Different Brake Challenges in the 6MT and GR-DAT
I drive both a 2020 GR Yaris RZ High Performance with the 6-speed manual transmission and a 2024 GR Yaris RZ High Performance with the 8-speed GR-DAT on track.
Although they are both GR Yaris models, the brake-related issues I have experienced on track have been quite different.
With the 6MT, the main issues have been brake bedding and judder. With the GR-DAT, the main issues have been brake heat load and what appears to be vapor lock.
I believe this difference is closely related to how much work the friction brakes are doing in the way I drive each car.
Repeated Brake Judder in the 6MT
When I first began driving the 6MT on track, I started experiencing brake judder under braking.
At first, I assumed the brake rotors were warped.
I replaced the rotors, had them resurfaced, and also replaced the brake pads.
Immediately after servicing, the symptoms improved.
However, after driving on track for a while, the same kind of judder would return.
There were also times when rotor runout measurements did not show enough variation to explain the severity of the symptoms.
Because the problem kept returning, I wanted to describe it accurately to the mechanic. Rather than relying on the explanation of an inexperienced track driver like myself, I decided to ask a professional driver to evaluate the car and describe what was happening in terms that another professional could work with.
A Professional Driver Evaluated the Car
I gave the 6MT to a professional driver while the judder was present and asked him to drive it on track for about an hour.
The result surprised me.
The judder disappeared.
What was even more striking was the condition of the rotor surfaces.
Before the session, the rotors showed uneven coloration and gloss. After the professional driver’s session, the entire surface looked evenly polished, almost as if the rotors had just been resurfaced.
The rotors and pads had not been changed.
With the same rotors and pads, about an hour of different driving changed both the rotor surface and the brake behavior.
Later, when I resumed driving the car myself, the judder gradually returned.
That experience made me strongly suspect a relationship between my braking technique and the condition of the rotor surface.
Brake Judder and Rotor Surface Condition
During brake bedding, the friction condition of the rotor surface changes as the pads and rotors are used together.
As pad material is transferred more evenly onto the rotor, the friction characteristics around the circumference of the rotor also tend to become more uniform.
If the friction condition varies around the rotor surface, braking torque can also vary as the rotor turns. That variation can be felt as brake judder.
In my 6MT, the condition of the rotor surface appeared to change together with the presence or absence of judder.
This experience taught me that when investigating brake judder, it is important to look not only at the physical geometry of the rotor, but also at the friction condition of its surface.
The 6MT and Engine Braking
When I slow the 6MT on track, I make considerable use of engine braking.
I brake, downshift, match the revs, engage the clutch, and reduce speed using both the friction brakes and the engine.
In the past, I also tended to use the brake pedal relatively gently over a longer period of time.
In other words, two things were happening at the same time:
part of the deceleration work was being handled by engine braking, while I was also applying the friction brakes relatively gently and for longer periods.
As a result, the amount of work being put into the pads and rotors over a short period of time was relatively small.
The fact that the rotor surfaces became much more uniform after the professional driver used the car suggested that my braking technique may not have been supplying the heat and pressure needed to maintain a consistent friction surface.
Braking Shorter and Harder
I therefore changed the way I brake on track.
I now consciously use a shorter and harder initial brake application.
I try to create the required deceleration on the straight, then release the brake as I transition into the corner.
Since practising this approach, the brake judder has been greatly reduced.
The rotor surfaces still show some variation, but they now tend to maintain a much more even appearance than before.
In my case, improving my braking technique also appears to have made the bedding between the pads and rotors more stable.
The GR-DAT Beds In Naturally
The situation is very different with the GR-DAT.
With the way I drive it, the friction brakes handle a noticeably larger proportion of the deceleration than they do in my 6MT.
The brake pads also wear much faster.
That means the pads and rotors are converting a larger amount of the car’s kinetic energy into heat.
In the GR-DAT, I do not make any special effort to maintain brake bedding, yet I have not suffered from the same kind of recurring judder that troubled me in the 6MT.
It feels as though normal track driving itself provides enough brake work to maintain a stable friction surface between the pads and rotors.
Moving the 6MT Rotors to the GR-DAT
There was another particularly interesting experience.
I removed a set of rotors from the 6MT while they were producing judder and installed them on the GR-DAT.
During the very first part of the session, I thought I could still feel a slight trace of judder.
However, as I continued driving, the sensation gradually weakened and eventually became almost imperceptible.
The same rotors that had been producing judder in the 6MT became progressively smoother as I drove the GR-DAT.
That time-dependent change closely resembled what had happened when the professional driver drove the 6MT for about an hour.
This strengthened my hypothesis that the friction condition of the rotor surface changes during use, and that the way the brakes are worked plays a major role in whether the judder appears.
Heat Is the GR-DAT’s Challenge
With the GR-DAT, the main challenge has been brake heat rather than bedding.
Because my driving style relies more heavily on the friction brakes, repeated attack laps can produce a soft or spongy pedal, similar to the feeling of air in the system.
Because it occurs when brake temperatures are high during continuous track driving, I suspect that the symptom may be related to vapor lock.
I have not experienced the same symptom in the 6MT.
This appears to be another sign of the difference in how much work the friction brakes are doing in the two cars when I drive them.
Balancing Attack Laps and Cool-Down Laps
With the GR-DAT, I have begun slightly reducing the number of consecutive attack laps and increasing the number of cool-down laps.
Allowing the brakes to shed heat before the next attack lap helps keep the braking system more stable.
However, track-day conditions sometimes make attack laps a higher priority. Session length, traffic, and the limited opportunity to obtain a clear lap can all make it difficult to devote enough laps to cooling.
In that situation, improving brake cooling itself may become worthwhile, for example by directing more airflow toward the brakes.
The more I drive the GR-DAT on track, the more important brake cooling appears to become.
Different Problems in the 6MT and GR-DAT
My experience can be summarized quite simply.
In the 6MT, I use engine braking aggressively, while my previous braking style also involved relatively long and gentle brake applications.
That combination meant that the friction brakes were doing less work over short periods of time.
As a result, I found that maintaining a good relationship between the pads and rotors required more conscious attention to how I used the brakes.
In the GR-DAT, the friction brakes do more work in the way I drive the car.
Normal track use appears to maintain good bedding more easily, but pad wear and thermal load are greater.
In the 6MT, my main issues have been brake bedding and judder. In the GR-DAT, they have been heat management and suspected vapor lock.
Even in two versions of the same GR Yaris, the brake problems I encounter on track are very different.
Conclusion: Brake Work Changes the Problem
In the 6MT, I initially interpreted brake judder as a warped-rotor problem and responded by replacing pads and rotors or resurfacing the rotors.
Yet the same symptoms returned after more track driving.
When I asked a professional driver to evaluate the car, the judder disappeared after about an hour and the rotor surfaces became uniformly polished.
After changing my own technique toward shorter, harder brake applications, the judder also became much less severe.
Then I moved a set of judder-producing 6MT rotors to the GR-DAT. There was still a slight trace of vibration at the beginning, but it progressively faded as I continued driving.
The GR-DAT, meanwhile, has presented a different challenge. Brake bedding has not been difficult, but thermal load is much greater, and during repeated attack laps I have experienced symptoms that I suspect may be related to vapor lock.
With the way I drive them, the GR Yaris 6MT and GR-DAT place different amounts of work on the friction brakes. That difference appears in different ways: bedding and judder in the 6MT, and thermal load and suspected vapor lock in the GR-DAT.
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