Brake bias and general brake questions..

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tiagosantos
Posts: 389
Joined: June 20th, 2010, 12:10 am

Brake bias and general brake questions..

Post by tiagosantos »

So yesterday was the last race of the season, and although I'm taking a couple weeks off of touching the car, I do want to start prepping it for next year as soon as possible. Knowing myself, if I start putting it off, it'll come April and I'll be running and screaming trying to make things work with a week to go.

The car is new to me, and a couple weeks ago while trying to adjust the brake pedal positioning to fit my legs a bit better, I lost count of the number of turns on the bias bar thingies, completely messing up my brake balance. So yesterday once the track dried off and I went out for a dry session, I hit the brakes at my normal brake point for one of the hairpins and went flying straight ahead. That happened again 3 more times at different points of the track - I was braking as hard as I could, it sure felt like something was locking up but both my front wheels were rotating and I was not slowing down much at all haha. After the session, I asked a couple other drivers who said they were hitting the brakes at the number 2 marker, I was having to brake at 4 to keep from going off the track.

Soooooo, I'm going to guess my brake balance is waaay off to the rear. My question is, is there a way to set this up, at least roughly, without having to waste precious track sessions with trial and error? I tend to "error" a lot before I get the right trial hehe.

Lastly, another brake question. My brake pedal (or rather the master cylinders, I guess) have a lot of slop in them. I can wiggle the pedal back and forth probably a little over an inch, which was more than double what I felt on other people's cars. Is there a way to get rid of this in the master cylinders? I am thinking of buying new master and slave cylinders etc, to both figure out how it all goes together, and to keep the old ones as spares while they still work. I'm hoping I have the energy to go through as much as I can this winter, without getting into more than I can chew ;)
brian
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Joined: June 26th, 2006, 12:31 pm

Re: Brake bias and general brake questions..

Post by brian »

There's two ways to static check brake bias. One is to install pressure gauges on both masters and read the relative pressures. The other, and one I use, is to use a ratchet tie down on the pedal and slowly tighten up on the pedal until the fronts no longer rotate then check the drag on the rears. Once the fronts are locked up there should be significant drag on the rear. Move the pedal adjustment until you can ballpark the above procedure. You can observe the location of the pivot point inside the pedal assy. to get an idea of which end is receiving reduced pressure. For the first time out I start with a 50/50 bias and move slowly to the front us9ing the cockpit adj. until the car is stable in the rear

It is not advisable to preload master cylinders by extending the pushrods. If there is play between the end of the pushrod and the master, extend the adjustment until the gap is closed, no farther. Make sure that each master cylinder pushrod is the same length. If the pedal still moves too far, double check the adjustment at the wheels. If the adjustment is ok, pump the pedal twice and see if the free play goes away. If it does, it may mean the system needs bleeding to remove air.

Every time I have a brake drum off I peel back the rubber boots on the wheel cylinders and look for moisture. If the inside of the boot is wet, the wheel cylinder is bypassing which will cause a soft pedal and it means that it will have to be replaced.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
tiagosantos
Posts: 389
Joined: June 20th, 2010, 12:10 am

Re: Brake bias and general brake questions..

Post by tiagosantos »

I was thinking of something like holding the brake pedal until the fronts were locked and adjusting the brake bias bar that way, but couldn't figure how in hell I'd make my helper keep constant pressure at the pedal! Ah, easy to do when the helper is an object rather than a person :) Genius..

Thanks Brian.. As for the free play in the master cylinders, when I was playing with it, I couldn't figure out how to reduce the gap at all.. It felt like the pushrods were just loose inside the rubber boot, rotating them didn't seem to do anything at all.. I might have to find an old master cylinder and take a better look at it!
hardingfv32-1
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Joined: December 1st, 2006, 8:01 pm

Re: Brake bias and general brake questions..

Post by hardingfv32-1 »

In general I'm not sure the master cylinder push rods will end up equal when you are done adjusting. I would say you want the balance bar perpendicular to the master cylinders when the pedal is depressed. I THINK the front master cylinder needs a little more movement because the front wheel cylinders are larger.

Brian
Matt King
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Joined: December 23rd, 2008, 1:44 pm

Re: Brake bias and general brake questions..

Post by Matt King »

tiagosantos
Posts: 389
Joined: June 20th, 2010, 12:10 am

Re: Brake bias and general brake questions..

Post by tiagosantos »

Great link.. Just cleared up a bunch of questions! From reading it, yup you're right Brian.. The bar should end up parallel with a depressed pedal, and ends up crooked while not pressing the pedal due to the different lengths in the pushrods :) I love it when things start to make sense!

Thanks guys, much appreciated..

Tiago
jpetillo
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Joined: August 26th, 2006, 2:54 pm

Re: Brake bias and general brake questions..

Post by jpetillo »

I'd go along with the bias bar being parallel when the pedal is fully pressed. But I believe it doesn't make any difference as long as the bias bar itself doesn't rotate enough that it interferes with the pipe it's sliding through. John
racing stuff
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Re: Brake bias and general brake questions..

Post by racing stuff »

The reason you want the brake bias bar, parallel to the front bulkhead, at full brake pressure, is so that if you have a brake failure on one end, you have maximum push-rod travel left for the other master.
A few years ago, a past employee who now works for an IRL team (CART at the time), told me that a number of the team engineers spent time during the winter months, looking at brake forces and the like. They came up with setting the brake bias shaft, at an angle when the pedal was at rest, that had the front master cylinder push-rod, noticeably longer than the one for the rear. This gave them a better brake curve. I asked, and was told yes, when max pressure was applied, the bias shaft was parallel to the front bulkhead, which is how I explained it to him many, many years earlier.
I guess they had to do something to earn their keep for the winter!
We have always done the static brake bias set-up as descibed, and some on my customer have not touched it since (although I don't recommend that).
Keith
Averill Racing Stuff, Inc.
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cendiv37
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Joined: June 25th, 2006, 7:29 pm

Re: Brake bias and general brake questions..

Post by cendiv37 »

Having the bias bar "level" or parallel to the bulkhead when the brake pedal is fully depressed probably means having the push rod longer on the front master than the rear when the pedal is "at rest". This is due to the larger wheel cylinders on the front brakes than the rears. But consider also the master cylinder diameters you are using. If the front and rear masters are the same (probably the most common set-up), then only the wheel cylinder sizes matter. If you use a smaller master on the fronts, the effect will be even greater. Go larger on the front master than the rear, and the effect will be less.

Another thing that might be worth adding is that because we have drum brakes that do not self adjust during a session, you might want to set up the pushrod just a bit longer on the front brake master than on the rear master, at full pressure on the pedal. This would be especially true if you are running "stock" shoes that wear fairly fast. It is less important if you use Carbotechs or something that wears better, at least on the front.

My experience was that the pedal went down a lot when I used stock (white box) shoes (though I haven't run stock shoes at the front for MANY years). This is mostly due to wear of the front shoes but is compounded by the fact that the fronts have larger wheel cylinders. The rears don't wear nearly as fast and the smaller wheel cylinders reduce the loss of pedal from wear they do see. Thus, as the pedal goes down due to wear, the bias bar will go further and further forward on the front master, changing the angle of the bias bar from the set-up you made with the brakes fully adjusted.

By this theory, you would want to have the bias bar parallel to the bulk head at half race. This could be simulated by setting it up that way after a shorter session like a qualifying session.

From a failsafe standpoint, you should probably test that at your final adjustment setting, you will in fact have front brakes if the rears fail (fluid leak) and have rear brakes if the fronts fail. This could be tested while bleeding the brakes. I believe that this will most often lead you to setting the front pushrod longer than the rear, again due to the larger wheel cylinders at the front.

Your mileage may vary.
Bruce
cendiv37
tiagosantos
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Joined: June 20th, 2010, 12:10 am

Re: Brake bias and general brake questions..

Post by tiagosantos »

Thanks for the comments guys :) I'll take the opportunity to ask about the carbotech shoes - most of the local drivers that I've talked to seem to be using stock shoes. Our local track is pretty hard on brakes, with 2 hairpins after long(ish..) straights and a couple other hard braking points. I've heard a couple drivers say the carbotechs go through the rotors a lot quicker..

I've been using the stock shoes and other than this race's problems (due to being way off in the bias balance, I assume..) I never had trouble with the brakes. And I gotta say the price feels right on the stock shoes! So would using carbotechs mean I'd be swapping rotors often instead of shoes?
FVartist
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Joined: June 25th, 2006, 11:59 am

Re: Brake bias and general brake questions..

Post by FVartist »

The Carbotect's are more abrasive and will wear the drums more. I always turn my drums within .005 of each other to avoid any possibility of a brake pull, then I mic the drum to arc the shoes, once I have bedded the brakes in, the drum and shoes become a matched set. I also flip the shoes from primary to secondary to ensure even shoe wear, this is usually done after each race or two.

Bruce
Left Coast Formula Car Board
http://norcalfv.proboards.com/index.cgi?
racing stuff
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Joined: January 30th, 2010, 11:08 am

Re: Brake bias and general brake questions..

Post by racing stuff »

I don't know if my customers do that flip the shoe thing, but we sell KFP Magnum, Carbon-Kevlar brake material on our F/Vee shoes. The KFP material is notoriously easy on brake drums/disks.
My big marketing problem with the KFP material, has been that many of my customers (F/Vee and FST included) don't want their competitors to know what they are using.
Keith
Averill Racing Stuff, Inc.
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http://www.racing-stuff.com
brian
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Joined: June 26th, 2006, 12:31 pm

Re: Brake bias and general brake questions..

Post by brian »

The Carbotech are ok on drums as well. Tell us more about your shoes. Price, wear factor etc.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
jpetillo
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Joined: August 26th, 2006, 2:54 pm

Re: Brake bias and general brake questions..

Post by jpetillo »

I side with what Bruce said. My point was simply that the brake force balance is not affected by the angle of that bias bar. However, what happens during wear and keeping the bias bar from contacting the tube is most important. Bruce's explanation was dealing with that. What happens during a brake failure is an excellent consideration so that you have the highest probability of still having full use of the non-failed end. Thanks for bringing that up. In that case would it be that the balance bar be straight during max braking or when both masters are collapsed. I'd guess the latter. John
Last edited by jpetillo on October 13th, 2010, 10:33 pm, edited 1 time in total.
racing stuff
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Re: Brake bias and general brake questions..

Post by racing stuff »

You want the balance bar to be "straight" under Max braking, so you have maximum stroke available on the good side when the other (hopefully never) side fails.
Keith
Averill Racing Stuff, Inc.
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tiagosantos
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Re: Brake bias and general brake questions..

Post by tiagosantos »

Ok so I'm almost convinced on buying some carbotechs front shoes or at least something better than stock. I think I'll be leaving the stock shoes in the rear though.

Can I expect a full season out of the carbotechs up front, if I rotate the shoes every weekend? A full season for me is 6 or 7 weekends, 6 sessions a weekend (2x practice, 2x qualifying and 2 races) I remember reading some stuff about making sure the shoes are sized (or arced..) appropriately for the drums - if I buy new drums will they match for sure? I was thinking of buying new drums anyway to keep the old ones as spares.
SR Racing
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Joined: June 24th, 2006, 1:58 pm

Re: Brake bias and general brake questions..

Post by SR Racing »

I will be happy to sell you Carbotechs, but I will try to talk you out of them. They are almost 5 times the price of stock VW brakes and are only marginally better. When you are finishing on the podium in all your races and you think your braking is keeping you from the win, consider the Carbotechs. Until then, spend your dollars on something else.
tiagosantos
Posts: 389
Joined: June 20th, 2010, 12:10 am

Re: Brake bias and general brake questions..

Post by tiagosantos »

I'm all for saving money.. I guess I can give the stock shoes another chance, I'm sure they won't be as bad once everything is properly adjusted. I am tempted by the promised reduction in pedal pressure, as the downside of being skinny is not having a lot of muscle :lol:
SR Racing
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Re: Brake bias and general brake questions..

Post by SR Racing »

I assume you have 5/8 Master cylinders (front and rear) with bias adj. If so, your pedal pressure should not be excessive if everything is set up right.
The stock VW system stopped a 2000lb car adequately. So, 1000 won't be a problem. Fade (gassing) is never a problem in a vee (unless you have a drum dragging).
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