I have one of each as shown in my signature. I plan on upgrading the small block to a Blueprint 396 sb later.

 

I'm not seeing a signature at all. Hence the question.

 

Back in the late 70's and early 80's I had a c3 bb car that all I did was tracked. I spent a ton of money with Dick Gulstrand and had Bilstein shocks, steel a frame bushings, solid mounted engine, trans, and rear end. I spent a lot of time on the phone with Gulstrand and he advised me to take the rear bar off. I didn't want to listen. He finally told me one day to just disconnect a link on the rear bar, run the course, and then compare my times. I did that. The car felt really loose and I was sure he was wrong. I was 2 seconds faster that run. I put the link back on. Next run I slowed back down. Took it off again and was 2 seconds faster. When I got home I took the whole bar off and never looked back.

 

 

I've run a wide range of front and rear bars on my 67 327 ragtop for autocross purposes. Eventually I went back to the OEM front bar and no rear bar. Whatever you're running now, adding a rear bar will increase oversteer.

 

The set up for a big track is VERY DIFFERENT than for autocross. If you compute the roll stiffness distribution for base and F40/41 you realize that F40/41 has more understeer bias. That's because it was designed for road racing on big tracks.

In autocross you need to get the car to very quickly develop a BIG yaw rate, so big negative front camber and toe-out gets the car to turn in fast and develop a high yaw rate then you steer with the throttle.

Take a car with a good autocross setup to Willow Springs and try to power through Turn 8 at triple digit speeds with the big boys and you'll be in the weeds every lap.

The first thing in setting up a car is replacing the normal trailer/van tires that most guys run with decent performance tires. The Avon CR6ZZ is the pick of the litter, but there are a handful of others that have decent grip and high speed ratings, but harder compounds for longer tread life.

The other thing to start with along with the tires are rebound adjustable shocks like Spax or QA-1.

C2 small blocks have a tendency to snap into oversteer at the limit, especially with low grip tires. This is due to compression of the front anti-roll bar link bushings, which reduces front roll stiffness that effectively transfers roll stiffness to the rear and too much rear roll stiffness can get you into oversteer. This can be at least mitigated if not eliminated by replacing the front bar rubber link bushings with hard urethane. The OE type rubber pillow block bushings are okay.

The next thing is to set alignment at the "sport" settings which is negative one degree camber and 1/32" toe-in at all four corners and as much positive caster, both sides equal, as you can get with the negative one degree camber.

Then go out and do some testing. The car should have mild understeer on turn-in and be neutral as you add power and approach the limit. Transient response is very important and must be evaluated. If you lift off in a constant radius corner the front should tuck in slightly. If the rear wants to come around, that's a problem that must be corrected. Same with a decreasing radius turn. If you go in too hot and lift and the rear comes around you may be in the weeds or over a cliff. If you're not an experienced test driver work up to these limits gradually.

You should also evaluate braking. Once the tires are heated up and at maximum grip can you do a maximum performance deceleration without the rear locking (The fronts should always lock first.), and can you trail brake into a corner without the rear getting loose?

Once you've done all the above then you can think about different anti-roll bars. Not only do larger bars reduce roll, but they are an easy way to tune front-rear roll stiffness distribution to get the understeer/oversteer balance you want, and since their contribution to roll stiffness increases with the FOURTH power of diameter a 1/32" diameter change will make a noticeable difference in both roll and handling.

Also understand that roll is a feedback mechanism, among others, that tells us how close we are to the limit. For a high performance road car that you drive in a sporting manner and maybe do do some track time events, a good objective is to allow about 3 degrees roll per g of cornering force, and with the OE springs that won't make it ride like a buckboard on public roads that are in reasonable condition.

Duke

 

IMHO 3 degrees is a lot of roll. It makes for very slow transitions. It takes a while to get settled. That is pretty much what a SB C2/3 would do with stock springs and bars. It will use up over 1.7 inches of suspension travel in a turn and as much as 2.7" accelerating out of a corner, or trail-braking. You will be on the bump stops already. And if you hit a bump it will shoot into oversteer, and that might not be a great surprise.

For optimum street handling, and certainly for autocross or track use, I would cut the roll angle back by some. Maybe by 25% - 33% or so. Cutting it to 50%, well that is like an autocross setup. It would also handle/feel more like a newer Vette, and not spend so much time rolling around.

Feel free to check out the excel spreadsheet I wrote just for our C2 / C3s. Dynamic handling gets incredibly complex. This should help most with the math. I did spend 27 years doing Pro-Solo, tuning suspension, and road racing..... so....Enjoy!

(If someone thinks of something you want me to tweak or add let me know)

 

So, as the OP none of this answers the original question. I have both a SB 1965 and a BB 1966 with stock springs in both. My question is what size bars to use in both cars to improve the handling for spirited street driving with Avon tires and QA1 adjustable shocks?

 

On a BB car it should not hurt a thing to step up to a slightly bigger front bar and use poly sway bar components. Leave the rear bar alone. I would stick with a GM bar as then you have a pretty good reference between it and your stock bar. If you go to aftermarket you really can’t be sure of what it represents compared to your stock bar.

 

Keep in mind a couple of things:

1. The nose heavy BB cars want to understeer at the limit. The rear bar was included to mitigate this tendency, making the handling at the limit more nearly neutral. Increase the size of the front bar and you bring back the tendency to understeer.

2. Even slight changes to bar size have a huge impact on the bar torsional stiffness because torsional stiffness is proportional the 4th power of bar diameter. In English, suppose the present bar is 15/16" and you replace it with a 1" bar, a 6.7% increase in diameter. That 6.7% increase in diameter will result in a 29% increase in torsional stiffness. That's huge.

If I had a BB car, I believe I'd leave the front and rear bars as they were engineered by GM.

 

Nobody knows, so you'll have to do the research and find out the answer. It took me about 2 years doing 4 events per year, 2 at Riverside, and 2 at Willow Springs, to get my Cosworth Vega totally dialed in back in the eigthtes. I started with tires, shocks, and alignment, and once I got that dialed in I worked on bars until I found the best setup.

You've received plenty of suggestions on tires, shocks, and alignment. So get to it, then start test driving. My bet is that you won't find any real need for different bars, but I suspect you just won't be able resist tinkering and will proceed to screw up the handling with aftermarket anti-roll bars.

Duke

 

I know Jim - my thinking is that on the street a little additional roll stiffness up front gives the perceived feel of better responsiveness in transition and a bit flatter cornering at low/medium speeds - which seems to me to be what he’s after. I didn’t hear that he plans to compete the car in any way. Not many of us - at least at my age - explore the limits of understeer/ oversteer of our car on normal street cruising. And in the event we misjudge a street corner or curve a little, we are better off with some understeer. Yes, on the track it may well not be as balanced or desired.

I probably should have put a 15/16 bar on my car as with aluminum heads/intake on the BB and no power or other weight accessories I’m probably not that far off of a SB car with a couple power accessories in weight. Either that or remove the rear bar. But I’ve made due with poly bushing on the front BB car. It’s close enough on the street.

 

I had a c3 bb car that I autocrossed back in the late 70's and early to mid 80's. I ran it at least 4 times a month and sometimes more. I changed almost everything on the suspension at one point or another. We found that with my car it was faster with the rear bar removed. It didn't feel as fast but it was. We'd disconnect the bar and run then reconnect and run. It was always faster with the rear bar disconnected. I can't remember which front bar we used and since we didn't have email back then but it was either the stock bar or one step up. I also had daytona springs and Bilstein shocks, solid a-frame bushings, and solid mounts for motor, transmission, and rear end.

 

So with a BB aluminum headed coupe in race trim what size bar or bars should someone start with?

 

Good info and great spreadsheet you've shared, thanks! A lot of work when in to that I'm sure.

Patrick

 

Good question, weight considerations…. In race trim? Something to keep in mind:
Weight, and weight distribution, is what should we consider in designing/selecting suspension components, not “BB“ per se…. I believe a big block coupe with aluminum heads and headers, and no power driver assist devices, weights virtually the same as a stock small block convert with hardtop and those devices (I’d have to check my data files to get the exact weight differences to validate ).

 

On my car '65 327 4spd. street car.
With stock springs, good quality replacement shocks and radial tires I found that replacing the stock front sway bar with a 1 1/8" bar made a good improvement in handling.
I didn't use the rear bar that came in the kit.

 

Wow! What kind of “improvement “ were you looking for? It seems to me that your change would have resulted in a significant increase in understeer.

 

I agree!!! See my arithmetic in posting #30.

 

I'm not sure what the difference is between understeer / oversteer. But I don't get so much body roll, much firmer ride, car likes to accelerate in the curves rather then roll over into the ditch. An improvement over what I had for sure.