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  • home-built steering project questions

    i am going to start a project over winter that i am slowly gathering parts for (otherwise known as dumpster diving). for one portion of it i am planning on taking a lawn tractor transaxle similar to this and adding steering:

    http://cgi.ebay.com/ws/eBayISAPI.dll...m=140151452430

    what i basically want to do is add a universal at the end of each axle and connect it to some steering knuckles i will have to build. anyone ever attempted such a thing? i don't think it will be that bad, but if someone has ever done it i am open to modifying their design for my own use. i have seen some 4-wheel steering garden tractors that already had steering on the rear axle, but the ones i saw were all hydromatic, and i want to use a plain old mechanical transaxle.

    thanks,

    andy b.
    The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

  • #2
    U-Joints need to be run in phased pairs to avoid acel/decel problems. They also have (generally) limited angularity limits. A small CV would work better, but if you want to use u-joints, look into the small u-joints used in some automotive steering shafts. Still limited angularity, but common and cheap.

    You'll also want to make sure the transaxle has a differential, otherwise turning can be, eh, "fun".
    Russ
    Master Floor Sweeper

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    • #3
      andy

      I wont say it is done every day but is relatively common.

      At normal off road tractor wheel speeds, phasing of the u joints on steering axles is a low priority.
      Real jeeps have lived for decades with unphased cardan cross joints on steer axles with no problem.
      They did phase them on the drive shafts. They worked at 5+ times the speed.

      Treat the rigging with fore-thought, you cant use the axle shaft to support the tractor because of the flex. You can use an extention of the axle housing, but you gotta keep the center of the U joint on the knuckle pivot center line.

      Rig your steering linkage so that all wheels roll approximately perpendicular to a radius from a common point.
      This is more important the tighter the turn.
      If you don't it will be scuff city.

      Jeep steer axles of Cardan Cross type did not exceed 27^ steer angle.

      Hth Ag
      Last edited by agrip; 08-31-2007, 11:35 PM.

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      • #4
        I would look for a small import 4x4 like a Suzuki Samuri and rob the frontend out from under it for the steering knuckes and joints.
        I just need one more tool,just one!

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        • #5
          You're right Ag, SFA 4x4s have done that for years. And I now realize my post was even further off track about that whole thing. When turned at the knuckle, the inboard joint won't match, so pretty much makes the whole thing about phasing irrelevant. <sigh>

          For some reason I got all muddled thinking about IS (and so needing two for the floating), phasing, having seen small and light u-joint steering knuckles like that which *do* noticeably oscillate at tighter angles, limited angularity, and so on. From that muddled train of thought my previous post appeared.

          I still say a small CV would be the cats meow with more angle available and negates most all the issues. And with low power requirements, should not be that hard to come up with from various applications.
          Russ
          Master Floor Sweeper

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          • #6
            Originally posted by wierdscience
            I would look for a small import 4x4 like a Suzuki Samuri and rob the frontend out from under it for the steering knuckes and joints.
            Or a 4x4 quad if you could find one junked. Closer to the right scale and much lighter.
            Russ
            Master Floor Sweeper

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            • #7
              While true that many 4x4's have used the u-joint for years, it isn't without problems. Where the tire doesn't have lots of grip, it can slip to compensate for the variable rotation rate of the outboard end of the joint. (I don't remember, is it twice or four times per axle revolution-) The more acute the steered angle, the worse this is. Your going to get some axle windup, and depending on the amount, something may have to give. Usually one or more tires will have to slip. If you're on slippery ground, that's going to alleviate the windup, but there's also another factor- suppose that to get over a certain bad spot you need all the grip you can get. If you're having to turn a tight corner at the same time, you'll be forcing one or more tires to slip- exactly the opposite of what you need at that point. Your 4x4 can be hopping and clawing at the ground where another vehicle with cv joints could just drive through with minimal problem. If you're on ice, you will lose traction sooner when cornering sharply if you use a u-joint instead of a cv joint.

              Edited to add- something else that does come into play in regards to axle windup in a 4x4 is when the front end and the rear end have different ratios. If I'm not mistaken, the front rotates just a little faster than the rear. Not the same periodic oscillation as with a front end, but a rotational difference nonetheless. It does have a noticeable effect on control.

              By the sounds of what Andy wants to do, no big deal. If one could purchase a cv joint that would 'bolt up' as easily as a u-joint, then (mho) it would be the better choice.
              Last edited by darryl; 09-01-2007, 03:51 AM.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                Many vehicles use a single U-joint on the steering axles. My Land Rover and Ford Ranger both do. They can and do wind up and for tight turns I usually take the Ranger out of 4WD for a moment. They hold up just fine though.
                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  thanks for all of the input! i understand the basic geometry of the entire mess (wind-up, knuckle pivot in line with u-joint pivot, etc.). this is for a low-speed 4-wheel drive application in loose soil, so most of the stress issues aren't much concern.

                  i like the idea of using steering shaft u-joints. i should be able to get tons of them from the local U-pull-it junk yard. i'll also check the ATV salvage yards. i'm not sure i've ever seen them with u-joints though, most of the ones my friends have all use CVs. while CVs are probably the better choice in some applications, my experience has been once the boot rips and crap gets into them, they wear out quickly. i don't plan on doing much maintenance to this thing (or using it daily), so cost is the overriding factor.

                  andy b.
                  The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

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