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Help with Ackerman??? How to avoid reverse ackerman???

Discussion in 'Shop - Suspension & Steering' started by atomicjoe23, Oct 6, 2009.

  1. atomicjoe23

    atomicjoe23 Well-Known Member

    Joined:
    Feb 2, 2008
    Location:
    Silverdale, WA
    I don't need ackerman angle explained. . .I understand what it is and it totally makes sense to me, plus there are already more than a few threads posted that explain ackerman.

    What I need help with is how to acheive the correct ackerman and not reverse ackerman?

    I ended up with reverse ackerman on the last buggy and I don't really know how to rectify it because I don't even really know how it happened!

    We put the R&P directly between the upper and lower A-arms and the tie rod pivot points were directly inline with the line formed between the upper and lower A-arm chassis side pivots. . .knuckle side pivot location was of course determined by the knuckle so there wasn't a lot I could do about that (was there???).

    Anyway I'm starting a new project and I don't want to end up with reverse ackerman and a front end that pushes because of it, so I'm hoping some of you can help explain how you actually get the inside wheel to turn more than the outside wheel (or the outside wheel to turn less than the inside. . .however you want to put it). . .I'm just not seeing how that's gonna happen.

    I'm not positive. . .but the knuckles could have been flipped so I'm assuming that this could be part of the issue. Ignoring everything else (like why someone would do that, etc.) let's say you have a rear steer knuckle and it is being run in a front steer application. Would that automatically create a reverse ackerman situation? If so, what can be done to rectify the reverse ackerman (besides flipping the knuckles), assuming anything can be done to fix the problem?

    Just in case it's important. . .this project isn't an A-arm vehicle, it's I-beams/radius arms with a swingset style steering set-up.

    Thanks!
     
  2. partybarge_pilot

    partybarge_pilot Well-Known Member

    Joined:
    Feb 19, 2002
    Location:
    Easton, KS
    Ackerman is great for driving around a parking lot and pulling into spaces..............
     
  3. atomicjoe23

    atomicjoe23 Well-Known Member

    Joined:
    Feb 2, 2008
    Location:
    Silverdale, WA
    Thanks. . .

    I built a buggy last year and the front end pushed like nobodies business!!!

    I think part of the problem was poor weight distrubution which resulted in poor front tire traction, especially in sharper turns, but when I looked at the tires at full crank later it appeared that the outside tire turned in more than the inside tire (reverse ackerman) and I thought/assumed that the reverse ackerman would be part of the problem as well.

    Are you suggestion that ackerman is not all that important in the dirt. . .therefore my problem was probably a weight distribution/traction problem?

    BTW, I'm fixing the weight distribution problem this time around. . .having only two months to build the last buggy I didn't have the luxury of lots of testing time, plus it was my first full build from scratch. I learned a LOT and that's what's important, plus it was fun!!!
     
  4. partybarge_pilot

    partybarge_pilot Well-Known Member

    Joined:
    Feb 19, 2002
    Location:
    Easton, KS
    It depends on a lot of things. How long your wheelbase is, spool or not, weight distribution, how much power the car has......

    In something like a pro-2 reverse Ackerman would almost be preferable as they are sideways most of the time. Something without a lot of power thats going to be used in tight trails you would want something with Ackerman. As you have all ready noted, it won't make much difference if there is no weight on the front. There is no correct amount, it's all just a game of compromises and trade offs.
     
  5. Triaged

    Triaged Member

    Joined:
    Nov 25, 2003
    Location:
    SoCal LA/OC border
    Give this a read. Then think about how it will apply to loose packed and/or hard packed dirt.

    Images at the bottom.
     
  6. atomicjoe23

    atomicjoe23 Well-Known Member

    Joined:
    Feb 2, 2008
    Location:
    Silverdale, WA
    Party_Barge. . .thanks for expanding on the subject, and Triaged you gave me my reading material for my down time (if there is such a thing) tomorrow.

    Greatly appreciate both of your inputs!
     
  7. loufish

    loufish Member

    Joined:
    May 7, 2005
    Location:
    Burbank, CA
    To Triaged.....Great article....BUT I think the OP was asking what things can get designed into steering systems the can un-intentionally produce "anti-ackerman"....
     
  8. scottm

    scottm Well-Known Member

    Joined:
    Feb 7, 2005
    Location:
    scottsdale AZ
    Are you talking about the buggy or in general? Understeer is also aggrivated by too little caster. With more caster, turning the wheels actually raises the front of the car, helping the tires bite. If caster is 6° or less, 8 or 9 will make a noticable improvement. BUT - if you are talking about a VW beam buggy, 8 is too much, and will make driving in reverse almost impossible!
     
  9. fly-n-hi

    fly-n-hi Member

    Joined:
    Jan 26, 2007
    Location:
    Phoenix, Az
    Don't use ATV spindles!!! The built in angle of the tie rod mount is not correct for under powered (Briggs) cars. They are built for an ATV, which steers by getting the rear end loose, it has the power to do it.
    We ran in to the same problem with mounting the rack in our Mini Baja car. We used Bombardier ds 650 spindles. After we found the proper height to avoid bump steer. We moved the rack forward and backward trying to find a position that didn't pull a staight line with the tie rod (at full lock). Once it pulls a straight line, you cannot exceed that angle. So, even though the inside wheel is not at full lock........ the outside is.

    I think the tie rod mount needs to be closer to the spindle.

    Good Luck.
     
  10. atomicjoe23

    atomicjoe23 Well-Known Member

    Joined:
    Feb 2, 2008
    Location:
    Silverdale, WA
    We are stuck with the ATV spindles for this year. . .I don't have a large enough budget to replace them this year, but I will put it on the list of priorities for next year (or possibly this year since I don't actually know the budget but am only anticipating it to be basically the same as last year).

    If positioning the tie rod mount closer to the spindle will help that is entirely possible for us to do. . .we are a welding program so we can weld pretty much anything and have a TON of different types and processes available to us, plus the experience and education to use them that some of the other more engineering focused schools might not be capapble of doing. . .no offense meant there flip side of the coin is that the engineering schools are better at SolidWorks, FEA, etc. than I am out this point, but I'm working on fixing thiat!

    Thanks for the suggestions. . .I knew part of the problem was the lack of power, but I didn't realize the ATV spindles would exacerbate the problem.
     
  11. Triaged

    Triaged Member

    Joined:
    Nov 25, 2003
    Location:
    SoCal LA/OC border
    Sure; but I don't think too much info is bad in this case. The how is in there but so is the why which is often harder to come by.
     
  12. 07FJRog

    07FJRog Well-Known Member

    Joined:
    Feb 26, 2009
    Location:
    Tonopah, AZ
    steering pivot points on the spindle determine this more than other things. IMO

    from center of spindle pivot to tie rod arm pivot should be inline with center or rear end.
    even with ATV spindles you can change the tie rod arm on the spindle to achieve what you need.

    like said though, if you use a spooled rear end or ATV axle, it will tend to push, because both rear wheels only want to go straight.
     
  13. TauMau

    TauMau Well-Known Member

    Joined:
    Jun 27, 2007
    Location:
    Roaming
    I attempted to clarify your statement for the listeners.
     
  14. Scott_F

    Scott_F Active Member

    Joined:
    Dec 28, 2003
    Location:
    Tucson, was Anaheim
    To clarify the clarifier, construct a plane from three points, the upper ball joint, lower ball joint, and center of rear axle. The outer tie rod end should lie on this plane. Clear as mud.
     
  15. Triaged

    Triaged Member

    Joined:
    Nov 25, 2003
    Location:
    SoCal LA/OC border
    To clarify the clarifier of the clarifier: that only is true for small angles of steering and should be ignored for large steering angles :)
     
  16. loufish

    loufish Member

    Joined:
    May 7, 2005
    Location:
    Burbank, CA
    Oh I agree....

    The most often I see anti-Ackerman in cars is where somebody not knowing better converts a rear steer car to front steer and simply swaps spindles....It happens more then you might think, they want to convert to R&P and their new car has the rack mounted up front, why can't their 62' Buick...:D

    I have some really basic general Ackerman concepts...
     

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  17. atomicjoe23

    atomicjoe23 Well-Known Member

    Joined:
    Feb 2, 2008
    Location:
    Silverdale, WA
    loufish. . .see I knew swapping the knuckles would do that (we are running them correctly this year), but is there something that can be done to correct for reverse ackerman if you absolutely HAVE to run your knuckles flipped.

    I would really like to know just for future reference. . .I don't plan on doing that again, but the more you know the better off you will be down the road.

    Thanks!
     
  18. loufish

    loufish Member

    Joined:
    May 7, 2005
    Location:
    Burbank, CA
    Well, you might be able to cut off the steering arms and re-weld/fab them to get some or all of your Ackerman back...If the steering arms bolt on, you might be able to install new arms to produce better geometry....These are just general ideas, each application is different and the answer might be a little different...

    Angling the arms in a front steer setup outward so that they form a line towards the center of the rear end isn't alway do-able...Many times the wheels are in the way...In cases like that, if the centerlink/rack is set-back (see last figure)you can get some Ackerman back...

    This should all be plotted out when needed for a particular application...
     
  19. fly-n-hi

    fly-n-hi Member

    Joined:
    Jan 26, 2007
    Location:
    Phoenix, Az
    Here is a few pics of our front end. It had good Ackerman (see 1st photo). The problem is that it would stop once the tie rod and steering mount were in a straight line. (See my sketch). Using spindles that have the tie rod mount in front of the spindle would definately help, unfortunately for us by the time we figured it out it was too late. You might be able to swap sides, but then your caster will be wrong. Maybe cut off the factory mounts and weld new ones on the front of the spindle. Don't you guys have a whole year before the next race??? I would make new spindles. Steering is huge part of the Mini Baja event....as you now know. Good Luck
     

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  20. spi6616

    spi6616 Member

    Joined:
    Aug 24, 2007
    Location:
    Phoenix, AZ. 85014
    No one seem to mention a very important contribution to steering that of "Kingpin Inclination" KI plays a large roll in the g-force tire patch roll on a steered vehicle. When you car is in a turning pattern the best way to get it to steer should be designed in the caster and KI that attacks the roadway to allow tire patch to flex its way to adhering to the road (asphalt or dirt). KI is the positive angle of the spindle to the pivots whether being a-arm or trailing arms (VW beam) A good number to use would be between 3 degrees 30 minutes (1/2 of a degree) which is VW to as much as 12 degrees. The more KI and caster you design in the vehicle the more corner attack you get with the tire!
     

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