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  • Oct. 17, 2010 9:28 a.m. petegossett SuperDork

    On a simple bellcrank steering setup(think gocart, lawn mower, etc.), is Ackerman still determined by having the steering arms at the wheels pointing back toward the center of the rear axle, or does the length of the bellcrank its self(and/or the spacing between the linkage pivots on the bellcrank) relative to the length of the steering arms have an effect?

    I haven't found any concise info online, but looking at several bellcrank setups, the pivot radius of the bellcrank is often longer than the pivot radius of the steering arms - however, all the ones I've found have been front-steer setups, and I'm working on a rear-steer one.

  • Oct. 17, 2010 9:39 a.m. Knurled HalfDork

    petegossett wrote:

    On a simple bellcrank steering setup(think gocart, lawn mower, etc.), is Ackerman still determined by having the steering arms at the wheels pointing back toward the center of the rear axle, or does the length of the bellcrank its self(and/or the spacing between the linkage pivots on the bellcrank) relative to the length of the steering arms have an effect?

    It's the angle of the steering arms.

    Well.... it's the angle of the steering arms relative to the tie rod. If the tie rod is straight, then that angle would have the steering arms meet at the rear axle centerline.

    (Basically, no matter what, angling the tie rods back from the wheels increases Ackerman)

    So, yes, if you had a bellcrank setup that altered the tie rod's angle, Ackerman would change.

  • Oct. 17, 2010 9:54 a.m. petegossett SuperDork

    OK cool, so if I'm understanding you correctly the radius from the kingpin to the outer tie rod pivot should be shorter than the radius from the bellcrank pivot to the inner tie rod pivot?

  • Jensenman

    Oct. 17, 2010 10:11 a.m. Jensenman SuperDork

    If the steering is behind the axle centerline (and most Pitman/idler systems are) the inside of the tie rod should be further forward than the outside. This is assuming that the Pitman/idler arms point toward the back of the car. They should be parallel to each other in the straight ahead position.

    If it's a single bellcrank system like an old VW, you got me. Never fiddled with one of those. You might look at this: http://www.muller.net/mullermachine/docs/ackerman.html Maybe that will help answer your question.

  • Oct. 17, 2010 10:11 a.m. Knurled HalfDork

    I forgot you were on a bellcrank setup, no rack or center link. I'm thinking that any deviation from equal length will increase Ackerman slightly, because of sine error.

    Part of me says that's wrong, though, because the steering arms themselves are also moving. Both sides are posting charts and graphs to support their positions, and I think the mental flamewar is reaching the point where demotivational posters are going to be deployed.

    I hate it when they argue.

    From a practical standpoint, once you factor other suspension geometry into the mix, and multiply by the differing loads on the front tires causing differing preferred slip angles, you may find that you're shuffling deck chairs on the Titanic.

  • Oct. 17, 2010 10:22 a.m. petegossett SuperDork

    Knurled wrote: Part of me says that's wrong, though, because the steering arms themselves are also moving. Both sides are posting charts and graphs to support their positions, and I think the mental flamewar is reaching the point where demotivational posters are going to be deployed.

    First, you've just described what's been going through my head for the last week in a manner far more eloquent and vivid than I could ever hope to achieve!

    Knurled wrote: From a practical standpoint, once you factor other suspension geometry into the mix, and multiply by the differing loads on the front tires causing differing preferred slip angles, you may find that you're shuffling deck chairs on the Titanic.

    Second, fortunately this is a low-speed vehicle with no suspension(it's a 26" reverse-trike...yes, I'll post pics soon), so no worries about slip-angles or bump-steer. I just don't want the inner tire to be skidding around the corner.

  • Oct. 17, 2010 10:28 a.m. petegossett SuperDork

    Jensenman - thanks for that link, that bellcrank on the bottom of the page is very close to the one I'm using. That at least tells me that I don't need my steering/pitman arms pointing to the center of the rear axle in order to achieve the Ackerman effect.

    I'm afraid I'll have to resort to Smith's "paper-doll" method to sort it all out...

 
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