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Thread: UTAS build 2015

  1. #91
    Quote Originally Posted by Z View Post
    Jonny,



    I sent a PM to Adam about the heavy steering issue.

    But for now can you give your numbers for,
    1. KPI (aka SAI = Steering-Angle-Inclination, from vertical, in end-view),
    2. Castor (angle in side-view),
    3. Offset (aka "scrub radius" = end-view distance between SAI-ground-intersection and nominal-centre-of-wheelprint),
    4. Trail (side-view distance ...),
    5. Degrees of full hand-wheel movement (lock-to-lock),
    6. Corresponding degrees of front-wheel movement (lock-to-lock).

    By FSAE standards you have more than average weight on the front wheels, plus biggish sticky tyres, but once the car is rolling a good steering geometry should give very light steering feel.

    Z
    Don't need to give away everything.

    Johnny,

    What's the steering ratio of the current car? As in (front tire degrees) / (steering wheel degrees) ?
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  2. #92
    Billcobb is correct that friction can play a large part in steering effort - we had a significant increase in our steering effort purely due to how we staked our spherical bearings in the a-arms increasing the friction.

    I would also recommend physically testing any of these changes - it can be quite easy to allow for a lot of these setting (kpi, castor, scrub, trail) to be adjustable during design. One thing we were told by some "experienced FSAE people" was that our scrub was too big and this was the cause of our heavy steering. This was easy to test with rebuilt wheels with different wheel shells increasing our scrub radius from ~60mm scrub to ~100mm. The outcome - it had no noticeable effect in steering effort. So ensure you test yourself!.

    We found that significantly reducing our mechanical trail (down to 0mm - which i'm sure ill get told is too low...) and slightly reducing the steering rate had the biggest impact on reducing our steering effort (and also significantly increased feedback to the driver). This corresponded to a ~1 second a lap improvement over a track ~60seconds long. This small change was likely one of the biggest improvements in the design of the car around those years (was much more significant than the ~15kg of weight dropped that year) - But don't take my word for it test it for yourself.

    Our settings back then were:
    1. KPI = 0
    2. Castor = 4.5deg
    3. Offset/Scrub radius = 60mm
    4. Trail (side-view distance ...) = 0mm (was adjustable 0mm to 40mm)
    5. Degrees of full hand-wheel movement (lock-to-lock) = 200deg
    6. Corresponding degrees of front-wheel movement (lock-to-lock) = cant remember this one from memory - was adjusted until we could easily make the minimum radius hairpin as per rules
    Curtin Motorsport Team
    2011 - 2014

  3. #93
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    spew

    Lock to lock references are irrelevant to the effort recipe. What we/you want to know is what the effective steer arm length is and what either the pinion radius or the pitman arm length is. If this is a State secret, post it on Hillary's server. On grease plates, you will be able to feel/measure the effect of rear suspension roll stiffness, especially with that much caster. That's not a criticism of your caster setting, for some tires, that's about right. The question is what the max steering wheel rim force your driver feels comfortable with and would you dare run a 300 mm rim diameter to get it (for example)?

  4. #94
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    MCoach,

    Given their "heavy steering", I don't think UTAS has any comp-winning steering-secrets to keep!
    ~o0o~

    Westly,

    Although it is feasible to adjust Trail independently of Castor, it is not common.

    Did your system allow this? Could you keep Castor fixed at 4.5 degrees, while varying Trail 0 - 40 mm?
    ~o0o~

    Bill,

    I agree that friction could be a contributing cause to their heavy steering, but I imagine Jonny and Co would have noticed that in their more than a year of struggling with it.

    Yes, "effective steer arm length [to] pinion radius" ratio is the important number to know, but that number varies a lot over the full range of steer-angles, so requires a curve to show the full range of ratios. Just knowing the two lock-to-lock numbers gives a good average ratio to start from.

    For example, if they had a total of 100 degrees of hand-wheel movement, AND they could also get around the hairpins (though I think they struggled there), then I would suggest they lower the ratio (ie. try 200+ degrees of hand-wheel rotation for same amount of front-wheel-steer).
    ~o0o~

    Anyway, I didn't get a close look at their geometry at comp, but I reckon the problem is a down to some of the above numbers.

    Z
    Last edited by Z; 12-18-2015 at 09:11 PM.

  5. #95
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    I Z your point

    OK, that makes sense to me, I'm not used to counting lock to lock because its just a rack length on a wide car with a specific wheelhouse size for a required turn radius a lot bigger that for these cars.

    I suggest an additional steer effort puzzle piece: screwed up (by design) or by careless assembly of U-joints in the steering column and intermediate shaft.

    It only takes a few mm of mis-position to create these effort changers. One condition might actually help if you can accept the reduction in gs by steering gain. Determining these geometricly induced ratio changes is just simple trig analysis of co-linear input and output shafts via Excel. A single u-jointed steering part actually has a lot of potential to address the steering gain and effort requirement/goal. There are quite a few production vehicles who use this technique instead of springing for an expensive nonlinear R/P gear.
    Attached Images
    Last edited by BillCobb; 12-18-2015 at 09:45 PM. Reason: spilling czech

  6. #96
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    Was that asymmetric one from the far east by any chance?

    Or west from your reference frame..

  7. #97
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    Ratio Diversity

    The pix I posted are from one of my simulations which accepts the coefficients for a ratio function usually determined from measurements of test cars. The function acknowledges ratio variation due to bad i-shaft geometry, bowed racks having designed in ratio nonlinearity, and asymmetry due to steer arm height and length choices or oversights. Test output data channels then become obfuscated by the nonlinear steering control.

    A summary from thousands of such tests reveals manufacturers who don't care, some who are anal about it, and some who we don't know about because somebody took the steering system apart to run cost and content studies on it and didn't have the proper tools or savvy to put the stuff back together correctly.

    Some Euro-sleds make use of a deliberate INVERSE valley (which corresponds to a steering torque valley) and others which attempt to address an on center understeer softness by using a valley ratio to increase the gain as you corner more. When there was an understeer test result with bizarre asymmetry, first thing to suspect would be a soft tire, or a bad suspension bushing or a wheel alignment problem. Not finding anything wrong, I started measuring ratio from a differential of the steering wheel to road wheels function. Then, correcting the understeer calculation for a new and improved ackermann gradient produced a perfectly wonderful looking symmetric set of cornering compliances and understeer. Correcting the steering parts then produced a much different feeling car, too. It didn't take long for the responsible Development Engineer to learn the relationship between how the car felt and what the likely steer ratio issues were. That's what's expected in the Profession.

    But, back to the message: a FSAE car with one Cardan type joint in the upper steering system can acquire quite a range of gain or effort attenuation if the design is implemented on purpose with forethought.

  8. #98
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    Play Time: Fun With Steer Radios

    See if you can get this working in Matlab. I've punched up a GUI deal to play the graphics. Run the '.fig' file. This generalized function can pretty much deal with the entire world of good and evil steering ratios. Credits to my buddy Keith Adams for dreaming this up. If any of you Rembrants come across some data for your car, try it out ! Use "lsqcurvefit" to punch out the coefficients.
    Attached Images
    Attached Files

  9. #99
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    Quote Originally Posted by BillCobb View Post
    But, back to the message: a FSAE car with one Cardan type joint in the upper steering system can acquire quite a range of gain or effort attenuation if the design is implemented on purpose with forethought.
    Yes, indeed. And I recall UTAS having two UJs in there...

    Given the many pages spent discussing UJs on the UTAS-14 thread, I hope they got this right. But if not, then easy fix.

    Just re-clock the UJs!

    (May require new hole drilled at 90 degrees to existing hole...)

    Z

  10. #100
    All this talk of steering on the UTAS thread! The car, as I drove in endurance, wasn't all that bad. There is reduced lock (hard to miss all cones in tight bits) and the steering is a bit quick, but not disastrous. (Quick steering confirmed from feedback on the driver swap day).

    This maybe a sore point for the team because some members are now against me, saying I have said wrong things or misrepresented the team on this forum. I'm not changing any of my comments, as my stories are my own account to the best of my knowledge, and if I quote a measurement from our car, it is often directly after measuring it with a ruler myself, even if an approximation is needed due to an awkward angle. I was "ergo" leader at the start of the year but other people followed that up as I became electrical leader.

    While different uni joint angles maybe interesting, I don't think it is directly applicable because we have a double cardan joint which I believe equals a CV joint. I/we tried to get an equal angle on each uni joint, however when looking from above the 2 joints are misaligned by about 2mm. I think I have the basics covered as far as looking for friction in different joints etc. When I place a rag under each front wheel the steering is significantly lighter. And it is not hard to calculate a ratio... Regardless of what the exact design was, I can now photograph and measure our actual car, as it was set for endurance. For your information...

    Steering wheel lock: 180 degrees total
    Steering rack travel: 56mm total (limited with spacers so rims don't hit wishbones).
    Toe-base: about 83mm +/- (I measure 63mm original plus 20mm spacer, this is disputed as I'm told I don't understand where to measure).
    Steering tyre angle: as pictured (not quite enough).

    columnbrace2.jpgIMG_2022.jpgIMG_2024.jpgIMG_2025.jpgIMG_2026.jpg

    You may see the steering column bearing is plain bearings instead of cartridge ball bearings, with a stainless steel column in IGUS plastic bushes, something I tried this year and it seamed to work well except for just a small amount of movement.
    University of Tasmania (UTAS)

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