ride height

Discussion in 'GT4 Tuning' started by nomis3613, Aug 29, 2010.

  1. nomis3613

    nomis3613 Premium

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    Calling all ride height gurus, could you please help explain ride height because I don't really know what I'm doing! I've read Scaff's guide for the basics, but I'd like to go into greater details. So if you could explain how you use the ride height for your track tuning, that would be great thanks.

    What I'm not understanding at the moment is
    1) How do you know when it's too low?
    The sparks are obvious, but I've had strange handling effects when I've lowered a car not enough to cause sparks. A friend was telling me about how IRL his car now oversteers after lowering the rear, because he now doesn't have enough rear travel in cornering, so the car rests on the bump stop. This effectively makes the rear really stiff so he oversteers. I was wondering if there's a similar effect in GT4.

    2) How do you use the front:rear height ratio to tune under/oversteer? What are the side effects to look out for? When would you use ride height instead of springs/dampers/stabilisers for your tuning?

    3) Scaff's guide talks about raising the rear to increase weight transfer during braking, wouldn't raising the front also increase the weight transfer?
    My understanding is that weight transfer is proportional to rate of deceleration and CG height. So an increase in ride height at either end will cause more weight to transfer forward.
     
  2. Dan360

    Dan360

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    1. You know when it's too low when you're flying off the kerbs! Some tracks have higher kerbs than others (such as Sarthe circuit having flat ones whereas fuji 90's is a lot higher. You can use suspension stiffness to help you keep 4 wheels on the ground.

    Im not completely sure about Q2 and 3 but i hope my reply helps.
     
  3. With a EG6

    With a EG6

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    Ride Hight , Dampers , Antil Roll bars and Camber , cant stress how important this when tuning , racing a car , LSD and balance dont really matter just get 1.5 way and turn off ASM !
     
  4. Dan360

    Dan360

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    A change of reply regarding ride height that is too low. Forget my advice! Of all the cars i've tested that have flown off kerbs, none of them have spun out. The same can't be said for B-spec. A 200mph le mans racer with lower ride height will be more likely to spin out than one with 90mm+ between the ground and the bumper, providing that you are in b-spec
     
  5. nomis3613

    nomis3613 Premium

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    Thanks for the replies.

    Amen about the ASM. Can't have stupid computers meddling around when you're trying to push the envelope.

    True, spinning out is kind of a problem(!), but I also think too low can slow you down even without spinning or seeing sparks. For example, bouncing between kerbs at Paris Opera, a lower car will get thrown around more so you can't go as fast. But I'm wondering if there's a way to tell when you've hit the bump stops of the suspension?
     
  6. Dan360

    Dan360

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    @Nomis3613: Try a higher ride height, and softer suspension stifness. The wheels would still lift off the ground, but it wont take the bumper along with it.
     
  7. Scaff

    Scaff Staff Emeritus

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    Your friend in quite correct in regard to the real life issue of suspension hitting the stops, at which point you effectively increase you spring rate massively.

    The question of GT simulating this is open to debate. Certainly something happens and can happen before you start seeing sparks. Given that I would recommend stopping lowering once anything weird starts happening, as a rough rule of thumb thi is of course going to happen sooner (i.e. at great ride heights) on bumpier tracks, El Capitan being a good example.



    Personally I don't generally use front:rear height ratio to tune for under/oversteer, tending to stick to springs/dampers/stabilisers for my tuning in this area.

    It is however a personally thing and at the end of the day its using whatever gets you the end result you are looking for.


    Yes and no.

    While you are quite right in stating that raising the ride height at either end will raise the CoG height, that is not the only thing it does to the CoG.

    Raise the rear height and it will raise the CoG and move it forward (slightly), raise the rear height and it will raise the CoG and move it backwards (slightly).

    Now what overall effect this has will depend on the static weight distribution of the car and how that places the static CoG both in terms of height and for/aft location.

    GT4 makes any testing on this area difficult due to the always-on ABS (which is why I'm looking forward to testing this on GT5 without ABS), however if you look at it in general terms, the only one that will (almost always) guarantee an increase in forward weight transfer in a lower front than rear.

    Think about this example, a fwd car with a typical static distribution of 65% front / 35% rear, if we increase the front height as much as we can then static distribution will not remain the same. As we have both moved the CoG up and back. No what effect that has on the car as differing forces start to act on it is another matter entirely, however that it will move up and to the back.

    You can carry out a simple test to show this. Take a ruler, three erasers and a ball bearing (or anything that is round and will roll). Place one eraser under each end of the ruler and then the ball bearing on top of it. The ball bearing will coem to rest (as long as you don't let it fall off) at the point at which the CoG is in regard to the static weight distribution (in will be in the plane of the CoG height but not actually at it - but its a good representation).

    Now if you mark this location and then add another eraser under the 'front' of your ruler and try and place the ball bearing on its mark you will notice two things. The first is that it is indeed now height than it was (and this matches your own observation), but also that it will now roll off the 'back' of your ruler. This indicates that the CoG has shifted backwards and we no longer have th same static weight distribution.

    Now its important to note that this only shows us a general trend, not the final weight distribution or exact CoG location. It does however illustrate that while raising the front or back will increase the CoG height, it also moves the CoG centre towards or away from the front of the object and thsi will also affect it when you try and slow it down.

    Hope that makes sense.


    regards

    Scaff
     
  8. nomis3613

    nomis3613 Premium

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    Hi Scaff, thanks for all the info.

    Of course, yeah it makes great sense to just avoid the uncertainty and raise the rear to shift the weight up AND forward. Thanks for clarifying.

    Actually I find that you can easily hear when the ABS kicks in, like when I overcook the BBC. So it could be tested by setting the BBC just below the ABS threshold, then playing with the ride height. Maybe I should try this... (puts it on the To Do list)

    Yep, makes great sense. Thanks for taking the time to explain.

    Just wondering, have you ever noticed
    higher grip tyres = higher cornering force = more body roll (assuming same susp settings) = need to adjust height (or spring rate) to avoid wierdness?
    I often find I can run softer/lower when switching to worse tyres. Is this real, or a placebo effect?

    Simon
     
  9. Scaff

    Scaff Staff Emeritus

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    I've tried many, many times to work around the ABS and after a lot of hours I must admit I gave up.

    GT5P (and therefore I assume GT5) have the ability to switch off ABS completely and it makes testing so much easier and also shows how much the ABS interferes with things before it kicks in.

    We have no way of knowing if or how much ABS interferes with things in GT4, but from my own experience it certainly makes things trickier.


    In regard to higher grip tyres what you are observing is quite real. In the real world simply fitting slicks to a car can actually cause damage to suspension components as they get subjected to far more force than they were ever designed to handle.

    As a general rule of thumb you would need to up the spring rates (and adjust the dampers) to compensate.

    So its not a placebo effect, but quite real (as in GT4 and the real world), less grip from the tyres equals less force on the car and assuming everything else remains the same, less body roll before the limit is reached.


    Regards

    Scaff
     
  10. nomis3613

    nomis3613 Premium

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    Thanks!

    If I may return to the question of braking vs rake angle (eg front and rear at different heights)...such as...
    So I got curious and had a look into this, firstly by dusting off my very rusty maths, then with some GT4 testing.

    MATHEMATICAL RAMBLINGS
    I'll spare you all the maths (cos I just realised it's all wrong anyway! :dunce:) but basically I calculated that the weight transfer due to raising the CoG
    = WB * sin (theta) where WB=wheelbase, theta = angle of rake due difference in f/r ride heights

    And the movement of static CoG forward/backward due to rake
    = WB * (1 - cos(theta))

    For typical rake angles (<4deg), the effect of the height is far greater than the movement forward/backward. But, like I said, I since realised that this maths is wrong (for a couple of reasons). So it's a good thing I also did some...

    ...GT4 TESTING
    STEP 1: DOES LONGITUDINAL WEIGHT TRANSFER EVEN EXIST?
    Yes, probably a trivial question. But I just wanted to check the basic assumption before I delved in too deep. Simplest way is with an FR car, test if higher ride height causes better acceleration (due to more acceleration weight transfer improving rear wheel traction). Here are the f/r ride heights and times (test car is Monaro on SM tyres with far too much power):
    148/148: 12.23
    138/138: 12.24
    128/128: 12.27
    108/108: 12.32
    89/89: 12.35

    So, yes, LONGITUDINAL WEIGHT TRANSFER IS MODELLED IN GT4

    STEP 2: HOW DOES LONGITUDINAL WEIGHT TRANSFER AFFECT BRAKING DISTANCE?
    Ok, so it works for acceleration, but will it produce a measurable difference in braking distances? An RR car will theoretically give the most difference, so braking distance was tested on a RUF BTR2 with RM tyres (multiple distances are quoted due to the variance in results- remember that the minimum increment is 0.5m). Brake bias was set f/r 16/0 so that braking distance would be determined by forward weight transfer, yet not so high that the ABS would kick in.

    79/79: 137.5-138.0m
    110/110: 136.0-136.0m

    Excellent! 1.5-2.0m difference, enough to give meaningful results.

    STEP 3: THE IMPORTANT QUESTION: RAKE ANGLE VS BRAKING DISTANCE
    Keeping the same "average" ride height as above (110mm), first the rear was lowered, then the front (wow it was a strange feeling to drive with the noise pointing at the sky!)

    79/144: 136.0-136.0m
    144/79: 136.5-137.0m

    STEP 3: CONCLUDE
    I reckon this means that raising the front still increases grip to the front wheels (eg the effect of CoG height IS greater than the rearward movement of the CoG), but not as much as raising the rear. Also, raising the rear will increase oversteer in cornering, so if you are trail-braking this will produce the same effect as increasing the forward weight transfer.

    Feel free to question my methods, I look forward to the discussion!
     
  11. -MOPAR-

    -MOPAR-

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    When I tune ride height I base it on how stiff or soft I want my suspension to be. If I'm going to be racing a very smooth track like Fuji or Suzuka I normally make the ride height low and suspension stiff. If I was going to race on Circuit De La Sarthe or the Nurburgring I would set the suspension at 90 to 110mm and soften up the suspension a bit. My main goal is to make the ride as smooth as possibe and keep the car from bouncing around because you can't steer if you are in the air.
     
  12. sukerkin

    sukerkin

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    A variable to bear in mind with ride height tuning in-game is that we don't actually know where the CoG roll centres of the car are when we start adjusting it.

    Scaff's advice on looking for anomalies in handling behavior is spot on the money because of that and only experimentation on a car-by-car basis will lead you to an approximation of the 'right' figures for a given chassis.

    There are a couple of rules of thumb that apply in this area mind you. If you get noticeable oversteer when lowering the ride height, it is a sign that the rear suspension is attaining infinite stiffness due to riding on the bump stops, for example. If you get entry understeer then that could well be a sign that the front end is too low, again because of achieving infinite stiffness.

    It messes with your head a little as most of the mantra's are that stiffness is 'good' for a race cars handling. But what is happening in these circumstances is that the car is acting as if it had no suspension and thus leaving all the work up to the tyres.
     
  13. nomis3613

    nomis3613 Premium

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    Sukerkin & Mopar, thanks for the replies.

    After getting sidetracked before with the braking stuff, I've returned to looking at the handling effects. I used an RX-7 at High Speed ring with minimum spring stiffness and Racing Qualify tyres (to maximise grip and body roll). Here are the lap times:
    Average height (125mm): 35.6s
    Max height (162mm): 35.6s
    Min height (89mm): 35.9s
    The max height felt more "lumpy", like the extra body movement was causing increased weight transfer. And the min height kinda felt like it was hitting the bump spot but I could have been imagining this! Also, there was large variation in my lap times. So I decided to throw the keys to B-spec Bob...
    125mm: 36.3-36.7
    162mm: 36.4-36.8
    89mm: 36.3-36.7
    Again, the large variation between laps makes the results not very useful. D'oh!

    <thinking aloud>
    The "scrolling text" talks about the tyres hitting the wheel well, or the body hitting the ground. You would expect this to slow the tyre/car down, so maybe I can try this at test track by looking at how much mid-corner speed is lost for each ride height
    </thinking aloud>
    Any other ideas?
     
  14. fbccars924

    fbccars924

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    +1 on this. I think This would be the way to go for tuning. But bear in mind this is for making the best grip possible. Maybe use the multi-tune storage system for this type of theory. i.e. A-smooth tracks, B-rough tracks, C-drift, rally, Top speed, etc.