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Thread: Kart steering in FSAE

  1. #1
    Couldn't find the old thread on this one, so here we go.

    I seem to remember from the previous discussion that the main reason most were going for rack and pinion over a kart-style system was bump steer considerations.

    I figure that if I can come up with a front geometry with the lower inboard a-arm points very close to the car centerline (think F1-style single keel here, but simpler and probably metal), and mount my kart steering arm such that the tie-rods are in the same plane as the lower-arms (or pretty close), I could get some very acceptable bump steer values.

    I guess what's really bothering me about such a design is what happens to the bump steer/ackermann/steering linearity once you turn the wheel.

    We've usually designed our steering to minimize bump steer to the outside wheel for some small steering angle. Our logic here is that a small change in toe will not have a significant effect in straight line performance, but a change in toe to the more heavily loaded outer tire in a high speed wide-radius turn would probably put the driver off. If anything, I'd think that a kart system could be designed such that you could decrease bump steer to the outside wheel. Do any other teams take a similar approach to ours for bump steer (i.e. design for some steering angle instead of straight ahead condition)? Has anyone looked into bump steer for kart steering systems?

    The major point that I think would be problematic would be steering linearity though. We see about an inch and a half of rack travel in either direction, so let's say with a 3inch arm on the kart steering system and each inner tie-rod point offset an inch to either side, we're looking at the inner tie-rod points moving longitudinally about an inch over all the wheel steering angle we want. What potential effects do you think would happen as a result of this. I'm pretty sure any ackermann effects would probably be very non-linear, but could it be possible to tune to vary the ackermann in the direction you want? Like ackermann steer in wide-radius high speed turns to minimize drag, and maybe more parallel in tighter turns to put some heat in the tires and increase your yaw moment.

    Mostly just thinking out loud, but I would like to know your take on this. Especially how everyone goes about determining where they want no bump steer.

    Matt Gignac
    McGill Racing Team

  2. #2
    Couldn't find the old thread on this one, so here we go.

    I seem to remember from the previous discussion that the main reason most were going for rack and pinion over a kart-style system was bump steer considerations.

    I figure that if I can come up with a front geometry with the lower inboard a-arm points very close to the car centerline (think F1-style single keel here, but simpler and probably metal), and mount my kart steering arm such that the tie-rods are in the same plane as the lower-arms (or pretty close), I could get some very acceptable bump steer values.

    I guess what's really bothering me about such a design is what happens to the bump steer/ackermann/steering linearity once you turn the wheel.

    We've usually designed our steering to minimize bump steer to the outside wheel for some small steering angle. Our logic here is that a small change in toe will not have a significant effect in straight line performance, but a change in toe to the more heavily loaded outer tire in a high speed wide-radius turn would probably put the driver off. If anything, I'd think that a kart system could be designed such that you could decrease bump steer to the outside wheel. Do any other teams take a similar approach to ours for bump steer (i.e. design for some steering angle instead of straight ahead condition)? Has anyone looked into bump steer for kart steering systems?

    The major point that I think would be problematic would be steering linearity though. We see about an inch and a half of rack travel in either direction, so let's say with a 3inch arm on the kart steering system and each inner tie-rod point offset an inch to either side, we're looking at the inner tie-rod points moving longitudinally about an inch over all the wheel steering angle we want. What potential effects do you think would happen as a result of this. I'm pretty sure any ackermann effects would probably be very non-linear, but could it be possible to tune to vary the ackermann in the direction you want? Like ackermann steer in wide-radius high speed turns to minimize drag, and maybe more parallel in tighter turns to put some heat in the tires and increase your yaw moment.

    Mostly just thinking out loud, but I would like to know your take on this. Especially how everyone goes about determining where they want no bump steer.

    Matt Gignac
    McGill Racing Team

  3. #3
    Matt,
    I would thank that the even if bump steer were not a problem, R&P steering would be far better with respect to ackerman than kart steering would be. R&P steering would not weigh significantly more.
    ----
    Mike Cook
    It's an engineering competition, not an over-engineering competition!

  4. #4
    I'm not sure how the device at the center of your steering linkage is going to affect your ackerman angle ??
    Cheers

    -Some people need to get an ice-cream sandwich,
    -Cement Legs needs to get an ice-cream sandwich

  5. #5
    Two issues I see with Kart steering:
    1) kart steering systems are designed to turn a much lighter vehicle, which makes me think that the steering arm would have to be vary large to be stiff enough for good steering feel and response
    2) Say you have a 3" arm like you talked about- that means you have to have a 6" diameter (at least) clearance area around the head of your steering shaft to package your legs, rollbars, whatever. In our car, the head is about 1 3/4" across. We would not be able to package such a system, at least from the quick and dirty solutions i tried to come up with on our new frame.
    "I couldn't find the sportscar of my dreams, so I built it myself" -Ferdinand Porsche

  6. #6
    this idea has also been considered by us. I don't think that that steering system, only the wheel, has to be in the vehicles roll planes, so if you determined you needed a 6" arm on your shaft, you could make it work.
    Mike Duwe
    UWP Alumni

    Former Drivetrain Leader and Team Captain

  7. #7
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by KURacing:
    1) kart steering systems are designed to turn a much lighter vehicle, which makes me think that the steering arm would have to be vary large to be stiff enough for good steering feel and response
    </div></BLOCKQUOTE>

    Steering arms don't necessarily have to be larger or stiffer. It just depends on your upright geometry and the rotary geometry on the "kart" steering system. Large moment arms on the spindle and the steering rotor would keep the forces down, so all you really have to worry about would be cone impacts if you are using a front steer setup.

    Also, seems to me like bump steer would be the same with a rotary system as it would be with a R&P type system. Just as long as the axis of rotation of the steering rotor is perpendicular to the ground, that way you can design the steering arms to be in the same horizontal plane as the upper/lower control arms.

    A kart rotary style steering system is definatly a cool idea and it gets rid of a heavy steel rack gear and pinion gear. And the slop induced from improper gear placement is removed.

    As far as ackerman goes, you could setup the rotor to be your ackerman adjustment or you could place your tie rod's so that the rotor has no effect on ackerman what-so-ever

    Seems like a cool idea, might look at using that for our 07 car. =]
    Erich Ohlde
    Jayhawk Motorsports
    FSAE 04 - 09

    All electrical components and wiring harnesses depend on proper circuit functioning, which is the transmission of charged ions by retention of the visible spectral manifestation known as "smoke". Smo

  8. #8
    come on now. what do u suspension guru's think?
    Erich Ohlde
    Jayhawk Motorsports
    FSAE 04 - 09

    All electrical components and wiring harnesses depend on proper circuit functioning, which is the transmission of charged ions by retention of the visible spectral manifestation known as "smoke". Smo

  9. #9
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">A kart rotary style steering system is definatly a cool idea and it gets rid of a heavy steel rack gear and pinion gear. And the slop induced from improper gear placement is removed. </div></BLOCKQUOTE>

    Another solution would be a properly designed and manufactured aluminum rack and pinion.

    When you spend a few hours each year designing your rack placment in ADAMS trying to get the exact right nonlinear ackerman curve without any bumpsteer, the rack and pinion is very easy to model, the gear calcs are a cinch, and it can be jigged extremely accurately (so you are confident that what you build is the same as what you designed).
    Buckingham

  10. #10
    Id suggest you want a fair bit of movement at the steering wheel, therefore the "kart linkage" isn't feasible

    We use -100deg left 100deg right at the steering wheel (200 deg total):

    Which corresponds to 35deg unloaded wheel and 25 deg loaded wheel.

    40 scrub 7 castor 20 trail kingpin = camber = -2 static

    Now you COULD sacrifice the amount of steer at the wheels a LITTLE on our car; but again I think not enough to make the "kart linkage" feasible.

    Regards

    Frank

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