Topic: 4-bar questions
posted October 02, 2000 08:00 PM
This is exactly what I did to my car last winter. It took me close to the whole season to figure it out, but it was pretty quick by mid-August.
I got most of my fabricating info off the Afco website. I downloaded a 7 page article on the 4-bar.. you could check, it may still be there. It was quite helpful.
I started out with 14" lowers and 16" uppers with the lowers angled down at 5 degrees, and the uppers at 15 degrees upward (to the front). The right side ended up that way working best there at the end of the year, while the left side had the upper at 15 and the lower angled UP at 5 degrees. The biggest thing you need to consider is the relationship between the 4-bar angles and the effective lift bar length. My 5th coil worked best at about 30" ahead of axle center, moving it to 32" for dryslick. That relationship I mentioned earlier has to do with your anti-squat. If you've got too much 4-bar angle working with a short lift bar length (the distance from axle center to the 5th coil), the rear of the car will lift all at once getting off the corner, bringing your rear suspension to full extension which will cause the car to bounce down the straightaway. This is bad because the rear tires are constantly unloading, which is hard on the drivetrain and hard to drive, not to mention the risk of losing your rear springs altogether...been there.. The idea seems to be to get enough lift to get the rear tires to bite hard on initial throttle and enough dampening through the 5th coil to keep it hooked up down the straight without getting the rear suspension to full extension.
My lift bar had an afco progressive rate spring on it, which I liked, but a lot of guys run about a 350# spring there, just personal taste, really. Turn enough preload into it to get about 3.5" of travel in it, checked after your race. On my progressive spring, that took anywhere from 3/8" to 3/4" of preload, depending on how far away it was from axle center. The further it is away, the less preload you'll need to get the desired 3.5" of 5th coil travel. I also tried a few different shocks on my lift bar. I started with a 10/90 (dampened braking forces with no compression resistance), and switched to a straight 7 valve, looking for some compression damping. It made the 5th coil work a bit smoother, but it was too stiff, causing the lift bar to actually work against the shock and not the 5th coil. I then changed to a straight 4 valve shock, and that worked much better.
Your panhard bar length and angle are also important. I started out the season with a 29" bar at 2 1/2" below axle center and 3 1/2" higher at the chassis. Don't go there.. I found that the car worked it's best with the bar a lot flatter.. only about 1" higher at the chassis. It got better every time I made it flatter until that point, which is where I left it, except to change the overall hieght of the bar at both ends. Move the bar up for a tackier track, down for dryslick, but leave the angle the same. I never had the bar any higher than 1" below axle center, or any different than 29" long. There are guys running shorter and longer bars, but this seems to be about average. Quite honestly, I couldn't tell you what a longer or shorter bar would do, having never run one.
I also had the brakes floated on both sides..I put the floater bars at 15 degrees upward and never touched them all year. The car never had any corner entry problems that couldn't be fixed with brake bias, although I did run into some rear wheel hop under braking on a track that had taken rubber one night late in the season. It only happened the one time and I never got another chance to try anything with the floater bar angles to get rid of it.
I'll give my best baseline set-up:
RF spring, 900#
shock, afco 6-2
2" offset wheel
camber 3 degrees inward
caster 11 degrees back
LF spring, 800#
shock, afco 5-3
4" offset, with zero front stagger
camber 1.5 degrees outward
caster 8 degrees back
(your total caster could be lower, it's the 3 degree split between the two that's important..I just like a lot of it for more steering feel)
RR spring 225#
shock, Bilstien 5050
Upper bar at 15 degrees upward
lower bar at 5 degrees downward
brake floater at 15 degrees upward
LR spring 200#
shock, Bilstien 4040
4" offset, with 3-3.5" stagger
upper at 15 degrees upward
lower at 5 degrees upward.
brake floater at 15 degrees upward
29" panhard bar mounted 1" below axle center, 1" higher at chassis left
lift bar spring at 30" ahead of axle center, preloaded for 3.5" travel, with afco 74 shock at 34".
Rear springs mounted under jackbolts, centered on the axle tube on rocking buckets that allowed the rear end to rotate about 10 degrees back without binding against the springs.
The RR shock was mounted on a clamp ahead of the axle, the LR clamped behind the axle... maybe could have been different... minor design flaw made this shock setup the only thing I could run.
It scaled with 50% cross, 56.5% left, 58.5% rear, and about 70# LR bite, full of fuel, with driver
This setup won some heat races against some fast cars. My feature setup would see the LR getting a 2" offset, bringing it to about 50# LR bite, keeping the same stagger; my 5th coil moving forward to 32"(with about 1/8" less preload) and the shock moved back to 28", and my panhard bar lowered to 2.5" below axle center without changing it's angle. It worked well, but I was never quite as fast on the dryslick with this car as I was with the old 3 link, set up to roll hard onto the RR. I always meant to try a setup that would load the RR hard with this car, but I never got done messing with so-called "conventional" 4-bar dryslick setups. I wish I'd had more laps on it.
Well, that's about all I know on the 4-bar...bought a Specialty Fabricating car (swingarm z-link, see my earlier post) 2 weeks ago..We all got beat by a lot of these cars all year, but couldn't let a season's worth of 4-bar research go to waste so good luck!
E-mail me if you have any more questions...it may take a few days, but I will write back...always happy to help.
posted October 13, 2000 10:28 AM
Thanks for all of your info Gnarly.
posted October 14, 2000 01:14 PM
Bought Dave Cain's car..Ran third in points right behind him at my Friday track (Princeton, MN) last year, and ran second to him in 2 features and 3 heat races last year. I guess if you can't beat him, the next best thing is to buy his car, eh? Shoe's gonna be on the other foot next spring...(Heh Heh)
Ran the car at this years last two shows.. had some bugs to work out at Ashland, broke in the heat and the B-main, but went to Fountain City with it the next weekend and showed up 10 minutes before the races started...working bugs out til the last minute. Started 9th in a 9 car heat and ran an honest 4th (no yellows)... This car does what it's told. Coulda run at least 3rd, but I was in the redraw so I didn't push things. Started 11th in the main, but could only manage a 9th place finish after the car developed a late race push. Called Jay McDonald the following Monday and found out that the front shocks Dave ran were not even close to his recommended dry-slick set-up, so I think we can get a lot faster next year.
Another thing to think about regarding your 4-bar... I always wondered what could have been different with the ability to move the lift bar side to side across the rear end. I'm sure there's a lot to be found in there, but I never got around to building a mount with those adjustments.