Turbo944,

I meant that I would just use shocks in the back instead of coilovers. I would leave the torsion bars in place. I don't even know what they are or what they do actually. I guess I need to look into that.

MPW/Matt,

Instead of races I should have said autocrosses. I've never been to a DE event but I plan on attending at least 1 this year. What car do you own and what is the color? If you are interested in autocrossing you should attend the Chesapeake club autocross training event in March. You can find more information about it at pcachs.org.

I look forward to seeing you at some events too.

Mickey

 

In this sketch, the inside rear wheels are off the ground. This is not an accurate depiction of body roll. In fact the situation you've depicted can happen with no body roll at all! Also, you can't have weight transfer if the wheels are in the air, because they aren't transferring weight!

In weight transfer induced body roll, the wheels remain in contact with the ground and are extended in rebound/droop on the inside and bump on the outside. Different suspension geometries will have different responses to this sort of movement, in terms of roll centre height, centre of gravity, etc. You need to measure your geometry exactly and then perform the calculations.

Has anyone done this for front engine Porsches? You seldom see applicable diagrams in books because although the front McPherson strut is covered, the rear trailing arms seldom are.

Clayton

 

Mickey, the torsion bars are just springs. You can remove them and use coilover springs of an increased rate (over none, or the standard 968CS one) instead. It makes the car easier to set up, particularly with respect to setting rear ride height. There's also the opportunity to remove weight and affect transient response of the springing effect. I've never done any of this. Lots of people on this list have, though.

Clayton

 

OK, Clayton, sorry my 2 minute sketch was as detailed as you would have liked... and the point of the sketch wasn't weight transfer anyways, it was to show the jacking force! I'll take a peak at the diagrams in the books I have, but many of them are independant of suspension types and are meant to emphasize certain other design points that are independant from suspension types, such as c.g.'s and the like.

...and we're not talking about compression braking unless you are trying to race your semi.

 

I have to admit I didn't look at any of your equations, and I'm still a bit confused about what a jacking force is.

By compression braking I mean all non-brake braking methods people have mentioned on this thread. Engine braking, or whatever it's called. I edited the past post to reflect this.

I was chased by a racing truck on a track during an open practice session once. 160km/h in a truck is great to watch and listen to, but not from the rear view mirror!

Clayton

 

Mmm: Page 6.
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Regarding the whole braking with the engine 'debate,' it seems that everyone but Kevin (aka FSAEracer03) agrees that it is not an effective method for slowing down a car on the track.

Kevin: again, I mean no insult, but please go back to the shop and ask your friends to clarify their braking technique. I have a feeling you have mis-interpretted some of what they have said.

I think many people have contributted to this thread and have explained proper braking technique for driving on the track. So far, you choose NOT to listen to this input and accept the fact that you may be wrong. Perhaps you need to reconsider the alternatives to your view.
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Now, back to the misconceptions and the current discussion about spring rates/shocks...etc. Here's another thought:
You don't need to have the fanciest cup car in order to drive well on a racetrack. Often after a few DE events, people will feel the only way they can go faster is if they upgrade to a better car. While initially their lap times may decrease, the truth is, they are only relying on a faster car, and essentially they've stunted their growth! A 'fast' car covers a multitude of mistakes on the track. Thus, one can drive such a car in a sloppy manner and still seem to be doing a great job!

However, with a slower, more 'stock-ish' car, like a slightly modified 944, a driver is forced to be more precise and smooth on the track! Such a driver truly learns the dynamics of driving a momentum car. (Momentum car is a kinder way of saying a slow car! ) The person who learns to extract the most out of such a car will be a faster driver, no matter what car he drives on the track.

So, if you relate this concept with shocks and springs, well then the answer is quite simple: JUST DRIVE! Learn to drive what ya got: when you can honestly say that you use 95% of your car on the track, then it is time to start considering the upgrades. Baby steps, I always say, baby steps!

Keep the comments coming!

Thanks, (and sorry for rambling....)
-Z-man.

 

I agree.

Also, there's nothing worse than seeing someone get a new car and immediately go about changing it before they've even had a serious attempt to drive it and understand it. I had some friends who's stories highlight this situation. One bought a 944T about the same time the other bought a 944S2 and I bought a 968CS. Mine is the only car that stayed standard. The first time I saw the 944T at the track, it was detonating so severely it was making people in the pits wince in pain. I had to tell him to stop driving it before it lunched itself. Why was it detonating? Because of some aftermarket boost control hack he'd had installed. Why did he do it? I don't know. He never even knew how quick it would go before he had it modified, let alone how to drive it well. And needless to say, with all that detonation he wasn't going faster - he was going home. He wasted his time and money, and to this day has never really gotten much satisfaction from owning a Porsche.

The other guy made lots of mods, but at least gave himself some time to decide where to make them, and took advice from others as to where the money should be spent. I presume he learned something along the way, too. He got considerable pleasure from his car, and developed a strong liking for Porsches in general as a result.

Interestingly, despite all the mods done to these other cars, mine still set faster lap times. They were full of excuses, of course. Apparently my 6 speed gearbox is worth about 150 horsepower. Yeah, right.

When you do finally feel that you're going as fast as you can possibly go with your current car, only then consider making alterations. And when you do alter it, make only one change at a time. Otherwise you won't know what change had what effect.

After owning my car for seven years, and tracking it regularly for the first few of those years, I feel I am now ready to make some performance enhancements. I also have very specific goals as to what I am trying to achieve with the enhancements. They will be introduced one at a time if possible.

Clayton

 

Hey Mickey,

Right now I'm driving a black 2002 Audi S4 that I'm trying to sell. I'm shopping for a 944 S2 and I've found what looks to be a cherry one close by. I'm hopefully going to check it out this week, we'll see. I actually attended one PCA autocross school last April, but I'm not sure if it was the Potomac or Chesapeake club. It was a blast though, hopefully this year I'll be there in a P-car. I've actually been autocrossing for the last two years with the Benz club in Winchester.

- Matt

 

I completely agree, I think I'm really going to like 944 ownership. This is happening a lot with the S4s now that the price is dropping to a point where more people can afford them. We're getting a ton of kids coming on the forums asking how to make their cars faster; some are buying Stage III cars puting out upwards of 400 HP with stock brakes. When you suggest that driving school should be their first "mod" so they can learn to drive the car they have, they take offense because they obviously already have Schumacher's skills. But I digress...improving the driver is almost always better than improving the car.

- Matt

 

LOL!! That's crazy! Yeah, I could imagine that would be a little unsettlig to the nerves... a concentration breaker to say the least (though it might give you a bit more incentive to get the hell out of his way and go faster, haha)

The so called "jacking force" is a bit hard to explain (damnit! outta luck once again). In words, it's the vertical component of the outside tires' reactant force from cornering. It lifts unsprung mass as well as the c.g... effecting weight transfer and overal cornering capability. And though technically, it isn't mathematically stated how I showed it in my sketch, it can be somwhat estimated that way. I should probably have thrown in a nice little proportionality constant in the equation... whatever! It's actually calculated using the angle of the contact patch (center) makes with the roll center... the subsiquent vertical component (cornering force*sin(angle)) is the jacking force... but because I am not as proficient in the art of roll center determination just yet (I'd need to know the car's suspension geometry and/or see a diagram), I used the c.g. to illustrate my point...

here is the actual, true mathematic representation and illustration of the jacking force (I even kept the inside tire on the ground for ya, lol):

 

I found this online. Since maths isn't a language I speak fluently, I haven't had time to absorb it yet, but it does mention jacking force, so I thought I'd post it.

http://www.cif.rochester.edu/~alain/ee399/roll.html

I can suggest this simplified definition of jacking force, though. If your roll centre is above the ground, the car will be jacked (lifted) up on the outside of the turn. If your roll centre is below the ground (rare for a road car, but possible), the car will be jacked down on the outside of the turn. This force counteracts roll about the roll centre, and is the vertical component of the force generated from the tyre contact patch to the roll centre. The closer the roll centre is to ground level, the smaller this jacking force will be.

This implies that some amount of roll centre positioning above ground level will help "flatten" a car in turns. However a roll centre that is too high can cause a catastrophic loss of wheel angle control, effectively tipping the car off its wheels. For more info, see Beavis and Butthead's Corvair washing episode and/or Ralph Nader's hobby horse, ridden to death in his seminal work "Unsafe at Any Speed". Now if Nader had said "Dude, that thing's roll centre is too high, you're gonna wreck!" we'd all take him more seriously.

Clayton

 

Theres a huge difference between a monster semi with its drum brakes and 50,000 pounds+ mass, and a little sports car with 4 discs and racing pads. Most big trucks cant use their brakes to slow down much, they get too hot, red hot and steaming even in 40C weather.

 

I agree... but for off-track use... I do use the engine braking to make small adjustments in speed for around town driving...

 

Is that s2 red? maybe with non-stock rims?

 

Nope, Silver, 104k, stock everything. Hopefully going to check it out today or tomorrow!