MAX LINKEAGE

Dis­pelling 4-Link Myths

Street Trucks - - MAX LINKAGE - IN­TER­VIEW BY MIKE SELF IMAGES COUR­TESY OF PEDANTIC PUB­LISH­ING

IN THIS WORLD OF BE­ING ABLE TO BUY A KIT FOR PRAC­TI­CALLY EV­ERY AS­PECT OF A TRUCK BUILD, YOU’D THINK A TRUCK COULD AL­MOST BUILD IT­SELF. Kits def­i­nitely make things tons eas­ier, but there is still a ba­sic level of knowl­edge nec­es­sary to prop­erly in­stall these parts. A per­fect ex­am­ple is 4-link kits. There are many bolt-on so­lu­tions, but if you don’t know what you’re look­ing for, you won’t know how a cer­tain kit will af­fect your truck’s ride and han­dling. If it was up to Max Fish, owner of Bio Kus­tumz and au­thor of Air Sus­pen­sion De­sign, Vol­ume 1, ev­ery­one would have a one-off sus­pen­sion tai­lored to their ex­act needs, but with the re­al­iza­tion that a lot of peo­ple buy pre-made, “univer­sal” parts, Max obliged us and shared some of his vast knowl­edge to help us bet­ter un­der­stand how to prop­erly set up a 4-link. We came up with a few com­mon ques­tions, and Max came through with his thoughts on 4-link de­sign. He even hooked us up with the help­ful il­lus­tra­tions seen here, which can also be found (along with many oth­ers) in his book. We have no doubt that you’ll find this in­for­ma­tion use­ful.

We see a lot of peo­ple set up their tri­an­gu­lated links at 45-de­gree an­gles, oth­ers at 30-ish de­grees. Does it mat­ter one way or the other?

Max: The amount of tri­an­gu­la­tion nec­es­sary to limit lat­eral move­ment of the axle has a lot to do with the ve­hi­cle’s in­tended pur­pose and the com­pli­ance [the amount of flex] of the bush­ings. For ex­am­ple, a ve­hi­cle that will spend its life rail­ing cor­ners around a track will need more tri­an­gu­la­tion than a dai­ly­driven com­muter. And as far as bush­ings go, a more com­pli­ant bush­ing, like a rub­ber bush­ing, will need more tri­an­gu­la­tion. While

a less com­pli­ant joint, like a heim joint, won’t need as much tri­an­gu­la­tion. How­ever, it still isn’t pos­si­ble to ap­ply a spe­cific num­ber to the ac­cept­able an­gle for a par­tic­u­lar ve­hi­cle. Ide­ally, 90 de­grees be­tween the tri­an­gu­lated bars is con­sid­ered op­ti­mum, but that isn’t al­ways pos­si­ble to fit into the con­fines of a rear sus­pen­sion.

What about up­per bar lengths? Lately, we’ve seen you post that it might not be as cru­cial to have the up­per bars su­per long. Can you elab­o­rate, and is it dif­fer­ent for tri­an­gu­lated and par­al­lel 4-links?

I see a lot of peo­ple who like to ap­ply rules to up­per bar lengths with no real rea­son why. The only the­ory that I will give credit to is the con­cept of equal length bars for both the up­pers and low­ers. This is gen­er­ally ac­cepted as the best way to keep the pin­ion an­gle per­fect through­out travel, but again, it’s not al­ways pos­si­ble to fit that into the con­fines of your ve­hi­cle. The re­al­ity is that there is no rule for how long the up­per bars need to be in re­la­tion to the low­ers. I’ve ac­tu­ally talked to road-race guys who ran longer up­per bars to get more trac­tion dur­ing corner­ing. Per­son­ally, I like to run the up­per bars shorter than the low­ers to help with pack­ag­ing. It also makes higher tri­an­gu­la­tion an­gles a lot eas­ier to pro­duce as well.

Can you elab­o­rate on the difference you’ll see be­tween set­ting up a 4-link at ‘ride height’ ver­sus set­ting one at mid-travel?

This one is tough be­cause my half-travel the­ory only works if the sus­pen­sion is de­signed prop­erly. If your sus­pen­sion ge­om­e­try is screwed off in such a way that it is only driv­able at the de­sign height, then you have no choice but to use ride-height as the set-up height. How­ever, if your sus­pen­sion is set up prop­erly, it should be use­able at any point of travel. So with that, to min­i­mize ge­om­e­try change through­out the travel, de­sign­ing it at the half travel point is the eas­i­est way to do that. This video does a pretty de­cent job of ex­plain­ing the con­cept: http://bit.ly/2o6l­bla.

Some par­al­lel 4-links have up­per and lower bars that are com­pletely par­al­lel to each other, while oth­ers con­verge slightly on one end and may ac­tu­ally have many ad­just­ment points built in. Is there a place for both, or is all of that ad­just­ment overkill for most peo­ple?

Again, the con­cept of equal-length bars that are par­al­lel to each other is a great way to keep the pin­ion an­gle per­fect, but do you re­ally think that’s what sus­pen­sion de­sign­ers are us­ing as their pri­mary fo­cus for de­sign: to first and fore­most keep the pin­ion an­gle happy? On a driven axle, the en­ergy is trans­ferred through the axle hous­ing into the sus­pen­sion com­po­nents and then to the chas­sis. The ori­en­ta­tion of the link bars and how they are at­tached to the axle and chas­sis will de­ter­mine how that en­ergy is trans­ferred. By de­sign­ing a sus­pen­sion with mul­ti­ple points for the bars, the sus­pen­sion can be tuned to have more or less trac­tion dur­ing ac­cel­er­a­tion, or in the case of a dirt track car, dur­ing corner­ing. I have heard a few dif­fer­ent the­o­ries as to how the con­cept works and even what it is called, but it is most com­monly called in­stant cen­ter or in­stan­ta­neous cen­ter. The the­ory is outside of the scope of this ar­ti­cle, but this video may help make some sense of things: http://bit.ly/2o6l­bla.

What is the big­gest mis­con­cep­tion peo­ple seem to have when set­ting up a 4-link? On a re­lated note, what’s the most com­mon mis­take you see re­gard­ing 4-links when some­one brings their truck into Bio Kus­tumz for re­pair?

Big­gest mis­con­cep­tion? That’s a tough one … prob­a­bly that they have it fig­ured out. Most peo­ple think they know what they are do­ing be­cause they looked at a ton of photos on­line, read a few ar­ti­cles, and know how to weld, but no one pays at­ten­tion to the sub­tleties. This is where the most com­mon mis­take comes in. A large ma­jor­ity of peo­ple don’t re­al­ize that roll-steer is a thing. And more im­por­tantly, it’s a scary thing when you have too much of it. The lim­i­ta­tions of most ve­hi­cles that are try­ing to lay frame on a big wheel only com­pound this is­sue. It’s un­for­tu­nate that poor ge­om­e­try isn’t as ugly as runs in a paint job or steel belts hang­ing out of a tire, be­cause more peo­ple would prob­a­bly be chang­ing their habits re­ally quickly. The halftravel ver­sus ride-height video ad­dresses roll-steer a bit as well.

Why does ev­ery­one think link bars need to be so long?

Be­cause most peo­ple have no idea how ge­om­e­try works, so they push things to the ex­treme to ‘min­i­mize front-to-back move­ment.’ Look at a ’64 Im­pala and tell me how long those link bars are.

Why don’t peo­ple like pan-hard bars?

I blame poor de­signs be­ing ac­cepted as cor­rect sim­ply be­cause they haven’t failed yet. There’s a big difference be­tween ‘strong enough’ and ‘prop­erly de­signed.’ A prop­erly de­signed pan-hard bar will keep the side-to-side move­ment very min­i­mal and is far eas­ier to de­sign than a Watts-link.

Thanks once again to Max for his amaz­ing in­sight on prop­erly set­ting up sus­pen­sions, and be sure to check out his book, Air Sus­pen­sion De­sign, Vol­ume 1.

ABOVE. ON THE RIGHT IS A SUS­PEN­SION THAT HAS NOT BEEN SET UP AT HALFTRAVEL. YOU CAN SEE HOW MUCH FRONT-TO-REAR MOVE­MENT THERE IS COM­PARED TO THE LEFT, WHICH HAS BEEN SET UP AT HALF-TRAVEL.

RIGHT. THIS EX­AM­PLE SHOWS HOW LIT­TLE DIFFERENCE THERE CAN AC­TU­ALLY BE WITH SHORTER LINK BARS, AS­SUM­ING THE SAME RANGE OF TRAVEL. A 12-INCH DIFFERENCE YIELDS LESS THAN 3/16-INCH OF DIFFERENCE.

THIS IS AN EX­AM­PLE OF 4-LINK BARS THAT HAVE BEEN SET UP TO HAVE AN IN­STANT CEN­TER JUST BE­YOND THE FRONT BUMPER AT HALF-TRAVEL.

AS ONE SIDE OF THE SUS­PEN­SION COM­PRESSES IN A TURN, YOU CAN GET A LOT OF REAR STEER­ING IN­PUT IF YOUR 4-LINK IS NOT SET UP COR­RECTLY. THIS CAN CAUSE UN­DE­SIR­ABLE, AND POS­SI­BLY DAN­GER­OUS, EF­FECTS WHILE DRIV­ING.

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