Downhill breaking

Hey All,

Is breaking on a steep decline at high speeds (65mph) with a loaded bed normal to shake badly? TIA

 

No.

 

Time to check your brakes.
Probably worn pads, shoes, and possibly rotors too.

 

Odd with only 32K on it.

 

Overheating the brakes has nothing to do with age or mileage. Most likely the brakes were overheated in the past and the heat cycle caused the rotors to slightly warp or there is excessive pad transfer on the rotors causing the pulsing feeling.

 

Thanks for this info, I appreciate it.

 

Take it out of overdrive? -S- that helps when loaded.

 

Yeah, did that to downshift manually.

 

I had a braking vibration that started around 30K miles. I could feel the vibration when braking from about 50mph to 35mph. I don’t normally tow or carry heavy loads.

Put PowerStop on two years ago and very happy with them.

$182 Power Stop Brake Kit For 2007-2014 Toyota FJ Cruiser, 2005-2022 Toyota Tacoma 6 Lug, 2003-2009 4Runner 12.56" Rotors - Front Carbon-Fiber Ceramic Brake Pads & Drilled Slotted Rotors Replacement, K137

https://www.amazon.com/dp/B005FKS28C?ref=ppx_yo2ov_dt_b_fed_asin_title

 

Sounds like the rotors either need to be turned or replaced. The idea when on long declines is to downshift BEFORE you start down to a gear low enough to maintain a safe speed with minimal brake applications. Load doesn't really matter. Depending on the grade and what is a safe speed I use either 3rd or 2nd gear. If either my speed or rpm's get too fast just a little tap on the brakes will get things copesetic. I've driven 30+ miles down mountain passes before and only had to touch the brakes 10-12 times.

That's how you prevent the rotors from getting warped. Constantly braking overheats and warps them.

 

It was 7% for 6-7 miles.

 

Don’t ride your brakes for anywhere near 6-7 miles. They could over heat, a fade, wear prematurely, and possibly become damaged. Use engine braking (s-mode if you have an auto).

 

Was probably 2 miles in before going to s mode.

 

I've heard of people warping their brakes on hills like that in Colorado. The transmission will automatically downshift a gear or two if you're riding the brakes, but it's usually not enough. I'll put it in S4, S3, or even S2 depending on the speed to get a bit more engine braking. Even with the engine around 4000 RPM the engine braking seems pretty weak compared to other vehicles I've driven.

 

As much as warped rotors are reported on here, you’d think that Toyota might add a brake rotor temperature warning light. There’s a warning light and/or beeper for just about every other mistake a driver or passenger might make. Why not this one?

Is it because brake rotors and pads are not covered under warranty?

 

i've noticed this when going down vail pass on i70 in colorado. my truck was probably around 30k miles then too. been awhile since i went over that way. i recently swapped my pads at 60k and haven't noticed the same vibration, but havent done any higher speeds down any passes.

 

what broke?

 

In my experience they don't actually warp, but they get uneven deposits of pad material. One time I had a single panic stop from around 45mph cause "warped rotors", and there's no way that created enough heat to get about normal operating temperature. In other words it's a little unpredictable when it happens.

The transmission will downshift for you to dummy proof it. It doesn't downshift far enough in my opinion, but it's enough that you probably don't need to ride the brakes very hard on a 7% grade. Depends on the load though. Are you fully loaded but within the payload rating, over the payload rating, or towing a trailer?

There could also be some tolerance in the grade warning signs. For example Eisenhower Tunnel to Frisco is I believe 6%, but it doesn't take much braking even without downshifts. I think Fremont Pass to I-70 says 7%, but in the steepest sections it feels like I need to burn off a lot of energy to maintain speed. Off the top of my head, that section of road is the hardest on brakes I can think of in Colorado because of the combination of slope and high speed limit.

 

thats a steep one there

 

A 4600 pound (~2000 kg) truck traveling at 50 mph (~20 m/s) down a 7% slope will need to dissipate approximately 27 kW of heat by my napkin math.

20 m/s x .07 = 1.4 m/s downwards.

gravitational potential energy = mgh = 2000 kg * 9.8 m/s * 1.4 m =~ 27k joules
power = energy/time = 27 kW

Estimating the temperature a brake rotor will reach in 6-7 miles is left to the reader (or me if I get bored enough).

EDIT: I looked up the heat capacity of steel to be 460 J/C and the typical brake rotor to be 7 kg. So making the approximation that the rotor does not dissipate any heat to its environment (terrible approximation) the rotor would get ~50 C hotter every second. 27 kW / 460 J/C.