Thanks ! Can anyone describe the handling carateristics of each configuration Upper pointed down to center- Uppers parallel with lowers Uppers pointed down to outside
Upper pic, the arms go negative cambered thru compression. Lower pic, the arms go more to a positive cambered thru compression. G.m. changed geometry on upper/lower arms in 1970. Before 70 they went positive, after 70 they went negative. You want it to go negative, it makes the car handle and drive better. When going thru dips the tires go negative and push against each other or the weight of the car depending on arm position at that moment. Going positive and it wants to pull the car to the positive side. That's why a 70 firebird handles way better than say a 68 firebird.
The center (second) picture will do pretty much what the third picture will do de to deflection of all of the chassis and suspension parts. If you are building...replicate the top picture...BUT...there's more to it that the above pictures...there's the "looking down" pictures not shown..! Mike
And this is what the "Shelby Drop" does for Mustangs and Falcons. It drops the mounting point of the upper control arm. It makes them plant the tires in corners, instead of unloading them.
Lostone is on the right track but the years vary. 2nd gen f-body was the first GM to use the revised geometry. A-body didn't change till the '73 restyle. Fullsize - 71(?) There are a couple good videos on youtube that go thru all this. One of them is a video of some seminar that the dude who started/own Fatman Fabrications did (can't remember his name). More useful to the pro-touring crowd but interesting & informative.
The upper example has the best cornering handling characteristics. During cornering there is weight transfer to the outside tyre which also requires the most grip. Having a dynamic roll centre because of shorter upper A-Arms causes induced negative camber upon suspension compression. This negative camber normally counteracts the bodyroll. BUT we also have to consider loads [Lateral acceleration at the intstant centre vs resistance at the tyre footprint] By raising the Lower A-Arm [figure 12] basically converts the whole suspension into a swing axle. The bottom of the tyre tries to swing outwards on compression but has an apposing force at the footprint. Figure 12 would try to "jack the car" on a corner ,with very little body-roll The set-up would is much better with the lower A-Arm pointing down at the centre [ lower roll centre] There is a Downside to this: Camber Thrust in a straight line. Camber causes the tyre to roll like a "Cone" ,so it will dart inwards on compression which makes it feel unstable in a straight line. Some designers will engineer a little bit of bump steer"toe-out" to counteract this. The middle example is almost the same as the upper example. There is less Camber gain on compression, and less camber thrust. The instant centre is only infinite at the ride height shown, but both directions of suspension travel will cause dynamic negative camber due to the shorter upper A-Arm [for cornering you want positive camber on the inside wheel and negative camber on the outside wheel] The lower example would cause positive camber on suspension compression ,and negative camber on suspension extension plus this is worsened by body roll. For cornering this would end up with negative camber on the inside wheel and positive camber on the outside wheel [ the opposite of the ideal situation] This suspension is designed for low performance drivers to cause under steer [and them to use the brakes] The average driver doesn't know how to use opposite lock, so manufacturers prefer a more stable feeling understeering car.
You may notice I've got two threads but separate questions. What's funny, every kit I've seen available (not that I've seen them all) has at least something or everything wrong. The one I'm taking off had everything wrong and the truck was almost un-drivable. But it sure fit nice.
Damn. You ask the BESTEST questions! And as a result you seem to get the bestest answers. Now if I could just understand all this stuff without making my brain hurt.
I wondered what was going on here. Thinking I might figure it out by looking at the thread in your signature line I started reading that. I'm into it 17 pages now and have shot most of the morning. Thanx a lot! It is funny how most of the kits do miss on something. But, if you have ever built an a-arm suspension from scratch, you find out that compromises are necessary. Unfortunately, many will compromise for looks or ease of installation rather than making it drive right. I built one for a IMCA modified car where the only requirements were that you had to use the stock lower control arm and steering gear. Most of what we did on that car doesn't apply here but let's just say that it didn't look good or fit easily, but he car just flew. Every piece in the front end was custom built and the LCAs were 'tweaked' a bit. (within the rules of course) Now, do you want to fill us in on the details?
Well your 1/2 way thru Elvis's build thread. Lol I'm sticking to general questions that everyone can benefit from. Not that it's a secret but details get threads deleted. But here's some details that might help. One of my strong points is getting metal from point A to point B cleanly and efficiently. Figuring out exactly where point B is, or wants to be, or needs to be, why it's there is where I spend time. In this case, when examining what was presented and looking for the "problem" it slowly became apparent that every point B was incorrect. Then the question became WHY and the quest to do it correctly. It's becoming crystal clear as to the Why. The frame's shape, width and elevation are not conducive to ideal geometry let alone fudged geometry. Two solutions present. 1 move the frame around, 2 use 2" taller spindles and 1/2" longer LCA with 1/2" shorter UCA and it all seems to fall into place. We did a custom front end on a 59 190sl that works flawlessly. Had to move everything to make room for a big ass V8.
The Jalopy Journal
