Pullbar help

[Wheelman33]

Is there anywhere I can read about pullbars? Angle, height, lengths, rearend placement(front to back), etc.... Bought this car from a guy that won a lot of races with the pullbar shorter, farther forward on rearend, and more angle than setup calls for. It just seems like it has a little too much instant traction. Don't want to get crazy with it cuz we are pretty good.

Any opinions?

[Duckhnter83]

If you lower the pull bar on the frame what affect does it have on entry is it a small or large affect and does it affect entry on throttle or off throttle?

[Lizardracing]

The basics...
More angle=More instant traction just as you pick up the throttle, can possibly cause some wandering on straights, loose entry. Too much and the rear tires wheel hop on entry
Less angle=Less instant traction when you pick up the throttle, more stable on the straights, tighter entry.
Frame holes are small adjustments.
Rear axle adjustments are big adjustments that can have several other effects depending on location of mount in relation to the axle center line. With the mount forward of the axle vertical center line the bar will gain angle as the rear axle wraps up. If mounted behind it looses angle. The higher the axle mount is the harder it is for the axle to wrap up(assumes same spring or bushing) and also the more dramatic the angle gain/loss. The lower the mount the easier for the axle to wrap up and less dramatic the angle change.
What's right is entirely dependent on driver preference (tighter feeling, looser feeling) and over all car set and design with importance put on balancing the front end with the rear end.
That pretty simplified but I hope that answers the questions you have.

[Duckhnter83]

Just what I was looking for thanks lizard

[Dirtrunner35]

The pull bar, is nothing more than a mechanical traction control device that utilizes the motion (wrap) of the rear end on acceleration to enhance the traction of the rear tires. As the pinion tries to crawl up the ring gear during acceleration the axle rotates/wraps. This motion is transmitted thru the pull bar to the chassis raising the chassis. A quick assumption would be that if the pull bar is lifting the chassis that it is taking weight off of the rear springs. That is true to a certain extent but you also have to go back to the rule of "For every action there is an equal and opposite reaction." This means that the weight that is taken off of the rear springs by the pull bar lifting the chassis is actually reapplied to the rear tires thru the pull bar into rear end housing into the wheels down thru the tires. So in actuality you don't directly gain or lose any weight on the rear tires by the lifting action of the pull bar acting on the chassis.

So at this point you're either thinking I'm full of it or you're asking yourself then why do we use pull bars at all. The answer to this question is that the pull bar keeps the center of gravity of the car higher during acceleration. If we did not use a pull bar the rear of the car would squat during acceleration effectively lowering the center of gravity. The center of gravity is important because the higher the center of gravity, the more weight will transfer from front to rear (I'm sure there is a limit to this.) So the purpose of the pull bar is not to directly add weight to rear tires during acceleration, but it is to indirectly allow more weight transfer from the front to the rear due to the higher center of gravity during acceleration.

A second function of the pull bar is to soften the application of torque to the rear tires when you jump on the throttle. This is why the pull bar is equipped with rubber biscuits, a coil spring, or both. These devices absorb some of the torque that is applied to the rear end by the engine to give the tires a chance to take a set into the track before they see full torque applied. If we didn't need to worry about softening the torque application to the tires the pull bar could just be a solid length of tubing connecting the rear end to the chassis. But since this would somewhat prevent the axle wrap motion, that means that 100% of the torque that is being produced by the engine would be passed onto the tires and give them one heck of a jolt on throttle application. That's okay if the track is tacky enough and the tires have already taken a set into the track, but if not and you're just getting back into the gas off of the corner you're just going to break the tires loose and spin down the stretch. Changing preload or spring rate on these torque absorbing devices is one way to control traction on different track surfaces. The preload determines how much torque will be delivered to the tires before the torque absorbing device begins to absorb some of the torque [i.e. if you have 250 lbs. of preload on the pull bar spring (which is about 1/4" of preload on a 1000 lb. spring), the tires will take the initial 250 pound hit of torque before the pull bar spring starts to compress and absorb some of the torque.]

Now on to the angle and height of the pull bar. The angle of the bar does have an affect on forward bite, but like Gene stated it's only up to a ceratin point. The more agressive you get with angle it's like asking the pull bar to lift the chassis higher - faster. This is okay if the tires have the traction to do this but if you're on a dry slick track the tires are just going to break loose and you wont get any lift out of the pull bar. The lift from the pull bar is a result of the tires staying adhered/stuck to the track and the pinion trying to climb the ring gear because of the tires traction. The net result is axle wrap. If you ask to pull bar to try to lift too much weight too fast it will actually direct too much torque to the tires too soon (before they take a set) and cause them to break loose. In other words and extreme pull bar angle on a dry slick track would not be a good idea. So angle becomes another tuning factor just like preload on the torque absorbing device.

Finally, the height of the rear pull bar attachment point from the centerline of the rear axle determines the amount of leverage the rear end has on the pull bar. The higher the pull bar attachment point from the center line of the axle, the less leverage the rear end has on the pull bar. The opposite of this also holds true. There are usually several holes to mount the rear of the pull bar in and again this becomes somewhat of a tuning tool. Another way to look at this would be to imagine yourself holding a two foot length of metal rod straight out from your body and hanging a five pound weight from the opposite end of the rod. You then try to lift the rod up and see how much resistance you feel. Do the same thing now with a four foot piece of metal rod with the same five pound weight hanging from the end. It will be harder for you to lift the weight with the four foot rod than it will with the two foot rod. This is the same affect that moving the rear attaching point of the pull bar (either closer or further away from the axle centerline) has on the leverage the pull bar feels. You might think that more leverage is always better, but again more leverage means more pull/lift which means the tires better have the traction to pull it off (no pun intended.)

[Wheelman33]

Thanks guys! This is exactly what I was looking for.

Any opinions on moving the rearend side mount back a hole? This would obviously make the bar longer, but does it change anything else?