96DXCivic
96DXCivic Reader
4/20/09 4:49 p.m.

I have several questions about brakes.

1 How can you figure out if a car will lock all four wheels under braking?

2 Does anyone have an experience with the Outlaw 1000 series caliper vs. the Wilwood Dynapro Single billet? And are there any other fixed two-piston calipers for under $150 that I should look?

3 What is the operating temperature range of the brakes of a FSAE car?

iceracer
iceracer Reader
4/20/09 4:59 p.m.

Remember, brakes stop the wheels, tires stop the car.

96DXCivic
96DXCivic Reader
4/20/09 5:03 p.m.

I am aware of that fact. These brakes are for a FSAE car. And one of rules states that you must be able to lock all four wheels under braking. I have figured the equations to find the brake torque but how do I know if this is enough to lock the wheels.

fiat22turbo
fiat22turbo SuperDork
4/20/09 5:06 p.m.

1) Without actually going out and trying it? You'd have to do a lot of math and model the car, the driver, the environment, the tires, etc.

If you want to how to tell when driving, try hitting the brakes hard, if the car brakes in a straight line and leaves four black marks, then you're in good shape. If it spins, then the rear is locking up first. If it slides relatively straight then it is probably only the fronts locking up.

2) Dunno

3) Assuming you aren't looking at carbon and/or ceramic braking systems, it should be the same as nearly any other car as the pads will have a working heat range and you have to balance the braking system components to work with this range. The amount of thermal mass you need to dissipate that heat between braking cycles is different given the mass of the vehicle.

AngryCorvair
AngryCorvair Dork
4/20/09 5:16 p.m.
96DXCivic wrote: I am aware of that fact. These brakes are for a FSAE car. And one of rules states that you must be able to lock all four wheels under braking. I have figured the equations to find the brake torque but how do I know if this is enough to lock the wheels.

brake torque divided by rolling radius of tire equals retarding force at contact patch. if that is greater than the product of the corner weight times the peak grip of the tire, then that tire will lock up.

all equations from pedal to contact patch are in the june '08 GRM, "Think Globally, Stop Locally"

96DXCivic
96DXCivic Reader
4/20/09 5:23 p.m.
fiat22turbo wrote: 1) Without actually going out and trying it? You'd have to do a lot of math and model the car, the driver, the environment, the tires, etc. If you want to how to tell when driving, try hitting the brakes hard, if the car brakes in a straight line and leaves four black marks, then you're in good shape. If it spins, then the rear is locking up first. If it slides relatively straight then it is probably only the fronts locking up. 2) Dunno 3) Assuming you aren't looking at carbon and/or ceramic braking systems, it should be the same as nearly any other car as the pads will have a working heat range and you have to balance the braking system components to work with this range. The amount of thermal mass you need to dissipate that heat between braking cycles is different given the mass of the vehicle.

We are a basically rookie team so we have no car to play with. So I was put in charge of the brakes and I am having to work thru all the equations.

billy3esq
billy3esq Dork
4/20/09 8:14 p.m.
AngryCorvair wrote: brake torque divided by rolling radius of tire equals retarding force at contact patch. if that is greater than the product of the corner weight times the peak grip of the tire, then that tire will lock up. all equations from pedal to contact patch are in the june '08 GRM, "Think Globally, Stop Locally"

I know I'm asking for it questioning a well known braking system article author and real life brake engineer, but does the corner weight used in your calculation above need to be a dynamic one that accounts for the weight transfer under braking? Or, is the effect of the weight transfer so slight for something like an FSAE car that it doesn't really affect the retarding force at each wheel? Or, is there some other reason I haven't thought of that the static corner weight can be used?

Full Disclosure: I have not reviewed my copy of "Think Globally" recently.

I'm not trying to be a smartass, I'm really curious. (That's the second time I've posted that disclaimer today.)

AngryCorvair
AngryCorvair Dork
4/21/09 9:02 a.m.
billy3esq wrote:
AngryCorvair wrote: brake torque divided by rolling radius of tire equals retarding force at contact patch. if that is greater than the product of the corner weight times the peak grip of the tire, then that tire will lock up. all equations from pedal to contact patch are in the june '08 GRM, "Think Globally, Stop Locally"
I know I'm asking for it questioning a well known braking system article author and real life brake engineer, but does the corner weight used in your calculation above need to be a dynamic one that accounts for the weight transfer under braking? Or, is the effect of the weight transfer so slight for something like an FSAE car that it doesn't really affect the retarding force at each wheel? Or, is there some other reason I haven't thought of that the static corner weight can be used? Full Disclosure: I have not reviewed my copy of "Think Globally" recently. I'm not trying to be a smartass, I'm really curious. (That's the second time I've posted that disclaimer today.)

Yep, it's a dynamic weight, so you'd use the peak mu of the tire as the decel in the weight transfer equation.

No, static isn't good enough. This is a race car, and if it ain't 100% right then it might as well be 0% right.

But seriously, in the real world, only the engineer on the car can say whether or not good enough really is good enough. Run the numbers with static and dynamic values, and see if the difference would force you to change component sizing or prop valve settings.

96DXCivic
96DXCivic Reader
4/21/09 1:10 p.m.
AngryCorvair wrote:
96DXCivic wrote: I am aware of that fact. These brakes are for a FSAE car. And one of rules states that you must be able to lock all four wheels under braking. I have figured the equations to find the brake torque but how do I know if this is enough to lock the wheels.
brake torque divided by rolling radius of tire equals retarding force at contact patch. if that is greater than the product of the corner weight times the peak grip of the tire, then that tire will lock up. all equations from pedal to contact patch are in the june '08 GRM, "Think Globally, Stop Locally"

Is the peak grip the same as the coefficient of friction between the road and the tire? (I don't have that issue but I am going to order from the store)

walterj
walterj Dork
4/21/09 1:19 p.m.
96DXCivic wrote: Is the peak grip the same as the coefficient of friction between the road and the tire? (I don't have that issue but I am going to order from the store)

I think you need the weight component - the calculated force of load smashing the tire into the road at full deceleration.

AngryCorvair
AngryCorvair Dork
4/21/09 3:00 p.m.
96DXCivic wrote:
AngryCorvair wrote:
96DXCivic wrote: I am aware of that fact. These brakes are for a FSAE car. And one of rules states that you must be able to lock all four wheels under braking. I have figured the equations to find the brake torque but how do I know if this is enough to lock the wheels.
brake torque divided by rolling radius of tire equals retarding force at contact patch. if that is greater than the product of the corner weight times the peak grip of the tire, then that tire will lock up. all equations from pedal to contact patch are in the june '08 GRM, "Think Globally, Stop Locally"
Is the peak grip the same as the coefficient of friction between the road and the tire? (I don't have that issue but I am going to order from the store)

yes. and like walterj said, it does depend to some extent on the vertical load on the tire.

since you're talking about FSAE, the tire supplier might give you some really valuable tire data, which they typically guard very closely. i can help you make sense of that data, and of course i would honor whatever non-disclosure agreement you have to sign with the tire supplier.

96DXCivic
96DXCivic Reader
4/22/09 11:28 a.m.

Ok so I got told by a tire manufacturer a ball park figure of a longitudinal frictional coefficient of 1.8-2.0. Is this the same as peak grip?

AngryCorvair
AngryCorvair Dork
4/22/09 11:41 a.m.
96DXCivic wrote: Ok so I got told by a tire manufacturer a ball park figure of a longitudinal frictional coefficient of 1.8-2.0. Is this the same as peak grip?

yes.

96DXCivic
96DXCivic Reader
4/22/09 11:46 a.m.

Thanks for all the help

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