What's stock braking power (not bias) on E46 M3?

[schwarz2]

Hey

I am running custom BBK: Brembo Z18 calipers from Cayenne and 996 rear calipers. From build journal brake bias guide, it shouldn't differ far from stock. I was recently experiencing a heavy rear braking (feels like pulling hand brake while driving) and decided to visit a shop to measure braking power on each axle.

The front outputted 2.72 kN and the rear outputted 2.59 kN, which is 611 lbf and 582 lbf respectively.

Now, are these values somehow close to the stock? Since only one axle was rotating at the time of breaking, is this expected that both front and rears are close together? Or... if my breaking bias was correct, the relation between front and rear would be closer to 60/40?

Trying to understand the nature of these results and whether it's my front that's too weak or rear that is too strong, or maybe it's something else.

[repoman89]

So this was measured at the tire one axle at a time via some kind of reverse dyno? Thats not measuring what you think it is if so. It’s measuring the tire’s ability to get force to the ground and those numbers shouldn’t be compared. When the brakes are applying torque to both axles spinning you may very well find that the front would read e.g 611 lbf while the rear reads lower for a given force at the brake pedal. This measurement sounds like it also doesn’t account for weight transfer toward the front leading to more friction capability for the front tire and less to the rear.

[liam821]

I'm not sure how you would measure such a thing, but if those torque numbers are to be believed your brake bias is 51% front 48% rear. I have no idea if the build journal numbers are accurate or not, but yeah, stock is around 60/40. Could be many things though, pads to rotor size, or perhaps you have some air in your brake system.

If you just recently started to have this problem, I'd start with bleeding the brake system and inspecting pad wear and go from there.

[schwarz2]

Thanks guys, I may have totally misunderstood the test and interpretation of its results. It was similar to dyno, I am not sure what's the actual tech behind this. This is what it looks like:



Anyways, hard for me to tell when this problem started, as I wasn't hard driving the car for past few years due to "stance" setup. Back to race one. Pads and rotors seem normal, I changed fluids recently. Could this be breaking force regulator needing some tweaks? Another suggestion today was soft rear suspension settings (running BC Racing). It's hard for me to believe a soft suspension would cause the rear to sit down by 1/2cm while breaking. Should happen with the front of the car if at all.

The only issue is when I press more than 50% of the pedal, it's when the rear "sits down". I know this, b/c my rear wheel rubs (I have to get smaller spacer, but it's unrelated). Otherwise, the breaking feels normal and nothing crazy. Pads are Ferrodo DS2500.

[oceansize]

Thanks guys, I may have totally misunderstood the test and interpretation of its results. It was similar to dyno, I am not sure what's the actual tech behind this. This is what it looks like:

Anyways, hard for me to tell when this problem started, as I wasn't hard driving the car for past few years due to "stance" setup. Back to race one. Pads and rotors seem normal, I changed fluids recently. Could this be breaking force regulator needing some tweaks? Another suggestion today was soft rear suspension settings (running BC Racing). It's hard for me to believe a soft suspension would cause the rear to sit down by 1/2cm while breaking. Should happen with the front of the car if at all.

The only issue is when I press more than 50% of the pedal, it's when the rear "sits down". I know this, b/c my rear wheel rubs (I have to get smaller spacer, but it's unrelated). Otherwise, the breaking feels normal and nothing crazy. Pads are Ferrodo DS2500.

I don't know about your problem but I'm thinking I need that setup in my garage and for my garage to look, generally speaking, like this. It's like Apple designed a garage, hence no tools, grease, or general uncleanliness. Do tools just appear when needed, like a little panel just slides out and over so you can pick what you need? Sheesh...

[repoman89]

Yeah that device is basically what I was picturing. I’m going to assume you’re running a square setup and what you measured was the friction capability of the tires. To really get at what the front/rear friction at the tire bias is you’d need to precisely control the force at the brake pedal and keep it the same for both measurements or measure both axles at once. For example if you find that 50 lbs at the brake pedal gets to your 611 at the front tire, then apply 50 lbs measuring the rear despite the fact that the tires can handle more. You’ll probably find the ratio is just about your brake bias if the system is working as it should. Then in real driving, weight transfer makes it so that the front/rear tires approach the limits of their altered grip at a similar rate.

This is why people just calculate it — it’s a simple system and not too hard to analyze to pretty good accuracy.

[schwarz2]

Thanks for the deep assessment, really appreciate it! The procedure was two fold.

Once the roller was spinning (< 10 km/h), I was told to gently press the pedal and once the gauges went slightly up, just floor it. Fraction of a second later, the roller stopped immediately. I think it was made to stop at the limit of the tire grip.

I am not sure if that’s what you meant by “same by both measurements” but in both cases, the pedal was in the floor and reading was right before wheel lock.

The technician said that either rear should be less, or front should be more. Since he wasn’t sure, I started researching what’s the expected value from a BBK (or stock) to decide which axle has issues before digging deeper.

[repoman89]

I am not sure if that’s what you meant by “same by both measurements” but in both cases, the pedal was in the floor and reading was right before wheel lock.

So there you have it. You measured what the brake system is capable of -- both front and rear calipers are perfectly capable of maxing out tire grip, and max tire grip is what you measured. The fact that you got the same front/rear is what led me to conclude that you're probably running a square setup. In this test the only thing that would make front to rear different at brake lockup assuming good measurements is different tire width. The test doesn't carefully control for force applied to the brake pedal, which is what ultimately directly translates to a friction force at the brake pads, torque at the rotor, and friction at the tire.

[bmwfnatic]

We use those brake testers here in Europe for technical inspection, you go onto them with one axle at a time and then brake hard, the purpose of this is to see if both L/R calipers are somewhat providing the same braking power.
So for instance if one of the calipers has a (somewhat) stuck piston, the delta between L and R will be way off.

I think the others are right and this is not a valid way to compare between front and rear axles, mainly because the vehicle has no actual velocity, it is still pretty much stationary, and both axles should be able to trigger abs and therefore 'max' out the braking system, leaving only the tires as variable.

[Mike RT4]

As an aside, if you're planning on tracking the car, you will need better pads than the DS2500s. They're basically a fast road pad and if use agressively on track in a heavy car (>3000lb), the glue within the pad material will melt and leave deposits on the discs (showing up as black shadows) that will then give disc run-out (felt as judder - like they are warped). If left on the discs, it will eventually warp them due to uneven heat application (the material on the disc acting as a barrier).

[schwarz2]

As an aside, if you're planning on tracking the car, you will need better pads than the DS2500s. They're basically a fast road pad and if use agressively on track in a heavy car (>3000lb), the glue within the pad material will melt and leave deposits on the discs (showing up as black shadows) that will then give disc run-out (felt as judder - like they are warped). If left on the discs, it will eventually warp them due to uneven heat application (the material on the disc acting as a barrier).

I will definitely need to upgrade pads and rotors to something more track focused and premium, but I am trying to asses whether those calipers are worth it or is better to replace with something better. I am trying to conserve unnecessary money spends until I reach profesional stage, so I don't think I would be looking at anything beyond OEM if it wasn't done by the previous owner already.

I think the others are right and this is not a valid way to compare between front and rear axles, mainly because the vehicle has no actual velocity, it is still pretty much stationary, and both axles should be able to trigger abs and therefore 'max' out the braking system, leaving only the tires as variable.

Yeah, looks like it. Thanks for educating me on this, it will save me a lot of time investigating false positives. I will report back once I figure this out, just in case someone else runs into it in the future.

[jvit27]

"BRAKES"
"BRAKING"