[rbx]

do you base your calculation on braking forces affecting the rear suspension by...

a)the difference in velocity between the rotor and the caliper?
(the caliper doesnt move in a synchronised manner with the rotor)

b)calculating the moment from the tire ground contact patch to the IC?

which one should be used?

 

[BRacing]

"IC"?
what does "IC" stand for?

Thanks
-Brad

 

[Silver]

Instant centre, I think.

(Keep in mind, I very nearly flunked out of introductory calculus. Could never get my head around it. If I'm wrong, I'm sure someone smarter than me will correct me)

 

[BRacing]

I'm not sure what exactally you're looking for. When you say "braking forces" I'm unclear what final result you are trying to achieve.

Are you looking for effectiveness for brakes? The necessary coefficient of friction to stop the bicycle? etc...

what will finally determine what stops a bicycle will be the tires and their contact with the ground.

-Brad

 

[BRacing]

Ok, so this started to bug me. I went back into my Automotive Theory books and found a few definitions that may help with whatever you are trying to figure out. Obviously these are for automotive applications, but bicycle disc brake are nearly identical.

Brake bias: The term used to indicate the ratio between the amount of brake torque exerted on the front brakes compared with the rear. Brake bias is normally expressed as a percentage of brake torque at one end of the car to the total brake torque, as in "60% front".

Braking efficiency: The ratio of actual deceleration achieved on a given surface compared with the theoretical maximum.

Braking torque: Braking torque in pounds feet on a single wheel is the effective rotor radius in inches times clamping force times the coefficient of friction of the pad against the rotor all divided by 12. Braking torque is the force that actually decelerates the wheel and tire. To increase the braking torque it is necessary to increase the line pressure, the piston area, the coefficient of friction or the effective rotor diameter. Increasing the pad area will not increase the braking torque.

Clamping force: The clamping force of a caliper in pounds is the brake line pressure multiplied by the total piston area of the caliper in a fixed caliper and two times the total piston area in a floating design. To increase the clamping force it is necessary to either increase the line pressure or the piston area. Increasing the pad area or the coefficient of friction will not increase clamping force.

Coefficient of friction: A dimensionless indication of the friction qualities of one material vs. another. A coefficient of 1.0 would be equal to 1g. The higher the coefficient, the greater the friction. Typical passenger car pad coefficients are in the neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. With most pads the coefficient is temperature sensitive so claims that do not specify a temperature range should be viewed with some suspicion. The optimum is to select a pad with a virtually constant but decreasing coefficient over the expected operating range of temperatures. As a result, the driver does not have to wait for the pad to heat up before it bites, and the pad fade will not be a factor so that modulation will be easy (see "plot shape").

Hydraulic ratio: The ratio of fluid displacement by the master cylinder to fluid displaced in the caliper pistons. Hydraulic ratio is an important factor in the pedal effort equation, the higher the ratio, the less pedal effort is required (and the longer the pedal travel to achieve a given clamping force). The stiffer the caliper and the stiffer the pad, the higher the hydraulic ratio that can be employed.

Mechanical pedal ratio: The brake pedal is designed to multiply the driver's effort. The mechanical pedal ratio is the distance from the pedal pivot point to the effective center of the footpad divided by the distance from the pivot point to the master cylinder push rod. Typical ratios range from 4:1 to 9:1.The larger the ratio, the greater the force multiplication (and the longer the pedal travel)

There will be a quiz next week. Class dismissed.
-Brad

 

[rbx]

Originally posted by BRacing
Ok, so this started to bug me. I went back into my Automotive Theory books and found a few definitions that may help with whatever you are trying to figure out. Obviously these are for automotive applications, but bicycle disc brake are nearly identical.

Brake bias: The term used to indicate the ratio between the amount of brake torque exerted on the front brakes compared with the rear. Brake bias is normally expressed as a percentage of brake torque at one end of the car to the total brake torque, as in "60% front".

Braking efficiency: The ratio of actual deceleration achieved on a given surface compared with the theoretical maximum.

Braking torque: Braking torque in pounds feet on a single wheel is the effective rotor radius in inches times clamping force times the coefficient of friction of the pad against the rotor all divided by 12. Braking torque is the force that actually decelerates the wheel and tire. To increase the braking torque it is necessary to increase the line pressure, the piston area, the coefficient of friction or the effective rotor diameter. Increasing the pad area will not increase the braking torque.

Clamping force: The clamping force of a caliper in pounds is the brake line pressure multiplied by the total piston area of the caliper in a fixed caliper and two times the total piston area in a floating design. To increase the clamping force it is necessary to either increase the line pressure or the piston area. Increasing the pad area or the coefficient of friction will not increase clamping force.

Coefficient of friction: A dimensionless indication of the friction qualities of one material vs. another. A coefficient of 1.0 would be equal to 1g. The higher the coefficient, the greater the friction. Typical passenger car pad coefficients are in the neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. With most pads the coefficient is temperature sensitive so claims that do not specify a temperature range should be viewed with some suspicion. The optimum is to select a pad with a virtually constant but decreasing coefficient over the expected operating range of temperatures. As a result, the driver does not have to wait for the pad to heat up before it bites, and the pad fade will not be a factor so that modulation will be easy (see "plot shape").

Hydraulic ratio: The ratio of fluid displacement by the master cylinder to fluid displaced in the caliper pistons. Hydraulic ratio is an important factor in the pedal effort equation, the higher the ratio, the less pedal effort is required (and the longer the pedal travel to achieve a given clamping force). The stiffer the caliper and the stiffer the pad, the higher the hydraulic ratio that can be employed.

Mechanical pedal ratio: The brake pedal is designed to multiply the driver's effort. The mechanical pedal ratio is the distance from the pedal pivot point to the effective center of the footpad divided by the distance from the pivot point to the master cylinder push rod. Typical ratios range from 4:1 to 9:1.The larger the ratio, the greater the force multiplication (and the longer the pedal travel)

There will be a quiz next week. Class dismissed.
-Brad

thanks BRacing for the good read but the info i need is not covered in automotive physics well maybe the coeifficent of friction can be usefull

 

[sub6]

rbx is trying to figger how you predict whether a given design will have brake jack or squat, and to what extent.

I do believe it has to do with the position of the IC - further forward, the less brake interference, something like that? I remember hearing DW or some such spout off on it.

Check the thread you posted about FSR hype; I think towards the end of that thread was where I remember seeing that......

 

[BRacing]

If it's brake jack that he was after, why not just say it?

Brake jack is simple (compared to all the other stuff I listed). Compress the rear suspension, does the brake caliper move in an arch (or rotate about the center)? If so, you have a "fixed caliper".

If you compress the rear suspension and the caliper stays located in the same spot on the rotor (or remains parallel) then you have a "floating caliper".

Generally, a floating caliper design will be less affected by suspension movement than a fixed caliper design.

-Brad

 

[BRacing]

And one more thing...

I believe most bicycle disk brake pads are rated between .7 and .8 coefficient of friction.

Hope all this helps.
-Brad

 

[sub6]

http://www.ridemonkey.com/forums/showthread.php?s=&postid=545095&highlight=brak*+forward#post545095

 

[rbx]

Originally posted by sub6
rbx is trying to figger how you predict whether a given design will have brake jack or squat, and to what extent.

I do believe it has to do with the position of the IC - further forward, the less brake interference, something like that? I remember hearing DW or some such spout off on it.

Check the thread you posted about FSR hype; I think towards the end of that thread was where I remember seeing that......

i know that some designer use the IC to calculate brake squat/jack and others use the difference in caliper/rotor position (velocity/coefficient of friction)...but wich one is right?are they both dependant to each other?

my theory is that if the IC is at infinity(true parralel floating brake)then the caliper movement will be synchronised with the rotor(because of linear motion of the caliper wich in turn is caused by the IC at infinity)then the acceleration delta between the rotor/caliper is nill which gives zero force going through the suspension.

but im pretty sure that i am way off!!!

 

[evilbob]

Originally posted by BRacing
If it's brake jack that he was after, why not just say it?

Brake jack is simple (compared to all the other stuff I listed). Compress the rear suspension, does the brake caliper
move in an arch (or rotate about the center)? If so, you have a "fixed caliper".

If you compress the rear suspension and the caliper stays located in the same spot on the rotor (or remains
parallel) then you have a "floating caliper".

Generally, a floating caliper design will be less affected by suspension movement than a fixed caliper design.

-Brad

Way to go Brad

 

[ohio]

Sorry I didn't catch this thread earlier rbx...

Both are correct, and both are the same measurement once you work out the geometry. However, the rotor/caliper angular displacement is really the RESULT of the IC position if you're iterating through the design process, and it's much easier to design with IC in mind than caliper/rotor. It's also easier to trackand quantify the changing effect throughout the travel.

Looking at the caliper and rotor is really just a useful way of explaining and visualizing brake effects.

 

[rbx]

Originally posted by ohio
Sorry I didn't catch this thread earlier rbx...

Both are correct, and both are the same measurement once you work out the geometry. However, the rotor/caliper angular displacement is really the RESULT of the IC position if you're iterating through the design process, and it's much easier to design with IC in mind than caliper/rotor. It's also easier to trackand quantify the changing effect throughout the travel.

Looking at the caliper and rotor is really just a useful way of explaining and visualizing brake effects.

exactly the reply i was looking for as always you have been a big help