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Author Topic:   Pullbar length ?
Bigdog
Member
posted March 18, 2001 10:42 PM
To maintain a constant amount of anti-squat the pull bar should be inversely proportional to the length of the trailing arms and the heights above the ground. By changing just the length and not the angle you are basically only effecting the amount that the anti-squat will vary as the suspension moves. If all the links were level you would actually have pro-squat where the rear would actually drop under acceleration, much like a drag race car.
Here is the problem with this rule, if your trailing arms are 14" long and 10" off the ground and the torque link is 20" off the ground it would need to be 7" long to maintain a consistant angle through suspension movement which is virtually impossible with a spring type torque link.
So what does all this mean, basically on most 3 link cars the torqu-link should be as short as possible. The angle of the torque link and the trailing arms is what determines anti-squat, and the length of the torque link controls the change in angle. Personally i like to run 3-links with the trailing arms level or just slightly up hill to minimize roll steer and the pull bar angled down at between 15-20 degrees dependant upon track conditions.


jammin
Administrator
posted March 21, 2001 06:31 PM
The shorter the bar, the more rear the car will pick up(anti squat..lol) anyway... the longer the bar, the more weight that your trying to pull back from the front.... It is basically preference in how your wanting your car to transfer weight. The longer bar will actually act more as a traction buffer because the decrease in angle that goes along with the longer bar. So, what this all means is that if you want more dynamic movement in the suspension, then go with the shorter bar, and if you want more of a static setup, then go with the longer bar.

hope this helps some.

jammin


dirttrackracer
Member
posted March 22, 2001 09:17 PM
How short or long are you talking Jammin?


jammin
Administrator
posted March 23, 2001 08:42 AM
This is what I go by....I would never go shorter than my trailing arms...it will cause some problems in the geometry in that it will be very erratic and will behave abnormally..but as far as length on the other end...I have seen some that reach up to the side of the tranny. It really doesnt do as much when your length is that long...because your losing your angle....just becomes a traction device only then.

jammin

[This message has been edited by jammin (edited March 23, 2001).]

wfoondirt
Member
posted March 23, 2001 11:48 AM
The length of the bar doesnt determine the amount of anti- squat that a car has. The angle of the bar determines that. The only effect that the length has is how the angle changes. The reason that the assumption is made about the long bars not creating as much anti-squat is because normially you are not able to acheive as steep of an angle because of packaging restraints.
Basically with a bar that is too long the instant center location will change more dramtically fore/aft with ride variations. This is applicable to most rear suspensions execpt a properly designed watts linkage where the svsa is of infinate length and level with the rearend.
Ok enough with the bs technically theory , in reallity the torque link length doesn't make a huge difference. Its one of the trick of the week deals. Most drivers that i have worked with cannot tell a noticable difference with all other things remaining the same. One observation that i have made is that a shorter bar tends to make the car more consistant unless its at an extremely steep angle.
I aggree with jammin to a point, a longer bar can make the car lift less under accel but its normially from the reduction in angle not the increase in length.
by the way, cgh is the biggest factor in for/aft weight transfer thus the reasoning behind the use of anti-squat in dirt cars. I'm curious jammin to your comment about pulling more weight with the longer bar, can you explain your theory a little further?


jammin
Administrator
posted March 23, 2001 10:44 PM
Well....if your pulling more weight from the front of the car, your going to have less anit squat(who came up with this term anyway) and if you have a shorter bar(which goes along with more angle, which moves the mounting point of the pullbar to the rear) will lift from that point instead of the front. Please read the post...I said your pulling more weight from the front...which means less from the rear....which means less anti squat. The longer bar will lose angle by nature..and the shorter bar will gain angle, if they are mounted the same height on the chassis. But one thing that has been left out of this scenario....spring rate plays an important part in anti-squat also... the shorter the bar with the harder spring will pick up more than with a softer spring that will give you less anti squat, but more traction control. Generally on a tacky track, you can run more spring and produce more body lift than what you would want on a slick track. The spring rates and angles all depict what you want the car to do, it is simply preference. Static or Dynamic setup.......not really a good or bad, just preference in setup.

jammin


rjs
Member
posted March 25, 2001 10:30 PM
Jammin is right, the problem comes from mis understanding what the pull bar is doing.


wfoondirt
Member
posted March 26, 2001 10:40 AM
The torque-link doesn't pull weight from anywhere. For the purpose of this discussion the geometry of the rear suspension will determine how the chassis lifts or squats under accel/decel. The purpose for anti-squat/pro-lift (whichever u prefer to use) is to raise the center of gravity height (cgh) which aids in fore/aft weight transfer.
The determing factor in the amount of anti-squat is the imaginary intersection of the suspension links, which is called the instant center. Idealy you want the angle of the svsa(side view swing arm) to remain constant with changes in ride heights. This will maintain a constistant amount of anti-squat through changes in ride. What happens with a torque link that is too long is that the angle of the svsa changes more dynamically which changes the anti-squat effect.
If i understand what your saying is that if a torque-link is made longer but mounted at the same height relative to a shorter bar the car will lift less and that is true because with a longer bar will have less angle. But a longer bar mounted at the same angle as a shorter bar will both have the same intitial percentage of anti-squat. The difference is how the percentage of anti-aquat changes dynamically.
You're also correct that the spring rate of the torque-link plays a roll in anti-squat effect also since anti-squat is created by the suspension converting torque into a downward force on the rear-end. The spring acts as a torque absorbing device, thus the more torque absorbed by the spring the less that is applied to the creation of anti-squat.


36k
Member
posted March 26, 2001 04:51 PM
Wouldn't both short or long bars loose angle?