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Multi-link rear suspension - effect of moving axle bridge forward.

BadDog

BadDog

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One of the things that has kept me from building a 4 (or simply multi) link for my truggy is the gas tank clearance. In my truggy, the front of the tank is actually over the axle and bump stopped to keep the two apart. There is simply no room for a traditional upper link bridge, and I would like to do a traditional triangulated upper or maybe double triangulated 4 link. I’ve put this off again and again due to the need for a custom tank, along with some other frame related issues.

So, now I’m wondering, why couldn’t I kick the bridge forward so the links mount about 4-5” in front of the axle center line. The keys for figuring the significant forces are generally intersection points and such that are not necessarily effect by the change. And axle control separation should also be sufficient as well, especially since I can foresee tying it to the pinion retainer as well.

Just something I was mulling over tonight, anyone see a major “forget it” issue?
 
I don't know much about link suspensions but is there room run the two upper links into the one link or two links to one brakcet and use the bolt on pinion snout of the 14bff instead of going to the axle itself? Just a thought, don't know if it will help.

Harley
 
Could you run a low bridge over the pumpkin and sacrifice some up travel for droop? That seems like a better option to me because you would be able to get away with running the same tank and still have a fairly stable truggy with tons of droop, and you will be able to keep the geometry better with the bridge going over center of the axle.

or another possibility would be to run a 3 link with 2 upper links mounted outboard with a single lower link mounted inboard close to the pumpkin but low. This way, if designed right can give you driveshaft protection with no loss in ground clearance or suspension travel. If you have seen the team purple buggy on PBB they have the suspension setup similar to that, but the only difference is that they run volvo portals, not a 14BFF in the rear.
 
Harley:
That's pretty much what I was thinking. Have the "bridge" come to the center from the axle tubes just like most of them do, but meeting up over the pinion retainer with a bracket that bolts onto the snout. This would put the link mounts about 4-6+ inches in front of the axle center line, rather than the more typical location directly above the center line.

Jason:
That's pretty much what I've already done with the leafs. I only have about 4-5" of up travel at the center with a bump stop to keep the axle out of the tank. It's worked well for the last few years. But with any sort of bridge up there, I'll have almost no up travel at all. That's why I want to go forward with the links.

I've also considered other link designs like you just mentioned. But the 3 link has some less that ideal characteristics, plus the track bar defines the roll axis and I can't mount that very high, again, due to the tank. The tank could go up, but then it raises all that weight, and interferes with what little storage area I have now. Still, these are viable options, I'm just trying to find the best compromise with the least negative effects. I'm also leaning a fairly strong toward the full buggy right now, but I want to look at all the options before giving up 6 months and $$$$ to a buggy build...

The only thing certain right now is that I’m NOT going out again on rear leafs. Killing a set of springs in under 4 hours has got to be a new record…
 
Are you killing the leaves from axle wrap? Couldn't you just use a antiwrap bar to stop the wrap?

Harley
 
I already have an anti-wrap bar that works fine, no wrap issues at all. My problems come in when climbing water falls and needing some "get on it!" When the rig is near vertical, the rear axle is basically supporting most of the weight of the vehicle on the front ends of the leafs. Putting the hammer down and bouncing a bit is more than these springs can support without bending.
 
I know you are not really wanting to run leaves but could a custom set of leaf springs help cure the problem? Something with a couble military wrap eye or something? I kinda want to know for my own info cause I am going to be going the route you took sometime with that Ford F250 I told you about. Thanks

Harley
 
Maybe, but there are other issues and compromises. For one, I don't want to give up my departure angle, so the back side of the spring still needs to be fairly short. For another, I don't want to go with custom leafs due to the cost and not *knowing* that they would hold in the abuse that I've been showing them. Links can handle what I want to do much better than leafs, and I can tune them which gives me much better flexibility and more capability (assuming I don’t screw it up). It's just a design limitation of leafs. Leafs work fine until you start wanting to climb the really gnarly waterfalls and such, then they just can’t do their job of locating the axle without becoming too stiff or otherwise limiting in other scenarios. At least that is my current opinion.

Edit: BTW, as long as you don't want to do full throttle assaults on vertical walls, and you keep the spring turned the right way (loosing departure angle) then they should work fine. Look at Matt’s orange Ford. He has a terrible departure angle running 63" GM springs and does quite well. Just depends on the goals and compromises you can live with... My current problem is a result of the compromises I chose to make, and I can no longer live with those choices, hence the pending change…
 
Four link the rear.
 
I think moving the upper links on the axle side forward would cause the pinion to drop down when the suspension drooped. Maybe you could compensate by making the upper links longer but that might run into mounting issues, maybe not.

Basically the concern would be the pivit point of the axle.
 
That's a good point, but I'm not really interested in tons of droop in the center. For the most part, articulation is what we need. I'll probably be strapping the center anyway to allow no more than about 4" of down travel. Just what ever it takes to handle the asymmetrical articulation (i.e. center needs to go down some due to limited up travel on the stuffed side)...
 
Well then, with the center limited and the pivit point forward would you get rear steer?
 
I don't think it will effect things for most suspension designs. For a parallel link system, the track bar defines the roll axis. For triangulated, it's the intersection point of the links. So technically you should be able to get the same results even though the physical mount is forward of the axle center line. I don’t see anything that would cause a problem here, but I’m no link geometry expert by a long shot…
 
Russ, the setup im talking about doesnt require a track bar. Many desert trucks run a similar setup, just reversed. You basically run a 4 link style setup at the frame but either the two lower links or upper links converge at one point to create a single link instead of 2 seperate mounting points. This way you still have the side to side control of a 4 link but have more options at the diff side for mounting options. Let me do some searching to see If i can find something similar to what im talking about.
 
No problem, I know exactly what your talking about. A wishbone upper... But I don't see how that helps since it still needs the mount on the top of the diff housing. One mount or two, I still need a mount on the top and there is no room for it without hiting the gas tank.

Edit: Oh, I think I see now. Your talking about running a wishbone link on bottom with the straight links on top. I don't think I would like that setup due to roll axis issues. Not sure how team purple deals with it...
 
Actually having parallel (in top view) upper links won't hurt the roll axis. The roll axis will be at the same slope as the parallel links. Upper links are alot easier to get flat (in side view) then lower links are. You would have the lower links converge at the t-case and if you make it a "Y" link you could put some driveshaft skids on it if you want.

If you try and play with some #'s in the 4-link calculator don't forget to make the links "not" parallel by a small amount (1/8" or 1/16" or so) or it won't calculate correctly
 
/forums/images/graemlins/doah.gif
I didn't even think about putting the converging point at the tcase... That would keep the roll axis fairly high with a slope matching the parallel links. Thanks for waking me up...
 
With the above discussion, I am not sure if the original question is even relevent any more, but here is my 2 cents FWIW. Again I am no 4-link expert by any means, but this is how I see it:

Depending on the angle of your upper links (typically fairly flat) the forces exerted on your connection point about the instant center (the axle tube) will be parellel to the link and cause no rotation at the connection point. As you move your connection point further forward the forces will become further from parellel to the upper link at the connection point, causing the axle to try to twist. I think this could be compensated for this with the angle and mounting location of the bottom links, but I am not sure it would cause enough twist to be an issue in the first place.

Again, I am not sure if you are even considering doing this any more, but just wanted to give you something to think about. /forums/images/graemlins/thinking.gif
 
Russ, you've seen Brandon and I's rear suspensions, that's EXACTLY what you need for this. The reason why I set mine up to converge under the T-case and have parallel uppers is to accomodate the exhaust and passenger compartment above the driveshaft and brandon used the same system but put his fuel cell in that same space. My fuel cell sits just behind the rear axle and on full stuff it comes close to contact, sounds just like your setup. The only problem with running the upper links on the outside and straight ahead is that it leaves limited room for the coil and shock but coilovers solve that and there's also room to mount a separate coil and shock behind the axle if the bracketry is built right. You also have to watch the angles of the lower links since they determine the roll center but that's not really a big deal. We have negligible roll steer since the upper links run level or very close to it. In fact, it felt funny driving the thing over the first rock with the links because it didn't steer like the leaf springs did.

There is no problem with running your trusswork over the front of the housing to mount the links, but it might make pinion angle control hard since the uppers would get kind of short without shooting them a long way up the chassis. You might also end up with some goofiness looking at anti squat characteristics under articulation and bump/droop motion. That part would take some modeling to figure out.
 
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