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Thread: Z8 brakes

  1. #1
    Team Z8 RRZ8's Avatar
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    Z8 brakes

    From what I understand (technical manual) the Z8 has a 2 circuit vacuum boosted brake system which is split in front and rear.
    Can anyone explain what happened to the diagonal split, and/or tell us more about the Z8 brake system ? Is it the same system used in the 7 series ?

  2. #2
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    Must be part of the retro design

    Sorry, that's a great question, and I wish I knew the answer.

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    The brakes used on the Z8 are supposed to be the same ones used on the armored version of the 7 Series sedan from the same era. Those cars weighed 7000 lbs. so their use on the Z8 was considered more than adequate. I believe the 750i used the same brakes. The front calipers are dual piston floating designs with a corresponding disc diameter of 13.1". The rear calipers are single piston floating designs and they grip 12.9" discs. The parking brake is a drum type contained within the rear hubs. All modern cars have a dual brake circuit, split between the front and rear brakes, to insure some braking ability is retained should one of the systems fail. I have never heard of a diagonal split in a brake system. Can you elaborate?

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    Nearly all brake circuits are split diagonally (at least on European cars), and have been for a long time. When the split design first came about, most were split front/rear. It was determined that a diagonal split was safer.

    No idea why the Z8 is split front/rear. There are 3 lines coming off the master cylinder, but I can't tell where they go (beyond the ABS unit)

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    Wow, you learn something new every day! However, I must take issue with some of your statement. Apparently, the diagonal split brake system was developed to complement front wheel drive vehicles, where it is now commonly used, but it is rarely used on rear wheel drive vehicles. That explains why I have not heard of it until now: I just don't pay much attention to front wheel drive vehicles. The diagonal split system may be safer on a front wheel drive vehicle but that is not the case with a rear wheel drive vehicle and as far as I know, the brake systems on all BMWs are split front to rear, aka vertical split. So the Z8 is not unique within the BMW line-up.

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    You're right. Too many years working on Volkswagens, I guess. I'd thought all cars were diagonal these days.

    Diagonal still makes more sense to me, even on front engined cars. I guess ABS makes it easier, but I'd hate to rely strictly on the rear brakes for a panic stop!

    Not that LF/RR or vise versa would be much better

  7. #7
    Z8 Ate My Homework! Norcal's Avatar
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    Brake Bias

    GM (or anyone else). Do you ever feel the brake bias is to much to the front end? I've always noticed, upon heavy breaking from high speed, that the front, not rear, can really skip around if the surface is not super smooth, or has any dirt on it, looseness etc.. It is very unnerving. It's never felt that way at the track, but rather on back roads etc.

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    Did some more research and found out that 2 of the leading car producers in the world, BMW and Mercedes Benz, use the front/rear split. I also understand that on heavy breaking with one circuit down in the diagonal split setup (front brakes 70% and rear brake 30%) the car tends to deviate towards the side with front system still intact possibly resulting in serious trouble (....)
    Of course in 'our' case, wih one circuit failing worst case scenario would be failing front circuit; resulting in a serious loss of braking power (more loss than the diagonal split system), but we will be able to keep our cars 'on the road' or stay clear from traffic left and right....
    I still do not understand what front wheel drive or rear wheel drive has to do with setting up diagonal or (as learned from Bruce) 'vertical' split.

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    Apparently, the reason the diagonal split (one front brake and one rear brake on the opposite side of the car are on the same circuit) is favored for front engined, front wheel drive cars is because of the resulting front weight bias found on this type of layout. Since a majority of the car's weight is on the front tires to begin with, when you add in the weight transfer to the front under braking, you end up with very little effective weight on the rear tires, therefor it makes sense to try to use one front brake to help stop the car. One of the problems with this set-up was mentioned by RRZ8, that being the potential for instability under severe braking since the single operable front brake will exert a much greater force than the one operable rear brake. From what I can gather, some of this instability is dealt with by the anti-lock braking system which remains operable even when one half of the brake system fails. The diagonal system also requires significantly more plumbing to work and makes ABS more difficult to employ. The vertical or front/rear system works better on front engined, rear wheel drive vehicles because, theoretically, they have a more even weight distribution which means the rear brakes will still be effective under severe braking. While braking in a straight line, the vertical system should be relatively stable but if just the rear brakes are applied while the car is turning, things could get pretty exciting! I assume Porsche, with its rear engine, rear wheel drive set-up, uses the vertical split. Audi, with their all wheel drive set-up, uses the diagonal system.

    As far as the brake bias is concerned (proportion of braking done by front vs. rear brakes), I too have experienced the "skipping" under braking on irregular surfaces Norcal mentioned, but I'm not sure it is related to bias. When the bias is optimized, the front and rear brakes will provide maximum braking potential while maintaining the directional stability of the car under braking. Improper bias generally manifests itself as a tendency for the car to rotate under hard braking, particularly while cornering. It can also show up as less than ideal stopping distances but that really is a different matter. I suspect the skipping of the front tires under hard braking on rough roads is a combination of the relatively unsophisticated ABS used on the Z8, the amount of deflection allowed in the front suspension by the elastomeric bushings, the somewhat stiff front springs, and the less than ideal damping rates of the shocks. It would be interesting to hear from Andrew or one of the Alpina owners as to their experience with that more compliant suspension set-up. Race cars often employ a mechanism to allow the front to rear brake bias to be adjusted manually and it might be enlightening to experiment under variable road conditions but I'm not sure it would resolve the problem. I would be more inclined to reduce unsprung weight as much as possible so that the front tires retain better contact with the irregularities of the road under braking and require less help from the ABS system.

  10. #10
    Z8 Ate My Homework! Norcal's Avatar
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    Thanks GM, I think we would need to find a known stretch of road and do repeated runs in various cars, including some with only light wheels, and some with the Alpina set up.

  11. #11
    I'm certainly familiar with the Z8's front brake shudder under very hard braking on poor surfaces. It was most pronounced on my first Z8 which had the ACS suspension, and under very hard braking the entire front wheel could be seen shuddering back and forwards a good three inches in the wheel well. Matt at StopTech and I took turns driving that car and looking at it from the outside to try to understand what it was doing. My recollection is we put it down to a combination of the bushings and the relatively slender curved lightweight aluminum front wheel mounting arms flexing as the ABS grab/released the traction/friction/tires. My best guess is that the Z8 ABS isn't rapid enough to keep things smooth at the very frantic end of the braking envelope, but I must also add that I've only seen/felt this while working on brake set up testing.

    I looked hard for this on my M3, my GT3 and on both my Lotus's, which also had aluminum suspension arms, ABS etc, but none of them shuddered in the same way.

    With my current wider tires, Brembo's and Alpina suspension I've never felt the shuddering, but that said I'm also driving my Z8's much more calmly these days, it is the Lotus that gets the track day duty now.
    Andrew Macpherson

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  12. #12
    Z8 Ate My Homework! Norcal's Avatar
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    From what I can tell, I think it is pre-abs intervention. Most recently, I was at about 125 on the freeway and hauled it down to about 80 very quickly. All this on the stock set up, not ACS.

  13. #13
    Z8Mania
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    I have also experienced the car wanting to "jolt" off track under very hard braking on regular streets. Mine doesn't shudder- its more like there is a big pull to one side. It could be that the ABS system was designed for the OEM run flats which had less traction than the sport tires I am running now and at the edge of their adhesion the tires are grabbing and the ABS/ EBD systems are not apportioning the braking evenly quickly enough....Interesting discussion.

  14. #14
    That sort of pulling/darting to one side sounds more like an alignment issue, I've never heard of that in any other Z8? I'm sure you had your alingment checked when you put in the PP but I'd suggest double checking it, and all the bushings and bearings in the steering parts.
    Andrew Macpherson

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  15. #15
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    Quote Originally Posted by Z8Mania View Post
    I have also experienced the car wanting to "jolt" off track under very hard braking on regular streets. Mine doesn't shudder- its more like there is a big pull to one side. It could be that the ABS system was designed for the OEM run flats which had less traction than the sport tires I am running now and at the edge of their adhesion the tires are grabbing and the ABS/ EBD systems are not apportioning the braking evenly quickly enough....Interesting discussion.
    Can you tell us more about the 'pulling to one side' ? Always same side ? During cornering ? Doesn't sound like ABS/DSC (...) Maybe one defective front brake ?

  16. #16
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    I've had this "pulling" associated with steering wheel shimmy on hard braking for at least two years. The pull was always to the right first, then the steering wheel would go left-rigit-left-right with about 1 inch of travel. The car would never veer off course and stayed straight even with very hard braking at high speed, but the steering wheel did a little dance. The dealer replaced both front rotors twice, rebuilt the calipers thinking that they were holding up, triple checked the balance and road force of the tires, and finally replaced the front bushings with urethane. The bushing change muted the problem to something I can live with, but I still wonder if there is a fault in the abs or computer controlling brake pressure or response time.

  17. #17
    Z8Mania
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    Thats it Ken, they just replaced my bushings and its exactly what you say - the issue has become muted- very much so. The truth is I don't really push the Z8 that much anymore so this doesn't really bother me but thats my experience.

  18. #18
    Z8 Ate My Homework! Norcal's Avatar
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    I find that worn tires make the car "Track" more, ie: follow contours in the road. Under braking this can be very pronounced. Do you have new tires? I've noticed when I get new tires it goes away. It may also be related to new alignment.

  19. #19
    Sport Button On - DSC Off jpklecker's Avatar
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    I noticed the same thing when my front tires were getting low on tread the car would constantly try to "track" any seams in the highway. Upon installation of new front tires this went away, even before the entire car had an alignment about a week later.
    John Klecker - 2002 Topaz / Crema (61782)

  20. #20
    Z8Mania
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    My tires have very light use on them though they are now 3-4 years old. Maybe 3k miles in that time. Btw- it was far worse before the PP install. The PP helped a lot. The worn down treads might mean youre actually getting more grip because you have more tire surface on the road. You are just in a bad spot if it rains since you have no tread to evacuate the road surface of the water.