Brake Pressure sensor

Jimster
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Brake Pressure sensor

Postby Jimster » Mon Mar 31, 2008 10:44 am

I'm looking for a cheaper alternative to a brake pressure sensor, one question I need to answer first is what pressures would a sensor in the brake lines (without servo or ABS) usually see? 100psi?
Thanks

Jim

tristancliffe
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Postby tristancliffe » Mon Mar 31, 2008 11:23 am

Most brake pressure sensors range from 0 - 100bar (1450psi), although most of that isn't likely to be used.

If you know how hard you press on the pedal, the mechanical advantage the pedal gives and the size of the master cylinder then you can work out the line pressure quite easily.

Assume 100lbs of force, with a 6:1 mechanical advantage (so the pushrod has 600lbs of force on it), on a 0.75" master cylinder, then you get 800psi.

If you have dual master cylinders (and 50:50 bias between them, say) then you half each. So with the above numbers you'd have 300lbs on each pushrod, which with a 0.75" front and 0.7" rear m'cylinder gives 400psi front and 428psi rear.

Numbers based purely on figures I plucked from thin air. You'd need to work them out for yourself.
Monoposto 2000 - Reynard 883 Toyota

faraday
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Postby faraday » Tue Apr 01, 2008 12:55 am

Tristan offers good advice as usual, but don't forget that brake failure is best avoided. A sensor in the hydraulic circuit should have an "over-pressure" or "burst rating" with a much higher factor of safety than a cheap sensor will provide, not that it's easy to find a cheap sensor in the range required.

Another factor is the sensor's ability to withstand the corrosive fluid and maintain its sensitivity and linearity at elevated temperatures.

If you want the best value from your efforts and you have a balance bar, you need two sensors.
If all you desire is a visual record of how hard and consistently the driver is applying the brakes, a potentiometer on pedal travel is the way to go.

Quality linear pots are not cheap, but sometimes space constraints makes a neat pulley on precision rotary pot, driven by a piece of string, a superior, affordable and elegant solution.
You probably only need a relative value. Accurate pressures are not much use unless you are a brake engineer needing to deduce and monitor friction co-efficients, for instance. To complete that story you'll need accurate pad temperatures.

Balance bars are not as predictable and reliable as we'd like. Keeping tabs on this and as an aid to car setup after system maintenance is justification alone for a serious racer to fit a pair of pressure sensors.

The performance of the driver will be evident visually from the speed and long G traces. You should bear in mind, however, that if the brake system is well designed for the application and sensibly maintained, the driver's skill makes all the difference. The difficulty of threshold braking increases with aero. Just have a look at F1 with the aids removed...

Jimster
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Postby Jimster » Tue Apr 01, 2008 8:15 am

I've just found this sensor which is good for 125 bar, what are your thougths?

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0164522285

faraday
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Postby faraday » Tue Apr 01, 2008 8:32 am

Sh*t.
F#%^ing cheap.
I might get a few.

Unfortunately, the over-pressure rating of 1.25 is probably not what most automotive engineers would consider adequate. I've used 3X. You may want to get a second opinion on this.

I would get the 250 bar version for peace of mind. The resolution loss should not be an issue.

Jimster
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Postby Jimster » Tue Apr 01, 2008 9:57 am

ok thanks for that, I've ordered the 100 bar one already now (oops) :roll:

I'll try this first and see what pressures I see, if I get anwhere near 100bar then I will change it. It's only a 500kg mini I have, so can't imagine huge pressures. I'll keep you posted

spaf
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Postby spaf » Thu May 29, 2008 2:14 am

Jimster, have you tried this yet?

Anyone else?

Jimster
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Postby Jimster » Thu May 29, 2008 3:27 am

I have tried it out, at looks like it works perfect. I havn't tried the car on the track yet, as it's still in bits in the workshop, but I tested it and it would be appear to be perfect

faraday
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Postby faraday » Thu May 29, 2008 5:08 am

So Jim, have you tried two?
I now know you like things in pairs, especially bigguns.

It's a great workshop tool (assuming a balance bar system) to have pressure of both front and rear.

Please tell this ignorant Aussie, is Wales to England as New Zealand is to Australia?
I.E. the males in the little places are regarded by their no doubt superior neighbours as Sheep Shaggers?

This is definitely a question about sensors:

How does a Kiwi find his sheep in the long grass?
:wink:

Jimster
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Postby Jimster » Fri May 30, 2008 11:07 am

I've not tried two, but now you mention it I think it would be a usefull tool
I now know you like things in pairs, especially bigguns.
LOL how do you know me so well??

I'd say you where pretty close with the wales - england comments

anyway whats wrong with sheep??
the team I'm part of www.sheepspeed.com

faraday
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Postby faraday » Fri May 30, 2008 12:41 pm

You have revealed, or your fellow sheep appreciating mates have for you, the fun you DDerive from simple pleasures.

Is the grease off wool good for calluses? I might need some.

So another thing the Kiwis have in common with you guys is a special racing series called Mini 7. They appear to be close to stock but run slick tyres.

Peter Carroll/Toronto
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I've had a lot of luck with a variable resistor.

Postby Peter Carroll/Toronto » Wed Jun 04, 2008 5:50 am

I bought a slide style 10Kohm potentiometer from an electronics store. I clamped it to the steering column shroud with a hose clamp and used a piece of coathanger to make a rod down to the brake pedal (attached with another hose clamp).

Works great. I hooked the potentiometer up to the ground and reference voltage on the DL1 and took the reading from the center tap.

It doesn't give brake pressure per se, but it gives me brake pedal angle which is close enough. It tells me what I asked the car to do and how much.

Peter

faraday
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Postby faraday » Wed Jun 04, 2008 6:12 am

Well done.
Obviously not appropriated for quantitative engineering analysis, but mosy of the gains to be had from an effort related braking variable are in the driver's skill at rapidly applying force and maintaining it during down-shifting.

:idea: You could still calibrate your affair against deceleration.

Jimster
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Postby Jimster » Wed Jun 04, 2008 8:54 am

faraday wrote:You have revealed, or your fellow sheep appreciating mates have for you, the fun you DDerive from simple pleasures.

Is the grease off wool good for calluses? I might need some.

So another thing the Kiwis have in common with you guys is a special racing series called Mini 7. They appear to be close to stock but run slick tyres.


DD's and wool! now your talking, lol

The mini's are still going strong in the uk


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