Question - Unsettling experience while braking

[rpatton]

Hi, I was recently driving on a very steep, straight and paved downhill road (westbound road through death valley) realized my speed was about 85/90 mph and starting applying my brakes - lightly. The front of the 4runner started shaking pretty violently, release brakes and it stops. I tried this several times, each time the front shook quite a bit. I'm not sure what to compare it with other than it felt like speed wobbles on a motorcycle, though up and down versus side to side. Pretty unnerving as I seemed to have no safe way to slow down.

I asked my Toyota dealership what it could be and they said it was the ABS. I've felt the ABS come into effect before, but only when I braked hard / slammed on the pedal. I certainly didn't do that in my situation, at that speed on a downhill I was very carefully easing the pedal, very light as I have no desire to flip. I had also just had the car's 20K service, fluids and brakes checked 2 days before that trip.

Any ideas what that shudder could be? It stopped happening as soon as the road leveled out, on the flat I could brake with no issues.

2010 LE in H4H.

Thanks!
Rob

 

[Okki]

My guess is that the speed and incline of the hill played a good role. Possibly the ABS thought you were in significantly more need of assistance compared to what you thought. Not sure if the Down Hill Assist played a role at all, but it may have been a factor.

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Okki

 

[philsey]

Well, at least he can spell brea .... err braking.

 

[westwind77]

realized my speed was about 85/90 mph

Not sure if the Down Hill Assist played a role at all, but it may have been a factor.

Wow 90mph in 4LO, how many times did the rpm needle spin around in a full circle?!?! Must have been creating one he!! of a smoke screen behind him too!

:hiding:

 

[jnance50]

Sounds like death wobble on an inverted y-steering setup on a live axle ...
yeah, I was a Jeeper

The only thing I can think of on the T4R is maybe a really badly slick spotted front brake rotor?

 

[icsa]

The front of the 4runner started shaking pretty violently, release brakes and it stops. I tried this several times, each time the front shook quite a bit. I'm not sure what to compare it with other than it felt like speed wobbles on a motorcycle, though up and down versus side to side.

Sounds like a warped brake rotor. If the 4runner is under manufacturer warranty, you should be able to get the dealer to replace it. They might refuse, but maintain your position that a rotor should not warp in less than 36K miles.

 

[knc1us69]

similar here

Similar experience while going uphill on windy north carolina mtn road (paved) but very tight turns and felt I was pushing my speed a little much trying to overtake a vehicle prior to two lanes merging to one. Probably pushing front end too hard but something similar to what you described which irritated me as I was under control and this vibration from abs or something else was more dangerous than if I had actually skidded. I have experienced one other time downhill at much lower speed but again didn't like the feel vs..no assist at all. Just my opinion.

 

[knc1us69]

not rotor in my case

Failed to mention mine was not rotor related. 1200 miles on new 2011 TE and rotors fine. Def something with abs or traction control of some sort but vibration was radical.

 

[1000MPH]

To the OP: did your engine rev up? The brake assist or whatever it is, will downshift to help slow the car on downhill braking. I'm not a fan of the way it works but never had any shaking from it.

knc1us69: that was most likely VSC and I agree it seems more dangerous than actually skidding.

 

[icsa]

Failed to mention mine was not rotor related. 1200 miles on new 2011 TE and rotors fine. Def something with abs or traction control of some sort but vibration was radical.

Abs should intervene only during lock up situations and the only vibration you should feel is in the brake pedal (different story on snow/ice). How do you know your rotors are fine?

 

[Jtraskell]

"It stopped happening as soon as the road leveled out, on the flat I could brake with no issues."

If it were rotors, I would suspect the OP would feel some sort of vibration every time he used the brakes.

OP, are you still running the OEM "soft sidewall tires"? If so, at what PSI are you running? I have seen videos before of what happens to front tires during heavy braking. They "squish" down. Some tires, will deform a surprising amount. Going down hill at that speed, that effect could be happening even with slight braking. Among other things, the effect causes momentary "flat spots" making the tire out of round. Could be the tires.....just a guess.

 

[knc1us69]

Abs should intervene only during lock up situations and the only vibration you should feel is in the brake pedal (different story on snow/ice). How do you know your rotors are fine?

30 years of working on braking systems including working in pits for both circle track and drag racing. That's been a while but still stay up to date on classic and muscle cars as well as my other rides. To be more specific there are no stress marks, no irregular pad wear, no rotor pitting or glazing plus rotors have 1600 miles on them total. Also, not an issue with slowing from speed quickly or otherwise, zero constant vibration only found in two situations where front end was mid turn under a little excessive speed and on attempted braking erratic vibration which could be felt heavily in steering wheel more than any other vehicle i own under the same scenario. My issue was an intervention system such as VSC mentioned above which was not a desireable response. Definitely not a rotor issue for my two instances.

 

[rouge1]

I have not had the VSC engage on my 4Runner yet, but did have do it once when I thought I was in complete control of the vehicle in my 07 Tundra. Scared the $h!t out of me, shook shimmied lights and enemy middle lock tone. The abs kicked in and out as the Tundra compensated for my driving. It also made the wonderful abs grinding sound.

 

[knc1us69]

bingo

All the same as my instances. I guess for some cases thats great but i was much less sure of myself with that happening than if i had been left alone. Thanks for the post. Great description.

 

[westwind77]

My issue was an intervention system such as VSC mentioned above which was not a desireable response. Definitely not a rotor issue for my two instances.

I would be shocked beyond belief if it was your VSC system that engaged! Have you tested your VSC to see the 'limits' when it activates? I have, and unless you were skidding (front sliding out, rear end sliding out, etc.) the system does not engage. When it does engage your will get an audible beep and dash 'idiot' light flashing, my wife calls it the 'stop driving like a a-hole' warning system If you were driving straight, on dry pavement, IMO, no way it was the VSC system, not from my experience playing with it. I agree with the rest, the issue is in your brakes or your tires, or a combo of both.

 

[Ukie]

"
If it were rotors, I would suspect the OP would feel some sort of vibration every time he used the brakes.

I would disagree with your statement. I'm experiencing vibration/pulsation while braking going steep downhills !!!only!!! It's happening on my Accord which is due for some TLC: front rotors resurface and pads replacement.

 

[rpatton]

Thanks for all the info, it is helpful. To the various questions or statements - I was not in 4LO, I was in H4F which I mistakenly gave as H4H is the original post. My tires are the original ones, had checked and made sure they were at the pressures on the door sill the day before, 32 psi I believe. The engine was at about 3k RPM, I wasn't on the gas at the time, I'm not sure whether that means a brake assist was taking effect. And I don't get any shake when applying the brakes normally. The point about vibration through brake pedal is interesting, I did not get that, while the front end was shaking I don't think the pedal had that.

One suggestion I had from someone is simply dirt on the rotors. I had been on some fairly rough unpaved roads through mountain passes that weekend.

Thanks all!

 

[knc1us69]

I would be shocked beyond belief if it was your VSC system that engaged! Have you tested your VSC to see the 'limits' when it activates? I have, and unless you were skidding (front sliding out, rear end sliding out, etc.) the system does not engage. When it does engage your will get an audible beep and dash 'idiot' light flashing, my wife calls it the 'stop driving like a a-hole' warning system If you were driving straight, on dry pavement, IMO, no way it was the VSC system, not from my experience playing with it. I agree with the rest, the issue is in your brakes or your tires, or a combo of both.

def not tires or rotors, I am very familiar with those during a failure or issue (these were at 1200 miles on new rotors and new BFG 285 A/T KO's. As I stated previously, I was pushing it too hard and at the point of loss of traction (front end should have started squealing lightly, and yes I heard the idiot buzzer and got the dash light warning for just an instant. I was merely saying if it was like my case the VSC or whatever, on the 4Runner is far more intrusive than on previous SUV's, our current Lexus (Yota product), or our Subaru. This was borderline so intrusive and frankly "weird feeling" that the outcome could have been undesirable. I was at the limits of the traction control and or whatever device was impeding as I stated and pushing it too hard but with other vehicles I would have more or less a loss of power as opposed to the abrupt shaking / loss of power / buzzer, etc. I called the dealership this a.m. who confirmed most likely this was performing as it should; I just DON'T LIKE IT PERSONALLY.

 

[knc1us69]

meant def not tires or rotors in my cases. have no way of knowing whether or not the OP's case was what I experienced or not, my bad as I didn't clarify in the post above I was only speaking to my instances, not OP

 

[Jtraskell]

I would disagree with your statement. I'm experiencing vibration/pulsation while braking going steep downhills !!!only!!! It's happening on my Accord which is due for some TLC: front rotors resurface and pads replacement.

That would be a first for me, if a rotor is warped, it is warped all the time. I have had plenty of warped rotors over the years. Yes, they were MORE pronounced as I braked "harder" going down hill.....but If I braked hard enough on a flat road, I would also feel it. Unless you tried hard braking on a flat surface and can not duplicate the condition, I am having a hard time saying it is your rotors. Could your abs be kicking in? That also causes brake pedal vibration.....just a thought. Be safe.