This isn't rocket science, guys. What you want is the pinion to be parallel to the crank/trans mainshaft under load. The goal is to have the same angle on the ujoints at the front and the back of the drive shaft. This generally requires a little downward bias on the pinion so that as the rear suspension flexes the pinion becomes parallel to the mainshaft.

Pinion angle won't make you hook better or any such nonsense. It can break parts if incorrect.

Instant center has a major impact on hooking, along with the shocks and springs, and you get much closer to rocket science here. The normal F-Body setup with lower control arms and torque arm fixed at the axle and pivoted at the front is a bit strange. You will not find an example like it in the literature on rear end setups, and I haven't seen it in any suspension software. It does not act like a four link at all. It acts more like the lift bars Ford invented for the Thunderbolts, but those LCAs are a kicker. The angle of the LCA has a much greater impact on the lift/squat characteristic than the torque arm does. The torque arm provides no longitudinal moment to the chassis - only vertical. The LCAs put in a combination of horizontal and vertical that varies with the angle of the LCAs with repsect to the ground. (The pictures I've seen of Madmans TA show it to be different. It is fixed at the front end and does provide longitudinal force input to the chassis, but binds because it is fixed at the rear). I am not convinced that the instant center applies to us (or in what manner it might) since I didn't take the math far enough yet. If it does apply, then the instant center will move forwards as the car squats and rearward as the rear rises. If you try to compute an instant center on a stock F-Body, you would come up with close to infinity, or at least way out in front of the car. Some upward angle on the LCAs would bring that back closer to the car, but it would take maybe 30* upward on the LCAs to bring that "instant center" anywhere near the CG of the chassis. I think 30* would cause the rear end to hop off the ground though.

Back to your regularly scheduled programming ...

 

So basically, most torque arms lift up, while LCA's can lift forward and up. On the few stock eliminator cars I've seen, they has stock type LCA's in as much as I can visually tell in stock type locations, ie no relocation brackets, but had the 'madman' style torque arm (much lower rear mounts compared to conventional). One more thing I noticed was the rear had mini upper control arms, which tied from the top of the rear axle housing to about the middle of the LCA.

What I'm curious is knowing is if the torque arm setup can bite too hard with the LCA's relocated and what the fix would be for that? (I think I know already) I have the LCA's at the lowest point now, and the car gets wheelie happy no matter what the shocks are set to. I'd like it to push forward more and not as high.

I'm kinda lucky the car has done decent for just throwing a bunch of off-the-shelf parts at it and not knowing exactly why its working.

 

You guys are getting close keep going.

 

It seems confusing.I always that it was the v between the driveshaft and the pinion but the of the 2 links,one says rear seal to pinion and the other says the v approach with a rear trans seal/drivetrain as a bonus.I have 1* up on the trans and 1* down on the pinion,so is that -2 degrees or 0? Do I need to have 3 degrees total down on the pinion so that it would be 0 under power? That sounds right...............

 

Damnit, you guys all confused me, I can't remember my numbers or how I calculated the pinion angle anymore and the car is apart, so I can't even go look!!!! Now i'm gonna have a hard time remembering what's supposed to be done, I used the bmr directions, but now I just read them and am confused, I shouldn't have read this post, hehe!!!!

 

Exactly correct. And the ratio of up to forward depends on the angle of the LCA.
Lower or higher mount to the rear end won't make any difference.
That has the effect of changing the LCA into a very short Thunderbolt style lift bar and would need very stiff rear shocks to keep the car from hopping.
Move the rear of the LCAs up a hole or two. That should help.

The other thing to do is to drop the car. Wheelies a pretty simple physics. The center of gravity of the car is above ground level, where the tires react again the earth. Push hard enough there and you overcome the force of gravity pulling down in the CG. If the CG were at ground level (impossible, I know) it would be impossbile to make it wheelie. If the CG was over the axle, it would be impossible to move without a wheelie. The torque of the axle can also contribute, but it's contribution is much less than simple interia until you get into high power/weight ratio vehicles.
Sometimes that happens

 

There is a lot of misinformation floating around on this subject. I recently saw an article where they made the point that the mainshaft and pinion should be parallel under load and ran tests that showed no significant change in ET or short times with pinion angle change. Sorry, I can't remember where it was. Mabye someone else saw it too.
3 degrees down compared to what? The mainshaft? No, that is too much.

Here is another thing to consider. If this is a street car, changing the pinion angle down will wear out ujoints and can cause vibration at highway speeds if you change it enough. The idea with this pinion angle thing is to make the pinion parallel to the mainshaft under load for drag racing, which requires a little negative alignment under static conditions. But this applies only to drag racing, not to streeet cars, not to autoXers. You want it parallel under static conditions for those.

 

check your PMs

 

Heres one to really confuse everyone

I have a 2000 Firebird PROMOD car that was built by Jerry Bickel in the shop right now. The pinion angle is set at 2 degrees down. With the car at ride height and thru out the arc of the suspension travel the pinion stays at 2 degrees down. This car has a 4 link and is a full tube chassis so I dont see much flex. Now this being the case and more pinion doesnt help and less takes 60ft away AND the driveshaft never comes in a straight line or 0 degrees as everyone thinks why would you measure the driveshaft or tailshaft in reference to the pinion???????

 

I would suggest that whatever you change that affects the short time is not pinion angle per se, but that pinion angle is a side effect.
I doubt that much 4 link wisdom transfers to stockish F-Bodys, but the driveshaft doesn't matter. All you want is the pinion to be more or less parallel to the crank/trans mainshaft. The job of the universal joints in the drive shaft is to make up for the fact that those two centerlines are not one (and obviously can't be). When a ujoint runs at an angle, the angular velocity of one side or the other changes. If both joints run at the same angle, the driveshaft absorbs this without much complaint. If they don't, some of that angular velocity difference is passed forward and backward into the trans/engine and rear end via the ujoints, which is why missalignment shortens their life. The trans/engine and rear end are not particularly happy about it either.

 

You are right 4link tuning versus ladder bar versus t/a suspensions all tune differently. The point is that pinion angle is exactly what it says "PINION ANGLE". It has no bearing on the crackshaft centerline or the tailshaft or the driveshaft.

On this same car the crank shaft is lower than the pinion in the car. The driveshaft runs uphill to the pinion. If we put 2 degrees in with your method or the driveshaft method the pinion would be way down.

 

The crankshaft being lower has no meaning in and of itself. Only the angle of the crankshaft does. Since the goal is not to point the pinion at the crankshaft, only to put the centerlines parallel.

Is the crankshaft level? Or is the front or the rear lower in this case? By about how much?

 

The motor is level in the car. The crank is 10" from the ground. The pinion is 15" from the ground.

 

So the lower the LCA's are the more the car would want to wheelie?


I need some relocation brkts (tryin to rip the bumper off)

 

If the crankshaft points up at 5 degrees towards the pinion then the pinion must point down 5 degrees to keep the driveline in line. If you wanted to account for any upward pinion movement under load (say 2 degrees) then the pinion would have to point down 7 degrees in relationship to the crankshaft.
And yes in this case the pinion would be pointing down in relation to the level ground.

 

The crank is at "0" degrees at ride height in relation to the ground.

 

The driveshaft must be pointing up at some degree (say +5 degrees) to meet the pinion. Whatever that angle is the pinion must be pointing down at that same angle (-5 degrees) to meet up with it if you want the driveline to be in line.
You do not have to run it that way but if you want it in line you do.

 

Excatly JNorris. You have proved my point. You are trying to set "driveline angle" not "pinion angle.

 

Carl, thats what I had planned next time I get to a decent track, racing at 5000DA on our local tracks suck. Its not a big issue currently being cam-only, but when I put the S/R engine back in is when I need it to work its best and/or if we put the bottle on it someday. Like I said, havn't had much time to make adjustments other than QA1 settings.

What about the location of the front torque arm mount? Does torque arm length come into play as much as we would think it does? Lifting in the stock type location -vs- lifting more towards the rear such as the BMR/Spohn designs.

 

Height plays a big part of the chassis setup. Height is what changes instant center. The length is not as important if you can move the height of the front mount.