1: it is not the camber curves (which are quite reasonable) 2: it is the rear roll center (and height thereof)
doesnt look too reasonable too me with some much inside tire neg camber on jounce travel on a street car. if your use the same 348 A-arms and same upright, wouldnt remounting the upper A-arm nearly an inch lower with same suspension components give a different camber curve? i believe so... i think the goal is both to lower the roll center and change the camber curve. it sounds like they lower the rear roll center to soften up the rear and alter the chamber curve for a better contact running off-loaded tire in corners the elise S1 had the same geometry issue and was changed on the S2 models to make it less twitchy. their approach was somewhat different using the same pick up points but different hub assembly in order to keep the more rubber on the ground with the unloaded inside rear tire. take a look at some track day mk1 and mk2 elises and clearly see the difference. early ones look like they have 3/4 day light under outer rear offloaded tire. btw how much did static roll centers moved with the later 348 pick up point locations? respectfully hf
Speaking of wheels, I don't really care for the directional wheels. On the 458, I think the 16M option wheels are much nicer. The directional wheels kind of work with the cheese graters on the 348, but no so much with the more modern cars from the 355 on... Image Unavailable, Please Login Image Unavailable, Please Login
I agree completely ........... directional wheels do not 'age' well ...... where symmetrical wheels like the 355's ...... are simply 'timeless' ......
The F355 actually has more camber gain in compression and extension than the F348. So, if you point is that the camber gain is what causes the F348 to have tricky handling, then the F355 should be all over the road. It is not. The difference between the early F348 and the later F348 post Speciale and the F355s is that the upper control arm was moved down ~0.25 inches at the chassis pick up point. This reduced the instant center from ~160 inches from the wheel to ~90 inches from the wheel. The smaller the instant center the greater the camber gain. This change also had the effect of raising the rear roll center and THIS caused much more of the lateral weight transfer to go frontward and promote soothinig understeer, calming the car. As you can see from the attachment, the lowering of the upper chassis pick up point (3)RAISES the rear roll center fromt (5) to (2). Image Unavailable, Please Login
Mitch Fantastic imagine, great post. More camber (neg) gain on compression on outside wheel is a good thing if needed but I can see no benefits of the inside wheel gaining more -camber on extension (jounce) in this case. why would your think more neg camber on the inside wheel extension is a advantageous ? I believe you may be mistaken in saying that the lower mounted upper pick up points of the 355 or late 348s with give you more extension (neg) -camber gain. that software youre running should confirm this pretty easly. It would be great to see the curves on another image plot. Ive done a quick and dirty sketch over your drawing. Smaller instant centers does not always equal more camber gain, its the combination up-right pivot points, unequal a-arm lengths and there mounting position locations that determines camber gain. But in this case you are correct about shorter the instant center will have more camber gain however this only applies on compression. The new location will have more -camber on compression (outside tire) but the tire will have less neg camber gain on extension (inside tire) which will put more rubber on the ground. So moving the pick up point down will keep the inside tire flatter on the ground. You do not want more neg camber gain on the inside extension tire like your implying even though the newer lower mount point dont really offer that. I believe the you are referring to Roll Axis when your mention more of the lateral weight transfer go frontwards when altering the roll axis by raising the rear RC? yes typically it does promote soothing understeers but usually just on turn in. once the car has stabilized an weight transfer begins to move to the rear on full power exits, the rear will be more twitchy, which is a compromise I believe the engineers may have chosen for a better camber curve in both compression and extension without then need to manufacturing new A-arms or uprights. im not saying that the 355 is more twitchy than the 348 in that respect but thats what happens with steeper roll axis. maybe the uprated tires, and dampers help on the tame the handling also. So dont lift in corners is a good rule of thumb when drive mid or rear engine cars. Remember raising the rear roll center will increase roll stiffness akin to putting bigger roll bars on . So basically what I have gather from effects of late 348 lower pick up mounts and 355 geometry of this is: More aggressive camber curve on compression side + good Less neg camber curve on extension side (more tire contact patch) +good Steeper Roll Axis for better turn in stability but looser on exit + good and - bad So 2.5 out of 3 aint bad? I believe so there are a ton of other variable that effects the car handling characteristics, electronic dampers, yaw rates, ride height, aero, anti dive and squat geometry ramps etc. but 348/355 basic suspension geometry does look fairly dated and straight forward to noodle with is a good thing I believe. All of this is just conjecture course without real world repeatable testing of course. Thanks for the lively conversation hf ps .....dang trying to up load an image but my attachment box is full...how do i emtpy that for more space??
You have impressive knowledge. I agree with your observations: The 348 has dodgy handling, while the 355 is clearly superior in every respect! +355!
