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Thread: High speed braking=SCARY

  1. #51
    Senior Member Abarth Five O's Avatar
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    Quote Originally Posted by ripper View Post
    I agree. Brake biasing is a good point that should not be overlooked as a means to improve the braking on our little Abarths. The Abarth appears to have its braking power naturally biased to the front. I am not an expert on the best solution; but, as with any track or race car, brake biasing (to attempt to transfer more braking force to the rear of the car) can be aimed at obtaining increased braking force while still getting the car to track straight when in straight line threshold braking situations (e.g., prior to entering a corner at high speeds). I put my Abarth (with all stock suspension and original P-Zeros) on the big track (High Plains Raceway, 2.6 mile road course) a couple of weeks ago. I was studying the track prior to putting our race car out (a modded RX7 with RWD). Felt the same "wiggle" tendency on a number of occasions with the Abarth when I came in too hot and set the brakes too hard coming into a corner--especially the downhill corners. The track surface was very smooth and the "wiggle" was managable but unsettling (to me and the car). As I put in more laps, I found that I had to pay attention to keeping the fore-aft weight shift under braking (i.e., to the front of the car) from getting excessive (e.g., by squeezing down on the brakes a bit slower at the onset of braking, and gradually increasing brake pressure to threshold as my speed decreased) in order to minimize the "wiggle" and get throught the corners faster. This kept the car more settled under braking and more planted entering the corners. With our race car, we have the brakes biased more to the rear and I get no such "wiggle" tendency in the car at all--even with higher entry speeds and even harder braking than with the Abarth. While this is the goal, brake biasing can probably only achieve somewhat of an improvement on a front wheel drive car (with a proportioning valve and/or alternative front and/or rear brake combinations). Braking in the Abarth can likely be improved with some rear-biasing, but only to the point where too much rear bias begins to reduce the overall braking capacity of the car, since the resulting reduction in weight on the rear tires under hard braking will only lock them up if the fore-aft weight shift is still too excessive for the bias setting. May be worth playing with though to see if this can be optimized to obtain higher overall braking performance, as lower lap times would be worth it. Finding the optimum with brake biasing may become even more feasible when combined with the other reasonable options being discussed in this forum (improving shocks, tires, etc.). In the meanwhile, I have installed a stiffer torsion bar on the rear and a set of stiffer vertical chassis braces on the front to see if this helps (although this was primarily aimed at improving cornering). Also, I am planning to change to the Direzzas as soon as I burn up the Priellis. I will report after my next track experience. But I may be looking at the brakes next....
    Good point. I was planning to upgrade the front w/ Wilwood BBK, but will instead upgrade all rotors, brake pads, and lines for the front and rear. Direzzas are also an excellent choice.
    2012 Abarth Grigio: visit my garage for list of mods and more pics: http://www.fiat500usaforum.com/dto_g...vehicle&v=1754

  2. #52
    Moderator Robert Nixon's Avatar
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    thanks for the comments above on your real track experience, that is certainly more useful than hitting the brakes on a freeway!

    But, on a street car, HOW do you bias your brakes front/rear? As far as I know there is no built in way to do it like a F1 car or whatever.

    I've read one tip where for track or even autocross you might affect the Front/Rear bias by say putting top of the line pads on the rear and medium line pads on the front, in an effort to increase braking in the rear! That at least is understandable to me, and since it was in a book I'm sure it works!

    My interest in brake performance right now is based on autocrossing this year, so if there was a simple way to drop times by better braking running stock, then that would really get my interest!

    EDIT: Just went back and caught what Crazy Otto said, so sorry for repeating his comment about pads!

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    Senior Member Crazy Otto's Avatar
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    Robert As I mentioned you can look into rear pads with more bite more friction.

    Ripper - agreed that more rear bias only goes so far to reduce stopping distances. The issue being addressed is the back end wanting to overtake the front end.

    All street cars have enough brake power to overcome tire traction. If you want to shorten braking distances then you have to increase tire grip. Only in repetitive applications of max brake will a typical car run into difficulty with brake fade due to thermal overload. I run stock calipers rotors with race pads motul600 and stinless lines on my race car and they are fine even without ducting. But car weighs 2700 lbs only.

    In general there is a lot of focus on brake power for race track but the typical driver will do much much better by attention on how the brakes are released and carrying more speed through the corner. Where the abarth is concered you will gain very little by the increase in max retard Going to a bbk. Pedal feel thermal management and esthetics are separate considerations.
    Last edited by Crazy Otto; 09-18-2013 at 03:51 PM.
    12 Fiat Abarth, 07 Mercedes ML63 AMG, 73 Mini 1275, 96 Ferrari F355 GTS, 98 BMW M3 NASA GTS3, 03 BMW M3 NASA GTS4 (2015), 06 Mercedes SLK 55 AMG.

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    Quote Originally Posted by Crazy Otto View Post
    Robert As I mentioned you can look into rear pads with more bite more friction.

    Ripper - agreed that more rear bias only goes so far to reduce stopping distances. The issue being addressed is the back end wanting to overtake the front end.

    All street cars have enough brake power to overcome tire traction. If you want to shorten braking distances then you have to increase tire grip. Only in repetitive applications of max brake will a typical car run into difficulty with brake fade due to thermal overload. I run stock calipers rotors with race pads motul600 and stinless lines on my race car and they are fine even without ducting. But car weighs 2700 lbs only.

    In general there is a lot of focus on brake power for race track but the typical driver will do much much better by attention on how the brakes are released and carrying more speed through the corner. Where the abarth is concered you will gain very little by the increase in max retard Going to a bbk. Pedal feel thermal management and esthetics are separate considerations.
    CO:
    Agreed, the issue is certainly the back end wanting to overtake the front end under heavy braking.
    I think that we agree that an improvement to rear brake stopping power, relative to the front brake stopping power, would only help reduce this tendency. Assuming that the tires are the same front and back, regardless of whether they are street stock or stickier racing tires, the situation would still require stronger rear braking relative to the front brakes to attempt to reduce the tendency of the back end wanting to overtake the front end. (In my experience I had no signs of brake fade with the Abarth on the track, and yeah, the Pirellis are part of the problem in my opinion.)
    Agreed that rear brakes with more friction appears to be the most obvious way to make an improvement of the car's handling under braking. This would be far easier than putting in a proportioning valve to separate the front brake lines from the rear brake lines so as to adjust the relative input to each from the master cylinder. (Although the latter is easier to tweak to taste after the mod.)
    Agreed, if you want to shorten braking distance you have to increase tire grip.
    Very much agreed that the best way to decrease lap times is to concentrate on carrying more speed into and out of the corner by attention to how the brakes are engaged and released. It would be nice, however, if the car didn't have the tendency for the back to overtake the front when I get a bit too aggressive or inconsistent--which appears to be happening even when I am not losing tire traction under braking. As you mentioned, braking power is otherwise generally overrated (or, at least a much lesser factor) as a means to go faster on the track.
    I am still on the fence on what I will do, if anything. All of the input from the forum is helpful and encouraging. I think I'll wait and see what the stickier tires do to my braking performance (and car stability under braking) before I commit to a plan. I am expecting that the stock Abarth will still have plenty of braking force to overcome tire traction with the Direzzas, unless others on the forum have experienced otherwise.

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    Member mjanowich's Avatar
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    Quote Originally Posted by Vlavsky View Post
    K so I was driving kind of fast the other day (90-95) when someone going the speed limit (50) decided to switch lanes. Long story short I had to brake pretty hard and it scared the **** out of me. The car felt so wobbly and just unstable. I was extremely dissapointed by this experience and to be honest don't feel very confident behind the wheel anymore. Is this normal? Has anyone else experienced thks? Hoping there is a quick fix for this (lowering?) Thanks
    As others have said, upgrade the springs to something with a higher spring rate to help control the weight transfer from back to front. There are other ways to control front/back weight transfer but springs would most likely be the cheapest. A little toe-in on the back wheels may help too.
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    Moderator Robert Nixon's Avatar
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    I'm not a physicist, but I read an entire book on braking, and one of the points they made is that changing the energy (heat) of forward motion is what stops the car, basically the ability of the braking system to absorb and get rid of the heat from braking. So while tire grip, pad performance, rotor interaction, etc are all factors, the biggest limitation is on having enough MASS in the system (rotors mostly I guess) to dissipate the heat.

    I'm not arguing either way since I'm not a scientist or in Formula 1 and NASCAR, but that's what the book was talking about.

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    I was at Summit point last weekend.

    Braking and especially trail braking is somewhat twitchy feeling.

    A couple points to consider.

    Mass and ability to absorb heat has nothing to do with this. That only is important on the track with repeated braking and relates to the vehicles ability to repeatedly brake without experiencing pad fade, cracked rotors and/or fluid boiling. On the street you will probably never have this issue and it has nothing to do with twitchy feeling.

    Increasing rear pad friction (adjusting brake bias towards the rear) will generally increase the chances that the rear will rotate. However, it is not that simple because with the lose suspension in the Abarth, weight transfer during braking also plays a role and increased rear bite might reduce forward weigh transfer.

    Couple easy suggestions to fix lose feeling under threshold braking include:

    1) alignment (zero or slight tow in at the rear)

    2) sticky tires will increase grip/traction in general and may reduce instability. Of course, sticky tires are also going to increase braking power and increase weight transfer forward which is not what you want to do.

    3) Suspension changes (shock and springs). Rear swaybar should not really do much here unless you are trail braking.


    I found the stock pads completely inadequate for track use (no surprise here). I got 4 laps before experiencing some fad, when I pitted in they were smoking terribly. Probably should have done longer cool down lap. Pad material was smeared all over the front rotors. Rear pad/rotors look fine. I generally never use street pads at the track but CarboTech has not yet made pads for the Abarth. Need to pull them and get some pics so they can start making these.

    My short term plans are better tires and track pads. The twitchy feeling might feel a little weird but the car generally does not want to rotate (even with the Neuspeed bar). I think that a short wheel base, narrow car with a high center of gravity is just never going to be the beast braking or handling car.
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    "Increasing rear pad friction (adjusting brake bias towards the rear) will generally increase the chances that the rear will rotate"
    "I think that a short wheel base, narrow car with a high center of gravity is just never going to be the beast braking or handling car."

    I completely agree with above statements

    I would expect that the reason of instability is just proportions (wheelbase vs track + height) and probably soft rubber bushings (you may improve it a little - at least in theory - changing into polyurethane one or uniball type, at the cost of comfort)
    Last edited by satural; 09-26-2013 at 09:10 AM.

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