333pg333

I've always had the same setup in terms of the KWs and the springs + tbar. When I switched from the Mo30 sways to the Tarretts I found that I could use more rear bar than before. If I switched from soft to even middle with the old Mo30 it would want to rotate very easily...so it always sat on soft.
I also had my car pretty low and I found eventually that the reason that I was bumping around some tracks and getting push understeer was that I had reduced the free travel and was riding the bump stops too much. So we raised the car and this alleviated the push, but now I think it's too high. So I want to get some different camber plates that will allow me to lower the front without getting closer to the bump stops, and we can lower the rears a little too. By reading some of the former advise here, we might try just lowering the rears first. I think I'll buy some stiffer springs for the front too.


Any thoughts on my other question about customising or changing suspension completely for a hypothetical project?

Dubai944

You can't beat double A-arms for adjustability and favorable camber curves. The page 1 suspension pics of the GTR Turbo thread show what can be done, but it's not really a 944 anymore Might as well go buy a real race car.

https://rennlist.com/forums/944-turb...fo-thread.html

You can improve the stock design a bit without going overboard. To be able to seperate toe and camber adjustment on the rear and relocate the mounting points to improve the trailing arm geometry would be worthwhile. Adjustability and limiting unwanted camber and toe changes is the goal. Also finding ways to reduce the unsprung weight.

At the end of the day these cars handle amazingly well for an old basic suspension design. I think it was Colin Chapman who said you can make even the worst suspension design handle if you stop it from moving!

Larry Herman

What's that they said about MacPherson struts? "Take a really good suspension and throw away half of it."

gt37vgt

i think Dubai knows not of bumps .... my f1 suspension tech buddy tells me that race tracks in the UAE are so smooth that they often think their sensors aren't working ..so i would seek more advice from eastern creek on spring rates than around the world .. and hill climbs are never that smooth either ..

Dubai944

lol... yer, what are those bump things??

Trucho-951

I wonder if anyone has ever adapted the front 928 suspension to a 951, don't 928's use double wishbone up front?

333pg333

Ok so today we went over the car's suspension settings. I found that it was a bit wonky in terms of ride height. The front left and rear right were higher than the front right and rear left. We tried to fix this up by adjusting the spring perches. Unfortunately to check the Rear Rebound on the KWs you have to drop the shocks/springs out but it was on the hoist so why not. It was at halfway so we decided to move it up to 3/4 full.
The Rear Bump/Compression was sitting on 10 out of 13 for full hard. We decided to wind it back to 5 just to soften things up a bit.

On the front we have 15 clicks of settings. It was on 11 so we again wound it off to a softer 5. The Rebound on the front was a bit strange. Out of 3.5 turns rate of movement in adj we had full hard on the RHS and 1 turn from Max on the LHS. We decided to go with 2 up from min, or 2 out of 3.5 turns towards full.

Coupled with the changes made in the geometry last week I think I am very much looking forward to having a crack at the GT3's at the next meeting. They were .5 to 2 seconds ahead of us last time when we were both on R specs. We hope to bridge the gap somewhat next time. Having syncro in 2nd will also be nice.

As an aside, we also found that we were running the software for the piggyback incorrectly and it was retarding the ign map quite a lot. I feel like a kid counting down the sleeps until X-mas...

Dubai944

You did corner weight it I hope?

333pg333

No scales but in effect isn't ride height essentially the same thing? In other words the only way to adjust corner weighting is via ride height adjustments, unless you want to put bags of cement in the various corners.

Van

Absolutely not!!

Corner balancing is essential to ensure the car's weight is evenly distributed on the ground through the 4 tires.

Due to manufacturing tolerances and bumps/scrapes a 25 year-old chassis will have gone through, I can 100% guarantee that the chassis is not uniform to the fraction-of-an-inch all around. Trying to set the ride height to chassis measurements will cause problems and an ill-handling car.

Please have it corner balanced - it's money well spent.

FWIW, when my car is corner balanced, the "ride height" measurements are all a little different - but the car drives like a dream.

Also, corner balance with about 1/2 tank of gas, weight in the driver's seat equal to you, and everything in racing trim.

gt37vgt

well perhaps if you do your ride hieght down to .5 mm resolution . bounce the car roll it back and forth and check again .. perhaps set it a shade high on the right for running anti clock wise and an extra cheeseburger for the driver . also by the look of the tank it will go right heavy as it go's to 1 thrid full .

333pg333

Ok I know I've simplified it by suggesting that corner balancing is just ride height with a different hat on, but then how do you redistribute the weight? We did some measurements off the body yesterday and I thought this could be open to speculation, so then we took some ride height measurements from underneath the car and pretty much came up with similar distributions. Having said all that, I'll get it corner balanced.

333pg333

Oh, here's a reply from a friend regarding this all. Thought I'd put it to the board:



"Dubai speaks truth. While I do think that your handling could be improved by a combination of a number of factors, there needs to be a starting point. And, while your turn-in and rear-end dartiness are separate issues, they are nevertheless still related.


First, definitely get your alignment to a solid baseline -- i.e., get your caster and toe settings equal and to the point where you're happy with them.


Second, I would look into adding a bit more toe IN on the rear. BUT, don't add toe out in the front. Generally speaking, yes: toe IN on the rear will make it more stable under braking, and toe OUT in front will give the car better turn in. But, adding toe out is more of a bandaid and has too many ill side affects that I wouldn't look into this unless all else fails (which I doubt it will! ). Turn in, steady-state, and transitional understeer can all be improved via other, more effective methods.


Third, are your shocks at least two-way adjustable (compression and rebound)? If so, there is the possibility that you're running too much rebound in the rear. Typical signs of too much rear rebound include a darty feeling when chopping throttle, a bumpy ride that skips out a bit too much, and when way too much rebound is used, the rear can actually begin jacking down.

I should add that there is the *slight* possibility of too much compression (in the dampers) in the rear as well, but I doubt it. I would avoid this for now. How much rear-end squat are you getting under acceleration?


Finally, I do agree about the spring weight difference issue (which helps define the Roll Couple, or plainly, each axle's wheel rate relative to the roll rate of the entire car). How easy is it to induce trail-brake oversteer? Under steady-state cornering, what is your bias? That said, turn-in understeer is another issue; the car can still be biased towards oversteer and yet understeer on turn-in.


As for bump steer, the solution is simple, and just as nearly everyone has been saying: ensure the angle of the tie rod, relative to that of the control arm is the same.

That said, I disagree somewhat about the control arm ball joint pin length. Let me explain (and thus begins the lengthy discussion about suspension engineering…sorry!:

The distance between the CG and the Kinematic Roll Center (which, it's becoming clear nowadays, is less meaningful than we think) helps define the Roll Moment. Meaning, with higher Roll Moment, the Elastic Weight Transfer increases, relative to the total weight transfer. It's argued that the EWT is the only thing that "truly" matters, given that it is the type of WT that is actually felt by the driver (and is manifested in the form of body roll).

Put simply, the larger the distance between the KRC and the CG, the more body roll that will be felt by the driver. And, as we know, body roll is bad.


BUT, this increase in Roll Moment does have its advantages. This comes in the form of a reduced jacking load. In fact, once the KRC is at ground-level, the jacking forces effectively become zero. This translates into more load being used for lateral cornering. And jacking is REALLY bad, especially for stiffly-sprung race cars.

So, the tradeoff begins: jacking is VERY bad, and so initially ('50s, '60s, and into the '70s), when suspension systems actually started to get designed from scratch, there was a huge push to minimize jacking as much as possible. But the problem remained that for production cars, the long-and-short of it is that reducing body roll (i.e., driver discomfort) is more important (the lateral loads they see aren’t high enough for jacking to be an issue).


***But as far as what’s ideal for your RACE CAR, it's actually a question of the Force-based Roll Center, its migration, and total distance the wheel itself travels.


Here's what I mean: Think of a high-end formula car. For very long Effective Swing Arm lengths (which our cars do not have) and for very small amounts of wheel travel and very high wheel rates (which our cars also do not have stock), the actual Force-based RC and KRC migration can be quite small. Furthermore, since the total wheel travel is not much, increasing the Roll Moment (i.e., lowering the KRC) will NOT have much negative effect on Body Roll, and since we care more about minimizing jacking, the roll center tends to be either at the bottom of the chassis or at ground level. This is true for nearly all formula cars nowadays, including F1.


So it should be fairly clear why the 951 was not designed this way: it has large amounts of wheel travel, the effective SA length is quite short, and the wheel rate is very low. Therefore, the engineers at Porsche wanted to decrease the car's body roll and make the cornering seem as "flat" as possible.


But, if we look at Dubai's race car, while the SA length is still small, the wheel rate has increased dramatically and the wheel travel has decreased a ton as well. Suddenly, the car starts looking more like a race car, rather than a sports car. At this point, the car is aided by dropping the KRC location (despite the increase in body roll) because jacking is reduced. Which, remember, is bad. And again, since the total wheel travel isn’t much, the amount of body roll felt also won’t be much.


So my point is this: when we lower the 944, it also has the effect of lowering the KRC. This is not necessarily a bad thing. As long as the suspension doesn't bind, it should improve grip of the car. But, this is only as long as we can also do something to the rear. Given that the rear has its own KRC, there is then what is referred to as a Roll Axis, or the line around which the car “theoretically” rolls (it passes through the front and rear axle's respective KRC's).


In summary (FINALLY!), lowering the KRC is a good thing as long as the change in the roll axis height/inclination doesn’t change the overall handling of the car in a detrimental way. Ideally, if one lowers the rear KRC, the roll axis would be back to “normal”, only lower, thus improving the handling of the car as a whole.


I’m very sorry for all this rambling. All of my thoughts were haphazard and quite disorganized, so my apologies. If you’d like, I’d be happy to talk about this over the phone or via email. I’d really like to also answer your question about “what would you change on the suspension design of the 944 if you could” because I have a number of ideas, ranging from “small” changes such as moving the KRC to big ones like adding much more Ackerman and even changing suspension geometries all together (e.g., dual A-arm). "

Larry Herman

IMHO your friend has some good advice based upon sound fundamentals.

Dubai944

Interesting 'friend' you have there Patrick.

With regards to corner balancing, the chair leg analogy is the easiest to understand. If you have a chair with uneven leg lengths it will tend to rock diagonally on two legs. The idea is to get it to rest evenly on all four feet.

It's important to understand you can't redistribute weight front to rear or left to right with corner balancing, you can only do that by physically moving something, but you can get the cross weights equal (LF+RR = RF+LR) so the car is not wedged unevenly across one diagonal like the wonky chair.

Unless you have a perfect 50/50 front to rear as well as 50/50 left to right you will end up with slightly uneven ride heights when properly corner balanced but that is not important as long as you have even diagonal weights to give even handling turning in both directions.

You can even purposely jack the car across one diagonal to favor one direction of turn. Nascar "wedge" their cars so the RF+LR is > LF+RR. Although it gives a bit more understeer turning left, it puts more weight on the inside left rear wheel allowing them to drive off left hand turns much better.

I haveexperimented running very small amounts of "de-wedge" on tracks where most of the turns are right hand, and am definately able to get back on the power earlier keeping the rear end planted. I have also use a bit of left hand thrust alignment offset to counter the right hand turn in understeer induced by uneven cornerweights. Of course if you do this the car doesn't do left hand turns as well, so there are diminishing returns and screwing around like this without your own scales and suspension tools is not practical.

In most cases it's best to set cross weights to 50% for tracks with both left & right corners. But definately get it done, and remember Van's advice to have your weight in the seat and half a tank of fuel while it's done.

Unfotunately you should do this first, before setting camber and toe as you will upset those settings when you alter the ride heights to adjust the weights. You may need to do another alignment after corner balancing.