Brake test

I am looking at it from years of watching Indy cars as well as NASCAR. Whenever they brake to a point of lockup, I have never ever seen the rears lockup. Only the fronts. The rears should do some braking, agreed, but really only control braking. I think if you succeed in obtaining your split at the percentages you are looking for, you will find your rears will lock and the car will spin. Having said that, not sure you are getting to speeds where the weight transfer to the front on heavy braking is so much that the rears will lock. On a road course, I think you will find this is not a preferred setup. I understand your line of thinking, but will have to see it I guess to believe it. Regardless of the soccer mom or not, they use the brakes hard too and engineers have produced millions upon millions of cars that brake safely.

Keep us updated on your changes, it will be interesting for sure :D
 
Well from a novice, this is a great thread with a ton of good information. Thanks to both of you.

I guess I can see the weight shift being a little greater percentage wise and maybe a little faster on our short wheelbased raodsters compared to the coupe, but I also kinda see what they are saying too about the brake bias. Tim you said "I am looking at it from years of watching Indy cars as well as NASCAR. Whenever they brake to a point of lockup, I have never ever seen the rears lockup."

and Mike quoted "As a general rule, 60% of your braking capacity should be on the front tires. Whatever the percentage is for your particular car, the front tires should lock up slightly before the rear tires."

So I think you are both saying and seeing the same thing. I think the difference is a small percentage of difference in brake bias and the style, tire width, down force and weight of the cars. I think the concept is a great starting point for any car. Thanks for keeping this good discussion going.
 
Great article Tim! I think tha author has a good grasp on the issues. Now we need to break it down to a specific car. In my case the FFR Coupe.

Here are the issues brought up by the stop-tech article:

1. "Weight distribution of the vehicle at rest". The brake system we use was designed for a very front heavy car. My Coupe has the same weight distribution as the Ferrari 458.

2. "CG height – the higher it is, the more weight gets transferred during a stop." My Coupe has a much lower CG height than the Mustang.

3. "Wheelbase – the shorter it is, the more weight gets transferred during a stop". Coupe is not much different in wheelbase.

4. Stiffness of suspension. Not addressed by the article, but plays a major role in weight transfer. A softly sprung car transfers much more weight due to the CG change caused by the diving of the front and the lifting of the rear. Ever see a old muscle car slam on the brakes and the rear bumper comes about 4' into the air. That is weight transfer caused by CG change.

5. "Typically, the auto manufacturers design their cars to be 5% to 10% more front-biased than optimum for maximum deceleration" On the track I am not willing to give up that extra 10% Ford built in for soccer moms.

6. "Your brakes do not stop your car. Your tires do stop the car." This does not apply to my Coupe with 315F/315R but does affect your roadsters with 255F/315R. The Mustangs never had this kind of tire imbalance. Just this factor alone accounts for a 20% difference front to rear in stopping power.

7. Driver capability - I am guessing that we as a group are much more capable drivers than the average soccer mom or beginning teenager. Why......because we are muscle car fanatics. We drive some of the most brutally fast cars ever made. We go looking for the edge of the envelope when most drivers cower from it.

___________________________________________________________________________
WEIGHT TRANSFER

Here some calculations I made comparing my coupe to a Mustang regarding weight transfer. These figures only account for point #1 above. The other 6 points are not considered.

url=http://phors.locost7.info/phors01.htm]Part 1: Weight Transfer[/url]
Comes from this: Brian Beckman's Physics of Racing Series

His calculations:

"Lf = 1600 + 3200 / 5 = 2240 lbs, Lr = 1600 - 3200 / 5 = 960 lbs

Thus, by braking at one g in our example car, we add 640 pounds of load to the front tyres and take 640 pounds off the rears! This is very pronounced weight transfer."
_____________________________________________________________________________
My calculations:

Lf = 1600 + 3200 / 5 = 960 lbs, Lr = 1600 - 3200 / 5 = -320 lbs
so 960 + -320 = 640 lbs?????
_____________________________________________________________________________
So on my car:

Lf = 1080 + 2400 / 5 = 696 lbs, Lr = 1320 - 2400 / 5 = -216 lbs
so 696 + -216 = 480 lbs of weight transfer in my coupe if it had the same wheelbase, springs, and CG as the example which it does not.

That means my weights would then be 1560 front and 840 rear = 2400 total
Lf now is 1560 = .65% front
Lr now is 840 = .35% rear
_____________________________________________________________________________
So on a Mustang that these brakes were designed for:
a 1998 SVT Cobra has 1950 Front and 1425 rear = 3375 total

Lf = 1950 + 3375/5 = 1065, Lr = 1425 - 3375 / 5 = -390
so 1065 + -390 = 675 lbs of transfer

Lf now is 2625 lbs = .78% front
Lr now is 750 lbs = .22% rear

Now there are some things to consider. Our cars are more stiff and have a much lower CG than a stock Mustang. You must also take into account the larger rear tires we use which will have more friction thus more stopping power.

IMPORTANT!!!!! Notice that in our brake pressure test we did on my Coupe the pressures were a 80%/20% split. Interesting that the laws of physics say in a 1G stop the SVT Cobra has a 78%/22% split in weight distribution. Also notice that the laws of physics say my Coupe is awfully close to that 60/40 split mentioned as a target for brake bias in the other article. Convinced yet!!!!!!
 
mmarshall said:
His calculations:

"Lf = 1600 + 3200 / 5 = 2240 lbs, Lr = 1600 - 3200 / 5 = 960 lbs

Thus, by braking at one g in our example car, we add 640 pounds of load to the front tyres and take 640 pounds off the rears! This is very pronounced weight transfer."
_____________________________________________________________________________
My calculations:

Lf = 1600 + 3200 / 5 = 960 lbs, Lr = 1600 - 3200 / 5 = -320 lbs
so 960 + -320 = 640 lbs?????
Click to expand...

Mike:

Are your ????? an indication of questioning his math? His is correct based on the fact that you do division first LF = 3200 / 5 = 640. Then addition 1600 + 640 = 2240. LR = 3200 / 5 = 640 1600 - 640 = 960.

Steve
 
Steve the ???? was to allow for / welcoming someone like yourself to correct me if my final calculations were incorrect. Do you see any issues with my final numbers?

Yesterday we were doing a correction for degration of climb gradient due to bank angle and I kept coming up with a bogus answer due to punching it in wrong. Garbage in = garbage out.
 
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mmarshall said:
Yesterday we were doing a correction for degration of climb gradient due to bank angle..
Click to expand...

And you want me to check your math? :p

My revisions ----

So on my car:

Lf = 1080 + 2400 / 5 = 1560 lbs, Lr = 1320 - 2400 / 5 = 840 lbs
which results in 480 lbs of weight transfer

-------------------

Hmmmmmm??? The 480 is the same # you arrived at.

I think I'll go back to work now.

Steve
 
I hate math, I'll leave that to ya'll. Seems to me, instead of using example cars and all, why can't we use real data? Here's a link to find your center of gravity - http://www.longacreracing.com/articles/art.asp?ARTID=22

What I think we need to establish is the COG, the affect of weight transfer and suspension anomalies and tire affects. My argument on this whole discussion is the weight transfer. If you can prove to me that you are effectively counteracting the weight transferred off your rear tires by increasing your front spring rate (which I believe you are) and show me how reducing your front brake capabilities is a good thing (which I don't believe) then I will be more convinced. Seems to me that if you want your brake percentages to be closer, you leave your front braking alone and change/increase your rear braking without affecting braking up front. I think you're on track for a nice ride on an autocross course where speeds aren't that great. Try and transfer the same system on to a road course where speeds are much higher and where you not only need to think about rear to front weight transfer, but left to right as well coming/going through a corner. How are you going to compensate for a full tank of fuel vs. a 1/4 tank of fuel? How are you going to compensate for a cold tire vs a hot tire? How are you going to compensate for max grip tire pressure vs. what is comfortable for the car to run at? Cold track (cloudy) vs hot track (sunny)? These are all things that major race teams chase each week. How do they compensate? I assume by finding a happy median between "soccer mom" and "race driver". Brakes, tires and suspension all make an average driver much better. We have scales, we have cars, let's get those together along with a good algebra calculator and get this figured out on paper, apply it and see how close the paper matches actual. Just sayin' :p
 
Well I havent commented on this thread yet. Tim if what you believe is true, then just take the rear brakes off and increase your fronts. I have said it before, with near equal f/r weight, these cars need way more rear brakes that any daily driver. The low center of gravity does not transfer weight forward like a family car. The pressure test you guys took are interesting, but really dont tell us anything. Far one thing, the rear pressure has to be lower because the rear rotors are smaller in diameter. The larger the diameter the more pressure required because of rotational torque. I dont know how to calculate the needed brake force, but if you want to stop in the shortest distant, all 4 wheels need to work to their fullest.
 
So the question is - if Mike takes the valve off the fronts and they run 100% - how can he increase the rears. That is what is needed correct?

If he is successful, and if the rears start to lock of first - then the valve (installed properly) could act to reduce rear pressures.

So - how can he increase the rears a bunch?
 
Rich, the rears are needed for control, wasn't saying that at all. I was going for what Mark said. Seems to me if you increase the braking area of the rear brakes either with a larger piston caliper, twin piston calipers, larger rotors and race pads, that would be better than dialing down the fronts. I disagree that there isn't weight transfer, or maybe I misread your post...even if there isn't nose dive from the weight transfer, there is still a forward motion that needs to be stopped. Newtons Law, right?
 
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Mark you are on to something here. My main contention is that the rear caliper is just too small to have balanced brakes. Even by reducing the fronts down by 1500 psi (hydraulic line pressure not caliper pad pressure) the rears still did not lock 1st.

I am thinking of mounting my 2 piston cobra brakes on the rear to try to give a little more power. Before anyone has a cow...just endure a little more of my math. Tim just look away for a minute!

1 piston GT caliper has a single 63mm piston which = 3117sq mm (currently on front)
2 piston Cobra has 2 38mm pistons which = 1134 sq mm x 2 = 2268sq mm (to be mounted on rear)

3117 + 2268 = 5385 sq mm total area
2268(rear) / 5385 (total) = .42 or 42% rear brake force
3117 (front) / 5385 (total) = .58 or 58% front brake force

This is awfully close to the magic 60/40 split. The 2 piston cobra calipers have double the surface area of the single piston cobra's that are currently on the rear which is exactly what my previous calculations indicated would be needed. This combo should get me within 10% of a balanced system which can easily be fine tuned with brake pad compounds and a proportioning valve if necessary.

BTW does anyone have access to a plasma cutter????
 
Check with disc brakes r us, seems they may have a bracket you are looking for. I really think this is the path to take, proportion the rears if necessary. That last math problem I understood :p
 
I have to wonder if there is an easier way to accomplish this. The problem seems to be not enough PSI pressure of the fluid - is that right?

Could it be the lines?

Could you need a booster of some sort?

Is there any way to check pressure before it gets to the caliper?

Is there anyway to improve it, before the caliper?

ALSO - does your engine make enough vacuum?


Sure would hate to see you spend money on parts with no improvement afterwards.
 
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Mark I already have a boosted brake system using a hydroboost. The rear line pressure is about as high as it can get. Money is not an issue on my plans as I already have the 2 piston caliper setup so the only expense will be for fabricating a bracket. If I stay with the current system it will still cost me $60 to replace the bad right rear caliper we found during testing. So now is the time to make a change

Tim I just spoke with disc brakes r us and they were very helpful. He said they only make a nonstandard bracket to put t-bird rear calipers on a 13" rotor which won't help me any. He did give some other suggestions on how to modify my current setup which I may try.

Also you are right there will always be weight transfer under braking. Even the F1 car in the article I posted has a 10% weight transfer. My only point there was that the calculated weight transfer of the mustang won't be the same as my Coupe and my Coupe will have more than the f1 car.

I would like to do most of the bias/balance without a rear proportioning valve. My 1st valve already failed. Had that been in the rear system the brakes would have gone from a reduced state (which is safe) to a non-reduced state (not safe). If this failure happens at the wrong time it could cause the rears to lock and cause bad things to happen. With the PV reducing the fronts to match the rears I did not even notice the failure due to how small the change was.

Thanks everyone for all the brainstorming and great ideas....keep then coming!

BTW any suggestions on a plasma cutter. I have access to a cutting torch but not a plasma cutter.
 
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If the Lincoln caliper would use the same bracket then it might be worth a try. 2 problems they brought up are limited brake pads available and you still need a custom bracket. Plus it is only a 40% increase which without some really aggressive pads would still be a little shy of a balanced system.

BTW I found out today that Disc brakes are us makes a bracket to mount the t-bird calipers with 13' rotors.
 
Mike, bolt pattern is the same for T-bird and Lincoln, and I'm sure you can get the pads. I have a set of Lincolns if you want to see if they fit.
 
I think it may be worth a try!

Here are the ratios I calculate:
Current brakes - 74%F / 26%R
W/2 piston cobra - 56%F / 44%R
W/ Lincoln rear - 64%F / 36%R

This is also getting awfully close to the magic 60/40 balance.

Rich PM me your info and I will swing by when I get back in town this weekend. I may grab the pressure gages from Tim and see what kind of pad pressures the Lincolns will produce.
 
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Same Calipers

The Thunderbird SuperCoupe and the Lincoln Mark VIII use the same rear brake system. Both use the same rotor and caliper. The caliper is 1.78 bore.

There is some mis-conception/wrong info floating around the internet about the calipers being different. NOT SO. The mistake is attributable to the Dorman parts catalog that shows the rear bore diameter as 54mm which it is not. That is the dimension for the front.

Brackets to mount that caliper onto a Ford axle, whether straight or IRS are readily available from multiple sources.

Regarding the proportioning valve/bias settings, I view those as a thing of last resort. The last thing you want to do is put something into the system that decreases the pressure. All that does is make you pedal effort higher. Wouldn't you rather have a lower pedal pressure for the same braking effort?

You are better off to mechanically configure the bias thru master cylinder sizes and balance bars. You then add an adjustable rear proportioning valve to make on-track adjustments to configure for fuel usage or track condition changes. As the fuel load gets lighter (remember, these cars used to carry 42 gallons of fuel), you could then dial in more bias so the rears remained in balance - the last thing you want is the rear breaking loose in a tight, high speed turn. When the driver felt the system getting squirrelly, he could increase the bias slightly to regain control.

My 2c

Paul
 
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