Pedal Ratio question

[Danny B]

Hey guys, im designing the brake system for hte University at Buffalo car and just needed a little feedback on pedal ratios. I have cranked out all of the calulations and basically we are looking at a 4 to 1 pedal ratio with a max foot force of 96 lbs. Is this foot force too low/ too high or just right??? If i were to make the pedal ratio a 3 to 1 the max foot force would go up to about 150 lbs. Also, will a 4 to 1 ratio result in too much travel? Any input would be awesome.

[CMURacing - Prometheus]

our braking effort is in the 85lb range, but 96 is good enough.

i think we have ergo data that says 75-110 lbs, so 150 is too much.

[jack]

we did some measuring a while ago. we found that a "stiff" pedal should lock the wheels at about 125lbs of force at the ball of your foot. the pedal should flex as little as possible up to about 450lbs, which is about as hard as the driver is going to slam the pedal in a panic situation.

my goal is to design the entire brake system to have as little pedal travel as possible (and still work well). instead of picking a pedal travel, and seeing what torques you get on the wheels, try it the other way around http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Danny B]

yeah it seems like we should stick with the 4 to 1 pedal ratio although maybe i will play around with the numbers and see if i can get a 3 to 1 ratio with a max of 120 lb's of force (different MC sizes). I want to keep pedal travel down to a minimum.

[Agent4573]

I think your right in the general ball park there with a 4 to 1. I'm almost positive(i guess i should be completely positive since im the brake guy) that we run a 4 to 1 with no problems at all.

[rjwoods77]

Is it better if given the chance to design it in to get as close to 1:1 as possible? I know larger pedal ratio requires more travel for teh same pressure. Is there a limit to the low end of this?

[jack]

Originally posted by Rob Woods:
Is it better if given the chance to design it in to get as close to 1:1 as possible? I know larger pedal ratio requires more travel for teh same pressure. Is there a limit to the low end of this?

the best (according to carrol smith) is to design for the least amount of pedal travel possible, where the brakes still work well. so do what you got to do with the hydraulic system and and mechanical system to get there.

[Steve Yao]

I've been straight told by a brake design judge that the driver should only have to exert somewhere around 90 lb(? have to find the notebook from that year) for 1G of braking. More than that and they become uncomfortable through a race....of course i have no idea what racing series or vehicle production he was most familiar with.

That year we had happened to design to 100 lb/1G so he said it was a bit high.

In my FSAE experience, pedal travel has been more a function of the stiffness of your system. i.e. how well the pedal is supported, line expansion, brake pad compression stiffness.

With a well built pedal assembly, quality braid lines, and maximal use of hardlines, any significant pedal travel is likely due to air in the lines.

I don't beleive you can get adequate performance from a 1:1 ratio unless you make your own tiny bore MC's or very-large-piston-area calipers(perhaps multiple calipers?)

[Agent4573]

rob, like sey implied, with a 1 to 1 pedal ratio your driver input force will be way too high. When i designed our brake system I started with the wheels and moved towards the driver from there. I started with the torque needed to lock the wheels, then through friction coefficient,pad area, and rotor size found the line pressure needed to lock the wheels, once you know your max line pressure you can look at different pedal ratio's/master cylinder piston sizes to see what works for you. We aimed for around 75 pounds of driver force to lock the wheels, which resulted in a .75 inch piston and a 4 to 1 pedal ratio. This seemed to work wheel for our system and no one complained. If we ran a smaller piston I probably could have went with a 3 to 1 ratio instead with a bit more travel, but I was trying for somewhere between a 4:1 and a 5:1 pedal ratio because that was optimum for packaging.

[rjwoods77]

What do you guys use for cf for tire? I know what an ideal would be but what have you planned for based on the rutty dirty surface,etc.

[Denny Trimble]

One and a half, plus or minus one http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Z]

I would suggest the smallest practical diameter M/C's, with a stroke long enough to take up pad clearance, hose expansion, etc, (+ some), then a pedal ratio as close to 1:1 as you can get. Probably only ~2:1 is achievable.

With a foot force of 100lbs, a 5:1 ratio gives 500lbs on the M/C pushrod, 500lbs on the M/C chassis mount, and 400lbs on the pedal pivot and its mount. A 1:1 ratio (if you could get it) gives 100lbs on the M/C pushrod and chassis mount, zero lbs on the pedal pivot, and no bending stresses on the pedal. Which one is going to have higher stresses (or more weight) and more flex?

SEY's design judge suggested a foot force of 90 lb/G. Since FSAE cars should be able to brake repeatedly at close to 2G (or higher G for quicker lap times http://fsae.com/groupee_common/emoticons/icon_wink.gif) that implies regular 180lb pushes from the driver. Better get those drivers down to the gym now. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[BryanH]

AP, Alcon etc pedal boxes are 5:1, production cars are 5:1 to 7:1. 4:1 on a very light car is at the edge of the envelope.

[Brian Evans]

I always looked at it from a system ratio perspective, pounds at the pedal relating to PSI at the caliper. As Z says, if you use small enough master cylinders and large enough caliper pistons, pedal ratio can be very low. If you stay close to what commercial race car designers are doing, then you can use off the shelf master cylinders and calipers. They seem to find that a 4 or 5 to one ratio is reasonable for the pedal, and it lets them use available parts for the rest of the system. There is probably some hydraulic design reason why 5/8" is the smallest easily available master cylinder size.

Brian

[jack]

DO NOT use really small MC's! learned that the hard way. we (tryed) to use 9/16 MC a few years ago, and there is NO WAY you can get enough air out of those tinny things to make them work well enough. ANY air in those and your screwed. personally i vote for as much fluid in the system as possible. the fact that small air bubbles will make less of an impact far out weighs the compressability of the fluid.

[CMURacing - Prometheus]

we've used 14mm master cylinders from AP racing the last two years, and aside from the exorbitant cost, they worked fine.

but i'd like to get rid of $300 worth of MC's...

[jack]

i guess that means you shouldnt use cheesy small MC's then..http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Z]

If you use small diameter M/C's AND a 5:1 pedal ratio, then you get higher than normal system pressures, so higher than normal fluid compression and line expansion. The extra M/C stroke required to push the compressed fluid into the expanded lines is then multiplied by 5 at the foot plate giving extra "sponginess". Bending of the 5:1 pedal adds further to total flex/sponginess.

Large caliper piston area (for given diameter discs/wheels) allows low system pressure so less flex due to fluid compression/line expansion. Low system pressure generally means more fluid moving back and forth, which can give slower response due to viscous friction. But this is not a problem on small cars like FSAE because of small calipers, short lines, etc.

Small M/C area allows most (or all) of the leverage to be done hydraulically. This means less mechanical flex in the pedal beam, and at the pedal and M/C chassis mounts. This mechanical flex at the pedal is often a large part of total flex, and it is what the judges (apparently) like to fuss over (caliper flex can also be large component of sponginess). A 1:1 pedal ratio would just about eliminate mechanical flex at the pedal.

Sponginess due to air in the lines is mainly about the location of the bleed nipples and the "refill" hole in the M/C, not about the diameter of any pistons. (Edit: Air can also get trapped in wrongly oriented T-fittings, M/C's at funny angles, and in any other "pockets" at the top of a loop that don't have an upward exit for the air.)

Z

[TomF]

The Delft 2004 car used like 1.5:1 PR. Off course it weighed only 125 kgs, which did help a lot. We did however experience some trouble with the braking pedal. It actually became a breaking pedal.... This ultimately lead to some difficulties concerning the pivot bearings, which introduced a lot of mechanical friction into the system, but it did perform.