Best sensor scheme for hot brakes? - Page 1
Forum section: electronics / projects

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 27, 2020, 11:26:57 pm
content: May or may not build something but the last couple cars I've had tend to develop dragging brakes after a while. Probably doesn't help that I park on a dirt/stone driveway and live in a northern climate. My guess is there're a lot of people not getting the best mileage due to dragging brakes, maybe more than from low tire pressure before tire pressure sensors. My Hyundai drops from about 38 to 35 when I know I have a problem. Less than that if it's a smokin' hot problem. Anyway, I've considered thermocouples mounted on the calipers, thermistors and diodes, all requiring wiring back to a box in the passenger compartment. I'd like an IR solution pointed right at the rotors, but that probably won't be cheap or easy. Curious what the hive mind can come up with. Any cheap wireless solutions? Something that can be monitored on the phone? If the automakers did it, how do you think they might go about it? Also, anybody else suffer from this?

author: Rerouter
author_rank: Super Contributor
timestamp: June 27, 2020, 11:32:14 pm
content: Thermocouple is still the best, but I would do it like wireless tyre pressure monitors, Those already can report trye temperature, so a dragging brake heats 1 tyre more than the rest, and there is a standard already to make it compatible with many cars. just broadcasting the temperature information.

author: mikerj
author_rank: Super Contributor
timestamp: June 28, 2020, 08:08:26 am
content: Non-contact sensing is going to be problematic, the sensor will quickly be obscured with dirt. An OEM would probably do something like integrate temperature sensing with the pad wear sensor, and the ECU would then compare pad temperature against integrated energy from brake use over some time period.

author: 2N3055
author_rank: Super Contributor
timestamp: June 28, 2020, 08:44:07 am
content: Breaking fluid temp will have at least some correlation with brake system temperature..Also brake calipers will have raised temp. Those can be contact measured..There are also pads with already embedded thermocouple sensors...

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 28, 2020, 06:33:38 pm
content: I see some interesting solution for the race people online, IR and thermocouples that you embed in the brake pads. Not practical for a street car though. Right now I'm thinking a sensor bolted to the caliper is the way to go.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 28, 2020, 07:03:39 pm
content: It's not a problem I have had but a thermocouple mounted to the caliper seems like the most reasonable solution.

author: james_s
author_rank: Super Contributor
timestamp: June 28, 2020, 07:11:39 pm
content: A thermocouple attached to the caliper where it contacts the backing of the brake pad is probably the way to go. It won't give you an absolute measure of the rotor temperature but a dragging brake will heat up the pad and caliper too so a relative measurement between wheels ought to tell you what you need to know.

author: duak
author_rank: Super Contributor
timestamp: June 28, 2020, 08:24:27 pm
content: One problem with measuring caliper temperature is the delay. Disk Brake systems are generally designed to minimize heat transfer into the caliper so I think measurement of the rotor temperature is best. Here's a complete overkill wired brake rotor temperature monitoring system:http://www.izzeracing.com/products/brake-infrared-temperature-sensors.htmlI can't imagine this working well during the winter.First idea: Add some sort of phase change, or rapidly expanding material or something using the Curie Point. We are surrounded by all sorts of things that do something at certain temperatures.The next idea was just put a thermal switch on the rotor and detect if it was open or closed by an inductive sensor.Then I remembered that tire pressure monitoring systems (TPMS) have been available for years:https://en.wikipedia.org/wiki/Tire-pressure_monitoring_systemI believe most are RF and so some sort of infrastructure has been worked out. Perhaps one could be modified to read temperature by coupling in to a small pneumatic reservoir and then attach it to the rotor. As the rotor heats it increases the temperature and pressure of the air in the reservoir. But then I thought why bother to do it aneroid barometer style? Are there any temperature reading devices that can respond to an RF chirp or bluetooth interrogation using an existing system? Or one that just reads digital inputs?I found one but it's In the go big or go home department, here's a wireless unit that has a TPMS too:https://www.craneae.com/Products/Sensing/Downloads/SmartStem%20OnBoard%20737%20Max.pdf

author: HighVoltage
author_rank: Super Contributor
timestamp: June 28, 2020, 08:43:34 pm
content: I would use thermocouples directly at the back side of the break pads.Drill a small hole in to the metal backplate and insert the TC with some heat resistant glue.Everything else would probably get far too expensive on a one off application.On the other hand, the tire pressure sensors do send out the temperature of the tire air.And a much hotter rotor disk will also cause the tire to heat up more.So, you might start by just reading the tire temperature from the existing sensor.

author: Benta
author_rank: Super Contributor
timestamp: June 28, 2020, 09:58:55 pm
content: Quote from: Conrad Hoffman on June 27, 2020, 11:26:57 pmDon't want to step on your toes, but perhaps you should review your driving style? I've never heard of this problem before, cold climate or warm climate. Brakes don't just "drag and get hot" unless they're defective, and then they should be fixed.Are you driving with both feet on the pedals? (assuming auto trans).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 28, 2020, 11:36:03 pm
content: A fair question.Manual transmission and , no, I don't ride the brakes or do anything odd. IMO, the problem is climate and design. We have a long wet season, winters with a lot of salt and I park on a stone/dirt driveway that's likely humid much of the time. Design-wise, I know there's an issue with the pad-to-frame clearance and even if the piston doesn't seize up, the pads can get very tight in the caliper frame. Some people have had problems with the calipers seizing on the pins they slide on, but I think I've avoided that. I've had the problem on the last two cars I've owned (Mazda3 and Hyundai Accent) and replacing calipers (and everything else brake related) has been a major expense when they get beyond 60,000 miles. General wisdom seems to be to service the calipers once a year, but I can't believe anybody really does that. Castrol LMA (low moisture absorption) brake fluid seems to help a bit with the pistons. The parking brake system is a whole 'nuther problem.edit/add- Pulled the caliper today and the pins and pads were fine. The piston wouldn't turn or move, so it looks like a new caliper is in order.

author: duak
author_rank: Super Contributor
timestamp: June 29, 2020, 10:27:57 pm
content: I'll echo what Conrad says about the environment. Vehicles from north eastern North America are usually in physically worse condition than those from the west coast of North America simply due to salt, sand and humidity. I remember looking at a brand new Mercedes (9 kM on the clock) and the underhood components were already showing signs of corrosion. And this from simply being shipped across the Atlantic, sitting on the docks and then taken by rail to Vancouver.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 29, 2020, 11:58:36 pm
content: At some mountain tourist destinations, they zap the wheels of every car coming down from the mountain with an IR thermometer or thermal imager and pull over cars that have overheated brakes.It seems to me that an IR thermometer pointed at the wheels every once in a while would let you keep an eye on things with the least hassle, and it would work on multiple cars too...

author: SilverSolder
author_rank: Super Contributor
timestamp: June 30, 2020, 12:01:05 am
content: Quote from: duak on June 29, 2020, 10:27:57 pmCalipers last about 7 to 10 years for me, but I clean them, re-grease, and even repaint them every time I change the brake pads (which is every couple of years).

author: coromonadalix
author_rank: Super Contributor
timestamp: June 30, 2020, 11:38:29 am
content: Dont want to be a drag, but is your problems due to poor brake maintenace, calipers pins rusted, calipers pins boots damaged or they let water in ?Are the brake pads moving correctly ? i've seen some people remove the stainless steel sliding plate / shim ... and the main piston (brake disk) is it getting stuck ??Never had this kind of problems unless damaged parts or poor maintenance or bad adjustments in the rear drums brakes

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 30, 2020, 01:45:26 pm
content: Quote from: Benta on June 28, 2020, 09:58:55 pmThat shouldn't damage neither the piston nor the pins. Brake rotors might get an axial runout and pads wear faster.The question is if the actual problem is all around or just the rear axle and if there is actually more damage to be seen than just wear (actual damage on piston or pins).Given a modern brake system engages the park brake via the main brake pads and has a self-adjusting mechanism for the rear axle, there might be several reasons for stuck brakes after parking. Environmental factors should not be an issue if only rear brakes have the problem, as these affect both axles (excluding the bowden cable for the park brake).One lesser known issue are problems with the self-adjusting wear compensation due to vibrations, which means the calipers close tighter than they should and can only be manually retracted (rotate them back in) when taking the pads out. As this is done when changing the pad (you usually can't reassemble new pads without doing so), it is considered "normal" but might as well be the cause.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 30, 2020, 02:14:20 pm
content: I've had the problem on all four corners at one time or another. Everybody I talk to says it's only me, so apparently I'm "special". The brakes were just serviced within the last year or so by very competent people. They replaced at least one caliper then for the same reason. The fluid was completely flushed with LMA fluid. I think it's a combination design issue and environmental issue. I also think a lot of people are driving around with slightly dragging brakes and don't know it. I like Thermoworks IR guns but none are on sale right this minute. Next time they do go on sale, I'll pick one up and start keeping records. BTW, if you do electronics, their CF-IR unit is great for small areas like chips, resistors and heat sinks.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 30, 2020, 03:32:26 pm
content: Quote from: Benta on June 28, 2020, 09:58:55 pmI don't think Conrad is arguing that it isn't a defect. The point is to monitor so that it can be noticed and repaired when it happens, similar to how TPMS is (supposed) to work. I have never heard of this being a common enough problem to warrant such measures, but then it isn't clear how I would have noticed if the only symptom is a few mpg worse fuel consumption and your brakes running hot even while cruising.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 30, 2020, 04:22:58 pm
content: I ususally notice it by the smell, or the ticking sound of brakes cooling - like after you've driven a car hard. (Not that I would ever do that, of course.)

author: SilverSolder
author_rank: Super Contributor
timestamp: June 30, 2020, 04:31:27 pm
content: Quote from: Conrad Hoffman on June 30, 2020, 02:14:20 pmOne of my collection of elderly vehicles is a hybrid, which gets a lot less use of the brakes than a gasoline car due to the regenerative braking. Here, the big problem is rusting brake disks due to lack of use... I have to buy the best, coated disks, and I'll still have to change them after about 3 years due to rust. If I buy the cheap disks, they are junk in one year! Got to love road salt etc.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 30, 2020, 05:04:06 pm
content: Interesting. Maybe I don't use the brakes enough or hard enough? I drive for mileage and have a manual transmission, so probably do less braking than most people.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 30, 2020, 05:29:11 pm
content: Quote from: Conrad Hoffman on June 30, 2020, 05:04:06 pmI think that pretty much explains it, especially if the car is not garaged.I have another ye olde car in the garage, a 1990 Miata, and the brakes were changed 15 years ago, and still look good! There is a dehumidifier in the garage, which is the smartest thing I've ever done in terms of keeping cars, tools, stored wood, etc., in good condition...

author: ejeffrey
author_rank: Super Contributor
timestamp: June 30, 2020, 06:51:09 pm
content: Quote from: Conrad Hoffman on June 30, 2020, 05:04:06 pmI have definitely heard of people with hybrids deliberately doing a "hard" brake a couple times in a row every few weeks just to keep their rotors clean. If you are relying heavily on engine breaking in a manual probably the same applies. It's never been a problem for me, but I just drive a hybrid like a regular car. I don't attempt to maximize regen. We don't have snow or salt here, but my family in Iowa also have hybrids and never have seen this problem. I have seen photos from hypermilers that don't do this, and it looks pretty nasty.

author: max_torque
author_rank: Super Contributor
timestamp: June 30, 2020, 07:07:09 pm
content: if you put the clutch in and "roll" you'll notice any serious drag, and if the car swerves to one side like wise. When you park, just carefully put your hand past a wheel, you'll feel heat if it's there. reliable, cheap and robust. No point in doing complex solutions to simple problems.....