I've run the no rear bar, stiffer front bar, 245x4, OEM suspension setup and agree that there is no rear end twitchiness in high speed corners.

 

Just occasionally. Maybe 3-4 times a year, when there is no race or track event on the calendar. Usually drive a prepared E36 M3 for the track, though. Not the S. Then outdoor go-karts when I have time during the week.

I respect autocrossers a lot. Although they drive at significantly slower speeds, the accuracy of their line has to be spot on. And things happen so much faster. Looking ahead becomes so much more important. And they have to be fast from the get go. No warm up lap.

Learning to autocross has made me significantly faster on the track. And so it applies to go-karting.

 

I'm sure you know this, but the dampers only provide a resistant force when they're being moved. On a smooth track, they do almost nothing when load is not transferring. So, say for example, you are brake-turning (I don't want to call it trailbraking because I'm asking you to imagine constant braking pressure) into a corner. Load has already transferred off the rears and onto the fronts, and now you're simply holding constant brake pressure at the cornering limit of the car, during corner entry. However the car behaves in this condition, whether it understeers or oversteers, whatever yaw angle it settles into, whatever slip angles the tires tend towards, everything about the car's behavior during this condition is determined by all parameters except the dampers (ie, springs, bars, and all the characteristics of the chassis).

Anyway, in a low-hp car like the OP's, the part of the corner from throttle application to track-out is very similar to the opposite of the situation I described above. You get to full throttle relatively soon, and the accelerative thrust is pretty constant; ie, you pick up the throttle somewhere mid-corner, and are full throttle (and therefore fully load-transferred) fairly soon thereafter. Unless you're going over a really bumpy section of track, the dampers aren't having nearly as much affect as things like alignment, springs, bars, ride height, roll moments, etc.

If you want to talk about dynamic load transfer, all I have to say is that it's a general truth that increasing the roll resistance at a particular end of the car increases the load transferred across the pair of tires at that end and decreases the overall tractive capacity of the pair. Increasing bump stiffness at whichever end increases the rate at which load is transferred across that pair of tires while the chassis goes into roll.

As for your particular situation, I have a few thoughts. You may disagree with this, but it takes an exceptionally 'tuned-in' and capable driver to give useful feedback. There are guys at the top levels of GrandAm and IndyCar that are very quick and capable, but can't describe in detail what the car is doing. Damper adjustments especially can have very subtle effects that are tough to describe or quantify. So the rest of this paragraph is just my opinion based on your "it felt less twitchy". When you increased bump & rebound, you made the rear move around less and move slower. This felt better to you, but I have no clue if it was a good thing or a bad thing. It could be simply making the rear suspension move and react slower like I said... Or it could have been slowing down the onset of some other geometry problem (slowing down how fast the outside rear compresses, therefore slowing down how fast it bumpsteers, making it feel less 'twitchy', or slowing down how fast the tire went past its ideal camber, therefore slowing down how fast it went beyond peak grip).... Who knows, it could be lots of things. My hunch is that it slowed down a geometry problem. If so, that doesn't mean that "Increasing rear bump gives a car more rear grip on corner exit."



You're right about the book-learnin'. S2000's are proper cars that usually respond as expected to textbook generalities.


- Question, debate, yell, scream, fling poo....

 

Oh cool, what kind of kart do you have? I have an HPV TonyKart that I need to make time for...

Do you do BMWCCA races...? Glad to hear you get out to the track

 

Yeah, I know. But weight is transfering. In an S2000, there is not nearly the acceleration of something like an IRL car, of course. But there is some weight transfer aft and laterally, just like the theory says. You can feel it, right? I know I can. My car is just as low HP as the OP's car is. Exactly as low, in fact.

Also, even though throttle is nearly wide open (I was actually still opening it), the power produced by the engine is such a peaky function of RPM that you can often slam the throttle open at 6K but the car responds as if you have been smoothly opening it all the way out to about 8K. (It's the old joke about VTEC being turbo lag without the turbo power.)

Since the springs are also a lot softer than those in an IRL car, even this lesser rate of weight transfer produces a reasonable rate of suspension travel, which is what the dampers work on. They are having some effect, even if it's not the "slam you upside your head" magnitude of effect in a high-powered race car.

Like I say, my experience in this is very limited. This was the one and only time I have ever made a damper adjustment on a car. I do have some experience with damping in other contexts, however.

But the bottom line is that after I tweaked my dampers a bit, I perceived the car to be a bit more planted. I don't have sophisticated enough data to know if this was anything other than perception, but my perception did match the theory, which is always a good sign.

(By the way, I was in fact going over a bumpy section of track. I also pinched off the corner. It shouldn't have been enough to loop me, but I looped. After changing the dampers, I haven't looped there again. Yet.)

 

Haha, I was not comparing the S2k to an IRL car... My point was that what you perceived as "less twitchy" could be due to a million different things, all of which I doubt you can detect from the driver's seat.

Anyway, you bring up some valid points, as the example I provided was somewhat theoretical. In the real world there are bumps, VTEC hits, driver doesn't apply power properly, etc... I'm definitely not saying that dampers have no effect; just that springs/bars/alignment/bumpsteer tend to overrule them (as they should).

VTEC - what are you doing out of it?? Downshift!

 

[QUOTE=blackey,Apr 10 2008, 05:50 PM]No validity, at least not with higher spring rates.

 

You can see the front end is a bit twitchy in this pic.

 

... saving this one.