Brake bias and testing

[AA-ron]

I finished my roadster about 3 weeks ago and everything feels great. I'd like to test the brakes beyond what I've done so far (a few hard stops here and there). I set my brake bias based on the build manual's recommendations but I'd like to put it to the test to verify things are working correctly. My question is this: can I test wheels locking up on a dirt road? Also, what's my target? Should all 4 lock up at the same time or should the front lock first? I ask about the dirt road because there's one next to my house and it would make things very easy. I have performed one test on this dirt road and the front wheels locked about 10-feet earlier with the rear only locking just prior to a full stop at about 15mph.
This car will never see a track I just want it to be as safe as it can be on the road.

[egchewy79]

the balance bar should be biased slightly towards the front wheels. get all 4 to lock up at the same time then slowly adjust the bias towards the front. and yes, dirt or gravel rd is fine to test.

[CraigS]

Disagree on the dirt/gravel road. Here is why. You get the brake balance perfect on a normal road but what happens in reduced traction conditions like dirt, gravel, rain? This is not intuitive so bear with me. As the traction drops off the car can't stop as hard. So the extra weight that is usually transferred to the front brakes is reduced. So now you have too much front brake power and one or both front wheels lock up. That's not great fun but it beats the heck out of the rears locking because that generally causes a quick, no way to counteract it, spin. I test brake bias on a good road at about 40mph. I adjust until the rears lock first, which can get exciting fast and is the reason I do it at 40. Then I adjust just enough to get the fronts to barely lock first. BTW I only do 3 hard stops in a row, and then I drive for 5-10 minutes trying to use the least brake I can so they can cool down. If you are using different compounds front to rear this is especially important. Different compounds have different friction, after all that is the reason we do that. But they often also have a different rate of change as temp increases so you could have perfect balance on the first stop, but repeated stops could cause the balance to change.

[phileas_fogg]

Craig,

Do you bed the brakes before you adjust for bias? Seems like bedding would come first.


John

[rich grsc]

Disagree on the dirt/gravel road. Here is why. You get the brake balance perfect on a normal road but what happens in reduced traction conditions like dirt, gravel, rain? This is not intuitive so bear with me. As the traction drops off the car can't stop as hard. So the extra weight that is usually transferred to the front brakes is reduced. So now you have too much front brake power and one or both front wheels lock up. That's not great fun but it beats the heck out of the rears locking because that generally causes a quick, no way to counteract it, spin. I test brake bias on a good road at about 40mph. I adjust until the rears lock first, which can get exciting fast and is the reason I do it at 40. Then I adjust just enough to get the fronts to barely lock first. BTW I only do 3 hard stops in a row, and then I drive for 5-10 minutes trying to use the least brake I can so they can cool down. If you are using different compounds front to rear this is especially important. Different compounds have different friction, after all that is the reason we do that. But they often also have a different rate of change as temp increases so you could have perfect balance on the first stop, but repeated stops could cause the balance to change.
If you're going to use dirt or gravel, hell, then you might as well try snow & ice too.:rolleyes:

[CraigS]

Yes, bed the brakes first.

[F500guy]

During my racing time, I have had the unfortunate experience of making contact with immovable barriers in hard brake zones due to not anticipating and correcting rear wheel lock up. Gravel will not give you the most weight transfer and grip on the front axle that you would experience in a panic type stop. The first second of the emergency braking will likely make the rear much lighter and have less grip, it only takes a second and when the rear comes around, watch out! I think Craig is is on the right track, but be conservative and keep more stopping power in the front.

[AA-ron]

This is exactly the type if info I was looking for. I appreciate all the explanations regarding weight transfer and why testing on pavement is essential. I do have a nice quite (semi-private) road where I can do some testing with the owners permission. I was thinking of using a video camera to help spot exactly when and where things lock up. I must say, I love the way the brakes feel in this car. Of the cars I've owned with manual brakes, the pedal effort is less on this one than any of the others. In a few of the hard stops I've made, it tracks perfectly straight all the way.
Thanks for all the advice!
Now, if we can just fix the brake dust clouding up my shiny new rims.... :)

[scrubs]

The primary way to adjust the bias will be the differential amount of rod into the clevis which will affect the total travel of the rod. The balance bat is more fine tuning.

I just went thru this and you can also reach out to MB750 who spent a while on the phone with Wilwood and this was the gist of it.

[MB750]

Thanks Scrubs

I had a recent chat with Dave at Wilwood about this balance bar mess. The FFR manual leaves a bit to be desired regarding how it exactly functions and I wasn't satisfied leaving well enough alone.

Summing up my conversation with Dave:

The balance bar is really only for very fine tuning, a couple percent change in bias at most. He says it's used mostly during racing to adjust trim to increase fuel mileage (or something to that effect). I'm not a racer so I don't fully understand that unfortunately.

The most bias adjustment comes from the pushrod length. You want the pushrod for the front MC to be longer than the rear at 0% brake pedal effort, then you want them even at normal braking (like for normal slowing down). Balance bar should be parallel to the footbox frame at this level of braking. You'll have to make adjustments to the pushrods, then test movement. I can't just say "give the front MC clevis two turns CCW" since your calipers might be smaller than mine which would lead to less travel in the MC pushrod. And your brake pedal might hit the chassis 3/4" tubing depending on where yours is positioned. Ultimately, get a feel for the pedal resistance during normal braking, then replicate that feel in your garage while you're adjusting your pushrods.

At 0% braking, the SS plate on the end of the brake pedal should be tilted back about 15 degrees from vertical. To clarify, when facing the side of the brake pedal from the drivers side, the top of the SS foot plate should be pointing a little forward. This is ultimately dictated by where the brake pedal is positioned based on pushrod length adjustment. My brake pedal can't go any more forward due to a 3/4" chassis tube and it's about 13 degrees using an angle finder app on my phone. Then at the onset of braking that SS foot plate should be vertical. If it goes too far your MC's are too small or your pushrods aren't adjusted correctly.

Smaller MC's will give you more braking pressure (hydraulic theory...) but will put more "sweep" in the brake pedal because less volume of brake fluid will transfer to the calipers during use.

If you've got 3/4" MC's, than your foot pushing down with 100 lbs of force puts 710 psi into each brake circuit. This is plenty for street braking.

His recommended way to adjust the balance bar is similar to braking on gravel, only you don't have to leave your garage but you need two friends to help: You sit in the car, one friend is at the front wheel, the other at the rear. They both start spinning the tires while you slowly start applying the brake. Ideal bias adjustment has the friend at the front wheel noticing the brakes dragging before the rear. This process is great in theory but when I try spinning my rear wheel I'm also spinning everything in the whole axle, the driveshaft, and half the transmission so there's a BUNCH of drag to begin with. And I don't have two friends. I'll stick with the gravel road test.

[CraigS]

Video could definitely be helpful. It is often hard to determine exactly which is locking first. If you have one front wheel locking first you can usually feel it in the steering wheel. Since the sliding tire has a little less grip, the wheel will usually get a slight jerk to one side. But, if they both lock equally, it is harder to feel. Now if the rear locks first that is easier. Hopefully you don't hold it long enough to get sideways. But if the rear locks, as MB750 alluded to, the whole driveline including the engine stops which puts a real obvious judder into the car. One other note I mentioned in a similar thread. Be careful dropping from 3/4 to a 5/8 MC. Over the years I had several rear brake systems on my mkII. Car always had 3/4 front and 5/8 rear MCs. When I did the IRS retrofit I used the OEM rear brakes. They were a much larger caliper than the old ones I had used. First drive I noticed that the pedal travel got longer. Later as I got everything set and started bias testing I realized that the rear MC was too small. It simply could not move enough fluid under max braking to lock the rears. Upgrading the MC size fixed that problem.

[J R Jones]

Long ago Mechanics Illustrated magazine had Tom McCahill covering auto evaluations. https://www.hemmings.com/stories/article/uncle-tom-mccahill
He described his technique as a sandpaper azz. He could feel what a car was doing tactilely.
Early in my work experience I was at a dealership where the service manager taught me to diagnose and troubleshoot auto performance. I did 30+ evaluations per day. I assumed that driving was critical to the task but more recently I navigated a vintage 54 Corvette in LA Carrera Pan America. The intensity of the speed sections resulted in "feeling" the car from the passenger seat and indeed I could feel the individual rear tires break loose as they crossed the centerline under maximum braking. It did not end well with that specific dynamic on a steep downhill into a hairpin that resulted in oversteer into a guardrail. We did not have enough set-up time and indeed there should have been less rear brake bias.
jim

186946

[MB750]

Fortunately for the vast majority of us building FFR Roadsters, slapping the FFR supplied brakes onto the car, putting the front MC clevis a little in front of the rear MC, and properly bleeding the system will result in a car that stops faster the harder you hit the brake pedal. You might even lock the tires up, and you'll look cooler than anyone else on the road mostly due to the car you're driving.

I'm a "tweaker", in that I love fully understanding all mechanical systems and tweaking them to work best for me and my application. If something has an adjustment, I want to know how the output is effected by the entire range of said adjustment. If I don't understand something, I dive in head first until I'm happy I know things to the best I can. That's where I went with the balance bar system.