I am a curious engineer\computer scientist who once worked in automotive powertrain control systems (1990s).

My search on regenerative and friction brake blending produced 500 irrelevant hits on this forum, mostly folks lamenting on brake performance issues. A waste of time. My search on "regenerative and friction brake blending" on this forum produced exactly 0 hits. No time needed to waste.

The reason for this query has to do with my first (7,500 mile) service, where I am skeptical of the reporting on my brake pad thickness. All is good (I think) but the details made me scratch my head. Before anyone responds about dealers please don't. I've played that game and likely will do so again. I suppose I could remove the tires one by one and perform the measurement myself but I thought I'd try here first.

I am interested in learning more about the schema GM uses to blend regenerative and friction braking. Blending is a fairly common term to describe how the car goes from regenerative toward friction braking and back again. For example, when slowing down for a stop sign, if your foot is not on the brake pedal the car will decelerate using whatever level of regenerative braking you have set, until a point where it (the car) decides to engage the friction brake because a) the recovered energy is becoming too small, not enough to justify the efficiency gain, and\or b) you come to a complete stop and the friction brake kicks in. Yes, I know the paddle enters into this dynamic and I am aware of and understand the obvious corner cases of cold temperatures, SOC, etc. that are mentioned in the manual.

As a curious engineer, the details of how blending is determined and implemented is intriguing. So far I cannot tell if there is a transition or not (a good thing?) while driving until I get to an obvious point in diving (like a full stop). I've had exactly two (2) instances where I'm fairly certain that the friction brakes were engaged in situations not associated with the car coming to a complete stop. One was safety related in a parking lot (Kudos to GM).

Another way to ask a similar question (going back to dealer maintenance visits) is what "should" the normal friction brake wear on an EV (Lyriq) be compared to an ICE equivalent (size, weight) under normal driving conditions? This query on the web produced nothing of value, mostly hits from companies trying to sell something, and none from any EV forum.

My 2016 Buick Enclave, where I was easy on the brake pads, gave me 50,000+ on the fronts and about 100,000 on the rear pads. I found these same numbers on the web for EVs. Doesn't make sense to me, the braking technologies are very different.

What should I expect on my Lyriq under similar driving conditions? 100,000 on the front and 200,000 on the rears? That's why understanding the brake blending scheme is important to me.

Thanks for any constructive ideas or pointers.

I’m not sure there is anyone who can match your experience here. I also suspect a lot of the information you seek may not have ever been released by GM, but that’s just a guess.

For most of us, understanding that the braking starts with regen and as more deceleration is requested with a strong pedal push, friction brakes start to become involved is enough.

As I’m sure you know, regen braking greatly reduces wear on the friction brakes, which is why your dealers comments about brake pads needing replacement so early are highly suspect. If they really are worn that much, I would suspect something is malfunctioning.

 

I am a curious engineer\computer scientist who once worked in automotive powertrain control systems (1990s).

My search on regenerative and friction brake blending produced 500 irrelevant hits on this forum, mostly folks lamenting on brake performance issues. A waste of time. My search on "regenerative and friction brake blending" on this forum produced exactly 0 hits. No time needed to waste.

The reason for this query has to do with my first (7,500 mile) service, where I am skeptical of the reporting on my brake pad thickness. All is good (I think) but the details made me scratch my head. Before anyone responds about dealers please don't. I've played that game and likely will do so again. I suppose I could remove the tires one by one and perform the measurement myself but I thought I'd try here first.

I am interested in learning more about the schema GM uses to blend regenerative and friction braking. Blending is a fairly common term to describe how the car goes from regenerative toward friction braking and back again. For example, when slowing down for a stop sign, if your foot is not on the brake pedal the car will decelerate using whatever level of regenerative braking you have set, until a point where it (the car) decides to engage the friction brake because a) the recovered energy is becoming too small, not enough to justify the efficiency gain, and\or b) you come to a complete stop and the friction brake kicks in. Yes, I know the paddle enters into this dynamic and I am aware of and understand the obvious corner cases of cold temperatures, SOC, etc. that are mentioned in the manual.

As a curious engineer, the details of how blending is determined and implemented is intriguing. So far I cannot tell if there is a transition or not (a good thing?) while driving until I get to an obvious point in diving (like a full stop). I've had exactly two (2) instances where I'm fairly certain that the friction brakes were engaged in situations not associated with the car coming to a complete stop. One was safety related in a parking lot (Kudos to GM).

Another way to ask a similar question (going back to dealer maintenance visits) is what "should" the normal friction brake wear on an EV (Lyriq) be compared to an ICE equivalent (size, weight) under normal driving conditions? This query on the web produced nothing of value, mostly hits from companies trying to sell something, and none from any EV forum.

My 2016 Buick Enclave, where I was easy on the brake pads, gave me 50,000+ on the fronts and about 100,000 on the rear pads. I found these same numbers on the web for EVs. Doesn't make sense to me, the braking technologies are very different.

What should I expect on my Lyriq under similar driving conditions? 100,000 on the front and 200,000 on the rears? That's why understanding the brake blending scheme is important to me.

Thanks for any constructive ideas or pointers.

I found this informative video on YouTube:

It has some math in it, but it answers the bulk of my question regarding brake blending and its relationship to brake pad wear.

It turns out that depending on what is being maximized (energy generation vs. stopping) the friction brakes will likely be activated simultaneously, and periodically (i.e. not necessarily;y staying engaged once they have been engaged) as a function of several variables (component and system losses, operating conditions (heat/cold), etc.). So knowing exactly how GM or any other EV manufacturer optimizes their algorithms for their systems is less important than understanding that it is rare that the vehicle is only regenerating (or only friction braking) during a slow down or stopping event. So while overall brake pad wear might be somewhat improved, it may be too optimistic to expect significantly more (double) life from brake pads.

 

Though interesting (and well produced), the video in post #3 doesn't get deep enough into the actual real world aspects where this would happen (speeds, decel rates, motor sizes, etc). I wouldn't say it is theoretical, but overly math heavy without applying to what an actual EV would do.

Sure there will be certain very specific scenarios where adding in friction braking early might yield slightly more regen and stopping. But is any EV really doing this - or just trying to yield the full regen and decel that would occur - albeit some small energy not able to be reclaimed? Or if some EVs do this friction blending earlier in the decel than might be assumed, would it really have any measurable wear on the pad life?

GM has done blended braking for years even before Ultium - Volt / ELR / CT6 PHEV / Honda Clarity / Bolt. Other standard hybrids also. Like other EVs, these vehicles have had excellent brake pad life far exceeding a gas car - with many people reporting never needing to touch the pads over the life of the vehicle. Even if the blended brake strategy is somewhat altered on Ultium, it is still the 'use max regen first' methodology. If you drive and decel normally, you can easily see the regen kW power on the dash cluster - and this is significant. The energy screen also shows how much 'miles' have been reclaimed (can convert to kWh). Also note how cool the rotors always are when you park the car after a drive with a lot of braking, unlike a non-regen vehicle. All this continues to support that the vast majority of the decel is coming from regen and not friction braking.

That said, if you are a so called "digital driver" (on/off with stop and go, late aggressive stops), you will see more wear on the pads than a driver who doesn't do this. But even if you drive 'badly' like this, the pads are still going to live longer than a traditional vehicle given the substantial regen mixed in with the friction braking. You don't have to be a 'Prius' style driver to get the large benefits of the pad life given the regen-heavy blended braking system.

We also have a lot of early 2023 Debut Edition Lyriq owners here, and mileage is likely substantial for some now almost 3 years into ownership. Have not heard of anyone needing to replace pads, but perhaps those with high mileage can chime in how things are going.

For those who want to confirm or map how the system is working exactly (piece of mind, curiosity, etc), the best way to do this is with an Ultium compatible OBD2 scan tool. The Brake Control Module has much data in this regard, such as pressure at each wheel, and overall. An example of what parameters to look at below. One is the commanded pressure, and the other is the measured pressure in terms of voltage (feedback).

Monitor parameters like this when performing various stops from various speeds. Believe you will see no action here until the final stopping & holding - when using any method (light brake pedal, paddle, one-pedal).

 

I use the paddle by the steering wheel all the time to brake, so I would expect very little if any wear on my brake pads and rotors..

 

I'm a former auto engineer 30+ years in the industry mostly in and around GM. The early part of my career was in chassis and brake controls integration but most of it was spent in the electrification. Since you’re a fellow engineer I’ll be technical, but hopefully not too technical.

You didn't specify what the reported brake pad thickness is or what is marginal/ concerning to you. I'll assume for this response, you think it might be excessive. No modern GM vehicle should be experiencing excessive brake wear at 7500 miles unless you are doing something unusual with the car - like frequent heavy trailer towing down steep grades/lots of high speed stops/track days/autocrosses/etc.

GM began rolling out "patented" FNC/long life brake rotors over 12 years ago. FNC=ferritic nitrocarburizing. FNC isn't a new technology, but the application was. Your 16 Enclave had FNC rotors too – did you replace just the pads @ those intervals or the rotors too? 75k miles is a reasonable life expectancy for FNC rotors for an average motorist. Pads are more variable, but 50/100k F/R aren't unreasonable not being able to actually measure them now. I might expect closer to 75k on the front on average too, but there are other reasons besides minimum pad thickness that customers replace them.

The Enclave and Lyriq have similar gross vehicle weight ratings (less than 5% heavier for the Lyriq depending on trim) and similar brake sizes. Under similar usage conditions the Lyriq brakes should have a similar life span before any regenerative braking considered. Regenerative braking is just substituting negative brake torque with negative motor torque under some conditions. The foundation brake systems - rotors/pads/calipers use the same basic technologies between EV's & ICE's. The sizing and tuning are unique for each vehicle regardless of powertrain w/mass being the most important consideration. EV's are heavier of course, but it's the max GCVW that sizes the foundation brakes - so your Enclave is a good comparison point for the foundation brake system. Although the Lyriq is midsized, its GCVW is slightly more than the larger Enclave due to the batteries.

The brake apply systems (pedal, booster, hydraulics) are what's different between your Enclave and Lyriq besides regen capability. Technically, the Enclave can generate negative torque too with engine braking but that's not how most people drive. The apply system doesn't affect pad wear typically. Regenerative braking via the motor reduces the frequency and output needs of the friction brakes in some conditions, mostly in typical traffic/low g decels. Your pads and rotors should last longer with regen under average usage/conditions vs the Enclave. The blending between regen and the friction brakes is mostly a concern for driver control/passenger comfort. Drivers want consistent/uniform brake feel and performance regardless of how it's achieved. Passengers like smooth drivers. It is a challenging control problem for sure, but one that is easier to solve with brake by wire like the Lyriq has. It eliminates the direct driver control of the hydraulic control path. Instead, the driver makes a decel request and the vehicle determines the negative torque required at the wheels, blending motor and brake torque based on strategy and conditions. There are some cases where regen might be limited like at 100% SOC or certain thermal conditions.

The exact figures might be slightly better today than when I was working in the space directly, but regen was limited to about .3 g's of decel. You can get more regen from the front axle due to weight transfer and stability issues, so FWD or AWD has a theoretical advantage over RWD for regen considerations. This is why the Lyriq AWD has a slightly better city number, it's getting more regen in the city, but looses some efficiency for additional motor at steady state/highway. Regardless, over a certain decel level the friction brakes ramp up the brake pressure and provide more of the decel taking motor torque to zero at some point. Below the transition region, the friction brakes might be “primed” to close the gap between the pad and the rotor w/o applying much friction. This increases the response time in case of a much larger decel request/panic stop. The brake system already does this as a safety measure if the vehicle detects a potential collision, like seatbelt pretensioners tighten the belts. Also, as the vehicle nears a complete stop, there's a transition to friction brakes for smoothness and there's very little energy in the very end of the stop anyway. Motor control is very difficult at zero/near zero rpm and you also have mechanical factors like gear lash and axle twist that make a pure regen stop unpleasant for little benefit.

 

This is a wonderful post and I thank you for your thoroughness. I'm retired now but your post is definitely not too technical and hopefully others can benefit from this conversation. I will try to answer the questions you posed, but know your comparison between my (now sold) Enclave and my new Lyriq is greatly appreciated, and also makes perfect sense.

First, what I did not report. There's no panic on my part, but dealers in the past have tried to tell me my brakes were worn when they really were not. I even caught a dealer trying to tell me they never changed brake pads on my vehicle in their shop when they actually had. What I am assuming as the typical GM multipoint inspection sheet has various checkboxes and a few measurements. Your experience tells me you are likely very aware about what these sheets look like. At my recent 7,500 mile service checkup my tire tread, where the three categories are 8/32" or greater (green), 7/32 - 4/32 (yellow), and 3/32 or less (red), were all documented as measuring 6/32". When I got home I measured them myself and got 9/32 on each tire. That was red flag number 1.

The green, yellow, red categories for the brakes are Front: 7mm or greater, 6 - 4mm, and 3mm or less. The rears are 4mm or greater, 3mm, and 2mm or less. I am assuming the brake pads are greater than 7mm (8 or 9?) on the front and greater than 4 (5 or 6?) for the rear. The rears were documented as measuring 6mm and the fronts were also documented as measuring 6mm. When I see data like this I get skeptical about how the measurement was performed and what might be in error. My driving style and what I know about how the Lyriq was driven during the first 7,500 miles also gave me pause. The data just seemed inconsistent. Thus began my curiosity with the overall braking system. I'm the type of person who tries to first determine if I am doing (or not doing) something to cause an anomaly.

To your question about when the brake components on the Enclave were changed. I know dealers did one or maybe two changes on the fronts, none on the rears while I had the car (nearly 10 years - it was purchased on late 2015). I no longer have the records but I'm pretty certain at least one brake pad change was likely unwarranted. I am the original owner. At 90,000 ish miles that dealer I mentioned earlier tried to tell me my front pads needed changing and they had never replaced front brake pads on my Enclave, but they had, so I was done with them. At 108,620 miles (I still have those records) I changed the pads and rotors on the Enclave myself. The rotors were the originals and those FNCs were still in good shape but since I was in there I just changed them out. The pads (all of them) were still fine, but at least the new owner of our Enclave is assured of having all new pads and rotors.

I'm so glad you took the time to compare and contrast the Enclave vs. the Lyriq in terms of weight and ride. My Enclave was FWD and our Lyriq is RWD. I really appreciated the reminder about the Enclave's negative torque. Our frequent road trips between Atlanta and Nashville TN go through the hills of the south end of the Cumberland Plateau, so the Enclave was routinely generating negative torque on the downhill stretches to maintain its set cruise speed. I never really had to apply the brakes in these situations and rarely would the transmission ever downshift. Also, I'm one of those drivers who tries to let the vehicle glide to decelerate as long as possible rather than applying the brakes, and when I do, it's gently. I can count the number of times I hit the brakes hard on the Enclave over ten years on one hand.

With the Lyriq we obviously get some extra miles on those downhill stretches and even around town, and I understand your point about the theoretical advantage of AWD with regen. We chose the RWD due in part to where we drive most (US southeast) and that we frequently road trip (steady state driving). And yes indeed, I really do appreciate the challenges of the control problem, so I have always tried to be one of those smooth drivers for my passengers sake as well as my own. Your comment about motor control at near zero rpm also made me reflect on my years working on three-phase AC motor controllers. I definitely recall the diminishing returns of near zero rpm regen.

The fact that the friction braking systems between the two vehicles are similar due to the factors you mentioned answers my curiosity regarding why the two vehicles "feel" nearly identical and why the pad wear should be better on the Lyriq.

Thank you again!

 

Engineer here. I share your frustration regarding a lack of technical information coming out of GM about these vehicles. I had a similar experience when I was trying to determine the best charging strategy for battery health (deep discharge or daily charging).

Definitely a very interesting convo here. It's rare to see such knowledgeable and comprehensive answers on an EV forum. I got about halfway down this rabbit hole myself trying to determine how regen affects brake pad wear before realizing two things:

1. Regen braking is not unlike engine braking. Less effective in an automatic, and it doesn't put gas back in the tank, but if you have driven a manual transmission vehicle, part of the fun is downshifting instead of braking. I imagine that the results on brake pad wear is comparable to this type of driving in a manual ICE vehicle.

2. Given how many of the specs of GM's EVs match their ICE vehicles, it's again likely that the brakes were spec'd to wear at a comparable rate to ICE vehicles.

That said, I am very interested to see where this thread goes and see who will be the first to break out the ODB and drive around with a multimeter on each the brake actuators.

--Doug

 

The most critical thing to avoid for battery health is time @ very high SOC and high temperatures. The battery life and controls have gotten quite good. If it's convenient, I tend to stick to 80-30%. However, I'm not afraid to use 100-0 if needed. In practice I don't often go below 10% and only to 100% if its more convenient for me, which is infrequent based on my usage profile.
Battery life is a complex mix of time, temperature, charge/discharge rates, and cycles. The Lyriq already pops up and recommends 80% as a typical max for longevity. Statistically, the narrower your SOC window, the lower the charge/discharge rates, and fewer thermal cycles, the longer the battery might last. However, the BMS largely keeps you outside of the danger zone by design. Having some occasional larger range in cell voltage helps the BMS stay calibrated, but this isn't the same magnitude of concern that LFP batteries have. They do need more frequent charges to 100% to stay calibrated. LFP batteries have a very flat voltage vs SOC curve, so the BMS needs to see more of the extremes of SOC to see a change in voltage & stay calibrated. Other chemistries like LMC have a larger change in voltage vs SOC so it's easier to detect/manage.
It doesn't sound like your use case requires you to use 100-0 frequently, so leaning towards more frequent charges if that's convenient, in theory, might get you better life, but I wouldn't organize my life around it.

 

I have been wondering how the blending is performed. I have a Lyriq RWD and I have OPD shut off. I use the regen on demand to slow the car, although the braking action isn't very strong, which means I have to start using the regen paddle far (say 1000 ft) from any stop when moving at 50 mph or use the friction brakes to bring the car to its final stop in a shorter distance. If the system is blending the friction brakes with the use of the regen on demand feature, it is using very little friction brake.

I just spent two weeks traveling with my son in his AWD Lyriq and when OPD is turned off the Regen on demand generates a lot more braking force than my car does. I don't know if this is because GM uses both motors as generators in the AWD version or uses one motor and blends in the friction brakes. With both cars, I have seen well over 50KW of power generation when using the regen paddle. But when driving his car, I wonder if it is also blending the friction brakes in with the regen on demand, as I can bring the car to a full stop in a much shorter distance from 50 mph. If there is supposed to be a blending of the friction brake, I wonder if my car is working properly, although if it isn't, I am not sure I would want to fix it. I plan my driving and rarely use the friction brakes. On my Tahoe and C7 Corvette (which has a manual transmission), I find myself reaching for a non-existent regen on demand paddle to slow them before remembering it doesn't exist with them.

Bill

 

I have seen a couple videos of EV's failing state inspections because the rear brake rotors were rusted/pitted from not being used.

 

My relative drives a Tesla Model S first generation, and it now has 192,000 miles on it. He replaced the brake pads at 160,000 miles. This is info can be used as a reference point.