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Thread: forward vs rear tack width.

  1. #1
    Is it necessary to have the front track wider than the rear track? What shall be the outcome of increasing the rear track width ? Because,as far as I believe ,rear track plays least role in the steer part at least
    Aseem Singla
    Joint Co-ordinator, 2008
    Suspension Co-ordinator, 2007
    Indian Institute of Technology, Delhi, India

  2. #2
    No.
    Increased rear grip.
    Maybe.

    The traditional reason for the front track to be greater than the rear track was that the rear tires were wider than the front tires. If you designed to the same over-all width, the wider rear tires resulted is a smaller track dimension.

    On front wheel drive cars it's not uncommon for the front track to be narrower, with wider front tires, for the same reason.

    You'll need to set your track dimensions based on your design criteria and goals.

    Brian

  3. #3
    not sure where I read this but (possibly a carrol smith book). The reason the rear track width isn't wider than that of the front is for stability, e.g. imagine steering hard on a tricycle. Also, the rear track follows a slightly tighter radius when going around a corner, this can cause problems such as clipping cones when the front/rear tracks are the same.
    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

  4. #4
    I cannot believe that hitting the cone on the inside rear has not been brought up. Those 2 seconds can kill an auto cross lap. Just one thing to keep in mind.

    EDIT: HAHA missed the last part of your post jayhawk...my bad indeed.
    John "Jack" Vinella
    University of Washington Alumni 06' 07' 08' 09'

  5. #5
    lol
    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

  6. #6
    A difference in trackwidth will affect both inelastic and elastic loadtranfer. Springrate and antiroll bars affect elastic loadtransfer and rollcenterheight decide the proportion of the inelastic loadtransfer. I guess you can play with this to get some desired transient characteristics. Correct me if I'm wrong...
    Sirius Racing
    www.formulasae.ltu.se

  7. #7
    You are correct but looking at the tire data and their load sensitivities, would a really wide rear track gain you much? Everything on the car is a compromise. Especially things in suspension.
    John "Jack" Vinella
    University of Washington Alumni 06' 07' 08' 09'

  8. #8
    At steady state maximum cornering I challenge someone to show me that the rear still tracks inside the front. In this case the outer tires, at least, are at max slip angle just like the front and are in fact yawed out.

    A truely over the top case is Nascar when a driver gets close to the inside white line. The fronts can be touching the line but the rears are 6-8" off the line. Some of what allows this is aerodynamics but if you take the aero away the rears would still not track inside the fronts.

    Now in transitions and slaloms chicanes ect. where the rear hasn't reached max slip angle yet or is returning to center is a different story.

    Also it has an effect on the diffs ability to transfer power effectively. Cars with spools are usually built with a measurably smaller track on the rear which lessens the speed difference on the inside and outside tires during cornering, and the corresponding push.
    'engine and turbo guy'
    Cornell 02-03

  9. #9
    Good points and interesting observations on where the rear goes 'engine and turbo guy'. I would propose though that you may have generalized a few observations too far the other way. If you reduce things back to a slow wagon going around a corner, clearly the rear cuts to the inside. By observation, you point out that some racecars actually do the reverse with the rear running around the outside. If you read up on my current ride, an old '92 Miata, you'll find that the engineers at Mazda intentionally tuned the compliance of the rear wishbone mounts to provide a certain amount of steering input from the rears as a result of side load on the wishbones in a corner. I suspect that the folks in NASCAR could be similarly savvy. I'm not certain that many of the FSAE teams are likewise as skilled at manipulating compliance to passively achieve steering input to control where the rears walk. That's not intended as a knock. I think most of us could agree though that most teams have bigger problems than the perfect steering balance and tracking. Keeping an amateur-built car running for 60 minutes can be a challenge in itself.

    I think Brian said it best at the top. Packaging with wide rears drives the whole situation. I can imagine some odd confrontations in passing if a car was appreciably wider at the tail than at the front. The only reason to widen the rear a great deal would be to increase the size of a flat bottom or tunnels between the wheels for a downforce improvement. Given the low CG and relatively wide stance of modern racecars, additional platform stability would come at the cost of increased unsprung weight. With the stiff springing of a downforce car, you also have a huge deal of roll stiffness before installing sways for front to rear roll stiffness trim. Maybe without downforce there's a stronger argument.

    For taking the hit in drag though by increasing frontal area for rear downforce, why not take the free width and widen the front likewise to improve downforce at the nose too? In most cases it seems downforce at the nose to maintain balance is more difficult to come by than downforce generated at the rear with a diffuser behind the rear axles or a wing on top. By the same argument, if you put the width in rear for roll stability, why not go for as much as you can up front too?

    -Not an expert, but I do see some sense in the traditional layout.
    Kevin

  10. #10
    VFR750R is dead on.

    At a certain point along the lateral acceleration line (0 to max), vehicle sideslip changes from positive to negative. Meaning the rears go from tracking along the inside to the outside. This zero point is dependant on the setup of the car but it always happens. Even in very transient courses like the ones we see at FSAE, this phenomenon is still observable.

    We chose track based on getting the car to hanlde the way we want it to. You can train good drivers not to nip cones. That said our rear track is thinner than the front...

    Bryan

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