Source
r/AskEngineers•
quadtodfodder

Software (or method) for designing steering linkages (for non-professionals)

Mechanical

I am building "reverse" tricycles (two wheels on the front).  Check 'em out here: ultropolis.net* (Patent pending!)

[Edit: the site seems to not always load, so maybe try this or watch this goofy video]

I am looking for a good ways to model their steering linkages*.  My trikes use a "dual crossed drag link" system, but my methods for deriving correct Ackermann Compensation are poor at best. The systems will eventually be complicated by adjustable an width mechanism and the need to run the linkages along irregular paths.

What I'm imagining would allow me to draw boxes, arbitrarily assign joints with other boxes, "pin" them down at points of rotation, and declare certain objects immobile.  Ultimately nothing more complex than your average physics based puzzle video game. 

•  I don't want to invest a the necessary time (or money) into rhino/autocad or blender/maya - I would like as shallow a learning curve as is realistic.

•  I am happy to make physical models, but my craft-store efforts in this direction are thus far are not accurate enough to generate useful geometry for building with - any guidance in this is also welcome.

• I don't need to model the trike on the road or anything like that, just the linkages themselves (although I certainly wouldn't mind something that did model the vehicle in motion)

• My ideal price point is "free" but "not very expensive" will do. I have an ok Mac and an older Ubuntu machine.

• If maybe there is some sort of guidance for grokking steering compensation over all, please point me at it!

In thanks, you may use the secret code-phrase "I engineered you from reddit" for free schwag and muffins at whatever trade show you find us at in a year or so. Thanks in advance!

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*The limited resources I've used so far include:Peter Elands steer sheets - This has been helpful, but feels very hit or miss. Also the sheets freak out with certain measurements.

This diagram shows ONE solution for applying the trick where you point your steering arms at the center of your real axle to dual drag links, however it does not account for variations such as steer arms of differing length or sweeping the overall steer arm angle forward or back.

Posted Jan 2, 2019, 4:40 AM UTC · edited

06
GregLocock•

What makes you think you want 100% ackermann? parallel steer works well for prototypes. Real production cars use 60-80% ackerman, some race cars use negative ackerman (ie outside wheel steers more than the inside wheel, and some use >100%, ie inner wheel steers too much.

Ackerman is way down on the list of priorities, bump steer and roll steer is more important.

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3
quadtodfodderOP•

>What makes you think you want 100% ackermann?

  1. Parallel steer is definitely not good, as the wheels trace visibly different radii. (the vehicles are short for their width). The wheels exhibit chatter when the compensation gets inaccurate at high turning angles. Indeed ideally the center of the turning radius could be at the rear axle (this would mean that the leading wheel steers rotates back slightly)
  2. What situations should one consider less than 100% ackermann? (other than compensating for tire flex). I understand negative Ackerman is a high speed thing, and these trikes are pedal powered!
  3. the trikes have no suspension other than the tires, so bump/roll steer (if I understand it correctly) is not where my thinking is at right now, but I will attempt to further familiarize myself with the concept - though at first glance it looks like something I want to minimize, if anything

What methods other than trial and error are there to get correct steering compensation? While I would like to include high speed tire effects (reverse ackermann I believe) and suspension effects (bump/roll steer) right now, I want to get the basic geometry under control!

Mechanical methods, software methods, "rule of thumb" methods, mathematic methods - Gimmie your methods!

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1
GregLocock•

Sorry, I hadn't realised you were after low speed stuff. Your website is broken.

1
quadtodfodderOP•

Hmm, the site seems to not load sometimes. Here's an alternate link, and here's a promo vid I made last year.

Any thoughts on how to figure out the Ackermann compensation?

1
GregLocock•

I agree you need accurate ackerman compensation. None of the simple methods I know of will necessarily work. If you aren't willing to get up to speed on a 3d cad system, then someone who is competent could probably knock that out in an hour. So find that someone.

1