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Thread: scrub radius poll

  1. #11
    Originally posted by Fyhr:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PatClarke:
    Of course it matters!!! Think about it.

    Pat
    The only trick you need to convince Pat is: Large toe-base (say 100mm), large mechanical trail (mmm, self aligning goodness! (100mm) and give it a horribly large rear scrub (100mm?) for an acceptable solution. Then extrapolate this down to, hey we run zero rear scrub (we did) a smaller mechanical trail (why not reversed?) and you can easily allow yourself a nice easy to package 10 mm toe base!

    But in all seriousness, refer to the videos above, calculate how your forces are affected by changing scrub and what you need to do to cope with those changes and you will be close to both a solution and answers to keep design judges happy. Remeber to read this too: http://www.formulastudent.de/d...ness-and-compliance/ </div></BLOCKQUOTE>

    1) Large toe base, to maintain the desired toe while coping with compliance.

    2) Why? Self aligning is desirable? It means both rear wheels will turn out in cornering, generating less lateral force. I can't see any beneficial effect, since in turn-in you won't have lateral force, then no turn-out (which could be useful in this phase). In mid-corner and exit under lateral load the behaviour will tend towards oversteer, beneficial? It depends, generally I would say not. Am I missing something?

    3) Again, why? Positive scrub radius? It causes toe-out under braking and toe-in under acceleration. These are beneficial in some cases (e.g. may help in turn-in provided you don't spin and reduce power oversteer in corner exit), but how much? Is it important to implement a large rear scrub? And what about instability under braking?

    Thank you for your contribution!

    Vittorio

  2. #12
    Please excuse my attempt at being funny, I'll try to provide serious input.
    Originally posted by Vittorio:

    1) Large toe base, to maintain the desired toe while coping with compliance.
    Yes, very desirable, are there negative effects?
    A larger toe base may lead to a smaller distance between wheel center and the lower a-arm mount giving higher forces.
    2) Why? Self aligning is desirable? It means both rear wheels will turn out in cornering, generating less lateral force. I can't see any beneficial effect, since in turn-in you won't have lateral force, then no turn-out (which could be useful in this phase). In mid-corner and exit under lateral load the behaviour will tend towards oversteer, beneficial? It depends, generally I would say not. Am I missing something?
    I don't think self aligning in the rear is desirable, assuming we are not trying to achieve any elasticity-steer effects. But again, it may be beneficial for packaging reasons and the effects of these decisions must be handled by the same reasoning you provided above.

    3) Again, why? Positive scrub radius? It causes toe-out under braking and toe-in under acceleration. These are beneficial in some cases (e.g. may help in turn-in provided you don't spin and reduce power oversteer in corner exit), but how much? Is it important to implement a large rear scrub? And what about instability under braking?

    Implementing a rear scrub may prove beneficial as it could allow a taller upright, feeding forces to a stiffer part of the spaceframe or monocoque and a less constrained suspension geometry. But again, what are the effects?
    Thank you for your contribution!

    Vittorio
    Pontus Fyhr - Lund University Formula Engineering alumn/assistant FA

  3. #13
    About rear scrub, yes, ridiculous large scrub might allow you an easier packaging (essentially the pickup points are not constrained by the rim size) but think about the force at the toe link/fifth member....

  4. #14
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    Problems with rear scrub radius?

    Use beam-axle, or swing-arms (lat, long, or semi) - no problems!

    Z

  5. #15
    Hi Everyone. My name is Viv from University of Toronto. I figured as my question relates to a few comments in this thread, I would start this thread again.

    I am looking to use tire data to determine the ballpark scrub radius I should be looking at. Above it was mentioned that Mx will give u an idea of tire deflection. So I took the Mx and divided by the normal load (I think the primary force causing the Mx) and this provided me the lateral tire deflection. Unsurprisingly, the tire deflects towards the direction of turn (by about 30 mm). The more interesting thing was the variation in this with braking and accelerating. The tire deflection varies from -10 to -70 mm in accelerating and braking respectively (as per the SAE J670 sign convention). What I was trying to figure out is what this would mean when the tires are on the car considering u don't know which way the tire is on the car in comparison to the testing (i.e. the tires aren't asymmetrical by design). I might be looking at some value here that is meaningless not sure though.

    The second question is related to the Mz result from tire deflection. Am I correct to assume that the Mz resulting from this tire deflection and the drag would be included in the tire data Mz?

    Thanks in advance.

  6. #16
    To my mind, I'd think that contact patch movement under acceleration or braking would be the result of static toe meaning that 'car longitudinal' (x) forces (accel, brake) have a 'tyre lateral' component which causes some contact patch movement. Failing that, I suppose with some static camber applied your tyre won't really behave in a symmetrical way anymore under longitudinal load. Failing that, I dunno, sorry!

    I've got a couple of semi-unrelated questions; who all approaches front scrub radius from a steering feedback perspective? I would think that scrub radius has a fairly strong effect on steering load under brakes - does anyone care? Why/not?
    MUR Suspension & Steering '11
    MUR Suspension advisory/annoyance '12
    BJR data engineer '13
    Maker of things

  7. #17
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    It's a major contributor to steering loads, so it's very important to analysis it from that perspective as well. We made a spreadsheet that used scaled tire data to calculate steering loads for old vehicles of ours and then compare that to our new designs.
    Trent Strunk
    University of Kansas
    Jayhawk Motorsports
    2010-2014

    Now in NASCAR land. Boogity.
    Opinions Are My Own

  8. #18
    Quote Originally Posted by theTTshark View Post
    It's a major contributor to steering loads, so it's very important to analysis it from that perspective as well. We made a spreadsheet that used scaled tire data to calculate steering loads for old vehicles of ours and then compare that to our new designs.
    Have you physically tested it?

    We did physical testing by changing the construction of some 3 piece wheels from 50mm scrub to 100mm scrub and found it had very little impact on steering input force required. (It did also adjust vehicle track at the same time though so wasn't a completely isolated test).

    Our Adams simulation also supported this - that there would be little increase in steering force.

    Based on this we ran 70mm of scrub radius.
    You mechanical and pneumatic trail will have a significantly greater inpact of steering force and
    Curtin Motorsport Team
    2011 - 2014

  9. #19
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    Scrub a dub-dub

    What test procedure did you use for the steering effort? With any kind of kpi, caster and rear roll stiffness, you should have picked up a tierod load gradient (slope of Force per SWA which transcribes to Force per g using a little math).

    There is a nice test procedure for a direct measure of scrub radius if you happen to be clever and can think inside of a box. [No electrons need be harmed in the pursuit of this information]. Make sure you measure BOTH wheels because ... [ well let's just say that things can get interesting when more than one of something is assembled by 'SelectCreature _Callback'.]

  10. #20
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    Hey Trent: Say "Hello" to Dave Rogers from Terri & me down there in Huntersville. Been a while...

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