Also check your alignment, particularly toe settings, a lot of guys get the front end worked on, or do it themselves and make the statement "I got it pretty close," as far as putting things back together. More often then not, "pretty close," is not close enough. I have a 2015 4C LE, at 104 mph (closed course, professional driver, yeah right) it got extremely interesting, the car would just about change lanes, with no input from me, reworked the front suspension with Alfaworks Race blocks, set the toe from the factory toe out, to slightly toed in, and I am happy to report that a recent blast to 120 mph, yielded a rock steady experience. So if you drop the front suspension, have it aligned by someone who knows what they are doing.
The Alfetta GT/GTV6 body is aerodynamic enough but the rear creates great lift at speed. That`s likely what you are experiencing. Along with air pressures on the sides of the car.
At Daytona my GT2 GTV6 racecar using a 3" airdam would tail wag at speeds around 100-150. Using a 6" airdam it would be much more stable. And about 5 mph slower due to drag.....
It would do that because of the light rear weight and the low pressures on the sides of the rear of the car, but when it wiggled to the side of the lowest pressure then low pressure was created on the other side.
You get used to it
Wind tunnel and road tests have shown that generally, the front develops far more lift than the rear. Long ago did a university study on the aero effect of race cars. One test by Alfa many years ago demonstrated that at ~100 mph, the Sprint GT/GTV body style developed ~250 lb lift at the front end and ~100 lb at the rear. Their tests showed that adding a Bobcor style front spoiler and rear trunk mounted spoiler lip reduced the front and rear lift by about a fourth, IIRC.
The 164 body style, more 'aerodynamic' than the Alfetta GT/GTV6 body, also demonstrates this same pressure distribution. A pressure distribution study by Alfa for the 164 shows in a profile diagram much more negative pressure at the front of the car hood than the rear, indicating substantial lift on the front suspension. Interestingly, the pressure distribution reverses at the base of the windshield which is why the hvac intake is located there, then it becomes neagitve again at the top of the car, thus why a sunroof exits inside air. Then it can become negative again at the rear, generating lift on the trunk lid. A rear spoiler can reduce the amount of reverse air flow (which develops some aero drag and some lift) coming back up onto the rear lid from behind the car.
Car suspensions are designed to develop understeer at lower speeds, eventually changing over to oversteer at high speeds. The book, The Racing Driver, while an older book, has some interesting diagrams and thoughts concerning this design philosophy. The sensitivy of this can be modified somewhat by changing tire pressures. Judging by my own experience with the GTV6, I would first fiddle with the front to rear tire pressures. Other factors, such as the ultrawide tires, may enter into a loss of steering weight normally generated by the front suspension caster. A slight increase in caster may be indicated.
"Aero lift, but I also think "we" are getting older and would not have even noticed this when 20 "
Lol, when I raced my Downton Mini when I was much younger, I didn't have to worry about lift, as that car was basically just a fast brick with a ton of drag but at least outcornering anything else.
"IMO cars should be designed for 0 lift at 100mph"
Nice thought, but almost impossible considering that they otherwise have to be functional. Many studies have been done, and the results are always unusable for the most part due to useful functionality and safety standards. F1 racers are of course at the other end of the spectrum. With tremendous negative lift, they are functional for one purpose only, ie, maximum cornering speed. The engine's sole purpose is to overcome the subsequent aero drag caused by this design. Does end up becoming a compromise between top speed and negative lift, different for each race course.
After re-reading your original post I'm wondering if you're suffering from a malady I've never experienced, but frequently read about called "Tramlining"
Tramlining - Wikipedia - Tramlining is the tendency of a vehicle's wheels to follow the contours in the surface upon which it runs Tramlining can usually be blamed on tires, and its incidence depends greatly on the model of tire and its state of wear. Although not normally dangerous, at very high speeds it can become a source of instability.. Vehicles with large and wide low profile tires are more prone to the effects ....
You're running 225 quite a bit up from the OEM 190. Is there another GTV6 owner in your local club who would lend you his front wheels for an A/B test; meaning do one high speed run with you your 225's and the anothere with his 190's ?
I second all the people saying add some toe in and a little more caster. I'm assuming that there's no play in the inner tie rods or the tie rod ends. Harder to test for, would be small wear in the steering rack, which would have the same effect as wear in the tie rods.
Hope this was helpful and let us know how things proceed.
I think we are not on the same page. You don't need wings and appendages to have 0 lift, just potentially flat under tray and the right shape. Wings are to produce drownforce which means negative lift, I just want 0, so the car is not dangerous. I don't want say 100 kg's forcing down or up, just 0 at 100 mph
That's the problem. According to the studies I've read and wind tunnel examples I've seen, the right shape for zero lift doesn't really give you a functionally adequate shape for reasonable use by the buying public market. Zero lift without wings or spoilers requires body shapes which have to be carefully tailored to eliminate the lift, and drag, developed by the body moving in close proximity, in a compressible medium, ie, air, to the road surface.
I became interested in the aero characteristics of vehicles when I was in college working at the local wind tunnel, analyzing the lift, drag, and pitching moments of various ariplane models from aircraft companies such as Boeing and Beechcraft. Lol, even the Seattle Space Needle model was tested (as I remember, the restaurant top blew off the tower and down the tunnel at 100 mph. Not strong enough). Also, a model of the infamous suspension bridge, "Galloping Gertie" was tested, after the bridge's famous wind induced failure. Now, the analysis of air flow around an object can be mathematically modeled to such an accurate degree that wind tunnels are not used as much in the design of aircraft, etc.
I think any of the raised front 105/115 and 116 cars imported to the US to meet bumper height requirements, will have issues at higher speeds based on their ability to flow more air under the cars, and aesthetically, ugh!
Umm, doesn't have quite the same drag coefficient. Close, though. Nor the range either, but still a remarkable car.
I remember the pictures of the Porsche Le Mans racer a few years ago which lifted off and did almost a complete flip, Porsche not getting the aero right on it, too much front end lift. Unlimited hydroplanes in the old days had the same problem, amateur racers not understanding the aero characteristics of those boats, getting the cg too far back thus letting the front end lift, the hydro doing a big flip. Killed drivers in a few races. The pickle fork design lessened the probability of a flip, but they would sometimes still get the aero incorrect and the boat could flip.
Happened twice I think. Must have been an exciting ride!
Yes a fascinating topic. With my last club race car I tried to be aero wise, i.e. it had a completely flat undertray that went up at the tail end (but should have had the vertical dividers like a diffuser), plus the passenger space (had to have that for the "road car" concept rules) I covered and only ran a little screen in front of me. I wanted to make a single roll bar hoop but the roll bars have to be certified so thought that was too much hassle ... I also put a lot of effort in to centre of gravity and ran a Sud flat 4 as far back in the front engine bar as possible. Ended up 48% of the weight on the front and 52% on the rear/driving wheels, but I was also sitting way back, which if I ever refound the car or built another I would like to change.
There is so much more to cars than engine power ...
I run 100+ on the highway in nevada all the time. Speed is posted at 80mph about 30 miles outside of reno next town of Lovelock is 70 miles away across the desert. 1976 alfetta gt feels good at 100mph+ but the roads are smooth. Ride height is about 4in from lowest part of car to surface (old LeMans rule works for me).Make sure your front end is at least 1” lower than the rear. My Lower A arms are level but I have at least 2 degrees negative front camber for track and hill climbs.
My older 1978 Alfa sports sedan was setup the same way and handled 115 mph (with some downhill) very smoothly and steady.
I’m surmising that If I routinely drive 100mph the feel of lifting goes away since you’re developing trust in the car. But occasional 100 mph in NJ with the bumpy highways, I would probably feel the “lift” too.
Maybe next year I’ll take my little 4 cylinder alfetta to bonneville UT and run a few 115 mph runs. Doubt if I can get any faster with the race setup I have on salt. I only use the little stock air dams in the front.