Brake Bias - FFCars.com : Factory Five Racing Discussion Forum

jimwallace · 10-09-2012, 08:01 PM

I have a Mark 4 with the Wilwood master cylinders, FF front and IRS rear brakes, and no power brakes. It takes more pedal pressure than I anticipated to get significant deceleration and I was wondering if the brake bias might be off. How do you test the brake bias? How do you adjust the brake bias? Thanks for any advice that you can give.

MikeKelly · 10-09-2012, 08:16 PM

Apologies for the long answer, but you really ought to read all this and understand it.

Brake bias concern and a warning for safety

No simple answers due to too many possible variations...

HTH,

Mike

dallas_ · 10-09-2012, 08:37 PM

Do you have the dual master cylinders?
If so, there is a front/rear brake bias adjustment at the pedals by the master cylinders.

Willwood also sells a cable and knob arrangement that makes it easy to adjust it from the cockpit.

ILM MK3 · 10-09-2012, 08:37 PM

I suggest the first thing you do to reduce pedal effort is put better pads on, its makes a HUGE difference. I am using Hawk HP+ and they are great, the pedal effort is much better than what came with the kit. I would still read the above as well to learn more about whats needed. Since you have the dual master cylinders (no proportioning valve) you just adjust the balance bar to set the front/rear bias.

Have fun!

d42davis · 10-09-2012, 08:52 PM

You will want to add the following so that you can more easily adjust the front/rear bias:
http://www.summitracing.com/parts/WIL-340-4990/
It is likely you will need to have the bias towards the rear as we need to work the rear brakes more because of the 49/51 weight distribution. More aggressive pads such as the Hawk HP+ pads will also help.
I am running the Wilwood dual masters with HP+ pads and have great manual brakes with the complete kit rotors.

rscocca · 10-09-2012, 08:54 PM

You may need to drive the car a bit to properly seat pads. New pads sometimes feel a little spongy and weak.

CHOTIS BILL · 10-09-2012, 10:02 PM

Jim,

There are several ways to check your brake balance. A quick but not very accurate was is to go to an empty parking lot and get going about 30-40 mph. Apply the brake and see which end locks up first and how much sooner than the other end. A better way is to put the car up on jack stands or blocks and have someone apply brake pressure until you just can’t turn one of the front wheels. At that point you should be able to turn one of the rear wheels but it should pretty hard to turn. I like to cut a stick that I can wedge against the brake peddle and the seat mount or frame tube and can apply any pressure I want including enough to locking the front wheels. I have made adaptors that replace the wheels and tires that have a ½” square hole in the center where I can attach a beam type torque wrench. I adjust the stick that is wedged against the brake peddle so it takes about 100 #ft of torque to turn the front wheels and then adjust the balance bar on the Wilwood brake peddle assembly so that it takes about 55-60 #ft of torque to turn the rear wheels. This number is from my race cars days and may be a little different for your car. It will take more torque to get the wheel to start moving but with the balance beam type of torque wrench you can watch and see what torque it take to keep the wheel moving at a constant rate.

To adjust the Wilwood brake peddle assembly there is a threaded rod that goes between the front and rear master cylinder with the brake peddle attached in between the two. By loosing the locknut on that threaded rod and turning the rod the threaded rod pivot moves closer to one master cylinder or the other. The closer the pivot is to a master cylinder the more pressure it develops and therefore those wheels will have more braking force.

HTH,

Bill Lomenick

CraigS · 10-10-2012, 12:49 AM

I always do the parking lot or country road at 40 mph technique. I keep adding more rear brake till the rears lock first, then back off a little. I notice that you haven't posted much here so also assume you have not too many miles on the car. Surely gettting the balance right will help, but also more miles so the pads get bedded in may also help. I wouldn't get new pads right now but you may need them in the future. Couple of questions will help us.
1- how many miles on the car?
2- what calipers do you have?

mmarshall · 10-10-2012, 01:29 AM

1st problem is the brakes we attempt to use were engineered to be used with power booster so they tend to be weak when used with a manual MC. As suggested the HP+ pads will help and fix the front.

The rear is another matter. The single piston caliper was designed to be used on a car that was very light in the rear. But we are using it on a car that is very heavy in the rear and has very large amounts of rear grip. Kind of like taking the brakes off a Toyota pickup and using them on a NSX. Here is how I fixed the weak braking in the rear of my Coupe. BTW I now have a source that is selling the brackets I built myself.

New brakes for the back

CraigS · 10-10-2012, 01:34 PM

Mike, I read through your post and then went to Summit to try to get an idea of pricing and availability of parts. The first thing I checked was to see if Hawk makes pads for the 94 cobra caliper. As I drilled down thru the options I got to where I need the FMSI reference number. D412,D627 and D410 are the options. Do you know which matches the calipers?

d42davis · 10-10-2012, 04:13 PM

From the Hawk pdf catalog:
Mustang Cobra 1994-1994 5.0 HB111x.610 D412 HB183x.585 D627

mmarshall · 10-11-2012, 01:13 PM

Quote:

Originally Posted by CraigS

Mike, I read through your post and then went to Summit to try to get an idea of pricing and availability of parts. The first thing I checked was to see if Hawk makes pads for the 94 cobra caliper. As I drilled down thru the options I got to where I need the FMSI reference number. D412,D627 and D410 are the options. Do you know which matches the calipers?

Craig if you are talking about the 2 piston 94 Cobra front calipers I put on the back of my Coupe then the part # should be HB111N.610

If you are speaking of the regular single piston 94 Cobra rears then it would be part # HB183N.585

These are Hawk's part #'s from their site: Hawk Performance Brake Pads: Race Proven, Street Legal

This is what I come up with from Summitt which is also the correct Hawk part#: http://www.summitracing.com/parts/HWK-HB111N-610/

Something is wrong about the D410 listing on summitt. The pads don't match the diagram on the box. I have no idea what they are for.

Curently I have the HPS on the rear of my Coupe as the 2 piston brakes are very strong. In the end with the adjustable bias of the Wilwood pedal setup it does not really matter what pads you put on the back as the 2 piston rears are very powerful. They really transformed the braking of my Coupe. It probably would be best to match the front and rear pads for consistent balance.

CraigS · 10-11-2012, 01:48 PM

Thanks Mike. I was looking for the front caliper that you put on the rear of your coupe. I already have the Mustang Cobra rear calipers.

tirod · 10-11-2012, 04:03 PM

Another factor in balancing brake bias is what pedal is being used. The donor pedals have a lower ratio due to the power brake booster doing much of the work. A manual brake pedal ups the ratio to 7:1 and increases leverage.

Donor pedals can be modified to put out more pressure by relocating the MC rod attachment.

Building the car is the easy 5%. The factory spends months changing the prototype to make it work better as a whole, and we can't just slavishly copy or use donor parts. We are the engineers who tune it, it's why some say it's never finished. What's really happening is we learn more about what it takes to finesse the details.

d42davis · 10-11-2012, 07:15 PM

Dual Wilwood masters equal complete kit Wilwood pedal box. There is no pedal mod for this setup. Pedal mods are for donor pedal boxes.

Bart Carter · 10-12-2012, 02:20 AM

You are going to find out that brake balance depends on tire traction. The more traction, the more weight transfer. The more weight transfer, the less rear bias is needed. You can only test for this dynamically. You would have a completely different bias for sticky track tires, street tires and driving in the rain.

A good brake proportioning valve has knee points that vary the bias according to how hard you brake and lets you drive under different conditions without having to adjust your bias manually.

My preference, even with dual masters, is to set up a proportioning valve to work with your system. You may be in for a good bit of testing to find out the perfect pedal ratio, cylinder size(s), proportioning valve setting, etc. But tuning is half the fun!