Topic: Radius arm setup

[OOPS]

Have you seen this one?

http://www.pirate4x4.com/forum/showthread.php?t=865762

[Phazertwo]

http://www.rmftc.com/howtos/extendedradiusarms/extendedradiusarms.html

this is my buddys truck/write up.  I wound't say that his truck flexes better than mine, but its stronger, the axle moves in better directions when it travels up and down. Most important, the thing handles better than most caddys on the road. When I get some time and money, this swap is happening to my truck!

PZ

I just forgot, he is on this board to, as Rock Ranger.

[82juguete [OP]]

sweet...only thing ive noticed is that the fords seem to flex pretty good and i think its because of those huge C bushings on the axle. (IIRC Tacorunner commented on this as well) My fear is that that type of bushing has a degree of "slop" that is allowing some extra flex that i wont see with a dual bushing setup like ive been envisioning.

[Phazertwo]

Well, there are HUGE bushings in there, so that may be.  Seems to work well on the LC's though, with the two bolt setup. Thats what I plan on doing, but over the top of the axle, instead of under it.

PZ

[82juguete [OP]]

yea i was actually referring to a setup similar to my little rendering i did on the last page....or like the blitzkrieg setup on the link i posted.  Its 2 bushings like the lc's have, but theyre both configured on the back side of the axle.

[TacoRunner]

Radius arms work better on road then off. They are good entry level systems, but my opinion is that they dont cost much less then a 3link, so why not just go 3link and be done. I dont think you'd save $100 on a radius arm set up.

[87pickup]

I wish my front was 3 linked.

[TacoRunner]

Its coming

[Phazertwo]

So I sat down and thought long and hard about the radius are set up binding.  It makes perfect sense that it does, but I could swear I could remember Rock Ranger saying something about how to make it better.  Than I remember that when he made longer radius arms, he brought them closer together on the frame side.  This makes sense because it will limit the bind.  The closer you get them together on the frame side the better, and if you can put them together, it should eliminate all of the bind.  This makes sense to me anyway.

PZ

[TacoRunner]

Sorry but no thats not "totally" true, that "mostly" helps the bind at the frame, not at the axle. If you want to eliminate bind at the axle you have to build it with one upper attachment point at the axle, like a three link is, not two points like "most" Radius Arms. Yes longer arms help because longer arms twist the housing less, which is the whole idea with a long arm suspension in the first place.

A Radius Arm suspension while articulating like in rock crawling will tend to twist the axle like a torsion bar. This is not lessend by converging the links like it would be in a Triangulated 4link. To help avoid binding at the frame converging links do help but some kind of flex joint should be used at the frame while a more solid mount is used at the axle, like a bushing. Usually the larger the bushing the more deflection it will handle and allow for more twist of the axle, which would help articulation, but could hurt streetability when those bushings become worn. This design is a fine suspension for street use offering little brake-dive because of its usual high anti-squat.

[82juguete [OP]]

Well I think I'm officially to the point of deciding rad arm vs. 3 link, no longer based on cost, but actually preference. But i am still unsure if my understanding is correct. What's throwing me off is the fact that blitzkrieg makes that badass radius arm setup, specifically for higher speed applications. Given that they could just as easily manufacture a 3 link setup (since its swapping from leafs, so no mounts are being reused) i am led to believe that radius arms provide a superior "soaking up" in the desert racing type scenarios. Now this may just be based on durability, simplicity, etc. and i understand that its all about the shocks, but there must be some sort of advantage to going that route.

As mentioned i do about 90% street driving at this point. When i do wheel its usually pretty moderate trails.... and i would love to be able to get it (more so than now) on the fire roads and in the dunes and stuff.

So basically i guess it comes down to the road manners of the different setups. Also, side by side, same shock setup, etc. what would ride better through a choppy, pot-holed mountain road; the radius arm setup i intend to build, or a properly setup 3-link?  Might sound dumb, but I'm really not sure.

[TacoRunner]

The performance down side to a Radius setup is the twisting of the axle, which you can fix by eliminating one upper axle mount and then the lack of any kind of adjustable anti squat. As well as a caster angle that changes rapidley under flex. On the plus side, the U-Joint angle stays very consistent.

As for soaking up bumps better at speed, I dont believe it, that sounds like someone had a setup that worked really well because of shocks and good bump stops, or maybe it was a fitment issue with the upper link that forced them to go with it. You have to remember the links aren't soaking up the bumps, its the springs, shocks, bumps and tires that soak it up, the links just keep the tire and axle in place. Or perhaps the vehicle did better because it was a radius arm system with large bushings like the Ford style that helped isolate vibrations.

[ntsqd]

I find it funny that the assumption is made that those Ford C-Bushings must allow a lot of flex when most of the Early Bronco guys complain about their lack of flex. That is what led them to first experiment with the Wristed Arms and then later with the extended arms. The longer the radius arms are, the easier job they have of twisting the front axle housing to increase the articulation. Either method increases front articulation, pick your poison.

For true high speed uses the radius arm suffers from a large and undesirable change in Castor angle. It is used because it is strong and lives in desert racing, not because it is the most ideal suspension design. Most are designed with a fair amount of Kinematic Progression, but the first inch or two of compression travel may actually have some regression. No way around that.

If the radius arm length and mounting is chosen carefully and a CV is used at the transfer case, then there is little to no change in the operating angle of the pinion u-joint, and little to no front drive-shaft length delta. Depending on driver style both of these are bonus' that may outweigh the Castor angle issue.

For my own use the radius arm design looks like the simplest solution. I'm not looking for 16" of wheel travel, though I do drive semi-fast in the desert. The added complexity of a simple, non-laterally locating 3 link to get a mild increase in control over the Castor angle doesn't look like it is worth it to me. For others I can see some advantages.

[LilGonzo86]

seems like your in a bind snico.... im in for the research  .....

[TacoRunner]

I find it funny that the assumption is made that those Ford C-Bushings must allow a lot of flex when most of the Early Bronco guys complain about their lack of flex. That is what led them to first experiment with the Wristed Arms and then later with the extended arms. The longer the radius arms are, the easier job they have of twisting the front axle housing to increase the articulation. Either method increases front articulation, pick your poison.

So a urethane bushing wont flex? You're right thats why they went with longer arms, but longer arms dont allow for more twist, they twist the axle less which increases movement. Or is that what you were trying to say???

[TacoRunner]

For true high speed uses the radius arm suffers from a large and undesirable change in Castor angle. It is used because it is strong and lives in desert racing, not because it is the most ideal suspension design. Most are designed with a fair amount of Kinematic Progression, but the first inch or two of compression travel may actually have some regression. No way around that.

Agreed completely 

[TacoRunner]

If the radius arm length and mounting is chosen carefully and a CV is used at the transfer case, then there is little to no change in the operating angle of the pinion u-joint, and little to no front drive-shaft length delta. Depending on driver style both of these are bonus' that may outweigh the Castor angle issue.

Again agreed. But why a CV, it wouldn't matter CV or U-Joint.

[TacoRunner]

For my own use the radius arm design looks like the simplest solution. I'm not looking for 16" of wheel travel, though I do drive semi-fast in the desert. The added complexity of a simple, non-laterally locating 3 link to get a mild increase in control over the Castor angle doesn't look like it is worth it to me. For others I can see some advantages.

One great advantage of to the radius is the lower amount of nose dive under hard breaking. Which is great for speed but not for crawling, a lower amount of brake dive helps in eliminating "Torque" loading of the axle while crawling. Anyone ever notice that lots of rigs that are linked and have a high Anti Dive or Anti Squat (same thing) in front tend to raise up when torque is applied through the axle. A lower Anti Dive will help eliminate a lot of that, and for crawling up step stuff that is a huge advantage not having your suspension push your nose up.

Im not trying to make enemies just trying to make a clear point. I dont disagree with most of what you said, maybe the bushing thing buts that about it. 

[ntsqd]

So a urethane bushing wont flex? You're right thats why they went with longer arms, but longer arms dont allow for more twist, they twist the axle less which increases movement. Or is that what you were trying to say???

The urethane C-bushings nearly always have some Caster compensation built into them - I don't know of any that do not. That reduces the bushing's thickness in one large area per bushing (there being 4 of them in a front suspension) and results in a stiffer bushing in one direction and a softer bushing in the other. Unfortunately those stiffer directions always gang up on you. Now factor in that the urethane C bushings are generally less compliant than the rubber C bushings, so even the 'soft' side arm in articulation is stiffer than with rubber. The guys who do a C-Bracket mod (= to a swivel ball rotation) and can run the over-all softer rubber bushings have greater articulation than any of those running urethane bushings.

The longer arms twist the axle less per unit of force, but have greater leverage to twist the axle with. It's win-win. This aspect is really no different than tuning a race car's sway bar.

Again agreed. But why a CV, it wouldn't matter CV or U-Joint.

Well, yes it does matter. If the operating angle at the pinion does not change or changes only a minute amount then w/o a CV at the transfer case the drive shaft's operating angles aren't the same and it will NOT be happy. Any drive shaft guy worth listening to will tell you that in such a situation the joint at the transfer case needs to be a CV. I suppose that you could run a std. UJ there, but the operating angle has the potential to get very large. Add needing some power at that high angle and the U-J will be done.

[TacoRunner]

The longer arms twist the axle less per unit of force, but have greater leverage to twist the axle with. It's win-win. This aspect is really no different than tuning a race car's sway bar.

Ok I'll see you, your U-Joint and bushing point of view and raise you this.

Your above statement is almost redundent. What I ment earlier by saying ("but longer arms dont allow for more twist, they twist the axle less which increases movement") is that the long arm changes its relative angle to the ground and axle less while moving. This means that while a short arm moves 10" and 20* a long arm that moves 20" may only move 10*. Which in the end will twist the axle less.

Im not sure I've said that right, but what the heck.

[ntsqd]

And what I've been trying to get across is that the long arms have more leverage to twist the axle housing. It's really just a torsion spring sway bar. If you want to decrease the sway bar's spring rate you either reduce it's OD (not likely since we're talking about an axle housing) or you increase the lever arm that the vehicle mass can use to twist the bar. Exactly what a long arm is.

You could get lost in theory about does the axle housing twist less or the same or what with longer arms for a given amount of articulation, but that only means something to us engineering dorks. Most guys just want to know how all of the mumbo-jumbo translates into how the vehicle will perform.
With a longer radius arm the torsional spring rate is reduced, which increases articulation.
We're pretty much in agreement. Maybe even violent agreement.