Brakes Wheels 2nd Gen Tires Turns out the rears lock first

Discussion in '2nd Generation: 2007+ R55 through R61' started by UKCoopeR, Jul 26, 2016.

  1. UKCoopeR

    UKCoopeR Active Member

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  2. BlimeyCabrio

    BlimeyCabrio Oscar Goldman of MINIs
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    Been there, felt that!

    Nice driving... I'll be glad when I get close to your level. I still gotta get my confidence up in the new car. A few more weekends and I'll be on the way.
     
  3. jasonsmf

    jasonsmf Active Member

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    Don't you hate it when that happens :p

    Scared a few corner workers myself with a faulted-out ABS. For the life of me, I can't understand why the system does that, I would expect the fail-safe to be front biased.

    Jason
     
  4. Crashton

    Crashton Club Coordinator

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    Even the nanny that lives within the MINI's wiring knows over steer is more fun. :arf:
     
  5. UKCoopeR

    UKCoopeR Active Member

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    #5 UKCoopeR, Jul 26, 2016
    Last edited: Jul 26, 2016
    Yeah I would have gotten an F on my exam (did vehicle dynamics in college) if I had proposed a system that locked the rears first.... I think the modern abs systems cover a lot of non standard baseline biases that flat out woun't fly on a car that didn't have abs.

    I have have massive front rotors (328x28mm) that should have biased more to front if anything. I could run less aggressive rear pads but the rear rotors get hot enough to need rear track pads so :fingerscrossed: into the braking zones :crazy:
     
  6. UKCoopeR

    UKCoopeR Active Member

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    Ugh looks like it was the controller not the sensors aka $$$ not quite able to justify a 15k bosch motorsport unit but still not a cheap part :-/ oh well such is the cost of beating on the car!
     
  7. BlimeyCabrio

    BlimeyCabrio Oscar Goldman of MINIs
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    I probably have an extra one... Maybe two...
     
  8. UKCoopeR

    UKCoopeR Active Member

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    Oh snap I should have know to ask. I actually just dropped the mini at the shop as I am recovering from a motorcycle crash (it was at the track so full leathers etc means just a dislocated arm and a few tiny things which more annoying than anything, obv can still drive the mini but I am favoring my stronger arm) and the garage is full of motorcycle bits and while I am almost back to normal (~85%) the crazy contortions needed to work on the mini seem like unnecessary risk till i am 100% again. I appreciate it though, will definitely ping you for spares next time :p Are you doing the trackdaze event at VIR in Nov? I will be there.
     
  9. BlimeyCabrio

    BlimeyCabrio Oscar Goldman of MINIs
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    Yep, I plan to be at that VIR event - glad you're on the mend!
     
  10. Rastoff

    Rastoff New Member

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    I don't think the brakes are designed so the rears lock first. That is just the natural action of any car. Of course the Mini has some dynamics that other cars don't have. Because the rear is so light, those are more likely to lock than the front.
     
  11. UKCoopeR

    UKCoopeR Active Member

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    Thanks man, been doing the car thing for years but still got tons of noob level stuff to learn on the bike side but it is making me faster in the car as the weight transfer and need for smoothness is 10x higher! See you there! :cornut:

    I even run stiffer springs in the front to counter this effect somewhat (to the slight detriment of handling). I am pretty sure it is a fact of the system relying on the abs module to control line pressure. Without that, way too much pressure gets sent rearward. When functional my rear brakes last 4x longer than the front brakes because there is so little going on back there. On a car without abs you have to use a prop valve or similar tech to mechanically limit the line pressure the rears will get so you don't have the bias changing with abs fails, meaning the fronts will lock first always. I would be shocked if a car with fully functional brakes had the rears lock first. When stuff breaks that obviously goes out the window.
     
  12. BlimeyCabrio

    BlimeyCabrio Oscar Goldman of MINIs
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    My rears would lock first, with ABS functioning properly, when I swapped to the 2nd gen JCW rear brakes. The larger rotors back there threw off the brake bias. Effect was that I'd get into the ABS very early, before the fronts were anywhere close to the limit. Swapped back to the smaller brackets / rotors and all was well.
     
  13. UKCoopeR

    UKCoopeR Active Member

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    Yeah that is why I thought my bigger front rotors would have resulted in it being harder to lock the rears.... maybe i should cut the rear pads in half haha. I know a few teams have done that in the fast to reduce rear brake bias and stay within the rules :p
     
  14. Rastoff

    Rastoff New Member

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    Hmmm, do Mini's have separate brake systems fore and aft? Most cars don't.

    In a close system, the pressure is equal throughout. So, whatever is applied to the front is applied to the rear. In the original ABS systems, the control would modulate the system pressure. Thus, it would reduce pressure to all wheels at the same time which would prevent any wheel from locking. In modern systems, each wheel is controlled independently, but only for releasing the pressure, not increasing it. At least that's how I understand it.

    If you take the ABS out of the mix, every car will lock the rear first. That's just simple physics. Under braking, the weight transfers forward which puts more force on the front. Because the weight is transferring forward, there is less force to the ground from the rear wheels. Thus the rears lock first.

    Absent a dual breaking system, which is easily possible, every car will have the rear lock first.
     
  15. UKCoopeR

    UKCoopeR Active Member

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    The mini has a 4 channel ABS though so line pressure won't be the same at each wheel, Yes, it is a closed system but there are multiple lines. Line pressure won't be the same front and rear even on non-ABS cars, a proportioning valve or similar tech will prevent that specifically to account for the weight transfer. There are even standard equations that help you figure out what spring strength to put in your system. There are a ton of inputs cg, rotor dimensions etc. but it is fairly basically to figure out what the safe max line pressure is for emergency braking. I agree with the physics I am just disagreeing that it isn’t already accounted for in mass produced systems unless someone screwed up big time. If an engineer showed me a braking system that had equal power front and rear when functioning as intended I would be shocked. Weight transfer has to be accounted for in the system design or everyone would have to be a finnish rally driver!

    Here are few quick reads that might help:
    http://www.drivingfast.net/technology/abs.htm
    https://en.wikipedia.org/wiki/Proportioning_valve
     

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