Hey you guys,

You see the latest Race Tech? The article on brake bias or whatever? Says in there that conventional balance bars have bias variance and bias shift, which they say might be attributed to hysterisis or...? And they say that fancy stuff like Tiltons spherical MC's and U-joint balance bars reduce but don't eliminate it.

Have you thought about this stuff?

You've seen that generic pressure chart showing front against rear pressure and optimal balance at threshold? And as the design g's go up, the balances needs shift frontward? So the graph is parabolic about the x axis. And you read somewhere that since we immediately go to max g's braking we just need to trim our balance to hit that spot?

Do you see something here?

As you trail off the pedal, trying to fill out the circle, what are you? Front biased - Duh.

So is the better answer a tandem MC and a prop valve? I'm thinking yeah. I'm also noting not unhappily that that's cheaper. And how handy that the rear calipers I want to use have lots more piston area than stock. Yeah, I know that Stoptech's white paper says hit the balance close with no prop. All within reason, and what comes a'what.

?

Scott, who has been visiting his kaa ocassionaly like a relative in a long term coma...

 

I thought you were dead.

 

I've been trying to grasp this brake bias thing for a while now. Anyone care to elaborate further on this subject?

 

Can you tell me what month it's in? I'd like to order it from them. Thanks!

 

I have been alive und well und living in Argentina.

It's the Feb 2009 issue of Race Tech. They promise to offer ways of improving the situation in the next issue.

Scott, who has to log off...I don't want them to find me...

 

me too....good to see you alive my west coast nemisis!!

chad

 

I never knew you saw me like that Chad. But then I am (or was) a man of many nemesi.

Scott, who feels a bit like dead what with not drivin for so long and all...I didn't think the good times would be so short...

 

Hi Scott. Darn, latest Race Tech issue I have is January 2009, with the cheap sensors and stuff.

But I have recently made a big spreadsheet on brake bias because I and and a friend of mine are designing a brake system for his Lotus and we have fitted some Alcon Advantage B calipers inside a Team Dynamics 15x7 wheel using 304x32 rotors we are getting custom made by HRP. And they fit on my ITR. So....

Brake bias and required brake pressure/torque for the rear vs front wheels is a point of confusion with us. We have sent our spreadsheet to AP Racing in England (cause we were at first thinking AP) and they said there are issues in brake systems of efficiency, which have to do with system flex, as in calipers, lines, etc. The spreadsheet has a balance bar option and an AP proportioning valve option. But required brake pressures on the rear to keep all tires at max grip depending on forward weight transfer don't seem to make sense. While books I have show charts of pressure requirement dropping as a function of g's in the rear, there are no calculations or example of the numbers. When we work it out, numbers look a little wrong and would make you select brake piston areas front and rear that don't make sense, All AP Racing guy wanted to say was not much!

 

You don't mention how you did the calculations, so we have no idea why the numbers you came up with look funny. Maybe if you explained more, somebody who was knowledgeable in Mechanics of Materials|Dynamics|Differential Equations|Physics 2 could help you out.

 

descartesfool, is your spreadsheet like this one? brake balance spreadsheet

 

is there a problem here?

 

No, it's much more comprehensive than that one. It includes charts of brake torque, pressure front and rear vs long. g's, brake proportioning valve with variable knee, etc.

The issue with the results is the apparent brake pressure required to keep rear tires at maximum grip throughout the braking sequence. The shape of the curve is correct, as brake pressure required at the front goes up while brake pressure required at the rear falls off compared to a straight line as g's increase. It is just that when you simulate OEM sizes for pistons for say an ITR, a CRX and an NSX for example, the rear brake pressure required is higher than the front pressure in many case, which does not make sense. If one assumes that the rear tires are not held anywhere near lock-up while fronts are, then brake pressures are much lower. The question is how close to max grip for the rear tires do OEM and racing brake systems. I have never seen a single book or magazine article that did quantitative calculations. That is why we asked for AP Racing's help, and sent them my stuff. They just did not seem inclined to respond, except to say that they thought we seemed to be having fun with charts and spreadsheets. It is certainly possible I have made an error in the spreadsheet, but we have checked it against a ton of other spreadsheets. Many of them have errors, but most agree with ours. It is just that they do not follow through the entire braking sequence with the calculations. Computing brake balance required simply at max grip tells you nothing about brake proportioning valve required.

I will try to post up some charts later.

I just picked up the Feb 09 issue of Race Tech at lunch. It shows the exact brake issue I found my friend had in the first chart. I brake very hard core, at over 1 g almost every corner, while he brakes at much less than 1G, and only gets to max on a couple of corners. But he corners very quickly. We both have DL-1's and I overlayed our longitudinal g's for same track, same day but different cars. My curve looks like the pro driver one while his looks like the non-pro driver. I am trying to help him stomp the middle pedal a little harder, and also design a brake system for his Lotus. Hence I wrote spreadsheet, and that got me wondering about proportioning valves and balance bars.

 

So are you just going to come on here every year or so Scott and post something up?

You should try to make it back out to the ITR Expo this year, in IL.

 

I dunno. I just had something on my mind, and after all the back and forth of the last few years, this is where many of the people I like to talk to have stuck or returned to.

Would love to go to the Expo. But not realistic under circumstances. Maybe the next one. I really don't want my car to become some kind of barn find in 50 years. Working on it.

Scott, who if he had the time would start to work on his cars endocrine system...

 

Here is a sample of data and chart for an OEM NSX setup to show issue of high brake pressure seeemingly required to keep rear tires near max g's.
I get similar charts for an OEM CRX or ITR:

 

Sweet Jesus, it's Rinde

 

I can't tell if this is a good thing, or a bad thing.

CLAUDE!!!!!!! You make my head assplode. I do get it about selection of disc diameters and piston area, but that graphic isn't the most helpful thing I ever saw. And since grip varies somewhat no equation if going to spit out The number. You're going to turn a **** or two and find where you want to be manually. You just need to get in the ballpark for a starting point.

Anybody got any thoughts to offer on my original question regarding the bias drift frontward on trailing brake, and it's inherency in vehicle dynamics, and the use of a tandem (like you stock Honda) master cylinder and an adjustable proportioning valve in lieu of the standard pair of master cylinders and a balance bar? Holes in my thesis large or small?

Johnny Mac? I know you can think about other things besides airfoils and Reynolds Numbers and such.

Scott,

 

Hey Scott,

I wanted to find a copy of that issue of Race Tech so I could read the entire story. If anyone knows where I can get a reprint of the story, I would be glad to pay for it. And yes, I know, I should be reading RT each and every month since some of the authors in RT are - in my opinion - some of the best in motorsports and they have plenty of wisdom to share with the rest of us. I have some ideas about the brake discussion, but I'd like to wait a bit do some thinking on this discussion.

 

First question to ask yourself is if your idea is so awesome, are the F1 people doing it?

To answer your question, yes, the shifting of brake bias as a function of total braking force is a good idea for stopping in a straight line over various surfaces, e.g. dry road, wet road, snow. As you INCREASE the braking, the bias should shift FORWARDS.

In terms of braking during a turn, you have it backwards. You do not become front biased, you become rear biased (relatively). If you brake during a turn, the front circle of traction grows and the rear one shrinks. This is why cars oversteer if you hit the brakes in the middle of a turn.

Why don't I be a pedantic *** and work thru an example.
First make some assumptions:
1) circle of traction for an individual tire is a circle (-it's not)
2) you are making a high speed turn, so ignore inside tire vs outside tire slip angle stuff
3) a tire's grip is linear vs weight, until some break away point ( -it's more complicated)
4) because of 3), you can assume that the car only has two wheels, a front and rear (like a motorcycle)

Also assume: Car weight 2000 lbs,
has 50/50 weight distribution,
has 90/10 brake bias
can threshold brake at 1g
cg height gives weight shift of +/- 800 lbs per g
coefficient of friction is 1

Driving in a straight line, the weight on each tire is 1000 lbs.

Under threshold braking, the tire weights are now
WF=1000+800g=1800 lbs
WR=1000-800g=200 lbs

The front circle of traction has radius rF=1000+800g=1800,
thus it makes 1800 lbs of force

The rear circle has radius rR=200, and makes 200 lbs force

200+1800=2000lbs = 1g, so good job, you set the proper brake bias to maximize both tires.

Now you start trail braking. You lift off the brakes a touch to make only 0.9g deceleration. How much cornering force can you make?

The front wheel weight is now 1000+800*0.9=1720, and so is the radius of the circle of traction.
The rear weight is now 280.

The front tire deceleration force (y axis) plus the rear decel force must equal 1800 lbs (0.9g)

FF_y+FR_y=1800
Note that FF_y^2+FF_x^2 =abs(FF)<=1720
Also, for the rear abs(FR)<=280

The other condition is that FF_x = FR_x (sideways forces much be equal or else the car will over/understeer)

Going back to your 90/10 brake bias:
FF_y=9FR_y
Thus FR_y=180 lbs
FF_y=1620

Now compare these values to their respective circle of traction:
Front circle of traction has radius 1720, and y component 1620. Busting out the Pythagoras theorem, the maximal x component it can make is sqrt(1720^2-1620^2)=577 lbs

For the rear circle, sqrt(280^2-180^2)=214 lbs

Oh no, it looks like the rear is limiting your total grip. But you knew this already, since trail braking tends to make the rear end come out. This is what I mean when I say that you are rear biased, relatively.

Now figure that you have a co-driver that will turn the bias **** for you. After you turn in, lets say he cranks the bias **** to 92/8 (yes, moves the bias even more forwards)

If you do the math again, you'll see that the front makes 1656 lbs decel force and the rear makes 144 lbs decel force. The radii of the two circles of traction are still the same as before since the deceleration is the same. Do that pythagoras stuff again, and you come up with a rear sideways force of 240, and front of 464. Hey, you can pull more sideways g's now! By moving the bias FORWARDS when you DECREASE the brakes. Hmm, that's the opposite of what I said before...

However, to have brake bias as a function of total brake pressure work as you would like, you would need some kind of 2D accelerometer, to know if you are trying to stop in a straight line, or stopping during turning. This is effectively what "stability control" does for you.

 

The only way you could know if there is this hysteresis/variable fiction/stiction effect is to measure the brake pressures and see if bias changes in terms of pressures as you apply the pedal a bunch of times. A tandem master cylinder would not have this effect, so it is simpler and cheaper, just not adjustable. A prop valve does not replace the balance bar, since it only reduces the pressure to the rear after you reach the threshold of the prop valve. So if you need to change the balance with the tandem MC, only easy way is to change the piston sizes, pad coefficient or disc radius. So you need pressure gages on the two circuits to know the answer.

The generic pressure chart is in that chart I posted. It shows front and rear pressures, front and rear torques, and effect of feeding front pressure into an AP adjustable prop valve. The generic chart, say from Puhn's book, is what I have plotted. Not true that you immediately go to max g's. G's build with time. Here is a chart of me (ITR with NSX front and stock rear calipers) and my buddy (ITR with Wilwood, unknown prop valve and some non original rear Honda calipers) on same track, same tires (PSC), same day, same lap.

So in the following data logger charts very similar to that shown in the Race Tech article, you can see the differences between two Hondas and two drivers. This is the sector at the end of the long straight at Calabogie track. You can see that I go faster into the corner (119 vs 108.4 mph), I brake later (229 metres vs 213) and harder (1.052 g's vs .599 g's), and that it takes time to get to max G's from initial deceleration point. My buddy is gun shy about his braking because he has turned car around a few times since his brake setup is non ideal, modified with unknown parts, and has unknown bias. We are trying to solve it using the spreadsheet.

 

There's nothing wrong with being a pedantic ***. But you gotta be on the right page, and you gotta be right.

Have you read the RT article under discussion?

Scott, who is remembering why he gave this all up...

 

I've always liked the word pedantic...

 

don't worry to much about him, he likes to ask question but he "already knows the answer". And acts like he has it all figured out.

 

I stopped reading your post right there, please please refrain from being a complete and utter douche, thanks.

good to see you back Rinde

I worked on My R just last night

 

sorry, i must have missed your point. I thought you were complaining that you become too front biased upon trail braking. never mind, then.