regard to front suspension camber. Under maximum suspension loading, none of them (BMWs and most all other cars) have what a tire manufacturer would like to see in terms of negative camber for maximum grip. Due to body roll of a couple of degrees coupled with static camber of maybe -1.0, the outside edges are punished. When you increase the front sway bar stiffness dramatically, you can reduce the body roll component of camber loss.

BTW, there is some negative camber gain with compression on a strut suspension along with some gain when the wheel is turned for high positive caster settings (which BMWs have a lot of). FWIW, on my E39 M5, the negative camber gain with compression is about 0.5 degrees/inch of compression, so for each inch of strut compression, it adds about 0.5 degrees more negative camber. I have Ground Control camber plates on it, and with static camber set (max'd out) at -2.3 degrees, I still need a tad more if I could get it for track/autox use. I'm using Dinan springs which of course are a compromise sport spring and hence not some crazy high rate you really need for street/autox peak performance. A stock M5 sits about 1" higher with softer springs and non-adjustable front camber of about -0.9 degrees up front.

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Chuck so you gain camber on compression and loose it on expansion. From that I'm drawing the conclusion that, side to side, camber change is a zero sum game.

So we are back to sway bars doing what they do best, keeping the contact patches more even side to side then without them. No great effect on camber.

Unless you have a car with as much body roll as my old Audi Quattro (it had a LOT of body roll!! Ask any of the old timer autocrossers.) I can't see a more modern car getting to the limits of its suspension travel (often or perhaps at all) where the camber changes from side to side may not be a zero sum game.

It's not a zero sum gain since the outside tire is doing VAST amounts of more mechanical "work" than the inside tire when cornering. Probably 80% or more even of the front cornering load at the limit of adhesion is due to the outside tire; hence, obtaining the best case camber situation for that tire under max load is what is desired. By using a large front bar you can significantly reduce the body roll which reduces the camber loss (think of the strut mounting point rolling 2 degrees outward and you can visualize how the wheel has now lost 2 degrees of favorable camber) on that outside front tire.

Heck, with a large enough front bar, the inside tire isn't even doing any work at the apex. Take a look at my daughter driving in our E46 ZHP in a ~40mph sweeper at Danville airport (i.e. inside tire):




The second part of the equation we've not even talked about is how a large front bar effects the transient response of the car in autocross situations, and that is also a significant contributor to reduced times. Imagine a slalom and the dramatic, transient chassis perturbations. Increasing that front bar stiffness along with shock damping increases can have a huge effect on slalom performance. Typical road racers would say you have way too much front bar stiffness if you're not loading the inside front tire properly/enough -- true, but in an autocross stock class car, you can have enough stiffness to pull the inside tire off the ground at the apex of turns but still gain autox performance going even stiffer -- all due to the gains seen in transient chassis behavior.

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First GREAT PICTURE! Am I seeing the right rear almost off the ground too?

My point on camber being zero sum is that you loose on one side what you gain on the other. No implication from that on what is desired just what happens.

The big difference, to me, from autocrossing to track is, for the track, to limit the amount of G load transfer in a corner so as to keep the contact patches as flat as possible. Obviously you give this up when autocrossing since the changes are more dynamic.

If your picture was from a track event I'd say way too stiff as you are loosing close to 50% (if the back is also off the ground) of your entire contact patch.

Ron

I think from looking at blow-ups of the original 8M picture that the rear is still carrying a lot of load, but it does look that way in this picture.

Here's a view from under/through the car with me driving a bit further along in that same corner. You can see the edges of the outside front taking their beating, the inside front off the ground, and the inside rear still down pretty good (since through this whole turn the throttle was flat out, and this car has a stupid open differential -- hence it wouldn't be going anywhere if that inside rear didn't have enough load to provide forward grip in addition to its cornering grip). BTW, these are pictures that my wife took with a Canon DSLR -- she claims she doesn't know how to take pictures.

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That makes MUCH more sense. Both right sides off the ground in an autocross is pretty scary to me!

Tell your wife not to change a thing. Pics are excellent.

Ron

The rules were designed many many years ago for British sports cars that didn't come with such things. In some cases, they didn't have *front* sway bars either (the rubber bumper MGs come to mind).



Carroll Smith has a good take on what happens at the contact patch in a turn with respect to amount of grip -- "any transfer of load from one tire of a pair to the other reduces the total tractive capacity of the pair."

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Or Wes, as I say in class rooms now, its not a zero sum game. The goal in every corner should be to minimize weight transfer for just that reason. You always loose more total contact patch then you gain.

For a while I couldn't come up with an easy way to prove it (that you loose more then you gain) until I recall what my car does in a corner: the one rear tire will likely be off the ground all the way through the turn. Of course Chuck's picture illustrates the point too: the right front completely off the ground. No argument can be made that the left front gained a full tire's width of contact patch.

A fine point to remember when driving on track.

Ron

Let me just say it this way: If you're driving a car that changes in STRUT TOWER BAR makes actual significant handling changes to your car that a relatively inexperienced driver can actually feel and isn't a placebo, well, you're driving such a piece of crap that all bets are off on what you should exactly be doing to "fix" it. Why? Because you have a fundamentally broken system. A chassis that flexes THAT much could cause all sorts of strange problems that could manifest themselves in strange and unpredictable ways, IMHO. That is to say that just because removing one from a Talon made it oversteer doesn't mean removing one from some other AWD car might not have the opposite effect. That's why I question the stance on adding/removing them having XYZ effect. Sure, you can usually say those sorts of things about swaybars and such, but not really on strut bars, IMHO.

As for why the SCCA stock class rules are as they are, Wes is mostly right. It's somewhat historical. Why not "just change it"? Because it would change the competitive balance of most of the stock classes DRASTICALLY. That is to say we wouldn't even know what the "car to have" is in most classes, but I dare say it wouldn't be the same in most cases. Given that, the members that compete don't want that kind of change. The only people that ever ask for changes like that are people that have a car that currently is NOT competitive and would now become competitive with the change. So it doesn't happen.

Also, most people ask in the form of "why not just add this one little thing?" Usually the "one little thing" is rear bars on FWD cars. That kind of thing usually gets turned down both because it would upset the competitive balance of a number of classes but it is also "yet another allowance in STOCK" class racing. It's hard to justify the work involved with THAT when every season there are always a number of letters (at least as many as asked FOR allowances) that ask for FEWER stock class allowances. But the same thing applies...it's always from someone who either isn't a serious competitor anyway -or- has a car that stands to benefit a lot more than something else from the change. Then there's the fact that just taking away the FSB allowance (for example) would change the competitive balance of so many classes at once -and- negatively impacts tire wear dramatically for so many.

Anyway, enough venting on why it is like it is. I might sound a little crabby, but I do welcome discussion on these issues. Maybe someone will come up with the magic bullet that will help with reform in a better direction. For now things are what they are and aren't changing without a comprehensive plan, not just "I want allowance XYZ" or "please take away allowance EFG."

(For those that don't know, I'm on the SCCA SEB, the seven person rules making board for the SCCA. And those are the rules THSCC uses. Want to change them? Write a letter to seb@scca.com, but you do need to be an SCCA member and include your member number.)


--Donnie

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Donnie I disagree completely with your premise that if adding or removing a strut tower brace makes a difference then the car is a piece of crap. Doesn't that go against your later statement about questioning making sweeping statements about XYZ?? Does that also imply that any car that comes with one is really just a crappy car with a bandaid attached?

As for the affect that removing a strut tower brace will have, and I'm only talking about AWD or FWD cars since those are the ones that I have experience on, it works as I've said every time I've seen or heard of it done. That is by no means conclusive but at least I have real world experience with it....as does Oscar White. It would be an interesting experiment to look at the FWD cars at an SCCA event and see how many had front strut bars. And then talk to the owners and see if they knew why? Or if its there because "that's how it was when I bought it".

On the other hand, I don't care if anyone or no one agrees. In fact I hope my competition decides to leave theirs on!

As for changes in stock class, I find it curious that you say that making this or that change would perhaps lead to folks not knowing which was the "car to have" for that class.

I thought the reason behind the classes being set as they are was to give any car in that class an about equal chance of winning. So the idea of the "car to have" seems to go against that thinking.

Of course I know that each class has a "car to have" because that car tends to win more often. Seems to me that should be a BIG red flag pointing out that there is an imbalance in the class.

For example, if a number of different drivers win in say, CSP with Miatas I'd say that, for what ever reason, the Miata has a competitive advantage over every other car in that class. Something needs changing.

Realistically I'm pretty sure I don't care because I don't compete in autocrossing any more. So for me the classification discussion is something that bugs me but not enough to care a lot:)

Ron