brake pedal force to lock up wheels 1

[damienmavis]

On a single seat 1400 lb. offroad race car what do you think would be a good brake pedal force to lock up the wheels. Its a two master cylindar system with no power assist. I was using a coeficient of friction of .8 assuming dry asphalt, which would be the highest friction situation this car would experience.

 

[patprimmer]

I think 40pounds sounds convenient.

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[GregLocock]

Typically when testing brakes efforts must not excced 50 kg in the event of boost failure. So that's an upper bound.



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Greg Locock

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[Warpspeed]

I recall 50 Lbs as being an acceptable design figure, but cannot now recall the source of where I read that.

 

[patprimmer]

Warp

Don't take my answer to seriously. I attempted to match the OP level of effort and clarity before posting.

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[Warpspeed]

O/k Pat, but I have definitely seen that 50Lb figure mentioned in one of my books on brake modification that discusses pedal and hydraulic ratios, along with brake material frictional coefficients.

The OP was merely seeking a sensible starting point for his own calculations. Greg's 110 Lb upper limit sounds about right too.

 

[patprimmer]

I would think most people could manage 50 Kg in an emergency.

A fit professional driver should be able to muster considerably more, but may not be able to maintain it for the duration of a race.

Of course an off road race car would rarely be on asphalt for any substantial time, and required forces would be lower on dirt.

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[NormPeterson]

0.8 sounds a little low, though it probably doesn't matter much whether you use 40# at 0.8g or 50# at 1.0g.


Norm

 

[Warpspeed]

Sure, most people could manage 50Kg in an emergency easily enough. But you would not design your braking system to require that much force as the "normal" design pedal pressure to slow the car before every corner.

There is a broad compromise between being over sensitive, and fatigue after perhaps hundreds of sequential brake applications on a demanding circuit.

The frictional coefficients of both the tires and the brake material will both vary over a range that cannot always be precisely known. But some basis, and a few assumptions for engineering a braking system from scratch, would have to be better than none at all.

 

[patprimmer]

Warp

There is a distinct lack of detail in the OP so we really are guessing.

Also, the system is NOT boosted, so there will be a real compromise of acceptable travel vs acceptable load.

Also being a race car implies it might use stainless steel braided (aircraft type) brake lines. These expand a lot more than rigid steel tube brake lines and lose pedal in the process.

Regards

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[damienmavis]

Thanks for the input! For clarification its a 107in. wheel base 150 hp off road buggy. Obviously with a some math, geometetry, and a few assumptions its easy enough to figure out where the lock up point is. I've been in cars with too light of a pedal pressure as well as too hard. I just wanted a ball park for the pedal effort so I dont have to buy new master cylindars at $100 each when I get it wrong. Thanks for the help, it seems like approximately 50lbs. force to lock up the wheels on pavement is a good starting point.

I tried to use a bathroom scale to test it out but the two I have are digital and dont work when they are vertical, who would have guessed? Its hard to find a scale with a dial anymore!

 

[Warpspeed]

Your local gym is certain to have a leg press machine. They may let you play with if you go there at an odd time when they are not particularly busy.

They will think you are completely out of your mind, but may humor you if you can come up with a really good story.

Or you could rig up a fifty pound weight to a rope and pulley with a stirrup on the end, and experiment in the privacy of your own home.

 

[swall]

At a brake supplier I worked for, we had a little gizmo that utilized a load cell in a simple housing that you would place between your foot and the pedal. Shouldn't be much trouble to make something like that.

 

[patprimmer]

You could even use an old master cylinder in the actual car chassis with the actual seat and pedal, then screw a pressure gauge onto the MC, plug any other ports, bleed the system and have the actual driver apply and reapply thr brake to the maximum level he can maintain at say every 10 seconds for a hour. Calculate force from line pressure.

If possible reduce results by 20% to correct for driver putting in extra effort to "pass" the test. It will end up a compromise between pedal force and pedal travel.

I expect a driver will adapt to lighter pressure easier than heavier pressure at the levels required for a disk brake unboosted system.

Regards

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[bentwings1]

Our little buggy is 800 lbs and 160 hp. The front brakes are dual piston ATV calipers on each side with a standard Wilwood caliper in the rear. Adjustable proportioning valve is installed on the rear. We use 5/8 MC. The linkage is a toggle joint style as the brake pedal is very short.

It is quite easy to lock up the wheels on this buggy even on pavement. The toggle joint provides light pressure for most use and the end of travel toggle action really clamps down when necessary.

Just from our testing I would guess there is about 50 pounds force for most use and maybe 75 for the full locking action. It is very easy to hold the brake pedal down by hand maybe 40 pounds and the buggy can be dragged with the wheels locked on pavement.

I tested 40 pounds hand force at about the same angle with the bathroom scale and that's about what the pedal pressure is. We did the scale measure test blind for "accuracy".



99 Dodge CTD dually.

 

[GregLocock]

Shouldn't be too hard to modify the leverage if you think about it in advance.



Cheers

Greg Locock

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