Brake parasitic drag
Brake parasitic drag
(OP)
Does anybody know a method of measuring parasitic drag in the disc brake (pneumatically operated)? It's a kind of parasitic torque that presents in all modern brakes. After applying the brake and when it's released there is still some local contact between the lining and the disc. But after a few rotations of the wheel this torque disappears. I need to find how big is this parasitic torque (it's considered to be pretty small) and is the possibility that this torque can appear not only applying the brake but when driving off-road. Currently I'm thinking of some kind of a sensor that could measure the force between the pad and disc - but I didn't find any appropriate type of sensor... I could try to use the brake dynamometer – but it will be the last thing I would try, cause I want to measure this torque right driving the truck depending on the road conditions.





RE: Brake parasitic drag
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Brake parasitic drag
RE: Brake parasitic drag
RE: Brake parasitic drag
RE: Brake parasitic drag
What Greg said was to put strain gages near the BRAKE CALIPER mounting bolts, not the wheel mounting bolts. The bracket that carries the caliper is stressed only by brake torque and by the weight of the caliper as magnified by bumps.
Mike Halloran
Pembroke Pines, FL, USA
RE: Brake parasitic drag
RE: Brake parasitic drag
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Brake parasitic drag
Driving off road may cause friction from deflection, but that action may also cause the pads to further retract.
Except for arguably the off-road part, would the subjective numbers tell much more than a simple but carefully controlled coast down test? Or something even simpler like turning the wheel by hand?
RE: Brake parasitic drag
Now I'm trying to figure out where to put the sensors: I think that's easier to put them on the brake disc. But somewhere in that part that Then I will try the idea to put them on the spindle near mounting points...
RE: Brake parasitic drag
RE: Brake parasitic drag
How about removing the wheel bearing seals for the duration of this test?
Then measure the rotational inertia of the tire/wheel/hub/rotor assembly (any good mechanical engineer knows how to do that, right? <grin>) and do a spin test with seals installed, giving you a measure of seal drag.
It could even be that seal drag exceeds brake drag under most conditions.
And Ace: I notice you're using air-applied brakes: would this be an Eaton study?
What *IS* the mechanism for retracting the pads when you don't have an hydraulic seal to do the work for you?
RE: Brake parasitic drag
RE: Brake parasitic drag
RE: Brake parasitic drag
2BobM3: You just guessed what I’m doing right now! We calculated that we can not just put the strain gauges – the strain is about 0.5microstrains and nobody knows any type of sensor to measure such a strain: it’s just below the normal error of such a measurement. So the minimum strain that can be detected is about 5 and I found out the new diameter of the brake disk hub and we will chisel it out to put the strain gauges there on the weakened surface. About the temperature: yes – that’s a problem. But we know the temperature distribution thru the brake disk and during the short test it won’t affect the results.
2Rob45: About the brake: this one on the test is made by Knorr-Bremse – this is the biggest brake company in truck industry. They also own Bendix. But I didn’t hear about Eaton’s air brakes. I know that Haldex and Marathon are making air disc brakes as well – but it’s pretty the same mechanism. Retracting mechanism is a spring. But normally it doesn’t help :) This spring moves out calliper but the pads are still in contact with a disk. They are not pressed but still are making some parasitic torque. During the next movement they are retracted by means of vibrations but still in some case might be pressed to the disc. This right what I’m measuring in this test :) About the seal drag: I believe it is constant – so will figure out how to find it and then just keep in mind.
About the last offer: in this case I think we won’t be able to measure this brake disc torque during the movement.
Any new ideas :)?
RE: Brake parasitic drag
Thanks for the explanation.
RE: Brake parasitic drag
You could add a small spring to the end of each pad for complete retraction. This is something that would be much more touchy to do with hydraulic disc brakes. It would not take long for mechanics to start throwing the springs away. But you still get to advertise the worlds only drag free disc brakes.
RE: Brake parasitic drag
RE: Brake parasitic drag
Interesting. I'd bet a dollar to a donut there are OEM's that don't observe that. And how effective is it on the billions of sliding calipers?
On the mechanical, if the components retract, you would not need to retract the pads unless they are a tight fit for rattling or some other reason. If they are dragging, something else is holding them there.
You could move the air quick release to each chamber and use the puff of air to move the pads back, cool them, and dust them off while they're at it.
RE: Brake parasitic drag
The flex in the piston to calliper body seals is normally enough to give suitable clearance, but only pulls the piston, not the pads back.
Single spot callipers make it more difficult as there is no device to centralise the calliper over the disk, so one side may drag.
I have seen pads where the pad clips into the piston and would be pulled back with the piston.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Brake parasitic drag
RE: Brake parasitic drag
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.