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  • Elmo Zoneball on 2012-Apr-03 18:14:03 Elmo Zoneball said

    anyone else find the tie rod to steering knuckle angle disturbing?

    Unless the rod end/knuckle connection is offset outboard of the Kingpin axis, I would think this will cause serious non-linearity in the force vs. steering angle response at large steering angles.

    At speed, if the buggy understeers at the limit, the driver will need to add steering angle to keep the buggy out of the hay bails in the corner, and the force required will probably go up dramatically. When the understeer stops (assuming contact with the hay bail has been avoided), the driver will need to rapidly reduce steering angle so as to not swerve to the right, but as she does so, the forces will be dropping radically, and might cause the driver to overcompensate, causing an driver-induced oscillation and potential lose of control of the vehicle.

    In short, is this design even safe? (Obviously, it would depend on the angles at the rod end/steering knuckle connection, which we can't see from the photo.) At slow speeds, it probably is, but at the limit, I'm not so sure.

    I'm not an expert, but what I'm seeing worries me.
  • Jerry on 2012-Apr-03 19:45:01 Jerry said

    Huh, that could definitely be an issue, depending on the geometry. I got a chance to look at Rubicon 2.0 closer at rolls, and the front axle is long on both sides, quite a bit wider than the other reverse-trikes. I believe the angle of the tie-rod appears very exaggerated here due to perspective. I feel fairly confident that the safety chair would inspect the steering's full range of motion as well, so hopefully there's nothing serious to worry about here.
  • Elmo Zoneball on 2012-Apr-03 22:00:54 Elmo Zoneball said

    I just did a quick back of envelop calc, assuming 24" track width (which I think is generous) and scaling the distance from the front of the buggy to the axle using the Xootrs as 7" dia, if the tie rod attaches anywhere near the nose of the buggy, they are looking at about a 60 deg angle at the rod end/steering knuckle intersection -- when going straight ahead. However many degrees of steering angle you need to get around the corner and over come understeer just adds to that value.

    This is not giving me a warm fuzzy feeling.....
  • Zatchmo on 2012-Apr-03 22:54:43 Zatchmo said

    It's a pretty fair argument, and one that should happen when designing a buggy, but honestly I don't non-linearity in steering as a huge threat to see another team using it. Especially for a buggy that won't exactly be flying down the course, I think that coaching the driver is more than enough to prepare them for what to expect. For the potential what-ifs, I don't recall ever seeing an issue where there was an oscillation build up through the chute - perhaps by the point where it is unstable, enough speed is scrubbed off by the sharp angles. As far as I think the safety chair should be concerned, if the steering set up is well built enough to hold up reliably and for a long time, he shouldn't be forcing design alternatives down their throats; this just doesn't fall in that realm of 'obvious problem that may cause an accident' where I would expect him to do his thing.
  • chayes on 2012-Apr-04 00:02:50 chayes said

    Whatever the case I think they did a damn good job and Rubicon (Can we consider it the same buggy anymore?) looks better than it ever has! Hope we can come out with something half this good!
  • the Pope on 2012-Apr-04 01:51:16 the Pope said

    I think the important thing is that the driver have sufficient mechanical advantage over the steering to be comfortably in charge of direction. I never really thought about a system with this type of geometry. Without running any trig, my guess is that the wheels only point the same direction for one 'position' and that any other position results in them fighting each other and scrubbing some speed. I too am a big fan of the rebirth of rubicon.
  • chayes on 2012-Apr-04 02:20:24 chayes said

    Isn't that a good thing? A perfect ackerman set up has the wheels turn at different rates to avoid slip, so if this will do that, then it is a good set up. I may not understand exactly what you mean, but I think SAE did a better job than we are giving them credit for. Definitely more ambitious than our 1st build
  • Shafeeq on 2012-Apr-04 04:08:23 Shafeeq said

    We can't see what the other end of the tie rod connects to, so maybe they were clever and the linkage between there and the driver's hands is non-linear in the opposite way?
    It does seem that as the tie rod pulls on the knuckle, at some steering angle, the tie rod will be in line with the kingpin and thus not be able to turn the wheel any further. Assuming Elmo's measurements, that's at most 30 degrees. One can look at buggies with faired wheels and make a similar back of the envelope calc about how far their wheel can turn and still stay inside the fairing.

    It seems like the bigger issue is having a seriously understeering buggy in the first place - even if the steering were perfectly linear, once it runs out of range, then there's nothing the driver can do except watch the haybales getting closer. Though at that point, there's probably enough drag that the buggy slows down and the understeer gets better.
  • The Cook on 2012-Apr-04 14:52:31 The Cook said

    This type of steering can be made to have nearly perfect aligment if the right acherman calculations are made. Steering angles on the course are very small and the errors versus perfect wagon steering are measure in the hundreths of a degree.

    I can't tell from the photo but if stud that the heim joints are puling on is horizontal that ain't good - you'll trash out the joints by hitting their misalignment maximum in push practice. The stud or bolt should be vertical like on a car's tie rods.
  • Mackin on 2012-Apr-07 14:59:33 Mackin said

    Hey Everyone, the conversation happening here is based on the wrong photo. All of the important dimensions are out of plane! I took a picture today that may be more helpful. (I added the yellow lines).

    http://cmubuggy.org/gallery/2011-2012/Spring-Rolls---April-7/photo-2

    I wouldn't normally get involved in this kind of thread, and as Safety Chair I absolutely wouldn't normally go out of my way to upload and discuss photos of a team's hardware, but I was upset to see the comments here after being so impressed and excited by the great job done by SAE in reinventing Rubicon. (Don't worry I asked permission to take/upload/discuss said picture).

    I've inspected the buggy thoroughly including its range and stability of steering motion, and can assure you it is not high on the list of least forgiving steering systems on the course. As you can see from the photo, the perpendicular level arm is actually quite large. The 'non-perpendicularity' of the physical material lever arm does introduce some non-linearity, but nothing dangerous or unstable within the required steering range.

    Good job, SAE. Turning an old ass standard tryke that rolled on ~12" pneumatics into reverse that rolls on xootrs is no trivial task.

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