I have been searching through the forum, and read a couple good posts. This is a really good one with great info:
Cycled my 4 link today.. Have a few questions
Most of my questions were answered in that one, but I do have a few more.
Using the Excel calculator, that is all over the internets,
I think, is for rear axle applications. Am I looking for the exact same properties when using the numbers on a front axle?
How much difference in length between the uppers and lowers is too little/too much?
Any other sound advice, tricks, secrets, etc. you have for a 3 link suspension?
TIA
Search for my other posts about link suspension.
Generally want less AS, when referring to the front suspension even though it becomes anti dive.
Anti-dive is a torque reaction force applied to the suspension under braking. The anti force with 0-100% anti dive, is the suspension resisting compression during braking. 100% AD would mean the front suspension doesn’t change position at all during braking.
0% AD would mean the suspension compresses unrestricted during the forward weight transfer due to braking and negative acceleration.
The big issue is when the torque direction reverses during 4wd use and the front axle is pulling the vehicle forward.
The same anti dive force that resists suspension compression during braking, will aid in suspension compression during throttle.
Too much AD under throttle, with decent traction will cause front wheel hop as the torque reaction force is trying to compress the front springs, and effectively reduce traction and the Tire contact patch. Once traction is lost, the suspension settles, traction is regained and the torque reaction force starts over, “trying to lift that tire”.
This process happening over and over rapidly equals wheel hop in the front during throttle.
Understand that unless using the newest calculator that has user defined torque and traction split %, the as/ad numbers in the calc represent all the torque and traction going to the one axle/suspension. This means when viewing the older calcs, the % numbers represent 2wd rear operation with 100% rear traction.
If 4wd, with 4 wheel traction and braking, the actual percentage is divided by 2.
The only time there is any torque reaction force is when there is torque applied with traction. If zero traction, then there is zero anti-anything.
As far as link lengths, try to get the pinion angle change to work with what style driveline and u joints you will use.
If double cardan, you want to try to keep the pinion pointed at the double cardan itself.
If single u joint, you want the pinion to not change angle thru travel, keeping the pinion parallel to the tcase output.
I️ try to keep total pinion angle change under 8 degrees total change. Obviously for high speed or endurance/ultra 4 racing, less is better for u joint survival. For crawling, you can probably get away with it as the rocks usually destroy the drivelines first, and joints are easy to change.
I️ don’t like zero degrees of pinion angle change with double cardan as it doesn’t keep all the needles moving in the caps to keep grease moving. The wear and shock load is then on the same needles in the same places all the time.
Next would be to try to keep the Anti forces from increasing greatly as the suspension travels.
This will keep behavior more consistent thru suspension travel.
This matters as the vehicle angle changes(pitch) such as climbing a waterfall,
The suspension will move towards droop, as more weight is on the links and less on the shocks/springs. This is because gravity doesn’t change, but the pitch does.
If the AS increases too much in the rear, in droop, then you can have the same wheel hop in the rear while climbing more vertical obstacles with decent traction.
All of this is why I️ say to shoot for 60-80% rear AS and no more than 10% increase in 7” of droop.
As for the front, opinion varies greatly, but I️ like to see AD under 40% and no more that 15% change as it travels 14”.
