Brake question of the month

[MCRacer]

I am still having stopping issues. Can't lock the fronts and my data shows about .7G max. I have Porterfield R4 shoes and I have checked and reworked my brakes at the wheels. I have not changed masters or flex lines. Now I'm wondering if I am using too much rear bias. I use the "ratchet tie-down" method and pull until I cannot turn the fronts. Then I adjust the rears until they can be turn with some effort. My question is how hard should the rears be to turn when biased correctly?

Thanks,
Mike

[SR Racing]

What you are doing should put you close to right. However the final adjustment has to be done on the track. Your spring rate, center of gravity and track/tire conditions will play a role. You should be able to lock up the fronts before the rears. If the rears lock up first, the car will be ill handling and you are giving up a lot of braking force. (This assumes straight line braking.) A street car bias is around 60/40 (F/R). A race car in the dry will be closer to 70/30.

[lucky13]

The "ratchet-tie-down method". Can someone please explain that in a bit more detail for me? My car's braking was fine but over the winter I've replaced all my lines (just being pro-active and replacing old lines that were showing a good bit of corrosion) and have new front brakes so I'm eager to get everything set-up properly at home so that I'm close when I get to the track.

[Dietmar]

It is a way to get your brake bias as close to "set" as possible without being on the track.

Jack up the car and place it on 4 jack stands. If you do not have a "helper" you will need some way to apply the brakes. This is where the ratchet tie down comes in. I use something different, but the idea is the same. After adjusting the brakes, you want to apply STEADY- increasing force on the brake pedal. When the front wheels no longer are able to be turned, the rears should just be grabbing with some force applied. You can also use the wife or the child if the latter can reach the pedal.

If you can not get the above result, play with the bias bar until you do and remember to have some free play on the rods that go to the masters.

Hope this helps

Dietmar
http://www.quixoteracing.com

[FV80]

Mike,
Be sure to take a look at your pedal geometry too. If the pedal goes past vertical under HARD braking, you will lose a significant amount of pressure at the wheel cylinders. Always adjust your pedal leaning TOWARDS you at rest - it should also STILL be leaning towards you at full pedal press. It makes a LOT of difference! Also .. if you have a bias bar, the bar should be straight across both master cylinders at full press - not at rest. The MC travel is generally different at front and rear and at rest the bar will NOT be straight.

Steve, FV80

[MCRacer]

After going through this intially with the new brakes I adjusted the pedal and bias bar as Steve suggested so I hope those are good. From Dietmar's comment and Jim's suggestion of 70/30 front/rear split I sounds like I may have too much rear bias. Would that leads to generally reduced braking force? I do not have a problem locking the rears ahead of the fronts as I never lock either, which given my general level of incompetence, seems unlikely if the brakes are really working right. So for a good starting point in the garage, at front lockup should the rears just be starting to grab or be close to lockup also?

[FV80]

I would put the preferred bias at about 80% front rather than 70. I would also say that, considering your situation, you would not go too much wrong to put 90% to the front at home and adjust at the track. If you have in car adjustability, it's not a big deal - you just adjust it as you drive. I would set mine so that it WOULD Lock up the fronts under hard braking.. then back off a tad till they DON'T lock up.

If you don't have 'in-car', then it's a LOT longer process through several sessions ... and you can't change it "on the fly" like when it starts RAINING during a race!

If you feel 'waivering' in the rear under heavy braking, you are likely locking up the rears.

Steve, FV80
PS - Please give us a report on the R4 shoes after you've had a chance to test them out (and tell us at what track and what lap times you're running)

[lucky13]

Thanks!

It is a way to get your brake bias as close to "set" as possible without being on the track.

Jack up the car and place it on 4 jack stands. If you do not have a "helper" you will need some way to apply the brakes. This is where the ratchet tie down comes in. I use something different, but the idea is the same. After adjusting the brakes, you want to apply STEADY- increasing force on the brake pedal. When the front wheels no longer are able to be turned, the rears should just be grabbing with some force applied. You can also use the wife or the child if the latter can reach the pedal.

If you can not get the above result, play with the bias bar until you do and remember to have some free play on the rods that go to the masters.

Hope this helps

Dietmar
http://www.quixoteracing.com

[tiagosantos]

Yup, I feel for us "newbies", it's sometimes pretty hard to tell that you're locking up the rears.

You do want the fronts to lock up first, you can see it/feel it much easier, and the car understeers. When you lock up the rears, you'll often not realize they're locked up. You're just flat spotting your tires and losing braking potential. Plus the rear can become loose, which is a bit harder to manage than understeer!

Under braking (opinions vary on the amounts, but bear with me..) a fairly big percentage of weight gets shifted onto the front wheels. It depends on the car's weight distribution, center of gravity, bla bla bla. Either way, it's quite a bit of weight. If you have (let's imagine..) 70% of the car's weight at the front wheels, you need to have at least 70% of your braking power at the front. Your rear wheels will be "light" having only 30% of the car's weight, which means they have less grip potential. If your braking bias is set to 50/50, your rear brakes will lock up at "30%" anyway and you'll be wasting the other 20% into tire smoke at the rear, and still be 20% off the ideal at the front.

I've used some creative and inaccurate "math" on the previous paragraph, it doesn't really work like that, but I'm sure you get the idea! Like Steve says, adjusting on the track with a cockpit bias adjuster would be faster. It also helps to have a helper on the side of the track to look at your rear wheels, if you can't tell if you're locking them up or not. If you get a chance to run a test day, bolt on a set of tires you don't mind ruining and add some stripes of tape to the wheels, so your helper can easily tell if and what you're locking up. Find a bit of clear track with no one coming up behind you and SQUEEZE the pedal hard! Something needs to lock up. Even with stock shoes, if you can't lock up either the rears, the fronts, or all four, something is wrong

Anyway, I'd start with the bias set reeeally far forward and make sure I was locking up the fronts easily. Then move the bias to the rear a turn or two at a time until the rears lock up instead, then back off to the previous setting or somewhere in between. The maximum braking force will be when all four wheels are on the verge of locking up, but I'd err on the side of caution (front bias)! It took me a while to be able to tell the difference on the track, but our cars really do brake quite well when everything's working right!

[MCRacer]

Even with stock shoes, if you can't lock up either the rears, the fronts, or all four, something is wrong

and that "something" would be? Seems I'm down to masters and lines if bias isn't it.

Steve, I do not think at my level (+10s to you at Barber) any input on shoe performance would mean much. I had CTs before and changed them because of some cracking. I went with Porterfield's because they were available - CTs being out-of-stock. I really expected a significant difference in braking feel and performance when I changed but I can't say that is noticeably different. I don't know if that is good or bad based on the old shoes but, as I said, I'm nowhere near the sharp end except when it comes up behind me.

[FV80]

and that "something" would be?

That "something" would most likely be one of 2 things....
1). Pedal geometry (addressed in my earlier post)
or
2). Improper sizing of Master Cylinder ... say something around 3/4" or larger? You need to have a 5/8 or maybe 0.70 to get the pressure you need on the wheel cylinders.

Steve, FV80

[BLS]

I've noticed in other situations that people often don't realize the larger master develops less pressure, or the difference in a 5/8 and 3/4 is almost a 50% pressure reduction...

[sabre1]

One aspect of pedal geometry that needs to be considered is WHERE your foot presses on the pedal, as this will define the actual geometry of how your foot pressure is applied to the master cylinders...

-Jim

[brian]

I have grown to rely on the proportioning valve that I have installed on my car. Several companies make a hydraulic valve that goes on the rear brake line to reduce fluid transfer, hence reducing shoe travel. There are several advantages to this system. It's easy to adjust on track and it's much more simple than a bias bar on the pedal to install. Usually if you have a lot of bias on the pedal, it's travel will increase to compensate for movement in the bias bar. With the valve, you can set up the pedal at 50/50 and adjust until you can feel the rear getting loose then back off a bit. If you can't get the car loose under braking you have other issues. If you can't stop the car from getting loose, regardless of what you have done to the valve, you will need to put some bias in the bar, but not as much as you would normally have.

By the way, manufacturers have been using these valves for many many years. If you've ever owned a car with front discs and rear drums, you've had a valve on your car. Many ABS systems use versions of these valves as well.

[tiagosantos]

Brian, I haven't thought that hard about how proportioning valves work, but I was always under the impression that, contrary to a bias bar, they always reduce the "efficiency" of the system.. That is, they reduce the amount of braking power in the rear without increasing the front. If you set the bias at 50/50 on the pedal assembly, that means 50% of the braking force is going to each end of the car. If you use a proportioning valve to change the bias, you're effectively making it, let's say, 50/30, for a total of 80%. Is that right?

Being the skinny little person that I am, every pound of leg force counts, which is why I went with a mechanical adjuster on my car! I have to agree though that the proportioning valve was much easier to work with.

[MCRacer]

That "something" would most likely be one of 2 things....
1). Pedal geometry (addressed in my earlier post)
or
2). Improper sizing of Master Cylinder ... say something around 3/4" or larger? You need to have a 5/8 or maybe 0.70 to get the pressure you need on the wheel cylinders.

Steve, FV80

1). Brake pedal starts out slightly towards me ands goes to vertical when ratcheted down to front wheel lockup
2). Front and rear are Girling 5/8

[FV80]

Pull that pedal towards you. you can put more force on that pedal with your foot (most likely) than with the ratchet strap. I think you'll be surprised at how much diff it makes.

Try to have the pedal still leaning towards you at about 75 degrees with your full foot force on it. As your brakes wear during a session, the pedal will move away from you. As you have it, before the end of a session (race), you'll likely be WELL over vertical away from you on the pedal.

Steve, FV80

[tiagosantos]

I'm not sure about this, but aren't there two different sizes of wheel cylinders that'll fit on our cars? I know front and rear are different sizes, but can't remember if they're interchangeable or if there are two styles of rears, or fronts, or whatever..

[BLS]

Tiago,

Accoding to Brian M, there are two sizes of wheel cyl for the rear, 19 and 17mm. Something I did not know. Probably depends upon what year model you source. The fronts are 22mm.

Brian's post here:
interchange/viewtopic.php?f=3&t=4721#p28960

Regards,
Barry