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Speed advisory signs (how they are determined)

Discussion in 'Politics, Laws, Government & Insurance' started by Chef, Mar 20, 2010.

  1. I've often wondered about this, so i was pretty chuffed when i came across this article and i thought i would share it.


    It's based in America, but I'm assuming they use the same system in Australia to determine ours.

    The article was written in 2007 and i highly doubt anything dramatic has changed since then, but according to the author, the system for determining safe cornering speeds was designed in the 1930's??? Certainly explains a lot.

    There's also some stuff in there about centrifugal force to keep the nerds happy, because a happy nerd is a good nerd :)

    Oh, and it appears i can't hotlink to the pretty diagrams and pictures, so you'll need to go to the source if you want to see them ;)

  2. Over to you Spots

    Oh Looky. I'm on 888 posts...ohhhhhhh
  3. From memory, speed advisory signs in Australia are based on the speed at which you have 5 seconds of visibility of the road ahead.
  4. Probably worth quote-boxing that slab, Chef - I for one read as far as the speed limit sign stuff before I realised it wasn't you talking anymore.

    I've been thinking about this recently actually, on a couple of rides through the RNP. The speed limit switches from 60 to 80 and back depending on the corners coming up, and there are advisory signs on pretty much every corner, from 75 down to 25, and even one 15 hairpin towards Bald Hill.

    My general approach (at my current level of skill and familiarity with the roads, and my new bike) is that an advisory of 55 means I don't need to back off the throttle at all and I'll make it through comfortably. 45 means roll off slightly, and 35 means a dab of brakes before I tip in.

    I'm not, of course, actually slowing to 55, 45, or 35, but they are good references. TBH, I don't actually know what speed I'm doing on corner entry - the speedo is the last thing I'm looking at. I'm doing a speed that feels safe (but still fun!) to me given all the circumstances of that particular section of road.

    The situation is obviously complicated with traffic, gradient, camber, surface changes, and series of corners - there are a few places where an advisory sign is lower than what seems applicable for the first corner, but there is a switch-back, double apex or tightening radius involved.

    I've often wondered how those advisories are worked out - I always assumed there was a far more complicated and mysterious mathematical equation involving radius, camber, surface etc than a fishing sinker hanging off the rearview mirror of an RTA engineers ute.

    Interesting reading anyway Chef, cheers,
  5. In Australia they currently use a measure of 0.5 lateral G. Buggered if I can remember who but I was speaking with someone who has a rellie who does this for a living.
  6. Fascinating.

    BTW - here's a quote form the Ride Skill Levels in the Netrider Calendar:

    Level 5 - I am a compentent and advanced rider.
    - I can easily double the Speed Advisory (Yellow) signs on corners

  7. try again

  8. Did you know that my email sends me the original post before it gets edited?

    Is there something you wanted to share with us Lilley?
  9. No I didn't know, but doesn't really bother me. Before I edited it, it read "rubbish".

    There is no way that advisory speed on corners here is rated at 0.5g's. 0.2 g's is more realistic, but even that isn't true.
  10. Okey dokey.

    ...and you're saying this because?....
  11. because you asked if I had something to share...

    I don't get where you're going with this mate :-s
  12. because you've called bullshit, and i'm curious as to why, and what you believe the system might be for judging speed advisory signs.

    i don't really want to dance with you Lilley, i enjoy your posts and what you contribute, if you have something to add to the thread, please do. I'm interested :)
  13. I don't have an iPhone, but I'm fairly sure I've heard that there is an app that means you can use it as an accelerometer.
    So for those with an iPhone and a car with cruise control, it would be pretty easy to check (roughly).

    T'would be interesting to see the actual guidelines used in various states though.
  14. sure. at 05 lateral g's lean angle on a bike would be roughly 25-30 degrees (off the top of my head with no calculation). Not sure about in vic, but around here if you doddled around at the advised speed you would almost be vertical. While looking at a protractor, 30 degrees doesnt seem like that much, when you are at 30 degrees to vertical its a hell of a lot more. I can push up to 2.5x advised speed of corners round my way on some roads and I doubt i'm barely at 40 degrees.

    Now for the real physics. Lets take a hypothetical 45 recommended corner.
    45 km/h is 12.5 ms^-1
    0.5 lateral g's is 4.9 ms^-2
    centripetal accel is v^2r^-1
    plug these values into the formula and we have a radius of 32 metres

    Now, taking one of these corners at 120 is completely possible. sooo, 120 is 33.33 ms^-1
    radius is 32 meters. plug these into given formula and out pops 35 ms^-2 centripetal acceleration. ie. 3.5g's.
    this corresponds to a lean angle of 74 degrees, which is, as I posted so briefly, rubbish. FWIW at 74 degrees my head would be 50 cm off the road if I were sitting bolt along the centreline of the bike. Blaringmike recently posted a pic of him managing 75 degrees, here it is:
  15. oh right, i'm with you now. if you haven't already, click on the link in the first post and it will take you to the sauce, the author has some neat tables that covers what you're talking about. In the original article he talks about .2g being used and being conservative.

    So a .5 would effectively halve our speed advisory levels compared to America if it were true. Is that right?
  16. so as I said, 0.2 lat g's would be much closer to the mark, but that is still wound up in red tape. 0.2g's doesn't change with time, but whenever the speed limit drops, so do corresponding advisory signs. Just as it goes anyway, I think 0.2g's is way over the mark for the same reason.

    taking the same 45 corner set at 0.2g's, at 120 you're at 55 degrees lean, which i dont think is accurate given the size of my chicken strips - the tyre does get quite steep toward the edge but I still don't think I have got anywhere near 55 degrees on my 'osung, i should probably be hunting down stoner if I was.
  17. yeh i looked at the link when you posted it.

    No, i think your getting it backwards. It would essentially double. If they were judged at 0.5 g's, ours would be well up above the yanks, and (depending on the road) above the speed limit. If the 0.2 g's claim was accurate (which I doubt), for a standard 45 corner, the new recommended speed for 0.5 g's would be 70 k's. I garbled that a bit, but hopefully its readable.

    Oh and stuh, if you're reading this, how's my science coming along?
  18. not sure if i agree with you on this, they've changed the speed limits on the spurs down here, but haven't altered the speed advisory signs. Which makes things damn interesting if you came off and they didn't follow policy, but there you go. we'd have to know what the policy is first before it came into play.

    Here's the thing though, the roads i ride aren't consistent in their markings, one corner is dead easy at 45, and then the next is a lot tighter but still posted at 45. It's this inconsistency that makes me curious about the whole thing.

    It also seems to be regionally dependent. That is, in some areas corners are marked more conservatively than in others. I imagine Vicroads is in charge of the measuring and deciding, so it doesn't make a lot of sense to a lay person like me. Perhaps camber comes into play?

    and i know i've never achieved 55 degrees, as i understand it the max lean that's been recorded is 63 degrees by a GP bike. Blaringmike is close, but he needs to be still stuck to the asphalt for it to count ;)
  19. They've done it in the natio. Not every corner but a few. The ones that are particularly awkward for them are where it has/did have a higher advisory than the new speed limit - there was one like that standing for (i think) over a year.

    oh, they achieve pretty insane angles going down the corkscrew at laguna seca, but that is relative to horizontal, not relative to the road.