Braking in rain

[JohnQ]

Had this same problem yesterday. Temps in the mid-20s. Had lightly snowed about an hour before. Roads were mostly dry. It's possible there was some ice on the rotors since the car had been sitting outside, undriven, for 3 days. Bumper to bumper traffic moving slowly (about 5-10 mph). Car in front slammed on the brakes, I slammed on mine. Pedal went to the stops and the car just kept rolling. It was like I had only pressed halfway. No chimes to indicate that I was on both pedals and I checked that I wasn't before I released my foot. I stopped very close to the tow hitch on the Jeep in front of me.

Once out of traffic I did a hard (but not panic) brake run or two to heat them up but never really tested them at the limits. Something I might try this weekend to figure it out.

 

[Doug_G]

If the pedal went to the floor, there's something up with the hydraulics. Air in the line or something. If it happens again try pumping the brakes once or twice.

 

[zax123]

I had this happen to me as well last week.

I had to pump the brakes like crazy to get any braking power, but it took a while. Thank goodness my regen was working, I would have hit a car in front of me otherwise.

 

[mitch672]

I have this issue as well. My Model S is going in to get the left side door handles replaced (left rear no longer opens, right side handles where already replaced). They just called to confirm my appointment and I mentioned this issue, they are going to take a look at the front brakes. I also have an issue in the morning, when driving out of the garage, the parking brakes "stick", and the car lurches forward.. They say that's a known issue and they recommend leaving regen low in the winter, so the brakes are used more often, seems like sand/salt get on the rotors and cause the stickiness on the parking brakes.

 

[AndreyATC]

Same here
Stopping power is greatly affected when it's raining
Didnt happen on BMW or Audi i've had in the past

 

[lolachampcar]

This sounds speciously like pad knock back which can happen when racing for other reasons (tank slapper on a motorcycle or hitting a curb the wrong way in a car). The next time you go for the brakes, the pedal goes to the floor and you have to pump once or twice to bring the pads in contact with the rotor. It can scare the crap out of you when it happens. Perhaps, and this is a big perhaps, water is causing unused MS brake pads (in comparison with ICE brake pads) to hydroplane off the rotor. On an ICE they are probably used sufficiently to keep the pads from pushing too far back but on MS the pads may have a chance to retract to the point where pumping is needed to bring them back into play.

.... just a guess as I've not had this problem on my MS.

 

[JohnQ]

Service Center called today as a result of my post here. Told them I was going to see if it happened again before taking it in. I'm due for my 12 month in about 7 weeks so will address then if it doesn't recur. Nice to know that they're keeping their eyes open.

Regarding hydraulics, that was my first thought but the pedal firmed up. I couldn't depress to the stops any more. Haven't messed around with it today but am going skiing tomorrow so will do a hard brake early on to see if it happens again. Had never heard of "pad knock back." Don't know if that was it or not.

 

[Doug_G]

I haven't heard of "pad knock back" on the track either, but I could see how it could happen. The brakes have to self-adjust as the pads wear away. When you change the brake pads you have to manually push the calipers back to make room for the thicker pads.

On some cars (e.g. Roadster) the piston is essentially a big screw that you turn with a tool, but for most you just push the piston back (I use a C clamp for this). It takes a bit of pressure but it's not especially difficult. Once you get the pads installed you have to pump the brakes a couple of times to push the pads back onto the rotor.

So I could see how the pad could possibly get knocked back, but I would think that on the road it would take a pretty violent event. Most street cars don't ever experience a tank slapper.

 

[lolachampcar]

The bikes have an issue with tank slappers (actually, it is a wild osculation in the front wheel) while cars can have issues with curbing. There is also an issue on some historic race cars that use one piece rotor that cup under thermal stress which can subside when cooled causing a gap. Every now and then you will get a footwell shot during a Grand Am or ALMS race and you'll see a driver reach over and gently stroke the brake on the straight after a particularly bad curb or a bump and run with another competitor. That is to make sure the pedal is there at the next corner. Normally brakes just slow you down but sometimes you need them so it is nice to make sure they are there

 

[Ven Rala]

I took my car in this week to the service center for an unrelated service issue and they said corporate told them I easy having a problem with my brakes. Since I never contacted Tesla directly. I guess they saw my post here. They said they would look into my brakes, but I didn't get a chance to ask them when I left if they found anything unusual, though I did notice my brakes were firmer when I got the car back.

 

[JohnQ]

I'm now convinced it's an issue with my hydraulics as Doug suggested. Pump a bunch of times and it firms up a bit but then, after not braking for a while, it softens up. Will be making an appointment at the SC to get my annual service 6 weeks early but 1,000 miles late.

 

[mknox]

I first noticed it coming out of a car wash. No problem with the hydraulics, but almost no stopping power until they dried out.

Had a bit of a scare today after driving for quite a few miles on a wet (salt brine) soaked highway. When I exited and hit the brakes... almost nothing. I gripped the wheel with both hands and pressed down on the brake as hard as I could. They dried out and gripped just in the nick of time. This is far worse than any other car I've driven. Lots of time I've driven for miles in the rain and used the brakes to slow down on an exit ramp and never seen anything like this. Once they dry out, the go back to working normally, but I will definitely mention it at my next service visit.

 

[techMology]

BMW have a feature called Brake Drying which uses the rain sensor to detect when it's raining, then, in intervals, lightly applies pressure to the rotor with the brake pad.

BMW Brake Drying - YouTube

I have this on my current car, however, I have noticed that in the winter, it's not always effective because the roads are nasty with slush without any actual new accumulation happening. The rain sensor doesn't think it's raining, in essence. I've had a couple of scary moments (nothing too dangerous) as a result. Pumping the brakes solves it.

Tesla should be easily able to institute something like this, or maybe not because perhaps the hydraulic system can't be controlled by the electronics. If possible, they could use regen setting as another input to figure out how often to intermittently dry the brakes.

It should be a high priority given the lack of use the brakes seem to have.

And for those wondering, I haven't had any higher wear on my brake pads with this feature than with other cars.

 

[lolachampcar]

I suspect there is a relationship between the accumulation of slight surface rust (a result of intermittent use when compared to ICE) and susceptibility to water. Perhaps the very light surface rust is capturing more water than would otherwise be retained by well used rotors.

 

[PBEndo]

I talked to my local SC guy about this. Apparently Tesla is aware of this issue and they are trying to figure out a solution.

If Tesla is aware of an issue specific to the Model S that unexpectedly results in loss of braking/increased stopping distance, they would have to issue an immediate warning to owners and/or a recall.

 

[Doug_G]

I'm now convinced it's an issue with my hydraulics as Doug suggested. Pump a bunch of times and it firms up a bit but then, after not braking for a while, it softens up.

I first noticed it coming out of a car wash. No problem with the hydraulics, but almost no stopping power until they dried out.

Okay, so just for the record - these are two completely different things.

mknox's problem is pretty common on all cars when you get the brakes wet. The conversation there is whether it is worse than other cars.

JohnQ's issue sounds like a mechanical problem.

 

[AmpedRealtor]

Drove through a pretty bad rainstorm yesterday here in AZ... it rained so hard that everyone on the freeway stopped and turned on their blinkers. Blinding sheets of water were coming down. I also drove through two large puddles where I could feel the force of the water hitting the underside of my car and shooting out the sides. I noticed no difference in regen/braking performance and my car made it through the deluge and ensuing flooding just fine, even after giving my battery pack and brakes a thorough drenching.

 

[mknox]

BMW have a feature called Brake Drying which uses the rain sensor to detect when it's raining, then, in intervals, lightly applies pressure to the rotor with the brake pad
.

Volkswagen advertises something like this as well. I'm not sure if my former Cadillac had anything like this, but I never once experienced brake fade due to wet conditions. Not once, ever. With the Model S its a semi-regular occurrence.

- - - Updated - - -

mknox's problem is pretty common on all cars when you get the brakes wet. The conversation there is whether it is worse than other car.

In my opinion, it's a lot worse, but I have no explanation as to why. The hydraulic brake system is no different than on any other modern car... is it?

 

[jerry33]

The hydraulic brake system is no different than on any other modern car... is it?

It's just Brembo brakes. Maybe the rotors need to be slotted.

 

[lolachampcar]

Wider tires will tend to hydroplane before narrower ones. The same applies to brakes and the Brembos have a lot of swept area.