4-bar setup question

[69speed]

I'm now in the process of setting up the rear suspension in my 69 camaro. I have the axle welded in at the ride height I am after (about 1 inch of the rear tire covered by fender). When I position my 4-bar setup the links are sloping down 10 degrees . This is a problem because I want to keep my rear seat, so raising the front mount will be more obtrusive. If mounted this way what effect will this have on handling and ride quality?

[Matt@RFR]

I'm not going to answer your question, but I am gonna say what I think the answer is to see if I know this or not.

The entire point of a 4 link is to keep pinion angle constant throughout the suspension's travel. From that, I would say that it doesn't matter where the bars are at ride height, as long as you've left enough room for travel, and that they're parallel (side view).

[Norm Peterson]

Best answer I can give without more details is that you'll have too much rear axle roll steer (making the car understeer more) and it'll ride a little harder (because as the wheel moves up into jounce as it steps over a bump it also moves forward, sort of "into" the bumps).

To try to put some numbers on the roll steer, a 10* down angle on the bars toward the chassis ends is in the neighborhood of 17%, give or take a few % depending on exactly what elevation a PHB or other main lateral locating linkage is positioned. For comparison, 7% is still a bit high and 3% to 5% is a better target.

In normal to moderate driving you may or may not particularly notice the roll steer, but probably would during more during hard maneuvers (such as slaloms).

Reducing the angle is likely the best way of improving the ride, as it also helps the roll steer. The simplest alternative is to use relatively soft bushings and give away some accuracy of axle location and pinion angle control.

There may be other solutions.

Norm

[MoeBawlz]

wait are you saying that the bars slope down 10* towards the rear end or do the bars slope down towards the chassis???

from how i read it the bars are sloping down towards the axle which under a bump wouldnt that push the axle away???

in either case... if the axle was pushing away how would that effect roll steer as opposed to it pulling towards the center of the car?

[Norm Peterson]

I read it as pointing down toward the chassis since he does not want to raise the front mounts up into the rear seat to fix it.

You do have the bump response to bars sloping down toward the axle correct. But that gives vehicle roll oversteer, which makes for a more demanding car to drive than one with vehicle roll understeer. While roll O/S may well be quicker around an 2nd gear auto-x course in some cars, it's not as stable and can be anywhere from spooky to downright evil at higher speeds. Better to have minimal vehicle U/S and have the initial bump response nearly vertical.

Norm

[MoeBawlz]

ok i just cant read... i see it now...

[Tony@AirRideTech]

Another thing you want to keep an eye on is your axle location in the fender well. If your bars are at a drastic angle, as the axle travels up it will continue to push the rear axle backwards until the four bars become parallel with the chassis.......

[Salt Racer]

I agree with Norm that roll understeer is better than roll oversteer.

One thing we found out, though, was that excessive roll under setup (ie, bars sloping down quite a bit towards chassis) can become quite handful under certain situations, like when you lift the throttle while cornering hard.

This is something you aren't supposed to do, but it happens sometimes especially on street - like a deer comes out of nowhere, or you see a cop coming while you're going around a blind corner fast. Even if you knew you shouldn't lift in your head, chances are your reflex would make you do so.

What happens is sudden load transfer takes place when you lift and rear end gets light (this is why you don't want to do this). When there's excessive roll understeer, this rear tire unloading unwind the rear axle and straightens it from understeer direction in sudden manner. This further increases the tendency of spin out.

When our car was tested on slalom course, it had roughly 10% roll understeer. The test driver was really pushing the limit, and he had to back off the gas on a couple of runs and span the vehicle. He's actually a pretty good driver (all of Primedia tests were done by this guy), and according to him, the car was really handful once it broke loose at the limit. My boss did the same thing on freeway ramp once, but he was able to recover since he wasn't going as fast. We revised the rear LCA angle, and the car is much more manageable now.

I had an opportunity to talk to one of BFG tire engineers a couple of month ago. He suggested that increase in rear shock rebound dampening also reduces the tendency of trailing throttle oversteer, which makes sense since it will reduce the sudden rear end unloading. Weight still transfers but increased shock dampening slows down the motion of suspension.

I'd keep the roll steer below 5% if you plan to do any kind of spirited driving. If you're using large diameter rims, more than likely you'll be able to drop down the link pivots on the axle side to decrease the link angle. Most 4-bar kit seems to be designed for 15" rims in mind. With 17s or 18s, you should be able to drop down the pivots on axle side by 1.0~1.5" without going below the scrub line.

[69speed]

Thanks everyone, you rock! that was just the info I was looking for.
What is the formula for determing the % slope when I have a degree measurment?

[73-TYPE-LT-LS1]

Thanks everyone, you rock! that was just the info I was looking for.
What is the formula for determing the % slope when I have a degree measurment?

Anyone have any input on the equation to figure out axle roll steer?

[Salt Racer]

Good job using the search function. I nominate you for the newbie of the month!

Equation is Tangent of roll axis angle.

Example: Your roll axis slopes down towards front by 10.0*
Roll steer % = tan (-10.0*) x 100 = -17.6 = 17.6% roll understeer