hard shimmy on the downhill when braking???

Thoughts on what it might be? I have a 2019 4wd tacoma. the past 6 months I have noticed a little shimmy when the brakes were applied on the steep downhill...haven't thought much of it till 4 days ago. Went to Death Valley and did 60 miles of hard offroad (awesome!). Coming out of DV on the pavement roads and steep downhill. WHen I applied the brakes, it was a violent shimmy, even at low speeds (35mph). When we came off the pass, I tried braking hard on the flats and it was fine. We are launching into diagnostics soon but I am wondering if anything springs to mind for anyone? TIA!! (BTW, Death Valley was epic!)

 

Sounds like warped brake rotors due to overheating.

 

When you are hard on the brakes the vibration is gassing of the brake material pushing the pads away from the rotor. The gas builds, then vents, build again etc, etc. I have experienced this as well. I adjusted my driving to prevent it. Upgrading to a slotted rotor would help if you plan more hard braking events like this in the future. The slots give the gas a place to vent quicker.

 

What trim Tacoma?
And you’re sure it wasn’t like the anti-locks kicking in?

 

that was my first thought but when I brake hard on the flat it's smooth.

 

My bet is pad build up on rotors, rotor rarely warp.

 

would that shimmy on the downhill but not on the flats?

 

I would suggest you inspect all 4 wheel brakes and look for uneven wear. I wonder if it’s the rear brakes. Good luck

 

Yes, the harder you brake the more extravated it will be.

 

Actually this just reminded me that this happened to me in a rental car in Maui coming down from Haleakalā. Was worried I totally screwed up the brakes on the descent as it was violently wobbly but then once we got back down to the flat land the brakes operated as normal. I just decided to not put any thought into it and turned the car in and went on my way so not sure what the deal was.

Sounds like @Speedfreak may have the right idea though.

 

I had the same thing happen to me recently, also in Death Valley, coming down Emigrant Pass. Interested to learn the cause.

 

I’d also look for any potential “play” in the suspension system after looking for dragging brakes.

1. Wheel bearings
2. Control arms
3. Bushings
4. Ball joints
5. Tie rods
6. Cracked or broken leaf spring
7. Loose leaf spring
8. Loose lug nuts

 

Was just wondering if you might be lifted ? Suspension drop out in the rear might be causing added driveshaft angle and vibration .

 

It's more than likely brake pad material transferred to the rotor. You could do a brake pad bedding sequence. Where you get it up to 50/60 then brake hard and constant down to 20mph. Repeat a few times. Then take for long ride to cool down the rotors. Sometime when we panic brake, or brake real hard , and leave our foot on the pedal, the pads transfer some material onto the rotor. I just had this with the Mrs. Accord. i was able to scuff off the transferred material, and the pulsing went away. So when braking real hard leave foot off the pedal. I put my vehicles in park when this happens to avoid the transfer.

 

Warped rotors. Try resurfacing them and reinstall.

 

It's your rotors. I was constantly getting rotors resurfaced on my 2nd generation.

 

As noted above, it's likely warped rotors. Happens all the time. Use the engine braking when coming down steep or long hills to avoid overheating them.

 

i got this problem too, even when vehicle was fully stock. happened on mountain passes in rockies both in a tacoma and land rover. maybe its from bad driving? braking in a way that overheats the things? i have manual and try to not wear transmission or brakes too hard but always happens on big road trips... so, subbed

 

I have it too, its more likely warped rotors than anything else.

 

Check rotors.
Check boiling point of fluid.