Welcome to ZRoadster.org - BMW Z1 Z4 Z8 Z3 Forum and Technical Database

If you want to join in with the discussion, and see the areas which are available only to members then sign up now!

  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Use of ARB's / What does it do?

Discussion in 'Z3 Roadster & Coupé' started by Stormy_be, Nov 22, 2013.

  1. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    I was not sure the other day of the effect of the ARB's.

    I know that it mean that it tries to make the left and right wheels make the some movement up-and-down.
    When the left goes up, but the right doesn't is will "fight" the upwards motion.
    Like in corners, the outside wheels are pushed upwards (the car moves down), the inside wheels have sell weight and move down (car moves up). This action is made harder by the ARB's that resist that.

    That's the physical work of one ARB's.

    But the thing I'm struggling with is the front/rear effect.

    (it wouldn't make sense, but just to give the extreme example)
    What if I would only fit an ARB to the front and not to the back?
    The fronts would stay more flat, the rear tires would have more effect of the weight shifting.

    Would that result in more over or more understeer?

    Could someone explain the logic behind the answer?

    Thanks, Koen
     
  2. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    Strange that there is so much discussion about this, but not anyone can explain the logic and working behind it....
     
  3. Mr-P

    Mr-P Zorg Addict

    Joined:
    Feb 10, 2013
    Messages:
    400
    Likes Received:
    185
    Trophy Points:
    62
    Location:
    Oxfordshire
    Model of Z:
    Sold Z3M
    The fitting of larger sway bars (rear and in general) has two main effects, vehicle balance in terms of understeer and oversteer, and increased roll resistance. Both of these can provide increased overall grip levels that can be achieved by the vehicle.
    As most factory vehicles are biased towards understeer, fitting of the larger rear sway bar will help in providing a more neutral characteristic in the handling at the limit. This is due to the increase in roll stiffness at the rear, which loads the rear wheels more unevenly and provides slightly less grip at the rear than previous.

    So the basic is Rear effects what happens at the front. Search the web and there are many expert articles.
    I the past on other cars I have just run a rear arb depending on what I wanted from the car.
     
  4. zedonist

    zedonist Guest

    The ARB's are there to control the sprung weight of the car and keep the drive wheels in contact with the road during cornering. It is correct above that road cars are set up with a neutral effect.

    On the Z3 there are three different thicknesses, the 1.9, the 2.0 - 3.0 and the //M. The 1.9 is the smallest diameter both front and rear, and the worst performing around corners, you can improve this by fitting the matched sets from any of the other two versions to keep a neutral effect.

    The ARB is in fact a big torsional spring, and it resists the effects of the weight of the body of the car and occupants as it transfers during cornering, on the 1.9 this means that cornering at the max cornering speed of an //M for example would mean that the cornering weight of the vehicle overcomes the resistance of the spring and the rear wheel will lift, causing a loss in traction. The //M does not experience the same effect as it's ARB's are thicker, I.e. A stronger spring and readily resists the body weight to a higher degree keeping both rear wheels on the ground at this speed.

    It is worth noting that it is the front ARB that is doing the work of keeping the rear wheels on the ground on a rear wheel drive car, hence thicker front than rear, it is the other way round on front wheel drive cars.

    H&R ARB's are only slightly thicker than the //M, but they offer a better response because they are made out of a different grade of alloy steel, and heat treated to a different toughness range, giving rise to stronger poundage spring over the //M.

    I have the H&R fitted to my 1.9, and the handling is phenomenal.

    I hope this helps, it us very simple terms, there is a lot more detail on the web, if you need it.

    Sometimes a picture helps:

    ARB PICTURE.jpg
     
    #4 zedonist, Nov 25, 2013
    Last edited by a moderator: Nov 25, 2013
    • Like Like x 1
  5. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    Thanks for this info, it does help.

    What I was triing to figure out is:
    On the H&R ARB's there are 2 setting.
    What is the effect when stiffing up the front or rear (without the other).

    Or as you mention it is the front ARB that keep the rear wheels on the ground, what if I was to replace only the front ARB to H&R and leave the rear standard?

    There is a click that I'm missing.
    May be try to search to web....but there is so much info (some even incorrect)...
     
  6. zedonist

    zedonist Guest

    Just changing one ARB, will put you in over steer or under steer, so it depends if you wish to drift round corners or drift into hedges.

    The adjustment on the H&R just shortens the lever effect and thus makes the spring stiffer adding a bit more resistence. I have mine on a neutral setting, but I am sure you can try different variations on track car to see which works best.
     
  7. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    I understand that doing 1 will result in over or understeer.
    But I don't grasp if have more anti roll in the front would give over or understeer (which of the 2?) and why.
    Same applies for rear...more anti roll at the rear --> result in over or understeer ?
     
  8. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    this is what I found after a couple of hours of searching (this 2nd part is what I was after)....now I still need to understand it...
    Anti-roll bars provide two main functions. The first function is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.

    The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer behavior can be tuned out by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total load transfer that the front axle reacts and decrease the proportion that the rear axle reacts. In general this will cause the outer front wheel to run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.
     
  9. zedonist

    zedonist Guest

    Probably comes down to the effect it has on the suspension geometry as a whole, and in extreme cases I.e. An under steer application the force if the ARB has overcome the coil springs and natural camber adjustment, which then reduces the contact patch of the tyre to the ground, and grip is lost at the front and the car slides.

    This is why you can never eliminate body roll, because at a certain point the ARB has a negative effect such as under steer.
     
  10. EnthuZiaZT

    EnthuZiaZT Zorg Guru (V)
    British Zeds East Anglian Crew

    Joined:
    Dec 6, 2011
    Messages:
    4,617
    Likes Received:
    2,214
    Trophy Points:
    193
    Location:
    Heacham Norfolk
    Model of Z:
    Z4 28i Msport
    Please remember that fitting thicker ARB's also reduces the movement of the suspension. While this may be an advantage on the race track or on a smooth road, it can be a disadvantage on normal bumpy roads. Stiffen up to much and you will find your self sliding more often. This can seem exciting, but can also be very dangerous on narrow country lanes in muddy or wet conditions. Larger engined Z3s are heavier at the front than 1.9s, the ARB's for these cars will be carefully matched to that weight.

    Mike
     
  11. Stormy_be

    Stormy_be Zorg Guru (I)
    Belgian Zeds

    Joined:
    Oct 29, 2013
    Messages:
    745
    Likes Received:
    470
    Trophy Points:
    97
    Location:
    Belgium, ieper
    Model of Z:
    Z3 2.8 Roadster
    My car is a track-only car. So it can get to extreme, but then it needs to be very extreem hard.
     
  12. zedonist

    zedonist Guest

    I would expect track conditions and layout would decide which ARB setting is required, and the ability to go from hard to soft would be advisable, it's the reason you get practice sessions before a race.

    For the road, I find it is best to drive within the cars limits, and the driving conditions, anyone going hell for leather down a muddy wet country lane, unless a rally driver, basically deserves to go in a bush.

    I have the Bilstein B12 kit and the H&R ARB's fitted to my 1.9, the ARB are thicker than the six pots and the neutral handling remains, but body roll is very much removed, giving a much more planted and confident feel around bends. it is also the same kit for all variations of the Z3 except the //M, which has a thicker rear, so one can assume they are not matched for weight, as I also assume this is taken up by the road springs which do have different poundage across the range.
     
  13. swamper

    swamper Zorg Guru (V)
    Supporter M Power

    Joined:
    Dec 4, 2011
    Messages:
    2,597
    Likes Received:
    815
    Trophy Points:
    181
    Location:
    Mossley
    Model of Z:
    z3m
  14. zedonist

    zedonist Guest

    Good buy if you can get them for a tenner
     
  15. swamper

    swamper Zorg Guru (V)
    Supporter M Power

    Joined:
    Dec 4, 2011
    Messages:
    2,597
    Likes Received:
    815
    Trophy Points:
    181
    Location:
    Mossley
    Model of Z:
    z3m
    do they fit on a non M?
    with the difference with the drop links....
     
  16. zedonist

    zedonist Guest

    I don't know, just think a good buy for a tenner for an M owner to get powder coated etc.
     
  17. swamper

    swamper Zorg Guru (V)
    Supporter M Power

    Joined:
    Dec 4, 2011
    Messages:
    2,597
    Likes Received:
    815
    Trophy Points:
    181
    Location:
    Mossley
    Model of Z:
    z3m
    i dont think they do...the drop links on the M are much longer IIRC
     
  18. zedonist

    zedonist Guest

    Yes they are, the front suspension on an M is completely different, the top mounts are offset to give more castor angle also.