3 link explanation/question

**Dirtrunner35** · 08-23-2011, 09:24 PM

Great reading

The pull bar, or the third link as you called it, 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.]

**wrengoodman114** · 08-24-2011, 04:35 PM

Great answer bud, I was just thinking if third links were angled downhill and pull bar was arranged to pull down it might put more weight on rear springs to force into dirt!

I just noticed that a car I just got had the left side link angled up hill around 17 degrees at ride height and the left side would only lift very little as if body roll was limited so is the bar angle good just get softer taller spring to enhance body roll and left rear traction!

I've always went up on spring to gain weight during weight transfer to left rear on non birdcage cars!

Wren

3 link explanation/question

3 link explanation/question

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